The Masterbuilder_July 2012_Concrete Special

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66 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Evolution of RMC in India

Concrete construction scenario in India has witnessed significant changes in the past decade. Traditionally, construction involving concrete has been a labor-

intensive activity, and even today, an overwhelming majority of concrete produced in the country is site-mixed, and most of it is volume-batched. However, thanks to the liberalization of the Indian economy and emphasis on the development of physical infrastructure, concrete construction scenario in India — especially in urban India — is undergoing welcome transformation in the recent years.

The demand for higher speed of construction, especially for residential and commercial housing, flyovers, highways, roads, aviation, etc. in metropolitan and other big cities of India, has necessitated the adoption of mechanized and semi-mechanized techniques of construction. The need for large volumes of concrete as well as faster speed of construction was felt. This was conducive for the development of ready-mixed concrete (RMC).

The concept of RMC was not new to India. Captive RMC plants arrived in the country in 1950s; but remained confined for application in mega projects. Thus, India missed ‘commercial’ RMC technology for nearly five decades! Early 1990s wit-nessed the beginning of RMC industry in India. The first commercial RMC facility was set up in Pune in 1992 and was quickly followed by establishment of similar facilities at other locations.

The growth of RMC started with metropolitan cities like Mumbai, Bengarulu, Delhi, Hyderabad, Chennai, Kolkata later then spread to other major cities, and it is now trickling down to tier II and III cities. No authentic data is available on the RMC industry in India. Based on rough estimates, it was reported that as on December 2008, there were around 450-500 commercial RMC plants producing about 25-30 million m3 of concrete per annum4. In addition, a large number of batching and mixing plants belonging to medium and large construction companies also operate as captive plants on a number of projects throughout the country. These plants are large in numbers and with the current emphasis on building physical infrastructure in the country, there seems to be a phenomenal increase in captive batching and mixing plants.

Major Hurdles to Growth

In terms of cement consumed through the RMC route, the total percentage is too low and it stands at around 7-8% of the total quantity of cement produced in India during 2010-11. In most of the advanced countries, this percentage is much higher, varying from around 48% (Europe) to 73% (USA). Thus, there is a great scope for the development of RMC industry in India. However, there many hurdles for the healthy growth of the industry. Some of these are listed below.

- Level playing field for RMC vis-à-vis site-mixed concrete: Higher rate of taxation on RMC is one of the major constraints for its faster growth.

- Land for RMC plants in urban areas at reasonable rates: Non-availability of land for setting up plants in urban growth centers is one of the major stumbling blocks in speedy growth. There is a need to reserve specified area in growth centers for setting up RMC plants at reasonable rent/cost.

- Regulation issues: Industry regulation through certification of RMC facilities is highly essential to ensure quality. In this regards, a good initiative is already taken by the Ready Mixed Concrete Manufacturers’ Association in evolving and implementing a Quality Scheme for RMC4.

- Lack of knowledge of applications of RMC.

- Tendency of too much dependence on labor-intensive techniques in construction.

- Education and Training of industry personnel and customers of RMC.

Two Indian experts have tried to estimate the future growth potential of RMC industry in the country. They have predicted that the growth of RMC in India may follow a path identical to the “slow” growth pattern followed by some of its counterparts internationally. Basing their estimate on the cement consumed through RMC route, they postulate that the percentage of cement consumption through RMC may reach the 10% mark by 2013-14. Thereafter, the growth may be faster and the cement consumption through RMC may reach the 25% mark by 2022, when the total number of commercial RMC plants in the country will reach 1,516

Vijaykumar R KulkarniMember Managing Committee & Principal Consultant, RMCMA Immediate Past President, ICI

RMC Industry

www.masterbuilder.co.in • The Masterbuilder - July 2012 67

Growth of Cement Industry

India has the distinction of being the second largest cement producer in the world, next to China. Based on the data from the Survey of Cement Industry, the Indian cement industry comprises of around 180 large plants belonging to 55 companies, with an aggregate capacity of 290 million tonnes as of 2010. Unofficial estimate predicts that the cement industry capacity will reach a figure of around 350 million tonnes by 2012-13. Historically speaking, the indigenous cement industry has generally achieved 1-2% higher growth rate than that in the GDP. Thus, even if we assume a very conservative GDP growth rate of around 5-6% during the 12th Five Year Plan (2013-2017), it will be safe to assume that the cement industry will grow at an average rate of 7%. With this assumption, the cement industry capacity would reach 442 million tonnes in 2017, Fig 1.

Size of Organized Concrete Industry

Based on the cement figures, let us now find out the approximate size of the organized concrete industry in India, which comprises of companies/organizations producing concrete using modern batching/mixing plants — for either commercial or captive consumption. Here, based on the previous discussion, it may be safe to assume that cement consumed through the RMC/batch-plant route will reach 7.5% in 2012-13 and 10% in 2017-18. Further, assuming that the average cement consumption per m3 of concrete produced is around 300 kg, the volume produced by the organized concrete industry in India will reach 87million m3 in 2012-13 and 147 million m3 in 2017. These are approximate estimates from a conservative angle. Yet, when compared with the production figures from other leading countries, the performance of the Indian industry seems impressive. By 2017-18, the organized Indian concrete industry will possibly rank the third largest concrete industry in the world, next to the USA and China.

As pointed out earlier, the RMC culture has now spread to around 50 major cities in India and the unofficially-estimated volume of concrete produced by the commercial plants during 2010-11 was of the order of around 40-45 million m3. If concrete produced by captive plants is added to this, the total

figure may reach around 80 million m3.

The organized concrete industry in India is unfortunately frag-mented. There are only a handful of commercial RMC players who have an all-India presence. Since the entry barrier to the industry are low, there is a preponderance of small players operating in local markets. However, being late-comer, the RMC companies in India have one advantage, in that most of the batching and mixing plants installed in the county during the past decade are of the state-of-the-art variety with computerized controls.

Regulatory Framework

Immediately after its formation in 2002, the RMCMA actively participated in revising the Indian Standard specification on RMC. The old standard, IS 49267, which was first published by the Bureau of Indian Standards (BIS) in 1968 and then revised in 1976, needed one more revision to incorporate the experience gained in the commercial operations RMC plants. The second revision of the BIS standard which was published in 2003 generally proved appropriate for the industry.

During the early years of its formation, RMCMA realized that in a country like India having a long history in the use of labor-intensive site mixed concrete, quality of concrete has indeed been one of the major concerns of customers. It therefore took the decision of creating a quality platform for winning over the customers using site-mixed concrete. In the absence of any national regulatory framework, RMCMA decided to develop an indigenous quality scheme for ready-mixed concrete in India. It also decided that the quality scheme shall rest on two strong pillars, namely, best international practices that are suitable for Indian conditions and strict observance of the codes of the Bureau of Indian Standards.

For evolving the quality scheme, RMCMA constituted a “Quality Team” consisting of senior representatives from member companies and eminent experts from the Indian construction industry. The quality team met on several occasions and after thorough discussions, decided to divide the quality in the scheme following two parts:

- Audit-based certification of RMC production facilities; and- Guidelines for quality control and quality assurance.

With the guidance from experts from construction industry, two detailed manuals were prepared covering the above-mentioned two parts4, 8.

The QC manual part I developed by the RMCMA contains an exhaustive “Check List” covering all the operational areas in RMC plant. It contains some 125 items. Out of these, conformance with some 110 items is considered to be strictly essential for achieving good quality concrete, and hence for getting the certification by the RMCMA.

While developing the ‘Check List’, it was ensured that the provisions in the same meet most of the stipulations in the

City-wise Certified Plants

18

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15

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3 42 1

12

2 1

19

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2 31

31 1

0

5

10

15

20

25

30

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Bangalore

Delhi(NC

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CoimbatoreNagpurNashikPune

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AhmedabadMysoreJaipurTrichy

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RMCMA

RMC Industry

68 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Indian Standard, IS 4926: 2003 and the other relevant codes on concrete such as IS 4569, IS 38310, IS 910311, etc. In fact, in certain cases, the RMCMA requirements are more stringent than those of IS 4926:2003 and other codes.

With a view to bring in transparency, enhance credibility and win over the confidence of customers, it was considered essential by the RMCMA to introduce a yearly audit of the RMC production facility by an external auditor. For this purpose a detailed audit procedure was drawn and the selection criteria for auditors were also finalized. The scheme was offered to members of RMCMA as well as others.

One more crucial feature of the RMCMA quality scheme is its adherence to the prevailing statutory norms in India. Before undertaking any audit, the auditor seeks and verifies certificates of compliance on the following three aspects from the RMC producer:

- Permission and consent to operate RMC facility from state Pollution Control Board;

- Permission from factory inspector confirming adherence to health and safety norms;

- Permission/license to operate plant from local body/municipal authority.

Till April 2012, RMCMA has certified around 250 RMC facilities throughout the length and breadth of country.

Realizing that mere certification based on the ‘Check List’ may not be sufficient to instill assurance on quality amongst customers, RMCMA prepared detailed Guidelines for QA and QC of concrete (Quality Manual Part II). The minimum benchmarks suggested in the this guideline document are based on the relevant provisions in BIS codes such as IS 4569, IS 49267, IS 38310, IS 910311, IS 381212, etc. In fact, certain benchmarks in the guidelines far exceed the provisions in different codes. Based on these guidelines, RMCMA encouraged its members to develop their own documents and

make the same available to customers on request.

Recent Changes in Regulatory Framework

After operating the quality scheme for RMC successfully for the past four years, RMCMA decided to raise the quality scheme to a higher pedestal. For this purpose, RMCMA recently signed an Memorandum of Understanding (MoU) with the Quality Council of India (QCI). The latter organization was set up in 1997 jointly by the Government of India and the Indian Industry represented by the three premier industry associations i.e. Associated Chambers of Commerce and Industry of India (ASSOCHAM), Con-federation of Indian Industry (CII) and Federation of Indian Chambers of Commerce and Industry (FICCI), to establish and operate national accreditation structure and promote quality through National Quality Campaign.

The MoU envisages that the quality scheme for RMC would be handed over to QCI, to be operated in an independent and impartial manner based on the best international practices. While QCI and RMCMA shall be the joint scheme owners, the governing structure of the new scheme shall be under a multi stakeholder steering committee and the scheme would be operated on a non-profit, but self-sustaining basis. It would have a defined consensus based technical criteria laid down for the ready mixed concrete plants which would be evaluated by competent third party certification bodies, who in turn, would be accredited by the National Accreditation Board for Certification Bodies (NABCB), which is a part of the international system of equivalence of accreditations and certifications, as per appropriate international standards.

A multi stakeholder Steering Committee as well as the Technical and Certification Committees have already been formed and the new scheme is expected to take a final shape by July 2012. With the new scheme, the regulatory framework is expected to get strengthened and this will go a long way in ensuring healthy growth of RMC industry in India.

Quality Schemes in RMC Industries: A

Comparison• USA

– 1913: Beginning of RMC production

– 1935: ASTM C 94 adopted first time

– 1965: Certification System commenced

• U. K.– 1930: beginning of RMC production

– 1950: BRMCA formed

– 1968: “Authorisation Scheme”

– 1984: QSRMC launched

• India– 1994: Beginning of commercial RMC

– 2002: RMCMA established

– 2008: Quality Scheme commenced

30 years

18 years

6 years

RMCMA

Proposed Structure of New Scheme

Quality Council of India

Steering Committee

Technical Committee Certification Committee

Certification Bodies accredited byNABCB of QCI

Joint CoordinationCommittee

RMCMA

RMC Industry

82 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Efficiency at Sites: The Prime Focus

With every fuel price increase and news on rupee devaluation against dollar, the thought of inflation naturally springs to our minds. As much as this

phenomenon cascades down to the common man, a con-struction project head seems to take a major brunt. Cost escalation has impacted construction business from all directions: materials, freights, labour, interest rates and alas! from the point of passing this on to the end consumer; for inflation consumes his disposable income. When there is this difficulty of passing on the cost escalation to the customer or alternatively keeping the project within the agreed rate levels, there are only few things a construction business can do. Options are; to find out the areas where there is money drain and seal it; to increase productivity. This effectively helps control costs as money saved is money earned. In this study, from different perspectives, I find that a little paradigm shift can help builders reduce their cost levels.

Cost savings

One of the main areas where the money gets drained is through delay in executing projects as per schedule. While delay can happen due to various reasons that are out of control, if the areas that are within control are recognized and controlled, the builder could make some savings. While some of the areas that are out of control are climatic disturbances, land acquisition delay, bureaucratic delays in approvals, the good news is: If proper suppliers are selected for materials, and if workers are kept in good spirits, it will do enormous good in controlling costs. Delays can happen due to non supply of material in time, its indifferent quality, repeat or corrective work and due to low productivity at site. These are areas which can attract a realistic focus and attention for a builder.

Many international research firms, when analyzing the risk of construction sector, came up with the conclusion that ‘delay of project execution’ and ‘increased working capital cycle’ are among the main criteria under consider. It is established

that delay directly eats in to the earnings of builders. Increased working capital requirement is necessitated by delay in recovery from the customers and also due to delay in project execution. Details of some of the infrastructure projects that are on schedule and delayed are as given below.

E RavishankarPh.D, Research Scholar, BITS-Pilani

Segment No: projects going as scheduled

No: projects delayed

Power 42 48

Road 39 80

Petroleum 22 30

Coal 20 18

Railways 18 22

Ports 8 12

Steel 5 5

Civil aviation 2 4

Telecom 6 4

Source: BMI, Ministry of statistics and programme implementationTable 1: Infrastructure projects details as of Jan ’12

The above table, which reflects only the infrastructure projects, shows that in majority of the cases the delayed numbers is higher than projects going on schedule. Another fact is that in all the cases there are projects which are delayed.

If we look at the changes in cost structure in residential building construction we get an idea of the tough picture:

Cost component 2008 2010 change

(% of the total cost)(% of the total cost)

Raw materials 19 22 3

Employee cost 17 18 1

Selling and admin 7 8 1

Others 57 52 -5

Source: ISI report on residential building sectorTable 2: Cost structure in residential construction

Project Management

www.masterbuilder.co.in • The Masterbuilder - July 2012 83

The table shows that between 2008 and 2010 there is increase in the areas of material, staff and selling costs, as a component of the total costs, for ‘residential building’ for the construction firms. The figures reflect a need for improving productivity with the help of material and staff which will effectively bring down the cost per unit, thereby increasing the earnings margin.

Increased working capital cycle makes degree of leverage increase for the construction firms due to combined reasons as can be seen below.

Factor Quarter 2 ‘2011

Quarter 3 ‘2011

Remarks

Avg days needed for receivables

114 125 High and increased

Avg days of coverage for cash& equalents

21 28 Increased

Avg inventory days 61 65 High and increased

Debt –to- Equity ratio % 95% 110% High and increased

Source: ISI report on residential building sectorTable 3: Details of Residential building construction sector as of Q3 2011

The above factors reflect blockage and delay in realizing cash which can result in higher leverage and financial costs.

Focus areas

All the factors put together gives us a view that ‘labour and supply’ are the areas of focus. There is a catch 22 situation here. On one hand the construction firms need to improve their supplier’s services; ensure that it does not become cause for delay; improve productivity of employees, and on the other hand they have to improve profitability. The answer comes as this: it makes business sense to buy products that are assured of quality, gets supplied in time and helps improve productivity. In construction activities many products like sand, aggregates, timber etc. are from un-organised sector and suffer from irregularities in quantity,

Urban population

‘2010

Percentage of total

Forecast % by 2030

Urban contrbn to GDP 2010

320 mn 26% 40% 60%

Source: AnandRathi, India construction sector, Mar ’12 reportTable 4: Importance of urban areas

quality, price consistency and supplies. Also many activities like ‘concrete’ and ‘mortar’ in the construction site are done manually by mixing these inconsistent materials. The difficulty gets more pronounced in urban areas, as these areas suffer from labour shortage and increasing costs more than rural areas. Furthermore, the construction activities are more in urban areas and can be expected to increase continuously in future for two decades as can be seen below.

Thankfully the un-organised sectors are converting into organized sectors, slowly but surely. Items like ‘Aerated concrete blocks’, ‘Jointing mortar’, ‘Ready -mix concrete’, ‘Ready mix plaster’, ‘Pre cast concrete’ are on the way up in to an organized business category, which can help assuage some of the problems faced by construction firms. If we consider for example the case of ‘Jointing mortar’, we get some insight in to how things can work favorably.

If one product, by becoming an organized sector, can bring in good amount of benefit by way of productivity, scheduled work, job satisfaction and cost savings, then if more products move in and get organized its benefit get more prominent to the construction industry.

Let us take two more related examples to see some of the ways in which the builder segment can benefit from using products from organized sector in lieu of taking the same from un-organized ones.

It is commonly assumed that the price of these organized sectors is likely to be slightly more than the un-organized sector. Given the benefits these bring with it, this myth gets unveiled. It needs to be remembered that in arduous climatic seasons, the un-organized sector can cost exorbit-

Usage Conventional mortar Jointing mortar Benefits

Height of wall Can be used for up to 3 ft wall in a day Can be used up to 6ft wall in a day Increased productivity

Subsequent work Needs a week’s curing before starting next work

Can be done next day Increased productivity, time and labour cost savings

Ease Manually work force has to mix materials to create it.

Ready to use Ease of work, improved job satisfaction, improved productivity

Quality Inconsistent Consistent Improved quality, productivity. Helps avoid rework. Improvement is even better if used with cement blocks.

Supplies Depends on supply of different materials by different suppliers

Consistent Helps project go as per schedule.

Table 5: Favorable aspects of ‘Jointing mortar’

Project Management

84 The Masterbuilder - July 2012 • www.masterbuilder.co.in

antly, almost paralyzing the construction activity and bring it to a halt. For example, it is common knowledge how difficult it is to get sand and bricks in rainy season. It will take a beating in quality, quantity, price and timely supplies during monsoon. It is reported that the Indian government has made a trillion dollar (@`48/$) commitment for infra-structure in its twelfth five year plan (2012-17). This plan envisages as many as 666 ‘Public-Private Partnership (PPP)’ projects, which will naturally put pressure on work efficiency and time. The time is ripe to place premium on efficiency at site, improving productivity, reduce delay and thereby control costs, earn better and take up more projects in the long run.

Usage Conventional Bricks Fly ash based Blocks Benefits

Reduction in load 2000 Kg/m^3 600 Kg/m^3 Weight on account of wall will get reduced by 2/3rds. If this weight factor is considered at design stage it can help in reduction of dimensions in structures which gives cost savings and additional space.

Work time reduction Physical mobility of 6 bricks for every block is a time consuming exercise

One block is equal to 6-7 conventional bricks

Increased productivity plus ‘time’ and ‘labour cost’ savings. Enables completion of work as

scheduled or even faster.

Job satisfaction Physically tiring Easy to use Ease of work, improved job satisfaction, improved productivity.

Quality Inconsistent in shape, quality and dimensions

Consistent in all the aspects of shape, quality and dimensions

Improves productivity. Improved shapes in structures gives workers improved confidence in

their work.

Supplies Depends on supply of different materials by different suppliers

Consistent Help project go as per schedule.

Sound filtering Less blockage of external noise Better blockage of external noise Improved usage in for construction of theatres and such other constructions needing external

noise filtering.

Fire resistance Gives far little time to escape in case of fire accidents

Has 2-4 hours fire resistance giving better chance for people to escape

Better safety.

Pricing Pricing is inconsistent throughout the year; sometimes due to climatic reasons

Pricing is more consistent Helps to plan the budget for the materials better.

Table 6: Fly ash based concrete blocks utility

Bibliography

- Business Monitor International, (2012). India Infrastructure report- Q3 2012, March 30, 2012, p.10-11.

- ISI Emerging Markets, (2012). India Q3 2011- Residential Building Construction. EcoTrends Sector Analysis, p. 12.

- ISI Emerging Markets, (2012). India Q3 2011, Residential Building Construction. EcoTrends Sector Analysis, p. 32.

- Manish Valecha, Jaspreet Singh Arora, (2012). Breaking the grid lock- India Construction Sector. Anand Rathi Research, March 9, 2012, p. 5.

- Manish Valecha, Jaspreet Singh Arora, (2012). Breaking the grid lock- India Construction Sector. Anand Rathi Research, March 9, 2012, p.12.

- NBM Media Pvt Ltd., (2012). US firms keen on infra spend in India. NBM&CW May 2012, p. 24.

Usage Conventional Plaster Ready to use Plaster Benefits

Proportion consistency Inconsistent, based on the mix done by different persons at work

Accurate and consistent Improves work quality and productivity.

Work time reduction Constitutes time of mixing the materials Ready to use Saves time and avoid work delay.

Job satisfaction Less satisfying Easy to use Ease of work, improved job satisfaction, improved productivity.

Thickness Physically mixed and becomes ticker on the wall Reduced thickness Helps get more appealing finish.

Wastage Manual work involves wastage Sometimes nil wastage due to consistent mix

Saves cost.

Rebound wastage Splashed on the walls and regularly gets rebound resulting in loss

Sticks to the wall and easy to use

Cost reduction, improved productivity, and better job satisfaction.

Table 7: ‘Ready to use’ plaster utility

Project Management

92 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Self Compacting ConcreteAn Economical Approach

High durability concrete will be discussed throughout this paper. It will be classed as that which has to be manually placed and include durability enhancers;

GGBFS, Micro silica and PFA in the mix designs. The high durability conventional concrete in question will have a total cementitious content of between 450 to 500kg per meter cube, and coarse to fine aggregate ratio will be as 60:40 percent, and concrete will be produced at water cement ratio of 0.40 (maximum).

High durable conventional concrete has to be well vibrated to achieve good compaction. It should be noted that with conventional concrete, water would tend to migrate to the surface of the coarser particles when vibrated causing porous and weak inter facial zones, which will affect the concrete durability. Therefore there will always be a difference in durability parameters of conventional concrete in a laboratory with tightly controlled conditions, compared to site batch concrete.

Awareness of self-compacting concrete has spread across the world and specifically in the Middle East and India, prompted by concerns with poor compaction and concrete durability.

In the U.A.E., and specifically in Dubai, there are a significant number of high-rise structures under construction and many more expected. Normal concrete technology has been

extended through SCC for easier placement of concrete, associated with other benefits. Specific instructions are necessary for designing, producing, transporting and handling of such concrete. Its innovative aspects lie in its fresh properties and the potential benefits to the contractors.

The UAE, Middle East and Far East markets including India are currently experiencing a change in concrete durability specifications, such that on-site only SCC mixes can meet the requirements with continued repeatability.

What is Self Compacting Concrete

SCC is a concrete, which in its plastic state, flows silently under its own weight and maintains homogeneity while completely filling the formwork of any shape, even around congested reinforcement. Compaction of concrete is achieved by its own movement properties.

Khayat et al defines SCC as

“A highly flowable, yet stable concrete is one that can spread readily into place and fill the formwork without any consolidation and undergoing any significant segregation”.

Materials & Mix Designs

SCC can be made out of similar materials as used for conventional concrete for structural use. Maximum size of

Dr. Y P KapoorManaging Director, Bang Associates & Director Editorial, The Masterbuilder

Self Compacting Concrete (SCC) has been promoted in the Middle East for the last eighteen years. Although targeted at the high quality end of the marketplace there has been little commercial success, despite the many advantages and savings in use of SCC. The majority of its applications have been small niche pours into congested rebar, domes, or thin wall sections. Self Compacting Concrete by its definition has to be fluid, self-compactible with high segregation resistance. To achieve these properties and to reach extremely high specification targets it is normal to use a high dose of new generation poly-carboxylate admixture plus a rheology modifier into the concrete mix. The use of these admixtures and change in mix design increase the cost significantly above that of conventional high quality concrete and this is seen as the major factor that has prevented wider use of SCC.This paper therefore studies the production of SCC with a specification for exceeding that of conventional concrete, currently in use in the Middle East, and gives a concrete mix with all the application advantages of SCC, without an excessive cost increase.

Concrete Self Compacting

www.masterbuilder.co.in • The Masterbuilder - July 2012 93

the coarse aggregate will be dependent on the minimum spacing allowed for reinforcement layouts. Even a 40mm size of aggregate may be used. Graded aggregates will be of a great help. Fine sand content less than 150 microns will be very useful for better cohesion and reduced segregation.

Mix Design, Admixture and Cement

The objective of traditional concrete mix design is always to work out the most effective proportioning of materials to achieve the concrete properties in plastic and hardened state.

But before looking at designing a mix for SCC, an under-standing is needed of the properties required for self-compaction and how it will be optimized using normally available materials. There are two main properties, which should be looked into, that is, as a highly fluid concrete, with segregation resistance.

To achieve highly fluid concrete, a low yield stress is required and for high segregation resistance, a highly viscous concrete is needed.

Fluidity and concrete viscosity may be varied by re-proportioning the aggregates or by using high-grade super -plasticisers in concrete mixes. Viscosity modifiers can also be used for segregation resistance, but it will increase the yield stress of the paste, thereby affecting the mobility of the mix.

Advances in superplasticiser technology have played a major role, and new generation superplasticisers based on polycarboxylate ethers promote good workability retention, and can be added at any stage of the batching cycle.

Fine particles play an integral part in SCC mix designs, as SCC mixes are made with high fines. On an average the SCC mix will have 400kg to 600kg of cement. This cement weight of 500kg/m³ of concrete can be replaced partly by pulverized fuel ash, ground granulated blast furnace slag and silica fumes. These cement replacements will enhance the concrete properties of the mix in the plastic state and hardened state of the concrete. These cement replacements will further add durability properties to the concrete. Even limestone powder is used extensively in Europe. All fillers must be assessed for their effect on water demand, to achieve the optimum level of water for the mix.

Typical guideline criteria are given in Table 1. These guidelines may be amended as per the country’s requirement of concrete specifications and locally available materials.

Water demand is very critical to SCC mix design and specifically related to segregation. It is advisable to design

Materials Properties Practical Guidelines

Coarse aggregate Well graded to reduce inter particle friction and prevent blocking

In practice it may vary between 700-900kg/m³

Sand Very well graded sand available at local source. Should contain fine materials passing through 150micron sieve

Should be >50% of the total aggregate content

Cement + Fillers High content required Typically 500kg/m³

Water Dependent on fillers and fines

Typically between 130litres to 150 litres (free water)

Water/(Cement+Filler) ratio

0.28 to 0.34 for tropical Middle East conditions

Admixtures Dosage to be tailored to meet the durability properties of Plastic and hardened state of concrete

May vary between 3.0 to 6.0 ltrs per m³. Follow manufacturer’s advice

Paste & Mortar Paste >40% by mix vol. Sand <50% by mortar vol.

Table 1

the mix conservatively to maintain the plastic state concrete properties.

Water demand and admixture dosage should be economically adjusted to retain the fluidity of concrete towards 90/120 minutes after batching.

A slump cone flow spread of 650 mm at the time of placing the concrete will be quite acceptable to meet almost all properties in plastic and hardened state of concrete.

Hardened state properties of SCC

Compressive Strength

At constant water cement ratios, the characteristic strength of SCC will be slightly above or at least equal to the

GGBS mix design

020406080

3d 7d 28d

Days

Com

pres

sive

Stre

ngth

N/m

m²

High DurabilityConcreteSCC

Fig 1

Concrete Self Compacting

94 The Masterbuilder - July 2012 • www.masterbuilder.co.in

conventional concrete of similar grade. (see Fig. 1&2)

Above results indicate that the characteristic compressive strength of 60N/mm² can be easily achieved. In addition to the above information, lower water to cementitious ratio with fines will help to achieve the rheological properties of concrete will make difficult to keep the strengths down.

PFA Mix Design

020406080

3d 7d 28d

Days

Com

pres

sive

Stre

ngth

(N/m

m²)

ConventionalConcreteSCC

Fig 2

The author of this paper was involved in re-proportioning the mix while using the similar quantity of fines, as mentioned in Fig.2, and could enable to reduce the admixture dosage within limits. The details of the compressive strengths achieved are shown in Fig 3.

Mix with Fly Ash

020406080

100

3d 7d 28d

Days

Com

pres

sive

Stre

ngth

(N/m

m²)

ConventionalConcreteSCC

Fig 3

Drying Shrinkage

Drying Shrinkage of SCC has been noticed to be similar to that of conventional concrete or lower, which is contrary to that expected from the lower aggregate content, which is partially expressed by the similar water content of SCC and high durability conventional concrete. The high fine content in the SCC mix may indicate slightly higher figure on drying shrinkage as compared to conventional concrete.

In the UAE and in Middle East most SCC mixes are designed on very low water cementitious ratio that enables the mix to achieve lower drying shrinkage as shown in Table 2. Attention to curing is very important for the tropical conditions prevailing in the Middle East, and India to ensure that the low drying shrinkage figures obtained in the laboratory are transformed to site.(Table 2)

Care should be taken to ensure the concrete is cured correctly.

Bond Strength

Bond Strength behavior of conventional concrete is depen-dent on the reinforcing bar location, deformation of bar pattern, fluidity of concrete mix and in general contact with rebar. Interlocking of aggregates in SCC is far superior in comparison to conventional concrete, which is due to the uniform distribution of aggregates over the full cross section and the higher volume of cement binder matrix. Therefore the bond strength between concrete and reinforcement for medium to high strength concrete in SCC is higher than that of conventional concrete.

Properties Conventional Concrete

Self Compacting Concrete

Drying Shrinkage % 0.0208 0.0224

Durability

Few indices of durability have been investigated in self-compacting concrete when compared to the same grade of conventional concrete. The results are listed in table 3, which are recorded at 28 days age.

Properties Conventional Concrete of high

durability

Self Compacting Concrete

Water absorption BS 1881:Pt 122

2% 1.0%

Water PermeabilityDIN 1048

10mm 5mm

Rapid Chloride Permeability (coulombs)AASHTO T277

1970 620

Initial Surface AbsorptionTest (ml/m²/sec)BS1881:Pt 208

0.02 0.01

Table 3

This indicates that durability parameters in self-compacting concrete are enhanced as compared to standard con-ventional concrete.

Summary and Economics

Self Compacting Concrete will be seen by the contractor as a material which will be useful, but demands different working practices and as such it has more advantages than disadvantages. These should be balanced against each other and economics can be worked, while listing and evaluating each parameter. In most of the cases and situations, the resultant effect will be in the favor of Self

Table 2

Concrete Self Compacting

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Compacting Concrete – as an improvement in ease and spread of placement, quality of finish and reduced overall cost in addition to the usual technical benefits of complete and assured compaction and minimizing the voids.

A summary of advantages can be listed as below

- Very less dependence on skilled work force.- Faster placing time – enhances total production period

to be reduced.- Improved surface finish – means cost of making good.- Noise elimination leads to safe working environment.- Some approximation of the offset costs can be calculated.

Estimates of these show the following in UAE Dirhams per cubic metre.

In summary, it is clear that self-compacting concrete offers some significant advantages over conventional concrete. Some of the economic applications may be listed, like,

Properties Conventional Concrete

SCC Saving

Man Power 2.00 1.00 1.00

Vibration 2.00 00 2.00

Finishing 2.00 00 2.00

Curing 2.00 2.00 0.00

Repairs 18.00 00 18.00

Formwork 15.00 20.00 -5.00

Plaster 5.00 00 5.00

Overheads 5.00 3.00 2.00

Total 51.00 27.00 25.00(Above are shown as Typical Values, which may vary from Country to country.)

precast elements, exposed walls and columns, water tight basements, congested reinforcement locations and column encasements.

References

- Okamura H. Self-Compacting High Performance Concrete – Concrete International, Vol. 19, No. 7, July 1997.

- Khayat K. Workability, Testing and Performance of Self-Consolidating Concrete. ACI materials journal, vol 96, No. 3

- Glavind M. How does Self Compacting Concrete contribute to implementation of sustainable/clean technologies in the construction industry. Proceedings of self-compacting concrete, Malmo, November 2000.

- SKARENDHAL A. – State of the art of Self Compacting Concrete. Proceedings of seminar of self-compacting concrete – Malmo, November 2000.

- R. Gaimster and N. Dixon – Self-Compacting Concrete – RMC Readymix U.K. Ltd.

- OKAMURA, OUCHI M. Self Compacting Concrete – Development, present uses and future. Proceedings of first Rilem International Sys perillum of Self Compacting Concrete, Stockholm – September 1999.

- Building Research Establishment – Practical guide for engineers using SCC.

- BARTOS, P. and GRAVER M. – Self-Compacting Concrete, Concrete Vol. 33 No. 4, April 1999.

Authors Bio

Dr. Kapoor has spent approximately 30 years in the field of Concrete Technology, while working in Middle East and Far East and advising customers, ready mix concrete manufac-turers ,suitable mix designs and appropriate use of concrete admixtures to achieve the highest degree of concrete durabil-ity for enhanced structural life.

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Metakaolin for High-Performance Concrete

Cement concrete is the most extensively used con-struction material. Maintenance and repair of con-crete structures is a growing problem involving

significant expenditure. As a result carried out worldwide, it has been made possible to process the material to satisfy more stringent performance requirements, especially long – term durability. High performance is generally assumed to be synonymous with high strength, although this is not true in every case. Unacceptable rates of deterioration due to environmental effects indicate that only compliance with strength requirements, although need, is not adequate to ensure long-term, durability, which is the primary requirement for high performance. It is generally accepted, that the high

performance of the very concrete contributes to low permeability, stronger and denser transition zone between aggregate and cement paste in the concrete. This also adds to the abrasion resistance of concrete. According to ACI “High Performance Concrete is defined as concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using con-ventional materials and normal mixing, placing and curing practices.

High Performance Concrete (HPC)

Concrete is probably the most extensively used construction material in the world. However, when the high range water

Special Correspondent

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reducer or super plasticizer was invented and began to be used to decrease the water/cement (w/c) or water/binder (w/b) ratios rather than being exclusively used as fluid modifiers for normal-strength concretes, it was found that in addition to improvement in strength, concretes with very low w/c or w/b ratios also demonstrated other improved characteristics, such as higher fluidity, higher elastic modulus, higher flexural strength, lower permeability, improved abra-sion resistance, and better durability. This fact led to the development of HPC. HPC is the latest development in concrete. It has become more popular these days and is being used in many prestigious projects such as Nuclear power projects, flyovers, multistoried buildings etc. Since 1990s, HPC has become very popular in construction works. At present, the use of HPC has spread throughout the world. In 1993, the American Concrete Institute (ACI) published a broad definition for HPC and is defined as the concrete which meets special performance and uniformity requirements that cannot always be achieved by using only the conventional materials and mixing, placing and curing practices. The addition of mineral admixture in cement has dramatically increased along with the development of concrete industry, due to the consideration of cost saving, energy saving, environmental protection and conservation of resources. However, environmental concerns both in terms of damage caused by the extraction of raw material and carbon dioxide emission during cement manufacture have brought pressures to reduce cement consumption by the use of supplementary materials. Mineral admixtures such as fly ash, rice husk ash, metakaolin, silica fume etc are more commonly used in the development of HPC mixes. They help in obtaining both higher performance and economy. These materials increase the long term performance of the HPC through reduced permeability resulting in improved durability. Addition of such materials has indicated the improvements in the strength and durability properties of HPC.

About Metakaoline & Its Role in Concrete and High Per-formance Concrete

Metakaolin differs from other supplementary cementitious materials (SCMs), like fly ash, silica fume, and slag, in that it is not a by-product of an industrial process; it is manufactured for a specific purpose under carefully controlled conditions. Metakaolin is produced by heating kaolin, one of the most abundant natural clay minerals, to temperatures of 650-900°C. This heat treatment, or calcination, serves to break down the structure of kaolin. Bound hydroxyl ions are removed and resulting disorder among alumina and silica layers yields a highly reactive, amorphous material with pozzolanic and latent hydraulic reactivity, suitable for use in cementing applications. Metakaolin reacts with portlandite (CH) to form calcium-silicate-hydrate (C-SH) supplementary to that produced by portland cement hydration. This reaction becomes important within the interfacial transition zone (ITZ) located between aggregate and paste fractions. This region typically contains a high concentration of large, aligned CH crystals, which can lead to localized areas of increased porosity and lower strength. Metakaolin can react with some of the CH produced by cement hydration, there by densifying the structure of the hydrated cement paste. The rates of pozzolanic reaction and CH consumption in metakaolin systems have been shown to be higher than in silica fume systems, indicating a higher initial reactivity.6 Because this reaction with CH occurs early and rapidly, metakaolin incorporation may contribute to reduced initial and final set times. In addition, this refinement in the ITZ can result in increased strength in metakaolin concrete. As portlandite in the ITZ and elsewhere in the paste is water soluble and is susceptible to deterioration in aggressive chemical environments, metakaolin has great potential for improving concrete durability. Also, because

(a) (b)Figure 1: (a) Metakaolin, (b) Self-consolidating concrete using metakaolin

Admixture Metakaolin

“As far as India is concerned metakaolin actually came into the market as a cheaper material than micro silica. It was only after reputed companies started using metakaolin that the product began to generate a buzz in the market.”

Jayant Basu RayMD, Constromat Consultancy & Services India PVT LTD

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the supplementary C-S-H formed during the pozzolanic reaction with metakaolin has a lower Ca/Si ratio than ordinary C-S-H, these products are believed to be better able to bind alkali ions from the pore solution, thus reducing concrete’s susceptibility to alkali-silica reaction (ASR). This potential beneficial use of metakaolin is particularly relevant, as silica fume agglomerates have been shown to contribute to ASR expansion in some cases. Metakaolin has also been shown to decrease concrete permeability, which in turn increases its resistance to sulfate attack and chloride ion ingress. Additionally, metakaolin may reduce autogenous and drying shrinkage, which could otherwise lead to cracking. Thus, when used as a partial replacement for Portland cement, metakaolin may improve both the mechanical properties and the durability of concrete. In general, metakaolin offers a set of benefits similar to those imparted by silica fume, including comparable strengths, permeability, chemical resistance, and drying shrinkage resistance. Physically, metakaolin particles measure approximately one-half to five micro meters across, making them an order of magnitude smaller than cement grains and an order of magnitude larger than silica fume particles. Both metakaolin and silica fume are typically used to replace 5 to 20 weight % of the cement. Metakaolin is white in color, whereas standard silica fume ranges from dark grey to black (although white silica fume is available at higher cost). This makes metakaolin particularly attractive in color matching and other architectural applications. For these reasons, metakaolin is increasingly used in the production of high-performance concrete. ASTM C618 and AASHTO M 295 classify metakaolin as a Class N (or natural) pozzolan. Figure 1 (a) and (b) shows metakaoline and self-consolidating concrete using metakaolin.

Metakaolin’s Contribution in HPC

- Improved Strength

Metakaolin’s reaction rate is rapid, significantly increasing compressive strength, even at early ages, which can allow for earlier release of formwork. As stated by Mr. Atil Parikh, Joint Managing Director of 20Microns “Globally metakaolin

is a very well accepted product. In India too the demand is fast picking up. Initially we had to educate people. We first targeted Mumbai– the high rise capital of India. This is because metakaolin finds use in the M60 or M 80 grade concrete that is typically used in high-rise construction projects”

(a) Control (b) With Meta Star 501Figure 2: (a) Control Mix floor, (b) Metakaolin mix floor

- Improved Durability

In addition to increasing strength, the densification of the microstructure that results from the pozzolanic and hydraulic reactions of metakaolin also leads to greater impermeability. In concretes containing metakaolin at 8 to 12% of the total cementitious materials, 50-60% decreases in chloride diffusion coefficient suggest that significant improvements in service life can be achieved through metakaolin utilization in chloride environments. In addition, metakaolin has been shown to be highly effective in mitigating expansion due to alkali-silica reaction (ASR) and sulfate attack.

- Improved Early Age Behavior

The relative fineness of metakaolin can result in decreased slump, but the use of water reducing admixtures or use in combination with fly ash in ternary mixes can compensate for this. Slumps of 125 to 180 mm have been achieved with metakaolin at water cementitious materials ratio (w/cm) of 0.36 to 0.38, using 25-35% less high-range water reducing admixture than comparable mixes. Metakaolin concrete tends to exhibit a creamy texture, resulting in better finishability compared to other finely divided SCMs. This quality also improves pumpability and can be used to impart detailed surface textures to cast surfaces. In addition, the cohesiveness provided by the metakaolin allows for relatively simple formulation of self-consolidating concrete, when using an appropriate dosage of polycarboxylate water reducer as shown in the photograph at the beginning of this article. Data on the potential contributions of metakaolin to chemical, autogenous, and drying shrinkage are inconsistent, with authors reporting both decreases and increases in each form at various ages and at various addition rates. For applications with restrictions on shrinkage, additional testing, including the assessment of shrinkage-reducing admixtures and fiber reinforcement, may be advised.

Admixture Metakaolin

“Globally metakaolin is a very well accepted product. In India too the demand is fast picking up. Initially we had to educate people. We first targeted Mumbai– the high rise capital of India. This is because metakaolin finds use in the M60 or M80 grade concrete that is typically used in high-rise construction projects”

Atil ParikhJoint Managing Director of 20Microns

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- Contributions to Sustainability

Because of the lower processing temperature compared to cement clinker, use of metakaolin can contribute to sustain-ability through energy savings, as well as reductions in greenhouse gas emissions. After examining various SCMs alone and in combination and considering performance, economic, and environmental criteria, metakaolin concrete was identified as a “very promising solution” for the precast industry for reducing clinker content in concrete.

Available Brands and Applications

1. MetaStar 501: METASTAR 501 is a metakaolin pozzolanic additive that:

- Improves strength, durability, and workability of Portland cement concrete

- Makes Portland cement easier to apply- Provides smoother finish- Has bright white color for white and color plasters- Reduces permeability, efflorescence, and cracking- Reduces the porosity of hardened concrete- Contains no undesirable impurities, such as carbon or

sulphur, which could affect the curing rate or strength of the final PC product

- Readily disperses in cement-based systems- Is safe and easy to handle

Refer Figure 2 (a) and (b) for judging the difference of control and Metakaolin mix.

2. In India 20Microns is considered one of the leading producers of white minerals. The company offers a wide range of products including functional fillers, extenders, and specialty chemicals. The company’s mines are spread across the country. The company offers a diverse range of customized products based on specific industry re-

quirements. 20microns has its calcined clay mines located in Gujarat, Bhuj. For the production of calcined kaolin the material has to be put through heating up to a certain temperature – around 1200oC. Metakaolin is an intermediary product which is produced at about 700-750oC during the process.

Figure 2: (a) Control Mix floor, (b) Metakaolin mix floor

3. PALAIS ROYALE, Mumbai: First project in India to use M80 grade of self consolidating concrete. The Project is around 300 metres high comprising of Duplexes and Villas in it. The developers have used metakaolin as one of the supplementary cementitious material.

Conclusion

The beneficial effects of using metakaolin is it reacts with calcium hydroxide almost as fast as it is formed in the cement during hydration. The overall effect of removing calcium hydroxide, refining the pore structure and densifying the interfacial zone is to reduce:

- Re-bar corrosion- Sulphate attack- Acid attack (e.g. silage clamps, food factories)- Freeze-thaw damage- Alkali-silica reaction (even when using active aggregates,

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high alkali cementsand where the concrete is exposed to salt solutions)

- Efflorescence

Till three-four years ago, hardly anybody in India was aware of the use of metakaolin in concrete. During these four years, the developments that have taken place include increased awareness of the huge potential of production of metakaolin in the country (with huge mineral resource, that is, kaolin availability across the country), start of indigenous commercial production and many investigations on the development of concrete mixes containing metakaolin. The metakaolin is a low cost, locally produced, highly effective pozzolonic material, particularly for the early strength de-velopment, without appreciable loss in workability. It makes finish easier, reduces efflorescence, increase resistance to sulphate and chlorine attack. It maintains colour of concrete, especially in white concrete. So it can be very well used in Architectural work. As silica fume is to be imported, it is dare necessity to find locally available material at an affordable price, substitute of it, Metakaolin could be an answer to it seeing its vast exploring opportunity in India. It can bring a huge export possibilities to India; as quoted by Mr. Atil Parikh, “The US is a high consumption market. It was during the last decade or so that people started to switch from micro silica to metakaolin there in a big way. Canada is another major market”.

Reference

- Justice, J. M. and Kurtis, K. E., “Influence of Metakaolin Surface Area on Properties of Cement-based Materials”, ASCE Journal

of Materials in Civil Engineering, September 2007, Vol.19, No. 9, pp. 762-771.

- Tafraoui, A. et al., “Metakaolin in the Formulation of UHPC,” Construction and Building Materials, Vol. 23, 2009, pp.669-674.

- Justice, J. M. et al., “Comparison of Two Metakaolins and Silica Fume Used as Supplementary Cementitious Materials,” Seventh International Symposium on Utilization of High-Strength / High Performance Concrete, Ed. Russell, H. G., Publication SP-228, Vol. 1, American Concrete Institute, Farmington Hills, MI, 2005, pp.213-236. Also on Compact Disc.

- Gruber, K. A. et al., “Increasing Concrete Durability with High-Reactivity Metakaolin,” Cement and Concrete Composites, Vol. 23, 2001, pp. 479-484.

- Khatib, J. M. and Wild, S., “Sulphate Resistance of Metakaolin Mortar,” Cement and Concrete Research, Vol. 28, No. 1, 1998, pp. 83-92.

- Ramlochan, T., Thomas, M., and Gruber, K. A., “The Effect of Metakaolin on Alkali-Silica Reaction in Concrete,” Cement and Concrete Research, Vol. 30, 2000, pp. 339-344.

- Garas, V. Y. and Kurtis, K. E., “Assessment of Methods for Optimising Ternary Blended Concrete Containing Metakaolin,” Magazine of Concrete Research, September 2008, Vol. 60,No. 7, pp. 499-510.

- Caldarone, M. A., Gruber, K. A., and Burg, R. G., “High-Reactivity Metakaolin: A New Generation Mineral Admixture,” Concrete International, Vol. 16, No. 11, November 1994, pp.37-40.

- Cassagnabere, F. et al., “Metakaolin, A Solution for the Precast Industry to Limit the ClinkerContent in Concrete: Mechanical Aspects,” Construction and Building Materials, Vol. 24, 2010, pp. 1109-1118.

- www.imerys-perfmins.com

- www.alcongoa.com/pdf/Alccofine_Tech_Info.pdf

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Concrete Durability of Structures

Durability of Concrete Bridges and Viaducts

A lot of concrete bridges, railway overbridges, flyovers and viaducts were constructed in recent years and many more are under construction. Such constructions

got boosts from the golden quadrilateral highway projects, east-west north-south corridor and dedicated freight corridor railway projects, the metro rail projects in the metro cities and the need for elevated expressways/carriageways within or between metro cities.

The nature of materials of construction (Fig.1) of such structures suggests that not much thought has been or is being given to the real possibility that the structures may not be available for their intended use a few decades after their construction.

This suggestion is made not only because of what is clearly visible (e.g. extensive corrosion in rebars even before concreting) or known (e.g. very high C3S/C2S ratios and inadmissible content of water soluble alkalis in cement)1-7 but also because of observations all over the world that concrete structures, built in recent decades, have shown signs of decay and distress fairly early in life. This phenomenon of concrete structures reaching states of distress early in life is in stark contrast to concrete structures, built before the decade of the 1960’s, which have proved to be durable, requiring no or little repair even 60 or 70 years after their construction.

Among abounding examples of old concrete structures with good performance, one could cite the concrete bridges which were built in the 1940’s over the highly polluted canals of Calcutta. None of these heavily travelled and overloaded concrete bridges in relatively corrosive environments has required any repairs till now whereas most of the concrete structures, built in recent decades, to carry today’s traffic, have required minor to major repairs (Fig.2).

In this context, it may be noted here that whereas the average

life span of modern concrete structures in Canada is thirtyseven years, the average life span of such structures in India is estimated to be twentyseven years. This is in the face of much longer theoretical or design life spans of bridges, tunnels and such other infrastructures.

Anil K Kar, B.Sc CE, MSCE, Ph.DEngineering Services International, Salt Lake City, Kolkata

Fig.1 : Corroded rebars in pier under construction for flyover; products of corrosion streaking down faces of pier

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It can no longer be overlooked that though concrete structures, built up to the decade of the 1960’s, have generally proved to be durable, the picture has changed over the years since that time. It is not uncommon for today’s concrete structures to show signs of distress even during the construction stage (Fig. 3). The writer recalls a particular day some years ago when prestressed concrete girders for railway bridges at half a dozen sites were found to have developed cracks. Though the girders, many of which were yet to be launched, were constructed by different contractors with different brands of cement, the result was the same.

Fig. 2 : A view of the deck of the Jogeswari flyover in Mumbai seven years after construction

Fig. 3 : Cracks in reinforced concrete shear walls for tall buildings in Calcutta even before a floor was completed (project has top designer, proof design consultant, top contractor and quality monitoring agent) in 2011

Though many may pretend not to see, the signs of decay in newly built bridges, elevated viaducts and other concrete structures of recent periods have become a matter of con-cern to many others.

It has thus become imperative to focus on durability, as

built structures, including concrete bridges and viaducts, are required to be durable. Such structures are required to provide service over long periods of time, that is variably set at 60 to 120 years.

This article is on durability aspects of concrete bridges and elevated viaducts. The principal causes for the lack of durability of concrete bridges, viaducts and such other structures of recent vintage are identified and solutions to the problem of early distress are recommended. Much of the contents of this article is applicable for concrete structures of all types.

Consequences of Not Doing Things Right

In a search for durability, a question could naturally arise. In order that bridges and viaducts may be durable, will it be sufficient to have a proper design for strength, to limit crack widths under service load conditions through conventional methods of design, to specify large cover to rebars and to maintain good workmanship while building bridges, viaducts and other structures ?

The answer would be in the negative. It will be seen in this paper that it will not be sufficient merely to have a good design and to maintain good workmanship in the case of bridges, viaducts and other concrete structures if such structures are to be durable.

A corollary would be the case of buildings (Fig. 4). On the evidence of durable concrete structures it was earlier considered sufficient to prevent visible water leakages through concrete roofs, water reservoirs, tunnels, etc. In contrast, it is seen in the case of concrete structures, built with today’s cement and rebars, that the mere arresting of visible water leakages is not sufficient for making such structures reasonably durable. As in the case of steel structures, all surfaces of today’s concrete structures, which may be exposed to the atmosphere, more particularly structures, which might be intermittently exposed to water, need be given surface protection, i.e. made waterproof. If this surface protection will not be provided, situations like those shown in Figs. 5 - 7 are likely to arise. This proposition is studied in this article.

The distressed conditions, depicted in Figs. 5 – 7, are

Fig. 4 : This is why people used to waterproof structures; in the case of today’s concrete struc-tures, it will no longer be suf-ficient to make merely the roof waterproof; there is a greater purpose to waterproofing than to arrest visible water leakages

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due to corrosion in rebars. It is easily perceived that this corrosion was possible because of the availability of moist environments around rebars and the absence (due to carbonation or chloride penetration) of the protective layers of passivation on the surfaces of rebars. Moist environments could have been avoided had the structures been given surface protection in the form of waterproofing treatment. Surface protection systems could have also prevented chloride intrusion and prevented or minimized carbonation and ingress of oxygen to facilitate corrosion.

The state of the structures in Figs. 5 – 7 would suggest that even though there may not be visible water leakages, one need to waterproof all surfaces of concrete structures,

Fig. 5 : Abandoned hospital building in the new township of Salt Lake City, Kolkata - a stigma on society

Fig. 6 : Distress in underside of deck slab of Buckland Bridge (Bankim Setu) over the platforms at Howrah Station near Calcutta prior to major repair and surface protection in the year 2005

including bridges and viaducts which may be exposed to weather, to the atmosphere.

Could the provision of surface protection systems have really helped ?

It has been a common experience to see that waterproofed containers of overhead water reservoirs may not show any water leakage or signs of distress but it is a different story with the unprotected staging (Fig. 7).

Fig. 7 : Distress in staging of overhead water reservoir due to corrosion in rebar

The same decaying process, that must have contributed to the conditions, as shown in Figs. 5-7, continues unabated in the case of unprotected concrete bridges and viaducts.

But because of good experiences in the past with con-crete bridges and viaducts, engineers, in the absence of visible water leakages of the type shown in Fig. 4, have been slow to realize and recognize the fact that the innate resistance of today’s concrete structures to the elements of nature is very different from such inherent properties of concrete structures of earlier periods.

The inadequate resistance of recent concrete structures to the causes of corrosion has resulted in the early development of conditions of distress in concrete structures of recent vintage, and this has become the norm today.

It is observed in Technical Circular 1/99 of Central Public Works Department, Government of India that : “while work as old as 50 years provide adequate service, the recent constructions are showing signs of distress within a couple of years of their completion.”8

Though today’s concrete structures in India have a pronounced

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susceptibility to early decay and distress, this lack of durability in the case of today’s concrete structures is a global phenomenon, and the occurrence of early distress started showing up in structures which were built since the 1970’s by which time rebars with surface deformations had come into the market and the constituents of cement had undergone some changes.

Papadakis, et al wrote in their paper, published in ACI Materials Journal in March-April 1991, “The last two decades have seen a disconcerting increase in examples of the un-satisfactory durability of concrete structures, specially reinforced concrete ones.”9

In a paper, published in 2007, Swamy too observed that “The most direct and unquestionable evidence of the last two/three decades on the service life performance of our constructions and the resulting challenge that confronts us is the alarming and unacceptable rate at which our infrastructure systems all over the world are suffering from deterioration when exposed to real environments.”10

Swamy further recorded that “What is most surprising is that this massive and horrendous infrastructure crisis has occurred in spite of the tremendous advances that have been made in our understanding of the science engineering and mechanics of materials and structures.”10

Matters have come to this pass and engineers are surprised because they failed to see or they refused to hear and admit the truth (Fig. 8).

It is known that there can be surprises only in the absence of knowledge. The absence of knowledge was exemplified in the case of aircrafts hitting the World Trade Center Towers (Fig. 9), which were required to be and claimed to have been designed for aircraft impact, and yet the aircrafts, wings and all, pierced through the columns and spandrel plates on the faces of the structures to land inside the buildings and start the big fires which ultimately led to the collapse of the towers.

The segmented facial elements of the World Trade Center towers had bolted connections whereas high speed aircraft impact being a postulated loading condition,

Fig. 8 : Like the proverbial three monkeys, engineers are in the denial mode

all connections should have provided continuity through thru-thickness groove welded joints so that the shock pulses could travel and the impact load get distributed and shared by the structural members over larger areas of the structures.

With welded connections, there could have been damages to the facial elements of the towers, but the fuel laden wings could have fallen on the ground below. The towers could have survived.

It is thus seen that there may be a gap between what we profess to know and what we really know. In such a scenario, more specifically, in an environment of tall claims of more durable constructions with newer cement and rebars, engineers have not only taken time to recognize the problem of early distress in concrete structures, they have also failed to understand its causes, and in the meantime the problem grew bigger and bigger.

And the consequences have been grave

It is not only that concrete structures of recent construction develop early signs of distress (Figs. 2, 3 and 5 - 7), today’s new constructions, may exhibit signs of distress at the construction stage itself even when such constructions may be carried out with utmost care (Fig. 3).

The situation being as it is, it cannot be disputed that structures perform the way they do, structures show signs of distress as early as they do, because these are built to do so.

The way, even major structures (Fig. 10) are built, can be seen in Figs. 1 and 11 - 14.

Fig. 9 : A plane, wings and all, disappeared into one tower of World Trade Centre; another plane on its way to enter the other WTC tower

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Causes of Early Distress

The first course in reinforced concrete teaches one that rebars should be free from loose mill scale, rust, dust and oil. But the way construction is carried out under expert eyes (Fig. 3) is reflected in Figs. 1 and 11 to 14. And that is part of the reason why structures reach distressed states (Figs. 3 and 5-7) early in life.

If in attempts to mitigate the problem of corrosion the

Fig. 10 : An elevated viaduct under construction

Fig. 11 : Rebars stacked at site for construction of viaduct

Fig. 12 : Casing with corroded rebars at site to be concreted for viaduct

rebars will be coated, and if such coatings will even be fusion bonded following prescribed surface preparation, the result is likely to be any of those shown in Figs. 2, 3, 5 - 7 and 1511.

Is it possible that the cases depicted in Figs. 1-3, 5-7 and 15, could have arisen only because of a nonchalant approach to construction, as depicted in Figs. 1 and 11-14, or is it possible that inherent shortcomings in materials of construction, viz., cement and rebars of recent periods, could have a role ?

It cannot be denied that properties of cement and rebars have changed very considerably over the years.

Thus, in the context of durability of concrete structures, one may be on the wrong track if in the face of incontrovertible evidence of early distress in concrete structures of recent vintage one will continue to depend on one’s experiences with durable concrete structures of the past and treat today’s materials at par with materials of the past and today’s concrete structures at par with concrete structures of yesteryears in terms of durability. If one does, the result can be like that depicted in Fig. 2 or like any other in Figs. 3 and 5 -7.

So, if one will agree that something has gone wrong with today’s concrete structures, a question would arise : what could have gone wrong with today’s concrete construction ?

The effect of any possible design flaw or change in aggressiveness of the environment can be discounted in the context of inadequate durability of concrete structures for the reasons that

- the examples shown in Figs. 1-3, 5-7 and 10-15 have nothing to do with design flaws

Fig.13 : Corroded rebars to be covered in concrete for construction of viaduct

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- old and new constructions in the same city or even on the same plot of land present strikingly different scenarios in terms of time dependent performance; older constructions performing better.

Engineers have overlooked the devastating effects of using modern cement. One of many such effects can be seen in Fig. 3. It is worth recognizing here that, unlike in the case of rebars, which underwent very significant changes in their properties virtually overnight, the constitution of cement has undergone significant changes gradually over the years.

Similarly, even though a close look at the observations of CPWD8, Papadakis et at9 and Swamy10 would have revealed that the problem of early distress in reinforced

Fig. 14 :Rusted rebar in the construction of the structure in Fig. 3 where an outside agency monitors quality and the contractor is a top name in India - a stain on profession

Fig.15 : Concrete easily separates from epoxy coated rebars under vibra-tory loading conditions; all structures are required to resist vibratory loads due to earthquakes

concrete structures started with constructions in the 1960’s, when high strength rebars with surface deformations (HSD) started arriving in the market, engineers have overlooked the possible effects of using such rebars with surface deformations, whether cold twisted (CTD) or thermo mechanically treated (TMT), and the result is evident in (Figs. 5-7). Also, may be if the bars were not as rusted as evident in Fig. 14, may be the cracks in shear walls (Fig. 3) due to excessive shrinkage in concrete with high alkali cement could have been arrested.

The changes in cement and rebars are results of various permissive provisions in the IS 45612 and other codes and guides in India and in other countries.

Furthermore, it is or it should have been common knowledge that rebars and cement, supplied and supported with manufacturers’ certificates of compliance with codal require-ments, frequently fail to meet codal requirements (Fig. 16 and Table 1) when these are put to test at independent laboratories.

In the particular case of the rebars in Fig. 16, the manufacturer claimed conformance of the billets with requirements of the relevant BIS code at the time of supplying the rebars. When provided with failed pieces of rebars, the supplier (a leading manufacturer of rebars and other steel products) produced for the rebars a certificate of compliance with the relevant BIS code for rebars whereas tests by the author had shown that the bars did not meet the requirements of the specific code. Representatives of the steel maker visited the construction site and when bars of their choice were tested in their presence, they had no option but to agree that the bars of all the different sizes (8 mm to 32 mm dia) were unfit for construction.

A casual approach with cement

Except for the period of curing, the concrete code IS 456:200012 permits, without any differentiation, the use of ordinary Portland cement (OPC), Portland slag cement (PSC) and fly ash or Portland pozzolana cement (PPC), as if cement, capable of hydration, were inert; as if all types of cement were equal; as if any cement is quality cement.

It is known to all that tests for conformance of cement with the requirements, set in the relevant codes, are performed after cement will have been used in construction and that cement manufacturers routinely come up with test certificates declaring all cements to have met all requirements of codes. This is in the face of knowledge that cement, when tested at independent laboratories, is occasionally found to be deficient.

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And yet no one has ever heard of any cement manufacturer having ever found anything wrong with cement it had made and sold.

No one has ever heard of any cement manufacturer having ever dumped cement it had manufactured.

It is easily perceived that cement manufacturers enter into routine contracts with fossil fuel power plant authorities for the procurement of flyash. Neither this procurement of flyash is based on any check for suitability of the flyash as a raw material for the making of cement, nor has anyone ever heard of any cement manufacturer having dumped or disposed of any unsuitable fly ash.

Furthermore, it is most likely for Indian cements to fail tests for requirements on limits of water soluble alkali as set in the Plain and Reinforced Concrete Code of Practice, IS 456:200012. The code requires, as many other foreign codes do, that the equivalent Na2O content in cement must not exceed 0.60%. Knowing very well that Indian cements of recent periods might not meet this requirement, the requirements for a limit on the content of water soluble alkali in cement have been withdrawn from latest versions of codes of the Bureau of Indian Standards on cement.

Fig. 16 :Brittle failures in 8-32 mm dia cold twisted rebars with surface deformations - supplied by a leading manufacturer of rebars and other steel products in India

Table 1 gives test results on some cements of well known brands in India.

OPC in India today may contain highest amounts of harmful water soluble alkalis.

In this particular case % Na2Oeq is 2.37 which is much in excess of quantities that could be considered safe,

whare % Na2Oeq = % Na2O + 0.658 (% K2O)

In comparison, in the year 2000, 140 samples of OPC in the USA yielded on an average:

Na2O (%) by mass of cement.... 0.173

K2O(%) by mass of cement ..... 0.571

i.e., % Na2Oeq = 0.55, compared to 2.37 in the above sample of OPC in India.

Note :

OPC – ordinary portland cement

PSC – portland slag cement

PPC – portland pozzolana cement (generally, flyash cement)

One consequence of the withdrawal of limits on water soluble alkalis from Indian codes on cement can be seen in Fig. 3.

Thus, it cannot be suggested here that the satisfaction of codal requirements on cement in India would have automatically guaranteed concrete structures with long life spans.

A casual approach with rebars

Since corrosion in rebars is a problem in a majority of cases of reinforced concrete structures of recent vint-age (Figs. 5-7), rebars of recent vintage must have a role in the context of durability of such structures.

As in the case of cement, IS 456:200012 permits equally the use of all types of bars and other steel elements as if strength alone was all that mattered and as if these materials were inert even when it is evident that corrosion in such elements is the cause of early distress in most or many cases of concrete structures.

In essence, the code IS 456:200012 suggests that the susceptibility of the different reinforcing elements to corrosion need not be a consideration. Manufacturers of rebars, engineers and constructors got the cue and no one bats an eyelid when bridges and viaducts may be constructed with corroded rebars (Figs. 1, 11-14 and 17)

If all bars were or are indeed equally suitable for use as rebars, a few questions could arise, viz.,

Table 1. Contents of Cao and Soluble Alkalis in Typical Cements Available in Kolkata, Year 2006

Chemical Content OPC PSC PPC

CaO (%) by weight of cement 56.8 43.27 46.59

K2O (%) by weight of cement 1.03 0.89 0.86

Alkali as (Na2O)(%) by weight of cement 1.69 1.17 1.07

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- why have the cold twisted HSD (high strength with surface deformations) rebars of steel, e.g., torsteel and other HYSD (high yield strength with surface deformations) rebars of steel (Fig. 18) yielded to TMT bars (HSD also denotes HYSD)

- what was the need to develop TMT-CRS (TMT-Corrosion Resistant Steel) bars

- what was the need to develop TMT-HCR (TMT-High Corrosion Resistant) bars

- what was the need to use stainless steel rebars

- what prompted recent efforts at increasing the ductility of rebars and that too as a tokenism ?

And why was it considered necessary to galvanize HSD rebars or to provide epoxy coatings or other surface treat-ments to HSD rebars at extra cost ?

Or why was it considered necessary to both galvanize and provide epoxy coating on the same bar (Fig. 19) at large additional expenses, if it were not a recognition, if not an admission, that HSD rebars (Fig. 18), without additional aid, could not give durable concrete structures ?

Familiarity breeds contempt

The ease of handling rebars, the easy availability of the ingredients (irrespective of the quality) of concrete and the easy formability of such concrete, coupled with the casual approach of IS 456:200012 and such other codes, have encouraged many to take liberty (Figs. 1 and 11-14) with the construction of concrete structures, more particularly with reinforced concrete structures. The consequences can be seen in Figs. 2, 5-7 and 15.

The Problem with HSD Rebars

What could have gone wrong with HSD rebars ?

In the case of rebars, all the emphasis is on strength. To have strength is good (Fig. 20).

Fig. 17 : Bridge under construction with corroded rebars

Figure 19 shows an advertised rebar with surface deformations that was galvanized as well as provided with an epoxy coating. The advertiser is a leading manufacturer of rebars in the USA.

No suggestion is made here that the costly protection to rebars, as shown in Fig. 19, would make concrete structures, constructed with such rebars, any more durable than a concrete structure with uncoated rebars. The structure in Fig. 15, showing easy separation of concrete from epoxy coated rebars, is a case in point.

Fig.18 : Hot rolled - HYSD rebars of steel

Fig.19 : Rebar, which because of its susceptibility to corrosion is given double protection; but the result, specially under vibratory load conditions, may be no different than that shown in Figs. 2 and 15

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But in the case of rebars, the emphasis has been on brute strength (Fig. 21) and nothing else.

Similar to a false feeling that a muscular body will necessarily lead to a long life, it was and it continues to be overlooked in the case of concrete structures that high strength of rebars alone may not guarantee durability of reinforced concrete structures.

and 24. Bend locations and the cut ends of the rebars in Fig. 23 show that if the strains/stresses will reach or cross the yield strain or stress levels, the rate of corrosion will increase uncontrollably. Figure 24 demonstrates it more vividly. The cold twisted deformed bars (e.g. Torsteel), where the surface strains/stresses crossed the yield levels at the manufacturing stage, corroded all over the surface whereas pieces from the same bar, which were not twisted, developed minor signs of corrosion at the roots of some of the surface protrusions. All the pieces of the bars were from the same rod and these were stored bundled together inside a polybag inside a room for about a month before inspection.

The writer13-15 has explained why stresses, and thus strains, in HSD rebars inside concrete are more likely, than plain bars, to reach or cross yield strain/stress levels and thus corrode early, thereby leading to distressed conditions in concrete structures early.Fig. 20 : It is good to have strength Fig. 21 : The emphasis in the case

of rebars has been on brute strength alone and almost on nothing else

Stronger and stronger rebars were introduced and continue to be used. Token improvement in ductility is a belated thought.

Besides the differences in metallurgical compositions, today’s high strength rebars, compared to plain round bars of mild steel of yesteryears, are made to have surface deformations. Presumably, the need for the provision of surface deformations arose from the desire to lessen the anchor or development/overlap lengths of high strength bars (Fig. 18)

But such high strength rebars with surface deformations, compared to plain round bars of earlier periods, have greater susceptibility to corrosion13-15 (Figs. 1, 12-14). In fact, HSD rebars are so highly prone to corrosion that even epoxy coatings, which may protect steel plates and other elements, fail to protect such rebars with surface deformations (Fig. 22). While this writer13-15 has explained why the presence of lugs on the surface of HSD rebars make such bars highly susceptible to early corrosion, Alekseev16, citing Russian work, stated that the durability of concrete structures with HSD rebars of steel was an order of magnitude less than the durability of structures with plain round bars of steel. A very damning observation indeed on the performance of rebars with surface deformations

Besides the inherent or chronic susceptibility of HSD rebars to early and accelerated rates of corrosion, the problem becomes more acute if the stress levels will be higher, particularly if the strains /stresses will reach or cross the yield limits. Examples can be found in Figs. 23

Fig. 22 : Corrosion in HSD rebars shows through epoxy coating

Fig. 23 : Fresh TMT bars (strain/stress beyond yield at bends and cut ends invite corrosion) with surface deformations on way to delivery at site

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Fig. 24 : Fresh CTD bars (with strain/ stress beyond yield invite corrosion all over the surface) and untwisted bars with surface deformations

Simply stated, today’s high strength rebars with surface deformations (Figs. 18, 23 and 24), which are susceptible to early corrosion, cannot give durable concrete structures, at least not without costly pretreatment or use of corrosion inhibiting cement or without other aids, e.g., waterproofing treatment to the surface of the concrete structures.

Pretreatment to HSD rebars can be costly and cumbersome. Furthermore, any pretreatment, if provided at the construction site, may not be reliable, and the pretreatment can get damaged during construction, thereby encouraging corrosion, and nullifying any claimed benefits of coatings. A negative aspect of a particular type of pretreated HSD bars is shown in Fig. 15.

As at or beyond yield the surface of a rebar becomes unstable, nullifying thereby the benefits of any layer of passivation (as is provided by Ca(OH)2 inside concrete with the right type of cement) or protective coating and since surface elements of HSD rebars inside concrete structures are likely to reach yield stress/strain levels under service load conditions, tests for effectiveness of corrosion inhibiting cement need to duplicate this condition of the HSD test sample by making repeated excursion beyond the yield strain threshold with varying loads.

Thus, the state of reinforced concrete structures of recent decades, constructed with HSD rebars, begs for a solution.

The problem with today’s cement

Cement is an important ingredient of concrete and the constituents of cement have changed over the years.

The main oxide compounds in cement include C3S and C2S.

Over the period of the last five decades, the C3S/C2S ratios have increased from 0.3 to 3.0.17

A dramatic change indeed

It is known that C3S, responsible for early strength of concrete, readily reacts with water, producing greater heat of hydration at the initial stages after concreting.

Thus, the increase in C3S at the cost of C2S, though beneficial in countries with colder climates, is generally harmful for concrete structures in most areas of India.

In addition, alkali content in today’s cement in India is far too high (Table1) for concrete structures to be durable (Fig. 3).

For OPC in the USA, % Na2Oeq = 0.55 (average)17; for the particular sample of OPC (Table 1) in India, % Na2Oeq = 2.37, robbing OPC of its virtues of the capability to produce copious quantities of Ca(OH)2 and the ability to seal cracks through a process of autogenous healing.

In India, with Na2O at percentages of about 1.17 and 1.07, and with K2O at percentages of about 0.89 and 0.86, i.e. % Na2Oeq of 1.76 and 1.64 for PSC and PPC, respectively, alkali contents in slag and flyash cement too are pretty high (Table 1).

Indian cement is thus not likely to meet the standards in other countries. In fact, Indian cement may also not satisfy the limit of 0.60% which has been set in IS 456:200012 for water soluble alkali content in cement.

The increased fineness of cement, including high C3A, the dramatic rise in C3S/C2S ratio and the unacceptably high alkali content have proved to be detrimental to concrete and concrete constructions (Fig. 3).

Besides the type of consequences of using cement of the type, available in India today, as can be seen in Fig. 3, high alkali cement makes concrete thirsty for water. This absorbed water, when passed on to rebar regions, makes the rebars corrode early, thereby making structures distressed early unless such structures will have been provided with surface protection systems, i.e., unless such structures will have been given waterproofing treatment to prevent any ingress of water.

Causes of the Problem

CPWD8 Papadakis, et al9 and Swamy10 have expressed their findings on the lack of durability of concrete structures which were built during recent decades. Alekseev16 put it bluntly when he observed that the durability of concrete

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structures with HSD rebars of steel was an order of magnitude less than the durability of structures with plain round bars of steel.

Obviously there is something wrong with HSD rebars. The writer found out in constructing a building that whether it were rebars or cement or/bricks, the basic materials of construction would most likely be found to be unfit for (durable) concrete construction6.

On the basis of the preceding, more particularly on the basis of information provided in Figs. 1, 12-14, 16 and Table 1, some factors, which play important roles in causing early distress in concrete bridges, viaducts and other structures of recent periods, can be identified as

- the use of cold worked high strength rebars with surface deformations

- the use of high strength rebars with surface deformations- the use of cement with high C3S/C2S ratios- the permitting of excessive quantities of water soluble

alkalis in cement - the permitting of use of cement with high C3A contents- the making of the grain sizes of ordinary portland

cement too fine- the lowering of the period of curing of concrete

The Solution

The best solution to the problem of early distress in concrete structures ought to include taking a step backward and using plain round bars of mild steel as rebars and ordinary portland cement with properties matching those of OPC of periods prior to 1970, or even 1960, and curing concrete over longer periods of time as in the past.

Another solution, involving additional expenditure, would be to improve the chemistry of rebars so that such bars would be less susceptible to early corrosion.

Another solution would be the use of high strength rebars with a plain surface but a deformed axis (Fig. 25)15 that can lessen the propensity of rebars for corrosion and enhance the durability of concrete structures without any added cost.

Yet another solution is to provide surface protection to concrete structures so as to prevent a moist environment inside concrete and/or ingress of CO2 and O2, thereby preventing carbonation of concrete and corrosion in steel elements inside concrete.

If the surfaces of concrete structures, exposed to the atmosphere, would not be given surface protection by

waterproofing treatment, conditions like those depicted in Figs. 5-7 could happen, as unprotected concrete will absorb moisture and water, and corrosion prone HSD rebars will corrode.

Fig. 25 : C-bar with a plain surface but a deformed axis as a solution to the problem of early corrosion at no additional cost

If concrete bridges, viaducts and other structures are to be made durable, particularly if improved rebars will not be used, and cement of the right constitution will not be used, one must then provide surface protection to all exposed surfaces. In other words, waterproofing can be a solution to the problem of early corrosion in HSD rebars that can be invited by the use of high strength rebars with surface deformations and cement with high C3S/C2S ratios as well as large contents of water soluble alkalis.

Engineers are waking up to the reality of today’s concrete structures slowly but surely. They have started learning that there is a greater purpose to waterproofing than to arrest visible water leakages (Fig. 4).

The writer has written extensively during the last two decades advocating this concept of achieving durability of concrete bridges and other structures through surface protection in the form of waterproofing treatment. It has been suggested that similar to steel structures, concrete structures can benefit from surface protection.

The effectiveness of surface protection systems in making concrete structures durable is not merely a theoretical proposition. This writer is privy to many structures where the worsening of structural conditions appeared to have stopped after the provision of effective waterproofing treatment.

In today’s scenario, any failure to take steps to prevent the ingress of water or moisture or alternatively carbon dioxide and oxygen through the provision of protective treatment on the surface of concrete structures can thus be a folly.

Today, Central Public Works Department, Bureau of Indian

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Standards, Indian Roads Congress and Indian Railways are among the organisations who have agreed and issued their guides, standards and circulars on enhancement of the life span of concrete structures through the provision of surface protection systems in the form of waterproofing treatment. The Bureau of Indian Standards too recognized the effectiveness of surface protection systems in enhancing the life span of concrete structures.12 Unfortunately, the engineering community appears to have overlooked the requirements set in these documents, and concrete bridges and viaducts, old and new, remain largely unprotected.

Waterproofing of Bridges & Viaducts

As in the case of steel bridges, concrete bridges and their supports require protection against corrosion in the steel elements.

Indian Railways and several other organisations have adopted specifications for specific systems of waterproofing which were developed by the writer. Figures 26-28 show three such items of waterproofing of concrete bridges which were developed by the writer. The detailed specifications can be found in Ref. 18.

The three specifications cover most cases of surface protection required for reinforced concrete bridges and viaducts.

The details in Figs 26-28, originally developed by the writer, relate to specification Nos. 22.14.1, 22.14.2 and 22.14.3 in the Indian Railways Unified Standard Specifications (Works & Materials) & S.O.R.18 A couple of typical bridges, given

Fig. 26 : Waterproofing treatment for substructure below water line and for sidewalks

Fig. 27 : Waterproofing treatment for superstructure and substructure above water line

Fig. 28 : Waterproofing treatment for bridge deck

surface protection in line with specifications as detailed in Figs. 26 and 27, are shown in Figs. 29 and 30.

Fig. 29 : Katakhali Bridge over the Goureswar River at Barunhat on Hasnabad-Hingalganj Road, North 24 Parganas, West Bengal, protected as in Figs. 26 and 27

Fig. 30: Bankim Setu (Buckland Bridge) over the platforms at Howrah Station, protected as in Fig.27

Concluding Remarks

Compared to concrete bridges and viaducts of earlier periods, such structures, built during recent decades, have suffered from early decay and distress.

In most cases, this decay is due to corrosion in high strength steel rebars with surface deformations. Prominent signs of corrosion in such rebars can be seen even before concreting.

Inside concrete, because of very high strains and stresses, rebars with surface deformations, compared to rebars without surface deformations, can fall prey to corrosion more easily or earlier.

The constituents of today’s cement, with high C3S/C2S ratios, high C3A and large alkali contents, make today’s concrete less capable of protecting steel rebars than concrete with cement of earlier periods was. Today’s concrete develops more cracks and such concrete also has greater capacity to absorb water.

The greater rate and depth of penetration of the carbonation front in concrete with today’s blended cement make rebars in such concrete more vulnerable to corrosion compared to rebars in concrete with OPC of earlier periods when alkali contents in cement were limited and C3S/C2S ratios were low.

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As solutions to the problem of early distress in today’s reinforced concrete structures several options are suggested in the following. Implementation of any one or more of the steps will lead to an enhancement in the life span of bridges, viaducts and other concrete structures.

- using plain round bars of mild steel as rebars and OPC having constituents as in the pre nineteen sixty era

- Using rebars of steel with improved chemistry/metallurgy that would be more resistant to corrosion

- Using C-bars (whether of low, medium or high yield strength) with plain surface and deformed axis as rebars instead of rebars with surface deformations

- Providing surface protection in the form of waterproofing treatment to all exposed surfaces of concrete structures instead of providing any coatings to rebars

- Improving the quality of concrete through the addition of admixtures, using low water cement ratios, achieving greater compaction and lengthening the period of moist curing.

The importance of waterproofing systems in making concrete structures durable can be found in Refs. 5, 7, 8, 12 and 17. Reference 17 suggests, “If water can be kept out of the system after curing, then the potential for durability related problems will be reduced markedly.” whereas Ref. 12 recommends, “The life of the structures can be lengthened by providing extra cover to steel, by chamfering the corners or by using circular cross-sections or by using surface coatings which prevent or reduce the ingress of water, carbon dioxide or aggressive chemicals.

References

1. Kar, Anil K., “The Ills of Today’s Cement and Concrete Structures”, paper No. 534; Journal of the Indian Roads Congress, Vol. 68, Part 2, July-September, 2007, New Delhi, www.irc.org.in.

2. Kar, Anil K., “Woe Betide Today’s Concrete Structures”, New Building Materials & Construction World; New Delhi, Vol. 13, Issue-8, February, 2008, New Delhi; and “Woe Betide Today’s Concrete Structures Part II”, New Building Materials & Construction World, Vol. 13, Issue-9, March, 2008, New Delhi, www.nbmcw.com.

3. Kar, Anil K., “Concrete Structures : A Tale of Reverse Technology”, RITES Journal, RITES Ltd., Vol. 10, Issue 2, July, 2008, New Delhi.

4. Kar, Anil K., and Vij, Satish K., “Yearnings of a Reinforced Concrete Structure”, New Building Materials & Construction World; New Delhi; Vol. 14, Issue-12, June, 2009, www.nbmcw.com.

5. Kar, Anil K., and Vij, Satish K., “Enhancing the Life Span of

Concrete Bridges,” New Building Materials & Construction World, Vol. 15, Issue 6, December 2009, New Delhi, www.nbmcw.com.

6. Kar, Anil K., “Construction Materials : Products of Our Education”, New Building Materials & Construction World; New Delhi; Vol. 15, Issue-8, February, 2010, www.nbmcw.com.

7. Kar, Anil K., and Vij, Satish K., “Waterproofing of Structures for Durability”, New Building Materials & Construction World; New Delhi; Vol. 15, Issue-10, April, 2010, www.nbmcw.com.

8. Durability of Concrete Construction, Technical Circular 1/99, Central Designs Organisation, Central Public Works Department, Government of India, No. CDO/SE(D)/G-29 dated 18.02.1999, New Delhi.

9. Papadakis, V. G., Vayenas, C. G., and Fardis, M. N., “Physical and Chemical Characteristics Affecting the Durability of Concrete,” ACI Materials Journal, American Concrete Institute, March - April, 1991.

10. Swamy, R. N., “Infrastructure Regeneration : the Challenge of Climate Change and Sustainability - Design for Strength or Durability,” The Indian Concrete Journal, The ACC Ltd., Vol. 81, No. 7, July 2007, Mumbai, www.icjonline.com.

11. Kar, Anil K., “FBEC rebars must not be used,” The Indian Concrete Journal, ACC Ltd. Vol. 78, No. 1, January 2004, pp. 56-62, www.icjonline.com.

12. IS 456:2000 “Indian Standard Plain and Reinforced Concrete, Fourth Revision,” Bureau of Indian Standards, New Delhi, July 2000.

13. Kar, Anil K., “Concrete Structures - the pH Potential of Cement and Deformed Reinforcing Bars”, Journal of the Institution of Engineers (India), Civil Engineering Division, Vol. 82, June 2001, Calcutta, www.ieindia.info.

14. Kar, Anil K., “Deformed Reinforcing Bars and Early Distress in Concrete Structures”, Highway Research Bulletin, No. 65, Indian Roads Congress, December 2001, New Delhi, www.irc.org.in.

15 Kar, Anil K., “A Rebar for Durable Concrete Construction”, The Masterbuilder; Vol. 13, No. 7, Chennai; July, 2011, www.masterbuilder.co.in.

16. Alekseev, S. N., “Corrosion of Steel Reinforcement,” Chapter 7 in Moskvin, V. (edited by), translated from the original by V. Kolykhmatov, “Concrete and Reinforced Concrete Deterioration and Protection,” 1990, English translation, Oxford & IBH Publishing Co. Pvt. Ltd. New Delhi, 1993, original Mir Publishers, Moscow 1990.

17. Johansen, Vagn C., et al, “Effect of Cement Characteristics on Concrete Properties, Portland Cement Association”, Skokie, Illinois, USA, 2005

18. “Indian Railways Unified Standard Specifications (Works & Materials) & S.O.R.,” Indian Railway Board, Ministry of Railways, Government of India, 2010.

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Effect of Concrete Temperature andFormwork Width on Variation PressureFormwork of Self-compacting Concrete

1Amir Hosein Bakhtiarain,2Morteza Askari

1The Bsc. Student of Islamic Azad University,2The faculty member of Islamic Azad University

The design of formwork systems for vertically cast elements

is controlled by the lateral pressure developed by the

flesh concrete. It is well established that concrete

consistency, method of placement, consolidation, type

of cement, temperature of concrete, maximum aggregate

size, head of concrete, pore water pressure, rate of

placement, and size and shape of the formwork have all

marked effect on the development of lateral pressure

[3-6-9].

Maxton (from Rodin [9]) studied the coupled effect of the

casting rate and concrete temperature on the lateral

pressure envelope for conventional concrete. Different

series of low-slump concrete mixtures placed at casting

rates varying between 0.6 and 2 m/h were investigated.

The concrete temperature varied from 4.5 to 27°C.

Maximum lateral pressure was found to increase with the

increase in the casting rate and/or decrease in concrete

temperature. Irrespective of the tested parameters, the

pressure envelope was reported to be hydrostatic from

the free surface to a certain maximum value, and then

remained constant until the bottom of the formwork.

For formwork design purposes, ACI Committee 622 [2]

proposed the following design equations for column and

wall elements, both of which take into account the rate of

casting and concrete temperature: For columns:

In this article two complete programs about effect of concrete temperature, formwork width, on lateral pressure formwork of Self-Compacting Concrete are discussed. For considering effect of concrete temperature concrete mixtures which are built under 10-30°c , are used and the result show that concrete temperature hasn't considerable effect on initial pressure (after casting finishing).But in time passing, pressure reduction is significant for surveying in formwork width effect, two columns with 200 and 920mmdiameter, are applied

For walls:

Where Pmax

: maximum lateral pressure, KPa

R: rate of casting, m/h

T: concrete temperature, °C

H: head of concrete, m

Effect of concrete temperature on formwork pressure

For investigation of effect of concrete temperature on

lateral formwork pressure, experimental research of Assad

[7] and his colleagues was used and described those

below:

Materials

The ternary cement contained 6% silica fume, 22% fly

ash, and 72% CSA Type 10 cement. The Type 30 cement,

Type 10 cement, and fly ash had blaine specific surface

values of 600, 325, and 410m2/kg, respectively. The silica

fume had a B.E.T specific surface of 20,250m2/kg.

Continuously graded crushed limestone aggregate with

nominal size of 10mm and well-graded siliceous sand

Form Work Pressure

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

were employed. The coarse aggregate and sand had

fineness module of 6.4 and 2.5, bulk specific gravities of

2.71 and 2.69, and absorption values of 0.4% and 1.2%,

respectively. Polycarboxylate-based high-range water-

reducing admixture (HRWRA) of 1.1 specific gravity and

27% solid content was used. A high molecular weight

cellulosic-based material was employed for the VEA to

enhance stability of mixtures proportioned with 0.40 w/cm.

Mixture proportion

As summarized in Table 1, the investigated mixtures were

prepared with 450 kg/m3 of binder content and w/cm of

0.40.The effect of concrete temperature on lateral pressure

variations was evaluated by testing mixtures prepared at

10, 20, and 30 ± 2°C for the TER-10, TER-20, and TER-30

mixtures, respectively. Ambient temperatures during the

sampling and testing were 14, 20, and 27°C, respectively,

to minimize heat loss of the tested concrete. The effect of

using Type 30 cement and set accelerating admixture on

the variations in lateral pressure was investigated, as they

have marked effect on the rate of cement hydration. The

dosage of the set accelerator was set at 1000 mL/100 kg

of binder. The T30-20 and TER-20-ACC mixtures prepared

with Type30 cement and set accelerating admixture,

respectively, were proportioned at 20 ± 2°C and tested

at 20°C ambient temperature. The VEA dosage was fixed

at 260 mL/100 kg of binder, and the sand-to-total

aggregate ratio remained constant at 0.46 for all tested

mixtures. The HRWRA and AEA concentrations were

adjusted to secure initial slump flow of 650 ± 15mm and

air content of 6 ± 2%.

Instrumented column systems

Two experimental columns were used to determine the

lateral pressure exerted by plastic concrete. The first

column measures 2800mm in height and 200mm in

diameter, and was used to evaluate pressure variations of

the plastic concrete. The lateral pressure was determined

using five pressure sensors mounted at 50, 250, 450, 850,

and 1550mm from the base. In order to enable the

evaluation of pressure variation up to the hardening of

the concrete, a shorter column measuring 1100mm in

height and 200mm in diameter was used. Three pressure

sensors similar to those employed in the former column

were mounted at 50, 250, and 450mm from the base.

Both experimental columns were made of PVC with a

smooth inner face to minimize friction with the concrete.

Fabrication and testing program

The slump flow, concrete temperature, unit weight, air

volume, L-box flow characteristics, surface settlement,

and setting time were determined, and the results are

summarized in Table 2.

Table 2. Properties of evaluated SCC mixtures

Fresh concrete properties

All SCC mixtures had L-box blocking ratios (h2/h1) greater

than 0.80 indicating adequate passing ability, and

relatively low surface settlement (<0.5%). Surface

settlement values are shown to decrease with the increase

in the initial concrete temperature. The maximum surface

settlement decreased from 0.48% to 0.34% and 0.32%

for the TER-10, TER-20, and TER-30 mixtures cast at

approximate temperatures of 10, 22, and 30°C,

respectively. The use of high early strength cement and

set-accelerator are also shown to enhance the static

stability of the plastic concrete. The mixtures prepared

with set-accelerating admixture and Type 30 cement

exhibited settlement values of 0.29% and 0.15%,

respectively.

Lateral pressure envelope with respect to height

A typical diagram showing the distribution of lateral

Pressure along the 2800-mm high experimental column

Slump flow, mm

Air content, %

Initial concrete

temperature, .C

Unit weight, kg/m3

h2/h1 of L-box test

Surface settlement, %

Initial set time, min

Final set time, min

TER-10

655

6.5

9.5

2230

0.84

0.48

690

780

TER-20

655

4.3

21.7

2265

0.81

0.34

610

705

TER-30

645

5.9

30.1

2190

0.85

0.32

585

660

TER-20-

ACC

645

4.5

20.8

2315

0.82

0.29

440

480

T30-20

640

6.2

21.7

2335

0.85

0.15

425

470

tempera-

ture, .C

10

20

30

20

20

Mixture*

codification

TER-10

TER-20

TER-30

TER20-ACC

T30-20

Ternary

cement,

kg/m3

450

450

450

450

-

Type 30

cement,

kg/m3

-

-

-

-

450

Water,

kg/m3 (w/

cm = 0.40)

180

180

180

180

180

Sand (0-

5 mm),

kg/m3

740

740

740

740

740

Coarse

aggregate,

(5-10 mm),

kg/m3

870

870

870

870

900

VEA, mL/

100 kg of

cement

260

260

260

260

260

Set-

accelerator,

mL/100 kg

of cement

-

-

-

1000

-

HRWRA L/

m3

3.8

3.8

3.9

3.7

3.3

AEA, mL/ 100

kg of cement

120

120

120

135

170

Table 1. Mixture proportions of evaluated SCC

Form Work Pressure

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

for the TER-30 mixture is given in Fig.1. The slump flow

values noted at various times are also indicated. Right

after casting, the concrete is shown to develop lateral

pressure close to the theoretical hydrostatic pressure. The

hydrostatic pressure (Phyd

) is calculated as: Phyd

= ρ × g ×

H; where ρ, g, and H refer to the concrete unit weight,

gravity constant, and head of concrete in the formwork,

respectively. The relative pressures compared to Phyd

at

the base of the column determined at end of casting and

then after 1, 2, and 3 hours were 91%, 77%, 68%, and 61%

respectively.

Fig. 1: Variations of lateral pressure envelope with time for the TER-30 mixture

Fig. 2: Effect of concrete temperature, cement Type 30, and use of set-

accelerating admixture on pressure variations determined at the bottom of the

2800-mm high column

Effect of concrete temperature on variations in lateralpressure

Variations of the P(maximum)/P(hydrostatic) values

measured along the 2800-mm column of the five SCC

mixtures placed at 10 m/h are plotted in Fig.2. Slump

values determined at the end of pressure monitoring are

noted. Mixtures prepared with ternary cement at initial

temperatures of 10, 22, and 30°C develop similar relative

pressures of 91% at the end of casting. This indicates

that concrete temperature has no significant effect on the

development of initial pressure. The maximum initial

pressure is rather affected by the degree of internal friction

that depends on the coarse aggregate volume and

mixture consistency. On the other hand, the rate of

pressure drop with time is significantly affected by

concrete temperature. For example, the time to reduce

the relative pressure by 25% decreased from 400 to 250

and 160 minutes for the TER-10, TER-20, and TER-30

mixtures, respectively.

Alexandridis and Gardner [1] reported that concrete cast

at higher initial temperature can exhibit higher cohesion

through the formation of a gel structure. This can enable

the plastic concrete to develop higher shear strength

capable of carrying a greater fraction of the vertical load,

thus resulting in increased rate of pressure drop with time.

It is important to note that higher initial temperature can

result in greater rate of loss in slump flow consistency,

thus reducing the degree of lateral pressure. For example,

slump values of 170 and 180mm were measured 5 and

3.5 hours after casting for the TER-10 and TER-30

mixtures, respectively.

The T30-20 and TER-20-ACC mixtures developed the

lowest initial relative pressures of 78% and 83%,

respectively, compared to 91% for those cast at 10 to

30°C initial temperatures and placed at similar casting

rates of 10 m/h (Fig. 2). The incorporation of set-

accelerating admixture in the TER-20-ACC mixture

resulted in the highest rate of pressure drop with time;

the elapsed period required to reduce the relative

pressure by 25% was 88 minutes. The increased rate of

cement hydration due to the incorporation of set-

accelerating admixture can lead to greater cohesiveness,

and hence sharper rate of drop in lateral pressure.[4]

Effect of section width on formwork pressure

For investigation of effect of concrete temperature onlateral formwork pressure, experimental research of

Khayat[8] and his colleagues was used and described

those below:

Materials

A ternary cement made with approximately 6% silicafume, 22% Class F fly ash, and 72% Type 10 cement was

used. A continuously graded crushed limestoneaggregate with nominal size of 10 mm and well-gradedsiliceous sand were employed. The sand had a fineness

modulus of 2.5. The bulk specific gravities of the aggregateand sand were 2.72 and 2.69, and their absorptions were0.4% and 1.2%, respectively. A naphthalene-based high-

range water reducer (HRWR) with solid content of 41%

Form Work Pressure

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

and specific gravity of 1.21 was used. A liquid-based

polysaccharide was used for the viscosity-modifying

admixture (VMA) to enhance stability of the plastic

concrete. A synthetic detergent-based air-entraining

admixture (AEA) and a carboxylic acid-based water-

reducing admixture were incorporated.

Mixture proportion

For the SCC mixture used in this study, a proven mixture

prepared using 490 kg/m3 of binder, 0.38 w/cm, and 0.44

sand to-coarse aggregate ratio was used. The VMA was

incorporated at a dosage of 1325 mL/100 kg of water,

and the HRWR dosage was adjusted at 6 L/m3 to secure

initial slump flow of 650 mm. A dosage of 150 mL/100 kg

of cementitious materials of the AEA was used. The unit

weight and air content were 2280 kg/m 3 and 6.1%,

respectively.

Instrumented formworks

As already mentioned, two experimental formworks wereused. The first measured 2100 mm in height and 200 mm

in diameter. The PVC tube had a wall thickness of 10 mmand a smooth inner face to minimize friction during andafter concrete placement. The stress in the diaphragm

caused by concrete lateral pressure was determinedusing five pressure sensors mounted at 850, 1250, 1650,1850, and 2050 nun from the top. The monitoring ofpressure distribution was stopped once the concrete had

an approximate slump consistency of 100 mm. Thesecond column consisted of a sonotube of 3600 mm inheight and 920 mm in diameter. The column was

adequately braced and reinforced. The lateral pressurewas determined using two pressure sensors located at2050 and 2880 mm from the top.

The monitoring of pressure distribution was stopped oncethe concrete had an approximate slump consistency of

100 mm. The second column consisted of a sonotube of3600 mm in height and 920 mm in diameter. The columnwas adequately braced and reinforced. The lateral

pressure was determined using two pressure sensorslocated at 2050 and 2880 mm from the top. In this case,the lateral pressure was monitored until the hardening of

the concrete.

Fabrication and testing program

Ready-mixed concrete was delivered to the experimental

site. The ambient and concrete temperatures were 16

and 19°C respectively. The slump flow, air content, JRing

and Lbox flow characteristics, and surface settlement were

determined for the SCC. The measurement corresponds

to the mean diameter of the spread concrete at the end of

flow. The JRing spread values was 600 mm and for Lbox

test the measure was 0.81 and maximum surface

settlement was 0.34%.

The concrete was directly discharged from the mixing

truck into the formwork from the top at the desired pouring

rate without stoppage or vibration. In the case of the 3600-

ram high column, the concrete was placed at a rate of

rise of 10m/hr. For the 2100-ram high column, the formwork

pressure was evaluated twice; once using a rate of

placement of 10m/hr and then at 25 m/hr for a second

column. The slump flow values determined upon the

arrival on site of the concrete and after 1 and 2 hours were

650, 635, and 450 mm, respectively. After 3 and 3.5 hours,

slump consistencies of 180 and 65 mm were measured,

respectively.

The initial and final setting times were determined in the

laboratory at 20°C in compliance with ASTM C403 and

are given in Fig.3. The adiabatic temperature was also

evaluated in an adiabatic calorimeter on mortar obtained

by sieving fresh concrete through a 4.75-mm sieve. The

heat evolved was determined by deriving the temperature

rise as a function of time. The time between the initial

contact of cement with water and that corresponding to

the beginning of the acceleration of temperature rise was

6 hours, as also shown in Fig.3.

Fig 3: Variations of hydration and stiffening kinetics with time

Lateral pressure variations

The variations of the lateral pressure envelope determined

on the 2100-ram high column along with the consistency

are plotted in Fig.4. Immediately after filling the formwork,

the concrete is shown to act as a fluid exerting almost

hydrostatic head. However, a gradual decrease in lateral

pressure takes place with time. The relative pressures at

the base of the column determined initially and after 1, 2

and 3 hours were 98%, 89%, 83% and 76% of hydrostatic

pressure respectively.

Results of the section width Influence on formwork

The effect of column diameter (200 vs. 920 mm) on

changes in lateral pressure is illustrated in Fig.5 by plotting

the variations of the P(measured)/P(hydrostatic) values

calculated at 2050 mm from the top of the formworks as a

function of time. It is important to mention that both

columns were cast on the job site at the same casting

Form Work Pressure

The Masterbuilder - July 2012 • www.masterbuilder.co.in146

rate of 10 m/hr. Initially, the mixture placed in the larger

column exhibited slightly greater pressure of 99% of

hydrostatic pressure compared to 96% for the 200-mm

diameter column. However, the rate of drop in pressure

was significantly different. In the case of the former

concrete placed in the 920-mm diameter column, the

time required to reduce lateral pressure by 5% of the

hydrostatic value was 20 minutes, resulting in a slope of

5.3 kPa/hr. Conversely, for the 200-mm diameter column,

this period was 38 minutes resulting in a slope of 3.3kPa/

hr. In general, the rate of drop in lateral pressure of plastic

concrete depends on the degree of thixotropy or shear

recovery [9]. This phenomenon causes a build-up of the

structure and an increase in cohesiveness soon after the

Fig 4: Variations of hydration and stiffening kinetics with time

material is left standing at rest without any shearing action.

In the case of the 200-mm diameter column, the arching

effect can be relatively more pronounced than that

resulting from the 920-ram diameter column.

Conclusions

- Variations in fresh concrete temperature have limited

effect on the maximum lateral pressure developed by

SCC at the time of casting. However, the rate of

pressure drop with time increases with the concrete

temperature that promotes faster development of

cohesion.

- The use of Type 30 cement or set-accelerating

admixture can lead to 10% reduction in the initial

pressure and accelerate the rate of pressure drop by

two folds compared to similar concrete prepared with

a ternary cement.

- The scale effect had an influence on the rate of drop in

lateral pressure of SCC with time; however, no

appreciable difference in the maximum initial pressure

was observed.

- Immediately after casting, the SCC placed in the 200-

ram diameter column was found to exert slightly less

pressure than that cast in the 920-ram column. This

can be due to an arching effect in the relatively

restricted section.

References

[1] ACI Committee 347 (2001) "Guide to formwork for concrete",

Farmington Hills, 32.

[2] ACI Committee 622 (1958)" Pressures on formwork", ACI Journal,

Proceedings, and 55(2):173-190.

[3] ACI Committee 622, "Pressures on formwork", ACI

Journal,Proceedings, 55 (2) (1958) 173-190.

[4] Assaad J, Khayat KH, Mesbah H (2003) "Variation of formwork

pressure with thixotropy of self-consolidating concrete." ACI

Materials Journal, 100(1):29-37.

[5] Bartos, P.J.M., "An appraisal of the orimet test as a method for

on-site assessment of fresh SCC concrete", Int. Workshop on

Self-Compacting Concrete, Japan, (1998) 121-135.

[6] Gardner, N.J. and Ho, P.T.-J., "Lateral pressure of fresh concrete",

ACI Journal, Technical Paper, Title No. 76-35 (1979) 809-820.

[7] Joseph J. Assaad · Kamal H. Khayat "Effect of casting rate and

concrete temperature on formwork pressure of self-consolidating

concrete",Rilem Materials and Structures (2006) 39:333-341

[8] K. Khayat, J. Assaad, H. Mesbah, and M. Lessard "Effect of

section width and casting rate on variations of formwork pressure

of self-consolidating concrete ", Rilem Materials and Structures

38 (January-February 2005) 73-78

[9] Rodin, S., "Pressure of concrete on formwork", Proceedings

Institution of Civil Engineers (London) 1 Part 1 (6) (1952) 709-

746.Fig. 5: Effect of the section width on lateral pressure

Form Work Pressure

168 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Rice Husk Ash – An Ideal Admixture for Concrete in Aggressive Environments

Rice husk is the outer cover of paddy and accounts for 20-25 % of its weight”. It is removed during rice milling and is used mainly as fuel for heating in Indian

homes and industries. Its heating value of 13-15 MJ/kg1.2 is lower than most woody biomass fuels. However, it is extensively used in rural India because of its widespread availability and relatively low cost. The annual generation of rice husk in India is 18-22 million tons and this corresponds to a power generation potential of 1200 MW4. A few rice husk-based power plants with capacities between I and 10 MW are already in operation and these are based either on direct combustion or through fluidised bed combustion.

Rice husk is characterised by low bulk density and high ash content (18-22% by weight). The large amount of ash generated during combustion has to be continuously re-moved for a smooth operation of the system. Silicon oxide forms the main component (90-97%) of the ash with trace amounts 2.7 of CaO, MgO, K2O ) and Na2O.

Manufacture

Rice husk ash is produced by burning the outer shell of the paddy that comes out as a waste product during milling of rice. Since they are bulky disposal of husk present an enormous problem. Each ton of paddy produces about 200kg of husk and this rice husk can be effectively converted through controlled burning. At around 500ºC a valuable siliceous product that can enhance the durability of concrete in the chemical composition of rice husk ash is

obtained. Variations in the burning temperature much above or below will drastically alter the silica content of the ash. It is estimated that one fifth of the five hundred million tons of world annual paddy production is available as rice husks. Only a small quantity of rice husk is used in agricultural field as a fertilizer, or as bedding etc. and stabilisation of black cotton soils.

The manufacture and batching of rice husk ash involves bulk handling of a light raw material and proper and a con-trolled burning methodology has to be adopted. Grinding of the ash is done after necessary cooling and can be done to any desired fineness. The author manufactures RHA and adopts a fineness value of around 4200blaine.There is another difficulty in the manufacturing of RHA. Namely burning of the raw husk to a high temperature for a sustained period makes it extremely difficult to cool the ash to normal temperature. This is also compounded by the inherent nature of raw husk to retain heat for a considerably long time. Therefore the method adopted is to allow the burnt husk to stay for some time and subsequently cool with water. However, when this is done the Ash is saturated with moisture and therefore grinding becomes a challenging task-especially with an abrasive material like RHA. Therefore drying of RHA is a must. Among the several methods that are possible normal sun drying and / or drying using paddy driers are the cheapest options.

Another point to be borne in mind is the variation in the raw material composition from different sources and therefore

Dr. R N Krishna, Ph.D (Civil Engg.)Concrete Technologist, Ex.Secretary General, Indian Concrete Institute, India. Proprietor, KC Contech – Manufacturer of Rice Husk Ash

The use of durability enhancing mineral admixtures or supplementary cementing materials has gained considerable importance the last decade or so as a key to long service life of concrete structures1. There are many mineral admixtures that are used in way through out the world but rice husk ash stands out as an eco-friendly, sustainable and durable option for concrete. This paper attempts to bring out the effectiveness of rice husk ash as a versatile concrete admixture and discusses some versatile application of rice husk ash concrete.

Concrete Admixtures

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

the material has to be tested for chemical composition.

However the durability enhancement properties of rice husk ash when blended with cement makes it the most eco-friendly versatile supplementary cementing material to concrete. The following properties of concrete are con-siderably altered when blended with RHA :

- Reduced heat of hydration – leading to minimal crack formation in higher grades of concrete.

- Reduced permeability at higher dosages.- Increased chloride and sulphate resistance/mild acids.

Therefore RHA can be used as an effective and green supplementary cementing material. RHA can be used for a wide variety of applications starting from a simple water proof coating to an admixture for cement to resist a wide variety of chemicals including mild acids like lactic acid(milk) alkalies, etc. in bathroom floors, swimming pools, industrial factory floorings, foundation concreting when concrete is exposed to both chlorides and sulphate attack and as an effective repair mortar to resist chlorides.

A small study was conducted to assess the chloride re-sistance of RHA concrete and proves that RHA can be used as an effective corrosion inhibitor in concrete:

An ideal corrosion inhibitor has been defined as a chemical compound, which, when added in adequate amounts to concrete, can prevent corrosion of embedded steel and has no adverse effect on the properties of concrete. Corrosion inhibitor admixtures are used to delay or retard corrosion of reinforcing steel in concrete.

In this part of the study, corrosion resistance of Ordinary Portland Cement concrete with commercially available zinc rich epoxy coatings for reinforcing bars was studied and compared with RCC specimens using rice husk ash. The commercially available rice husk ash (Hyper 2000 manu-factured by the author) was used for the study to prove its efficacy as an effective corrosion inhibitor. Details of chemical composition are given in Table 1 for three different samples from different places in the country.

M 30 and M 35 grades of concrete with Ordinary Portland Cement (OPC) and Rice Husk Ash Cement (replacing OPC 30% by weight) have been used. Polymer based coating and Zinc rich coating were the two different coatings applied on the surface of the steel rebars.

Chemical Composition

Preparation of Specimens

Beam specimens of section 100 mm x 150 mm and length of 1000 mm were selected. For the beam specimens, main reinforcement of 2 numbers of 10 mm diameter, hanger

reinforcement of 2 numbers of 8 mm diameter and shear reinforcement of 6 mm diameter two legged stirrups of HYSD bars were used. The surface of rebars was derusted using wire brush and rust clear solution in order to remove any loose rust particles. The coating material was applied over the surface of the rebars with a brush and was allowed for curing. The initial weight of the rebars was taken and then the rebars were tied together to form the skeleton reinforcement cage. Twelve numbers of beam specimens were cast during M30 and M35 grades of concrete. After curing them for a period of 28 days, the specimens were transferred to Fiber Reinforced Plastic (FRP) tanks and were subjected to accelerated corrosion process.

Sample Marked as : 1 2 3

(Percent By Mass)

Loss on ignition 8.71 7.00 3.07

Silica (as SiO2) 83.60 84.00 90.47

Aluminium (as Al2O3) 3.05 3.84 3.13

Iron (as Fe2O3) 1.10 0.60 0.32

Titanium (as TiO2) Nil Nil Nil

Calcium (as CaO) 1.80 2.85 1.96

Magnesium (as MgO) 1.28 1.35 0.35

Sodium (as Na2O) 0.17 0.13 0.22

Potassium (as K2O) 0.29 0.23 0.48

Table : 1 Sample : Rice Husk Ash

Acceleration of Corrosion Process

Galvanostatic method was used to accelerate the corrosion process. In this method, the embedded steel acts as anode and an external stainless steel plate acts as cathode. The beam specimens were impressed a selected current in-tensity under low voltage conditions. The specimens and the stainless steel plates were immersed in an electrolyte solution of three-percentage sodium chloride concentration to simulate the conditions of seawater. The current was applied using a regulated D.C. Rectifier. The accelerated corrosion process was carried out for a period of fifteen days. Thereafter, concrete samples from the cover portion near main rebars were collected to determine the pH value and chloride content. Then the beam specimens were broken and the rebar grid was taken out separately, from which the main rebars were separated. Then the rods were cleaned and wiped. The weight of rebars was determined to estimate the weight loss. (Table 2).

Discussion of Test Results

For the M30 grade contol concrete with RHA, the beam specimen with uncoated rebars showed 50% less loss of weight than that of the similar control concrete specimens with OPC. The same trend was observed for specimens

Concrete Admixtures

170 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Fig. 1 - SEM of OPC Concrete

of M35 grade of concrete also. The beam specimen with uncoated rebars showed loss of weight of 41 % less than that of the specimens with OPC for M35 grade.

For coated rebars with and without rice husk ash concrete, it is observed that the use of rice husk ash contributes to a significant reduction in weight loss when compared to OPC concrete with the same coatings.

Beam Designation Initial Weight (gms)

Final Weight (gms)

Loss of Weight (gms)

Loss in Percentage

M30 Grade OPC Concrete

M30 OPC control 1086.9 942.4 144.5 13.29

M30 OPC Z* 1108.6 1062.0 46.6 4.20

M30 OPC IP** 1062.3 1047.5 14.8 1.39

M30 Grade RHA Blended Concrete

M30 RH control 1063.5 993.4 70.1 6.59

M30 RH Z* 1092.7 1051.0 41.7 3.82

M30 RH IP** 1073.5 1065.8 7.7 0.72

M35 Grade OPC Concrete

M35 OPC control 1086.7 1004.2 82.5 7.59

M35 OPC Z* 1105.7 1089.9 15.8 1.43

M35 OPC IP** 1075.6 1063.3 12.3 1.14

M35 Grade RHA Blended Concrete

M35 RH control 1065.5 1017.4 48.1 4.51

M35 RH Z* 1091.5 1076.6 14.9 1.37

M35 RH IP** 1074.8 1068.3 6.5 0.60

* zinc rich primer** polymer based primerTable: 2. Weight Loss of Rebars

Fig. 2 -SEM of RHA

paste. Fig.1 and 2 show Scanning Electron Micrograph of OPC and RHA respectively.

SEM of rice husk ash blended concrete clearly shows large number of silicon fibers in concrete. These silicon fibers are seen to be very effective in substantial resistance to corrosion of RHA blended cement concrete. Earlier experiments com-bined with SEM observations suggests that the structure of rice husk ash is similar to the composite material with silica tubes filled with cellulose material with the matrix consisting of lignin2. These silica fibers constitute the greatest advantage when using rice husk ash in concrete and are responsible for its impressive performance in corrosive environments.

Miscellaneous Applications of RHA

Rice husk ash has been effectively used as simple cementitious coatings for concrete surfaces to act as a waterproofing barrier coupled with higher chemical resistance. Photograph shows a typical coating application of RHA in a water-treatment plant. Fig.3

Rice husk ash has also been extensively used as an effective

The chloride content of cover concrete in the beams cast with rice husk ash cement is less than that of beams cast with Ordinary Portland Cement by 29 % in case of M 30 grade of concrete. For M 35 grade of concrete with RHA, it is 12 % less than that of specimens with OPC (Table 3). It shows that the rice husk ash cement is very effective in controlling the entry of chloride ions into the concrete. This reduction in chloride content can be considered to be of significant order.

Concreting with RHA poses no difficulties at all in fact the consistency / plasticity of the mix shows considerable improvement. However, the initial slump of fresh concrete is slightly reduced.

Microstructure studies of RHA

Scanning Electron Micrographs (SEM) of OPC and RHA cement concrete samples were taken for manification of 2000 X. The samples were prepared out of hardened cement

Concrete Admixtures

172 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Beam Designation pH Value Percentage of chloride content mg/g of concrete

dust

M30 Grade OPC Concrete

M30 OPC control 11.3 0.600

M30 OPC Z 11.6 0.540

M30 OPC IP 11.3 0.490

M30 Grade RHA Blended Concrete

M30 RH control 11.7 0.340

M30 RH Z 11.7 0.490

M30 RH IP 11.4 0.325

M35 Grade OPC Concrete

M35 OPC control 11.5 0.485

M35 OPC Z 11.4 0.620

M35 OPC IP 11.5 0.285

M35 Grade RHA Blended Concrete

M35 RH control 11.8 0.340

M35 RH Z 11.9 0.630

M35 RH IP 11.7 0.255

* zinc rich primer** polymer based primerTable 3 - pH Values and Chloride Content Cover Concrete

repair mortar without the use of SBR latex /Acrylic polymer bonding agents as seen in the photographs. Fig.4

Field observations done by the author for concrete blended with RHA (Hyper 2000) at 10% by weight has shown that RHA is intact on the concrete floor subjected to constant salt water usage – Even after more than 2 years of exposure.

Fig. 4: Beam

Fig. 3: TankNeither plastic shrinkage cracks nor long term drying shrin-kage cracks were observed.

Conclusion

The production and use of RHA in India should be considerably increased given the fact that RHA contributes significantly to a green building. It not only reduces the consumption of cement due to blending but also solves waste disposal problem.

Rice husk ash therefore can be effectively used as a sustain-able concrete option in severe environments and can be considered a class apart from all other mineral admixtures due to its unique microstructure and the resultant benefits in concrete and its multi various application possibilities. Considerable study needs to be done on applications of RHA as repair mortars, coatings and soil stabilization.

Reference

- Role of Rice-Husk Ash and Silica Fume in Sustainable Development” by P.K.Mehta and I.Nakagawa, Proceedings of the International Symposium on Concrete Technology for Sustainable Development in the twenty first century, Hyderabad, India, Feb.1999

- “Pyrolysis of rice husk” by Anshu Bharadwaj, *, Y. Wang, S. Sridhar and V. S. Arunachalam ; Center for Energy and Environment Studies and Department of Material Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

Concrete Admixtures

194 The Masterbuilder - July 2012 • www.masterbuilder.co.in

Performance of Concrete Flexural Elements Reinforced with C-Bars

Compared to concrete structures of the first-half of the previous century, concrete structures of recent decades have suffered from early decay and distress.

In most cases, this decay shows up as spalling of concrete or cracks in concrete structures, which are most often caused by corrosion in rebars in the case of reinforced concrete structures.

There can be many different reasons for early corrosion in rebars of the last four or five decades. It should not be difficult to recognize that chemical properties of the material of the rebars must have a role in the process of corrosion. But what could have accelerated the process of corrosion in rebars of recent decades?

Just as it could be easily recognized that the chemistry of the material of the rebar would have a role in the process of corrosion, it could also be recognized that the problem of early distress in reinforced concrete structures came into limelight following the introduction of high strength (Fe415 and higher) rebars with surface deformations in the 1960’s and 1970’s. These bars could be easily recognized by the presence of lugs or protrusions on their surface (Fig.1).

Mohammed, et al.1 observed in tests that, compared to rebars with a plain surface, rebars with surface deformations corroded faster.

Alekseev2 too recognized the greater (compared to the case of rebars with a plain surface) propensity of rebars with surface deformations for early corrosion. Alekseev2 could be quoted thus : “In accelerated tests, the durability of reinforcement specimens with a stepped (deformed) profile may be roughly an order less than that of smooth specimens since the former have space concentrators on the surface of the bases of projections which represent sites of preferential formation of cracks.”

Kar3-5 has shown that besides cold work (Fig. 2 as in Torsteel, Tiscon, etc.), the presence of surface lugs/protrusions/ribs on the surface of rebars of the present generation predisposes such bars (Figs. 1 and 2) to early corrosion.

Dr. Anil K Kar1, and Dr. M.S. Haji Sheik Mohammed2

1Proprietor, Engineering Services International, Kolkata 2Professor, Department of Civil Engineering

Fig. 1.Typical high strength rebars with surface deformations but of straight line configuration, which replaced plain round bars starting the decade of the 1960’s

Cold twisted deformed (CTD) bars (Fig. 2), which were the most highly susceptible (to corrosion) among rebars, have virtually been withdrawn from the market. Engineers, however, still continue to use rebars with surface deformations (Fig.1).

In order that strength capacities of rebars can be increased without costly change in chemistry, bars are generally given thermo mechanical treatment, as in the TMT process, in India and in some other countries.

The preference for surface lugs has been due to the perceived need for limiting the anchor/bond or lap length of today’s high strength (Fe 415 or greater) rebars. As

Fig. 2. Typical cold twisted deformed (CTD) rebar with lugs and protrusions on the surface and stresses beyond yield on the entire body

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stated in the preceding, the lugs invited early corrosion in rebars and early distress in reinforced concrete structures.

Thus, with an objective of using high strength (Fe 415 or higher) rebars without surface lugs and yet limiting the lap length within reasonable limits, Kar6-8 had proposed C-bar (Figs. 3 and 4), characterized by its plain surface and a deformed (wavy/undulated) axis.

Fig. 3. View of partial lengths of some C-bars, characterized by deformed axes and the absence of any surface lugs

Reservations were, however, expressed by individuals that because of its curved shape, C-bar, under tension, could push off concrete in the tension zone of flexural members, e.g., reinforced concrete beams.

In such expressions of reservations, it was overlooked that, as an element in a composite construction, a rebar, bonded to concrete, performed differently than a bar would have under tension if it were to pass through an opening of a curved shape.Nevertheless, tests were performed to see if indeed there could be spalling of concrete if beams were to be reinforced with C-bars and loaded transversely.

Test Results

Though limited in number, independent load tests on reinforced concrete beam elements at three universities have shown that there is no discernible ill effect of using C-bars (Figs. 3 and 4). On the contrary, the use of C-bars could possibly make reinforced concrete structures much more ductile than concrete structures which may be reinforced with conventional (with straight axis) rebars.

The test beams had no ties or stirrups that could have prevented lateral thrust, if any, by the curved bars in tension due to transverse loads on the beams.

Under two-point load tests (Fig. 5) on beams with equal (except for differences in configuration of rebars; Fig 4) reinforcement, the crack and failure patterns at ultimate loads were identical (Figs. 6 and 7).

There was no sign of spalling of concrete when C-bars were used as reinforcing bars. It needs to be noted here that in order to avoid any possible confining effect of ties/stirrups, no such tie or stirrup was used in the beams in Figs. 5-7.

Tests, monitored at another test centre for response under

Fig. 4. Proposed rebar (in the front) with a plain surface and a deformed axis

Fig. 5. Two point load test on reinforced concrete beam

Fig. 6. Crack patterns in the bottom face of beam reinforced with proposed C-bars (plain round bars with deformed axes (Figs. 3 and 4))

Fig. 7. Crack patterns in the bottom face of beam reinforced with conventional plain round bars with straight axis

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increasing three point load, showed that deformation at failure of elements, reinforced with C-bars, could be much higher than the ultimate deformation of conventionally reinforced elements (Figs. 8 and 9).The ultimate capacity of the beam with conventional plain round bars (Figs. 7 and 8) was reached when there was failure of bond between rebars and their surrounding concrete. No stirrups were used and there was no spalling of concrete when C-bars were used as rebars. There were no end hooks for rebars which were used in preparing the beams in Figs. 6 and 7.

Additional Confirmatory Test

1.1 2.2 3.3 4.4 5.5 6.6 7.7 8.8 9.9 11.00

6.0

12.0

18.0

24.0

30.0

36.0

42.0

48.0

54.0

60.0

Displacement, Mm

Load

,Kn

Fig. 8. Load deflection response of beam reinforced with plain roundbars

6.0

12.0

18.0

24.0

30.0

36.0

42.0

48.0

54.0

60.0

Load

,Kn

7.10 14.20 21.30 28.40 35.50

3.56 10.55 17.75 24.85 31.95Displacement, Mm

Fig. 9. Load deflection response of beam reinforced with C-bars

To dispel any lingering doubt, yet another pair of tests on beams (Figs. 10 and 11) were conducted at Structural Engineering Laboratory, B.S. AbdurRahman University, Chennai to see if the use of C-bars in flexural elements could in any way lead to any spalling of concrete. To study the spalling effect, if any, of using curved rebars, one of the beams involved an extreme example in the form of a simply supported beam having a rectangular cross section and a central camber of 30 mm in the tension zone, i.e. at the bottom (Fig. 11). No ties/stirrups were used.

Objectives of the Test

- To determine the Modulus of Rupture of the normal rectangular beam (Fig. 10) and rectangular beam with 30mm central camber (curved beam) (Fig. 11)

- To study the spalling/pushing out effect of C-bars during flexure test.

Test Method

ASTM C 293-02 : Standard Test Method for Flexural Strength of Concrete (Using simple Beam with Center-Point Loading)

Details of Test Specimen

- Normal rectangular beam of size 1270mm (L) X 200mm (W) X 150mm (D). The beam is reinforced with 2 – 10mm dia. M.S. rebars with bends at end and spaced 100mm apart in the tension zone. The reinforcement was provided with an effective cover of 30mm at bottom and 20mm at sides. The 6mm dia. M.S. spacer bars were provided near supports to maintain stability.

- Curved Beam : Rectangular beam of size 1290mm (L) X 200mm (W) with height of the beam varying from 180mm in the bearing length (150mm) at either supports to 150mm at centre which gives a central camber of 30mm. The beam is reinforced with 2 nos. curved M.S. rebars of 10mm dia. with a central camber of 30 mm. The reinforcement was provided with an effective cover of 30mm at bottom and 20mm at sides. The 6mm dia. M.S. spacer bars were provided near supports to maintain stability.

Concrete of M25 grade made from 53 grade ordinary Portland cement, 20mm downgraded aggregate, locally available river sand and potable water was used for the study. The mix design was done as per IS 10262 : 1982 - Recommended guidelines for concrete mix design and the mix proportion was 1 : 1.567 : 3.27, w/c ratio : 0.45 with hand mixing.

Test Description

The test was conducted in the 3000kN capacity digital U.T.M. with computer interface facilities as per guidelines

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outlined in ASTM C 293-02. The specimen was placed over supporting rollers with an effective length of 880mm. The width of the roller was just sufficient to accommodate the width of the specimen. The load applying block consisted of rectangular metal block with a curved assembly. The specimens were placed such that the finished surface faced upwards and capped with plaster of paris in the central region to level irregularities in the surface.

The load was applied centrally and gradually without shock at a constant rate until failure of the specimen. Sensitive dial gauge was fitted near central portion in the compression side (which is the possible location to fit dial gauge) to capture load deflection behaviour. The U.T.M. was interfaced with a computer to capture the overall load-deflection behaviour.

Figure10 shows the Test set-up and failure pattern of the Normal Rectangular Beam and Figure 11 shows the Test set-up and failure pattern of the Curved Rectangular Beam. The ultimate loads, loads at first and second cracks and the failure patterns were observed carefully. The modulus of rupture was calculated using the formula

Modulus of Rupture, R (MPa) = (3PL) / (2bd2)

Where P = Maximum Applied Load, N L = Span Length, mm

b = Average width of specimen at fracture d = Avg. depth of specimen at fracture

Results and Observations

Table 1 shows some observations on flexural strength test. It can be observed that ultimate load for the Normal Rect-angular Beam (Fig. 10)was observed as 37.91kN. The first and second cracks were observed at 21kN and 29 kN,

respectively with locations near center (15mm and25mm from center) in the tension side, direction parallel to the applied force. The failure pattern was complete cracking of cross section under the application of load which is an expected typical failure as shown in Fig. 10 (b).

The Curved Beam (Fig. 11) uses two special C-bars which had a single curvature for the entire length of the beam. During the application of load, no pushing out or spalling effect on concrete was observed. For the Curved Rectangular Beam, the ultimate load was observed beyond 42.50 kN. The first and second crack loads were observed as 21.50kN and 28.50kN, respectively. The cracks originated in the tension zone at around 75 – 80mm from centre on either side and those progressed towards the loading point in the compression side. The failure pattern was typical cone shaped failure under the applied load as shown in Figure 11(b).

Fig. 10. (a). Test set-up of Normal Rectangular Beam

Fig. 10. (b) Failure pattern of Normal Rectangular Beam

Fig . 11 (a) Test Set-up of Curved Rectangular Beam

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No pushing out or spalling effect on concrete in the deformed locations of C-bar was observed when the specimen was subjected to transverse loading.

Fig. 12 shows the Load-Deflection behaviour at central zone in the compression side during flexure test. It can be seen that almost a linear behaviour exists up to 17.50kN for the Normal Rectangular Beam followed by appreciable increase in deflection upon further increase in load up to 33kN. For the Curved Rectangular Beam, linear behaviour was observed up to 14kN followed by appreciable increase in deflection upon small increases in load up to 32kN.

The test results confirm that there need be no concern for any pushing out or spalling of concrete as tension in proposed C-bars (rebars with a plain surface and a deformed axis) increases under increasing load.

Increased Energy Absorbing Capacity and Ductility

It was observed from tests at the second test center, where the performance of a rectangular concrete beam, reinforced with a pair of C-bars (Figs 3 and 4) was compared to the performance of a rectangular concrete beam, reinforced with a pair of conventional rebars of the same diameter

Fig. 11 (b) Failure pattern of Curved Rectangular Beam

Specimen Details Ultimate Load (kN)

First crack load (kN) Second crack load (kN)

Failure Pattern Modulus of Rupture (MPa)

Rectangular Beam

1270x200x150mm

37.95 21.00 29.00 Complete cracking of

cross section under the

applied load

11.13

Curved Beam

1290x200x180mm (with

30mm central camber)

More than 42.50 21.50 28.50 Typical cone shape failure

under the applied load.

12.47

Table 1: Observations on Flexural Strength Test

Fig. 12. Load - Deflection Behaviour at Central Zone in the Compression side during Flexure Test

and plain surface, that the use of C-bars, which are less prone to corrosion than rebars with surface deformations are, could lead to a several fold increase in the ductility of reinforced concrete elements (Figs. 8 and 9).

Test results at the third test center too have shown the same trends as can be seen in Fig. 13, which is a replotting of Fig. 12.

Fig. 13 Load - Deflection Behaviour at Central Zone in the Compression side during Flexure Test

Standard Beam

Ductility = 1.52/0.39 = 3.89Area under load-deflection curve = 12 kN-mm

Curved Beam

Ductility = 3.41/0.22 = 15.5Area under load-deflection curve = 37 kN-mm

Curved BeamPlain Beam

Deflection (mm)0

01 2 3 4

Load

(kN

)5

10

15

20

25

30

35

40

Deflection (mm)0

00.5 1 1.5 2 2.5 3 3.5 4

Curved BeamNormal Beam

Load

(kN

)

510152025303540

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The computations under Fig. 13 show that there is a great potential for a significant increase in both energy absorbing capacity and ductility if curved rebars, as in C-bars,will be used in lieu of conventional rebars with a straight axis.

Concluding Remarks

As a solution to the problem of early distress, and thus directly as a means for the construction of structures with enhanced life span and indirectly as a step towards minimization of the carbon foot print and global warming, C-bars were proposed by Kar6-8 for use as rebars in reinforced concrete constructions; C-bar costs nothing more to manufacture than what it takes to manufacture any other bar of the same chemistry and it is amenable to any special treatment, if it will be so desired.

Concerns were raised by some that because of their curved axis, C-bars, if used as rebars, could lead to spalling of concrete in the tension zone of reinforced concrete elements.

Tests at three universities have conclusively demonstrated that the use of C-bars, which corrode much less than rebars with surface deformations do and which cost no more than any other bar of the same metallurgy to make, would not lead to any spalling of concrete

Tests have further revealed that the use of C-bars in concrete elements can greatly enhance their ductility and thus lead to very significant increase in their energy absorbing capacity; all at no added cost.

Acknowledgement

The authors acknowledge the help of Mr. S. ShafeerAhamed, Assistant Professor; and Mr. S.C. Jayakumar, Mr. H. Sheik Mohamed, Mr. R. Sridhar, Ms. V.Harini, Ms. C. Hemalatha,

Ms. R. Jeyalakshmi, Ms. V. Sofia, M.Tech. Students, B.S. AbdurRahman University and attached to ‘ Dr. Haji’s Research Group’ in the conduct of the tests depicted in Figures 10-13 and in Table 1.

References

1. Mohammed, T.U., Otssuki, N., and Hisada, M., “Corrosion of Steel Bars with respect to orientation,” ACI Materials Journal, American Concrete Institute, March-April, 1999.

2. Alekseev, S. N., “Corrosion of Steel Reinforcement,” Chapter 7 in Moskvin, V. (edited by), translated from the original by V. Kolykhmatov, “Concrete and Reinforced Concrete Deterioration and Protection,” 1990, English translation, Oxford & IBH Publishing Co. Pvt. Ltd. New Delhi, 1993, original Mir Publishers, Moscow 1990.

3. Kar, A. K., “Concrete Structures --- the pH Potential of Cement and Deformed Reinforcing Bars,” Journal of the Institution of Engineers (India), Civil Engineering Division, Vol. 82, June 2001, Calcutta, pp. 1-13.

4. Kar, A. K., “Deformed Reinforcing Bars and Early Distress in Concrete Structures,” Highway Research Bulletin, No. 65, Indian Roads Congress, December 2001, New Delhi, pp. 103-114.

5. Kar, A. K., “Deformed Rebars in Concrete Construction,” New Building Materials & Construction World, Vol. 12, Issue 6, December 2006, New Delhi, pp. 82,83,86,88,90, 92,94,96,98,100 and 101, www.nbmcw.com.

6. Kar, A. K., “Improved Rebar for Durable Concrete Constructions,” New Building Materials & Construction World, Vol. 16, Issue 1, July 2010, New Delhi, pp. 180-199, www.nbmcw.com.

7. Kar, A. K., “Rebar for Durable Bridge and Other Concrete Constructions”, Indian Highways; Vol. 39, No. 3; The Indian Roads Congress (IRC), New Delhi; March 2011pp. 59-65.

8. Kar, A. K., “A Rebar for Durable Concrete Construction”, The Masterbuilder; Vol. 13, No. 7, Chennai; July 2011, pp. 224-236, www.masterbuilder.co.in.

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Self-Healing Concrete

Concrete will continue to be the most important building material for infrastructure despite being prone to cracking. Tiny crackson the surface of the

concrete make the whole structure vulnerable because water seeps in to degrade the concrete and corrode the steel reinforcement, greatly reducing the lifespan of a structure. Concrete can withstand compressive forces very well but not tensile forces. When it is subjected to tension it starts to crack, which is why it is reinforced with steel; to withstand the tensile forces. Structures built in a high water environment, such as underground basements and

marine structures, are particularly vulnerable to corrosion of steel reinforcement. Motorway bridges are also vulnerable because salts used to de-ice the roads penetrate in to the cracks in the structures and can accelerate the corrosion of steel reinforcement. In many civil engineering structures tensile forces can lead to cracks and these can occur relatively soon after the structure is built. Repair of conven-tional concrete structures usually involves applying a concrete mortar which is bonded to the damaged surface. Sometimes, the mortar needs to be keyed into the existing structure with metal pins to ensure that it does not fall

Sonjoy Deb, B.Tech,’Civil’

Associate Editor

Tiny cracks in concrete do not necessarily affect structural integrity in the short term, but they do allow water and other chemicals to seep into the structure, which may cause problems over time. Self-healing concrete has embedded clay particles that contain dormant bacteria and a food source. When a crack appears in the concrete, water seeps in and activates the bacteria. When they wake, the bacteria eat their packed lunch and then conveniently excrete chalk, which fills the crack.

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away. Repairs can be particularly time consuming and expensive because it is often very difficult to gain access to the structure to make repairs, especially if they are underground or at a great height.

Self-Healing of Concrete

Self-healing concrete is a product that will biologically produce limestone to heal cracks that appear on the surface of concrete structures. Specially selected types of the bacteria genus Bacillus, along with a calcium-based nutrient known as calcium lactate, and nitrogen and phosphorus, are added to the ingredients of the concrete when it is being mixed. These self-healing agents can lie dormant within the concrete for up to 200 years. However, when a concrete structure is damaged and water starts to seep through the cracks that appear in the concrete, the spores of the bacteria germinate on contact with the water and nutrients. Having been activated, the bacteria start to feed on the calcium lactate. As the bacteria feeds oxygen is consumed and the soluble calcium lactate is converted to insoluble limestone. The limestone solidifies on the cracked surface, thereby sealing it up. It mimics the process by which bone fractures in the human body are naturally healed by osteoblast cells that mineralize to reform the bone. The consumption of oxygen during the bacterial conversion of calcium lactate to limestone has an additional advantage.Oxygen is an essential element in the process of corrosion of steel and when the bacterial activity has consumed it all it increases the durability of steel reinforced concrete constructions. The two self-healing agent parts (the bacterial spores and the calcium lactate-based nutrients) are introduced to the concrete within separate expanded clay pellets 2-4 mmwide, which ensure that the agents will not be activated during the cement-mixing process. Only when cracks open up the pellets and incoming water brings the calcium lactate into contact

Figure 1: Schematic of self-healing process in bacterial concrete. (A) water enters from the left into a micro crack activating the embedded bacterial spores. (B) the active bacteria seals the cracks with the production of limestone, protecting the embedded steel reinforcement (brown bar) from attack and erosion

with the bacteria do these become activated. Testing has shown that when water seeps into the concrete; the bacteria germinate and multiply quickly. They convert the nutrients into limestone within seven days in the laboratory. Outside, in lower temperatures, the process takes several weeks. Refer Figure 1 for self- healing process.

History of Self-Healing Concrete

Bio-concrete was first introduced as a way of sealing Mount Rushmore. The idea of bacteria-mediated concrete was first introduced by a US research group led by Prof Sookie Bang in the late 1990s. She had the idea of using it as a sealer on Mount Rushmore, which was subject to the effects of the climate. The team at the South Dakota School of Mines and Technology developed a bacteria/glass-bead system that it believed increased the strength of concrete by 24 per cent. Unfortunately, the application of the theory was never taken forward due to a lack of interest among the commercial engineering sector at the time.

Finding the right bacteria

The starting point of the research was to find bacteria capable of surviving in an extreme alkaline environment. Cement and water have apH value of up to 13 when mixed together, usually a hostile environment for life: most organisms die in an environment with a pH value of 10 or above. The search concentrated on microbes that thrive in alkaline environments which, can be found in natural environments, such as alkali lakes in Russia, carbonate-rich soils in desert areas of Spain and soda lakes in Egypt. Samples of endolithic bacteria (bacteria that can live inside stones) were collected along with bacteria found in sediments in the lakes. Strains of the bacteria genus Bacilluswere found to thrive in this high-alkaline environment. Backat Delft University the bacteria from the samples were grown in a flask of water that would then be used as the part of the watermix for the concrete. Different types of bacteria were incorporated into a small block of concrete. Each concrete block would be left for two months to set hard. Then the block would be pulverized and the remains tested to see whether the bacteria had survived. It was found that the only group of bacteria that were able to survive were the ones that produced spores comparable to plant seeds. Such spores have extremely thick cell walls that enable them to remain intact for up to 200 years while waiting for a better environment to germinate. They would become activated when the concrete starts to crack, food is available, and water seeps into the structure.This process lowers the pH ofthe highly alkaline concrete to values in the range (pH 10 to11.5) where the bacterial spores become activated. Finding a suitable food source for the bacteria that could survive in the concrete took a long time and many different nutrients were tried until it was discovered

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that calcium lactate was a carbon source that provides biomass. If it starts to dissolve during the mixing process, calcium lactate does not interfere with the setting time of the concrete.

The Basic Mechanism

The basic mechanism of bacteria remediate cracks is shown below-

Interest from Industry

When the idea of bacteria mediated concrete was first mooted by US academics in the late 1990s by the research group of Professor Sookie Bang, testing and application of the theory was not taken forward because there was a lack of interest from the commercial engineering sector for such a product.The R&D process still has some way to go but several big industry players have created partnerships with Delft University to develop applications of self-healing concrete. Investment funding from industry is now forthcoming. The concept is to engage with one major player from each concrete sector. Delft is therefore developing self-healing concrete products for specific civil engineering markets that will not be in competition with one another. Products will be developed for sectors such as tunnel-lining, structural basement walls, highway bridges, concrete floors and marine structures. Pure concrete products will take two years to develop and products with steel reinforcement will take four years. Refer Figure 2 for the before and after pictures of a slab of self-healing concrete.

HOW DOES BACTERIA REMEDIATE CRACKS?CHEMISTRY OF THE PROCESS

Microorganisms (Cell surface charge is negative) drawcations including Ca from the environment to deposit onthe cell surface. The following equations summarize the

role of bacterial cell as a nucleation site.

2+

Ca +Cell Cell -CaCell -Ca +CO Cell -CaCO

2+ 2+

2+ 2-

3 3

The bacteria can thus act as a nucleation site whichfacilitates in the precipitation of calcite which caneventually plug the pores and cracks in concrete

(a) (b)Figure 2: Before-and-after pictures of the surface of a slab of self-healing concrete. The crack is visible in the left-hand image (a) and on the right (b), the white limestone has filled up the gap

Some Research Studies

(1) An engineering student Michelle Pelletier from the University of Rhode Island (URI) announced that she has developed a self-healing concrete that would be inexpensive to produce.Michelle Pelletier, collaborating with URI Chemical Engineering Professor Arijit Bose, created a concrete matrix that was embedded with a micro-encapsulated sodium silicate healing agent. When cracks formed in the concrete, the capsules ruptured and released the agent into the adjacent area. The sodium silicate reacted with the calcium hydroxide already present in the concrete, and formed a calcium-silica-hydrate gel that healed the cracks and blocked the concrete’s pores. The gel hardened in about one week.When Pelletier’s concrete was stress-tested to the point of almost breaking, it proceeded to recover 26% of its original strength. By contrast, conventional concrete only recovers 10%. Pelletier believes that she could boost the strength of her mix even higher, by increasing the quantity of the healing agent. Refer Figure 3.

Figure 3:Michelle Pelletier with her self-healing concrete

2. Researchers at Northumbria University in the U.K. are developing a “self-healing” concrete. Dr Alan Richardson, a Senior Lecturer in Construction in the School of the Built and Natural Environment is using ground-borne bacteria – bacilli megaterium - to create calcite, a crystalline form of natural calcium carbonate. This can then be used to block the concrete’s pores; keeping out water and other damaging substances to prolong the life of the concrete.The bacteria is grown on a nutrient broth of yeast, minerals and urea and is then added to the concrete. With its food source in the concrete, the bacteria breeds and spreads, acting as a filler to seal the cracks and prevent further deterioration. It is hoped the research could lead to a cost-effective cure for ‘concrete cancer’ and has enormous commercial potential. While further research is needed, Dr Richardson

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is hopeful that the repair mortar will also be effective on existing structures. So-called ‘concrete cancer’ may be caused by the swelling and breaking of concrete and is estimated to cost billions of pounds worth of damage to buildings .Dr Richardson said: “This project is hugely exciting. The potential is there to have a building that can look after itself.”

3. At Delft University, Dr. HenkJonkers is developing a biological concrete that uses specially selected bacteria of the genus Bacillus, alongside a combination of calcium lactate, nitrogen and phosphorus, to create a healing agent within the concrete.If untouched, these agents can remain dormant in the concrete for centuries. But if water begins to seep into the cracks, the spores of the bacteria start to germinate and feed on the calcium lactate. This consumes oxygen, which in turn converts the calcium lactate into limestone that solidifies and seals the surface. The removal of oxygen also improves the durability of the steel reinforcement. They are using clay pellets that are around 2-4mm wide to make sure that the agents are not activated during the mixing process. The problem with this is there is need to use relatively high volumes of this porous aggregate within the concrete mix which results in gain of self-healing but lose of strength of the concrete. The clay pellets make up 20 per cent of the volume of the concrete that would otherwise be made of a harder material. This is estimated to weaken the concrete by around 25 per cent, which is far too much for applications that require high compressive strength. Jonkers is now working on using a compressed powder instead of pellets that will hold the self-healing agent in less than one per cent of the volume of the concrete.

4. Researchers at Ghent University are using the micro-organism Bacillus sphaericus with urea as a nutrient source to create calcium carbonate. Researcher Dr. Nele De Belie said they have first discovered the bacterium as it was causing problems closing up water pipes. They realised the same bacteria could help enhance the durability of concrete. Instead of using a porous aggregate to hold the self-healing agent, the Ghent team opted to place the material in a hollow glass capsule with an internal diameter ranging from 0.8 to 4mm. As the concrete cracks, the capsules break, releasing the self-healing agent. This method eliminates the need for porous aggregates and retains the strength of the concrete. During the course of its research, the team found that the bacteria struggled to fill cracks of more than 300mm. It has since developed a solution that is purely synthetic, by using polyurethane capsules, which foam in moist environments, and an accelerator that shortens the reaction time. Initial tests

have shown that the foam can expand 25-30 times more than a bacterial solution. But the team haven’t given up on biological process yet.

Environmental Advantage

Self-healing concrete could reduce the significant CO2 emissions that result from concrete production. Because the production of concrete is very energy intensive – when mining, transportation and concrete plants are considered – the industry is responsible for about 10 per cent of all CO2 emissions in the US. If self-healing concrete can lengthen the life of the concrete and reduce maintenance and repairs, it will ultimately reduce the production of excess amounts of concrete and result in a decrease in CO2 emissions.

Conclusion

Self-healing concrete is a kind of smart concrete and becoming one of the research focus both in material and civil engineering field. Self-healing concrete is likely to provide the greatest global benefit over the coming years. Current technologies under development include autonomic systems, where cracks are automatically repaired with internally released resins, as well as more natural autogenic systems, such as swelling and hydration of cement paste into cracks. This can be significantly enhanced by systems that help to close up the cracks, such as activating embedded shrinkable polymer bars. Considering that over 6 billion cubic meters of concrete is used annually, and that the majority of its durability problems originate from cracking, then the development of smart cementation material with the ability to self-heal cracks offers potentially massive savings to the annual amount of money spent on repair and maintenance of concrete structures.

Reference

- http://www.gizmag.com/student-creates-self-healing-concrete/15237/

- h t t p : / / w w w . s c i e n c e d a i l y . c o m /r e l e a s e s / 2 0 1 2 / 0 4 / 1 2 0 4 2 6 1 0 5 0 0 1 . h t m ? u t m _source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+%28ScienceDaily%3A+Latest+Science+News%29

- http://www.torontosun.com/2012/04/27/self-healing-concrete-could-save-millions-research

- http://www.guardian.co.uk/artanddesign/2011/sep/11/architecture

- http://www.theengineer.co.uk/in-depth/wise-crack-self-healing-concrete/1008203.article

- h t tp : / /www. theeng inee r.co .uk /news /se l f -hea l i ng -concrete/1002629.article

- h t tp : / /www. icev i r tua l l i b ra ry.com/con ten t / re la ted /advert?advert=05072011

- http://www.scientific.net/AMR.250-253.405

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Accelerated Short-Term Techniques to Evaluate Corrosion in Reinforced Concrete Structures

In recent years an increased research effort has been focused upon corrosion of reinforcing steel in concrete and upon techniques whereby such damage can be

reduced or haulted. Although the problem has been most apparent on reinforced concrete bridge decks, it also occurs on support structures (girders, piles, piers, etc), industrial plants and marine docking facilities.

Steel embedded in concrete is normally subjected to an alkaline environment which subsequently passivates the steel, forming a stable oxide film. The corrosion protective properties of concrete are dependent upon maintaining an alkaline environment in intimate contact with the steel. Any condition which disrupts the environment around the steel can result in disruption of the passive film and initiate

Sonjoy Deb, B.Tech,’Civil’

Associate Editor

The phenomenon of corrosion of reinforcement bar in concrete is a time dependant process. Under severe environmental conditions also, it takes years for the steel reinforcement to be corroded and to cause deterioration of reinforced concrete (RC) structures. However when it becomes imperative to evaluate the relative performance of different types of steel and binder in a short time, the accelerated corrosion test can be adopted. The accelerated corrosion test does not always reproduce the actual corrosion still it can simulate to some extent the steel corrosion found in real RC structures.

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corrosion process. Marine environments and deicing salts both provide significant concentrations of chlorides which can penetrate the concrete to destroy the passive film on the reinforcing steel and reduce the pH of its environment. As the corrosion process proceeds, an accumulation of solid corrosion products forms at the metal-concrete interface. The corrosion product formed absorbs water and increases in volume. The volume of corrosion products (rust) is 2.5–6 times larger than the volume of the steel used in concrete which is the reason behind cracking and spalling of the concrete in the corrosion propagation phase as this increase in volume of rust products exerts enormous stress on the surrounding concrete. These stresses have been reported to be as high as 450MPa.

Numerous variables have been determined to influence the occurrence of reinforcing metal corrosion and concrete cracking. These may be categorized as pertaining to (i) nature of the embedded metal, (ii) nature of concrete, and (iii) nature of the environment. Chemical and electro chemical properties of the embedded metal itself are important with regard to the first category. Also included here is the influence of caotings, either metallic or inorganic, applied to the reinforcing metal. Variables in category two are the concrete mix design, including special admixtures or inhibitors, the water-cement ratio, the cement type and the depth of cover. The foremost factor of interest in the last category has been the chloride ion concentration.

Particularly lacking in the testing of reinforced concrete for its corrosion protective properties has been an accelerated test or test procedure. An exemption to this is the technique developed by Tremper et.al. in 1958, which involves exposure of a freely corroding, partially submerged reinforced specimen. The principle of this technique is that the region near or below the water line becomes anodic due to Cl- penetration; and corrosion of this area is driven by the cathodic portion of the reinforcing steel, which is in the air. A disadvantage to this test procedure is the length of the time required for data to be evolved. This is inspite of relatively small concrete cover the reinforcing metal (Approximately 0.75 inches 0r 1.9 cm). For greater cover even longer times should be required. One of the famous test that is being adopted these days is accelerated corrosion test or Impressed voltage test. The test has gained popularity because of its simplicity and more user friendly behavior.

Accelerated Corrosion Test

The setup for accelerated corrosion test (also known as impressed voltage test) is shown in Figure.1 and Figure.2. It consists of a DC power supply, two stainless steel plates, a data logger, test specimen and the container containing the required dosage of NaCl solution. Beam specimens

with a centrally embedded steel bar are used for the accelerated corrosion test. The specimens are tested at the required age after preparation. The steel bar (anode/working electrode) of the beam specimen is connected to the positive terminal and the stainless steel plates (cathode/counter electrode) are connected to the negative terminal of the DC power source. The corrosion process is initiated by applying a constant voltage to the system. The current response is continuously monitored and recorded by the data logger. In addition the specimens are daily inspected visually for the onset of cracks. The data logger is set at a sampling frequency of 1min for recording the corrosion current of the circuit. The accelerated corrosion test is terminated after cracking of the specimen when the rate of increase of corrosion current with time was negligible.

Figure 1: Schematic test set-up for accelerated corrosion test

Figure 2: Test set-up for accelerated corrosion test

Data Analysis

An important facet of this test is definition of specimen failure. In all cases test is considered to commence at the time of application of the impressed current. Specimen failure is defined as that time corresponding to onset of a large current increase. The example in Figure 3 illustrates schematically the generalized nature of current variations in the constant voltage test method. Initially it can be seen that some relatively small amplitude variations are encountered; but after that at later stage a large increase in current occurs. It is considered that this corresponds to growth of a crack within the concrete and to a corresponding decrease in electrical resistance. Crack growth is sub sequently incremental and is comprised of successive stages of propagation, which relieves tensile stresses, and

ComputerDigital multimeter withsingle channel data logger

D.C.PowerSupply

Beam Specimen

ContainerNaCl Solution

Steel Plate

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arrestment, which gives rise to additional solid corrosion products accumulation. Time to failure also have been shown in this example. Figure 4 shows a cracked beam specimen after the accelerated corrosion test.

Figure 3: Corrosion Current Vs. Time Graph

Figure 4: Cracked beam specimens

Other Parameters Determined

There are various parameters of corrosion performance of reinforced concrete that can be evaluated from the accelerated corrosion test. This includes (a) free chloride content at cracking measurement, (b) mass loss measurement, (c) critical corrosion current, (d) post depassivation cracking time. Hence it can be seen that one test can give so many parameter study opportunities.

(a) Determination of free chloride content of cracked beam specimens-The beam specimens after accelerated corrosion test and half-cell potential test are analysed for free chloride content, since free chloride content is mostly responsible for corrosion of steel reinforcement in concrete. The concrete powder is collected by drilling on all four sides of the beam specimen i.e. cracked side and other three uncracked sides. The free chloride content is determined by potentiometric titration using

an automatic titrator (Make Metrohm, model: 848 Titrino plus). The titrator is equipped with a screen display that can display the chloride content automatically when appropriate inputs are fed. For determination of free chloride, three grams of the powdered concrete sample is transferred to 100ml beaker and 50ml of distilled water is added. The sample is then heated gently and thoroughly mixed by a stirrer. The solution is then cooled and filtered using Whatman No.1 filter paper and the free chloride content is then determined by titrating against 0.1 M AgNO3 solution in the automatic titrator. The results are expressed as percentage of free chloride by mass of concrete.

(b) Mass Loss analysis of the corroded reinforcement bars- After the beam specimens are corroded and the measurement of half-cell potential and free chloride ion content are taken, the beam specimens are jackhammered to remove the corroded longitudinal steel bars. The corroded bars are cleaned with a wire brush to ensure that they are free of any adhering concrete or corrosion products. After this the procedure stated in ASTM G 1-03 is adopted for the cleaning of the corroded steel specimens and for determination of mass loss. For the purpose of cleaning, the cleaning solution used is 500 ml of hydrochloric acid with 3.5 gm of hexamethylenetetramine added with reagent water to make it 1000 ml. Each specimen is cleaned several times with this solution and mass loss is noted after

Figure 5: Corroded steel bars obtained after breaking the beam specimens

2 4 6 8 10 12Time, Days

Curre

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illamps

2

46

8

10

12

14

16

18

20

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ack

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each cleaning. The mass loss corresponding to the cleaning time after which no significant increase in mass loss observed is used as final value of the mass loss. The clean bars are then weighed and the percentage mass loss for each bar is calculated. Refer Figure 5,6 for corroded steel specimens after breaking of the beam and before carrying out the mass loss analysis. A typical way of mass loss calculation is shown in Figure 7.

Discussion on the Accelerated Corrosion Test

The test can be carried out by varying the impressed DC voltage, the result would be increased or decreased time to failure. Various researchers used various voltages, however six volt is found to be an adequate value to complete the tests within a reasonable length of time. The beauty of the test is that it continues 2hours a day and 7 days a week continuously, the only attention that needs to be given is frequent checking of the crack appearance period. Early research indicated that three test specimens are adequate for each type of mix or material. Test data indicate that sufficient accuracy could be obtained since the data analysis method used here averages the three values. Judgement and expertise should be used where premature specimen failure occurs. If one of the three specimens fails the first few days and the other two continue for a long period, the data from the prematurely failed specimen should not be used. Visual inspection of this one specimen will probably show improper preparation of the specimen. Longer time-to-failure and higher resistance are indicative of improvement over the standard mix and the ability to better deter the intrusion of chloride to the reinforcing steel. Preliminary research results established that a continuation of testing to a point where cracks became 0.79mm and larger caused pollution from seepage of corrosion products into the water. For the purpose of these tests various researchers used various chloride concentrations, based on the exposure level the concrete structure will be exposed when put on use.

Two methods of monitoring and testing have been used viz. the Constant Current Method and the Constant Voltage Method. The Constant Current Method consists of maintaining a constant current output to reach specimen and visually determining the time at which the specimen fails. Bothe the Constant Current and Constant Voltage Methods have certain limitations but can be varied to accommodate various materials tested. It must be recognized that these test procedures are sensitive only to the cracking tendency which results from a given induced rate of corrosion of the embedded reinforcing steel.

In this test, the prismatic specimen is accompanied in some instances by development of an aqueous solution on top of the specimen surface. The pH of the top liquid has been found to be approximately 1.9, presumably due to hydrolysis of initial corrosion products. Because of this and also because of the relatively low electrical resistance of the top liquid, corrosion can occur pre-ferentially at this location. Occurrence of this can lead to erroneous conclusions. In order to permit this liquid to drain away from the reinforcing steel, the specimen preparation procedure specifies a 100 slope to the top surface.

Figure 6: Specimen after breaking (spots of corrosion are visible)

Mass Loss CalculationData Series 2 Equation: y = 0.0048x + 1.4777

R2= 0.9991

00.20.40.60.81

1.21.41.61.8

0 2 4 6 8 10 12 14 16 18 20Time (minutes)

MassLoss(grams)

Data Series 1Data Series 2

Figure 7: A typical plot of mass loss versus cleaning time obtained

(c) Critical Corrosion Current- From Figure 3, the current corresponding to the failure stage is noted and is called critical corrosion current.

(d) Post depassivation cracking time- From Figure 3, the time corresponding to the specimen failure is called post depassivation cracking time.

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Conclusion

- This Procedure provides an accelerated method of testing reinforced concrete for relative corrosion protective characteristics.

- Because of the flexibility of the method, other materials such as claddings and coatings on reinforcing steel, and concrete coatings and sealants can be tested and compared.

- This procedure should not be used to establish the life expectancy of reinforced concrete materials since the actual voltages found in the field can vary drastically. As mentioned earlier resistivity varies as the corrosion process proceeds. Changes in temperature and salinity found in the filed may greatly affect the time to failure. Until a correlation between corrosion rates in the field and in the laboratory for specific materials and mixes is established, this procedure should be used as a method for comparison only.

- If properly worked upon this test can be utilized for finding various parameters related to concrete durability. This test can also be used for comparison between various building materials. This test can be utilized for simulating various aggressive field conditions by varying the chloride content and voltage applied.

Reference

- IlkerBekir Topçu, Ahmet Raif Boga. Effect of ground granulate blast-furnace slag on corrosion performance of steel embedded in concrete. Materials and Design 2010; 31: 3358–3365.

- Mohamed Ismail, Masayasu Ohtsu. Corrosion rate of ordinary and high performance concrete subjected to chloride attack by AC impedance spectroscopy. Construction and Building Materials 2006; 20: 458–469.

- T.A. Soylev, M.G. Richardson. Corrosion inhibitors for steel in concrete: State-of-the-art report. Construction and Building Materials 2008; 22: 609– 622.

- B. Elsener. Macrocell corrosion of steel in concrete: implications for corrosion monitoring. Cement & Concrete Composites 2002; 24: 65–72.

- A. Poursaee, A. Laurent, C.M. Hansson. Corrosion of steel bars in OPC mortar exposed to NaCl, MgCl2 and CaCl2: Macro and micro-cell corrosion perspective. Cement and Concrete Research 2010; 40: 426–430.

- BuluPradhan, B. Bhattacharjee. Performance evaluation of rebar in chloride contaminated concrete by corrosion rate. Construction and Building Materials 2009; 23 2346–2356.

- A. Poursaee, C.M. Hansson. Reinforcing steel passivation in mortar and pore solution. Cement and Concrete Research 2007; 37: 1127–1133.

- Marco Ormellese, Luciano Lazzari, Sara Goidanich, Gabriele

Fumagalli, Andrea Brenna. A study of organic substances as inhibitors for chloride-induced corrosion in concrete. Corrosion Science 2009; 51: 2959–2968.

- Bulu Pradhan, B. Bhattacharjee. Corrosion zones of rebar in chloride contaminated concrete through potentiostatic study in concrete powder solution extracts. Corrosion Science 2007; 49: 3935–3952.

- A.A.A. Hassan, K.M.A. Hossain, M. Lachemi. Corrosion resistance of self-consolidating concrete in full-scale reinforced beams. Cement & Concrete Composites 2009; 31: 29–38.

- IlkerBekirTopçu,AhmetRaifBoğa,FatihOnurHocaoğlu.Modelingcorrosion currents of reinforced concrete using ANN. Automation in Construction 2009; 18: 145–152.

- Ha Minh, Hiroshi Mutsuyoshi, KyojiNiitani. Influence of grouting condition on crack and load-carrying capacity of post-tensioned concrete beam due to chloride-induced corrosion. Construction and Building Materials 2007; 21: 1568–1575.

- M.M. Al-Zahrani , S.U. Al-Dulaijan, M. Ibrahim, H. Saricimen, F.M. Sharif. Effect of waterproofing coatings on steel reinforcement corrosion and physical properties of concrete. Cement & Concrete Composites 2002; 24: 127–137.

- Abdul-Hamid J. Al-Tayyib and Mesfer M. Al-Zahrani. Corrosion of Steel Reinforcement in Polypropylene Fiber Reinforced Concrete Structures. ACI Material Journal 1990; 87: 108-113.

- Mustafa Sahmaran, Victor C. Li, and Carmen Andrade.Corrosion Resistance Performance of Steel-Reinforced Engineered Cementitious Composite Beams. ACI Material Journal 2008; 105: 243-250.

- Tae-Hyun Ha, SrinivasanMuralidharan, Jeong-HyoBae, Yoon-Cheol Ha, Hyun-Goo Lee, Kyung-Wha Park, Dae-Kyeong Kim. Accelerated short-term techniques to evaluate the corrosion performance of steel in fly ash blended concrete. Building and Environment 2007; 42: 78–85.

- ErhanGuneyisi, TuranOzturan, Mehmet Gesoglu. A study on reinforcement corrosion and related properties of plain and blended cement concretes under different curing conditions. Cement & Concrete Composites 2005; 27: 449–461.

- M. Maslehuddin, M.M. Al-Zahrani, S.U. Al-Dulaijan, Abdulquddus, S. Rehman, S.N. Ahsan. Effect of steel manufacturing process and atmospheric corrosion on the corrosion-resistance of steel bars in concrete. Cement & Concrete Composites 2002; 24: 151–158.

- ASTM C 876. Standard test method for half-cell potentials of uncoated reinforcing steel in concrete.Annual Book of ASTM Standards, American Society for Testing and Materials; 1991.

- IS 383-1970 (Reaffirmed 2002).Specification for coarse and fine aggregates from natural sources for concrete. New Delhi: Bureau of Indian Standards.

- Neville AM, Brooks JJ. Concrete technology. Delhi: Pearson Education; 2004.

- ASTM G 1-03. Standard practice for preparing, cleaning, and evaluating corrosion test specimens.West Conshohocken, PA; 2003.

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The aggregate fraction of concrete is often thought of as an essentially inert filler material, extending the active cement paste to more economic volumes. In

practice this is an oversimplification, with most aggregates likely to react to some degree with the highly alkaline en-vironment created by hydrating cement. Ordinarily this is beneficial; partial dissolution of the aggregate particles’ surface enhances bonding with the cement phase. However, in extreme cases the chemical reactions that can occur are sufficiently disruptive to pose a threat to the structural integrity of the concrete.

The most common damage-causing interaction between cement paste and aggregate is ‘alkalisilica reaction’ (ASR), the name reflecting the essential interaction between the alkali-

rich pore fluid of the concrete and aggregate containing the mineral silica (SiO2). Susceptible rock types have a mineralogy in which silica is present in either an amorphous phase or a highly strained or crypto-crystalline form.

The reaction sequence is broadly as follows

- Initial depolymerisation and dissociation of reactive silica minerals in the aggregate under highly alkaline conditions.

- Hydrolysis of the resulting dissolved silica, by sodium and potassium hydroxide derived from the pore fluid, to produce a solid alkali-silicate gel.

- Hydration of the gel.

Alkali-Silica Reaction in ConcreteSonjoy Deb, B.Tech,’Civil’

Associate Editor

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The presence of the alkali-silicate gel is not damaging to the concrete per se. However, gels of the appropriate formulation have the capacity to absorb large quantities of water. The consequent volume expansion may generate internal tensile stresses in the vicinity of 6–7 MPa, sufficient to crack both the affected aggregate and surrounding cement paste. ASR typically manifests itself through extensive surface map cracking, although restraint due to structural loading or re-inforcement may modify the observed cracking pattern. The progress of the reaction can be extremely slow, and signs of distress may only appear when the concrete is years to decades old. The tensile strength and elastic modulus of the concrete are compromised as the severity of the reaction develops, but compressive strength is usually little affected. This has led to suggestions that ASR problems may be more aesthetic than structural, providing the concrete is well-reinforced, but the potential durability impact must also be borne in mind.

Typical map cracking in a concrete damaged by alkali silica reaction (ASR)

ASR Mechanisms

Most researchers agree that the main reaction of ASR is the reaction between certain forms of silica present in the aggregates and the hydroxide ions ( OH-) in the pore water of a concrete. Very early in the hydration of cement calcium ions are incorporated in the hydration products but potassium and sodium stay in solution and eventually they are partially incorporated into calcium silicate hydrate (C-S-H) and monosulfate. Hydroxide ions from the hydration of portland cement result in a pore solution having a pH of at least 12.5. Soluble alkalies raise the pH to about 13 or higher. Also, the amount of alkalis present in the pore water is related to the amount of soluble alkalis present in the cement. The hydroxide ions will attack a silica surface. If the silica is well crystallized the vulnerable sites are only at the exterior surface of the aggregate, but in the case of poorly crystallized silica, there are many vulnerable sites in the aggregate structure, leading to disintegration of the silicate network. To keep a neutral charge balance,

the cations Na+ and K+ difuse toward the hydroxide ions to react with them and the resulting product is a gel-like material. According to Powers and Steinour the migration of cations of Na+ and K+ is slow, therefore the migration of Ca2+takes place. If the gel is high in calcium then the gel is not expansive when exposed to water and, therefore, may not induce cracking in concrete. This theory rests on the assumption that calcium could be available. Diamond found that there is very little calcium in the pore solution. This is expected since the high pH causes the volubility of Ca(OH)2 to be depressed. Nevertheless, calcium could be dissolved from the solid phase of cement paste to produce a gel. Most researchers do not mention the distinction between “safe” and “swelling” gel but there are acknowledgments that there are more than one composition of gel produced by ASR.

Old concrete pier heavily damaged by ASR. Trondheim, Norway

The formation of the gel per se is not deleterious. The deterioration of the concrete structure is due to the water absorption by the gel and its expansion. R is reported that the RH must be higher than 80% for the gel to swell although it can be formed at lower relative humidity’s. As the tensile strength of the system is exceeded, cracks will form and propagate. As there is not a preferential direction for cracks to propagate and also the sites of crack initiation are randomly distributed in the specimen, map cracking will be characteristic of ASR deterioration. The sites of the cracks are determined by the location of the reacting silica on the aggregates and the availability of OH- in the vicinity.

Aggregate reactivity testing

The obvious solution for avoiding problems with ASR is simply to use a non-reactive rock type. Unfortunately there is no single test that can be unreservedly recommended as a fool proof method for screening potential aggregates for which no historical performance records exist. Consequently a conservative engineering approach is suggested, checking the aggregate performance by more than one technique and taking appropriate measures where the test results indicate caution is justified. The first step in the evaluation

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of a new aggregate source should be the commissioning of petrographic examination in accordance with ASTM C 295 (or an equivalent method). This determines whether the rock contains any of the reactive silica, which indicate potentially deleterious performance. The examination should preferably be carried out by a geologist with previous experience in the assessment of the mineral constituents of concrete aggregate, and must also include an inspection of the quarry site to assess variability in the rock’s composition. If the petrologist’s report indicates a potentially reactive aggregate then either the aggregate should be treated as deleterious and appropriate mitigation measures taken, as detailed below, or further testing should be undertaken to clarify the significance of the risk posed. A wide variety of test methods have been proposed for determining ASR susceptibility, most of which involve using the suspect aggre-gate to produce mortar or concrete beams and measuring their eventual expansion under humid conditions. Due to the slow-developing nature of ASR, the test methods necessarily accelerate the reaction by artificial means such as elevated temperature and alkalinity, potentially distorting the result. This, combined with the fact that many tests historically originated as performance discriminators for the aggregates in one particular geographic region, means the potential for generating false positive or false negative results is considerable. Tests recommended for New Zealand, on the basis of both relative speediness and reasonable fidelity to aggregate behaviour observed in the field, include the ASTM C 289 ‘chemical test’ and the ASTM C 1260 ‘rapid mortar bar method’.

ASTM C 289 measures the dissolution of silica and cor-responding reduction of alkalinity that occurs when a pulverised sample of the test aggregate is mixed with a 1 mol dm-3 solution of sodium hydroxide at 80ºC for 24 hours. When plotted graphically, this pair of results can be used to classify the aggregate’s likely behaviour as ‘innocuous’ or ‘deleterious’ according to boundaries derived from the in-service performance of previously tested aggregates. The method has been criticised for giving misleading results, primarily with aggregates derived from sedimentary sources, but has been found to correlate well with observed site concrete performance for the volcanic rocks that represent the bulk of New Zealand’s potentially reactive aggregate. In the ASTM C 1260 test the suspect aggregate, crushed as necessary to achieve the specified grading, is made up as sand-cement mortar bars of a fixed water-to-cement ratio. The bars are stored in a strongly alkaline solution, maintained at 80ºC for a minimum of 14 days, and their increase in length is measured periodically. A variety of criteria have been suggested to delineate potentially harmful performance, with a maximum permissible expansion of 0.1% after 14 days testing being the most usual quoted. Some authorities recommend longer testing periods to

distinguish between non-reactive and slowly-reactive aggre-gates. It is generally accepted that tests of this type may be somewhat over-sensitive, identifying aggregates known to perform acceptably in the field as potentially deleterious. Variations on this method such as ASTM C 1567 are also used to evaluate the effectiveness of SCMs (supplementary cementitious materials) for the suppression of ASR.

Supplementary cementitious materials. From left to right, fly ash (Class C), metakaolin (calcined clay), silica fume, fly ash (Class F), slag, and calcined shale.

Mitigation techniques

Avoidance of susceptible aggregate is not always a practical or economic possibility. An understanding of the chemistry governing ASR allows for the development of approaches that allow potentially reactive aggregate to be safely used. Feasible mitigation techniques broadly encompass:

- ecreasing the pH of the concrete’s pore solution to suppress the initial silica solubility

- educing the free alkali metal ion (sodium and potassium) concentrations present to restrict gel formation

- educing the permeability of the concrete to restrict water ingress, hence preventing the gel from expanding.

New Zealand has followed a de facto route of controlling the alkali content of concrete, courtesy of an informal agreement amongst local cement manufacturers to restrict the free alkali content of their cement to less than 0.60% Na2O equivalent. Figure 3 demonstrates just how sensitive the development of ASR can be to this parameter. Note that cement is not the only source of soluble alkalis in concrete, however, and contributions from the aggregate, mix water and chemical admixtures also need to be assessed. Normal concrete, as defined by NZS 3104: 2003, made with a potentially reactive aggregate, can be expected to be durable for 50 years, suffering only cosmetic damage if a maximum concrete alkali limit of 2.5 kg/m3 is observed. Where alkali limits of less than 2.5 kg/m3 are not achievable, or the concrete performance requirements are particularly demanding, SCMs can be used to successfully suppress damaging levels of expansion.

SCMs function via a number of beneficial modes: all

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typically reduce the permeability of the concrete, limiting its capacity to imbibe water. By replacing cement, they also physically dilute the amount of free alkali metal ions available in the concrete. SCMs with true pozzolanic characteristics react with the calcium hydroxide produced by the cement hydration reaction, helping lower the alkalinity of the concrete below that of the pH needed to support the formation of the alkali-silicate gels. It is also believed that these gels can only be formed in the presence of calcium ions, whose concentration is reduced by the pozzolanic reaction. Paradoxically, finely divided and highly-reactive amorphous silica in the form of silica fume or equivalent natural silica sinter is one of the most effective means for reducing the risk of

ASR. This can be explained by the fact that where reactive silica contents are initially low, adding additional silica, e.g. in the form of a reactive aggregate, ordinarily increases the potential for gel formation. However, if the silica addition is great enough, and the particles’ size fine enough, the ratio of free alkali metal ions to the surface area of the reactive silica becomes unfavourable, suppressing the formation reaction. This mechanism is also assumed responsible for the phenomenon of ‘pessimum’ proportions observed with certain aggregates, whereby the severity of the ASR developed has a clear maximum which is not found to correspond with a ratio of reactive to unreactive aggregate in the concrete equal to 100%.

Conclusion

From this brief overview of the existing literatures on ASR, the following summarizes present knowledge on ASR:

- ASR is a reaction between the OH in the pore solution with amorphous or poorly crystallized silica in the aggregates.

- The reaction product imbibes water and expands.- The presence of water or RH higher than 80’% is

necessa~ for the gel formed to expand and induce concrete cracking.

- Some siliceous mineral admixtures deplete the alkalis horn the pore solution, lowering the pH, therefore decreasing the likelihood of ASR.

- The aggregate type and size distribution play a significant role in the expansion measured in concretes

- Other factors influencing the cracking due to ASR include air entrainment and possibly W/c.

A combination of currently identifiable trends will likely increase the need for concrete suppliers and specifiers to take precautions against ASR damage in the immediate future. These trends include: economic and environmental pressures encouraging reduction of the kiln temperatures used for the manufacture of cement, thereby increasing its alkali content; depletion of aggregate sources in urban

areas requiring the use of potentially deleterious material (e.g. Taranaki andesites); and routine recycling of wash-water in ready-mix plants allowing alkali levels in mix water to exceed those normally encountered in potable town supplies. However, the industry is fortunate to have access to a wide range of SCMs capable of both enhancing the fresh and hardened properties of concrete and reducing the risk of ASR.

Referencen

- Hobbs D. W., Alkali-Silica Reaction in Concrete, Thomas Telford, London 1988

- Diamond S., Penko M., “Alkali Silica Reaction Processes: The Conversion of Cement Alkalis to Alkali Hydroxide” ,G. M. Idorn Inter. Symposium, Durability of Concrete ACI SP-131, 1992

- Diamond S., “Alkali Reactions in Concrete Pore Solutions Effects”, Proc. 6th Int. Conf, Alkalis in Concrete, Idom G.M. and Rostam S. eds. 1983, p. 155-166

- Figg J., “An Attempt to Provide an Explanation for Engineers of the Expansive Reaction between Alkalis and Siliceous Aggregates in Concrete”, 6th Int. Conf Alkalis in Concrete Copenhagen 1983.

- Helmuth R., Stark D. “Alkali-Silica Reactivity Mechanisms”, in Materials Science of Concrete III, J. Skalny cd., ACS 1992

- Stark D., “Alkali-Silica Reactions in Concrete”, in Simificance of Tests and Properties of Concrete and Concrete Makirw Materials, Klieger P. and Lamond J. eds. ASTM STP 169C, 1994

- Idom G.M., Johansen V., Thaulow N., “Assessment of Causes of Cracking in Concrete”, Materials Science in Concrete III, Amer. Ceramic Sot., New York, 1992

- Alkali-Silica Reaction and High Performance Concrete, Chiara F. Ferraris, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, August 1995.

- ASTM C 289–03. Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method). American Society for Testing & Materials, Philadelphia, USA.

- ASTM C 295–03. Standard Guide for Petrographic Examination of Aggregates for Concrete. American Society for Testing & Materials, Philadelphia, USA.

- ASTM C 1260–05a. Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method). American Society for Testing & Materials, Philadelphia, USA.

- ASTM C 1567–04. Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar- Bar Method). American Society for Testing & Materials, Philadelphia, USA.

- Freitag SA, Goguel R and Milestone NB. 2003. TR 3 Alkali Silica Reaction: Minimising the Risk of Damage to Concrete. Guidance Notes and Recommended Practice. 2nd Edition. Cement and Concrete Association of New Zealand, Wellington, New Zealand.

- Mindess S and Young JF. 1981. Concrete. Prentice-Hall Inc, New Jersey, USA. Neville AM. 1995. Properties of Concrete. 4th Edition. Addison Wesley Longman Ltd, Essex, UK.

Photo Courtesywww. robertdmoser.wordpress.com, www.bam.de, www.bam.de

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Concrete masonry has many proven sustainable benefits including low maintenance requirements, long lifecycle, high recyclability, high reusability

potential, and lower energy cost over life span. The concrete masonry industry could become even more sustainable by

reducing the use of Portland cement, whose production generates approximately one ton of carbon dioxide per produced ton. A possible way to achieve such a vision is to increase the substitution levels of fly ash and ground granulated blast furnace slag for Portland cement in

Masonry Grout for Sustainable DevelopmentSonjoy Deb, B.Tech,’Civil’

Associate Editor

The use of High Volume Fly Ash (HVFA) in construction is a solution to environmental degradation being caused by the cement industry. The concept very much fits into the era of sustainable development. As cement industry, itself, is responsible for 7% of world’s carbon dioxide emissions, responsible for global warming, attention needs to be drawn by construction industry to solve the problem. High Volume Fly ash mix contains lower quantities of cement and higher volumes of Fly Ash (up to 60%). From the literature available, it is found that the proportions of Fly Ash in Concrete can vary from 30% - 80% for various grades of concrete.

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masonry grout – low substitution levels have already been used for many years. The high volume replacement of Portland cement will most likely not cause a decrease in cement’s production, but it will cause a better use of available resources. There are several benefits of increasing the substitution levels of fly ash and slag for Portland cement in masonry grout. The benefits include:(a) using 100% recycled materials, (b) reducing their disposal in landfills, ponds, and (in many places around the world) in river systems, (c) making construction more affordable because less expensive materials are used, (d) possible construction industry expansion without increasing green-house gases emission, (e) making the masonry concrete construction more competitive, and (f) alleviating the demand for Portland cement, especially in developing countries where masonry construction is the preferred construction method. All these benefits, however, can only be achieved if these materials can be used without compromising building code requirements.

History

The earliest literature available on the use of Fly Ash is in 1932 which was carried out by Cleveland Electric Illuminating Company and The Detroit Edison Company. However, the use of Fly Ash in concrete was first carried out by Davis and his associates in University of California in 1937. Though extensive research was carried out throughout the world to promote the use of Fly Ash in construction, only a few milestones could be achieved till1960 and that too in developed countries only. As far as India is concerned, the first ever study on use of fly ash in concrete was carried out in 1955 by CBRI, Roorkee, in the form of a review of American and Australian research work on Fly ash. Later, Fly ash was used in small proportions in mass concreting for dams and other hydraulic structures.

Fly-Ash sample as taken from as ash impoundment

About Fly Ash

Fly ash is a fine, light-brown powdery waste product obtained from the dust control equipment of coal-fired power plants. A large volume of this waste product is produced every year. It is a pozzolan which combines with calcium hydroxide in the presence of water to form cementitious compounds. Though some secondary uses for it are found, much of it finds its way to land fills. For over 60 years, fly ash has been employed in Portland Cement concrete as a supplementary cementing material. Fly ash for use in concrete products must meet the requirements of ASTM C618, which defines two classes of fly ash: Class F (which requires a source of calcium hydroxide such as cement or lime) and Class C (self-cementing). Class F is typically used in concrete products. In concrete products, fly ash slows the rate of compressive strength gain and acts as a plasticizer, so it improves the workability of plastic grout. Replacement of up to 15% (typically by weight)of Portland cement by Class F fly ash is currently a common practice in grout mix designs.

Need of High Volume Fly Ash Addition

For a variety of reasons, the concrete construction industry is not sustainable. First, it consumes huge quantities of virgin materials. Second, the principal binder in concrete is Portland cement, the production of which is a major contributor to greenhouse gas emissions that are implicated in global warming and climate change. Third, many concrete structures suffer from lack of durability which has an adverse effect on the resource productivity of the industry. Because the high-volume fly ash (HVFA) concrete system/masonry grout addresses all three sustainability issues, its adoption will enable the concrete construction industry to become more sustainable.

Characteristics of HVFA Grout/Concrete

The characteristics defining a HVFA concrete mixture are as follows:

- Minimum of 50% of fly ash by mass of the cementitious materials must be maintained.

- Low water content, generally less than 130 kg/m3 is mandatory.

- Cement content, generally no more than 200kg/m3 is desirable.

- For concrete mixtures with specified 28-day compressive strength of 30 MPa or higher, slumps >150 mm, and water-to-cementitious materials ratio of the order of 0.30, the use of high-range water-reducing admixtures (super plasticizers) is mandatory.

- For concrete exposed to freezing and thawing environments, the use of an air-entraining admixture

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resulting in adequate air-void spacing factor is mandatory.

- For concrete mixtures with slumps less than 150 mm and 28-day compressive strength of less than 30 MPa, HVFA concrete mixtures with a water-to-cementitious materials ratio of the order of 0.40 may be used without super plasticizers.

Mix Proportions

A leading researcher has given typical range of component materials for different levels of strength of HVFA concrete as shown in Table 1. The most important point to be noted here is the amount of water which is varied within a narrow range between 100-130 kg/m3 by using a combination of one or more tools such as a super plasticizing admixture, a high qualityfly ash, and well-graded aggregate. Depending on the desired strength levels, the content and the fly ash/cement ratio of the binder can be varied. As the water content between the different strength levels does not vary much, it is necessary to increase the cementitious materials substantially to achieve higher strength. When very high strength is needed at an early age, it can be obtained by adopting one or more of the following methods: a higher ratio between Portland cement and fly ash, substitution of a high-early strength Portland cement for ordinary Portland cement, and replacement of a portion of the fly ash with a more reactive pozzolan such as silica fume or rice-husk ash. Figure 1(a), (b) shows grout samples during casting. Figure 2 shows curing of grout specimens in dry and wet condition.

Compression Test Results and Discussion

Compression test samples were made from the grout specimens by saw cutting the grout specimen to the dimensional requirements of ASTM C1019 (as shown in Figure 3). The test samples were cut two days prior to testing. After cutting, samples were returned to their

Strength level(MPa) Low Moderate High

28 days 20 30 40

90 days to 1 year 40 50 60

Mix proportions (kg/m3)

Water 120-130 115-125 100-120

Cement, ASTM Type I/II 100-130 150-160 180-200

Fly ash, ASTM Class F 125-150 180-200 200-225

Water/cement 0.40-0.45 0.33-0.35* 0.30-0.32*

Coarse aggregate,19 mm max

1100-1200 1100-1200 1100-1200

Fine aggregate 800-900 800-900 800-900

Table 1: Typical mix proportions for different strength levels

Figure 1: (a) Grout samples during casting, (b) Grout placement

Figure 2: (a) Grout specimen curing in dry condition, (b) Curing in wet condition

specific curing environment until testing. The specimens were capped and tested in compression in accordance with ASTM C1019 as shown in Figure 4.Grout specimens were cured in either dry or wet conditions. The average compression strength (of three specimens) for the dry cured grout specimens made by replacing 0, 20, 30, 40, 50 and 60% of Portland cement with Class F fly ash, and tested at 0, 7, 14, 28, 42 and 56 days after casting, are shown in Figure 3. Corresponding compressive strengths for wet cured grout specimens are shown in Figure 4.

Figure 3: Saw-Cutting of Grout Specimens from Concrete Masonry Unit

Mechanisms by which Fly Ash Improves the Properties of Concrete

Fly ash as a water reducer-Too much mixing-water is probably the most important cause for many problems that are encountered with concrete mixtures. There are two reasons why typical concrete mixtures contain too much

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mixing-water. Firstly, the water demand and workability are influenced greatly by particle size distribution, particle packing effect, and voids present in the solid system. Secondly, to plasticize a cement paste for achieving a satisfactory consistency, much larger amounts of water than necessary for the hydration of cement have to be used because Portland cement particles, due to the presence of electric charge on the surface, tend to form flocs that trap volumes of the mixing water. It is generally observed that a partial substitution of Portland cement by fly ash in a mortar or concrete mixture reduces that water requirement for obtaining a given consistency. Experimental studies have shown that with HVFA concrete mixtures, depending on the quality of fly ash and the amount of cement replaced, up to 20% reduction in water requirements can be achieved.

Figure 4: Grout Specimens (a) During Compression Test and (b) After Testing

Figure 5: Compressive strength of dry cured specimen

Figure 6: Compressive strength of wet cured specimen

Drying Shrinkage

Perhaps the greatest disadvantage associated with the use of neat portland-cement concrete is cracking due to drying shrinkage. The drying shrinkage of concrete is directly

influenced by the amount and the quality of the cement paste present. It increases with an increase in the cement paste-to-aggregate ratio in the concrete mixture, and also increases with the water content of the paste. Clearly, the water-reducing property of fly ash can be advantageously used for achieving a considerable reduction in the drying shrinkage of concrete mixtures.

Thermal Cracking

Thermal cracking is of serious concern in massive concrete structures. However, with a HVFA concrete mixture con-taining 50% cement replacement with a Class F fly ash, the adiabatic temperature rise is expected to be 30-35oC. As a rule of thumb, the maximum temperature difference between the interior and exterior concrete should not exceed 25oC to avoid thermal cracking. This is because higher temperature differentials are accomplished by rapid cooling rates that usually result in cracking. Evidently, in the case of conventional concrete it is easier to solve the problem either by keeping the concrete insulated and warm for a longer time in the forms until the temperature differential drops below 25oC or by reducing the proportion of portland cement in the binder by a considerable amount. The latter option can be exercised if the structural designer is willing to accept a slightly slower rate of strength development during the first 28 days, and the concrete strength specification is based on 90-day instead of 28-day strength.

Water-tightness and Durability

In general, the resistance of a reinforced-concrete structure to corrosion, alkali aggregate expansion, sulfate and other forms of chemical attack depends on the water-tightness of the concrete. The water-tightness is greatly influenced by the amount of mixing-water, type and amount of supplementary cementing materials, curing, and cracking resistance of concrete. High-volume fly ash, when properly cured, are able to provide excellent water-tightness and durability.

Properties of HVFA Grout

- Easier flowability, pumpability, and compactability.

- Better surface finish and quicker finishing time when power finish is not required.

- Slower setting time, which will have a corresponding effect on the joint cutting and lower power-finishing times for slabs.

- Early-strength up to 7 days, which can be accelerated with suitable changes in the mix design when earlier removal of formwork or early structural loading is desired.

- Much later strength gain between 28 days and 90 days

0%, Dry Cure20%, Dry Cure30%, Dry Cure40%, Dry Cure50%, Dry Cure60%, Dry Cure

Percent Portlandcement replaced withClass F Fly Ash

0-day 7-day 14-day 28-day 42-day 56-day

Grout Age

40004500

350030002500200015001000500

0

Com

pres

sive

stre

ngth

(psi)

4000350030002500200015001000500

00-day 7-day 14-day 28-day 42-day 56-day

Grout Age

Com

pres

sive

stre

ngth

(psi)

0%, Dry Cure20%, Dry Cure30%, Dry Cure40%, Dry Cure50%, Dry Cure60%, Dry Cure

Percent Portlandcement replaced withClass F Fly Ash

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or more. (With HVFA concrete mixtures, the strength enhancement between 7 and 90-day often exceeds 100%, therefore it is unnecessary to overdesign them with respect to a given specified strength.)

- Superior dimensional stability and resistance to cracking from thermal shrinkage, autogenous shrinkage, and drying shrinkage. In unprotected concrete, a higher tendency for plastic shrinkage cracking.

- After three to six months of curing, much higher electrical resistivity, and resistance to chloride ion penetration, according to ASTM Method C1202.

- Very high durability to the reinforcement corrosion, alkali-silica expansion, and sulfate attack.

- Better cost economy due to lower material cost and highly favorable lifecycle cost.

- Superior environmental friendliness due to ecological disposal of large quantities of fly ash, reduced carbon-dioxide emissions, and enhancement of resource productivity of the concrete construction industry.

Conclusion

Throughout the world, the waste disposal costs have escalated greatly. At the same time, the concrete construction industry has realized that coal fly ash is relatively inexpensive and widely available by-product that can be used for partial cement replacement to achieve excellent workability in fresh concrete mixtures. Consequently, in the modern construction practice 15%-20% of fly ash by mass of the cementitious material is now commonly used in North America. Higher amounts of fly ash on the order of 25%-30% are recommended when there is a concern for thermal cracking, alkali-silica expansion, or sulfate attack. Such high proportions of fly ash are not readily accepted by the construction industry due to a slower rate of strength development at early age. The high-volume fly ash concrete system overcomes the problems of low early strength to a great extent through a drastic reduction in the water-cementitious materials ratio by using a combination of methods, such as taking advantage of the super plasticizing effect of fly ash when used in a large volume, the use of a chemical super plasticizer, and a judicious aggregate grading. Consequently, properly cured high-volume concrete products are very homogenous in microstructure, virtually crack-free, and highly durable. Because there is a direct link between durability and resource pro-ductivity, the increasing use of high volume concrete will help to enhance the sustainability of the concrete industry. In conclusion, the high-volume concrete offers a holistic solution to the problem of meeting the increasing demands for concrete in the future in a sustainable manner and at a reduced or no additional cost, and at the same time reducing the environmental impact of two industries that

are vital to economic development namely the cement industry and the coal-fired power industry. The technology of high-volume fly ash concrete is especially significant for countries like China and India, where, given the limited amount of financial and natural resources, the huge demand for concrete needed for infrastructure and housing can be easily met in a cost-effective and ecological manner. The test results from reported literatures as discussed in this paper indicate that up to a 50% replacement of cement in grout may be a sustainable alternative for masonry. In addition, high volume replacement of Portland cement with fly ash in concrete products acts as a plasticizer, which in grout mixes may help increase the flow ability of grout in concrete masonry wall construction. Testing of both grout samples and composite prisms may be considered when using grout mixes with high replacement of Portland cement with class F fly ash.

Reference

- Malhotra, V.M. “High-Performance, High-Volume Fly Ash Concrete.” Concrete International 24(7), 2002, pp. 30-34.

- Malhotra, V.M. “High-Performance, High-Volume Fly Ash Concrete.” Concrete International 24(7), 2002, pp. 30-34.

- Owen, P.L. “Fly Ash and Its Usage in Concrete.” Journal of Concrete Society 13(7),1979, pp. 21-26.

- Jiang, L.H., and V.M. Malhotra. “Reduction in Water Demand of Non Air-Entrained Concrete Incorporating Large Volume of Fly Ash.” Cement and Concrete Research 30,2000, pp. 1785-1789.

- Hanle, L., Jayaraman, K., and Smith, J., CO2 Emissions Proile of the U.S. Cement Industry,U.S. EPA 05-03-2006, 2006.

- Hogan, F, Meusel, J., and Spellman, L., Breathing Easier With Blast Furnace Slag, Rock Products: Cement Americas, Jul/Aug 2001, 11-15.

- Malhotra, V.M., High-Performance High-Volume Fly Ash Concrete, Concrete International, V. 24, No. 7, July 2002, pp. 30-34.

- Bouzoubaa, N. and Malhotra, V.M., Performance of Lab-Produced High-Volume Fly Ash Cementsin Concrete, Concrete International, V. 23, No. 4, April 2001, pp. 29-33.

- Cross, D, Stephens, J, and Vollmer, J., Filed Trials of 100% Fly Ash Concrete, Concrete International, V. 27, No. 9, Sep. 2005, pp. 47-51.

- ASTM Standards, ASTM International:

- C143/C143M, Standard Test Method for Slump of Hydraulic-Cement Concrete, 2010

- C476, Standard Specification for Grout for Masonry, 2010.

- Mehta, P.K. “Concrete Technology for Sustainable Development.” Concrete International 21(11), 1999, pp. 47-52.

- Mehta, P.K. “Durability: Critical Issues for the Future.” Concrete International 19(7), 1997, pp. 69-76.

Photo Courtesywww.explow.com

Fly Ash Concrete

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The Impact of Basaltic Fibre on Selected Physical and Mechanical Properties of Cement Mortar

Basalt is of the family of igneous rocks which means it melts when heated up, as a thermoplastic material. Basalt is volcanic magma which has solidified in the

open air. Basalt has always been used for its hardness in road surfacing and in construction as a filling stone. Also after brought to the molten stage, it is molded into construction parts, as for example floor tiles and as internal lining in steel pipes transporting abrasive and hot compounds. Basalt stones come with different chemical compositions, and only particular chemical compositions and physical

characteristics of basalt allow its extrusion into continuous thin filaments with useful properties. The nominal diameters of these continuous filaments now come in the range 9 to 24 μm. It is mainly used (as crushed rock) in construction, industrial and high way engineering. One can also melt basalt (1300-1700°C) and spin it into fine fibres. When used as (continuous) fibres, basalt can reinforce a new range of (plastic and concrete matrix) composites. It can also be used in combination with other reinforcements (e.g. basalt/carbon).

Sonjoy Deb, B.Tech,’Civil’

Associate Editor

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www.masterbuilder.co.in • The Masterbuilder - July 2012 287

Brief history

Credit has to be given to a Frenchman from Paris, Paul Dhé, who in 1923 got a US patent for extruding filaments from basalt. It is known that in the 1950/60’s in Moscow and in Prague, in today’s Russia and Czech Republic – among other places, research in this field started. In the 60’s and 70’s, intensive R&D efforts took place in the North-West of the USA –which by the way has large basalt deposits. In the 1960/70’s the defense ministry of the Soviet Union got interested in the potential of this technology for military and space applications. All R&D was concentrated in Kiev, Ukraine. Budgets were unlimited. This research eventually became successful. The technology was kept secret and the object of little publication. The research institutes and production plants dealing with it were off limits. It is in 1990/ 92, in Perestroika, the technology was declassified. This allowed its application in the civilian field.

General Basalt Fiber’s Technical Features as Belowt

- Permanent flame retardant resistance: Limiting oxygen index (Loi) >70

- Extraordinary high softening temperature (point): >1200 Celsius degree

- Operating temperature range: from -260 to 760 Celsius degree

- High tensile strength (breaking strength): 3200 MPa- Low elongation at break: 3.1 %- High elastic modulus: 89 GPa- Density: 2.7 gram/cubic centimeter- Low thermal conductivity: 0.035 W/m•K- High sound absorption coefficient: 0.95- Low moisture absorption: 0.1 %- High specific volume resistance: 1x1012 ohm•m- Radiation proof lead equivalent: 0.0073 mm Pb

Water-absorbing capacity of basalt fiber is much less than 1%, of fiberglass - up to 10-20%. For comparison, industrially manufactured fiberglass, especially of neutral composition, absorbs substantial amount of moisture in humid air, which weakens its physical-technical and longevity properties and eventually leads to fiber damage. On con-trary, low nonvolatile water absorbency of basalt fiber ensures stability of thermal and physical characteristics in case of continuous service. Basalt fiber has high chemical stability and pertains to the first dimming class and greatly exceed fiberglass in acid, alkali and steam resistance characteristics. The disadvantages of fiberglass compared to basalt fiber are spinosity of threads, and discharge of the finest dust at disintegration of thermal insulation at thermal-cycle loads. Due to high elastic modulus, basalt fiber strength is 35-40% higher than that of fiberglass - the fiber is more elastic, non-spinous. Materials of basalt fiber

have a greater operating life as compared to materials of fiberglass. Super-thin basalt fiber is firmly knitted by natural cohesive attraction. Basalt fibers are chemically stable to exposures of aggressive means and steam and do not accumulate radiation. Costs of basalt fiber production are markedly lower (by 15-20%) compared with other mentioned fibers manufacture owing to one-stage basalt fiber production scheme. Yield of basalt fiber from basalt is 100%. Notice also that basalt fiber producing facilities are compact, environmentally safe and waste-free (only products of cumbistion of natural gas, cooled and cleaned in filters, are emitted to atmosphere). The sole factor, hindering wide application of basalt fiber in Russia is very low volume of their commercial production.

The present story reports the study on cement mortar with basalt fibre intrusion for its mechanical and physical properties by Institute of Structural Engineering at Poznan´ University of Technology.

German Marina Norderney gets first basalt fibre-reinforced docks. In a multi-phase project, The German marina will be protected from the area’s typical gales and high seas by ground-breaking 21st-century technology. The first such installation in the world, X-Line basalt fiber-reinforced docks from SF Marina are replacing 30-year-old aluminum units.

Outcome Experimental Investigation Carried out in Institute of Structural Engineering at Poznan´ University of Technology :

Tests have been carried out in the laboratory of the Institute of Structural Engineering at Poznan´ University of Technology. The aim of the tests was to check the influence of added basaltic fibre on some selected physical and mechanical properties of cement mortar. An attempt was made to calculate the optimum amount of the basaltic fibre, allowing the best mechanical properties of the mortar to be achieved. For mortar preparation cement CEM I 32.5R and standard quartz sand were used (according to PN-EN 196-1). The tests were carried out to check bending

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strength and ultimate compressive strength after 3, 7 and 28 days. The measurement of shrinkage in the first 28 days of mortar curing was also made. The tests were performed on standard cement mortar prisms with dimensions of 4 x 4 x 16 cm. The results of preliminary tests show that the effective impact of the basaltic fibre on the change of basic physical and mechanical properties of the mortar decreases in case of the fibre amount higher than 2% and lower than 0.2% of the mortar weight. That is why samples for the tests were made by adding to the mortar the basaltic fibre totalling 0.3, 0.8, 1.3 and 1.8% of the mortar weight. The basaltic fibre used for tests was cut into pieces of approx. 6.5 mm. Base prisms with no fibre content (0.0%) were also made up. During the whole period the samples were stored under laboratory conditions in the temperature of 18°C and a relative air humidity > 90%.

Basalt Fibre strands for Concrete

Test results for the bending strength

The results achieved during the tests are presented in (Table 1). The presented results are the arithmetic average of measurements made on six samples. Standard deviation of the obtained results of bending strength is between 1.6 to 8.3%. The analysis of the data included in (Table 1) demonstrates that the addition of basaltic fibre increases the bending strength by 13% on average. The achieved increase is practically independent from the amount of added basaltic fibre. After 7 days of mortar curing the bending strength is highest. The highest increase of the bending strength (of approx. 6.5%) in comparison with the base mortar is achieved by adding basaltic fibre equalling 0.8% of the mortar weight. After 28 days of curing the achieved bending strength was lower than after 7 days. The reason for this was probably the use of cement of high initial strength for tests. For prisms with an additive of fibre equalling 0.3%, 1.3%, 1.8%, the bending strength

decreased by another 7.5% in comparison with the base prisms (0.0%). The lowest decrease (approx. 4.5 %) was noticed in samples with the fibre content totalling 0.8% of the volume. It may be assumed that the main reason for this is high fragility of the used basaltic fibre, its relatively small elongation at break and high adherence to the mortar. The mortar which was used to make samples is characterized by fairly high shrinkage. After more than twelve days of curing, the shrinkage strength causes cracking of the basaltic fibres. This would explain a high increase of bending strength after 3 days of curing and a lower increase after 7 days as well as a significant decrease after 28 days of mortar curing. Observation of the prisms’ cross-sections after the tests revealed that the basaltic fibre was diffused in the mortar at random and spatially. During breaking of the samples, approx. 90% of the fibre got broken and the remaining 10% were torn out of the mortar. This demonstrated considerable adherence of the mortar to the basaltic fibre.

Bending Strength

Basaltic Fibre Content (%)

0.00 0.30 0.80 1.30 1.80

After 3 Days 5.3 6.0 5.8 6.0 6.1

After 7 Days 7.8 8.0 8.3 8.1 8.2

After 28 Days 6.7 6.2 6.4 6.2 6.2Table 1: Bending Strength of Mortar Prism

Ultimate compressive strength

The achieved test results are presented in (Table 2). The presented results are the arithmetic average of measurements made on 12 samples. Standard deviation for achieved results of ultimate compressive strength is between 3.0 and 8.1%. After 3 days of curing a significant increase of ultimate compressive strength (of 10%) was observed only for the amount of 0.8% of the fibre in the mortar. For the remaining tested prisms, with the fibre content of 0.3%, 1.3% and 1.8%, the ultimate compressive strength remains on the same level in comparison to the base samples or is slightly higher, which is within the measurement error. After 7 days of curing we can observe stabilization of the strength for 0.3%, 0.8% and 1.8% of the fibre content. In case of the fibre amount equalling 1.3%, we can see a slight decrease of the ultimate compressive strength. However, the reason of such results might be a measurement error. After 28 days of curing, test results are much more differentiated. For small contents of fibre we can observe a significant increase (over 20% for 0.3% of the fibre content and almost 8% for 0.8% of the fibre content). For the fibre amount equalling 1.3%, the strength compared to that of the base samples did not change and for the content reaching 1.8% there was a visible decrease of the strength (of over 15%). The mortar with 1.8% of the fibre content was characterized by much worse workability

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Ultimate Compressive Strength

Basaltic Fibre Content (%)

0.00 0.30 0.80 1.30 1.80

After 3 Days 30 31 33 30 31

After 7 Days 40 41 40 37 41

After 28 Days 39 47 42 39 33

Table 2: Ultimate Compressive Strength

and worse possibility of its thickening than others. This might have had a decisive impact on its lower ultimate compressive strength.

Basaltic Fibre Content (%) 0.00 0.30 0.80 1.30 1.80

Shrinkage (mm/m) 0.077 0.058 0.060 0.046 0.027

Table 3: Average Shrinkage of Mortars after 28 days of curing

Research Outcome

From the analysis of the test results as well as the observation made during these tests the Researchers concluded that: The addition of basaltic fibre causes noticeable increase of 3 and 7 day bending strength; after 28 days of curing, there is a decrease in bending strength and The highest bending strength in comparison to the base samples was achieved in case of 0.8% of the fibre content after 7 days of curing and the highest ultimate compressive strength was achieved

in case of 0.3 to 0.8% of the fibre content in the cement mortar after 28 days of curing and exceeding the amount of the fibre addition by 1% results in significant worsening of mortar workability and possibility of its thickening and adding the basaltic fibre to the mortar causes a smaller shrinkage, proportionate to the fibre content. Researchers recommended an optimum amount of the basaltic fibre in the mortar, allowing the best mechanical properties to be achieved, ranges from 0.5% to 0.8% of the cement weight. The addition of basaltic fibre in the content as mentioned above will cause the decrease of shrinkage of the cement mortar by approx. 15 to 20%.

Basalt fiber is a modern XXI-century material, combining ecological safety, natural longevity, and fire safety (incombustibility). Good understanding of its useful mechanism continues to be gained. These fibers, alone or combined with others, go through the first transformation step, producing: e.g. wovens and nonwovens, braids, knits and chopped fibers. Coatings can be applied for outdoor use, for oils and greases resistance, for impermeability to water and permeability to air, for anti-slip and anti-soiling, for continuous skin contact, for high abrasion resistance, for heat insulation through intumescence, for colored or fluorescent appearances, etc. Basalt has advantage over glass fibres interms of mechanical, thermal and chemical properties. In a time where we are looking towards fibre’s in concrete, Basalt fibres can be a very good choice considering its beneficial characteristics. However more detailed research may be needed to explore its other effects on the cement concrete/mortar.

Reference

- ‘Aketoma – Basalt fabrics, tubes, prepregs, rods etc.’ http://www.laseroptronix.com

- Jiri Militky, Vladimir Kovacic; ‘Ultimate Mechanical Properties of Basalt Filaments’, Text. Res. J. 66(4), 225-229 (1996)

- Stephen Cater; ‘Editorial’, International Composites News, (40) March (2002)

- ‘Basalt Fibre Products’, http://www.mendex .de/services3.html

- Tengiz Chantladze; ‘Industrial assimilation of the effective type of fibre with multicomponent charge’, http://www.tctv.ne.jp

- ‘Basaltex, The thread of stone’, http://www.basaltex.com- Sergeev et al.; ‘Basalt Fibers – A Reinforcing Filler for

Composites’, Powder Metallurgy and Metal Ceramics, 33(9-10), 555-557 (1994)

- Bednár M., Hájek M.; ‘Hitzeschutztextilien aus neuartigen Basalt-Filamentgarnen’, Technische Textilien, 43 (November), 252-254 (2000)

- http://basaltfiberworld.blogspot.in/2011_05_01_archive.html

Photo Courtesy

www.diytrade.com, www.charterworld.comhoping521.en.ec21.com

Test results concerning shrinkage

The shrinkage values given in (Table 3) are the arithmetic average of measurements made on 3 samples. The measurements were made with the accuracy of ± 0.005 mm. From the achieved results we may see that the highest shrinkage value is achieved in the specimens with no fibre content. Shrinkage after 28 days decreases proportionately to the increasing fibre content in the mortar. Measurements made after 3, 7 and 14 days of curing show that the proportionality of the shrinkage to the fibre content is maintained during the whole 28-day period of measurements. It proves that the phenomenon of breaking of the basaltic fibre, observed during the tests of bending strength after the first week of mortar curing, practically has no impact on weakening of its anti-shrinkage properties. The fibre, broken into small pieces, considerably decreases the shrinkage of the mortar. The data from (Table 3), shows the dependence between the fibre content in the cement mortar and its 28-day shrinkage. It was conclude by the reasearchers that in the tested range of the basaltic fibre content in the mortar, increasing the fibre amount by 0.1% of the cement mortar weight, results in lower shrinkage of the mortar by approx. 0.0024 mm/m, i.e. by approx. 3.1% in comparison to the mortar with no fibre content.

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