MB Precast Concrete Buildings Dec07

7/29/2019 MB Precast Concrete Buildings Dec07

1/20

Precast Concrete

in Buildings

A GUIDE TO DESIGN AND CONSTRUCTION


2/20

Precast Concrete in Buildings

PAGE ii

The use of precast concrete elements is well established as a construction

method throughout the world and provides solutions for a great variety

and complexity of layouts, shapes and faade treatments.

INTRODUCTION

Precast concrete can be incorporated into everybuilding type. Whether the building has a regular oran irregular shape, the entire structure or elementsof that building, such as frame, floors, walls, stairs orbalconies, can all be precast. Precast construction isvirtually unlimited in its application and is suitablefor single and multi-storey construction. In fact,precast building elements should be considered as anoption for every construction project.

Bespoke designs can be achieved using standardprecast components, which need not imply a

modular appearance. Precast elements, includingfloors, stairs and wall panels combine seamlesslywith non-precast elements to produce free-flowingspaces. Curved precast panels with a wide range ofattractive and durable finishes can meet the mostchallenging of design requirements.

An error to be avoided is to take an all precast orno precast approach to design. The key issue fordesigners is to identify which construction method,or mix of construction methods and materials, ismost appropriate for the specific requirements ofthe building.

The most economical solution might well consistof a mix of cast in-situ and factory produced,precast units. Preliminary structural investigationmay identify solutions such as beams and floorslabs fabricated off-site being erected on cast insitu columns. These structural elements are thenintegrated as a composite structure when the in situstructural topping is placed. Thorough considerationof construction options at an early design stageis critical to optimise speed of build, structuralperformance and delivery of the most economicalframe package for each project.

Efficient structures are just one way of providing asustainable building. The precast concrete industryhas recognised the importance of sustainabilityand is funding a research programme to delivera sustainable strategy for manufacturing whichcomplements the progress that is already beingmade. Key issues are being targeted including:

Health and safety

Employment

Supply chain

Social/community

Energy

Waste

Resources

Further up to date information can be found atwww.sustainableprecast.com

Contents1 Benefits of Precast

Concrete

4 Precast Concrete Buildings

6 Precast Concrete Floors

8 Precast Concrete Elements

10 Architectural CladdingPanels

12 Joints and Connections

13 Production14 Site Erection

15 Maximising the Benefits ofPrecast Concrete

16 Case Studies

17 References

More London hotel, London. This 250-room building was constructed using twinwall construction.Courtesy of John Doyle Construction.

Cover pictures:

Main: One Coleman Street, London.Courtesy of Decomo.

Inset top: Malmaison Hotel, Liverpool.Courtesy of Buchan Concrete Solutions.

Inset bottom: Precast beams at London School ofEconomics. Courtesy of Thorp Precast.


3/20


PAGE 1

To obtain best value,designers shouldconsider earlyinvolvement of theprecast concretemanufacturer whowill have considerableexpertise that canreduce cost andmaximise value when

harnessed early in thedesign process.

The advantages of factory production combined withthe inherent benefits of concrete provide compellingreasons to use precast concrete.

In assessing suitability, designers and costconsultants should consider the benefits discussed inthis section:

Cost and programme

Performance in use

Quality

Design

Pre-manufacture

Sustainability

Cost and Programme

EconomyUsing precast elements reduces requirements forformwork and access scaffolding, this saves costthrough reduced resources and by shortening theprogramme. There is less reliance on wet trades,which can be delayed by unfavourable weatherconditions. There are also benefits in using precastelements for specific areas of the building such asstairs, where safe access is immediately availableonce installed.

Speed of ConstructionSpeed of construction and tight constructionprogrammes are primary considerations in mostbuilding projects. To maximise the speed of

construction with precast elements, two criticalfactors should be taken into consideration:

The building layout should be designed tomaximise repetition of precast units.

Construction details should be designedto maximise the number of standardisedcomponents.

Installation times for precast units vary with eachproject, but indicative rates of installation (based onone erection crew) are shown in Table 1.

Table 1: Indicative installation(based on one erection crew)

TYPE OF UNITS NO OF UNITS

Single storey columns 12 to 14 per day

Spine or edge beams 12 to 15 per day

Wall panels 12 to 16 per day(up to 150m2. per day)

Floor units 250 to 350m2. per day

Stairs or landings 12 to 15 per day

BuildabilityPrecast elements are designed by specialists withexperience in ensuring that the structure can beerected quickly and efficiently.

Whole building costsThe Concrete Centre has commissioned independentcost model studies for various types of buildingswhich have demonstrated that the choice ofstructural frame has cost implications for a numberof other elements of the building. For example, aconcrete stability core also provides a division wallbetween the circulation space and the useablespace. A stability system comprising steel membersrequires additional partitioning to create the divisionwall, which increases the comparative cost.

Whole life value

Frame choice and design can have a surprisinglyinfluential role in the performance of the finalstructure, and importantly, also influence peopleusing the building. Therefore, although concretecan often be cheaper, cost alone should not dictateframe choice. Many issues should be consideredwhen choosing the optimum structural solutionand frame material that give best value for theconstruction and operational stages. Inherentbenefits fire resistance, sound insulation andfabric energy storage (thermal mass) mean thatconcrete buildings tend to have lower operatingcosts and lower maintenance requirements. This isalso particularly important when considering theenvironmental performance of a building.

BENEFITS OF PRECAST CONCRETE

The Woodview development in Birmingham utilised precast crosswall and hollowcore floors toachieve quick, economical and high-quality housing. Courtesy of Bison Concrete.


4/20


PAGE 2

Performance in use

Inherent fire resistanceConcrete has inherent fire resistance, which ispresent during all construction phases, and isachieved without the application of additionaltreatments. It is also maintenance free. Concrete hasthe best European fire rating possible because it doesnot burn and has low heat conductance. Furtherinformation can be found in Concrete and Fire [1]byThe Concrete Centre.

AcousticsConcretes qualities make it good for acoustics meaning additional finishes can be minimised. Precastcomponents can meet the highest standards forresistance to sound transmission. Buildings employingprecast components are included in the RobustDetails accepted by Robust Details Ltd under Part Eof the Building Regulations. Further information canbe found in Concrete and Sound Insulation [2]by The

Concrete Centre.

Air-tightnessPart L of the Building Regulations requires pre-completion pressure testing. A building failing thesetests will have to undergo a time-consuming jointsand interfaces inspection process, resealing wherenecessary. Precast cladding improves air-tightnessbecause the large units reduce the number of joints.These joints are also easier to seal because the edgesof the units are flat surfaces.

Vibration controlFor concrete buildings, vibration criteria for mostuses are covered without any change to the normaldesign. For some uses, such as laboratories orhospitals, additional measures may be needed, butthese are significantly less than for other materials.

A study by Pavic et al[3] demonstrated that thevibration criteria for a laboratory with a grid of6.6m x 7.3m could be met with a 400mm deephollowcore unit and screed. The less onerous hospitalvibration criteria could be met with a shallower unit.

Information on how to design structures to controlvibration can be found in The Concrete CentrespublicationA Design Guide for the Footfall InducedVibration of Structures[4].

QualityOff-site production provides a high quality productfor the following reasons:

Accuracy

Precast elements are cast to close tolerances, andchecked in the factory before delivery to site.

Quality control systems, a consistent well trainedworkforce, and widespread use of self-compactingconcrete ensure a high standard of workmanship.

High quality finishesHigh quality finishes are generally achievedthrough the use of robust, purpose made formworkand dedicated concrete mix designs in a factoryenvironment. Sample finishes can be approvedby the client as a benchmark for the projectrequirements. Acceptability of finishes can beconfirmed prior to leaving the factory.

A wide choice of precast concrete cladding finishesand facings is available, including:

Surface retarding and wash-off

Rubbing

Abrasive blasting

Bush hammering

Mechanical grinding and polishing

Acid etching

More information on architectural finishes can befound on pages 10 to 11.

Consistency of concrete supplyFor visual concrete, consistency of colour and textureis important. Precast factories have dedicatedconcrete supplies ensuring consistency of supply andgiving greater control of the constituent materialsused.

Controlled environmentProduction takes place in an enclosed space, givingprotection from the weather, allowing manufactureto occur in all conditions.

Precast concretecan meet the designrequirements forbuildings:

High qualityfinishes

Fire resistance

Long clear spans

Long life

Acousticperformance

Air-tightness

Vibration resistance

Sustainabilityperformance

Precast concrete balcony, complete with finishes, services, fixings and connections, ready to be placedon-site. Courtesy of Marble Mosaic.


5/20


PAGE 3

Design

Long clear spansReducing the number of columns is often importantin developments such as offices, sports stadia andcar parks. Prestressing the concrete can deliver theselonger spans or shallower construction depths.

Proven designs and methodologiesPrecast construction incorporates proventechnologies and methodologies which have beendeveloped over many years.

DurableConcrete designed and built to the requirements ofBS 8500 will have a working life of 50 years, or 100years if required.

Also, as concrete is such a hard wearing material, itcan be utilised in tough environments such as schoolcorridors.

MouldableConcrete can be formed into any shape. The onlylimit is the designers imagination. Repetition ofelements can make even complex shapes affordablefor projects which are cost-driven.

Pre-manufacture

A reliable servicePrecast concrete manufacturers offer a completeservice from design through manufacture toinstallation. The production facilities are in anenclosed environment which ensures continuityof prefabrication. The facilities are managed tomaximise output and meet programme requirement.

Reduction of noise is a further benefit, as precastelements can be erected quietly on-site, minimisingdisruption to neighbours.

Health & SafetyOnce precast floors are installed, they provide a safeworking platform for site operatives. Simultaneousinstallation of precast stairs offers safe and easyaccess between floors once handrails have beeninstalled. Off-site manufacture generally reduces thelevel of activity on site and this can enhance safety.

The Architectural Cladding Association, PrecastFlooring Federation and Structural PrecastAssociation have each published Codes of Practicefor Safe Installation of their respective products.

SustainabilityThe environmental, social and economic impactsof developments are increasingly being consideredduring initial design. Concrete has many sustainablebenefits during both the construction and operationof a building.

Thermal mass/fabric energy storageA concrete structure has a high thermal mass.Exposed concrete, typically floor soffits, allows fabricenergy storage (FES) to regulate temperature swings.This can reduce initial plant costs and ongoingoperational costs, while converting plant space tousable space. With the outlook of increasingly hotsummers, it makes sense to choose a material thatreduces the requirement for energy intensive, highmaintenance air-conditioning. Precast with its highquality concrete finish is well suited to providinguseful thermal mass on exposed surfaces. Furtherinformation can be found in publications from The

Concrete Centre[5,6,7].

Locally sourced materialThe vast majority of precast concrete used in the UKis manufactured in the UK. All the constituents ofconcrete are usually locally sourced:

99.9% of aggregates used in the UK are sourcedin the UK (80% are used within 30 miles ofextraction).

90% of Ordinary Portland Cement is produced inthe UK (there are cement kilns throughout the UK).

100% of UK sourced reinforcement is producedfrom UK scrap steel.

Less wastageStrict control of materials and efficient machineprocesses in a factory environment minimiseswastage and therefore costs. A recent research reportby WRAP [9] concluded that waste sent to landfill isless than 1% of the total material weight.

Other sustainable benefitsConcrete is durable, frequently allowing buildingreuse, rather than replacement. If the building isto be demolished, precast units are increasinglye-tagged: an electronic chip is embedded in theunit and contains the design information. This willallow the unit to be reused in the future. Concretethat cannot be reused is 100% recyclable, as arereinforcing bars.

Precasting reduces the noise and waste from theconstruction site to the factory where it is easier tomanage.

Further information on the sustainability credentialsof concrete can be found in the publicationSustainable Concrete[9].

The Lawn Building, Paddington Station, London. Precast concrete was used to give a high-qualityfinish, enabling the thermal mass of concrete to be exposed. Courtesy of Trent Concrete.


6/20


PAGE 4

PRECAST CONCRETE BUILDINGS

There are three frequently used forms of precast

building construction in the UK: Car park frame and deck (floors)

Crosswall construction

Volumetric construction

In addition, precast elements may be combinedwith in-situ concrete to deliver hybrid construction.Further details are given on page 5.

Precast concrete frames can also be used for:

Single-storey industrial sheds

Multi-storey offices

Public buildings

Some examples are given in the case studies onpage 16.

Car Park Frame and DeckPrecast columns and beams with precast decks

are commonly used for car parks. They can beindependent, free standing or form part of a mixeddevelopment.

Car parks tend towards a standard bay size of15.6m x 7.2m, the longer dimension being usedto avoid columns between car parking spaces.Prestressed precast deck units are ideal for the longspans and two solutions are regularly used:

400mm thick hollowcore units

600mm deep double tee units

The latter can be combined with double tee rampswhich can span up to 16m.

A number of options are available for spanning theshorter distance, including:

Precast beams

In-situ beams (i.e. hybrid concrete construction)

Precast spandrel panels

Spandrel panels are combined beam and wall panelunits. Spandrel panels also act as internal vehiclebarriers or as external vehicle barriers when used atthe perimeter of the structure.

Crosswall ConstructionCrosswall construction, using precast floors and load

bearing walls, is normally associated with multi-storey buildings. This type of construction is idealfor buildings of a cellular nature, for example hotels,student accommodation, housing and apartments.

In crosswall multi-storey structures the wallsare designed as the means of primary support.Longitudinal stability is achieved by external wallpanels and/or diaphragm action involving the floorsand roof, connected back to lift cores or staircases,which may also be formed by precast wall panelsor shaft units. The system provides a structurallyefficient building with main division walls offering ahigh degree of sound insulation between adjacentdwellings or rooms.

Crosswall construction has all the advantages ofprecast concrete construction with the followinghighlights being particularly beneficial:

High quality finishes often it is only necessaryto have a skim coat on the ceilings and walls.

Thermal mass there is a significant thermalmass which is easy to utilise because of theminimal finishes.

Bathroom pods these can be easily integratedinto the structure and be fully fitted out.

Acoustic performance tests have shownthat crosswall exceeds the Part E acousticrequirements by a significant margin.

The cost savings from these and other benefits, suchas inherent fire resistance, should be fully consideredwhen comparing the costs with alternativestructures.

Further information on crosswall construction can befound in a guide from The Concrete Centre[10].

Churchill Square car park, Brighton. Prestressed concrete double tees with curved soffits were used togive long clear spans supported by circular and elliptical columns. Courtesy of Tarmac.


7/20


PAGE 5

VolumetricConstructionProjects such as prison cell blocks can benefit frommodular precast construction which offers particularbenefits, including:

Robustness

Off-site fitting out

Rapid assembly on-site

Independence from extremes of weather projectcertainty

The on-site construction phase is substantiallyreduced by using concrete modules cast asfive-sided boxes (usually four walls plus a roof)in purpose-made steel moulds. The modules will

generally be delivered to site on low-loaders, withthe ground floor units being erected onto a preparedground floor slab. The subsequent units are thensuccessively erected onto the roofs of the unitsbelow.

Units are generally fitted out at the factory withwindows, vents, bathroom and other fittings, plusplumbing and electrical fixtures and fittings. A majorbenefit of the factory production process is that itcan be carried out largely unaffected by weatherextremes. Once on-site, in addition to a reducedconstruction period, there will also be a substantialreduction in site labour requirements.

Hybrid ConcreteConstructionThe combination of precast concrete with in-situconcrete can be seen to make best use of theadvantages of each, which are given in Table 2.

Added to the inherent benefits of thermal mass,durability and good fire resistance, hybrid concreteconstruction can provide straightforward andquickly-built structures that are of high quality andextremely economic.

The use of precast concrete for the major part ofhybrid concrete structures will reduce the overallconstruction time, the amount of traditionalformwork which has to be used and the number

of operatives engaged in wet-trades on-site. Safeworking platforms are created by the adoption ofprecast floor systems, enhancing the level of safetyon-site.

Information on the various precast elements thatcan be used for hybrid concrete construction isgiven on the following pages. More details ofthe options, design and procurement of hybridconcrete construction can be found in a number ofpublications from The Concrete Centre[11,12,13].

Table 2: Benefits of Hybrid ConcreteConstruction

PRECAST IN SITU

Quality Economy

Excellent Finish Flexibility/versatility

Consistency Bespoke situations

Speed of build Continuity

Accuracy Robustness

Toyota (UK) headquarters. Precast concrete,combined with in-situ concrete, was used to givehigh-quality office space. Courtesy of Trent Concrete.

A concrete bathroom pod being lifted into crosswall student accommodation at the University of the West of England, Bristol. Courtesy of Buchan.


8/20


PAGE 6

Hollowcore FloorsHollowcore slabs derive their name from the voids

or cores which run through the units. The corescan function as service ducts and significantlyreduce the self-weight of the slabs, maximisingstructural efficiency. The cores also have a benefitin sustainability terms in reducing the volumeof material used. Units are generally available instandard 1200mm widths and in depths from110mm to 400mm. There is total freedom in lengthof units and splays and notches can readily beaccommodated.

Hollowcore slabs have excellent span capabilities,achieving a capacity of 2.5 kN/m2 over a 16mspan. The long-span capability is ideal for offices,retail or car park developments. Units are installed

with or without a structural screed, depending onrequirements. Slabs arrive on-site with a smoothpre-finished soffit. In car parks and other openstructures, pre-finished soffits offer a maintenancefree solution.

Prestressed units will have an upward camber

dependent upon the span, level of prestress, etc. Thiswill be reduced when screeds/toppings or other deadloads are applied.

Hollowcore units with reinforcement are alsoavailable, generally 225mm deep and 1200mmwide. They have a shorter span capability butdo not exhibit upward camber. They can also bemade available with an integral layer of expandedpolystyrene on the soffit to provide insulation forground floor situations.

TermodeckTermodeck is a specialist application of hollowcoreslabs. The voids within the slab are used as part of

the ventilation system. Air is circulated throughthe voids before being discharged into the room.This enables the benefits of thermal mass to bemaximised through active measures in addition tothe passive benefits.

Solid Prestressed FloorsSolid prestressed units, 75mm or 100mm thick, are

often produced on the same prestressing beds ashollowcore floors. These units are designed to beused compositely with an in-situ concrete structuraltopping between 75mm and 150mm thick.

Coffered Floor UnitsThe increasing importance of reducing operationalor energy in use and the need to expose the highthermal mass of soffits has led to the developmentof coffered floor units of various shapes. Beingindividual, there is usually a cost premium, but withcareful planning the moulds can be reused manytimes, making them more cost effective. The units

are designed to be aesthetically pleasing and cancarry conduits for services.

PRECAST CONCRETE FLOORS

IL= 2.5 kN/m2

IL= 5.0 kN/m2

IL= 7.5 kN/m2

IL= 10.0 kN/m2

Key

Slabdepth,mm

Span, m

a) Hollowcore

350

400

300

250

200

150

10054 76 98 10 11 141312 15 16

350

300

250

200

150

1003.0 4.0 5.0 6.0 7.0 9.08.0

Slabdepth,mm

Span, m

b) Lattice girder slabs

c) Beam and block

350

300

250

200

150

1003 4 5 6 7 98

Slabdepth,mm

Span, m

500

400

300

2506 8 12

Slabdepth,mm

Span, m

800

700

600

7 9 1110 151413 181716

d) Double Tee unit

Span to depth graphs for various precast elements


9/20


PAGE 7

Lattice Girder SlabsLattice girder units comprise a thin precast concrete

biscuit into which a lattice girder made of steelreinforcement is cast. The units are usually 2400mmwide and can be supported with in-situ or precastconcrete beams.

Once in position, reinforcement is fixed to the topof the lattice girder and an in-situ concrete toppingis poured which acts compositely with the precastconcrete. The overall floor depth is generally in therange 150mm to 300mm.

The floor slab can be designed to act continuouslyacross several spans. Void formers can be introducedin the form of polystyrene blocks or spheres madefrom recycled plastics. Different systems are

available from various manufacturers. The voidformers reduce the quantity of concrete used andalso the self-weight of the slab.

Beam and Block FlooringBeam and block flooring consists of extruded orwet-cast prestressed beams between 150mm and225mm deep, together with blocks of various types.These may be purpose-made blocks with rebates tosuit the shape of the beams (tray blocks) or may bestandard concrete masonry blocks which have beentested and certified for use in floors.

Also commonly used are specially shaped extrudedor expanded polystyrene blocks which provide a highdegree of insulation for ground floors.

The use of beam and block is well established inground floors, particularly for housing, with domesticand commercial upper floor use a growing marketsector.

The standard spacing of the beams is to suitthe length of masonry walling blocks (440mm)interspersed with the beams. This may be reduced to215mm when the walling blocks are turned through90. Beams may be placed in pairs to accommodateloading from partitions and in the extreme, under

heavy loading or for long spans or other line loads,beams may be placed abutting each other over thewhole floor.

Double-Tee Floor UnitsDouble-tee floor units are ribbed precast prestressed

concrete units. They can be procured in a variety ofdepths from 300mm to 800mm and even beyondbut the most common unit is 600mm deep as thisconveniently carries office loading over 12m andcar park loading up to 16m. The top flange is usually50mm or 60mm deep and the ribs taper from aminimum of 140mm at the base, widening upwardstowards the underside of the top flange, the taperof 1 in 20 each side allowing for easy lifting out of afixed mould.

Double-tee floor units are produced in standardwidths of 2400mm. They offer greater structuralcapacity at longer spans than hollowcore or latticegirder but often require a deeper floor zone.

The ribbed soffit profile can provide improvedaesthetics in many situations. Account should betaken of the camber of the units, particularly forlonger spans.

Easy to install beam and block flooring.Courtesy of Cemex.

Hollowcore units quickly provide a safe working platform.Courtesy of Hanson.

Double-tee floor units. Courtesy of Tarmac.


10/20


PAGE 8

Precasting elements in concrete can be used to

speed up construction, provide high quality finishesor reduce the costs for specific elements of theframe. The biggest benefits usually come fromrepetition.

Particular elements that are regularly used incombination with other forms of construction are:

Columns - for a quality finish or to reduceprogramme.

Stairs - for a quality finish or for safety.

Balconies - to allow pre-assembly in a safeenvironment.

ColumnsPrecast columns are generally square, rectangular orcircular, although other shapes are possible and canbe cost-effective where there are a large number ofrepetitions. Increments of 50mm on the dimensionsof faces of square and rectangular columns arepreferred. The preferred increment for the diameterof circular columns is 50mm.

Circular columns are routinely cast vertically,limiting them in most cases to single-storey height.Rectangular and square columns can be casthorizontally and the maximum height of columnswithout splices is generally between 20m and 24malthough 15m to 16m is often more economic.Where the columns are continuous through one ormore floor levels they can have corbels or structuralinserts to provide support for beams.

BeamsPrecast concrete beams are reinforced with either

steel reinforcement or prestressed with steel strand.They may be designed to act compositely with thefloor. They can also be designed to be monolithicwith columns especially where these are in-situelements.

Where the beams are supporting precast concretefloor units the beam profiles are generally invertedT-beams or L-beams with the nib designed tosupport the floor unit. However, other profiles canbe manufactured.

Twinwall

Twinwall consists of two precast concrete panelsheld apart by a lattice girder manufactured fromsteel reinforcement. The precast concrete panelsform both a permanent shutter for the in-situconcrete and contribute to the final structuralelement. The surface finish of the panels are goodquality and usually only require a skim coat ofplaster.

The advantage of using an in-situ concrete infill isthat the elements can be readily tied together toform a robust structure.

Twinwall panels can be used for:

Basement walls.

In combination with lattice girder slabs to formcellular structures.

Core walls or lift shafts.

Residential structures with load-bearing partywalls.

PRECAST CONCRETE ELEMENTS

Using precast concrete elements can:

Speed-up construction

Provide high-quality finishes

Reduce costs

Salvation Army headquarters, London, makesuse of precast concrete for exposed structuralelements.


11/20


PAGE 9

StairsPrecast concrete stairs offer a quick method of

providing safe access routes during construction.They remove the need for complicated on-siteshuttering and provide a high quality finish. Theygenerally do not require temporary propping and areoften connected to floors and landings using steelangle joints. Other connections such as continuoushalving joints and intermittent halving joints arealso used. Combined stairs and landing units are alsoavailable.

Precast concrete stairs are particularly cost-effectivewhen duplicated or based on manufacturersstandard mould sizes. The greater the number ofidentical units required, the lower the cost.

Bathroom PodsThe structure for a bathroom pod can bemanufactured in precast concrete. The structuregenerally consists of thin concrete walls and floorwith a single layer of reinforcing mesh. Servicessuch as electrical conduits and pipework can beincorporated into the concrete structure.

After casting the concrete pod the bathroom is fullyfitted out, including all the finishes. The finished podis delivered to site and lifted into position ready forfinal connection of the services.

BalconiesPrecast concrete balconies are manufactured mainly

for use in apartment complexes. Units have steelreinforcing bars projecting from the back which tie-in with the steel reinforcement in the concrete floorstructure. Balcony units are temporarily supporteduntil the structural floor or screed has been placedand reached sufficient strength.

Precast concrete balcony units typically have integraldrainage slots to receive drainage outlets and anupstand to facilitate proper weatherproofing detailsat door thresholds. They may also incorporate tiledupper faces and cast-in fittings for balustrades.

There are proprietary systems available to minimisecold bridging which can be incorporated into the

precast balconies.

TerracingPrecast concrete is used extensively for terracing ingrandstands, stadia and auditoria. Precast concreteprovides a strong, durable and versatile terracing unitthat is quick and easy to install. Importantly, it caneasily be designed to meet the vibration criteria forsports grounds.

There is a large range of associated productsincluding stairs, vomitories, steps, raking beams andcolumns that will enable the structure, as well as

terracing, to be constructed in precast concrete ifrequired.

St Georges Wharf, London.Courtesy of Marble Mosaic.

Precast concrete stairs provide quick, safe access. Courtesy of Tarmac.


12/20


PAGE 10

The use of precast concrete cladding panels offersmany advantages:

Panels are produced by skilled craftsmen inpurpose-built factories and each stage ofmanufacture is inspected in accordance with anindependently certified quality system.

Finish and dimensional accuracy are verified priorto delivery.

Panels are produced off-site while the foundationand frame construction proceed, enabling them tobe delivered and installed on a just-in-time basis.

Panels are erected by teams of specialists whohave been trained in their safe handling and fixing.

External scaffolding is generally not required asfixings are accessed from the rear of the panels.

Panels can be delivered with windows andinsulation fitted in the factory thus furtheraccelerating the work of following trades.

Negligible waste is produced during productionof units as they are fully engineered in thefactory. Sustainability is further enhanced bythe ability to dismantle the cladding at the endof the economic life of the building with panelspotentially being refurbished for further use orcrushed to provide recycled aggregate and scrapsteel.

Variety can be introduced with clever use of the

moulds.

Self-finished PanelsThe most cost-effective cladding panels are thosewith self-finishes, often using carefully selectedmaterials to create an appearance intended to mimica particular natural stone. The surfaces producedmay be textured or highly polished and the surfacetreatments adopted to achieve them include:

Bush hammering

Abrasive blasting

Acid etching

Mechanical grinding and polishing Surface retarding

Rubbing

Selffinished precast concrete cladding panels aretypically 150mm thick and their size is limited onlyby site cranage and/or transportation constraints.These are frequently overcome by use of low-loadertrailers which allow storey-height panels to bedelivered ready for off-loading and hoisting directlyinto place on the structure.

Applied-finish PanelsTypical factory applied finishes include terracotta,glazed bricks, brick-slips and tiles and stone facingssuch as granite, limestone and slate (used in

thicknesses from 30mm to 50mm depending uponthe stone). An individual panel may incorporate inexcess of 100 pieces of stone or more than 1000bricks. Panels may also include a mix of applied andself-finishes that on-site would demand separatetrades or skills with attendant sequencing andmanagement.

Individually SupportedPanelsThe panels are designed to span either from columnto column or floor to floor (see diagram (a) below),

allowing large areas of the structure to be rapidlyenclosed and subsequent weather-dependant tradesto proceed.

The panels are fixed to the frame with bracketsthat are designed to allow for adjustment in threedirections. Usually there is a bracket at each cornerof the panel.

ARCHITECTURAL CLADDING PANELS

Factory produced precast concrete cladding offers almost unlimited scope for architectural expression. A wide variety of low maintenance

and extremely durable surfaces are available, including self-finished options and a range of applied materials. The panels can be either

supported by the frame, be self-supporting, restrained by the frame or be designed to support the floors.

Self-supporting Panels(stacked faades)Cladding panels are often 150mm thick andtherefore have considerable strength to carryvertical loads. An efficient system is therefore todesign the panels to be self-supporting by stackingthem on top of each other (see diagram (b) below)and using the frame to tie them in the lateraldirection. The advantage is that the frame carriessignificantly less load and can be lighter. Differentialmovement between the frame and faade must be

accommodated by the restraint system. They aresometimes referred to as structural panels.

Load-bearing StructuralPanelsAlternatively on-site the panels can be designed sothat they act as part of the structural frame (seediagram (c) below). The cladding panels (usuallysandwich panels) at the perimeter of the buildingsupport the floors, slabs and beams. The advantage isthat there is no requirement for perimeter columnswhich increases the floor area and gives a flush wall

profile. It does however require close co-ordinationby the project team and the cladding systembecomes part of the critical path for the frameconstruction.

InsulationThe insulation required to meet the requirements ofPart L of the Building Regulations can be pre-fixed tothe concrete panel in many ways:

Fixed to the back of the panel, ready for internalfinishes to be added on site.

Fixed between concrete and the applied finishesin the factory.

Fixed between two layers of concrete (sandwichpanel) in the factory.

Panel support options

SS angle supporting

panel fixed to frame

Structural

frame

Horizontal

restraint bracket

Cladding

panel

Shims for

Bearing shims

for tolerance

vertical

tolerance

Horizontal

restraint bracket

Structural floor

Horizontal

restraint bracket

Claddingpanel

Interface between floor

and supporting panel varies

depending on floor system

a) Individual panel supported by frame b) Panel supported by panel below c) Load bearing panel

Horizontal

restraint bracket

Cladding

panel

Cladding

panel

Shims for

vertical

tolerance

Shims for

vertical

tolerance

Cast-in

socket

Cast-in

socket

Cast-in socket

Cast-in

socket

Structural

frame


13/20


PAGE 11


PAGE 11

Polished concreteat Beetham Tower, Manchester.

Courtesy of Trent Concrete.

Precast concrete panel showing exposedaggregate.

Brick pre-fixed to precast concreteat 77 Grosvenor Street, London.


Precast concrete with cast in flintsat West Quay car park, Southampton.

EXAMPLES OF ARCHITECTURAL FINISHES

Bush hammered

Light grit blast

Acid etch

Heavy grit blast

Aggregate transfer

Medium grit blast

Reconstituted stone finishat St Georges Battersea Reach, London.

Courtesy of Marble Mosaic.

Slate pre-fixed to precast concreteat Swansea Museum.



14/20


15/20


PAGE 13

PRODUCTION

The precast industry is constantly developing, as shown at Bison Concretes state-of-the-artprecast factory at Swadlincote, Derbyshire. Courtesy of Bison Concrete.

This section explains the systems and techniquesused by the precast industry and will increaseunderstanding of the processes.

Component DrawingsDrawings are produced by the precast concretemanufacturer for every element showing all relevantinformation, such as reinforcement, the position offixings, penetrations, cast-in items, openings andlifting anchors.

MouldsThe main mould types include:

Adjustable long-line mould systemsThese can be used to cast a variety of beam andcolumn sections. The flexibility to cast sections, ina range of sizes, from one mould, ensures optimumproductivity and facilitates the quick turnaroundof precast components required on fast-trackconstruction programmes.

Flat table mouldsThese are generally used to form panel members.Formers are quickly fixed to the steel faced mouldwith magnetic clamps. After casting formers areremoved and the panel is lifted off the mould.

Tilting table moulds

Their use reduces the handling stresses on panelsand can therefore reduce the amount of handlingreinforcement which has to be cast into the unit.

Battery mouldsThese consist of a series of steel faced sections withvariable perimeter formers positioned to create therequired dimensions. These sections open apart toallow preparatory work and are then mechanicallyclosed and clamped together to form a multi-castmould. Typical applications would include retainingwall panels and other panel sections complete withdoor and window openings, suitable for apartmentor housing developments.

Specialised or bespoke mouldsThese are manufactured to produce a specificone-off range of products. Moulds can bemanufactured from different materials such as timber,steel or fibreglass and may be lined with a range ofpurpose-made patterned liners to imitate naturalfinishes. The choice of mould material is usuallydetermined by the number of casts required andthe complexity of the shape and size of the finishedproduct. Specialised moulds can be made to beadjustable and hence may be used for similar projectsin the future.

Production Techniques

FloorsHollowcore, solid prestressed units, lattice girderunits and beams for beam and block floors aremanufactured on either long-line steel castingbeds or in purpose made steel moulds, often usingautomated casting techniques.

The steel beds used for prestressed elements arethoroughly cleaned prior to use and a releaseagent is applied to produce a quality surface finish.Reinforcing strands are placed on the bed andhydraulically tensioned. The concrete is then placedusing either extrusion, slipform or wetcast machines.All necessary slots and openings are marked and cut.

Once the concrete has obtained sufficient strength

the strands are released, thus prestressing theconcrete. The strands at the end of each unit arethen cut. Where long line beds are used saw-cuttingis used to produce individual units to the requiredlengths.

Wall panelsPanels are produced in flat beds, vertical batterymoulds, horizontal tilting tables or carousel systems(carousel systems allow units to be moved aroundthe factory for each stage of manufacture). Wallpanels produced in vertical battery moulds have asmooth surface on both sides. Panels produced onflat beds and tilting tables have one moulded faceand one side with a trowelled finish. A trowelledfinish is used where walls have further finishes to beapplied or where the face of the panel is concealedinside a cavity wall. Lifting points are cast-in whichare used in the de-moulding process and whenerecting the finished units on-site.

StairsA number of systems are available for casting stairswhich can be cast in bespoke moulds or in adjustablemoulds. Units can be cast in the upright position orin the inverted position. Alternatively, units can bevertically cast on their edge giving a mould finishto the top and underside of the stairs and leavingone side to be hand finished. Simple stair units, or a

combined stair and landing unit, can be produced ina variety of finishes.

CuringThe curing process is an important part of

component manufacture. Heating the concreteaccelerates curing. Heat is applied in various wayssuch as steam or hot water running through anetwork of piping. Other methods include the useof hot air and the application of electrical currentthrough reinforcing strands which act as heatingelements. Covering the components with insulatingsheets to retain heat and moisture helps the curingprocess.

QualityConsiderable emphasis is placed on quality controlat all stages in the production of precast concrete

components. Precast concrete manufacturersgenerally manufacture in accordance with ISO 9001standards or with other internal quality systems.Key areas of quality control include:

Test certificates for materials

Compressive strength testing

Consistence (workability) testing

Mould standard and quality checks

Correct preparation of reinforcement cages/strands check

Cast-in components and fittings checks

Dimensional checks both before and aftercasting

Assessment of early age strength

Quality of finish inspection

Handling and StorageWhen the units have reached the required strength,they are removed from the mould and labelled forlater identification. They are then stacked on bearersplaced at suitable locations or in the case of wallpanels, sometimes in rack systems.


16/20


PAGE 14

The speed of construction is rapid and must beplanned in advanced to ensure an efficient and safeprocess. This section explains the erection processand pre-planning that is undertaken.

Method StatementAt the commencement of each project, a methodstatement confirming how the elements will bemanufactured, transported and erected should beprepared.

The headings covered in this statement shouldinclude:

Safety (including the mandatory safetystatement)

Handling/cranage and transportation (with due

consideration to the weight of the units) Site erection (procedure, programme, sequence)

The design for temporary conditions during erectionshould take into account overall stability and thestresses in individual elements and joints.

Load paths through a partially completed structuremay be different for those in a completed frame.An example is the temporary state when floor unitshave been placed on one side only of an internalbeam. Here the connection should be checked for itsresistance to torsion and if necessary, propped untilthe slabs on the other side of the beam are placedin position.

The design and positioning of any temporarypropping and of the bases are critical to thesuccessful erection of a precast structure. Fixingpoints for props may be incorporated in the designand provided in the precast elements.

FloorsBefore installing floors, passive fall protection (by

means of, for example, crash-deck, air bag or nettinginstallation) should be in place.

HollowcoreUnits are craned individually into approximateposition and then finally positioned by one of thefollowing methods:

lifting with beam clamps

lifting via integral lifting points

barring (final positioning using a crowbar)

Bearings must be sufficiently robust to withstandthese operations. When placing units on masonry,the mortar must be allowed to achieve adequate

maturity before installation commences. The outerleaf of cavity walls must be built up to within225mm of the inner leaf and fully tied.

The units are then grouted using a small aggregateconcrete to provide initial stability and the joints sealedbefore any subsequent floor finishes are applied.

Beam and blockUnits are either delivered in bundles and theirposition adjusted manually or positionedapproximately with finger grabs. The infill blocks/trayblocks should then be installed manually and thefloor grouted before placing screeds or other floorfinishes.

Lattice girder/solid prestressed floorsUnits should be craned into position and groutedbefore topping concrete is placed.

StairsAs with floor installation, adequate provision to

prevent falls should be in place before installation ofprecast stairs commences. Consideration should alsobe given to installation of temporary handrailing tothe stair units to avoid undue risk to the operativescarrying out the work.

Access and CranageThe following are some of the issues whichwill be taken into consideration by the precastsupplier before choosing a crane and finalising theconstruction sequence:

Public safety and on-site safety

Component sizes and weights

Maximum reach of the crane from set-up positionto final component installation

Any constraints such as overhead power lines

Availability of secure standing areas for cranage

Ground bearing pressures for crane loads

SITE ERECTION

University of East London. Five seven-storey and four three-storey structures, creating 788 bedrooms, were erected within 33 weeks using crosswall construction.Courtesy of Bell and Webster.


PAGE 14


17/20


PAGE 15

MAXIMISING THE BENEFITS OF PRECAST CONCRETE

Early Involvement ofPrecast SupplierThe UK precast concrete industry has years ofexperience working on a vast range of projects. Toobtain the maximum benefit of this experience,it is advisable to involve the precast concretemanufacturer at the earliest opportunity. The precastindustry is pleased to give initial advice and contactdetails can be obtained from the trade associations(see below). If a hybrid concrete structure isbeing considered then reference should be madeto Best Practice Guidance for Hybrid ConcreteConstruction[16], for guidance on procurement.

Component

StandardisationWell-designed frame elements in standard sizes canfacilitate economic construction. The dimensions ofbeam and column sections should be standardisedwherever possible, allowing the precast designer tofully utilise available moulds.

The structural grids may have offsets whichform curved or other irregular shapes, withoutcompromising the general uniformity of thestructural grid. This technique is used to good effectin many buildings where a curve is required in onesection of the building or in some of the elevations.

The precast industry can give advice to help achievestandard components.

Lead-in TimesThe design team should be aware of the lead-in

times for the type of precast concrete they areintending to use. Some elements can be obtained ina short period because they are relatively standard(such as hollowcore units) whereas other elementsare more bespoke and a longer period should beallowed for co-ordination, design, mould production,casting and delivery. Precast suppliers can adviseappropriate lead-in times for individual projects.

Co-ordinationBefore the final design of precast concrete can becarried out, the design requirements should be fullyco-ordinated with the design team. A key area to

resolve is the location and size of service voids.

Once on-site, precast elements are installed quickly.It is therefore important to programme the work tomaximise the speed of construction and avoid stop-start erection.

Precast Concrete TradeAssociations

British Precast (www.britishprecast.org) is theumbrella body for the UK precast concreteindustry. It has three product associationsspecialising in precast systems for buildings.

For information on other precast products visit:

Structural Precast Associationwww.structural-precast-assocation.org.uk

Architectural Cladding Association

www.architectural-cladding-assocation.org.ukPrecast Flooring Federationwww.pff.org.uk

Wembley Park Station Capacity Enhancement, London, uses reconstituted stone to frame its curved facades.

Courtesy of Decomo.

The UK precast

industry has a wealthof knowledge and canadvise on:

Maximisingstandardisation

Lead-in times

Detailing Surface treatments

Erection


18/20


PAGE 16

Twickenham SouthStand, TwickenhamA new South stand was required at Twickenham tomatch the profile of the existing East, North andWest stands constructed up to 15 years earlier.

Why precast was chosenPrecast concrete was chosen to match the existingin-situ concrete while, at the same time, providinga fast erection programme. The lower tier of thestructure was erected in just eight weeks and wascritical to enabling this part of the stand to be usedfor an important international rugby match. The2,200 units were erected in that period, includingcolumns, beams, terracing, vomitory walls and shearwalls.

ConstructionTwo-span precast raking beams, which weigh 10tonnes, span from ground level at the perimeterof the pitch onto a central circular precast columnand then onto an in-situ concrete Vierendeel swayframe. Precast concrete beams span between therear two bays of precast flooring providing spectator

concourses on two levels.

The last 10 bays at each end of the South standcurve round to meet the East and West standswhich support the end bay of terracing. Thereforethe precast structure connects to concrete castpreviously from ready-mix concrete on four edges.This required careful control of tolerances duringdetailing, manufacture and erection to achievea good fit of the structure within its peripheralconstraints.

A further 1,800 precast units were used for themiddle and upper tiers as well as 18,500m2 ofprestressed hollowcore and solid slab flooring

together with 220 stair flights.

Project teamPrecast concrete contractor: ABC StructuresPrecast concrete supplier: Bison Concrete Products

Chessington College,KingstonA precast concrete structural frame was chosenfor the teaching block at Chessington CommunityCollege, a one-school pathfinder project for theRoyal Borough of Kingston-upon-Thames. Thethree-storey college facility was constructed byComposite Structures Ltd in two phases.

Why precast was chosenWhere feasible, the project architect - Initiatives InDesign (IID), made a feature of the precast concreteframe by leaving it exposed. This allowed IID to meetKingston RBs key requirements for an innovativelearning environment, including space flexibility,energy efficiency and an ability to adapt to advances

in technology and education during the next 20 to30 years.

ConstructionThe precast concrete hollowcore units are 450mmdeep for spans up to 15.5m and 260mm deep forspans up to 9.6m. They are supported on precastbeams, which, in turn, are supported by precastconcrete columns springing from the foundations.The whole structure is erected very quickly, givinga safe working platform for follow-on trades.The precast concrete finish is suitable for directdecoration.

Project teamArchitect: Initiatives In DesignProject manager: Tuffin Ferraby TaylorMain contractor: Willmott Dixon ConstructionPrecast concrete supplier: Composite Structures

Waste Treatment Centre,Frog Island, East LondonThis is a new waste treatment facility for recyclingand disposal of household rubbish for the boroughsof Havering, Barking and Dagenham. A precastconcrete building has been built to house the stateof the art treatment facilities.

Why precast was chosenAs the waste treatment equipment was suppliedby Ecodeco, which has other precast operatingplants across Europe, it was decided to use thesame construction solutions at the East London site.Concrete also offers a huge level of durability in theharsh environment.

ConstructionThe facility comprises a Mechanical BiologicalTreatment facility (Bio-MRF) which processes180,000 tonnes that was previously sent to landfill.

The Bio-MRF process turns 50% into recoverablefuel and also separates metals and glass to be reusedin industry. The roof of the building consists of1800mm wide double tee units. These are 900mmdeep and span 21.25m. These units are supportedby precast perimeter beams, which in turn aresupported on precast columns. Precast concretecladding units were also used to provide weatherprotection to the building.

Project teamClient: Shanks (East London Waste Authority)Main Contractor: Kier ConstructionContractor: EcodecoPrecast concrete supplier: Tarmac Precast Concrete

Composite Structuress precast concreteteaching block takes shape at ChessingtonCollege, Surrey.

Precast terracing units and raking beams atTwickenham South Stand.

Precast cladding provides weatherprotection to the building.Courtesy of Tarmac.

CASE STUDIES


19/20


PAGE 17

REFERENCESTo download or access many of these publications, visit www.concretecentre.com/publications

1. Concrete and Fire, TCC/05/01, The Concrete Centre, 2004

2. Concrete and Sound Insulation, TCC/04/03, The Concrete Centre, 2006

3. Pavic A, Reynolds P, Prichard S and Lovell, M, Evaluation of mathematical models for predicting walking-induced vibrations of high-frequency floors, InternationalJournal of Structural Stability and Dynamics Vol. 3, No. 1, 107-130, 2003

4. Wilford, MR and Young P, A Design Guide for Footfall Induced Vibration of Structures, CCIP-016, The Concrete Centre, 2006

5. Thermal Mass, TCC/05/05, The Concrete Centre, 2005

6. Thermal Mass for Housing, TCC/04/05, The Concrete Centre, 2006

7. De Saulles, T, Utilisation of Thermal Mass in Non-residential Buildings, CCIP-020, The Concrete Centre, 2006

8. Waste & Resource Action Programme, Waste Reduction Potential of Precast Concrete Manufactured Offsite, WAS003-003, WRAP, 2007

9. Sustainable Concrete, TCC/05/03, The Concrete Centre, 2007

10. Crosswall Construction, TCC/03/26, The Concrete Centre, 2007

11. Hybrid Concrete Construction, TCC/03/10, The Concrete Centre, 2005

12. Goodchild CH and Glass, J, Best Practice Guide for Hybrid Concrete Construction, TCC/03/09, The Concrete Centre, 2004

13. Taylor HT and Whittle R, Hybrid Concrete Construction Design Guide, CCIP-030, The Concrete Centre, due 2008

The entire concreteand cement industry in

your office

The Concrete Centre provides continuing

professional development at your fingertips.

A wide range of presentations, all of which

are CPD-certified with approved learning

outcomes, are free of charge and can be

delivered in your office by our expert team of

regional engineers.

For more information visitwww.concretecentre.com/cpd

If you have a general enquiry relating to thedesign, use and/or performance of cement andconcrete in construction please contact ournational helpline.

Advice is free and available Monday to Fridayfrom 8am to 6pm.Call 0845 812 0000Email [email protected]

AGUIDETO DESIGNANDCONSTRUCTION

ConcreteFramed Buildings

Concrete Framed BuildingsAt the start of each project, a decision is made about the form and material for thestructural frame. This publication sets out to help the designer come to an informeddecision, giving likely structural options for a concrete frame, with useful points tonote written by engineers for engineers. The publication also discusses issues facingdesigners and provides background information on sustainability, innovations inconcrete and best practice.

Publish date: 2006

TCC ref: TCC/03/024

ThermalMass

A CONCRETE SOLUTION FOR

THE CHANGING CLIMATE

Thermal MassOur climate is already changing and will continue to change significantly withinthe lifetime of buildings designed today. This publication provides a general guideto understanding thermal mass and fabric energy storage (FES). It outlines theapplication of FES techniques using cast in-situ and precast concrete floor slabs innon-domestic buildings and gives readers full references to facilitate further reading.

Publish date: 2005

TCC ref: TCC/05/05

Hybrid Concrete ConstructionHybrid Concrete Construction (HCC) combines precast concrete and cast in-situconcrete to take best advantage of their different inherent qualities.This publication provides an overview as to how this can be done.

Publish date: 2005

TCC ref: TCC/03/010

HIGH PERFORMANCE BUILDINGS USING CONCRETE FRAMES AND CLADDING

Crosswall

Construction

Crosswall ConstructionCrosswall is a modern and effective method of construction which uses precast,cellular concrete components to achieve structurally robust, fast, economical mediumand high-rise buildings. This publication explains the benefits of using crosswallconstruction and includes case studies of projects which have benefited from its

effectiveness.Publish date: 2007

TCC ref: TCC/03/26

Listed below are other publications in this series. To download or order free hard copies of any of thesepublications visit www.concretecentre.com/publications.


20/20

www.concretecentre.com

Ref. TCC/03/31

ISBN 1-904818-51-x

First published 2007

The Concrete Centre 2007

The Concrete Centre,

Riverside House,4 Meadows Business Park,

Station Approach, Blackwater,

Camberley, Surrey GU17 9AB

National Helpline

Call 0845 812 0000

Email [email protected]

All advice or information from The Concrete Centre is intendedfor those who will evaluate the significance and limitations ofits contents and take responsibility for its use and application.

No liability (including that for negligence) for any loss resultingfrom such advice or information is accepted. Readers shouldnote that all The Concrete Centre publications are subject torevision from time to time and should therefore ensure that

they are in possession of the latest version.

CI/SfB

UDC

Precast stairwells at Swansea Liberty Stadium.Courtesy of Tarmac.

MB Precast Concrete Buildings Dec07

Documents