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Computational Tools Appliedto Urban Engineering

Armando Carlos de Pina Filho,Fernando Rodrigues Lima,Renato Dias Calado do AmaralFederal University of Rio de Janeiro (UFRJ), Brazil

The objective of this chapter is to present some ofthe main computational tools applied to urbanengineering, used in diverse tasks, such as:

conception, simulation, analysis, monitoring andmanagement of data.

In relation to the architectural and structural project,computational tools of CAD/CAE are frequently used.One of the most known and first software created toPersonal Computers (PCs), with this purpose, was theAutoCAD by Autodesk. At first, the program offered 2Dtools for design assisted by computer, presentingtechnical and normalisation resources. After that, theprogram started to offer 3D tools, becoming possiblethe conception and design of more detailedenvironments. The program is currently used forconstruction of virtual environments (or virtual scalemodels), being used together with other programs forsimulation of movement and action inside of theseenvironments.

Another software very used currently is the ArcGIS,created to perform the geoprocessing, in which toolsand processes are used to generate derived datasets.Geographic information systems (GIS) include a greatset of tools to process geographic information. Thiscollection of tools is used to operate information, suchas: datasets, attribute fields, and cartographicelements for printed maps. Geoprocessing is used inall phases of a GIS for data automation, compilation,and management, analysis and modelling ofadvanced cartography.

In addition to the programs of CAD and GIS, otherinteresting technology is related to BuildingInformation Modelling (BIM), which represents theprocess of generating and managing building dataduring its life cycle using three-dimensional, real-time,dynamic building modelling software to decreasewasted time and resources in building design andconstruction. Some of the main software used for BIM

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are Autodesk Revit Architecture and Vico Constructor.

Computational tools for monitoring and management arevery important for the urban development. Several urbansystems, such as: transports, water and sewerage system,telecommunications and electric system, make use ofthese tools, controlling the processes related to eachactivity, as well as urban problems, as the pollution.

Therefore, in this paper we will present details about thesetechnologies, its programs and applications, which will tserve as introduction to the use such computational toolsfor study and solution of urban problems.

2. CAD (Computer-Aided Design)

It is a technology largely used in the conception of projectsof Engineering and Architecture. It consists of a softwaredirected to the technical drawing, with severalcomputational tools. Amongst the areas in which the CADis applied, we have the Urban Engineering.

Urban Engineering studies the problems of urbanenvironments, emphasising the creation of plannedenvironments to be sustainable, considering the balanceof economic, territorial, and social factors. Theinfrastructure urban systems are subject of study,searching to optimise the planning of the environment,sanitation sectors, transports, urbanism, etc. It is in thiscontext, that we can begin to understand the use of CADprograms in assisting urban projects.

In respect of development of CAD software, we observethat without the postulates of the Euclidean Mathematics(350 B.C.) it would not be possible to create thiscomputational tool. Thousand of years later, morespecifically at the beginning of the 60th decade of the20th century, Ivan Sutherland developed, as thesis of PhDin the Massachusetts Institute of Technology (MIT), aninnovative system of graphical edition called "Sketchpad".In this system, the interaction of the user with the computerwas perform by "Light pen", a kind of pen that was useddirectly in the screen to carry through the drawing,together with a box of command buttons. It was possibleto create and to edit 2D objects. Such system was alandmark in computer science and graphical modelling,considered the first CAD software.

In the beginning, the use of CAD software was restrictedto companies of the aerospace sector and automobileassembly plants, as General Motors, due to the high costof the computers demanded for the systems. Suchsoftware were not freely commercialised in the market.The Laboratory of Mathematics of MIT, currently calledDepartment of Computer Science, was responsible forthe main research and development of CAD software. Inother places, as Europe, this type of activity was started.

Other prominence developers were: Lockheed, withCADAM system, and McDonnell-Douglas, with CADDsystem.

Fig. 1. Example of virtual scale model: Hospital Metropolitano Norte, Pernambuco,Brazil (http://acertodecontas.blog.br)

From the 70th decade, CAD software had passed to befreely commercialised. The first 3D CAD software, CATIA -Computer Aided Three Dimensional InteractiveApplication, was developed in 1977 by French companyAvions Marcel Dassault, that bought the Lockheed,revolutionising the market. The investments, as well asthe profits, vertiginously grown. In the end of the decade,programs for solid modelling already existed, as, forexample, the SynthaVision of the Mathematics ApplicationGroup, Inc. (MAGI).

From 1980, with the development of the first PersonalComputer (PC), by IBM, the Autodesk released, inNovember 1982, the first program of CAD for PCs, the"AutoCAD Release 1". In 1985, the Avions Marcel Dassaultreleased the second version of CATIA. In this same decade,the workstations (microcomputers of great efficiency andhigh cost, destined to technical applications) weredeveloped, using the operational system UNIX. In the 90th

Fig. 2. Interface of AutoCAD software

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decade, specifically in 1995, the SolidWorks companyreleased the SolidWorks 95 3D CAD, revolutionising themarket for used the operational system Windows NT, whilethe majority of the programs developed was destined toUNIX. In consequence of this, SolidWorks 95demonstrated to be a software with good relation of cost-benefit, when compared with the competitors, excessivelyexpensive.

In the following years to present time, the technologycomes being improved and the software became veryaccessible around the world, with open access versions(freeware). An important application of the 3D CADprograms is the creation of virtual environment, also knownas electronic or virtual scale models (Fig. 1). Suchapplication is largely used in architecture projects.

2.1 Working with CAD

As previously said, we had a great development of CADsoftware in the last decades. Amongst the main programsof CAD, the AutoCAD (http://www.autodesk.com.br) isdistinguished. The software developed by Autodesk hadits first version released in 1982, and recently, theAutodesk released the AutoCAD 2010.

Fig. 3. Project in SolidWorks (http://www.danshope.com)

The AutoCAD (Fig. 2) is a 2D and 3D modelling programwith several applications, such as: mechanical, civil,electric, and urban engineering projects; architecture;industrial manufacture; and HVAC (heating, ventilationand air conditioning). It is important to notice that theAutoCAD is also largely used as tool in academicdisciplines of technical drawing.

AutoCAD have commands inserted by keyboard, makingpossible a practical creation of entities (elements of thedrawing), at the moment of the conception of the desiredmodel, optimising the work of the designer. Such

commands substitute the necessity of navigation with themouse to manipulate the toolbars.

Fig. 4. Example of project of Civil Engineering - a highway (http://usa.autodesk.com)

The program generates diverse types of archive, whichcan be exported to other programs. Some examples:DWG (*.dwg); 3D DWF (*.dwf); Metafile (*wmf);Encapsulated (*.eps); and Bitmap (*.bmp). DWG archiveis an extension shared for several CAD programs.AutoCAD is capable to import archives of the type 3DStudio (*.3ds), from Autodesk 3D Studio Max. User ofAutoCAD is able to associate with your projects, programsmade by programming languages, such as: Visual Basicfor Applications (VBA), Visual LISP e ObjectARX. AnotherCAD software largely known is the SolidWorks (http://www.solidworks.com).

Developed by SolidWorks company, from group DassaultSystèmes, is a 3D CAD program for solid modelling,generally used in the project of mechanical sets (Fig. 3).

SolidWorks can also be used as CAE software (Computer-Aided Engineering), with simulation programs, such as:SolidWorks Simulation, and SolidWorks Flow Simulation.SolidWorks Simulation is an important tool of analysis oftensions in projects. The program uses finite elementmethods (FEM), using virtual application of forces on thepart.

SolidWorks Flow Simulation is a program of analysis ofdraining, based on the numerical method of the finitevolumes. This program allows the professional to getreasonable performance in analysis of the project underreal conditions.

SolidWorks is compatible with DWG files generated byAutoCAD, being able to modify 2D data or to convert into3D data.

Other interesting CAD programs include: CATIA(Computer-Aided Three-dimensional InteractiveApplication), developed by Dassault Systèmes andcommercialised by IBM (http://www.3ds.com), and Pro/

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ENGINEER, developed by Parametric TechnologyCorporation (http://www.ptc.com).

2.2 Application of CAD

CAD software have as main use the aid in projects of CivilEngineering and Architecture for urban environment, suchas: buildings, roads, bridges, etc (Fig. 4). CAD also iswidely used in the project of transmission lines of electricenergy. Such practice consists in optimise the allocation

Fig. 5. Schema extracted from ArcGIS Reference manual showing the three viewsof GIS

of transmission towers and wires, in accordance with thetechnical norms. An important characteristic is thetopography of the land.

Other applications in Urban Engineering include: themaintenance and update of sanitary networks, and theenvironmental recovery in urban areas. In the first case,CAD is used to update the database of the sewer networkof the city, supplying detailed information. In the secondcase, CAD is used for mapping of a region, with the aid ofa GPS system (Global Positioning System), identifyingenvironmental delimitation (sources of rivers, roads,buildings, etc)(Mondardo et al., 2009).

There are several other applications of CAD in urbansystems and areas related to Urban Engineering, and it isimportant to note, in practical terms, that CAD is nearlyalways associate to other technology: GIS (GeographicInformation System), that it will be seen to follow.

3. GIS (Geographic Information System)

Engineering problems were on the last 40 years gradually

directed to employ computerized solving techniques.Precision and increasing speed for calculating multi-variable operations are a good reason to usecomputational resources, but the quite unlimitedpossibilities to organize, simulate and compare dataturned computer sciences on a strong allied for researchand design activities.

The final claim to say that now we are living in aninformation systems age is the large accessibility ofhardware and software, the diffusion of personal systemsand all related facil it ies: servers, networks,telecommunications, etc.

An information system can be defined as an organisedquiver of tools and data that can be used to answer on asystematic way questions structured by specialists. Asthese questions can be classified in patterns, it shouldbe possible to build on artificial intelligence to make thesystem learn and deliberate by itself.

If the answer to a problem employs variables associatedto geographic information, it's recommended the use adataset structure to implement and model graphic objectsthat represents all on earth, natural or artificial. AGeographic Information System (GIS) is a set of tools thatwork with data presenting three basic concepts (Fig. 5):Geodatabase, Geovisualization and Geoprocessing(Harlow, 2005).

Geodatabase represents the set of spatial data that canbe expressed by rasters, vector features, networks, etc.,and every rule to control their creation and management.Geovisualization is an action performed on spatial databy intelligent maps and views, from which we can viewthe database for querying, analysing and editing.Geoprocessing is the term used to designate operationson datasets that obtain outputs of analyses and generatenew information.

Fig. 6. Vector features overlays raster satellite image

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Some engineering projects have territorial themes asindustrial projects, social benefits, general infrastructure,logistics, demography, and other geo/urban/environmental aspects. On those cases the solutioninvolves studying geographic elements and their availableinformation, in order to perform technical analyses. So aninformation system for geographic data organisation,visualisation and processing will be appropriate to thoseproblem solving. To be efficient as a GIS, the system mustperform some general tasks:

acquire, convert, organise and project the geographicelements; import, organise and extract imagery, numericand textual information; process geographic elementsand information with data integrity and operationalefficiency; display appropriately the data and relatedoperations (geoprocess techniques); perform simulationsand comparison of alternatives; present support forprogram language and custom computational routines;generate new data based on selected results; publishmaps and all sort of documents for project discussion;and permit data interchange with other systems.

As we can observe, GIS is designed to manage spatialdata, and the geographic representation of this data canuse many types of elements for plotting the information(Fig. 6): vector based features classes, as points, linesand polygons; raster datasets, as digital elevation modelsand imagery; networks, as roadways, pipelines, hydrologyand other interlaced elements; survey measurements, astopographic annotation; and other kind of information, aspostal codes, address, geographic place names, etc.

These elements can be organised by layers, and couldbe selected by pointing or grouping for edition tasks orcustom display. The selection methods could also beperformed from spatial analyses or statistic classification.Georeferenced co-ordinates and related data tables ofGIS elements help to improve these tasks.

Geographic data representation has integrity rules(Harlow, 2005), performed by spatial relationship patternsbetween elements, as topologies and networks.Topologies are used to manage boundaries behaviour, toapply data integrity rules, to define adjacency andconnectivity properties, to structure creation and editionof new geometry, and to express other topologicaloperations. They are used to represent area contours,parcels, administrative boundaries, etc. Networks areused to represent graphs and their connections,controlling paths, barriers and flows. They are used torepresent behaviour of pipeline, transportation, traffic, etc.

Although organisation and management of spatial datacan be well attempted with modern GIS programs, thereis until an important aspect: how to deal with data quality.

Fig. 7. Example of a workflow model for GIS based research on industrial location

The cartographic databases can be generated from oldcharts or maps digitalisation, or from satellite and aerialimagery treatment. The numeric and textual databasesmust be converted into tables, and quite often comesfrom census and researches output. A great variability ofdata procedures can be observed world-wide whenintegrating data obtained from different fonts, places andscales. The periodicity of data actualisation is anotherdeal to GIS users.

The problems don't result ever from confidence, trustedfonts may have different methodological approaches, andpersonal interpretation can also give different validoutputs. Professional development of GIS operators canhelp them to detect, evaluate and work that variability,and a methodological approach is needed to treat itsuitable to each research task.

3.1 Working with GIS

Many users can be satisfied on using GIS as a datasetmanagement tool for generating maps and classify data,but nowadays GIS is turning on a knowledge approach,where models incorporate advanced behaviour andintegrity rules. The ultimate development on GISprocedures is directed to intelligent use of geoprocessingfor built, explore and share the possibilities of geographicinformation. Users now are able to structure schemas andworkflow models in order to improve their geoprocessingtasks, as import, check, integrate and compose data(Fig. 7).

As GIS is the best way to work data from local to globallevel, an efficient DBMS (Data Base Management System)is needed to perform data integration, actualisation,access and sharing. As result, GIS catalogue portals basedon Web nodes are increasing in number and theirinteroperability is part of a concept called SDI (SpatialData Infrastructure). Servers are used to host enterpriseGIS and their databases, and to provide multi-user access.Geographic Databases are employed to control anddevelop published data, as maps, features and tables.They are known as Geodatabase, have a proper logic to

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work with datasets by applications and tools, and performaccess and management tasks.

But GIS capabilities can also provide single users tocustomise their data. A Personal Geodatabase (PGDB) isan example of option to collect and organise featuresand tables attempting to user needs, using desktopcomputers at low cost and with feasible results. If you area adviser or researcher and are in charge of studyingurban problems, you can go ahead on mounting yourPGDB, however some steps must be observed.

The start point is to structure correctly your problem,identifying the factors and conditions that impacts on, amethodological approach to face it, and a technicalprocedure to get alternatives and produce results.

First, you must study what kind of information you need,identify the sources and think about layer and featuresorganisation. Next, you must acquire geographic datafrom GIS portals or institutional sources. Many researchand administrative institutions provide download of vectorand raster data from their DBMS, or send it by request. Ifthere is no available geographic data, it will be necessaryto digitalis existing map and imagery, but for this task isrecommended a professional with advanced knowledgeof geodesics, cartography and geoprocessing.

After getting the appropriate geographic information isimportant to know that vector data is usually related to atable, which has a column whose contents link the graphicrepresentation to a register. Raster image has pixelposition attached to a co-ordinate value. Vector featuresas point, line or polygon has as code number for the systemlink requirements, but can also have a code for geographiccadastral purposes (Fig. 8). Geocode is a tendency onGIS procedures and has the advantage to make easylater joins and relates of table data with none geographicplot.

In other words: if you get a basic data of shapes withrelated table presenting geocode column you canaggregate new data from other ordinary tables that hasalso this geocode column. GIS also enables visualisationof each element by selecting it from geocode, and permitsediting the tables to insert new columns containing yoursown information. Second, you must organise your features

Fig. 9. Use of GIS in the mapping of water and sewer ducts (http://www.gis.com)

and tables in a dataset, defining co-ordinate systems andimporting independent features and tables to the PGDB.This modality of data organisation provides more securityand flexibility, increasing edition and analysis tasks.

Working with stand alone features can face restrictionsthat are not present on a PGDB structure, as it works moreproperly with layers, overlays, projections and co-ordinates. Third, you must know what to do to improveyour queries on GIS ambience. It is a lost of potential touse a GIS only for data visualisation or map creation, thereis more than this. Both DBMS or PGDB can generate dataperforming spatial analysis or statistic classification. Asyou have the demands of your research well structured,GIS can help you to answer by crossing multi-layerinformation, selecting and editing data from SQL(Structured Query Language) statements and processingnew features containing partial and conclusive results.

Finally, you must obtain a valid output for your problemsolving, and communicate it to others on a suitable way.GIS can help you on producing thematic maps, analyticalgraphs and technical reports. You can also get community,representatives and specialists to work in a participativemode using GIS to generate and validate output ofdecision sessions. Some people have difficulties toidentify and interpret geographic elements, and GIS canhighlight and detach text and visual information formaking it easier.

Fig. 8. Vector features as point, line and polygon with associated table containinggeocode

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Application of GIS

GIS technology is much used in Urban Engineering toanalyse, in a detailed way, characteristics related to urbanplanning. In addition to CAD, GIS presents solutions forseveral problems, and it is applied, in a integrated way, inprojects of Civil Engineering and Architecture, includingthe most diverse urban systems (Fig. 9), making possiblethe maintenance and update of service networks, as wellas the environmental recovery in urban areas.

Nowadays, the accessibility of GIS technology stimulateseducators to work in a new concept, called GeographicalInclusion, which can be performed on basic educationclass in order to provide young students with geographicvisualisation and interpretative capabilities. We are livinga age of saving resources, environmental care andsustainable actions, and GIS with his solving problemdesign and participative net work potential is the moststrong partner in managing data for this purposes.

Concluding the technologies presented in this chapter,we will see to follow the BIM technology (BuildingInformation Modelling), that it represents, in a certain way,an evolution of CAD technology, previously presented.

BIM (Building Information Modelling)

It is a technology that consists in the integration of alltypes of information related to conception and executionof a project of Civil Engineering. Such information, storedin efficient database, not say respect only to design or tomodelling of plants and virtual environments, but also tomanagement of execution time of project, geographicinformation, quantification of material used in all building,detailing of the constructive processes, sustainability, etc.In short, the technology makes possible that the work teamhas an integrated vision of the project. This allows, forexample, that engineers and architects idealise andexecute the project sharing the same base of information.This technology has been spread together with thepractice of Urban Engineering.

In a certain way, BIM is seen as an evolution of 3D CADtechniques. In fact, this technology is defined as 4D CAD,where the fourth dimension is not physical, but the set ofinformation that go beyond the engineering concepts,used in the development of the project.

The use of BIM can mean an effective optimisation of timeand increase of the productivity levels. Other importantcharacteristic is the easiness to perform modifications inthe project, in any phase of execution. BIM makes possiblethe meeting of information, such as: the documentationof licensing for building, the established environmentalconditions, and other legal aspects that are of extremeimportance for execution of the project. Thus, the

technology allows to greater efficiency in the taking ofdecisions during the elaboration of the project, easinessin the emission of building documents, establishment ofdeadlines, estimate of costs, information about theanalysis of risks and management of the operationalconditions of the installations.

Fig. 10. Interface of Autodesk Revit Architecture (http://images.autodesk.com)

Using a CAD software in an engineering project, thedesigner inserts detailed specifications through theheadings, for example: specification of the material usedin the confection of a wall, manufacturer of the material,necessary amount. In the case of BIM technology, suchinformation is directly inserted in the drawing at themoment of the modelling.

Working with BIM

In BIM technology, a set of tools provided by one or moresoftware is used for: modelling of surfaces; modelling andstructural calculation; management of the building;manufacture management; environmental analysis;estimate of costs; and specification.

Fig. 11. Interface of Google SketchUp (http://www.crackvalley.com)

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Autodesk Revit Architecture (usa.autodesk.com)(Fig. 10),is one of the main BIM software, having: tools of 2D and3D drawing; co-ordinated database, in such way thatalterations performed in the information are automaticallyupdate in all model, reducing the possibility of errors and/or omissions; associative sections of divisions table;libraries of details, that can be created and be adaptedto the patterns of the project team; parametriccomponents, that function as an open graphical systemfor design concern and shape creation; inventory ofmaterials, that allows the calculation of detailed amountsof material, updating while the project evolves, on thebasis of parametric alterations; etc.

project; exportation of DWF files (used in CAD programs);and navigation in real time.

Other popular freeware is the Google SketchUp (http://sketchup.google.com)(Fig.11), much used in theacademic area, presenting modelling by means ofsurfaces. Such software presents limitations comparedto the programs already cited. SketchUp works efficientlywith information related to the localisation, size anddesign, reason for which is used in the confection of modelsthat can be exported to programs, as for example, theAutodesk NavisWorks Review.

There are several other programs related to BIMtechnology, as for example, Vico Constructor (http://www.vicosoftware.com), presenting diverse chara-cteristics and resources, such as: the structural analysisof the building; the constant update of the information,correcting possible errors of execution; the estimate ofcosts of the enterprise; etc.

Application of BIM

As well as CAD and GIS technologies, BIM presents aseries of applications in the area of Urban Engineering,and currently it comes substituting CAD, in a effectiveway, because it presents advantages in relation to themanagement of the projects.

A current example of BIM application is the NationalCentre of Swimming of Pequim, China (en.beijing2008.cn/46/39/WaterCube.shtml). Seat of the competitions ofswimming during the Olympic Games of 2008, known asWater Cube, the place have a useful area of 90,000 m²,five Olympic swimming pools and capacity for 17,000spectators, and BIM was used in all phases of the project(Fig. 12 and 13).

Other example of BIM application is the InternationalAirport Maynard Holbrook Jackson Jr., Atlanta, UnitedStates (Fig. 14). This airport is in construction phase with

Fig. 12 and 13. Assembly of the structure of the Water Cube, and aerial photo of theplace (http://comunicacaoexponencial.com.br)

There are other BIM programs by Autodesk, as AutodeskNavisworks (usa.autodesk.com), that it does not presenttools of environment modelling, being destined to therevision of 3D projects or visualisation of models, that isthe case of the freeware NavisWorks Freedom. The maintools include: aggregation of files and 3D data; revisiontools; creation of 4D table; object animation; managementof interference and detention/correction of conflicts in the

Fig. 14. Model of the Airport (http://bim.arch.gatech.edu)

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characteristics and with diverse applications in urbanprojects, providing better results in relation to theplanning, management and maintenance of the systems.

In relation to presented software, it is important to notethat the authors do not have any connections with thecited companies. The programs were shown only ascomputational tools that use the presented technologies,and there are many other commercial software andfreeware that can be used in works involving CAD, GIS orBIM. Therefore, the work presented here does not representany intention of marketing for no one of cited softwareand/or companies.

References

- Autodesk (2006). Orange County Sanitation District - CustomerSuccess Story. Autodesk Infrastructure Solutions

- Autodesk (2009). Langan Engineering & Environmental Services- Customer Success Story. Autodesk Infrastructure Solutions

- Ford, K. (2009). Maynard Holbrook Jackson Jr. InternationalTerminal. Holder Construction Group LLC, Georgia Tech

- Harlow, M. (2005). ArcGIS Reference Documentation. ESRI:Environmental Systems Research Institute Inc., Redlands

- Kymmell, W. (2008). Building Information Modelling - Planningand Managing Construction Projects with 4D CAD andSimulations. The McGraw-Hill Companies, Inc

- Mondardo, D., Bellon, P. P., Santos, L. B., Meinerz, C. C. &Haoui, A. F. (2009). Proposta de Recuperação Ambiental naÁrea Urbana da Microbacia do Rio Ouro Monte. 2nd InternationalWorkshop - Advances in Cleaner Production, São Paulo, Brazil

- Sutherland, I. E. (2003). Sketchpad: A man-machine graphicalcomunication system. Technical Report 574. University ofCambridge, Computer Laboratory, p. 20

- http://acertodecontas.blog.br/ Accessed in August 13, 2009

- http://bim.arch.gatech.edu/ Accessed in December 04, 2009

- http://comunicacaoexponencial.com.br/ Accessed inDecember 04, 2009

- http://en.beijing2008.cn/ Accessed in December 04, 2009

- http://images.autodesk.com/ Accessed in December 01, 2009

- http://usa.autodesk.com/ Accessed in November 27, 2009

- http://www.3ds.com/products/catia/ Accessed in August 24,2009

- http://www.autodesk.com.br/ Accessed in August 21, 2009

- http://www.crackvalley.com/ Accessed December 03, 2009

- http://www.danshope.com/ Accessed in August 22, 2009

- http://www.gis.com/ Accessed in August 14, 2009

- http://www.ptc.com/products/proengineer/ Accessed in August24, 2009

- http://www.solidworks.com/ Accessed in August 22, 2009

- http://www.vicosoftware.com/ Accessed in December 03, 2009

a stipulated deadline for 2011. In this project, of greatmagnitude, BIM is extremely necessary in the optimisationof execution time, since the old airport of Atlanta isoverloaded. The estimated cost of the enterprise isapproximately US$ 1.4 billion (Ford, 2009).

Conclusion

This chapter looked for to present the main details onthree technologies much used in Urban Engineering: CAD(Computer-Aided Design); GIS (Geographic InformationSystem); and BIM (Building Information Modelling). As itcan be seen, each one of them presents specific

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The world is changing, the economy is changing, and

the architectural practice is changing. Designing

energy- and resource-efficient buildings, in many

locations, is no longer optional, but mandatory. While

owners have always sought designs that are cost-effective

to operate and that will command premium lease values,

research shows that green buildings (for example,

LEED®-certified) are more likely to deliver on these

criteria. A 2008 report from McGraw Hill Construction finds

a 13.6 percent decrease in operating costs from green

building and a 10.9 percent increase in building values

as reported by architects, engineering firms, contractors,

and owners over the past three years. (McGraw Hill

Sustainable Design Analysis andBuilding Information Modeling

Manideep Saha

Head, AEC & Geospatial, Autodesk India

Construction September 19, 2008) More pressing is the

growing number of local and national regulations that

mandate targets for energy and resource efficiency as

well as carbon emission reductions in new and renovated

buildings. These government initiatives are certainly put

in place to help reduce greenhouse gas emissions and

slow our impact on climate change, but they are also

instituted to reduce dependence on unpredictable

markets for oil as an energy source and, most recently, to

help stimulate the global economy.

Sustainable Design in Practice

Design decisions made early in the process can deliver

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significant results when it comes to the efficient use of the

vital resources. Employing sustainable analysis tools

helps architects and engineers to make better informed

decisions earlier in the design process and enables them

to have a greater impact on the efficiency and performance

of a building design. Historically, analysis softwares were

complex and required special training-making them

unsuitable for infrequent users such as architects or

designers. Sustainable analysis tools, such as Autodesk

Ecotect Analysis helps users to become proficiently faster

by providing access to immense stores of data and the

ability to more quickly iterate for optimal sustainable

designs.

Designing and delivering more sustainable projects can

be complex. It requires close coordination across different

project stages, from design through construction and

operation. Many firms are looking for the best way to

integrate building information modeling (BIM) technology

with sustainable design and analysis tools. BIM is core to

Autodesk's sustainable design approach for building

performance analysis and simulation.

Analyzing a Building Design

BIM enables architects and engineers to use digital

design information to analyze and understand how their

projects perform before they are built. Developing and

evaluating multiple alternatives at the same time enables

easy comparison and informs better sustainable design

decisions.

A computable Autodesk® Revit® Architecture design

model is devised for sustainability analyses-even during

early conceptual design. As soon as the layout of a

building's walls, windows, roofs, floors, and interior

partitions (elements that define a building's thermal zones)

are established, the information employed to create a

Revit® model can be used to perform analyses.

Performing these analyses in a CAD workflow is a fairly

difficult undertaking as the CAD model has to be exported

and carefully massaged to work with analysis programs.

Using the Autodesk Ecotect Analysis to analyze early

building designs emerging from a Revit-based BIM

process can simplify the analysis process.

Whole Building Energy, Water and Carbon Analysis

The Autodesk® Green Building Studio® web-based

service enables faster, more accurate whole-building

energy, water, and carbon emission analyses and helps

architects-the majority of which are not specially trained

in any of these analyses-to evaluate the carbon footprint

of a Revit-based building design with greater ease.

Built specifically for architects and using green building

extensible markup language for easy data exchange

across the Internet, the web-based service was one of the

first engineering analysis tools to deliver easy-to-use

interoperability between building designs and

sophisticated energy analysis software programs such

as DOE-2.

The link between the Revit platform and the Green

Building Studio web service is facilitated through a plug-

in that enables registered users to access the service

directly from their Revit Architecture design environment.

Inline Energy Analysis

The Autodesk Green Building Studio web-based service

enables architects and other users to perform faster

analyses of a Revit-based building design, from within

their own design environment, directly over the Internet.

This helps streamline the entire analysis process and

enables architects to get faster feedback on their design

alternatives-making green design more efficient and cost-

effective.

Based on the building's size, type, and location (which

drives electricity and water usage costs), the web-based

service determines the appropriate material, construction,

Figure 1: The Autodesk Green Building Studio web-based service enables faster,

whole-building energy, water, and carbon emission analyses of a Revit-based

building design. The building location (being defined here) drives the resulting

electricity and water usage costs.

Figure 2: The link between the Revit platform and the Autodesk Green Building

Studio web-based service is facilitated through a plug-in that enables registered

users to access the service directly from their Autodesk Revit Architecture design

environment.

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The Masterbuilder - March 2012 • www.masterbuilder.co.in78

system, and equipment defaults by using regional

building standards and codes to make intell igent

assumptions. Using simple drop-down menus, architects

can quickly change any of these settings to define specific

aspects of their design; a different building orientation, a

lower U-value window glazing, or a 4-pipe fan coil HVAC

system.

The service uses precise hourly weather data, as well as

historical rain data, that are accurate to within 20

kilometers of the given building site. It also uses emission

data for electric power plants across the United States

and includes the broad range of variables needed to

assess carbon neutrality.

Analysis Results

Usually, within minutes the service calculates a building's

carbon emissions and the user is able to view the output

in a web browser, including the estimated energy and

cost summaries as well as the building's carbon neutral

potential. Users can then explore design alternatives by

updating the settings used by the service and rerunning

the analysis, or by revising the building model itself in the

Revit-based application and then rerunning the analysis.

The output also summarizes the water usage and costs,

and electricity and fuel costs; calculates an ENERGY STAR

score; estimates photovoltaic and wind energy potential;

calculates points toward LEED daylighting credit; and

estimates natural ventilation potential. Unlike most

analysis output, the Autodesk Green Building Studio

report is easier to understand-giving architects and other

users actionable information they need to help make

greener design decisions.

Detailed Environmental Performance

The desktop tools in Autodesk Ecotect Analysis provide

a wide range of functions and simulations, helping

architects and other users to understand how

environmental factors will impact building operation and

performance in the early design phase.

Working with the Environment

To mitigate a building's impact on the environment, it is

important to first understand how the environment will

impact the building. Built specifically by architects and

focused on the building design process, Autodesk Ecotect

Analysis is an environmental analysis tool that enables

Figure 3: Architects and other users can explore design alternatives by updating

the settings used by the Autodesk Green Building Studio web-based service and

rerunning the analysis, or revising the building model itself in the Revit-based

application and then rerunning the analysis.

Figure 4: The Autodesk Revit-based software application user views the output of

the analyses in a web browser, including the estimated energy and carbon

emission summaries (shown left) and a detailed LEED water efficiency guide

(shown below).

Urban Engineering BIM

The Masterbuilder - March 2012 • www.masterbuilder.co.in80

designers to simulate the performance of their building

projects right from the earliest stages of conceptual

design. Autodesk Ecotect Analysis combines a wide array

of analysis functions-including shadows, shading, solar,

lighting, thermal, ventilation, and acoustics-with a highly

visual and interactive display that presents analytical

results directly within the context of the building model.

access, and visual impact.

Revit-based design models can be exported to gbXML

format and imported directly into Autodesk Ecotect

Analysis for simulation and analysis throughout the design

process. At the onset of the design process, very early

stage Autodesk Revit Architecture massing models can

be used in combination with site analysis functionality in

Autodesk Ecotect Analysis to help determine the optimal

location, shape, and orientation of a building design-

based on fundamental environmental factors such as

daylight, overshadowing, solar access, and visual impact.

As the conceptual design evolves, whole-building energy,

water and carbon analysis can be conducted using the

integrated access to Autodesk Green Building Studio in

order to benchmark its energy use and recommend areas

of potential savings. Once these fundamental design

parameters have been established, Autodesk Ecotect

Analysis can be used again to rearrange rooms and zones,

to size and shape individual apertures, to design custom

shading devices, or to choose specific materials-based

on environmental factors such as daylight availability,

glare protection, outside views, and acoustic comfort.

Visual Feedback

Perhaps the most unique aspect of the software is its visual

and interactive display of the analysis results. The inability

of the designer to easily interpret the results of analyses is

often the biggest failing of building performance analysis

software. Autodesk Ecotect Analysis provides actionable

feedback to the designer in the form of text-based reports

Figure 5: Early stage Autodesk Revit Architecture models can be analyzed with

Autodesk Ecotect Analysis to help determine the optimal location, shape, and

orientation of a building design-based on basic environmental factors such as the

overshadowing of a particular building (highlighted in red) shown here.

Figure 6: Autodesk Ecotect Analysis can also be used for detailed design analysis. For example, the

visibility analysis displayed here shows the amount and quality of views to the outside mapped over the

floor area of an office.

This visual feedback enables the software to

communicate complex concepts and

extensive datasets more effectively and helps

designers quickly engage with multifaceted

performance issues-at a time when the design

is sufficiently "plastic" and can be easily

changed.

Analyzing a Design in the Context of BIM

Revit-based design models can be exported

to gbXML format and imported directly into

Autodesk Ecotect Analysis for simulation and

analysis throughout the design process. At

the onset of the design process, very early

stage Autodesk Revit Architecture massing

models can be used in combination with site

analysis functionality in Autodesk Ecotect

Analysis to help determine the optimal

location, shape, and orientation of a building

design-based on fundamental environmental

factors such as daylight, overshadowing, solar

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The Masterbuilder - March 2012 • www.masterbuilder.co.in82

as well as visual displays. These visual displays are more

than just charts and graphs. The analysis results are

presented directly within the context of the model display:

shadow animations resulting from shadow casting

analysis; surface-mapped information such as incident

solar radiation; and spatial volumetric renderings such

as daylight or thermal comfort distribution in a room.

This type of visual feedback lets designers more easily

understand and interact with analysis data, often in real

time. For instance, a designer can rotate a

view of surface-mapped solar radiation

looking for variations over each facade, or

watch an animated sequence of solar rays to

see how sunlight interacts with a specially

designed light shelf at different times of the

year.

Ongoing Building Performance Analysis

During conceptual design, Autodesk Ecotect

Figure 7: Using Autodesk Ecotect Analysis, architects can see the results of their

analysis displayed in the context of a building model, such as the surface-mapped

results of this solar radiation analysis.

Analysis and the Autodesk Revit Architecture model can

be used for a variety of early analysis. For example, the

designer can perform overshadowing, solar access, and

wind-flow analyses to iterate on a form, and orientation

that maximizes building performance without impinging

on the rights-to-light of neighboring structures.

As the design progresses and the elements that define a

building's thermal zones are established (the layout of

the walls, windows, roofs, floors, and interior partitions),

the Revit model can be used for room-based calculations

such as average daylight factors, reverberation times, and

portions of the floor area with direct views outside.

Eventually the Revit model can be used for more detailed

analysis-such as shading, lighting, and acoustic analysis.

For example, the designer can use Autodesk Ecotect

Analysis in conjunction with a shading louver design

modeled in Autodesk Revit Architecture to simulate how

the design will work under different conditions throughout

the year. Or the architect can use Autodesk Ecotect

Analysis to help assess the acoustic comfort of a Revit-

Figure 8: Autodesk Ecotect Analysis software also displays analysis results using spatial volumetric

renderings, such as this analysis of the visual impact of a building within an urban site.

based design, and then adjust the location

of a sound source or adjust the internal wall

layout or the geometry of sound reflectors for

optimal comfort.

Summary

The consistent, computable data that comes

from Autodesk Revit Architecture combined

with the breadth of performance analysis and

meaningful feedback of Autodesk Ecotect

Analysis work in combination to help reduce

the cost and time to perform energy modeling

and analysis. The feedback from these

analyses helps architects and other users to

optimize the energy efficiency of their designs

and work toward carbon neutrality earlier in

the design process-a key ingredient not only

for incorporating energy efficiency into

standard building design practices but also

for mitigating the carbon footprint of our built

environment.

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Mechanisms of Deteriorationof Reinforced Concrete Structures

Dr. Manu Santhanam

Associate Professor, Department of Civil Engineering, IIT Madras

Durability of hydraulic-cement concrete is defined

as its ability to resist weathering action, chemical

attack, abrasion, or any other process of

deterioration (ACI). Durable concrete will retain its original

form, quality, and serviceability when exposed to its

environment.

Concrete is an inherently durable material. Reinforced

concrete structures are expected to be maintenance-free

during their service lives. However, there is evidence of

premature deterioration of modern structures. The

resultant costs to the economy reach 3 - 5% of GNP in

some countries (and up to 50% of construction budgets).

This occurs because existing knowledge not adequately

applied.

As shown in Figure 1, durability of concrete depends on

two primary factors - the concrete system, and the service

environment. The concrete can be further subdivided into

the materials and the process, while deterioration in

service conditions can be through physical or chemical

means.

Concrete has to function in different types of environments,

some of which are aggressive or degrading to the concrete

quality. Typical aggressive environments are: Seawater

(or close to sea), Polluted soils (due to industrial or

agricultural effluents), Freezing conditions, to name a few.

Design of concrete for these environments has to take

into consideration the alterations that cement paste (or

concrete) may undergo upon interaction with the

environment.

The common durability problems in concrete are:

- Corrosion of steel in reinforced concrete

- Sulphate and other chemical attack

- Alkali aggregate reaction (more of a material problem

than environmental)

- Freezing and thawing damage

- Carbonation

Concrete characteristics affecting performance

Porosity and permeability

Durability of concrete is related to its performance in the

service environment. Concrete is subjected to a host of

durability problems, which typically result in:

- Progressive loss of mass from the surface

- Volume changes, which can be of three types: (1) both

paste and aggregate expand, (2) the paste expands,

while the aggregate is inert, or (3) only the aggregate

expands.

Water is common to all the durability problems in

concrete. The presence of water, or its involvement in the

reactions is necessary for the problems to occur. Thus,

the durability of concrete is intrinsically related to its water-

tightness, or permeability.Figure 1. Constituents of concrete durability

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Permeability of concrete is a function of the permeability

of the cement paste, of the aggregate, and of the

interfacial transition zone. The permeability of these

components is in turn related to the porosity. Paste

capillary porosity is typically 30 - 40%, while normal

aggregates have a porosity of 2 - 3% (and rarely greater

than 8 - 10%). The transition zone is highly porous due to

the presence of flaws such as microcracks and bleed-

channels.

Figure 2. Porosity and permeability: A is highly porous compared to B, butprobably less permeable due to the poor interconnectivity of pores

Mix characteristics - w/c and presence of admixtures

Both porosity and permeability increase with an increase

in the water to cement ratio. The permeability also

depends on the degree and nature of curing, and the

presence of mineral admixtures, which can act as fillers

densifying the transition zone. Additionally, the pozzolanic

reaction of mineral admixtures contributes to the

resistance of concrete. Chemical admixtures such as

corrosion inhibitors and air entraining chemicals enhance

the performance of concrete during corrosion and

freezing, respectively.

Type of cement and aggregate

Blended cements perform better (combined benefits of

pozzolanic reaction and reduced permeability) in all

environments. Special cements such as Type V (sulphate

resistant) and Sulpersulphated cement are good for

sulphate resistance.

As far as aggregates are concerned, low density material

is susceptible to freezing damage. The bond with cement

paste will govern the quality of interfacial zone. Some

aggregates have better bond than others.

Presence of cracks

Cracks in concrete could be structural or non-structural

(thermal effects, shrinkage etc.). Most often, the non-

structural cracks occur as a result of poor material

selection, lack of adequate quality control, etc. There is

no use having HPC between cracks, since cracks will serve

as channels for the ingress of water and other chemicals.

Good concreting practice is the only way to minimize

unwanted cracking.

Importance of the cover zone

The importance of a good concrete cover for reinforcing

steel cannot be overemphasized. The cover concrete is

primarily responsible for its response to the service

environment.

Sulphate attack on concrete

Sulphate attack is the deterioration of concrete by means

of reactions between sulphate ions and hydrated cement

products. Generally, sulphate attack is divided into two

categories: External and Internal. External sulphate attack

is when the source of sulphate ions is external to the

concrete, such as when it is from ground water or seawater.

Na2SO

4, MgSO

4, CaSO

4 and (NH

4)

2SO

4 are some

detrimental sulphate sources that are primarily found in

ground water contaminated with industrial effluents and

agricultural products. Internal sulphate attack, on the

other hand, occurs when a late release of sulphates within

concrete takes place. In this case, the formation of

ettringite occurs after the concrete has hardened, and

this results in distress.

Sodium sulfate (N ) and magnesium sulfate (M ) can react

with CH to produce gypsum (C H2), sodium hydroxide

(NH) and magnesium hydroxide (MH, or brucite). It is not

fully understood if gypsum formation causes any

volumetric expansion. The formation of gypsum, however,

is reported to render the structure soft, which leads to a

decrease in strength of the structure.

The formation of gypsum is closely linked to the formation

of other products of sulfate attack, as gypsum can combine

with other hydration products to produce ettringite. This

phenomenon is called ettringite corrosion. The formation

of ettringite is said to be expansive, although the

mechanism of expansion is still debated by researchers.

Numerous theories have been postulated to explain the

expansion due to ettringite formation: (1) crystal growth,

either by topochemical (when the products form at the

reactant sites itself) mechanism, where ettringite crystals

grow on the surface of aluminate particles, or by through-

solution mechanism; (2) swelling due to imbibition of

water, because of the high surface area of ettringite.

The damaging effects on the C-S-H gel are only due to

the action of magnesium sulfates.

The MH and the silica hydrate (SHy) formed in this reaction

further react to produce magnesium silicate hydrate (M-

S-H), which is reported to be non - cementitious, and

leads to complete disinitegration. The phenomenon of

progressive reduction of the C/S ratio within the C-S-H

gel is called decalcification. This process does not

actually begin until the pH drops to very low values (<10).

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The conversion to M-S-H is a very advanced stage of

deterioration. The M-S-H and gypsum formed from the

above reactions are frequently observed to be deposited

in bands parallel to the exposed surface. The

decomposition of the C-S-H gel into a non-cementitious

M-S-H can be achieved only by M . The decomposition

by the M doesn't stop with the formation of M-S-H but

continues further. The action of M renders a low pH to the

Figure 1. Cement mortar in sodium sulphate solution showing deposition ofettringite (E) in the cracked surface zones; the dark region represents decalcifiedC-S-H

pore solution. Hence the C-S-H releases some CH into

the solution in order to stabilize itself at a higher pH. But

since there is M in the surrounding environment, the

deterioration cycle repeats itself beginning with the

gypsum corrosion.

Some scanning electron micrographs that depict the

attack of cement mortars by sodium and magnesium

sulphate solutions are presented below in Figures 1 - 6.

Figure 2. Large deposit of gypsum formed in cement mortar in sodium sulphatesolution

Figure 3. Layer of gypsum surrounding sand grain in cement mortar stored insodium sulphate solution, suggesting a conversion of CH to gypsum; smalldeposits of ettringite are also visible

Figure 4. Layers of M-S-H and gypsum on the surface of concrete subjected tomagnesium sulphate attack

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Figure 5. Formation of a surface double-layer of brucite(B) and gypsum(G) in acement mortar stored in magnesium sulphate solution

Figure 6. M-S-H and gypsum in a highly deteriorated surface zone of a cementmortar stored in magnesium sulphate solution

Protection from sulphate attack

" Use of low C3A cements: The philosophy of prescribing

low C3A cement to improve resistance to sulphate attack

hinges on the need to minimize ettringite formation after

the concrete hardens. When the C3A content is low, most

ettringite will be formed in the plastic state. The use of

very low C3A content, however, is not good in the case of

attack by chlorides. C3A can bind the chlorides that

penetrate into concrete, thus reducing the free chloride

content that can cause corrosion. Thus, a moderate C3A

content should be prescribed in such cases. Lowering of

C3S might also help, since this would reduce the amount

of CH that forms.

- The best protection against sulphate attack is to have

a low w/c in concrete. Blended cements, that lead to a

consumption of CH, need not be good in cases of

magnesium sulphate attack.

- Use of high alumina cement: HAC is good for sulphate

resistance if the conversion of its hydration product

does not occur.

- Supersulphated cement: In this cement, all the

available aluminates are converted to ettringite during

hydration. Thus, there are no excess aluminates

present to react with external sulphate ions.

Sea water attack

Sulphate attack can also take place in seawater. However,

the mechanism may be altered due to the presence of a

high concentration of chlorides. Typically, seawater attack

is characterized by the formation of higher amounts of

brucite compared to groundwater attack. In addition to

the chemical reactions involved in sulphate attack,

physical deterioration of the concrete may also occur due

to cycles of drying and wetting. The tidal zones in

concrete structures are especially susceptible to

alternate drying and wetting, which may lead to the

crystallization of salts in the surface pores, and the

development of expansive pressures that may cause

spalling. The action of waves can further aggravate the

surface concrete.

Acid attack

Attack by sulfuric acid occurs most commonly in sewers,

where a lot of sulphide gases exist owing to the large

degree of microbial action. Sulfuric acid creates an acidic

environment in the concrete, in which the primary cement

phases (C-S-H, ettringite) are extremely unstable. Gypsum

formation occurs when sulfuric acid reacts with CH. The

loss of integrity and softening of the structure occur as a

result of gypsum formation and destabilization of C-S-H.

Carbonation

Carbon dioxide diffuses into the pores of concrete and

reacts with calcium hydroxide; as a result, the alkalinity

(pH) of the concrete is reduced. Reduction of pH causes

the passivity of reinforcing steel (protective layer) to be

destroyed.

Delayed Ettringite Formation (DEF)

Under certain initial storage or curing conditions, such as

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for steam-cured concrete that is subjected to high

temperatures, the ettringite that forms in the process of

cement hydration gets destroyed. The formation of

ettringite in the early stages may also not occur if there is

a late release of sulfates. The reformation of ettringite in

hardened concrete in the presence of moisture leads to

the generation of expansive pressures, and cracking of

concrete. This is called delayed or secondary or late

ettringite formation.

Most researchers believe that three elements are essential

for DEF to occur: presence of microcracks, late sulfate

release, and exposure to water. If cracks are present in

concrete, DEF causes the deposition of ettringite in the

cracks. The late-released sulfates, or the sulfates from the

deteriorated ettringite, go into the structure of C-S-H.

These are released later by moisture, which carries them

to the aluminate phases, resulting in the formation of

ettringite. This ettringite often effectively masquerades

as ASR, since it forms in extremely small crystals.

DEF is more of a problem with modern cements, since

the clinker SO3 levels have increased dramatically over

the years, and the use of sulfur fuels has also grown. Thus,

it is all the more essential nowadays to restrict the

temperature rise in concrete, not only from steam curing

but also from the use of rapid hardening cements. Most

European countries have adopted standards restricting

concrete temperatures.

Alkali aggregate reaction

Many sil iceous igneous (opal, chalcedony) and

sedimentary rocks (chert) possess a glassy or amorphous

texture. In alkaline environments the silica structure can

get dissolved from these aggregates. The resultant

reaction between silica and alkalis results in the formation

of a gel that is expansive. This phenomenon is called the

alkali-sil ica reaction. Strained quartz present in

metamorphic rocks may also be susceptible to damage

by alkali-silica reaction (ASR).

The alkalis may come from the cement, chemical and

mineral admixtures, impurities in aggregate or water. The

reaction itself needs the presence of moisture.

SiO2 + KOH (in the presence of moisture) →Alkali silica

gel (no definite composition)

The first step in this reaction happened on the surface of

the aggregate, where the Si-O bonds are dissolved by

OH-. Thus, the silica becomes available to combine with

the alkalis to form alkali-silica gel.

The alkali silica gel formed from the above reaction could

also contain some Ca2+. The ratio of Ca2+ to the alkalis

(Na+ or K+) in the gel determines its expansive nature.

Usually, the higher the Ca2+, the lesser expansive the gel.

If the alkali hydroxide concentration falls below 0.3N, the

reaction tends to slow down and stop. The rate of reaction

cannot be determined from the amount of gel forming,

since it does not have a distinct composition.

The amount of expansion in ASR depends on the type of

aggregate. For some aggregates, a pessimum type

relation is observed between the % expansion and the %

of reactive aggregate, while for others, the % expansion

increases consistently with an increasing proportion of

reactive aggregate. The decrease of expansion beyond

a certain limit occurs because when there is too much

reactive silica, the gel can form at very early stages when

the concrete is still in the plastic state. At high alkali

contents, the gel that forms has got a low viscosity, and is

thus not able to generate high expansive pressures. In

the case of aggregate size, when the size is too small, the

reaction occurs in the plastic state of concrete, and thus

does not lead to any expansion. On the other hand, for

very large aggregates, the surface area to volume ratio

becomes too small for the reaction surface to be a

significant factor.

Mechanism of expansion

Various theories have been proposed to account for the

expansion that occurs as a result of ASR. These are:

- Absorption (swelling) theory proposed by Vivian: The

imbibition of pore water and the resultant swelling of

the alkali-silica gel causes expansion. The aggregate

grows outward and puts the paste in tension.

- Osmotic pressure theory proposed by Hansen: The

alkali-silica gel acts as a semi-permeable membrane

that allows only an inward diffusion of OH-, Na+, K+,

and Ca2+ from the pores to the aggregate surface.

Thus the aggregate exerts osmotic pressure against

the surrounding paste. Lea modified this theory and

stated that there is actually a preferential diffusion of

some species - Na+, K+ - over others such as Ca2+.

Manifestation of ASR

ASR is a very slow reaction and may take many years to

show up at the surface of the concrete and get detected.

Cracking due to ASR generally shows up as a map pattern

on the surface. Irregular small cracks form at the surface.

These are unsightly, but are rarely the cause of a structural

collapse. However, expansion associated with ASR can

cause misalignments.

Surface aggregates can often pop out of the concrete

due to expansion. The alkali-silica gel can ooze out to the

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surface and get carbonated if wet. The resultant hard

white gel that forms resembles carbonated calcium

hydroxide.

Microstructurally, reaction rims are often visible on the

surface of reactive aggregates due to a slow dissolution

of the silica. However, the rim could also be due to

weathering, Certain ASR reactive aggregates even do

not form rims.

Protection against ASR

Due to circumstances, it is not possible to change an

aggregate source locally even if prior knowledge about

the reactivity of the aggregate is available. Thus, other

methods have to be adopted in order to prevent ASR.

The primary measures for protection are:

- Use of low alkali cement (< 0.6% equivalent Na2O).

- Preventing access of moisture.

- Using coatings (such as silane, which allows water

vapour to go out of concrete, but does not permit

water to come in) or waterproofing agents.

- Use of chemical admixtures such as Lithium salts

(LiNO3, LiOH, etc.) or alkyl alkoxy silanes, which bind

the reactive silica into a non-expansive product.

- Use of mineral admixtures such as silica fume. Mineral

admixtures can act in two ways: (1) by reducing the

penetration of water, and (2) by binding the alkalis

within the unhydrated glass. The alkali silica gel that

forms in mineral admixtures is also high in Ca2+, and is

thus not very expansive.

Freezing and thawing related damage

The damage due to freezing and thawing (F/T) is a physical

problem, unlike the chemical issues that were discussed

earlier. F/T can cause three types of failures:

- Paste failure: This is related to the failure of the paste.

Parallel cracks form in the paste and proceed inward

from the places where concrete first becomes highly

saturated with water. Sometimes, scaling of the top

surface can occur. Scaling is exacerbated when

deicing salts are used.

- Aggregate failure - D-cracking: This relates to the

failure of the paste when it is subjected to expansive

stressed by the aggregate. It shows up as parallel

cracks proceeding inward from the point of saturation.

The pattern of cracking on a jointed concrete

pavement or slab appears like the letter D, as shown

in the figure below.

- Aggregate failure - popout: Popouts are caused when

porous aggregates on the surface of concrete are

Figure 7. D-cracking in a concrete pavement slab

subjected to expansion on freezing. A part or the whole

of the aggregate piece cracks and pops out.

Sometimes, a mortar flake can also pop off as a result

of the expansion of an underlying aggregate.

Mechanism of freezing and thawing

Water expands by 9 - 10% upon freezing. Thus, the critical

saturation of a pore in concrete is about 90%. It must be

understood that freezing point in small pores is depressed

to a large extent. In fact, in some of the small pores in

concrete, freezing does not occur until temperatures as

low as - 40 oC. Also, the presence of other ions in the pore

solution also depresses the freezing point. If the concrete

remains frozen through its lifetime, then not much of a

problem occurs. The deterioration occurs only if there are

successive cycles of freezing and thawing.

The expansion and damage associated with F/T is

explained using various mechanisms. Let us first consider

the case of paste failure (see Figure 8).

Figure 8. Movement of water inside capillaries

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Water in the large pores near the surface of the concrete

is the first to freeze. The freezing of concrete proceeds in

a front parallel to the surface. The expansion of water in

the large pores on freezing drives out the unfrozen water

into the paste. The travel of water through the paste

generates a hydraulic pressure. The longer the path of

flow of water, the higher the pressure generated. If the

flow path is longer than a critical distance (0.2 mm for

concrete) then failure occurs.

The use of air entraining agents is the best remedy for

paste failure. Air bubbles serve as closely spaced

reservoirs into which the unfrozen water can migrate. The

spacing between air bubbles should be smaller than twice

the critical distance (0.2 mm) for the air entrainment to be

really effective. Thus, it is not just the amount of air

entrained, but also the dispersion of air that is important.

The larger the coarse aggregate size, the lower is the air

entrainment required.

When high strength concrete with low w/c is used, the

pore size can be so small that freezing does not even

occur at service temperatures. Thus, air entrainment is

sometimes not necessary in such cases. The use of

mineral admixtures, which act as pore refiners, can thus

be beneficial. However, mineral admixtures tend to

increase the scaling problem in concrete.

In the case of aggregate failure, the type of failure is

dictated by the porosity of the aggregate. If aggregate

porosity if very high, then the expansive stresses

generated by the aggregate are not critical. In other

words, the expansion is accommodated by the aggregate

elastically. Popouts are caused by aggregates that have

a moderately high porosity. Such low-density aggregates

(cherts are especially susceptible) are subjected to high

internal pressures due to expansion. The problem gets

worse when the aggregate size is large. The remedy is to

screen the concrete aggregate for low-density elements.

In the case of D-cracking, the aggregate porosity is not

high. Thus, and expansion of the water in the aggregate

causes the unfrozen water to move into the surrounding

paste, resulting in hydraulic pressures. Air entrainment of

the paste can help in this case to a certain extent.

Corrosion of reinforcing steel

The corrosion of steel in reinforced concrete is a problem

of mammoth proportions. It is estimated that 5% of a

developed nation's GDP is utilized for repair of corrosion-

related damage. The yearly cost of repairs for reinforced

concrete bridge decks in the US alone is estimated to be

$ 50 - 200 million.

Corrosion is an electrochemical problem. The overall

mechanism can be broken up into the anode reaction

and the cathode reaction, as shown in Figure 9. An

electrical current flows through the aqueous medium

Figure 9. Reactions of corrosion

Figure 10. Current flow during corrosion process

opposite to the direction of flow of the electrons (see Figure

10). In addition to the electron current, there is also an

ionic current. The flow of current resembles a battery cell.

This system is thus known as a 'galvanic cell' and the

process is also known as 'galvanic corrosion'.

The ferrous and hydroxyl ions combine to form the rust

products.

2 Fe2+ + 4 OH- → 2 Fe(OH)2 (greenish rust)

The greenish rust, upon further reactions with O2 and OH-

, can form Fe2O

3 (red rust) and Fe

3O

4 (black rust). The rust

often accumulates at places other than the reaction sites.

Likelihood of occurrence of corrosion

Anodic sites can be created on steel due to a multitude of

reasons:

- Compositional variances on the steel surface

- Presence of dust/dirt etc. partially on the steel surface

- Presence of local differences in applied stress

- Microstructural variations in the steel: (1) The ferrite

phase is more active than the cementite phase, (2)

Grain boundary atoms are more active compared to

the bulk

- Strained zones produced during cold working of

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metals may be more active

- Presence of stress concentrations

- Differences in oxygen concentration at different sites

on the steel: generally, the site with a lower oxygen

concentration becomes anodic

Figure 11. Rust formation and delamination of concrete

Figure 12. Relative volumes of corrosion products

Manifestation of corrosion

- Formation of rust causes expansion and cracking

(some forms of rust are 6 - 7 times the volume of

undamaged steel) - see Figures 11 and 12.

- Loss of sectional area of rebar - reduction in load

carrying ability

Factors controlling the rate of corrosion

The following are the principal factors that control the

rate of corrosion:

- Availability of dissolved oxygen and moisture at the

cathode: In order for the cathodic reaction to occur,

both oxygen and moisture are necessary. Due to the

concrete cover, both these elements have to reach

the steel surface by diffusion. This slow diffusion

produces a significant reduction in the potential

difference between the anodic and cathodic areas.

This phenomenon is called 'concentration polarization'.

- Resistivity of the medium (concrete and its pore

solution): The flow of ions has to occur through the

medium of concrete and the pore solution. Thus, the

resistivity of the concrete can have a significant bearing

upon the easy flow of ions.

- Passivation of steel: In an alkaline environment, the

surface atoms of the steel get oxidized to form an thin

oxide layer (thickness of about 10 nm). This film is

stable at the highly alkaline environment of concrete.

The stability of the film is enhanced when the steel

contains a large amount of alloys. This phenomenon

of the formation of a protective layer around the steel

is called 'passivation', and is made possible by the

high concentration of OH- in the concrete pore solution.

The level of OH- required to maintain passivation is

not a constant value, but depends on the presence of

other ions, especially Cl-. The ratio of OH- to Cl- is very

important. Depassivation can occur by a number of

mechanisms: (1) Consumption of OH- by carbonation

and other reactions; when the pH falls below 11.5, the

film is no longer stable; (2) Presence of a high

concentration of Cl-: In addition to lowering the pH

due to ionic balance with OH-, Cl- can react with oxide

films of Fe(OH)2 (that have not been converted to the

Figure 13. Some factors governing the rate of corrosion

stable oxide film because of lack of availability of

oxygen) to form iron chlorides. This results in pitting

corrosion. A threshold concentration of Cl- has to be

exceeded before corrosion can take place, and this

concentration is a function of the OH- concentration or

pH. Limits on Cl- concentration have been stipulated

in various codes.

Figure 14 shows the rate of occurrence of corrosion. As

shown in the figure, the initiation stage lasts until the

depassivation of steel. Beyond this stage, the propagation

of corrosion occurs at an almost constant rate. Finally, the

corrosion process enters an acceleration stage where the

rate is high.

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Protection mechanisms against corrosion

- Galvanization: this process involves the plating or steel

with Zinc. Zn, having a higher electrochemical potential

compared to Fe, becomes the sacrificial anode, and

Fe is protected as the cathode.

- Cathodic protection: An external voltage or current is

Figure 14. Rate of corrosion

supplied to the steel to keep it cathodic and

preventing oxidation from occurring.

- Use of stainless steel (very high Cr): Produces a stable

passivating film.

- Use of epoxy coated steel.

- Use of corrosion inhibitors (see chapter on Chemical

Admixtures).

- Adequate depth of cover.

- Good quality concrete with low permeability.

Summary

Good concrete performance in aggressive environments

can only come about with the combined action of a

number of factors:

- Proper mix design

- Reduction of cracking

- Optimum cover thickness

- Adequate compaction and curing

- Quality of construction

- Correct maintenance

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Corrosion of Steel in Concrete& Assessment Techniques

The corrosion cell

To demonstrate the principles of electrochemical

corrosion, let us consider the simple "Daniel-cell". This

consists of zinc immersed in ZnSO4 solution and copper

immersed in a CuSO4 solution. The two electrodes are

connected to a variable resistor R, voltmeter V, and

ammeter A (Fig, 1). The potential difference (emf) between

the two electrodes when no current is flowing is 1.1 V. If a

small current is allowed to flow through the external resistor

(I1 in Fig. 2), the measured potential difference falls below

1.1 V because both electrodes polarise, Zn to b and Cu to

e (Fig. 2). As the resistance is decreased, the current

increases and the potential difference decreases until,

when the system is short-circuited (the resistance is very

small), maximum current flows and the potential difference

is almost zero (Imax

). The Zn anode polarises along the line

abc and the Cu cathode along the line def. The full

polarisation of Zn in volts is given by c-a and for Cu by f-d.

The anodic reaction in the "Daniel-cell" is:

Zn → Zn2+ + 2e-

where Zn corrodes and goes into solution and the cathodic

reaction is:

Cu2+ + 2e- → Cu

where copper is deposited from the CuSO4 solution.

In the case of steel in oxygenated water, the simplified

anodic reaction is:

Fe→ Fe2+ + 2e-

George Sergi, Ph.DTechnical Director, Vector Corrosion Technologies

Iron atoms undergo oxidation (electron loss) to form F++

ions which pass into solution. The excess free electrons

left in the metal are consumed, converting oxygen and

water to hydroxyl ions in a process of reduction (electron

addition) according to the following cathodic reaction:

½ O2 + H

2O + 2e- → 20H -

Both the anodic and cathodic reactions occur at adjacent

Figure 1 Polarised copper-zinc cell (Daniel-cell)

Concrete is a porous material whose pores contain an electrolyte made up primarily of sodium and potassium hydroxides.1Steelreinforcement is normally protected in such an electrolyte owing to the formation of a dense and uniform passive oxide film.2

Carbonation of the concrete (neutralisation of the alkali constituents by CO2 gas from the atmosphere), or infestation of the

concrete with salt from seawater or from deicing agents leads to the breakdown of the protective oxide film and to corrosion of thesteel.3,4 Corrosion of steel in concrete is an electrochemical process whereby anodic and cathodic reactions occur simultaneouslyon the surface of the steel resulting in the dissolution of the metal at the anodic sites.5

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sites simultaneously on the surface of the steel (Fig. 3). As

was the case with the "Daniel..cell", the nodic and cathodic

reactions of steel in water can be represented in a

polarization diagram (known as an "Evans" diagram), as

shown in Fig. 4. The intersection point of the two lines

corresponds to the electrode potential at which the rates

of the anodic and cathodic reactions are equal and is

termed the corrosion potential, (Ecorr), the value that steel

adopts when corroding freely. The magnitude of the

anodic and cathodic reactions is represented by the

number of electrons flowing per second between the

anodic and cathodic sites on a unit area of the metal

surface and is a measure of the corrosion rate, Icorr. It is

often referred to as the corrosion current density and

expressed in electrical units (amps/m2 of steel area or

more commonly, mA/m2 ). It is the current at the equivalent

Figure 2 Polarisation diagram for copper-zinc cell

Figure 3 Mechanism of corrosion in oxygenated water

point of intersection in Figure 4. Faraday's laws of

electrolysis may be used to obtain corrosion rates in more

familiar terms (average rates of mass loss per unit area)

and these values may be simply converted to average

rates of loss of thickness of metal from knowledge of the

density of the steel.

Figure 4 Evans diagram for iron corroding in oxygenated water

Factors affecting corrosion rates

Both Icorr and Ecorr can vary depending on the degree of

polarization of either the anodic or the cathodic reactions.

When polarisation occurs mostly at the anodes, (i.e. the

anodic process is for some reason hindered) it is said

that the corrosion reaction is anodically controlled.

Similarly, if the cathodic reaction is polarised, the corrosion

reaction is said to be cathodically controlled. Since the

operation of a corrosion cell depends on three processes

occurring in series, viz. the anodic reaction, the cathodic

reaction and the flow of currents through the intervening

electrolyte and metal the overall corrosion rate is not only

governed by the anodic and cathodic reactions but also

by the magnitude of the resistance of the electrolyte which

can hinder ionic conduction. This is represented

graphically in Figure 5.

Corrosion of steel in concrete When steel is immersed in

an alkaline solution such as sodium or potassium

hydroxide, its surface becomes coated with an adherent,

insoluble oxide film (γ-Fe2O3) which is stable over a

range of potentials. Its polarisation curve is modified to

that shown in Figure 6 and shows that the corrosion rate

over intermediate potentials becomes very small.6 Such

a situation is termed passivity, one of the states in which

the metal can thermodynamically exist at variable pH

levels of the electrolyte as shown by a simplified Pourbaix

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diagram for steel (Fig. 7). The pore electrolyte of concrete

is primarily a mixture of sodium and potassium hydroxides

and as such, the embedded steel attains passivity. The

condition is usually characterised by a value of Ecorr

higher than about -250mV (measured relative to a

standard copper/ copper sulphate reference electrode-

CSE scale).

When carbonation of the concrete occurs, whereby the

pH of the pore electrolyte becomes neutral, passivity can

Figure 5 Effect of resistance of the electrolyte on the corrosion rate of the

reinforcement

Figure 6 Evans diagram for passive iron in oxygenated alkaline solution

no longer be thermodynamically stable and the steel

corrodes as it would in an equivalent neutral solution (Fig.

4). The rate of steel corrosion in carbonated concrete is

subject to anodic resistance control.78, It therefore

depends critically on the moisture content of the concrete

since this is primarily what determines the electrolytic

resistivity of the material. The corrosion potential of the

steel in this condition is typically in the range -450 to -

600mV (CSE scale) when corrosion is occurring.

Figure 7 Pourbaix diagram for steel

The other main cause of corrosion of reinforcing steel is

the presence of chloride salts in the concrete. In sufficient

concentrations, they can undermine the passive film of

the steel locally and bring about pitting corrosion. This is

illustrated by a modified Pourbaix diagram (Fig. 8) and

by a series of polarisation scans of steel in concrete

contaminated with chloride (Fig. 9). It is evident from

Figure 9 that the corrosion potential becomes more

negative and the corrosion rate increases as the chloride

concentration increases.

Figure 8 Modified Pourbaix diagram for steel in solutions contaminated with

chloride

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Figure 9 Evans diagram for steel undergoing pitting corrosion in chloride

contaminated concrete

Durability assessment and corrosion detection of steelreinforcement

There are a few obvious and simple techniques that should

be carried out initially in order to obtain an overall picture

of the condition of the reinforced structure. A good visual

survey is probably the most important step in the

assessment. It is essential to visit the site and look for tell-

tale signs such as cracking of the concrete, rust staining,

delamination but perhaps more importantly, signs of water

retention, inadequate drainage and, particularly in flat

surfaces such as car parks, formation of paddles. Edges

and crevices should be studied carefully. An assessment

of possible environmental factors such as exposure of

structure to direct rainfall, sun and wind should be made.

Then, key regions that may require detailed investigation

can be selected.

Chloride and carbonation survey

As was discussed earlier, both carbonation and chloride

infestation can lead to corrosion of the reinforcement. If

chlorides are suspected, dust samples should be

collected from selected nodes on a grid at increasing

depths into the concrete. This can be done simply with

the use of a drill. A concentration gradient of chloride

would suggest that the source was external (deicing salts,

seawater) whereas a fairly constant concentration may

suggest that the chlorides were added to the concrete at

the mixing stage. The depth of carbonation (neutralisation

of concrete alkalinity by acidic gasses such as CO2) can

be determined by spraying phenolphthalein indicator

either on freshly broken concrete or on drilled powder at

increasing depths.

Potential mapping

It was shown above that the potential of steel

reinforcement can give a good indication of its condition.

Potential mapping is used, therefore, as a major

investigative technique for the detection of steel

reinforcement corrosion. The set-up for carrying out a

potential map is simple and it involves a few easy steps.

First of all, electrical continuity of the reinforcement is

checked by measuring the resistance between two points

where the steel has been exposed. A resistance of less

than 1Ω would normally signify continuity of the steel.

The potential on the steel is then measured relative to a

Figure 10 Simple potential mapping set-up

Figure 11 Potential map of concrete section showing several areas of low potential / high corrosion activity

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standard reference electrode (normally copper/copper

sulphate or silver/silver chloride) by resting the reference

electrode on the appropriate position on the surface of

the concrete as shown in Figure 10. A potential map can

then be constructed (Figure 11) which can show

equipotential contours on the surface of the concrete.

Regions of high corrosion activity can normally be

identified as they show characteristically low potentials.

It is generally assumed that the probability of corrosion of

the steel reinforcement increases as the potential

diminishes. There is, however, a lot of uncertainty in the

middle range of potentials (-200 to -350mV). The problem

arises primarily from the way the potential of the steel

reinforcement is determined in practice. Figure 12 shows

diagrammatically the current distribution around a

corroding site (the anode) through the surrounding

concrete to the adjacent cathode regions. These constant

current flux lines, give rise to equipotential lines distributed

through the concrete as shown in Figure 12. In a region

above the anode the "apparent" potential is Ea (Fig. 12)

and at some distance away at the cathode it is Ec. The

value of Ea is normally less (more -ve) than that of Ec so

local corrosion sites can be identified. It can be seen,

however, that the potential measured on the surface is

not necessarily that of the anode. This "surface" potential

is influenced by parameters such as the concrete cover

depth and the resistivity of the concrete which can alter

the constant current flux lines and subsequently the

equipotential lines. Wetting of the concrete will inevitably

reduce the resistivity of the concrete and therefore change

significantly the potential characteristics.

Figure 12 Current and potential distribution in concrete near anodic corrosion

A further complication which gives rise to a common

misunderstanding, is the fact that steel embedded in

submerged or waterlogged structures can exhibit

potentials which are more negative than -0.35 V (CSE

scale) whilst suffering negligible corrosion. This is caused

by the inability of oxygen to penetrate the cover depth in

sufficient quantities which leads to the polarisation of the

cathodic reaction as shown in Figure 13. Provided the

condition of the concrete is taken into account, potential

mapping, when used on its own, can provide a reasonable

assessment of the corrosion activity of the reinforcement

at the time of the survey. It cannot, however, provide

information on the rate or extent of corrosion.

Figure 13 Evans diagram for passive steel in uncontaminated concrete containing

different concentrations of oxygen

Resistivity measurements

As was shown earlier (see for example Fig. 5), the

resistance of the cover concrete can have a significant

effect on the corrosion rate of the reinforcement. The rate

of corrosion at the anode is dependent on the ease with

which ions can pass through the concrete pore electrolyte

between the cathode and anode. Hence a large potential

gradient between the anode and cathode associated with

a low concrete resistivity will normally signify a high

corrosion rate of the reinforcement.

The 4-point Wenner technique, developed from its use in

geotechnical surveying, is a technique that can measure

the resistivity of concrete. It involves passing an

alternating current between the outer pair of four

equispaced probes in contact with the concrete surface,

as shown in Figure 14. The resistivity of the concrete can

be calculated from the measured voltage between the

inner probes from:

Where, a, is the spacing between the probes.

The resistivity of the concrete will normally be affected by

many factors including moisture and salt content of the

concrete, mix proportions, water /.cement ratio, type of

cement replacement etc. As a general rule, if the resistivity

of the concrete is lower than 10,000 Ωcm, the corrosion

rate of the reinforcement which is suspected from a

potential mapping survey to be corroding, is likely to be

high. Higher resistivity values would normally signify a

lower corrosion risk.

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In theory, it should be possible to estimate fairly accurately

with the use of computer modelling the corrosion current

of steel reinforcement from a combination of potential

mapping and resistivity measurements. In practice, there

are, however, limitations as to the accuracy of the obtained

potential values, as was shown earlier, and of the resistivity

values owing to the inhomogeneity of concrete. Increasing

the spacing between the probes will diminish the effects

of localised non homogeneity but there is a l imit

determined by the concrete cover depth as the highly

conductive steel provides an easy path for the input

current. Furthermore, resistivity on the surface layer of the

concrete is different to that of the bulk owing to curing

effects, differential wetting or drying and particularly

carbonation of the surface layer. It is common practice to

drill shallow holes at the positions where the probes will

be located so that these surface effects are minimised.

Linear polarisation techniques

The actual corrosion rate of a section of steel reinforcement

can be determined by linear polarisation. The technique

essentially involves shifting the corrosion potential of a

known area of steel reinforcement in either the positive or

negative direction with the use of a potentiostat and

measuring the current that flows between the steel and

an external auxiliary electrode placed in close proximity

to the steel and in contact with the concrete via the

potentiostat. This can be done in single steps of potential

shifts or by scanning the potential over a range of typically

± 20mV with respect to the corrosion potential.8 The

increase in current at such a small shift in potential is

essentially linear and the gradient of the current- potential

plot gives the polarisation resistance, Rp, which can be

related to the corrosion rate by the Stern and Geary

equation:

where βa and βc are the anodic and cathodic Tafel

slopes respectively.The slopes can be determined

experimentally but they are normally assumed to be about

120 mV/decade so that the above equation is simplified

to: Rp = 26 / Icorr

As the corrosion rate can vary by several orders of

magnitude such an assumption is acceptable and will

only introduce comparatively small errors in the calculated

corrosion rates. Laboratory work has shown that corrosion

rates of steel in concrete calculated in this way are in

good agreement with weight loss determinations of the

same steel.9 The biggest limitation of this technique is

the inherent difficulty of applying it to large structures

where the steel reinforcement cage is electrically

connected and the true area of the steel being polarised

during the test is diff icult to establish. Recent

developments10 have attempted to concentrate the

polarisation over a small calculated area of the steel

reinforcement with the use of a special anode

arrangement (Figure 15) but problems are always likely

to exist because of variations in the concrete cover depths,

the geometry of the structure and the distribution of the

steel reinforcement bars.

Figure 15 Arrangement of insitu linear polarisation device

Final assessment

A combination of the assessment techniques described

above, should give a fair indication of the present

condition of the reinforcement. They do not, however, show

how the corrosion varies with the change of environmental

conditions. That information can be obtained by frequent

visits to the site or by continuous monitoring with the use

of embeddable reference electrodes and other probes.

Neither do they reveal the extent of the corrosion and the

length of time that corrosion had been occurring. Figure

16 describes the different stages of the corrosion process.

There is a virtually corrosion-free period before initiation

occurs (a), followed by a period of corrosion propagation

(a-b) before the reinforced structure totally loses its

serviceability. A survey of the kind described earlier can

Figure 16 Stages of structural element deterioration related to reinforcement

corrosion

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easily detect whether you lie either before or after point a

of the graph. Determining exactly where you are on the

propagation stage is much more difficult and would require

exposure of the reinforcement and a detailed structural

assessment.

If the correct choice of assessment techniques is made,

the limitations of the techniques employed are well

understood and the results are interpreted wisely, it is

possible to determine whether the structure is sound or

whether some remedial measures are required. The

assessment should also assist in the choice of

rehabilitation techniques but that is practically a different

discipline.

References

1. Barneyback R. S. & Diamond S, "Expression and analysis of

pore fluid from hardened cement pastes and mortars", Cem.

&Concr. Res. 11, 1981, 279-285.

2. Arup H "The mechanisms of the protection of steel by concrete",

Crane A. P. (ed), 'Corrosion of Reinforcement in Concrete

Construction', Ellis Horwood, Chichester, 151-157, 1983.

3. Gonzalez A., Algaba S. & Andrade C. 'Corrosion of reinforcing

bars in carbonated concrete" Br. Corros. J.,15, 1980, 135-139.

4. Hausmann D. A "Steel corrosion in concrete" Mater. Prot. 6,

1967, 19-23.

5. Burstein Q.T. "Passivity and localised corrosion" Shreir L . L.,

Jarman R. A. & Burstein G.T, (eds), 'Corrosion', 3rd edn., 1994,

1.118-1.150.

6. Page C. L. & Tteadaway K. W. J. "Aspects of the electrochemistry

of steel in concrete, Nature, 297, 1982, 109-116.

7. Glass G. K., Page C. L. & Short N. R. "Factors affecting the

corrosion rate of steel in carbonated mortars" Corros. Sci., 32,

1991, 1283-1294.

8. Sergi G., Lattey S. & Page C.L. "Influence of surface treatments

on corrosion rates of steel in carbonated concrete" Page C. L.,

Treadaway K. W. J. & Bamforth P. B. (eds), 'Corrosion of

Reinforcement in Concrete Construction', SCI / Elsevier, London,

1990, 409-419.

9. Andrade C. & Gonzalez J. A. "Quantitative measurements of

corrosion rate of reinforcing steels embedded in concrete using

polarization resistance measurements", Werkstoffe und Korrosion

29, 1978, 515-519.

10. Rodriguez J., Ortega L. M., Garcia A. M., Johansson L. & Petterson

K. "On-site corrosion measurements in concrete structures"

Construction Repair, Nov./Dec, 1995, 27-30

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Recent Developments in Mitigationof Rebar Corrosion in Concrete

M N Ramesh, CEO

Savcor India Private Limited

Corrosion of reinforcing steel in concrete is a

widespread and enormously costly problem

worldwide. Numerous concrete structures

including bridge decks and substructures, parking

garages, balconies and others are deteriorating as a result

of reinforcing steel corrosion. Virtually any reinforced

concrete structure is susceptible to the ravages of

corrosion if subjected to a conducive environment. The

corrosion process that takes place in concrete is

electrochemical in nature, very similar to a battery.

Electrochemical corrosion is a phenomenon accompanied

by a flow of electrons between cathodic and anodic areas

on a metal surface. In concrete the electro-chemical

corrosion reactions are most often triggered when three

factors- chloride, oxygen and moisture-meet at the

reinforcing steel surface. A sort of natural battery develops

within the reinforced concrete structure, generating a low-

Fig.1 Cracking due to corrosion of reinforcement

level internal electrical current. The points where this

current leaves the metal surface and enters the concrete

electrolyte are called anodes. The current leaving the

concrete and returning to the steel does so at the cathodes.

Corrosion or oxidation (rust) occurs only at anodes. When

corrosion of reinforcing steel occurs, the rust products

occupy more volume than the original steel, causing

tensile forces in the concrete.

Since concrete is relatively weak in tension, cracks soon

develop as shown in Figure 1, exposing the steel to even

more chlorides, oxygen and moisture-and the corrosion

process accelerates. As corrosion continues, delamination-

separations within the concrete and parallel to the surface

of the concrete occur. Delamination is usually located at,

or near, the level of reinforcing steel. Eventually concrete

chunks break away or spall off. Visual signs of corrosion-

induced damage on many types of reinforced concrete

structures are becoming more and more prevalent. In many

parts of the country one can hardly drive across a bridge

or enter a building that doesn't have some degree of

corrosion damage!

The rate of concrete deterioration at any given time is

dependent on many factors including corrosion rate,

reinforcing steel concentration, concrete properties, cover

and the environment, to name a few. Once corrosion has

begun there is one thing for certain-it will only get worse

and it will do so at an ever-increasing rate (see fig.2) .

Ultimately, if corrosion is allowed to continue, structural

integrity can be compromised due to loss of section of the

reinforcing steel and/or loss of bond between the steel

and the concrete, and replacement may be the only

solution. In order to mitigate or control a corrosion problem

(provide low future maintenance and long term protection)

specific information is needed for any given structure.

Fortunately, proven technology and scientific methods are

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Fig 2. Time dependent rebar corrosion based on various factors

available to evaluate corrosion of reinforcing steel (and

other embedded metals) and associated damage on

reinforced concrete structures. These techniques are

designed to determine the extent of damage, define the

corrosion state of steel in undamaged areas, evaluate the

cause, or causes, of corrosion, and determine the potential

for the steel to corrode in the future resulting in further

damage. It is only after this information is obtained through

a detailed corrosion condition evaluation that a suitable

repair and protection specification can be developed for

a corrosion-plagued structure. It is important to point out

that concrete; itself can deteriorate, regardless of the

condition of embedded reinforcement. Examples of this

include freeze/thaw damage, sulphate attack and alkali-

silica reactions. Various concrete tests are therefore often

conducted as part of an overall evaluation. Although there

are similarities between corrosion of conventionally

reinforced concrete structures and prestressed concrete

structures, this paper deals with conventionally reinforced

concrete structures only, particularly with respect to the

applicability of Electro-chemical protection techniques

that are developed recently in the area of corrosion

engineering.

Electro chemical method of rectifying carbonation:

This method is a recent development in concrete

restoration. This addresses the root cause of carbonation

and not its symptoms. Hence, this is a permanent solution.

The principle of the solution is driving an alkaline solution

into the carbonated concrete by an electro chemical

process. It comprises of installing an external anode kept

inside an alkaline electrolyte solution on the surface of the

concrete. This solution can either be sodium carbonate or

sodium bicarbonate. A direct current rectifier with its

positive terminal connected to external anode and its

negative terminal is connected to the reinforcing steel

making it cathode. Upon switching on the system the

alkaline solution will move inside concrete cover and will

reach the reinforcing steel making the entire section and

around the reinforcing steel alkaline. Thus, this process

corrects the carbonation by realkalining the concrete, and

forming hydroxyl ions around the steel. The schematic

details of the process are illustrated in the sketch below:

Electro-chemical remedial measures for ChlorideContaminated Concrete:

The problem associated with the chloride contaminated

concrete is pitting corrosion of rebars. The traditional

methods, to remedy the problem, has been isolating the

rebars from the concrete and replacing the removed

concrete by a new material free from chloride such as

polymer-modified mortar or micro concrete. This method

however suffers the impracticability of removing concrete

behind the bars and reinstating the same with new

material, if a very large extent of concrete is affected by

Fig 3. Realkalisation setup for correcting corbonation of concrete Fig.2. Setup for desalination or chloride extraction of concrete

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chlorides. In order to overcome the above problem the

recent development in the area of remediation to the

chloride attacked concrete is "desalination" and "cathodic

protection". These methods being electro chemical in

nature address the root cause of the problem and not the

symptoms. Electrochemical Chloride Extraction The

Electro Chemical Chloride Extraction (ECE), extract

chloride ions from contaminated concrete and is used to

reduce chloride concentration near steel reinforcement to

a level below the corrosion threshold. Hydroxyl ions are

also generated at the reinforcement, which allows the

passive oxide layer on the steel to be reformed after

treatment. An externally placed anode in the electrolytic

solution is connected to the positive pole and the

reinforcing steel is connected to the negative pole of the

rectifier. When the system is powered, all the negatively

charged chloride ions will be repelled by the cathode, i.e.

steel, and are forced migrate towards the anode which is

place outside the concrete. These ions will be neutralized

by the anode thereby extracting all the chloride ions from

the concrete. This method is also known as Desalination

Process. The schematic of the process setup is illustrated

in fig 2 This method is recommended for the concrete

structures which are located in the chloride laden

atmosphere. After the process is completed, further

diffusion of chloride ions in to the concrete should be

prevented by providing appropriately selected protective

barrier concrete surface coatings.

Impressed Current Cathodic Protection (ICCP)

Impressed current cathodic protection is achieved by

driving a low voltage direct current from a relatively inert

anode material, through the concrete to the reinforcing

steel. Figure 3 shows the basic layout required for

impressed current cathodic protection systems. Direct

current of sufficient magnitude and polarity is applied, so

as to oppose the natural flow of current resulting from the

electrochemical corrosion process. The direct current is

supplied by an external power source, most often a CP

rectifier. Recently, the use of solar power has received

attention and further research is going on for its adaptation

in actual structures. Impressed Current Cathodic

protection is a widely used and effective method of

corrosion control. Many people, engineers included, think

cathodic protection is so complicated and expensive that

it has no practical use in the concrete rehabilitation

industry. It is also believed that CP doesn't work or that it is

unreliable in the long term. The fact is ICCP is not so

complicated, is often the most cost-effective technique,

and has practical application on reinforced concrete

structures and that it most definitely works. Of course,

performance of ICCP systems, like all other corrosion

protection systems, is directly dependent on sound

specifications, proper installation, and monitoring and

maintenance. With ICCP, one cannot simply install it and

forget it. Good long term performance of all ICCP systems

requires good monitoring and maintenance procedures,

often due to this fact is discounted as a corrosion protection

system. Every corrosion protection system requires

Fig 3. Schematic of impressed current cathodic protection of reinforcement in

concrete

periodic inspection and maintenance. Impressed Current

Cathodic protection has been successfully used to protect

pipelines, ship hulls, off shore oil platforms, heat

exchangers, underground tanks, and many other facilities

exposed to a corrosive environment for many decades. Its

first application to steel in concrete was only in 1973. Since

then, many r c structures are protected with ICCP.

Recognizing that the corrosion process generates electric

currents, Impressed Current Cathodic protection supplies

a source of external current to counteract the corrosion

current. Hence, corrosion stops, or at least is greatly

minimized.

After an intensive research in the areas of corrosion of steel

in concrete, cathodic protection evolved as the only

technique which could positively arrest corrosion of steel

in existing concrete structures. In fact, cathodic protection

is the only rehabilitation technique that had proven to stop

corrosion in chloride affected concrete regardless of its

concentration in concrete. It should be noted, however,

that ICCP is not always needed nor is suitable on all

structures.

To select and design a proper repair and protection

scheme it is imperative that the causes of the distress are

properly diagnosed, fully understood, and the extent of

damage is determined Hence the first step is to have a

concrete and corrosion condition survey conducted in

order to define the cause and extent of the problem.

Electrical continuity of the reinforcing steel to be protected

is also a primary factor in considering ICCP. A closed

electrical circuit (unbroken electrical path) between all

Concrete Corrosion Reinforcing Steel

The Masterbuilder - March 2012 • www.masterbuilder.co.in122

reinforcing steel is required in order for the ICCP system to

function properly. Electrical continuity testing can be done

during the condition survey. The chloride concentration in

the concrete throughout the structure is also important. If

sufficient chlorides are present at the reinforcing steel depth

in many areas of the structure, ICCP may be the

economically viable alternative Based on the results of the

condition survey determination is made on the type of

repair and protection method to use. One advantage of

ICCP is that removal of sound concrete is not required,

thus a considerable cost savings may be realized. It may

be a viable alternative to removing two or three inches of

concrete over a large area in order to prevent future

corrosion. Cathodic protection is usually most cost effective

when long term performance is desired.

Galvanic, or sacrificial anode, cathodic protection is based

on the principles of dissimilar metal corrosion and the

relative position of as shown in Table.1. No external power

source is needed with this type of system and much less

maintenance is required. Such systems also provide

protective current primarily to areas on the steel surface

which need it the most. However, the relatively high

resistivity of concrete results in driving low voltage

provided by such systems would be inadequate for

cathodic protection of steel in concrete especially in the

splash zones of marines rc structures subjected to drying

and wetting cycles. Sacrificial anodes can be used to

mitigate corrosion in certain circumstances especially for

patch repair, specifically, if corrosion activity is low. In case

that pitting corrosion has not initiated or propagated but

in a situation prone to active corrosion, such as splash

zone, a full active impressed current system is the only

technically sound and cost effective option of corrosion

mitigation However, if the chloride content is relatively low,

or if the chlorides are generally located only in isolated

areas of the structure, sacrificial anode system may be

most appropriate. Basically sacrificial anode may enhance

the repair work but in most cases does not deliver the

protection in accordance to CP standards. So in theory it

can't be called as CP. In any real corrosion problem

situation where pitting corrosion has initiated and

propagated, high current density, may be around 20mA/

m2 of steel surface, is required to bring the potential of

rebars to certain level at which corrosion is arrested. This

cannot be delivered by sacrificial anodes.

Galvanized rebars

The zinc on galvanized reinforcing steel functions as a

sacrificial anode much the same way as zinc in a sacrificial

anode system does. In this case, the steel is protected by

the zinc from the day the rebar is galvanized. However,

once all the zinc is consumed, the base steel will be

susceptible to corrosion in the same way as plain

reinforcing steel.

Summary

Reinforcing steel corrosion causes extensive damage to

concrete structures. Various NDT methods are successfully

employed to carryout condition survey to evaluate

corrosion of reinforcing steel and the associated damage

on reinforced concrete structures. These tools help in

determining the extent of damage, define the corrosion

state of steel in undamaged areas, evaluate the causes of

corrosion and determine the potential for the steel to

corrode in the future resulting in further damage.

The recent development in the electro-chemical methods

are aimed at addressing the cause of the problem there

by ensuring a long-lasting solution to the corrosion

problem. Cathodic protection is a widely used and effective

method of corrosion control for reinforced concrete

structures. Cathodic protection supplies a source of

external current to counteract the corrosion current. Hence,

corrosion stops or minimizes to a greatly low level.

Almost any atmospherically exposed reinforced concrete

structure or structural members of almost any geometry

can be catholically protected. However, existing structures

must be considered individually with regard to the need

for the applicability of CP. Not all structures are good

candidates for CP, but ICCP is the only system that can

truly retard or mitigate corrosion in the harshest of the

environment.

References:

- Atef Cheaitani, M N Ramesh- Corrosion prevention

considerations to achieve a 100 year design life for reinforced

concrete structures in marine environments, NCCI

Seminar,2010

- Atef Cheaitani, M N Ramesh- Maintaining infrastructure - the

latest development in the repair and maintenance of reinforced

concrete structures, Asian Conference on ecstasy in concrete

2010, Indian Institute of Technology Madras, Chennai

- Ali Sohanghpurwala and William T. Scannell -' Repair and

protection of Concrete Exposed to Seawater

Element

Electrode Potential

Magnesium

Electrode P-2.38

Aluminium

-1.67

Zinc

-0.78

Chromuim

-0.58

Iron/Steel

-0.44

Nickel

-0.25

Tin

-0.14

Hydrogen

0.00

Platinum

+1.2

Gold

+1.80

Table 1: specific metals in the galvanic series

Concrete Corrosion Reinforcing Steel

The Masterbuilder - March 2012 • www.masterbuilder.co.in124

Repair Principles for Corrosion DamagedReinforced Concrete Structures

MG Alexander and JR Mackechnie

Department of Civil Engineering, University of Cape Town

Corrosion is the inevitable process that occurs when

refined metals return to their more stable combined

forms as oxides, carbonates and sulphides. The

corrosion process may be defined as the surface wastage

that occurs when metals are exposed to reactive

environments. Costs associated with corrosion damage

and control can be substantial, being as much as 3.5% of

the GNP of some industrial countries.

Reinforced concrete structures have not been immune to

the ravages of corrosion despite the protection that

concrete provides to embedded steel. Reasons for the

increasing incidence of corrosion damage to reinforced

concrete structures include the use of deicing salts and

calcium chloride set-accelerators, increased construction

in aggressive environments, fast-track construction

practices, changing cement composition resulting in finer

grinding and lower cement contents, lower cover depths

and poor construction practice including inadequate

supervision.

Reinforcement corrosion is particularly pernicious in that

damage may occur rapidly and repairs are invariably

expensive. Furthermore by the time visible corrosion

damage is noticed, structural integrity may already be

compromised. There is currently considerable debate

about the merits of the various systems for the repair of

reinforcement corrosion. This monograph attempts to

clarify some of the important issues by drawing on

international experience as well as local findings.

Ultimately the effectiveness of repair systems should be

measured in terms of cost, risk of failure and long-term

performance. As such no single system is appropriate for

all repairs but will depend on the type of structure, service

conditions, level of deterioration and financial constraints

of the project.

This monograph focuses on repair principles rather than

dealing with issues of detail that have been competently

published by others. Repair options can only be rationally

compared when the corrosion process and its influence

on concrete are fully understood. The document also

focuses on South African conditions and experiences,

derived from almost ten years of research on concrete

durability and repairs at the University of Cape Town.

Corrosion Fundamentals

Steel reinforcing bars will corrode freely when exposed to

moisture and oxygen under ambient conditions. When

steel is embedded in concrete however the high alkalinity

(pH of 12.5 or higher) stifles corrosion by the formation of

a passive ferric oxide film on the steel surface. The ferric

oxide layer forms a dense, impenetrable film that

suppresses further corrosion by limiting the movement of

cations and anions near the steel surface. This passive

ferric oxide film on embedded reinforcement may be

disrupted by a reduction in the alkalinity of the concrete

(principally by carbonation) or by the presence of

aggressive ions such as chlorides and sulphates.

Depassivation of the steel occurs as follows:

- in carbonated concrete, insufficient hydroxyl ions are

available to repair pits in the passive film

- in salt contaminated concrete, chloride ions break

down the passive layer at localized pits and encourage

metallic dissolution

Once depassivating conditions exist in concrete either

by a reduction in alkalinity (pH <10.5) or by the presence

of sufficient chloride ions (termed the corrosion threshold

value), corrosion may occur. For corrosion to occur at a

significant rate the following conditions are required:

- a reactive metal that will oxidise anodically to form

soluble ions

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- a reducible metal that provides the cathodic reactant

(typically hydroxyl ions)

- an electrolyte that allows ionic movement between

the material and environment

It is important to note that the establishment of

depassivating conditions at the steel (i.e. carbonation or

chlorides) is not necessarily indicative of a high probability

of corrosion damage since other factors (e.g. oxygen

availability, moisture content) will largely determine the

rate of corrosion. A schematic diagram of the corrosion

process of steel in con crete is shown in Figure 1.

Figure 1: Schematic of corrosion of steel in concrete

Four states of corrosion may be defined for reinforced

concrete depending on environmental conditions1:

- Passive state where minute levels of corrosion are

needed to sustain the ferric oxide film (typical of

embedded reinforcement in sound, alkaline and

uncontaminated concrete).

- Pitting corrosion causing local breakdown of the

passive film, usually due to the presence of chloride

ions. Adjacent steel acts as the cathode, being

considerably larger in area than the anode (typical of

steel embedded in chloride contaminated concrete).

- General corrosion due to an overall loss of passivity

that results in multiple pits along the steel surface

(typical of steel in carbonated concrete or concrete

containing high chloride concentrations).

- Active, low potential corrosion that occurs slowly when

insufficient oxygen is available to sustain the passive

film despite the high alkalinity of the concrete (typical

of reinforcement embedded in concrete underwater).

Clearly only pitting and general corrosion represent a

threat to the reinforcement and their severity will depend

on a number of internal and external factors which need

to be assessed when doing a corrosion survey. Internal

factors include concrete microstructure, cover depth and

moisture condition. External influences such as stray

currents and microbial activity may introduce a new

dimension into the corrosion system, but are not

considered here.

The nature of steel corrosion in concrete depends on local

conditions at the surface of the bar. High resistivity

concrete with relatively deep covers tends to favour micro-

cell corrosion where anode and cathode are close together

and cause localized pitting. Conductive concrete

contaminated with salt is often able to sustain more widely

spaced anode and cathode sites, termed macro-cell

corrosion.

Corrosion Damage

Once the passive layer on the reinforcing steel has been

disrupted and corrosion is activated, the chemical

reactions are similar whether the corrosion was initiated

by chloride attack or by carbonation. Steel dissolves into

solution and gives up electrons at the anode.

Anodic reaction: Fe ↑Fe2++ 2e- . . . . . . . . . . . . . . . . . . . . . (1)

The excess electrons are used up at the cathodic site

where water and oxygen are reduced to hydroxyl ions.

Cathodic reaction: 2e- + H2O + ½O

2 ↑2(OH)- . . . . . . . . (2)

These two reactions are necessary for electrochemical

corrosion to proceed. Little distress would be caused to

the surrounding concrete however if steel merely dissolved

into the pore water without further oxidation. Several more

oxidation stages occur which form expansive corrosion

products or rust capable of causing cracking and spalling

of the surrounding concrete. The oxidation stages may

be described as follows:

Fe2+ + 2(OH)↑Fe(OH)2Ferrous hydroxide . . . . . . . . . . . . (3)

4Fe(OH)2 + O

2 + 2H

2O ↑4Fe(OH)

3 Ferric hydroxide . . .

................... . (4)

2Fe(OH)3 ↑Fe

2O

3.H

2O + 2H

2O Hydrated ferric oxide . .

................... . (5)

The expansion associated with rust is mostly due to

hydrated oxides that may swell up to ten times the original

volume of the steel. The type of corrosion product formed

at the steel depends on environmental con ditions:

- red or brown rust forms under high oxygen

concentrations, forming flakey rust which is relatively

soft and easy to dislodge from the rebar

- black rust forms under low oxygen concentrations,

forming a relatively dense and hard layer that may be

difficult to remove from the parent steel

Two major consequences of reinforcement corrosion are

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commonly observed, cracking and spalling of the cover

concrete as a result of expansion of the corrosion product,

and a reduction of cross-sectional area of the rebar by

pitting (usually only a problem in prestressed con crete

structures). Manifestations of corrosion depend on a

number of influences that include:

- geometry of the element (large diameter bars at low

covers allow easy spalling)

- cover depths (deep cover may prevent full oxidation

of corrosion product)

- moisture condition (conductive electrolytes

encourage well-defined macro-cells)

- age of structure (rust stains progress to cracking and

spalling)

- rebar spacing (closely spaced bars in walls and slabs

encourage delaminations)

- crack distribution (cracks may provide low resistance

paths to the reinforcement)

- service stresses (corrosion may be accelerated in

highly stressed zones)

The loss of serviceability of corroded reinforced concrete

structures may be described by a three phase damage

Figure 2: Three-phase corrosion damage model

model shown in Figure 2 2.

The different phases are defined as follows:

- An initiation period, before corrosion is activated by

either carbonation or chloride attack, during which

negligible concrete deterioration occurs.

- A propagation period in which active corrosion

commences and cracking of the cover concrete

occurs due to the formation of expansive corrosion

products at the steel surface.

- An acceleration period of damage where corrosion

increases due to easy access of oxygen and water

through cracks in the cover concrete, resulting in

spalling of concrete.

Unfortunately most reinforced concrete structures that

exhibit cracking and spalling have gone beyond the point

where simple, cost-effective measures can be taken to

restore durability. Condition surveys are therefore an

important strategy to identify and quantify the state of

corrosion of a structure timeously.

Condition Surveys

A detailed corrosion or condition survey is vital in order to

identify the exact cause and extent of deterioration, before

repair options are considered. Various diagnostic sheets

are given in the Appendix for guidance during condition

surveys.

a) Visual assessment

Corrosion damage may be identified and defined using a

systematic visual survey. Classification of visual evidence

of deterioration must be done objectively, following clear

guidelines that define damage in terms of appearance,

location and cause. Defects may be defined in terms of

cracks (caused by corrosion, temperature, shrinkage or

fatigue), joint deficiencies (joint spalls, upward movement,

lateral movement, seal damage) surface damage

(abrasion, rust stains, delaminations, popouts, spalls),

changes in member shape (curling, deflection, settlement,

deformation) and textural features (blow holes,

honeycombing, sand pockets, segregation).

Visual assessment of deterioration can provide useful

information when done in a rational, systematic manner

but the data may come too late for cost-effective repairs.

Rebar corrosion damage is often only fully manifest at the

surface after significant deterioration has occurred. Early

evidence of distress can sometimes be detected by an

experienced engineer before major distress takes place.

b) Delamination survey

A hammer survey or chain drag is a simple method of

locating areas of delamination in concrete. Hollow

sounding areas can be marked up on the concrete or

recorded directly in a survey form. Delamination surveys

often under-estimate the full extent of internal cracking

and should not be considered as definitive. Radar and

ultrasonic instruments may provide a more sophisticated

approach to locating areas of delamination, particularly

at greater depths.

c) Cover surveys

Cover surveys are routinely done to locate the position

and depth of reinforcement within a concrete structure.

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Covermeters use an alternating magnetic field to locate

steel and any other magnetic material in concrete (note

that austenitic stainless steels are non-magnetic). Cover

measurements may be unreliable when:

- rebar is at deep covers (e.g. covers greater than 80

mm)

- measuring regions of closely spaced bars

- measuring differing bar types and sizes (unless

specifically calibrated) o other magnetic material is

nearby (e.g. window frames, wire ties, bolts) To ensure

reliable cover depths from a survey, direct

measurements of rebar depths should be made by

exposing a limited number of bars. Calibration can

then be made for site specific conditions such as rebar

type, concrete and environmental influences.

d) Chloride testing

The presence of sufficient chloride at the surface of

reinforcement is able to depassivate steel and allow

corrosion to occur. Chlorides exist in concrete as both

bound and free ions but only free chlorides directly affect

corrosion. Measuring free chlorides accurately is

extremely difficult and water-soluble chloride tests are

unreliable, being strongly affected by the method of

sample preparation. Further, bound chlorides may be

released into solution under carbonating conditions or

by dissolution, making all chlorides in concrete potentially

corrosive. Chlorides are therefore most commonly

determined as acid soluble or total chlorides in

accordance with BS 1881 3.

Chloride sampling and determination in concrete is

illustrated in Figure 3 and is usually done in the following

manner:

- concrete samples are extracted as either core or

drilled powder samples

Figure 3: Chloride content determination and typical chloride profile

- depth increments are chosen depending on the cover

to steel and the likely level of chloride contamination

(increments are typically between 5 and 25 mm)

- dry powder samples are digested in concentrated

nitric acid to release all chlorides

- chlorides are analysed using a colorimetric or

potentiometric titration

- chloride contents are generally expressed as a

percentage by mass of cement

- chloride profiles may be drawn such that chloride

concentrations may be interpolated or extrapolated

for any depth (see Figure 3)

- future chloride levels can be estimated from Fick's

second law of diffusion

Table 1: Qualitative risk of corrosion based on chloride levels

Chloride content by mass of

cement (%)

< 0.4

0.4 - 1.0

> 1.0

Probability of corrosion

Low

Moderate

High

The corrosion threshold depends on several factors

including concrete quality, cover depth, and saturation

level of the concrete. The probability of corrosion may be

assessed from the following qualitative rating shown in

Table 1 for acid-soluble chloride contents.

Limitations of chloride testing of concrete are asfollows:

- presence of chlorides in aggregates may give

misleading results

- chloride contents in cracks and defects cannot be

accurately determined

Figure 4: Schematic of the carbonation process

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- slag concretes may be difficult to analyse with

colorimetric titration methods

- relatively large samples are required to allow for the

presence of aggregates

e) Carbonation depth

Carbonation depth is measured by spraying fresh

concrete with a phenolphthalein indicator solution (1%

by mass in ethanol/water solution). Phenolphthalein

remains clear where concrete is carbonated but turns

pink/purple where concrete is still strongly alkaline (pH

> 9.0). Carbonation moves through concrete as a distinct

front and reduces the natural alkalinity of concrete from a

pH in excess of 12.5 to approximately 8.3, with a pH level

of 10.5 being sufficiently low to depassivate steel. The

progress of the carbonation front is shown in Figure 4.

Environmental conditions most favourable for carbonation

(i.e. 50 - 65 % R.H.) are usually too dry to allow rapid steel

corrosion that normally requires humidity levels above

80% R.H. Structures exposed to fluctuations in moisture

conditions of the cover concrete, such as may occur during

rainy spells, are however vulnerable to carbonation-

induced corrosion.

Limitations to carbonation testing are as follows:

- phenolphthalein changes colour at pH 9.0 whereas

steel depassivation occurs at a pH of approximately

10.5, hence the corrosion risk is slightly under-

estimated

- some concretes are dark (e.g. slag concretes) and a

distinct colour change is difficult to discern visually

- phenophthalein may bleach at very high pH levels

(e.g. after electrochemical realkalization)

- testing must be done on freshly exposed concrete

surfaces before atmospheric carbonation occurs

f) Rebar potentials

Chloride-induced corrosion of steel is associated with

anodic and cathodic areas along the rebar with

consequent changes in electropoten tial of the steel. It is

possible to measure these rebar potentials at different

points and plot the results in the form of a 'potential map'.

Measurement of rebar potentials may determine the

thermodynamic risk of corrosion but cannot evaluate the

Rebar potential (-mV Cu/CuSO4)

< 200

200-350

>350

Qualitative risk of corrosion

Low

Uncertain

High

Table 2: Qualitative risk of chloride-induced corrosion 4

kinetics of the reaction. Rebar poten tials are normally

determined in accordance with ASTM C876 using a

copper/copper sulphate reference electrode connected

to a handheld voltmeter 4. The qualitative risk of corrosion

based on rebar potentials is shown in Table 2. Note that

the technique is not recommended for car-bonation-

induced corrosion where clearly defined anodic regions

are absent.

The procedure for undertaking a rebar potential survey is

as follows:

- mark up a grid pattern in the area of measurement

(not more than 500 mm centres)

- make an electrical connection to clean steel by coring

or breaking out concrete

- check the steel is electrically continuous over the

survey area using a multimeter

- wet the concrete surface with tap water if the concrete

appears to be dry

- take and record readings either manually or using a

data logger

- check data on site to correlate with visual signs of

corrosion

- Rebar potential measurements are relatively quick to

perform but have the following limitations:

- interpretation of results must be done with caution

(preferably by a specialist)

- rebar potentials from carbonated concrete are difficult

to interpret (the reading is a mixed potential of anodic

and cathodic sites)

- delaminations may disrupt the potential f ield

producing false readings

- environmental effects will influence potentials (e.g.

temperature and humidity)

- rebar potentials cannot be directly correlated with

corrosion rates

- stray currents may affect measured potentials

Absolute values are often of lesser importance than

differences in rebar potential measured on a structure. A

shift of several hundred millivolts over a short distance of

300-500 mm often indicates a high risk of corrosion.

g) Resistivity

Concrete resistivity controls the rate at which steel

corrodes in concrete once favourable conditions for

corrosion exist. Resistivity is dependent on the moisture

condition of the concrete, on the permeability and

interconnectivity of the pore structure, and on the

concentration of ionic species in the pore water of concrete

Corrosion Repair & Rehabilitation

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such that:

- poor quality, saturated concrete has low resistivity (e.g.

less than 10 kOhm.cm)

- high quality, dry concrete has high resistivity (e.g.

greater than 25 kOhm.cm)

Measurement of resistivity is done with a simple in situ

Wenner probe connected to a portable resistivity meter.

The outer two probes send an alternating current through

the concrete while the inner two probes measure the

potential difference in the concrete. Once the concrete

resistivity is known a rough assessment of likely corrosion

rates can be made as shown in Table 3. This assessment

assumes conditions are favourable for corrosion.

Resistivity measurements are simple to perform on site

but have several limitations:

- measurements are affected by carbonation and

wetting fronts

- surface conductive layers and rebar directly below

the probe should be avoided

- readings may be unstable in concretes with high

contact resistance at the surface

Table 4: Qualitative assessment of site corrosion rates

Corrosion rate

(? A/cm2)

> 10

1.0 - 10

0.2 - 1.0

< 0.2

Qualitative assessment of corrosion rate

High

Moderate

Low

Passive

h) Corrosion rate measurements

Corrosion rate measurements are the only reliable method

of measuring actual corrosion activity in reinforced

concrete. A number of sophisticated corrosion monitoring

systems are available, based primarily on l inear

polarization resistance (LPR) principles. These

techniques require considerable expertise to operate

reliably. Corrosion rate measurements on field structures

are most commonly done using galvanostatic LPR

techniques with a guard-ring type sensor to confine the

area of steel under test. Experience indicates that

corrosion rates fluctuate significantly in response to

environmental and material influences and single readings

are generally unreliable. Table 4 shows a qualitative guide

for the assessment of corrosion rates of site structures 5.

Epair Strategies

Numerous repair options are available and new

technologies continue to make an impact in the field of

concrete repairs. The suitability and cost-effectiveness of

repairs depends on the level of deterioration and specific

conditions of the structure.

a) Patch repairs

Before patch repairs are considered it is important that

the distinction between chloride- and carbonation-

induced corrosion is appreciated. As a general rule

chloride-induced corrosion is far more pernicious and

difficult to treat than carbonation-induced corrosion. This

often dictates a completely different approach to

repairing damage due to the two types of corrosion.

Carbonation-induced corrosion causes general corrosion

with multiple pitting along the reinforcement. Carbonated

concrete tends to have fairly high resistivity that

discourages macro-cell formation and allows moderate

corrosion rates. Steel exposed to corrosive conditions will

therefore show signs of corrosion that can be easily

identified (e.g. surface stains, cracking or spalling of

concrete). Repairs are generally successful provided all

of the corroded reinforcement is treated.

Chloride-induced corrosion is characterized by pitting

corrosion with distinct anode and cathode sites. The

presence of high salt concentrations in the cover concrete

means that macro-cell corrosion is possible with relatively

large cathodic areas driving localized intense anodes.

High corrosion rates can be sustained under such

Figure 5: Formation of incipient anodes after patch repairs

Resistivity (kOhmcm)

< 12

12-20

>20

Likely corrosion rate given corrosive

conditions

High

Moderate

Low

Table 3: Likely corrosion rate based on concrete resistivity

Corrosion Repair & Rehabilitation

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conditions resulting in severe pitting of the reinforcement

and damage of the surrounding con crete. Much of the

reinforcement may be exposed to corrosive conditions

without showing any signs of corrosion, this is particularly

noticeable when corroded structures are demolished.

Localized patch repairs of areas of corrosion damage are

popular due to their low cost and temporary aesthetic

relief. This form of repair has limited success against

chloride-induced corrosion as the surrounding concrete

may be chloride-contaminated and the reinforcement is

therefore still susceptible to corrosion. The patched area

of new repair material often causes the formation of

incipient anodes adjacent to the repairs as shown in

Figure 5. These new corrosion sites not only affect the

structure but often also undermine the repair leading to

accelerated patch failures in as little as two years.

Consequently, it is necessary to remove all chloride-

contaminated concrete from the vicinity of the

reinforcement.

Complete removal of chloride-contaminated concrete,

where it is possible should successfully halt corrosion by

restoring passivating conditions to the reinforcement.

Mechanical removal of cover concrete is usually done

with pneumatic hammer, hydrojetting or milling machines.

This form of repair is most successful when treating areas

of localized low cover, before significant chloride

penetration has occurred. If repairs are only considered

once corrosion damage is fairly widespread it will be

expensive to mechanically remove chloride-

contaminated concrete from depths well beyond the

reinforcement.

Patch repairs consist of the following activities that are

briefly described below:-

- removal of cracked and delaminated concrete to fully

expose the corroded reinforcement

- cleaning of corroded reinforcement and the

application of a protective coating to the steel surface

(e.g. anti-corrosion epoxy coating or zinc-rich primer

coat)

- application of repair mortar or micro-concrete to

replace the damaged concrete

- possible coating or sealant applied to the entire

concrete surface to reduce moisture levels in the

concrete

b) Coating systems

A variety of coating and penetrant systems are available

that are claimed to be beneficial in repairs of concrete

structures. Barrier systems attempt to seal the surface

thereby stifling corrosion by restricting oxygen flow to the

cathode. In large concrete structures, corrosion control is

theoretically unlikely due to the presence of oxygen

already in the system. In practice barrier systems are

generally ineffective due to the presence of defects in the

new coating during application and further damage

during service. Such an approach is more likely to promote

the formation of differential aeration cells further

exacerbating the potential for corrosion.

The application of a hydrophobic coating (sometimes

referred to as penetrant pore-liners) may be used to reduce

the moisture content of concrete and thereby

electrolytically stifle the corrosion reaction. The drying

action works on the principle that surface capillaries

become lined with a hydrophobic coating that repels water

molecules during wetting but allows water vapour

movement out of the concrete, to facilitate drying.

Hydrophobic coatings using silanes and siloxanes are

gen erally most effective on uncontaminated concrete,

free from cracks and surface defects. The feasibility of

such an approach is questionable for marine structures

where high ambient humidity, capillary suction effects and

presence of high salt concentrations all interfere with

drying.

Figure 6: Sorptivity results from bridge cores

The long-term effectiveness of hydrophobic systems

applied to new construction is not known but local studies

suggest reasonable performance over 10-15 years service.

The Storms River bridge was coated with a silane system

in 1985 and concrete cores were extracted from several

parts of the structure in 1996 for analysis 6. The effect of

the hydrophobic coating on absorption was determined

by sorptivity testing at increasing depth increments into

the concrete. Sorptivity results are shown in Figure 6 for

arch and column concrete. The sharp increase in sorptivity

at depths between 0.5 and 3 mm may be ascribed to the

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Table 5: Likely performance of migrating corrosion inhibitors in concrete

Likely

inhibition

Good

Moderate

Poor

Corrosive

conditions

Mildly corrosive,

low chlorides or

carbonation

Moderate levels of

chloride at rebar

(i.e. <1%)

High chloride levels

at rebar (i.e. > 1%)

Concrete

conditions

Dense concrete

with good cover

depths (> 50 mm)

Moderate quality

concrete, some

cracking

Cracked, damaged

concrete, low

cover to rebar

Severity of

corrosion

Limited corrosion

with minor pitting

of steel

Moderate

corrosion with

some pitting

Entrenched

corrosion with

deep pitting

presence of the silane in the concrete near-surface zone.

c) Migrating corrosion inhibitors

A corrosion inhibitor is defined as a chemical substance

that reduces the corrosion of metals without a reduction

in the concentration of corrosive agents. Corrosion

inhibitors work by reducing the rate of the anodic and/or

cathodic reactions thereby suppressing the overall

corrosion rate. The effectiveness of migrating corrosion

inhibitors is generally controlled by environmental,

material and structural factors, shown in Table 57.

Migrating corrosion inhibitors are generally organic-based

materials that move through unsaturated concrete by

vapour diffusion. Organic corrosion inhibitors such as

amino-alcohols are believed to suppress corrosion by

primarily being adsorbed onto the steel surface thereby

displacing corrosive ions such as chlorides. The adsorbed

organic layer inhibits corrosion by interfering with anodic

dissolution of iron while simultaneously disrupting the

reduction of oxygen at the cathode.

When assessing the suitability of repairs with migrating

corrosion inhibitors, two important issues must first be

considered:

- the likely penetration of the material into the concrete

needs to be determined

- the severity of the corrosive environment at the

reinforcement must be quantified

Migrating corrosion inhibitors are designed to move fairly

rapidly through partially saturated concretes that allow

vapour diffusion. Penetration has however been found to

be poor in near-saturated concretes typically found in

partially submerged marine structures. This poor

penetration performance may be ascribed to high

moisture and salt levels that prevent significant vapour

diffusion through the concrete. It is critical therefore that

satisfactory penetration of corrosion inhibitors is checked

before undertaking full-scale repairs.

The performance of migrating corrosion inhibitors in

controll ing chloride-induced corrosion is largely

dependent on chloride levels at the reinforcement. Work

done by Rylands indicates that effective inhibition is not

possible at chloride levels above 1.0% at the

reinforcement 8. This can be seen in Figure 7 where ribbed

steel bars embedded at 25 mm in a grade 40 portland

cement concrete were subjected to wetting and drying

cycles with a salt solution for a period of 18 months.

Concrete blocks were either controls (CON) or contained

organic corrosion inhibitor, either admixed during casting

(ADM) or coated after 30 cycles (CTG). The chloride

content at the level of the reinforcement was approaching

2% at the time of application of the migrating corrosion

inhibitor and resulted in poor inhibition. Better inhibition

is possible if treatment is done earlier when chloride

contents are lower.

The effectiveness of migrating corrosion inhibitors appears

to be enhanced when used in combination with

hydrophobic coatings to reduce moisture levels in

concrete. This has been noted in both laboratory trials

and field monitoring of repairs. Such an approach has

also been found to be effective in the repair of carbonation-

induced corrosion damage.

d) Electrochemical techniques

Corrosion of reinforcement in concrete is an

electrochemical process that occurs when embedded

steel is depassivated by a reduction in concrete alkalinity

or the presence of corrosive ions such as chlorides. Two

repair techniques, electrochemical chloride removal and

realkalization, attempt to restore passivating conditions

by the temporary application of a strong electric field to

the cover concrete region.

Realkalization is the process of restoring the original

alkalinity of carbonated concrete in a non-destructive

manner. The electrochemical treatment consists ofFigure 7: Corrosion rate measurements with time for grade 40 concrete

Corrosion Repair & Rehabilitation

www.masterbuilder.co.in • The Masterbuilder - March 2012 133

placing an anode system and sodium carbonate

electrolyte on the concrete surface and applying a high

current density (typically 1 A/m2). The electrical field

generates hydroxyl ions at the reinforcement and draws

alkalis into the concrete. Alkaline conditions may be

restored in the concrete in as little as one to two weeks

using the system.

Electrochemical chloride removal (ECR) is a more time-

consuming and complex technique and its suitability

needs to be carefully assessed. Chloride removal is

induced by applying a direct current between the

reinforcement and an electrode that is placed temporarily

onto the outside of the concrete. The impressed current

creates an electric field in the concrete that causes

negatively charged ions to migrate from the reinforcement

to the external anode. The technique decreases the

potential of the reinforcement, increases the hydroxyl ion

concentration and decreases the chloride concentration

around the steel thereby restoring passivating conditions.

Figure 8 shows the basic principles of ECR.

The effectiveness of ECR depends on several factors that

include the following:-

- extent of chloride contamination in concrete

- structural configuration including depth and spacing

of reinforcement

- applied current density and time of application

- pore solution conductivity and resistance of cover

concrete

- presence of cracks, delaminations and defects

causing uneven chloride removal

Figure 8: Schematic illustration of electrochemical chloride removal technique

Figure 9: Chloride profiles before and after ECR treatment for 8 weeks

ECR typically takes 4-12 weeks to run at current densities

within the normal range of 1-2 A/m2. Results from ECR

trials performed in the laboratory are shown in Figure 9

and indicate that complete extraction may take longer

than 8 weeks at a current density of 1 A/m2 9. In some cir

cumstances chlorides beyond the reinforcement may be

forced deeper into the concrete during the process. There

is a risk that chlorides left in the concrete may diffuse

back to the reinforcement and cause further corrosion

with time.

The feasibility of using ECR depends on several factors

such as:-

- the presence of major cracking, delaminations and

defects that will require repair before ECR

- large variations in reinforcement cover that will cause

differential chloride extraction and possible short-

circuiting

- reactive aggregates requires special precautions to

avoid possible alkali silica reaction; lithium salts

should be used in these cases

- prestressed concrete structures may be susceptible

to hydrogen embritt lement after ECR; special

precautions are needed to eliminate this risk

- temporary power supplies of significant capacity are

required during application of ECR

e) Cathodic protection systems

Cathodic protection systems (CP) have an excellent track

record in corrosion control of steel and reinforced concrete

structures. The principle of CP is that the electrical

potential of the steel reinforcement is artif icially

decreased by providing an additional anode system at

the concrete surface. An external current is required

between anode and cathode that diminishes the corrosion

rate along embedded reinforcement. The current may be

produced either by a sacrificial anode system or using an

impressed current from an external power source.

Sacrificial anode systems consist of metals higher than

steel in the electrochemical series (e.g. zinc). The external

anode corrodes preferentially to the steel and supplies

electrons to the cathodic steel surface. Sacrificial anode

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The Masterbuilder - March 2012 • www.masterbuilder.co.in134

systems are most effective in submerged structures where

the concrete is wet and resistivity is low. Warm

temperatures are also generally required for sacrificial

CP systems (i.e. above 200C).

CP systems more commonly use an external electrical

power source to supply electrons from anode to cathode.

The anode is placed near the surface and is connected to

the reinforcement through a transformer rectifier that

supplies the impressed current (see Figure 10). Anodes

may be conductive overlays, titanium mesh within a

sprayed concrete overlay, discrete anodes or conductive

paint systems. Anode systems are usually designed for a

minimum service life of 20 years but may last in excess of

50 years.

Before CP repairs are undertaken several factors needto be considered:

- reinforcement must be electrically continuous

- concrete cover must be uniformly conductive and free

of delaminations

- alkali reactive aggregates and prestressing steel need

special treatment

- power must be available to drive the impressed

current in the structure

CP repair of concrete structures requires a thorough

corrosion survey by a specialist and the design needs to

be undertaken by a corrosion expert. Reliable CP systems

are fully controlled and monitored by a series of

embedded sensors in order to ensure optimum

performance. This is essential since under or over-

protection of the reinforcement may be potentially harmful

to the structure or the CP system. Continuous monitoring

of CP systems is usually done remotely by modem and

the power consumption during operation is extremely

small.

The first major CP repair of a reinforced concrete structure

in South Africa was done at the Simonstown Jetty in 1996

10. The structure was almost 80 years old and in an

extremely poor condition with widespread chloride-

corrosion damage. Several previous patch repairs had

failed and the concrete was contaminated with chlorides

making conventional repairs unfeasible. An impressed

current CP system was installed with metallic ribbon

anodes protected within a sprayed concrete overlay. The

structure has been restored to full serviceability and

should require no further repairs for at least 40-50 years.

f) Demolition/reconstruction

Deterioration of reinforced concrete structures is often so

advanced that demolition and reconstruction becomes

viable. This option should only be considered as a last

resort since the total cost (capital costs plus loss of service

and temporary works) is usually well in excess of repairs

costs. Corrosion damage is also generally confined to

near-surface regions and engineers often over-estimate

the extent of damage to corrosion-dam-aged structures.

Recent demolition of several bridge-decks along the Cape

coast revealed that actual corrosion damage was less

than anticipated.

Demolition and reconstruction is often preferred by

engineers who have limited repair experience or lack

confidence in new repair systems. It is crucial nevertheless

that lessons are learnt from the old structure when

designing the replacement. Guidance about ensuring

durable reinforced concrete structures is given in

Monographs 1 and 2.

Economics of Repairs

Repairs of reinforced concrete structures damaged by

corrosion have often proved to be unsuccessful with further

damage occurring after repair. Reasons for the poor

performance of repairs include:-

- lack of understanding of deterioration processes

- inadequate investigation and testing prior to repairs

- inadequate funds to undertake satisfactory repairs

- ineffective or inappropriate repairs being specified

- poor supervision and implementation of repairs on site

Repairs are not generally anticipated by owners and funds

for repairs are nearly always extremely limited. Economics

largely dictate the timing and scale of repairs but

unfortunately only short-term costs are often considered.

Whilst corrosion damage is to some degree unique to

each structure some basic tenets hold for most cases.

- Performance of the concrete structure prior to

Figure 10: Typical cathodic protectipon layout

Corrosion Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in136

treatment often dictates the likely performance after

repair. Structures with high levels of damage and rapid

rates of deterioration require more substantial repair

than those less seriously affected.

- The timing of treatment is crucial since corrosion rates

and damage increase with time. A structure that has

been neglected and allowed to reach an advanced

level of damage will not respond to 'quick-fix' solutions.

Conversely a structure that is repaired early enough

may be restored to full serviceability relatively cheaply.

- The effectiveness of treatments in retarding corrosion

is not equal and may range from highly effective to

detrimental (e.g. cathodic protec tion versus patch

repairs)

considered, a practical example is given in the Appendix.

Closure

The notion that reinforced concrete structures require no

maintenance or repair during their service life is gradually

being dispelled. It has been said that owners will have to

pay for durability at some point in the life of a structure.

Inadequate designs with excessive cost-cutting will

merely transfer the savings in capital costs to much more

expensive repairs at a later stage. While accountants may

encourage some deferment of capital costs into

maintenance, experience suggests that investments in

the form of design and construction for durability bring

better rewards than allowing for maintenance. Despite

this evidence, economic imperatives that attempt to

maximise short-term profits, often impact detrimentally

on the durability and service life of infrastructural

developments.

Repair of reinforced concrete structures needs to be

undertaken in a rational manner to guarantee success.

An increasing number of repair options are available that

must be considered in terms of cost, technical feasibility

and reliability. Engineers need to understand all the

relevant material, structural and environmental issues

associated with concrete repairs in order to make

intelligent choices.

High quality repairs require a thorough investigation into

the causes of deterioration, appropriate repair

specifications and competent execution of the repair work.

This can only be done when structural investigations are

carried out by independent experts, specifications are

drawn up by engineers with specialist repair expertise

and repairs are undertaken by competent contractors.

Appendix 1:Repair example

A 60-year old bridge structure is in need of major repairs

arising from widespread corrosion damage. The bridge

spans a tidal estuary with direct exposure to seawater

splash and spray action. Concrete is heavily contaminated

with salt and chloride levels at the reinforcement are

around 1.0% by mass of cement. Damage in the form of

cracking, spalling and delaminations are widespread

over much of the structure and are the result of chloride-

induced corrosion. Urgent repairs are essential to restore

full serviceability to the bridge.

Rough estimates of service life of the various options are

based on recent experience in South Africa and specialist

publications13,14,15. Whilst the projected performance

of the various repairs is a subjective assessment, the

figures serve to illustrate the many issues that need to be

considered when costing repairs.

Importantly, repairs costs need to be compared in a

rational way by comparing life-cycle costs of the structure.

Scott showed that when life-cycle costs are compared, a

maintenance-free structural design is cheaper than

cutting initial costs and deferring some money for repair

and maintenance at a later date (data shown in

Table 6) 11.

Strohmeier showed that repair costs escalate dramatically

as deterioration proceeds and that repairs should be

done as soon as distress is noted 12. This research helped

quantify what many engineers had long realized; that

durability-based designs are cost-effective in the long-

term and that delays in repairs cause an exponential

increase in costs.

Engineers considering repair of concrete structures do

not have the freedom to change either the original design

or the timing of the repairs. Repairs therefore need to be

considered on the merits, logistics, costs and risks of the

many options that are available to rehabilitate the

structure. To illustrate some of the issues that need to be

Option

Original

design

Repairs/

main-

tenance

Relative

costs

1

60 MPa 30%

fly ash 55

mm cover

None

1.0

2

60 MPa 30%

fly ash 30

mm cover

Surface

treatment at

10-year

intervals

2.0

3

60 MPa 30%

fly ash 40

mm cover

Patch repairs

after 20 and

35 years

2.3

4

60 MPa 30%

fly ash 40

mm cover

Cathodic

protection

after 20 after

20

3.0

5

60 MPa

100%PC 75

mm cover

Patch repairs

after 15, 25

and 35 years

3.5

Table 6: Total life cycle costs of typical beam members exposed to marine

environment

Notes on repair options:-

Option 1. Durability design for maintenance free 40 year service life

Option2. Based on anticipated life of surface treatment

Options 3-5. Based on the likely stage at which spalling damage becomes

excessive

Option 5. Design required by SABS 0100:1992

Corrosion Repair & Rehabilitation

www.masterbuilder.co.in • The Masterbuilder - March 2012 137

For the purposes of costing the repair options, the

following assumptions are made:-

- unescalated 2001 costs are used due to uncertainties

about future discount, inflation and tax rates

- site establishment costs are fixed at R 250 000 for

each repair option

- total area of concrete under repair is 2000 m2

- unit rates for repair include allowance for labour,

materials, access and supervision

- repairs are focused on chloride-induced corrosion

damage only

A The following repair options are considered for the

bridge. A Localized repairs of corrosion-damaged

areas with only cosmetic con sequences. Assuming

15% of the structure requires patching and that

concrete is only broken back to the reinforcement, a

unit rate of R250/m2 is used. Given the limited nature

of the repairs and the likelihood of incipient anode

formation an effective life of 8 years is con sidered

possible.

B More extensive mechanical break-outs and patching

are done with all corroded reinforcement being

exposed, cleaned and a good quality repair material

used for patching. Approximately 30% of the structure

is treated at a unit rate of R280/m2. Despite the effort

made to repair the structure, corrosive conditions still

exist at the reinforcement and further corrosion

damage limits the effective life to 12 years before more

repairs must be considered.

C Conventional corrosion repairs are done but a

migrating corrosion inhibitor is applied to the repaired

concrete surface together with a hydrophobic coating

(silane/siloxane). Mechanical breakout is limited to

damaged areas of concrete and not all corrosion on

reinforcement is removed resulting in a unit rate of

R300/m2. This includes the cost of the migrating

corrosion inhibitor and coating at R40/m2. The chloride

level at the reinforcement (1.0%) is at the upper level

for corrosion inhibitor performance resulting in an

effective service life of only 15 years.

D Electrochemical chloride extraction is applied to the

concrete to remove chloride from around the steel.

The cost of the system is approximately R750/m2 for a

six week application and includes repair to damaged

concrete. Unfortunately not all the chloride is removed

from the concrete resulting in an effective service life

of 25 years.

E Cathodic protection is applied to the structure to

protect the embedded reinforcement. The cost of the

system is R900/m2 at installation and a nominal

maintenance and monitoring fee of R5000 per year.

The anode system is designed to last 50 years thereby

dictating the effective life of the system.

Present value costs for the various options are shown in

Table A1. From these findings it is clear that initial repair

costs and total repair costs over 40 years vary significantly.

Timing

Initial

20 years

40 years

Option A

0.75

2.25

3.75

Option B

0.81

2.43

3.24

Option C

0.85

1.70

2.49

Option D

1.75

1.75

3.50

Option E

2.05

2.15

2.25

Table A1: Total present value costs (million rands)

Item

Structure

name

Location

Environment

History

Date

inspected

Surface

condition

Early

cracking

Concrete

quality

Rebar cover

Structural

effects

Surface

damage

Staining

Cracking

Rebar

Condition

Carbonation

Delamination

Previous

repairs

Example

Background data

Identification, reference number

Physical address or location

Severity and type of exposure

Age, design data, repairs

Date

Original condition

Honeycombing, bleeding, voids, popouts

Plastic settlement or plastic shrinkage

Surface hardness, density, voids, colour

Covermeter survey, mechanical breakout

Overloading, dynamic effects, structural cracking

Present Condition

Abrasion, chemical attack, spalling, leaching

Rebar corrosion, AAR gel, effloresence, salts

Width, pattern, location, causes of cracking

Visual examination of bar, rust and pitting damage

Indicator test on cores or mechanical breakouts

Size, frequency, severity of delamination

Integrity of repairs, signs of damage near repair

locations

Observa-

tion

APPENDIX 2: Diagnostic sheets

Table A2: Checklist for investigation of structural deterioration

Option A is most cost-effective when only short-term costs

are considered but most expensive in the longer-term.

For a structure that only has to last another 20 years, option

C may be preferable whereas for 40 years further service,

option E is most economical for the hypothetical example.

Corrosion Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in138

References

- Arup, H., 'The mechanisms of the protection of steel by concrete',

Corrosion of reinforcement in concrete construction, SCI, 1985.

- Miyagawa, T., 'Durability design and repair of concrete structures:

chloride corrosion of reinforcing and alkali aggregate reaction',

Magazine of Concrete Research, 43(156), 1991, pp 155-170.

- British Standards Institute, 'Chloride content determination for

concrete', BS 1881 Part 124, 1988.

- American Society for Testing and Materials, 'Standard test

method for half-cell potential measurement of reinforcement in

concrete', ASTM C876, Philadelphia, 1991.

- Broomfield, J.P., 'Corrosion of steel in concrete: appraisal and

repair', Chapman and Hall, 1997.

- Hoppe, G.E. and Varkevisser, J., 'Long term monitoring of the

effectiveness of the silane impregnation of a concrete arch bridge

to inhibit further effects of alkali aggregate reaction', FIP

Symposium: The Concrete Way to Development, CSSA, 1997,

pp 777-786.

- Mackechnie, J.R., Alexander, M.G. and Rylands, T., 'Performance

of Ferrogard corrosion inhibitor in chloride environments',

Unpublished report, University of Cape Town, 2000.

Type of

corrosion

Chloride-

induced

Carbonation-

induced

Stray current

Chemical

induced

Secondary

forms

Artificially

induced

Environment or

causative conditions

Marine environments Industrial

chemicals Admixed chlorides

(older structures)

Unsaturated concrete Polluted

environments Low cover depths

to steel

DC power supplies Railway

systems Heavy industries,

smelters

High sulphate groundwaters

Fertilizer factories Industrial

plants Sewage treatment works

Primary cracking due to alkali

aggregate reaction, delayed

ettringite formation, structural

cracking

Bimetallic corrosion Partial

sealing of concrete High

temperatures (>2000 C) Patch

repairs of corrosion

Significant features of

deterioration

Rapid and severe corrosion

Distinct anode & cathode

regions Corrosion damage

may affect structural integrity

General corrosion along rebar

Moderate corrosion rates

except when wet & dry faces

are close Corrosion damage

generally only affects

aesthetics

General corrosion of rebar

exposed to moist conditions

Corrosion not confined to

low cover depths Large crack

widths possible

Corrosion generally

associated with near saturated

conditions Concrete

deterioration occurring

together with corrosion

Corrosion localized in regions

where cracks intersect rebar

Other forms of distress

evident in concrete (i.e. AAR

gel deposits)

Generally very localized

intense corrosion due to well

defined anode/cathode

regions

Table A3: Conditions and features of different forms of reinforcement corrosion

Type of

deteriora-

tion

Reinforcement

corrosion

Alkali

aggregate

reaction

Shrinkage/

creep

Chemical

attack

Softwater

attack

Fire damage

Structural

overload

Visual evidence /

associated factors

Rust stains, cracking along

reinforcement, spalling of

cover concrete, delamination

of cover concrete

Expansive map cracking,

restrained cracking following

reinforcement, white silica

gel at cracks

Characteristic cracking,

excessive displacements,

time dependent movements,

exposure to drying conditions

Surface attack, salt deposits

on surface, expansive internal

reactions causing cracking,

exposure to aggressive waters

Surface leaching of

concrete, exposed

aggregate, exposure to

moving waters in conduits

Surface discolouration,

concrete spalling, thermal

cracking, buckling, loss of

strength, microcracking

Major cracking in areas of

high stress, localized

crushing, excessive

deformations and deflections

Confirmatory testing

Cover depth of rebar

Carbonation & chloride

testing Exploratory coring

Electrochemical testing

Core analysis for gel and

rimming of aggregates

Petrographic analysis

Aggregate testing

Concrete core analysis

Loading and structural

analysis Aggregate and binder

analysis

Chemical analysis of

concrete Core examination

for depth of attack and

internal distress

Chemical analysis of water

Core examination for

leaching damage

Core examination for colour

variations, steel condition

Petrographic analysis

Specialist techniques

Loading and structural

analysis Core testing for

compressive strength and

elastic modulus

Table A4: Diagnostic sheet for concrete deterioration (all forms)

- Mackechnie, J.R., Alexander, M.G. and Rylands, T., 'Laboratory

trials with an organic corrosion inhibitor', 14th Int. Corrosion

Congress, Cape Town, 1999, CD-ROM.

- Mackechnie, J.R. and Le Maire, H.R.A., 'Electrochemical

extraction of chlorides from OPC and fly ash concrete', Concrete

Beton, 82, 1996, pp 9-17.

- Stevenson, C.E., Unpublished MSc thesis in progress, University

of Cape Town, 2001.

- Alexander, M.G. and Scott, A., 'Designing reinforced concrete

structures for durability and economy in marine environments',

SAICE Journal, 41(4), 1999, pp 15-21.

- Strohmeier, J.H. and Alexander, M.G., 'Deterioration, repair and

maintenance of reinforced concrete structures in the Cape

Peninsula', Concrete Beton, 81, 1996, pp 14-21.

- Mackechnie, J.R., 'Observations from case studies of marine

concrete structures', SAICE Journal, 40(4), 1998, pp 29-32.

- Addis, B.J. and Basson, J.J., 'Diagnosing and repairing the

surface of reinforced concrete damaged by corrosion of

reinforcement', Portland Cement Institute, Midrand, 1989.

- Standards Australia, 'Guide to concrete repair and protection',

ACRA, Homebush, 1996.

Corrosion Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in140

Concrete Repair & Rehabilitation

Repair and Rehabilitation of CorrosionDamaged Concrete Structures

P. Srinivasan

Principal Scientist, ACTEL, CSIR- Structural Engineering

Research Centre , CSIR Campus, Taramani

Corrosion of steel reinforcement in concrete

structures is a techno-economic problem for

several reasons. Technically (i) it poses challenges

in research and development to discover methods and

materials either to control or prevent corrosion (ii) inspite

of considerable research work world-wide, it is now well

recognised that corrosion in plain carbon steel can only

be controlled and a total prevention is nearly an impossible

task (iii) corrosion of reinforcing steel in concrete is peculiar

in the sense that the corrosion product, because of its

volume growth, causes cracking to the concrete. This

physical effect together with sectional loss of reinforcing

bars affect the load carrying capacity, serviceability, and

the service life of a structure (iv) rehabilitation of corrosion

damaged structures is often cumbersome requiring high

technical expertise and competence. This paper

highlights the materials and techniques for the repair and

rehabilitation of corrosion damaged concrete structures.

Few case studies are also presented.

Steel is passive under high alkalinity environment, and

therefore corrosion will not occur. Then, there are two ways

to initiate the onset of corrosion. One is by the penetration

of carbon dioxide from the environment into the cover

concrete and the other is the penetration of water

containing dissolved salts through the concrete cover or

through a concrete crack. In the first case, the alkalinity of

the concrete surrounding the steel could be reduced by

carbon dioxide which reacts with calcium hydroxide in

cement paste to form calcium carbonate (called

carbonation of concrete) and further to form carbonic acid

with the pore solution. The reduction of calcium hydroxide

leads to a low pH value. This creates an environment for

the corrosion of steel to take place. In the case of chloride

diffusion, the alkalinity of concrete is not reduced, but when

the chloride ion concentration is high enough, reaching a

certain ratio with the hydroxyl ions (Cl/OH), the

deapssivation of steel takes place and corrosion of steel

may start.

The required treatments for restoring the protective

environment for steel depend on the extent and cause of

the corrosion damage:

Carbonation-induced corrosion damage. Under such

conditions, carbonated concrete should be removed and

new concrete should be installed, re-passivation is

provided by the new repair mortar or concrete.

Chloride-induced corrosion damage. Under such

conditions, if chloride has penetrated to the level beyond

the steel reinforcements, removal of chloride around steel

bars does not guarantee re-passivation as chloride ions

may diffuse back from the deeper part of the concrete to

the new concrete cover. This is the so-called redistribution

of chloride after the repair. In this case, the repaired

concrete will become cathodic and the rebar will be the

anode. The corrosion will occur in the bars immediately.

Other factors may influence the re-passivation of steel, for

instance, coating of the steel reinforcements, and the

application of membranes or sealers to limit the moisture

content. In most cases, the strategy of repair is either a

comprehensive or a partial repair of the concrete member.

These strategies are common in the rehabilitation of

concrete and they depend on the structural system,

external environmental factors, and the degree of structural

degradation.

Steps in executing repair

There are several regular steps in the repair of all structures

exposed to corrosion.

- The first step is to strengthen the structure by

performing structural analysis and designing a suitable

www.masterbuilder.co.in • The Masterbuilder - March 2012 141

location for the temporary support.

- The second step is to remove the cracked and

delaminated concrete. It is important to clean the

concrete surface and also the steel bars by removing

rust. After rust is removed by brush or sand blasting,

the steel bars should be painted with epoxy coaling or

replaced; additional steel rods shall be added if

necessary. Then new concrete can be poured.

- The final step is to coat the concrete member with

concrete surface coating as external protection. These

steps will be explained in detail in the following

sections.

Materials

The materials adopted for repair of corrosion damaged

structures are described below:

Polymer Concrete

Three to four fold increase in strength up to 140 N/sq mm

has been obtained using polymer in concrete.

Corresponding increase in tensile strength of concrete is

also achieved by polymer impregnation. The durability of

polymer impregnated concrete is substantially increased

when exposed to freeze-thaw cycles. This has been the

strong reason for its application in cold regions, and against

corrosive salts and acids. These properties can be fully

utilised in repairs and renovation of old structures

damaged due to heavy wear and tear and by corrosion

due to marine atmosphere. The development of

techniques for such applications is in progress. Materials

(monomers and polymers) used for the impregnation are

Styrene, Polypropeleyne, Methyl Methacrylate (MMA) and

Poly Methyl Methacrylate (PMMA).

Epoxy Grouts, Mortars and Coatings

Epoxy resin is a product of Epichlorohydrin and Bisphenol

with or without additives such as plasticisers and dilutants.

To get a cured epoxy resin product, a hardener (usually an

amine) is blended with the epoxy resin at ambient

temperature. The resin mortar may be obtained by adding

fillers, such as, coarse sand or calcined bauxite grit. They

develop excellent strength and adhesive properties, and

are resistant to many chemicals. They have good chemical

and physical stability; they harden rapidly and resist water

penetration. In all, they provide a toughness that couples

durability with crack resistance.

Latex Modified Concrete

The third group of materials which can be used for repair

Materials

Portland Cement Mortar

Portland Cement Concrete

Microsilica Modified

Portland Cement Concrete

Latex Modified Portland

Cement Concrete

Polymer Modified Portland

Cement Mortar with

Non-sag Filler

Magnesium Phosphate

Cement Concrete

Preplaced- Aggregate

Concrete

Epoxy Mortar

Methylmethacrylate (MMA)

Concrete

Shotcrete

Ingredients

Binder

Portland cement

Portland cement

Portland cement

Portland cement

Portland cement

MagnesiumPhosphate

cement

Portland cement

Epoxy resin

Acrylic resin

Portland cement

Additive

Micro- silica

Non-Sag fillers

Pozzolans

Pozzolans

Admixture

Water reducer

Air-entr

Water reducer

Air-entr

HRWR Air-entr

Latex SBR

Acrylic latex

Fluidifier

Water reducer

acceler latex

Application Requirements

Thickness Limitation

in/cm

Curing

Wet 7 days

Wet 7 days

Wet 7 days

Wet 3 days

Sheet 45 min-

2 days

Wet 7 days

-4 hrs.- 2 days

1 hr.- 6 hr.

Wet 7 days

Installation Tempera-

ture 0F/0C

Table- 1 Repair and Overlay Materials

Concrete Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in142

purposes are the latex or acrylic-modified mortars. These

are conventional patching mixes to which is added a

synthetic latex. These additives actually give a mortar

greater internal strength. For this reason they are usually

preferred where strength or heavy loading is an important

factor, for example, on bridge decks or for factory floors

subject to heavy wheel loads. Both compressive and

tensile strength are improved, while flexibility of the patch,

a major factor influencing its durability, is increased

substantially. Resistance to alkalies and dilute acids is

good; the concrete has low water absorption properties

and freeze-thaw stability is improved over a conventional

patch. Bond strength of the latex modified mortar is said

to be greater than the shear strength of the old concrete.

Table- 1 gives the summary of the repair and overlay

materials with the properties.

Structure Strengthening

One of the most dangerous and important first steps

necessary for the repair is selecting the temporary support,

which depends on the following:

- evaluating the state of the whole structure

- determining how to transfer loads in the building and

its distribution

- determining the volume of repair that will be done

- determining the type of concrete member that will be

repaired

- the repair process must be carried out by a structural

engineer with a high degree of experience who has the

capability to perform the structure

Removal of damaged concrete

There are several ways to remove the part of the concrete

that has cracks on its surface and shows the effects of

steel corrosion. These methods of removing the

delaminated concrete depend on the ability of the

contractor, specifications, the cost of breaking, and the

whole state of the structure. The selection of the breaker

methods is based on the cause of corrosion; if it is due to

carbonation or chlorides, then one must also consider

whether cathodic protection will be performed in the future.

In this situation, the breaking work would take place on the

falling concrete cover; it would be cleaned and all the

delaminated concrete and cracked concrete parts

removed. Then, high-strength, nonshrinking mortar would

be poured.

If the corrosion in steel reinforcement is a result of chloride

propagation into concrete, most specifications

Drying

Shrink - age

Moderate

Low

Low

Low

Moderate

Moderate

Very low

Low

Moderate

Moderate

Compressive StrengthCoeff. of thermal

Expansion

Equal to substrate

Equal to substrate

Equal to substrate

Compat w/substrate

Compat w/substrate

Equal to substrate

Equal to substrate

(1.5-5) *concr.

(1.5-5) *concr.

Equal to substrate

1 HR

0

0

0

0

0

1 Day 3 Day 28 DayElastic Modulus

psi/Mpa

Permeability

(Con-

crete=10)

9

9

6

5

5

9

10

1

1

6

Freeze Thaw

Resis- Tance

Good

Good

Good

Excellent

Excellent

Excellent

Good

Excellent

Excellent

Good

Non-Sag

Quality

Moderate

NA

Good

NA

Excellent

Low

NA

Moderate

NA

NA

Exo-

Therm

Low

Low

Low

Low

Moderate

High

Low

High

High

Low

Com-

ments

ACI 30

4R - 23

ACI 503.4

Vapor mayCause

ProblemsinConfined

areas

ACI 506

R - 90

Table- 1 ( Contd.,)

Concrete Repair & Rehabilitation

www.masterbuilder.co.in • The Masterbuilder - March 2012 143

recommend removing about 25 mm behind the steel and

making sure that the concrete on the steel has no traces of

chlorides after the repair process. The difference between

good and bad repair procedures is shown in Fig. 1.

The difference in the procedure of breaking the

delaminated concrete is due to the difference in the causes

of corrosion. Therefore, a careful study to assess the state

of the structure and the causes of corrosion is very important

to get high quality after the repair process. The evaluation

process is the same as illness diagnosis. It is necessary

and important to remove concrete for a distance greater

than the volume required for removal of defective concrete

so that proper steel can be reached. This will be important

later in the repair process. Several methods are commonly

used for breaking and removing the defective concrete

and these are explained next.

Manual Method

One of the simplest and easiest methods is to use a

hammer and chisel to remove defective concrete. This is

considered one of the most inexpensive ways, but it is too

slow compared with mechanical methods. However,

mechanical methods produce high noise and vibration,

have special requirements, and need trained labor. Using

the manual method makes it difficult to spare concrete

behind the steel. Any worker can manually break the

concrete, but it is necessary to choose workers who have

done repair work before as they must be sensitive in

breaking the concrete to avoid causing cracks to the

adjacent concrete members.

Pneumatic Hammer Methods

These hammers work using compressed air; they weigh

between 10 and 45 kg. If they are used on the roof or walls,

their weight will be about 20 kg. They need an attached

small power unit to do the job, but in large areas may

require a separate, bigger air compressor. This machine

requires proper training for the worker that uses it. The use

of pneumatic hammers is more economical when a small,

rather than large, area is to be removed.

Performance rates are about 0.025-0.25 m3 per hour using

hammers weighing 10-45 kg, respectively.

Water Jet

This method has been commonly used since it was

introduced to the market in the 1970s. It relies on the

existence of water at the work site and on the removal of a

suitable depth of concrete in a large area. It removes

fragmented concrete, cleans steel bars, and removes part

of the concrete behind the steel bars, as shown in Fig. 2

The water jet is used manually by an experienced worker

who has previously dealt with the hose, which is pushing

water under high pressure Very high safety precautions

need to be applied to the worker who uses it and the site

around it.

Grinding Machine

This is used to remove concrete cover in the case of large,

flat surfaces. The grinding machine is usually used after

the water gun or the pneumatic hammer to obtain final

concrete breakdown around and under the steel

reinforcement. Therefore, one must take into account

whether the thickness of the concrete cover is equal. The

rate of removal of the concrete by this machine is very fast.

Clean concrete surfaces and steel reinforcements

This phase removes any remaining broken concrete with

a process of cleaning. At the same time, the process of

assessing the steel and cleaning up and removing

corrosion takes place.

Concrete

The stage of preparing a surface by pouring the new

concrete is one of the most important stages of the repair

process. Before application of the primer coating, which

provides the bond between the existing old concrete and

the new concrete for repair, the concrete surface must be

well prepared, and this takes place according to the

materials used. The concrete surface must be clean and

not contain any oils, broken concrete, soil, or lubricants.

The surface must be cleaned completely through sand

blasting, water, or manually using brushes. This stage is

very important and very necessary, regardless of the type

of material used to bond the new concrete with the old.

Clean Steel Reinforcement Bars

After removal of the concrete covers and cleaning the

surface, the next step is to evaluate the steel reinforcement

by measuring steel diameter. If the cross-sectional area of

the steel bars is found to have a reduction equal to or more

than 20%, additional reinforcing steel bars must be added.

Before pouring new concrete, one must be sure that the

Good Repair Wrong Repair

Fig.1

Concrete Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in144

development length between the new bars and the old

steel bars is enough, as shown in Fig. 3 It is usually

preferable to link the steel by drilling new holes in the

concrete and connecting the additional steel on concrete

by putting the steel bars in the drilled hole filled with epoxy.

However, in most cases the steel bars are completely

corroded and need to be replaced.In the case of beams

and slabs that need to add additional steel reinforcement

bars, it is preferable to connect the steel bars with concrete

by drilling new holes in the concrete and making the bond

of the steel bars in the holes by using adhesive epoxies.

For beam repairs, the additional steel bars are fixed in a

column that supports this beam. In the case of slabs, the

steel bars are fixed in the sides of the beam that is

supporting the slab, as shown in Fig. 3

Dry Pack

Dry pack is a mortar mixture with a very low water-cement

ratio, applicable for small area of repair. It is normally placed

by hand. Materials commonly used in drypack are Portland

Cement, sand, and water. Other types of Portland cement

can also be used.

Pre-placed Aggregate Concrete

This essentially involves first placing the coarse aggregate

in the forms, and thereafter filling the voids by pumping in

cement grout (sanded or unsanded). This has been found

to be suitable on areas where accessibility is a problem.

Joint Sealers

Joint sealers are very important in concrete structures as

every concrete structure has joints (or cracks). Joint sealers

should ensure the structural integrity and serviceability. In

addition, they should serve as protection against ingress

of harmful liquids, gases, or other undesirable substances

which would impair the quality of concrete.

Jacketing

Jacketing is the process of fastening a durable material

over concrete and filling the caving with a grout that

provides needed performance characteristics. The

materials used for jacket are metals, rubber, plastics, and

concrete. This restores structural values, protects the

reinforcement from exposure to the harmful elements and

improves the appearance of the original concrete.

Jacketing materials may also be secured to concrete by

means of bolts, screws, nails, or adhesives; by bond with

the existing concrete; or by gravity. The method of securing

employed, will depend upon the exposure, the material

Fig. 2 Concrete Surface after removing with water jet

Fig. 3 Installing additional steel

The techniques for the replacement of the cover concrete/

damaged concrete are given below.

Shortcrete or Gunite

Shotcrete or Gunite is mortar or concrete conveyed

through a pressure hose and applied pneumatically at

high velocity onto a surface. This material has found wide

applications in several major repair works as it can be

applied on vertical, horizontal or overhead surfaces, with

the area to be repaired being either reinforced or

unreinforced. For the purpose of design, gunite may be

considered as good quality concrete of grade M.20.

Shotcrete mixed with steel fibres can also be used.

Fig. 5 Use of concrete collars for strengthening concrete compression member

Concrete Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in148

used and the positioning of the jacketing material. Fibre

glass reinforced plastics, ferrocement, and other hard

materials such as polypropylene can be used for jacketing.

A few examples of guniting, jacketing, and strengthening

are schematically shown in Figs. 4-7 respectively.

Cathodic Protection

As discussed, the corrosion of reinforcement in concrete

is an electrochemical process. Cathodic protection is a

technique by which the electrical potential of the steel is

increased to a level at which corrosion can not take place.

It is widely used for both steel and concrete offshore

structures, while on land it has been used for the protection

of pipelines and similar structures. It has been used on a

limited scale, for concrete structures as discussed below.

Two different methods are employed, an impressed current

and the use of sacrificial anodes. In the first the structure

is connected to the negative terminal of a DC power source,

ideally using an anode which does not corrode. In the

second the reinforcement is connected to anodes with a

more negative corrosion potential than steel, such as zinc

or aluminium. The current is reversed and corrosion now

takes place at the anode, which is gradually used up. In

both cases, electrical continuity of the reinforcement is

required. Fig. 8 shows the schematic setup for the cathodic

protection.

Use of FRP Wrapping

The growing interest in FRP systems for strengthening and

retrofitting can be attributed to many factors. Although

the fibers and resins used in such systems are relatively

expensive compared with traditional strengthening

materials like concrete and steel, labor and equipment

costs to install FRP systems are often lower. Fiber-

reinforced polymer systems can also be used in areas with

limited access, where traditional techniques would be very

impractical-for example, a slab shielded by pipe and

conduit. These systems can have lower life-cycle costs

than conventional strengthening techniques because the

FRP system is less prone to corrosion.

Fiber-reinforced polymer (FRP) can serve as an alternative

to the use of steel sheets. The use of FRP has a wide range

of advantages and offers an alternative to the steel used in

the strengthening process. There are different types of

FRPs; the famous type is carbon fiber-reinforced polymer

(CFRP), which is most commonly used as appropriate in

practical applications and because of its unique properties

in terms of resistance and the resistance with time, as well

as resistance to stress.

Corrosion Inhibitors

It has been shown that certain admixtures can be used to

inhibit corrosion of the reinforcement in the presence of

chlorides8. One that shows promise is calcium nitrite. When

corrosion takes place in untreated concrete, the ferrous

ions at the anode pass into solution and, in a secondary

reaction, are converted to rust. With the calcium nitrite,

Fig. 6 Beam strengthening with steel plates

Fig. 7 Replacement of concrete using pressurized forms

Other Methods

Other techniques employed on repair of corroded

concrete structures include removal of chloride ions,

cathodic protection, use of Fibre reinforced Polymer

Wraps, corrosion inhibitors and concrete coatings. The

details are given below

Concrete Repair & Rehabilitation

www.masterbuilder.co.in • The Masterbuilder - March 2012 149

ferric ions are formed which are insoluble and hence stay

on the surface of the reinforcement, preventing further

corrosion. The addition of calcium nitrite extends the time

to corrosion initiation The corrosion rate, once corrosion is

initiated, is less with calcium nitrite.

Corrosion inhibitors can be classified based on their action

and their chemistry and function:

Different types of coatings are available such as

chlorinated rubber coating, vinyl coatings, epoxy coatings,

acrylic based, polyurethane, etc. Sealers are an

intermediate application between penetrants and

coatings. They protect concrete by blocking the pores.

Sealers are more viscous than penetrants and generally

form a thin film on the surface of concrete.The effectiveness

of surface treatment materials in preventing the ingress of

aggressive ions depends on the penetrability of the

material to provide protection of the concrete matrix.

Various organic polymers are used as sealers and coatings.

The most widely used penetrating materials tend to be

siliceous, which line the pores of concrete forming silicone

resins and thus provide protection through water repellent

properties. Silane/siloxane primer with aliphatic- acrylic

top coat gives good protection. CECRI also have

developed and implemented concrete coatings.

Case-studies

Many corrosion-affected structures were investigated by

the author at SERC for the its condition assessment through

NDT & PDT and repair measures were formulated to

increase its service life. One of the structure is the prill

tower for the manufacture of urea and the age more than

30 years ( Fig.9) The structure is a RCC shaft and the

thickness is 230 mm. Since the structure was constructed

in marine environment, a coating was provided on the

surface right from its construction. After the detailed

investigation it was found that the carbonation depth was

only 12 mm and chloride content in the concrete was within

allowable limits. The reinforcements are found to be in

very good condition. The test results have proved the

efficiency of concrete coating.

The other structure is a 30 year old RCC water tank

constructed in Bangalore (Fig. 10) and was affected by

Fig. 8 Schematic diagram of CP including modern link and remote monitoring

Anodic inhibitors

Cathodic inhibitors

Ambiodic inhibitors

Suppressing the anodic corrosion reaction

Suppressing the cathodic reaction

Suppressing both anodes and cathodes

Inorganic inhibitors

Organic inhibitors

Vapour phase or

volatile inhibitors

nitrites, phosphates and other inorganic

chemicals

amines and other organic chemicals

a subgroup of the organic

inhibitors (generally Amino alcohols) that

have a high vapour pressure

By their action:

By their chemistry and function:

Coating to Concrete

Surface treatment materials are often used to protect

concrete from deterioration due to reinforcement corrosion.

These materials are classified as Penetrants, Coatings,

and Sealers.

Penetrants are low viscosity liquids designed to penetrate

into concrete and line its pores. They protect concrete by

forming a hydrophobic layer and thus repel moisture, but

they facilitate the evaporation of water vapor and other

gases from the interior of the concrete mass. Coatings

provide protection to concrete by forming a thick,

protective film on the surface. However, due to minimal

breathability, these materials may contribute to concrete

deterioration.Fig. 9 Prill Tower ( RCC Shaft- 30

Years Old)

Fig. 10 Water Tank repaired and coated

with Concrete coating

Concrete Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in150

corrosion. After detailed investigation, remedial measures

were formulated. The repair measures consist of

strengthening and finally a concrete surface coating was

provided. It was found that after seven years of exposure,

the carbonation depth was almost nil.

Conclusions

Since corrosion is a complicated problem, the cause has

to be diagnosed and proper material and technique has

to be adopted. The cost of repair will vary with the type of

technique being adopted. Repairing of a corroded

damaged structure requires skilled personnel. The

repaired structure has to be monitored periodically for their

performance.

References

- Page C.L, Treadway, KWJ and Bamforth PB (Editors) - Corrosion

of reinforcement in concrete; Society of Chemical Industry;

Elsevier Applied Science, May 1990.

- IS 13620 -1993, "Specification for fusion bonded epoxy coated

reinforcing bars".

- British Standards institution, BS6744: 1986, "Austenitic Stainless

Steel Bars for the Reinforcement of Concrete".

- British Standards institution, BSEN-10088-1:1995, "Stainless

Steels, Part 1 - List of Stainless Steels".

- Mani, K., and Srinivasan, P., "Service life of structures in corrosive

environment : A comparison of carbon steel and SS bars", The

Indian Concrete Journal, July 2001, pp 452-456.

- John L. Clarke (Editor),(1993), "Alternative Materials for the

Reinforcement and Prestressing of Concrete", Blackie Academic

& Professional, First edition,

- Jones et al., (1995), "Concrete surface treatment : Effect of

exposure temperature on chloride diffusion resistance", American

Concrete Institute, Materials Journal, pp.197-208.

- Srinivasan,P., Firdows M.Z.M., Prabakar J., and Chellappan, A., A

simple accelerated test method for rapid assessment of chloride

penetration of concrete with and without surface coating, The

Indian Concrete Journal, January 2007, pp 43-47.

- Srinivasan, P., Mohd. Firdows, M.Z., & Mani, K., "Surface coatings

for protection of concrete in marine environment - performance

evaluation through laboratory experiments", National Seminar on

Harbour Structures, (NASHAR- 2003), IIT Madras, Chennai, 21-

22, February 2003, pp 341-350

- Alonso, C. and Andrade C. (1990), "Effect of nitrate as a corrosion

inhibitor in contaminated and chloride - free carbonated mortars",

American concrete Institute, Materials Journal, pp.130-137.

- John Broomfield, (1999) "Corrosion inhibitors for steel in concrete",

pp.45-47.

- Berkeley, K.G.C. and S. Pathmanabhan, "Cathodic Protection of

reinforcement steel in concrete", Bulter works, London.

- Mohamed A. El-Reedy, "Assessment and Repair of

Corrosion",CRC Press, London, New York, 2007

Concrete Repair & Rehabilitation

The Masterbuilder - March 2012 • www.masterbuilder.co.in168

Formwork Failure: Cases & causesSpecial Correspondent

Collapse of concrete structures during construction

has been happening since concrete has been

placed in formwork. Cases and causes of these

type of failures have been documented and recorded in

many texts, articles and journals. This article will try and

focus on a few of them from the available reports, starting

with The New York Coliseum on May 9, 1955, 2000

Commonwealth Ave. on January 5,1971, Skyline Plaza in

Bailey's Crossroads on March 2, 1973, The Harbour Cay

Condominium in Cocoa Beach, Florida in March 1981 and

ending with The Tropicana in Atlantic City on October 30,

2003.The focus will be on what has been learned over time

from these failures and what has been done to keep these

type of tradgedies from occurring in the future.

Although there were many cases of concrete failures during

construction prior to the New York Coliseum collapse as

illustrated in (McKaig 13-27, 1962), only a few will be

looked at after this point because of the changes and

progressions being made in the construction industry at

this time in history.

(A) New York Coliseum on May 9, 1955

Pic source: http://www.ppconstructionsafety.com

Formwork Failure

www.masterbuilder.co.in • The Masterbuilder - March 2012 169

The construction method was a flat plate waffle slab with

solid slabs at the column caps. It was one of the first times

the use of motorized buggies had been used in the pouring

of this type of structure. The floor that collapsed was the

first floor above grade supported on two tiers of shores at

a total of 22' high. It can be seen from Figure 1 how collapse

happened. The buggies weighed about 3000 lb loaded,

ran at about 12 mph, and there were eight of them at the

time of the failure with about 500 cubic yards of concrete

already placed. The investigation that followed put the

blame solely on inadequate provisions in the formwork to

resist lateral forces, it even went on to say that "if there had

been sufficient diagonal, horizontal, and end bacing of

the temporary supporting structure, the collapse could

have been prevented entirely,...", (McKaig 15-16, 1962).

After the collapse the district attorney called attention to

the lack of inspections and made recommendations to

revising the building code with respect to formwork

because of the new advances.

(B) 2000 Commonwealth Avenue: January 5, 1971

This was a progressive collapse of a cast-in-place

reinforced concrete flat-slab structure. Punching shear was

determined to have been the triggering mechanism but

the real problem was in the numerous errors and omissions

by every party involved in the project (Delatte 133-143).

The investigating committee determined that if the

construction had had a proper building permit and had

followed codes, then the failure could have been avoided

(Delatte 142) (See Figure 2 and 3 how failure occurred).

Some of the problems leading to the collapse are

- Not following the structural engineers specifications

for shoring and formwork

- Lack of concrete design strength

- Lack of shoring or removed too soon

- Improper placement of reinforcement

- Little construction control on site

- Owner changed hands many times

- Almost all jobs were sub contracted

- No architectural opr engineering inspection done

- Inadequate inspection by the city of Boston

- The general contractors representative was not a

licensed builder

- Construction was based on arrangements done by the

subcontractors

- No direct supervision of subcontractors

Figure 2: Typical flatplate with uniform distributed loading

Figure 3: Punching shear failure diagram

Figure 4: Skyline Plaza at Bailey's Crossroads, National Archives

Figure 1: N.Y. Coliseum Collapse, National Archives

Formwork Failure

The Masterbuilder - March 2012 • www.masterbuilder.co.in170

(C) Skyline Plaza: March 2,1973

Skyline Plaza (See Figure 4) in Bailey's Crossroads is an

example of a catastrophic collapse of a 30 story cast-in-

place reinforced concrete structure. This was also a flat-

plate design structure that failed due to punching shear

on the 23rd floor and resulted in a progressive collapse.

Some of the reasons for this failure again were 1) premature

removal of shores and reshores, 2) insufficient concrete

stength, 3) no preconstruction plans of concrete casting,

formwork plans, removal of formwork schedules, or

reshoring program (Kaminetzky 66-67).

(D) Harbour Cay Condominium: March 1981

Built just 10 years after 2000 Commonwealth Ave. and 8

years after Skyline Plaza, was another cast-in-place

reinforced concrete structure that collapsed during

construction. It was determined that the most important

factor towards its failure was a design error coupled with a

construction error of the wrong size rebar and chair height.

The designer never performed any calculations to check

for punching shear, the most common form of failure in

these type of structures (Feld & Carper 18).

Figure 5: Tropicana Casino; Parking Garage Picture taken fromwww.CTLGroup.com

(E) The Tropicana Casino parking garage in AtlanticCity, N.J.: October 30,2003

The structure collapsed during construction killing another

four construction workers and and leaving more than 30

others injured. Larry Bendesky, Mongeluzzi's partner of the

Philadelphia law firm Saltz, Moongeluzzi, Barrett &

Bendesky, P.C, the lead counsel for the litigation with Paul

D'Amato of the D'Amato Law Office and a member of the

trial team, said that "the simple explanation of the cause of

the collapse is that the floors were not connected to the

walls with the required reinforcing steel. Built without the

necessary steel, it is no wonder it collapsed like a house of

cards." (pr newswire) The vertical columns left standing

and the fact that the floors were not connected implies

that this was another punching. Refer Figure 5 for the

collapse picture.

Codes & Regulations

Codes in Place

ACI, The American Concrete Institute's origins started in

1905 with its first building code published in 1910 and

changing its name to the current designation in 1913. ACI's

first design handbook came out in1939 and the first

building code titled ACI 318 came out in 1941. The

beginning volumes of ACI were less tha fifty pages with

the current code specification being nearly 470 pages of

design specifications and commentaries (ACI 318). This

clearly shows the history of ACI is closely tied to the ever

changing demands of concrete construction and

technology. The ACI sees itelf as an expanding, alert,and

informed organization prepared to stimulate imaginative

applications of concrete and better knowledge of its

properties and uses, and will take an increasingly active

part in solving problems affecting the public welfare

(History of ACI).

Lessons Learned

(A) New York Coliseum on May 9, 1955

From this failure the construction industry learned that

shoring systems should be well braced to resist lateral

loads and to consider the effect of power or motorized

buggies/carts on the formwork (Auburn University).

(B) 2000 Commonwealth Avenue: January 5, 1971

From 2000 Commonwealth Ave. the industry learned that

this type of failure is a critical failure mechanism for flat-

plate-slab concrete construction. Structural safety

depends on adequate slab thickness, proper placement

of reinforcement, and adequate concrete strength (Delatte

144).

(C) Skyline Plaza: March 2,1973

Six lessons learned from the colloapse of Skyline Plaza at

Bailey's Crossroads are listed in (Kaminetzky 67)

- the contractor should prepare formwork drawings

showing details of the formwork, shores, and reshores.

- The contractor should prepare a detailed concrete

testing program, to include cylinder testing, before

stripping forms.

Formwork Failure

www.masterbuilder.co.in • The Masterbuilder - March 2012 171

- The engineer of record should ascertain that the

contractor has all the pertinent design data (such as

live loads, superimposed dead loads, and any other

information which is unique to the project).

- Inspectors and other quality control agencies should

verify that items 1 and 2 above are being adhered to.

- Uncontrolled acceleration of formwork removal may

lead to serious consquences. 6) Top and bottom rebars

running continuously within the column periphery must

be incorporated in the design.

(D) Harbour Cay Condominium: March 1981

The Harbour Cay Condominiums presented the industry

with six more lessons learned in this type of construction

also listed in (Kaminetzky 77-78). This tradgedy happened

only eight years after the Skyline Plaza tradgedy and yet

some of the same lessons are listed again, they are

- A punching shear strength check s critical to the

success of a flat-slab, since punching shear is the most

common failure mode of concrete slabs.

- Minimum depth of a flat-slab must be checked to

assure proper strength and acceptable deflections.

- Reinforcing bars, both at the top and at the bottom of

the slab, should be placed directly within the column

periphery to avoid progressive collapse. This can easily

be accomplished routinely in all flat-slab jobs at no

additional cost at all.

- Proper construction control must be provided in the

field, including design of formwork by professionals.

This must include shoring and reshoring plans,

procedures, and schedules, with data on minimum

allowable stripping strength of the concrete.

- When there are failure warning signs of any type on a

construction site, work must stop. All aspects of the

project must be carefully evaluated by experienced

professional help. Immediate evacuation of the

structure must be considered.

- Special care must be taken during cold weather to

evaluate the actual in place strength of the concrete. It

is also a fact that the level of construction carelessness

increases in the winter months.

(E) The Tropicana Casino parking garage in Atlantic City,

N.J.: October 30,2003

The Tropicana lessons learned have not yet been

published in any documented form, but from articles such

as the one from ASQ Newsletter published in the summer

of 2004, one can reasonably determine that all of the above

lessons learned will be revisited. The article states that all

of the errors were remarkably simple engineering error.

Contractor failed to tie rebar in the frames floor beams to

the columns and shear walls in several places was only

one reason as listed in (ASQ Newswire 11-12).

Statistics

Statistics released in 1984 by the National Safety council

reported over 2200 deaths were reported for the

construction industry for that year, and 220,000 disabling

injuries, the largest total for the eight major industries

surveyed (Carper 312).

Over $1.6 billion is lost annually in the U.S. due to

construction accidents (Carper 312). Forty-nine percent

of falsework collapse happens during concrete placing

(Hadipriono & Wang 115).Untimely removal of falsework is

the second most significant event related toconcrete failure

(Hadipriono & Wang 116). Investigations prove that many

accidents causing thousands of dollars worth of damage

could have been prevented if only a few hundred dollars

had been spent on diagonal bracing for the formwork

structure (University of Washington).

Conclusions

OSHA, ASCE, and ACI have all responded to these as wellas many other accidents and issues with activities,publications and codes aimed at improving constructionsafety and the welfare of our construction workforce;

however, these organizations alone cannot be responsiblefor all construction related activities and failures.

The safety record in the construction industry can be and

must be improved in all phases. As C. Roy Vince has stated,many construction accidents are the result of ignorance,carelessness, and greed (Carper 133). The lessons learned

from above being repeated over and over again can onlypoint to the fact that this statement is precisely true. "Aslong as structures are constructed by humans, using

imperfect materials and procedures, failures are likely tocontinue. Many of these failures will occur during theprocess of construction, endangering the lives of

construction workers." (Carper 143) There is no way to breakeveryone of their bad habits but awareness has to be raised

and the consequences have to be sharply increased.

More focus has to be placed on required education of all

construction personel beyond certain levels of

responsibility, this is to include the workers themselves

who are actually assembling these structures. Better

licensure requirements, more stringent inspections, and

increased factors of safety during construction (because

it is at this time when the structure will be likely to see its

most significant loading), should also be considered to

help prevent these tragedies from reoccurring. From the

initial design phase to maintenance of the structure after

Formwork Failure

The Masterbuilder - March 2012 • www.masterbuilder.co.in172

completion everyone involved needs to pay strict attention

to all details and warning signs of impending failures. There

can be NO SHORTCUTS if we are to protect the safety and

lives of the individuals who provide us with all of the

essential structures in our lives.

Most often it is not their mistake that cost them their life

and the misery of the families who lost them too soon.

References

- American Concrete Institute. "History of ACI" <http://

www.concrete.org/members/mem_info_history.htm> (October

10, 2009)

- ACI Committee 318, (2008). ACI 318-08 "Building Code

Requirements for Structural Concrete and Commentary"

pp. 81-82

- ACI Committee 318, (1963). ACI 318-63 "Building Code

Requirements for Structural Concrete and Commentary" pp. .

- Arthur H. Nilson, David Darwin and Charles W. Dolan, (2004).

"Design of Concrete Structures" pp. 12-17

- The ASQ Newsletter. "Extracts from Engineering News Record"

OSHA Report Claims that Atlantic City Garage Contractors Failed

to Tie Rebar and Properly Shore <http://www.library.illinois.edu/

archives/e-records/ASQ%20Archives/1182001_Division_General/

DesignDiv/Design-News-Summer2004.pdf> (summer 2004),

(October 10, 2009)

- Auburn University. "Lateral Stability of Structures" New York

Coliseum <https://fp.auburn.edu/heinmic/StructuralStability/

newyork%20coliseum.htm> (2009), (Sept. 18, 2009)

- Charles D. Reese and James Vernon Eidson, (2006). "Handbook

of OSHA Construction Safety and Health" pp. 181-183

- Fabian C. Hadipriono,1 M. ASCE and Hana-Kwang Wang2,

(March/April 1986). "Analysis of Falsework Failures in Concrete

Structures" J. Constr. Engrg. Mgmt. 112(1), pp. 112-121.

- Jacob Feld and Kenneth L. Carper, ((1997) "Construction Failure"

pp. 242-274 Kaminetzky D. (1991). "Design and Construction

Failures" Lessons In Forensic Investigations pp. 67-78

- M. ASCE, (August 1987). "Structural Failures During Construction"

J. Perf. Constr. Fac., ASCE, 1(3), pp. 132-144.

- McKaig T. (1962). "Building Failures" Case Studies in Construction

and Design Norbert J. Delatte Jr., Ph.D., P.E. (2009). "Beyond

Failure" Forensic Case Studies For Civil Engineers pp. 129-155

- PR Newswire. "$101 Million Settlement in Deadly 2003 Tropicana

Parking Garage Collapse That Killed Five Workers" < http://

www.prnewswire.com/news-releases/101-million-settlement-in-

deadly-2003-tropicana-parking-garage-collapse-that-killed-five-

workers-58264282.html> (October 10, 2009)

- University of Washington. "CM 420 Course Lecture 1" Temporary

Structures <http://www.courses.washington.edu/cm420/lec1/

lec1.ppt> (Spring Quarter 2002), (Sept. 18, 2009)

- Zallen Engineering. "Collapse of Flying Formwork During Concrete

Placement" <http://www.zallenengineering.com/On-Line_Issues/

OL-8.pdf> (July 2002), (Sept. 18, 2009)

- http://failures.wikispaces.com/2000+Commonwealth+Avenue+-

+Boston

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The Masterbuilder - March 2012 • www.masterbuilder.co.in180

Fabric Formwork: Sky’s the LimitSpecial Correspondent

Picture Source: www.matsysdesign.com

Fabric formworks for reinforced concrete construction

and architecture is an emerging technology with

the capacity to transform concrete architecture and

reinforced concrete structures. The natural tension

geometries given by flexible fabric membranes provide

extraordinarily light and inexpensive formworks, some

using hundreds of times less material than conventional

formworks, and some providing zero-waste formwork

systems. The flexibility of a fabric formwork makes it

possible to produce a multitude of architectural and

structural designs from a single, reusable mold. The use of

a permeable formwork fabric produces improved surface

finishes and higher strength concrete as a result of a filtering

action that allows air bubbles and excess mix water to

bleed through the formwork membrane.

A brief history

According to the International Society of Fabric Forming,

the first practical applications for fabric formwork were

introduced in the mid-1960s for erosion control and to line

ponds, although there are several patents for 19th- and

early 20th-century fabric forms. In the 1970s, the Spanish

architect Miguel Fisac used thin plastic sheets as formwork

for textured wall panels. In the late 1980s and early 1990s,

three men, each on his own, invented a variety of

techniques for fabric-forming aboveground structures.

Kenzo Unno, a Japanese architect in Tokyo, invented a

fabric formwork system for in situ cast concrete walls. Rick

Fearn, a builder and businessman in Canada, invented a

number of fabric formwork techniques. This led him to

develop a series of foundation footing and column

products now manufactured and sold by Fab-Form

Industries in Surrey, British Columbia, Canada. He is

president of the company. Mark West - an artist,

architectural educator and builder who is now the director

of the Centre for Architectural Structures and Technology

Focus Fabric Formwork

The Masterbuilder - March 2012 • www.masterbuilder.co.in184

(CAST) at the University of anitoba's Faculty of Architecture

in Winnipeg - invented a series of techniques for

constructing fabric-formed walls, beams, columns, slabs

and panels. CAST is the first research center dedicated to

fabric formwork technology and education.

Visualizing the end result

"Fabric is so much more efficient than plywood (for forms),

but the industry is slow to change," says Rick Fearn. "It's

staggering how long it takes to get new ideas into the

marketplace." He thinks the biggest stumbling block to

fabric formwork's acceptance is that many contractors

cannot picture the end result before they start. "(Unlike

rigid formwork), it's just a loose piece of fabric. What you

get is not what you see." To help contractors visualize an

end product, Fearn has a computer program that predicts

the shapes fabric forms will produce. He's hoping that as

more contractors accept computer-generated virtual-

reality scenarios, fabric formwork will grow in use. "Fabric

is a tension membrane," Fearn says. "If you use a different

fabric, it will give you a different texture, but the shape will

be the same." Also, some fabrics aren't coated, so they let

excess water bleed out, he notes. This can make fabric-

formed concrete products stronger than those made with

traditional lumber forms. In a world where resources are

dwindling, he notes, fabric forms, like the ones he sells for

columns, just make good sense. Fast-Tubes, made from

high-strength polyethylene, come in 120-foot rolls that

easily fit behind the seat of a truck and can be cut to any

length with minimal waste. Fabric formworks are such a

green product and so efficient. They take up 1% of the

space cardboard does and they are 1/10 the weight. Also

unlike cardboard, there is no waste to be hauled to the

landfill after the column forms are trimmed to size or when

the forms are stripped. "Fast-Tubes can be put under a

slab after they are stripped. They act as a moisture

protector." Besides allowing contractors to form sturdy

columns of varying lengths - Lawton used Fast-Tubes to

make 29-foot columns for a treehouse he built in Vermont

- Fearn's fabric-formed columns can be easily decorated

by simply tying ropes or putting bands around the forms

while the concrete is still wet.

Flexible fabric vs. hardened forms

The primary differences between both the formwork is ease

of errection.While rigid formwork needs more time to errect.

Also lot of staging and design work is needed for rigid

formwork which Is not required for flexible formwork. One

more striking advantage with flexible formwork is that any

shape can be designed and made using fabric formwork.

The same fact is supported by the all the Figures in the

entire storey. Use of fabric formwork saves lot of manpower

cost and saves lot of energy for preparation of the rigid

formwork. Morover where space is a concern, stocking and

keeping of rigid formwork will be a major concern. Since

most of the fabric formworks are made for one time use

only, they can be kept after concreting which will facilitate

in curing of the concrete. If any kind of aesthetic treatment

is required to be given in the structure fabric formwork is

the only option as it is very tough with rigid formwok and in

some cases it is impossible also.

End product using fabric formwork

A flexible fabric mold awakens concrete to its original wet,

plastic nature by naturally producing concrete members

with complex sensual curvatures. The sculptural and

architectural freedom offered by this method of

construction is matched by new possibilities for efficiently

curved structures. Research at CAST has produced simple

methods for forming beautiful and efficient beams, trusses,

panels, vaults, slabs, and columns.

The Centre for Architectural Structures and Technology

(C.A.S.T.) is fundamentally interested in finding simple ways

to reduce the amount of material consumed in

construction, while at the same time, making these

constructions more beautiful. C.A.S.T. is also committed

to making these methods accessible to as many people

as possible.

The end product is divided here into two parts viz. (A)

Architectural application, (B) Strutural Application.

(A)Architectural application

Fabric formwork can be used to give tough architectural

shapes to the structural member very easily. Figure 1 shows

typical surface of a fabric cast panel and Figure 2 shows a

branched column made with fabric formwork.

Fabric forms can be used to produce complex concrete

Figure 1:Surface detail of a fabric-cast

panel

Figure 2:Branching column formed in

a geotextile form-liner

Focus Fabric Formwork

www.masterbuilder.co.in • The Masterbuilder - March 2012 185

shapes that would be extremely costly or nearly impossible

to create with traditional rigid formwork. Anne-Mette

Manelius, an architect and doctoral student in

Copenhagen, Denmark, made this chair as part of her thesis

work on fabric formwork for concrete. She wanted the soft-

looking chair to fool sitters (Figure 3).

Figure 3: Chair produced with fabric formwork

Green, clean, relatively inexpensive and incredibly

practical, fabric formwork can be used with concrete to

produce structurally efficient and architecturally

compelling components in all shapes and sizes, ranging

from footings, columns and beams to walls, sinks, furniture

and an array of accessories

"It's allowed us to create masonry architecture using very

simple skills," says Sandy Lawton, owner of ArroDesign, a

design/build construction company in Waitsfield. With a

background in carpentry, Lawton says, he found rigid

formwork complicated and labor intensive. "Fabric

formwork has given us the freedom to do complicated

structural work in a very different way that's not complicated

at all. That's the bigger advantage. There's a lot more

flexibility with this system." Fabric formwork also has

benefits from a sustainable viewpoint, Lawton says. "Fabric

formwork basically reduces the amount of everything

required to construct something - placement, storage and

even building the forms. There are huge savings every

step of the way."

Also, he points out, depending on the type of fabric you

use for the formwork, you can get a really nice finish. "You

don't have to go behind and refinish." Instead of using rigid

forms made from lumber, plywood, cardboard, steel or

aluminum, fabric forms use a flexible textile membrane to

form concrete in place. Wet concrete is poured into a

tensile membrane, which produces efficient structural

curves and extraordinary surface finishes. The shape is

determined by how the material is restricted. This can

happen in a number of ways, from creatively using form

ties to make "buttonholes" to placing a brick under a fabric

form to make a relief.

Kenzo Unno, a Japanese architect in Tokyo, devised

methods to cast beautifully shaped walls with thin, flexible

textile sheets. These methods are collectively called "Unno

Reinforced Concrete (Shown in Figure 4)."

Figure 4: Walls casted with Fabric formwork

Figure 5 shows a thin GFRC stingray sink created by

students of Brandon Gore of Gore Design Co.

Figure 5: The 1-inch-thick GFRC Stingray Sink

(B) Strutural Application

Here the use of fabric formworks in various structural

members is shown. Figure 6 and Figure 7 shows casting of

a isolated footing and slab footing using fabric formwork.

The fabric comes in rolls of certain widths and it is simply

cut on site to suit the size needed. Apart from normal tools

for cutting and fixing the braces and perimeter frame, the

only extra items are a Stanley knife and a staple gun. The

fabric is cut neatly with the knife and staple to the timber.

There is a very simple method of cutting the fabric at the

Focus Fabric Formwork

The Masterbuilder - March 2012 • www.masterbuilder.co.in186

corners, and when it is simply stapled in position that it, in

effect, holds the corners together just as strongly as normal

methods. Before the pour, a sheet of standard plastic

vapour barrier is laid on top of the fabric to stop the footing

absorbing moisture if it is required.

It can be noticed from Figure 6 and Figure 7, that no

movement at the top and a slight bulging at the bottom is

there in the freshly concreted isolated footing.

Figure 6: Fabric formwork used for casting isolated footing in a construction site

Figure 7: Fabric formwork used for casting isolated slab footing in a construction

site

A system for forming round concrete columns usingfabric formwork

Figure 8 and Figure 9 shows round various stages of casting

of round concrete column. It can be very easily seen the

end product finish in Figure 9 and also the ease of casting

from the other Figures ( from Figure 8 (a) to (c) ).

Figure 8: (a) Column ready to be poured, (b) Column pouring in progess, (c)

column pouring completed

This method of casting column is beneficial because offollowing reasons

- The fabric come ready made up in tube sections to

form the desired diameter of the column.

- The fabric tube is

simply cut to length

with a Stanley knife.

- In the manufacture,

tabs are made

vertically along a

center line.

- The loose sleeve of

fabric is fitted over

the rebar.

- The tabs are then

nailed to a straight

length of 4" x 2"

timber.

- The 4" x 2" timber is

then positioned, and

braced to hold it

plumb.

- For the first foot or so a guy hold the base of the tube in

the correction position with a boot on either side.

- During the pour, it is possible for a guy to feel and guide

the rebar cage, to make sure that it is in the correct

position.

- Unlike conventional formwork, because this is a throw

away, one off system there is never any reason for undue

haste to strip the formwork.

- Therefore the fabric can be left in position to act as a

perfect curing membrane

However if there is a doubt that whether this system can

work for higher columns, then Figure 10 shows the 20ft tall

column ready to be poured in one hit. The project for a

church in Nicaragua in Central America.

The concrete was mixed by hand on the site and lifted up

by hand. In itself, this was probably a good thing as the

slowness of the pour would mean that the concrete at the

bottom would be stiffening up nicely as the height

increased, reducing the theoretical hydrostatic pressure.

Conclusion

It is very essential to use fabric forms and rebar in an area

where wood is scantily available. Fabric is a very forgiving

material.However one should remember that fabric

formwork is not as uniform as standard formwork. Engineers

had to create some structure to give the appearance of

what they wanted, but in the same breath it gives us a lot

of design freedom. It is really an exciting medium. As for

fabric formwork's limitations, "It's wide open. No one have

tested its limits yet.

Figure 9: Fabric Formwork - Stripped

column

Focus Fabric Formwork

The Masterbuilder - March 2012 • www.masterbuilder.co.in188

The Influence of ConstructionChemicals on Tunnel Durability

Willie Kay

Managing Director of WAK Consultants Pte Ltd /

WAK Technologies Pte Ltd , MC Bauchemie Muller GmbH & Co

Injection Systems

Injection systems in tunnels and underground

constructions are now often considered in the planning

and design stage. They can be a means to simplify

construction, enhance safety, and control potential leaks

or many other applications.

The reason for this change is due to advances in materials

in terms of set times in resin to particle sizes in cement

suspension. Equipment technology in mixing has

improved and pumps are now capable of handling just

about any material even at tropical ambient temperatures

around 35oC.

Engineers and clients need documentary proof of materials

consumed and at what pressure to ensure correct grouting

and this equipment is now readily available.

Injection resins based on polyurethane have been around

for more than thirty years. In general these were a single

component with an accelerator and reacted with water.

There were and are many manufacturers with varying

quality and properties. Figure 1 show a high quality water

reactive resin foamed to approximately 35 times its original

volume.

Newer technologies have two part polyurethane bases and

have properties from highly elastic to highly rigid elastic.

New technologies in gels allow swelling of up to 30% with

This paper looks at the role construction chemicals in the Tunnelling Industry. Advances in both Tunnel boring machine technology(TBM) and ground conditions have accelerated the need and growth of specialised material.

Specialised additive and admixtures have revolutionised the durability and production of precast segments. The advancement ofAlkali free shotcrete accelerators has enabled much safer working conditions. The uses of supplementary cementitious additiveshave allowed high build high strength concrete tunnels by robotic spraying. This paper however will look at the role of injectionresins in tunnels with case histories.

Figure 1

negligible pressure on the substrate. Many of these

products have both CE and REAch compliance. Table 1

shows some typical properties of a gel material.

Thixotropic Gels

Swell up to 30%

Excellent adhesion to most substrates

Ductile up to 300% (see figure 2)

High tear resistance

Variable set times from less than 10 second to minutes

Table 1. Typical Properties

Certification

REACh is the uniform chemical legislation with a strong

Tunnelling Construction Chemicals

www.masterbuilder.co.in • The Masterbuilder - March 2012 189

focus on the protection of human health. Companies

registered can be checked on the internet by contacting

Helsinki. All the injection products we have been

discussing all have REACh certification.

Polyurethane Injection Resins (Elastomer)

Polyurethane and Gel Technology have made major

advances due to understanding the critical nature of mix

ratio, mixing technology and advancement of twin line

pump technology.

The term polyurethane is very generic and does not reflect

the technical changes that have taken place over the last

twenty years. The term elastomer is adopted to describe

the material as it technically describes the material

function. To many people, polyurethane is a brown liquid

that foams and stops leak. This statement is simplistic, as

it does not reveal some of the key properties of a water

reactive resin. In order to fill a void and stop water ingress,

of the following properties are needed.

- Expansion of the material in contact with water

- A stable dense foam

- Non Shrinkage after foaming

- Closed cell structure to prevent water permeation

To achieve all these properties with a single component

water reactive resin is impossible under all conditions. The

foam density will depend on the amount of water and

reaction time. The expansion will vary with the specific

environmental conditions at each project. Due to these

constraints, Europe and specifically Germany have

adopted a two-stage process of injection to ensure

Figure 2. - Example of deformation

permanent leak sealing. In applications of high water inflow

a water reactive open cell foaming resin is first injected as

initial seal. This is ten followed by a second injection using

a two part elastomer resin, which will penetrate the open

cell and give a permanent watertight seal. This method is

adopted from the German Training Council and German

Concrete and Construction Association Deutscher Beton

UndBautechnik Verein e.V. (DBV) for injection of water leaks.

Two part elastomer resins have customisable stiffness

properties and can be engineered from elastic and flexible,

to strong and semi-rigid.

The ability to adjust the setting time is of great importance

to ensure complete penetration of the crack as void

viscosity is another critical factor and this will be discussed

later in his paper. Table 2 shows some basic properties

achievable in the market today.

Std

30 secs

60

Differing Properties of Elastomer Resin

Pot Life

Elongation

Strength (N/mm2)

Viscosity (mPas)

Long Life

45 mins

60

UW

43 secs

80

NV

35 mins

100%

Compressible

Rigid force transmitting

Table 2

Hydro-Structure Resins

The name hydro-structure is used to dissociate these resins

from the toxic acryl gels, which has caused major

environmental problems in Europe. All the resins discussed

and described in this paper comply with the highest

standards of non-toxicity in contact with potable or

drinking water. These resins cross-link and depend on

water migration for long-term performance. The latest

generation has "thixo" or skinning effect which makes them

an ideal solution for buried leaking joints in car parks,

stations and other underground structures. The ability to

be pumped into very specific locations and then set, gives

an ideal method of repairing joints and damaged

membranes. The viscosity of these materials is very low

thus making penetration into tiny voids and fissures very

quick, which is impossible to achieve with a high viscosity

resin. Table 3 lists some key properties.

Solidification

-

Sealing flexible

++

Sealing swelling

+++Hydro-Structure

Resins

+ dry ++ wet +++ water pressure

Table 3

These properties have simplified the repair of leaky

segment joints. "Steps" often occurs when building tunnel

rings in precast concrete and this can lead to failure of the

gasket with subsequent leakage. The hydro structure resins

Tunnelling Construction Chemicals

The Masterbuilder - March 2012 • www.masterbuilder.co.in190

with the thixo agents will be able to rebuild a membrane

behind the joint and effectively waterproof the ring. Skill is

needed in packer selection, gel time of the resin and pump

pressure. The use of Twin Line pumps with the correct mix

head technology is essential.

Equipment

Advances in equipment technology in the last twenty years

have enable resin injection to provide a long-term durable

repair where previously demolition and rebuilt would have

been the only answer. Twin Line pumps with varying

pressure and volume outputs allow correctly trained

applicators to repair almost all leak problems in tunnels.

The reason why Twin Line pumps are so important and

especially in tropical climates are as shown in Figure 3.

Figure 3. - Pump pressure versus injection duration

From this table one can see that the resin penetration is

dependent on three factors; viscosity, time and pressure.

Too high a pressure often causes more damage to the

structure by re-cracking or worse. Time is something we

cannot keep extending as the viscosity is increasing and

the injection costs keep rising. Imagine a situation where

each injection port requires a 15 minutes injection. Spacing

of the injection ports could be at 250 mm centre so each

linear metre of crack would take one hour to inject. The

duration is also dependant on the thickness of the concrete

structure.

The answer is the Twin Line equipment where the resin is

mixed only at the point of discharge and this enables the

lowest possible injection viscosity at the packer. This allows

filling of the crack in the shortest possible time and to the

finer parts of the cracks.

Twin Line pumps are only part of much bigger technical

break through as both mix head technology and online

monitoring have become available. Resins which have

different viscosities or mix ratios require different degrees

of mixing. Some resins can be mixed in 60 seconds with a

shear mixer while others require 3 minutes for complete

mixing. Each resin type has a specific mixer length and

this is critical if the mixed resin is to achieve the designed

property.

On many projects the Engineer would like to predetermine

the pressures at which injection is taking place, others

would like to restrict the volume of resin pumped into each

packer. Other sites require a list of packers used and record

of the volume, pressure and duration when the resin was

pumped. All this information can be made available by

using the German made control device.

This equipment pictured below Figure 4 comprehensively

monitors the injection process. It ensures that the machine

is calibrated and should the mixing ratio be out of margin

it will stop and sound an alarm. Given that the machine is

in good working order it will start pumping and record

pressure volume and time. At the end of a shift the tagged

packers are photographed and the information down

loaded. This is then transferred to a computer and a report

is generated automatically. This can be co-related to the

site by grid reference and crack mapping showing an as

built and as repaired document.

The equipment can also be used with water to carry out

void surveys in structures with very heavy reinforcement

when other techniques may not be suitable.

Figure 4

Applicators

With the sophistication of materials and equipment

technology, a new approach to applicator training has

evolved. Companies licensed to use the materials and

equipments are required to have a government backed

independent certification. This requires attending a two

weeks residential course in Europe taking and passing an

exam supervised by impartial and independent bodies.

Manufacturers are not allowed to give this independent

overview in a training course. The course is operated by

the BZB Akemie and the course topics include Basics of

concrete and steel, repair of concrete construction parts,

Tunnelling Construction Chemicals

www.masterbuilder.co.in • The Masterbuilder - March 2012 191

polymer and spray polymer repair mortars, and injection

of cracks cavities, joint repair, surface protection systems

and strengthening using carbon fibre laminates. An

examination occurs at the end of the course and if

successful a certification is given. After which, these

licensed operators then attend specific product and

machine training to ensure the total system Man, Materials

and Machinery works.

Case Histories

Brisbane Road Tunnel - Case Histories

North South Bypass Tunnel - NSBT

The SMART Project provides a storm water diversion

scheme including floodwater storage and a 10 km, 11.8 m

diameter bypass tunnel, sufficient to save the city from

flooding in the foreseeable future. With no major flood event

most of the year the tunnel a dual use was engineered,

with double road decks built into the central three kilometre

section, relieving traffic congestion by providing 2 x 2 traffic

lanes for cars connecting the city centre to the southern

gateway, the KL - Seremban Highway.

Suspended slab / Segment detail Application

TBM Segment Installer

The flood water is diverted at the confluence of the Klang

and Ampang rivers into a Holding Pond. From there the

water passes through the tunnel into the Taman Desa

Attenuation Pond and via a box culvert discharges into

the Kerayong River.

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MC was involved in supplying admixtures for both the

backfill grout and the road deck concrete. We were also

involved with all grouting to stop water ingress from within

the tunnel.

Figure 5 and Figure 6 show two specialised injection

systems. Figure 5 shows how we repaired damaged

gaskets using specially developed packers and Figure 6

shows a specially developed packer for resealing leaking

grout sockets.

Area of Application

Application Preparation

Shaft & Joint Sealed

Full Depth Penetration

SMART Tunnel Malaysia - Case Histories

Summary

As tunnel technology advances new materials have been

developed to keep up with these advances and no doubt

will continue in the future.

Figure 5

Figure 6

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Tunnel Repair and RehabilitationUsing ShotcreteSpecial Correspondent

The art of rehabilitation of tunnels has flourished and

developed significantly over the last couple of

decades. Several hundred railroad, highway, and

conveyance tunnels have been successfully rehabilitated,

converted, and/or enlarged. Much of this development

can be attributed to the successful use of steel fiber

reinforced shotcrete. One of the major attributes of

shotcrete is excellent bond to the substrate, usually

superior to the bond achieved with cast-in-place concrete.

This has made shotcrete particularly well-suited for repair

and or rehabilitation work of vertical and overhead

surfaces. The flexibility and adaptable nature of steel-fiber

microsilica shotcrete is ideal for rehabilitation of tunnels.

Many developments in shotcrete technology during the

1980s have enhanced shotcreting capabilities. These

include advances in shotcreting materials technology and

improved methods for batching, mixing, supply, and

application. The developments have stemmed largely

from the desire of engineers and contractors to improve

the quality and durability of inplace shotcrete, increase

View of a shotcrete operation. Look closely and you can see the stream of wet concrete being

blasted onto the rock surface

Pic courtesy: http://thelaunchbox.blogspot.in

Tunnel Engineering Repair & Rehabilitation

www.masterbuilder.co.in • The Masterbuilder - March 2012 205

shotcreting productivity and economy, and expand the

range of shotcrete applications. It is because of these

developments in shotcrete technology, enlargement and

rehabilitation of tunnels without fully taking the tunnel out

of service is not only technically but also economically

feasible considering the cost of other alternatives

Enlargement was usually accomplished by raising the

crown but some have been enlarged by lowering the invert,

which is much more difficult and time-consuming.

Harvey Parker & Associates, Inc., in Bellevue, WA have

rehabilitated several tunnels that were over a century old,

allowing these tunnels to begin their second century of

service. This long life represents a huge life-cycle benefit

for the tunnel owner, and this cost advantage can be

maintained by conducting an occasional rehabilitation

from time to time. The increase in the number and type of

tunnels being rehabilitated over the last few decades was

largely made possible by the continued development of

ground support methods using rock bolts and steel fiber-

reinforced shotcrete. Repair and Rehabilitation is done for

several reasons. Sometimes rehabilitation work is done

simply to extend life or to improve future performance,

such as reduction of maintenance or to improve safety.

Generally, highway tunnels, such as the one illustrated in

Fig. 1, fall into this category. Other reasons for rehabilitation

include: 1) enlarge the tunnel to increase clearances or

capacity or 2) change the type of tunnel from one use to

another. On the other hand tunnel gets damaged because

of following reasons needing urgent repair to bring back

the traffic into operation through it again, these are: 1)

Damage due to lack of maintenance, 2) Damage due to

fire, 3) Damage due to natural calamity such as earth quake,

4) Damages due to unexpected operational problems.

Examples of damage occurring due to reason 4 are shown

in Fig. 2.

The introduction of double-stack container cars and other

special or extra-large cars (for example, tri-level auto racks)

created a need for enlargement of most of the tunnels in

the United States and Canada. This is an ideal example of

Figure 1: Shotcreting for rehabilitation of highway tunnel.

tunnel rehabilitation to satisfy a need for larger tunnels

and better service rather than just to extend their lives.

Many of the railroad tunnels in the west and several on the

east coast have been enlarged by increasing clearance in

the crown. Clearances were improved mostly by crown

mining, which consisted of either cutting a notch in the

existing lining or rock walls, as shown in Fig. 3, or by

Fig. 2. Some failed tunnels at Jiulongkou Coal Mine.

Figure 3: Tunnel clearance notch in a railroad tunnel.

complete or substantial removal of the brick or concrete

lining.

Importance of fast recovery of the Tunnel foruninterrupted service:-

Tunnels are vital to keeping our transportation systems

going, and interruptions of service are rarely permitted.

Rehabilitation that requires invert work usually shuts the

entire tunnel down. It is better to concentrate tunnel

rehabilitation on the crown and sidewalls if at all possible.

Typically, there are no alternate routes so tunnel work must

be done with the least disruptive effect on paying traffic.

This is done by either temporarily shutting down one lane

or one track in multiple lane/track tunnels or by managing

traffic to permit work windows that might last from 1 to 8 h.

Yes, work can be accomplished in windows of 1 or 2 h; it is

not very efficient but sometimes that is all the time one can

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get in any one work window. Rehabilitation work while

keeping the tunnel in service requires enormous planning,

coordination, and selection of proper construction methods.

Advances in Shotcrete Materials for Tunnel Repair/Rehabilitation job

Before the 1980s, most shotcrete used for repair and

rehabilitation in North America was made of conventional

portland cement and sand mixtures applied by the drymix

shotcrete process. Some polymer- modified shotcrete was

used for remedial work in aggressive exposure conditions.

There also was limited use of wet-mix shotcrete, primarily

for large- volume projects. Today, both dry- and wet- mix

shotcretes often contain supplementary cementing

materials, such as fly ash and silica fume, as additions or

partial cement replacements. These materials improve

shotcrete workability and performance. In the early 1970s,

a major advance in shotcrete technology was the

development of steel-fiber- reinforced shotcrete (SFRS).

SFRS is particularly useful for remedial applications in

aggressive chemical or marine environments because it

resists corrosion better than shotcrete with conventional

steel reinforcement. As long as the shotcrete matrix retains

its inherent alkalinity and remains uncracked, deterioration

of SFRS is unlikely. Corrosion of the discreet steel fibers

occurs only to the depth of surface carbonation in the

shotcrete. If corrosion of the surface fibers is aesthetically

objectionable, a flash coat of plain, unreinforced shotcrete

can be applied. SFRS has another advantage: It's more

user friendly and less prone to problems caused by

inadequate workmanship. For example, it eliminates the

shadowing and voiding problems sometimes encountered

in conventionally reinforced shotcrete repairs

(Refer Fig. 4).

Fig. 4. An extreme example of shotcrete improperly applied

to mesh reinforcement shows build-up of shotcrete on the

face of the mesh and shadowing and voids behind.

Steel-fiber reinforcement addition rates vary from about

60 to 140 pounds per cubic yard, depending on job

requirements and fiber type and size. Generally, higher

fiber addition rates are used in structures subject to severe

stresses and strains imposed by:

- Impact or explosive forces.

- Heavy, repeated, dynamic cyclic loading.

- Large exposed surfaces, which are more susceptible

to shrinkage cracking

Advantages of Steel Fiber Reinforced Shotcrete

Steel fiber-reinforced shotcrete offers the flexibility needed

to adapt to rapidly changing ground conditions and

uncertain work window schedules. In some projects, due

to the remote location, a concrete batching plant is not

available. Shotcrete dry mix including steel fibers and

microsilica can be purchased in prepackaged 1 y3 (0.75

m3) bags (sling bags) and conveniently stored at the site

until needed (refer to Fig. 5,6,7). Usually the dry mix is

batched at a centrally located plant where the quality of

the shotcrete mixture can be controlled before shipping

to the site. Shotcrete from sling bags can be placed by

the dry or wet method. When placing steel fiber-reinforced

shotcrete in tunnels, costly steel or wood arch forms, and

even rebar or mesh, are not required. Time is not wasted

while erecting, curing, and removing forms or hassling with

mesh. Shotcrete will conform to the rock surface and

smooth out the irregularities caused by blasting. In cases

where the tunnel rock is locally unstable, the design ground

support can be increased to carry the unbalanced load.

Additional shotcrete and rock bolts are placed as

necessary to stop movements as documented by

monitoring. Shotcrete can be finished with a trowel to a

smooth surface equivalent to a form finish. In a pedestrian

tunnel, shotcrete was placed in the steel reinforced arch

of the horseshoe-shaped tunnel and elegantly finished to

a smooth surface. In tunnel sidewalls, the presence of steel

Fig. 5: Shipment of prepackaged Shotcrete in 2205 lb (1 metric tonne) bags to an

underground mine

Tunnel Engineering Repair & Rehabilitation

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Fig. 6: Shotcrete storage area at Falconbridge Raglan property

Fig. 7: Sling bags of shotcrete mixture and work train at a railroad siding

fibers on the surface could cause scratches on the arms of

pedestrians. In these situations, the last 2 in. (5 cm) of

shotcrete are placed without steel fibers. Typical shotcrete

specifications for mixture proportioning indicate that each

cubic yard contains a minimum of seven and a half sacks

of cement (420 kg/m3), 80 to 100 lb (50 to 60 kg/m3) of

steel fiber, 80 lb (50 kg/m3) of microsilica, and a coarse

aggregate/total aggregate ratio of 0.4. The compressive

strength of these mixtures exceeds 5000 psi (34.5 MPa) in

28 days. The fiber content can be adjusted higher or lower

as necessary to accommodate the ground conditions.

How Rehabilitation is done Keeping Tunnel in Service

Rehabilitation work while keeping the tunnel in service

requires enormous planning, coordination, and selection

of proper construction methods. The flexibility of shotcrete,

Fig.8. Typical Railroad Work Train (Schematic Diagram)

Figure 9: Railroad tunnel clearance excavation: single to double track

especially with volumetric mixing, is extremely valuable to

tunnel rehabilitation. Usually, all work is done from work

platforms designed specifically to make all the work

(including handling muck and rebound) done as efficiently

as possible. A schematic of a special work train that is

used for railroad tunnel rehabilitation is shown in Fig. 8.

Examples of Tunnel Rehabiltation Using Shotcrete

- A railroad tunnel in the eastern United States was

enlarged from a single-track tunnel to a twin-track

tunnel. Originally lined with brick, the tunnel was taken

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out of service and enlarged to obtain the double-track

clearance. The new liner consisted of steel fiber-

reinforced shotcrete and rock bolts. Shotcrete

thickness varied from 4 to 12 in. (10 to 30 cm). Typical

rock bolt lengths were 12 ft (3.7 m) but in places ranged

up to 18 ft (5.5 m) long. Figure 9 shows the excavation

process and the shotcrete liner. The tunnel

encountered open abandoned coal mine works, and

the flexibility of utilizing shotcrete and tensioned rock

bolts was invaluable in advancing the work through

difficult ground.

- A highway tunnel on the west coast was rehabilitated

because of the limited clearance and continued

deterioration of the timber lining. The liner consisting

of timber sets and lagging was replaced with 4 to 5 in.

(10 to 13 cm) of steel fiber reinforced shotcrete in the

arch and 2 to 4 in. (5 to 10 cm) of concrete on the

sidewalls. Rock dowels anchored with epoxy resin

cartridges were installed after an initial layer of shotcrete

was installed.

- Repair of a deep-mine permanent access tunnel using

bolt, mesh and shotcrete Jiulongkou Coal Mine, China).

Shotcrete prevents the failed rock mass from falling

and further weathering. The total thickness of shotcrete

applied was 120 mm on average and was sprayed as

three layers. The first and second layers together were

70 mm in thickness. This allows the dilatancy

deformation to be released. Sometimes there were local

failures in the first and second layers. The final layer

was 50 mm in thickness and was sprayed after the

surrounding rock mass deformation became stable.

Steel mesh was used together with shotcrete to

increase the tensile and bending strengths of the

shotcrete. Steel wire with a diameter of 6.5 mm was

selected to form a 125x125-mm2 mesh. Application of

rock bolt, steel mesh and shotcrete to repair seriously

deformed tunnels of the Jiulongkou deep coal mine

shows that the support approach and techniques

based on the loosening zone concept were very much

successful.

- Restoration of a Tunnel Damaged by Noto Offshore

Earthquake in coast of Suzu city in Japan in 1993 was

carried out using steel-fiber-reinforced shotcrete

(SFRS). Spray of steel-fiber-reinforced shotcrete was

adopted because it was considered to increase

bending tensile strength and ductility under uncertain

additional loads from the ground loosened under the

influence of the earthquake. The SFRS design thickness

was 150 mm and mean extra thickness provided was

50 mm. The restoration procedure is shown in Fig. 10.

Conclusion

In the 80 years since the shotcrete process was developed,

shotcrete has played a valuable role in repair and

rehabilitation projects. One of its major attributes is

excellent bond to the substrate, usually superior to the

bond achieved with cast-in-place concrete. This has made

shotcrete particularly well-suited for repair of vertical and

overhead surfaces. The use of steel fiber-reinforced

shotcrete made the rehabilitation of railroad and highway

tunnels practical and economically viable. The strength

and durability of steel fiber microsilica shotcrete in

combination with tensioned or untensioned anchor bolts

can handle almost any type of tunnel ground loading.

Shotcrete can be installed utilizing the wet or dry methods

and can be installed to sculpt any tunnel shape without

the use of costly forms or the need for rebars or mesh.

However advanced research is still going with with other

varieties of shotcretes with polypropylene fibers and other

polymers.

Reference

- H.W. Song, S.M. Lu, Tunnelling and Underground Space

Technology 16 (2001), pg. 235-240.

- M. Kunita, R. Takemata and Y. Lai, Tunneling and Underground

Space Technology, Vol. 9, No. 4, pp. 439-448, 1994.

- The are of Tunnel rehabilitation with shotcrete, Harvey Parker

et al.

Fig. 10. Restoration Procedure

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Durable Concrete forTunneling Applications

Eugenkleen

MC - Bauchemie Mueller GmbH and Co. KG

Over the past decade, the use of concrete

admixtures, especially plasticizers and

superplasticizers, is showing upward trend in

India. The advent of concrete pumps and transit mixers

has also contributed to this, as the use of superplasticizers

enables trouble-free pumping operations and minimizes

pipe blockages. With the advent of Major Metro Projects

across India, durability of concrete used especially for

tunneling segments is of prime importance. The earlier

attitude of taking recourse to the use of admixtures only

after facing problems is changing fast, and now, in most

tuneling projects, high performing admixtures are already

included in the specifications and the mix is designed to

achieve the necessary properties.

The concrete for tunnel segments necessitates the

concrete to have the following properties:

- Compressive strength

- Workability

- Surface Finish

- Durability

As part of the durability requirements, concrete is or should

be generally tested for the following properties:

- Chloride migration

- Sulfate resistance

- Water absorption

- Acid resistance

- Porosity

- Freeze Thaw Resistance

This can be achieved using the latest technologies

available for concrete. Concrete is now no longer a material

consisting of cement, aggregates, water and admixtures

but it is an engineered material with several new

constituents like PFA, GGBSF, Microsilica, Metakaolin,

Colloidal Silica and several other Binders, Fillers and

Pozzolanic materials. The concrete today can take care of

any specific requirements under most exposure

conditions.

The mix designs are getting relatively complex on account

of interaction of several materials and mix design calls for

expertise in concrete technology and materials. High

Performance Concretes will have to be adopted for

tunneling segments, considering special properties as well

as low cost maintenance strategies.

What type of Concrete do we use?

Concrete used in tunneling applications need the following

outstanding properties viz. Compressive Strength, High

Workability, Enhanced Resistances to Chemical or

Mechanical Stresses, Lower Permeability, Durability

etc.This will necessitate the useof High Performance

Concrete. SomeHPC types which will hold the key for

tunneling applications, can be classified into:

- Self Compacting Concrete / High workability concrete

- Concretes resistant against aggressive media

1. Self-Compacting Concrete (SCC)

Self-Compacting or Consolidating Concrete (SCC) as the

name signifies should be able to compact itself by its self-

weight under gravity without any additional vibrations or

compaction. Self Compacting Concrete should be able

to assume any complicated formwork shapes without

cavities and entrapment of air. The reinforcement should

be effectively covered and the aggregates should be fully

soaked in the concrete matrix. In addition, the concrete

should be self-leveling type and self-defoaming without

any external compaction. Figure 1shows SCC.

The formulation of Self Compacting Concrete has the latest

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concrete technology and it requires in-depth knowledge

of materials and meticulous testing procedures before the

concrete is designated as Self Compacting Concrete

(SCC).Self Compacting Concrete has the following special

advantages.

- Saving of costs on machinery, energy and personnel

for vibrating the concrete

- Considerable improvements to exposed surfaces (Fair

Faced Concrete), less efflorescence.

- Marked improvements in durability on account of better

compaction

- Extremely suitable for slim and complicated moulds

- Covers reinforcement effectively.

- Better adhesion between cement binder and

aggregates.

- Reduction in demoulding time

- Advantage with respect to sound pollution

Figure 1: Flow of Self Compacting Concrete around reinforcement

Therefore while calculating the costing and economics of

Self Compacting Concrete all the above mentioned

advantages should be converted to cost parameters. This

kind of concrete can give advantage of good Compressive

Strength, workability and finish to the tunnel segments

and may prove suitable.

2. Durable Concrete resistant against aggressivemedia

One major application of HPC is to increase the durability

of concrete where aggressive underground conditions are

anticipated. This can be achieved physically by resorting

to very dense aggregate packing. The packing curve is

shown in Figures 2a and 2b. This is practically possible by

selecting a very smooth sieve line from largest aggregate

to the smallest grain of Mineral Additives like Microsilica

or New Generation Aluminosilicate slurries. Chemically,

cement by itself is not acid resistant. The acid resistant

binder is formed by combination of cement, microsilica /

aluminosilicate and flyash.To control permeability very low

water cement ratio has to be adopted. So as to provide

the essential concrete properties a high-performance PCE

(polycarboxylate ether) needs to be incorporated in the

mix. By adjusting the particle size distribution on a micro

scale the permeability of the concrete is reduced which

minimizes the penetration of aggressive substances.

Depending on the degree of dispersion these material

particles more or less completely fill the spaces between

the cement particles. During hydration the pozzolanic silica

reacts with the free calcium hydroxide to form calcium

silicate hydrates. This gives a denser concrete structure.

Figure 2a: Densest packing grading curve

The main materials, which can help change normal

concrete to durable aggressive media resistant concrete,

are:

- New Generation PCE Based Admixtures

- Condensed Silica Fume or Microsilica Slurry or

- Latest Generation Aluminosilicates

a. PCE Based Admixtures: Most of the new generation

superplasticizers are from the Acrylic Polymer (AP) family.

Polycarboxylate is a common term for the substances that

are specifically used as Polyacrylate or Polycarboxylate

ethers (PCE). The PCE based Super Plasticizers are by far

superior to the conventional ones with respect to initial

slump as well as slump retention with time. The efficient

working of these plasticizers is due to the new type of

Figure 2b: Pictorial representation of Densest packing of aggregates in Concrete

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molecule designs. PCE based superplasticizers produce

excellent properties when used with cementitious

materials. The disadvantages associated with longer

setting times of conventional superplasticizers is offset by

PCE based super plasticizer and therefore its use in

concrete can also attain high early strengths.Figure 3

shows the structure of PCE molecule and its working

mechanism - steric hindrance. The development of highly

effective superplasticizers with long and consistent

duration of action is therefore an important precondition

for the production durable concrete, due to low water

contents and high early strength requirements.

Figure 3: Structure of PCE Molecule and its Mechanism of action

Concrete additives based on PCE offer advantages like:

- Significant reduction of the water demand of the mix

- Little loss of consistency

- Short setting times

- High early strengths

- Low tendency to segregation

The advantages of these New Generation polymers are

very clear, not only in terms of performance but also in

terms of the dosages used for similar conditions and this

factor balances the disadvantages in economy, as New

Generation Superplasticizers are relatively expensive per

unit price.Figure 4shows workability comparisons of MSF/

SNF against PCE. Figure 5 shows comparative

development of compressive strengths and the dosages

required are very low.

b. Condensed Silica Fume / Microsilica:The term

"Microsilica" is adopted to characterize the silica fume,

which is used for the production of concrete. Microsilica or

Condensed Silica Fume (CSF) is a by-product resulting

from reduction of high purity quartz with coal in the Electric

Figure 4: Workability Comparison of MSF/NSF against PCE at lower dosage

Arc Furnaces used in manufacture of Silicon, Ferrosilicon

and other alloys of silicon.

There are three main reasons for the incorporation of Silica

Fume as an additive for HPC. Microsilica has a filler effect

i.e very fine particle distribute itself in the space between

the materials in the concrete in a homogenous way to give

rise to more dense concrete. Silica Fume improves the

strength of the transition zone between cement paste and

aggregates. CSF is highly pozzolanic in combination with

Portland cement. Figure 6 shows structure and effect of

Microsilica.

During cement hydration there is surplus of Calcium

Hydroxide. The Added Condensed Silica Fume's SiO2

reacts with surplus of Calcium Hydroxide. This results in

greater amounts of Calcium Silicate Hydrate, which are

denser and stronger than Calcium Hydroxide. The

pozzolanic reaction and the fil ler-effect lead to a

compaction of the cement paste and the conversion of

CH crystals into CSH gel leads to a homogeneous paste.

This phenomenon of dense packing in the interface zone

of aggregates also contributes to increase in strength of

the concrete on account of aggregates fully contributing

their strength to the set concrete. Therefore the high

strength of concrete with silica fume is greater than those

of the matrix, indicating the contribution of the aggregate

to the total strength.Experience shows that slurry forms of

Microsilica (50:50 with water) have all the benefits in

transportation, dispensing methods, mixing times and

dispersions to get the desired effect in durable concrete

for tunneling segments.

3. New Generation Aluminosilicates:New generation

aluminosilicates based on special nano-crystallizers have

been developed. These new materials improve the

properties that are crucial for the durability of high-

performance concrete. In addition to reducing chloride

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migration, an exceptional chemical and resistance to

aggressive media of the concrete can be achieved with

Aluminosilicates. The concrete structure is simultaneously

reinforced right down to nanoscale, density is improved

and compressive and flexural strength as well as abrasion

resistance of the high-performance concrete is increased.

There is also a significant reduction of micro-crack

formation,which makes it particularly suitable for the

production of tunneling concrete. Aluminosilicates reduce

the proportion of portlandite by way of a pozzolanic

reaction that changes it into the aluminosilicate crystals

into calcium silicate hydrate. In addition to the unique

resistance against acids a crystalline micro-reinforcement

within the concrete structure is achieved. This reduces

the risk of micro-crack formation, rendering concrete

impermeable.

Due to high homogeneity and reduced tackiness

compared with microsilica-basedconcrete, workability is

improved significantly. In many instances this enables the

production of high-performance concrete that can be

pumped. In addition, a distinct improvement of the building

structure's aesthetics is gained due to the fair appearance

Figure 5: Strength Comparison of PCE versus MFS/NFS at lower dosage

of the concrete surface.Aluminosilicates performs over the

some of the disadvantages of Microsilica:

- Graded for dispersion in concrete

- Graded particle size

- Optimizes mixing time within concrete

- Good dispersion reduces unreacted material in the

mix and increases passivation by C-S-H gel on

aggregate surface

Figure 6: Structure and Microfiller effect of Microsilica in Concrete

- Material if agglomerated improve strength of the mix

- Reduces risk of Alkali Silica Reaction by Agglomeration

of aluminosilicate particles

Table 2 shows some of the key differences between

Microsilica and Aluminosilicate slurries. Figure 7 shows

the comparison of strength development between

Microsilica and Aluminosilicates.

All in all the use of PCE Admixtures and Microsilica or

Aluminosilicate Slurries in addition to the standard

ingredients in concrete, plus excellent mix-design

practices can facilitate the production of high performance

concretes resistant to aggressive media, suitable for use

in tunneling applications.

Microsilica

By-product of the Ferrosilicium- &

Silicium production, not specifically

produced for concrete

Quantities are depending on the

metal industry and the economic

development

Quality of the product has a higher

deviation because it is only a by

product

Aluminosilicates

Manufactured product, it is only

produced for use as concrete additive

Quantities are not depending on other

industries and are unlimited, the

reforereliable availability

High quality standards for end

product because every step in

production is controlled

Table 2: Key Difference Between Microsilica and Aluminosilicates

Figure 7: Comparison of Strength Development between Microsilica and

Aluminosilicates

Tunnel Engineering Concrete Admixtures