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The Architectural Cladding Association

By Susan Dawson

CAST INCONCRETEA guide to the design of precast concrete and reconstructed stone


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Foreword

A short history of Foreword

For me, concrete has its own rhetoric. In architectural and structural terms it is unique. It canachieve structural strength with very low porosity and can be moulded to create three-dimensional shapes and finished with textures which range from rugged to highly refined andpolished.The practice has explored the potential of precast concrete; at St. Johns College,Oxford, we created an underworld of enclosed spaces, suggesting the idea that they were hewnout of the ground; precast concrete was the material which allowed us to achieve this.

Concrete has a complex cultural status. It was a key material in the early days of the ModernMovement; Frank Lloyd Wright used desert concrete for the base of Taliesin in Arizona.Thesedays Tadao Ando has demonstrated its potential for sculptural form; it is also now associated withthe industrial aesthetic of conspicuous thrift.

Yet it suffered from decades of unpopularity.Why was this? It was associated with cheap, badlydesigned social housing, poorly specified and not designed to cope with problems of water run-off.The key to weathering is to use cornices or concealed drainage, as we did in our officebuilding at Crown Place, to throw water back from the faade and to prevent rain falling on ahorizontal surface and draining onto a vertical surface.

Precast concrete is not a substitute.The term reconstructed, although used commonly todescribe a finish with similar characteristics to stone, implies a material pretending to besomething else. Precast is more than that; it is a refinement of concrete, achieving a dimensionalprecision and surface quality by off site manufacture, enabling architects to create components ina way that no other material allows them to do.

Architectural solutions arise from the potential of a material: in the case of precast it is itsthree dimensional adaptability, its strength, and the quality of its surface textures. Understandingthese potentialities is the key to achieving an architectural language of concrete.

Sir Richard MacCormac

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The Architectural Cladding Association

The Architectural Cladding Association (ACA) is a product association within the national body of the precast industry, the British Precast Concrete Federation.

The members of the ACA are the major providers of precast concrete cladding and specialarchitectural products for structural applications in the UK, together with one internationalmember who is based in Hong Kong.The members are fully resourced and experienced toprovide a complete service of advice, design development, manufacture and site erection.

ACA member companies lead the way in top quality factory engineered concrete solutions.Precast fabrication is safe and sustainable. Both cost and programme are predictable and the use

of just in time delivery is much faster than traditional construction methods.

The overriding objective of ACA members is to provide quality with true value.

Members of the Architectural Cladding Association

Histon Concrete ProductsRedland Precast Concrete Products, Hong Kong

Techrete (UK)The Marble Mosaic Company

Trent Concrete

Published by the Architectural Cladding Association,60 Charles Street, Leicester, LE1 1FB

Tel: 0116 253 6161

ISBN: 0 9536773 3 8

Susan Dawson, 2003

All rights, including translation, reserved. Except for fair copying, no part of this publication may bereproduced, stored in a retrieval system or transmitted in any form or by any means, electronic,

mechanical, photocopying or otherwise, without the prior written consent of the ACA. Every

effort has been made to ensure that the statements made and the opinions expressed in thispublication provide a safe and accurate guide; however no liability or responsibility of any kind can

be accepted by the publishers, the authors or the Architectural Cladding Association.

Printed and bound in Great Britain by Brown & Son, Hampshire.

An imprint of the British Precast Concrete Federation.ACA acknowledges the valued support of the British Cement Association in the publication of this book.


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Contents

Contents

6 Introduction

Design

8 Principles of faade design

9 Design and the manufacturing process

12 Structural design

13 Transport restrictions14 Fixings

17 Joints and sealants

18 Design development and drawing process

19 Precast concrete finishes

21 Brick and stone-faced precast concrete

Design for construction

22 The construction process

25 Precast and sustainability

26 Health and safety issues

Weathering

27 Causes and types of weathering

28 Weathering and design

A short history of precast materials

33 The beginnings of concrete

34 The discovery of Portland cement

36 The development of precast materials

Case studies

44 Armagh arts centre, Northern Ireland

48 St George Wharf, London

52 Merrill Lynch headquarters, London

57 Clearwater Court, Reading

60 Central Library, Hong Kong

62 St Anthonys primary school, Singapore64 Office campus, Leatherhead, Surrey

66 Office buildings, Slough

68 St Johns College, Oxford

71 The Lawn Building, Paddington Station, London

74 Office conversion, London

76 Paribas headquarters, London

78 Office building, Paternoster Square, London

80 Toyota headquarters, Epsom, Surrey

82 Housing,Timber Wharf, Manchester

84 Sainsbury headquarters, Holborn, London

86 North stand, Ipswich Football Club

87 Extension, Royal College of Obstetricians &

Gynaecologists, London

88 Office building, Crown Place, London

89 Swimming pool, Oxfordshire

93 J C Decaux headquarters and warehouse, London

95 Bibliography

96 Acknowledgements


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Precast concrete is a building material with gravitas. It hassolidity and strength, factors which recall traditionalconcepts of enclosure, yet it has all the advantages of amodern prefabricated product.

Precast is uniquely versatile. Its composition, based onstone aggregate mixes, can be altered to produce a varietyof colours, textures and finishes; in addition, as a castproduct with high strength, it can be shaped and used ascladding panels to enclose a building, used to createloadbearing structural panels and components, wholestructures or hybrid structures.

The most common use of precast on buildings is as

cladding panels and this is reflected in the contents of thisbook.As a structural material, precast is ideal for this use

it can be shaped to form mullions and spandrels orstorey-height panels. In most cases precast cladding panelsare cast from a mix which will produce the appearanceand texture of natural stone a specification generallyknown as reconstructed stone. Such mixes make thematerial acceptable in environmentally sensitive areaswhere new projects are required to blend in with existingtraditional stone buildings.Precast cladding panels can alsobe faced with brick, natural stone and terracotta tiles.

Introduction

A short history of Introduction

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Principles of faade designEven at the basic design stage,of a faade, the decision to useprecast panels has important implications. To realise thefaade design in a creative way the architect should be awareof potential pitfalls.For example,the design decision to placea panel joint in a certain position sets up a chain of implications - how the panel is made, how it is fixed on site

and how the contract is managed; these in turn will haveimportant results on cost and speed of construction.

Key stages in the design of a precast concrete faade aredetermining where joints between panels will be positioned,and whether windows will be set within the panel or framedby spandrels and mullions. As the panels have to besupported and restrained by the structure, floor levels andcolumn grids will determine to a large extent where jointswill fall.The most economic design results come from theuse of panels as large as practically possible the lowestnumber of panels, joints and fixing hardware giving thefastest construction programme. But this must be balanced

by the limitations of site cranage capacity, transport and, inexceptional circumstances, factory resources.

Precast offers the opportunity to prefabricate theexternal faade in the factory rather than on the site, withall the advantages of economy and speed of constructionwhich that entails.Design decisions on window positions willaffect this. A panel with inset punched windows gives the

opportunity for factory-installed window frames and glazing,whereas windows set between spandrels and mullions needwindow openings which have to be formed by at least 2 andoften 4 panels; frames and glazing would have to be installedon site.

Insulation,when detailed to be applied to the inside faceof the cladding, is also best fixed at the factory.This avoidssite work and the difficulties of fixing insulation around edgebeams and columns.

Design and the manufacturing processIt makes sense for an architect involved in the design of

Design

Holistic approach to design & procurement methodsToo often in the past, with traditional procurementmethods leading to sequential appointment of tradecontractors, it was impossible to arrange for designissues to be addressed in a holistic manner.The resultwas technical problems with compromised details,wasted time and money. In the end it was the clientwho paid. Sir Michael Lathams 1994 report

Constructing the Team addressed this inefficiency andcalled for a radical change in attitudes and culture.Theindustry must change from confrontation to co-operation. By driving out inefficiency and waste, Lathamreported, construction costs could be cut by 30%.Partnering was a key recommendation in the report; itwas a way to manage the design process moreeffectively, with all the key players being appointed earlyenough to exchange information and ideas.Theseproposals were further emphasised in Sir John Egans1998 report, Rethinking Construction.The targets set byEgan are well known and include a demand for moreeffective procurement using strategic alliances. Both

Latham and Egan demand higher quality and greatercertainty of cost and time.Two principles emerge, whichare relevant to all specialist contractors, not only to theprecast cladding industry.1 Abandon lowest capital cost as the value comparator2 Involve specialist contractors and suppliers in designfrom the outset.For precast cladding the key interfaces are with

structurewindows/curtain wallM & E servicesmaterial suppliers eg brick, stone etc

No matter what procurement route is followed, it isvital to have the interface specialist appointed earlyenough to enable detailed design meetings at whichmaterials, specifications and design decisions can betaken against a background of expert knowledge. In thisway the architect, engineer, precaster, glazing contractorand services engineer can contribute to achieving amore effective result for themselves individually andultimately for the client.

Design


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Design

precast components to understand how they aremanufactured. Understanding leads to an efficient andbuildable design; it may also inspire a creative approach.

The economies of mould usePrecast units are cast in purpose - built moulds which maybe constructed of steel, GRP, timber and even concrete.Some manufacturers use tilting steel vibrating tables aspart of their mould strategy. These are very suitable forcasting flat panels and the mould can be tilted to a verticalposition to act as a strong back for handling, reducing

stress and allowing panels to be thinner.Timber is the material most frequently used. A single

timber mould can be used to cast about 30 identical units(tolerances are difficult to maintain after 30 castings).As atypical mould for a complex cladding unit costs severalthousand pounds, efficient mould use is important.

A steel mould is capable of casting several hundredunits but costs about three times dearer than that of atimber mould. Therefore to achieve a similar costamortisation, the steel mould would need to cast at least90 units. Manufacturers seldom come across projects with90 or more units sufficiently alike to justify a steel mould,nor do they see lead time programmes which would allow90 casting days - assuming a daily casting cycle.

In any manufacturing operation, repetition is the keyto economy. 30 identical casts from a timber mould wouldgive an optimum unit cost,but in practice precasters work with much lower repetition and frequently have only ahandful of identical panels.

Design of precast cladding elements should aim for anaverage repetition of more than 10.Although any complex

project will have elements with few or no repeats, theobjective is to raise the average. Similar factors apply to

precast units clad with granite or other stone slabs; theslabs must be easy to handle and of simple design. To

reduce the effects of low repetition, precast elements canbe designed to form groups of relatively similar units, ableto be cast by making small alterations to the mould. Atypical example of how a basic mould can be adapted isshown (above top). A more complex example, the steelmould for floor units of Michael Hopkins & PartnersInland Revenue building in Nottingham, is shown in (abovelower).The mould was made so that a series of differenttimber ends could be inserted where required. Precastersprefer to cast in a sequence of largest to smallest: this mayhave cost and lead-time implications.

Finishing processes affect panel costs and wherepossible alternative acceptable finishes should becompared (see section on precast finishes).

The graph illustrates the effect of repetition of casting on cost. For atypical unit cast in a timber mould, the cost of 30 identical units istaken as unity. As repetition reduces to 10 castings the cost risesgently; below 10 castings it rises rapidly.


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The manufacturing processIn general precast mixes will contain aggregates of lessthan 20mm diameter and a higher than average finescontent to allow a relatively smooth surface finish.Commonly used aggregates, selected from sources in theUK and abroad, include granite, limestone and basalt. Allprecast works have a large aggregate store withcomputer-controlled batching plants to give precisecontrol of the mix.

The precast production process starts in the CADdrawing office where every unit is drawn and thereinforcement is designed. The drawings then go to themould shop, a key stage in the process which requires ahigh degree of skill, as the mould must be strong enoughto resist deflection under the strain of the casting process.

Some manufacturers also use vibrating tables as thebasis of the mould unit.These are smooth-polished steeltables which act as the base of the mould; the sides are

Precast units are detailed in the drawing office Stone facings are positioned face-down in the base of the mould

A timber mould under construction The bending of steel reinforcement has been automated

The reinforcement is laid in the mould Stainless steel fixings are fabricated for cladding components

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Design

formed by timber moulds clamped with jacks.The amountof carpentry work is reduced, and units with commonfeatures can be produced by altering the positions of thetimber sides.

The completed mould is then fitted with itsreinforcement cage, which is usually in the form of deformed high-tensile steel bars.

Cast-in threaded lifting and fixing sockets are alsopositioned at this stage.The mix is poured in and vibrated

to fully compact the concrete.The units must then be left in the mould for at least16 hours or until they have developed sufficient strengthfor handling.Their final strength will range from 40 to 70N/mm

2.

Most precast units are finished to remove surfacelaitance and to expose slightly the underlyingaggregate/cement matrix. This is done by acid etching,rubbing or by grit blasting (see precast finishes).

After casting, precast units are craned into the yard

A mock- up unit ispropped in the yard

The mix is poured into the mould...

and vibrated to fully compact the concrete

Definitions: What is precast, reconstructed and cast stone?Precast is also described as reconstructed stone or reconstituted stone. It should not be confused with what is generallyknown as cast stone.Precast is produced by the wet-cast method, as described above, and is an extremely strong structural material with alow absorption rate and a variety of finishes and textures.Cast stone is generally produced by the semi-dry method, also known as the 'moist earth' mix method.As the nameimplies, the material has a low water content and has the same texture as moist earth when freshly mixed. It isconsolidated in the mould by ramming or tamping. Semi-dry cast stone has a surface texture and colour closelyresembling those of some natural stones. It tends to have relatively lower strength and higher porosity, and can only bemanufactured in fairly small unit sizes.


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Structural designThe most common use of precast on a building is as non-structural cladding panels, but it can also be used asloadbearing structural panels and components and as wholehybrid structures.

Cladding panelsThe panel must be strong enough to resist site-appliedloads (in most cases from wind) and handling stresses(which are usually the greater force).

The panel connections, joint widths and sealants mustaccommodate both movements in the structure andthermal movements of the panel.

Design responsibility rests with the precast specialistwho will design in accordance with BS 8297:2000 Designand installation of precast concrete cladding.

Panel thickness is determined by structural design, bythe need to provide adequate cover to the reinforcement,and by the need for sufficient thickness to contain andretain fixings and lifting devices. As a general guide, a

4metre high and 3metre-wide storey-height panel would

usually be 150-180mm thick; a spandrel 4 - 6metres longwould be 140-160mm thick. Panels with applied finishessuch as brick or stone will be correspondingly thicker.

Loadbearing panels

Precast concrete is of a very high specification and itsstructural properties can be used to advantage either as loadbearing wall units or as part of a complete structure. Loadbearing units are designed in accordance with BS8110:1997The structural use of concrete.

Precast structural wall panels can provide an efficient

structure solution and increase lettable floor area in officebuildings. Precast crosswalls were used at Timber Wharf,Manchester (see case study), to form party walls betweenapartments. The walls were cast with integrated serviceducts and a smooth surface finish so that they did not needto be concealed behind a secondary lining but could be leftexposed.

Precast panels can also be used to provide bracing inframed structures. (See J. C. Decaux case study where thepanels are also cast with integral insulation in a sandwichconstruction).

Precast structures

Precast combines the potential to be used as a structuralcomponents with the ability to achieve a high quality finishwhich needs no additional treatment or fire-proofing. (seeprecast structure case studies)

Hybrid structures

Hybrid structures are those in which structural precastcomponent are combined with structural steel or with in-

situ concrete.In the case of steel hybrid structures, steel stanchions

and beams are pre-encased, in most cases with areconstructed stone mix, and used in combination withprecast components with the same mix.

Concrete hybrids are those in which precastcomponents are combined with cast in-situ concrete.Theprecast, with its high quality finish, is exposed.The cast in-situ concrete,which is generally in the form of beam stripsto give structural continuity, is hidden.

Both forms of hybrid structures are accurate and fastto build. Hybrid structures allow fabric energy storageprinciples to be used, contributing to the energymanagement of the building.

Design

Top: structural panelswith integrated insulationat J.C. Decaux Right: astructural componentwith a high quality finish.Below: hybridconstruction at Toyotaheadquarters building


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FixingsThe primary purpose of fixings is to support the dead loadof a precast cladding panel and to restrain it from thedirectional movement caused by applied loads.

Although the design of fixings varies widely dependingon the type of cladding, the size of the panel and thestructure of the building, it follows a number of generalprinciples. In order to achieve a safe, efficient and cost-effective fixing scheme, a number of basic factors shouldbe addressed at an early stage.

The support of a panel is provided by loadbearingfixings which transfer its weight on to the structuralframe. Loadbearing fixings can take the form of concretenibs cast integrally with the panel.If the raised access floorzone has insufficient depth to accommodate nibs, thepanel can be supported on a pair of stainless steel angles,

each set on shim packs to allow for any adjustment of level. To reduce the tendency for the panel to falloutwards, it should be supported in line with its centre of gravity. It is also better to support panels at their basesrather than to top-hang them, as concrete whensupported remains in compression.

Restraint fixings are intended primarily to resist windloads and allow adjustment for both line and plumb. Fourrestraint fixings per panel are usually used, set as close tothe corner as is practical. Restraint fixings can take theform of a grouted dowel, but are more often designedwith an angle, or plate, which allows the panel to beattached positively to the structural frame as soon aspossible after installation.

The design of restraint fixings must also allow forpermissible deviations in the manufacture and erectionof the panels and the construction of the structuralframe. Each restraint fixing must allow for adjustmentsof typically + or - 25mm in all three dimensional planes.The adjustment may be achieved by the use of shimpacks, cast-in channels and/or slotted holes in the steelfixing angles or plates.A structural frame of concrete orconcrete-encased steelwork may also incorporate cast-in channels. The pair of restraint fixings furthest fromthe panels support fixings should allow for movementcaused by thermal effects and deflection, by using PTFEwashers. Angles, plates and washers with inter-lockingserrated faces will be required in situations where anyload acts in line with a slotted hole.

The support and restraint functions of a fixing can

be combined. A concrete nib may include a hole for adoweled connection, and a support angle may be boltedto a cast-in channel, socket or drilled hole in thestructural frame.

Grade 1.4301 (previously Grade 304) austeniticstainless steel is generally used for both support andrestraint fixings. Grade 1.4401 (previously Grade 316)stainless steel is more suited for use in industrial, highlycorrosive or marine environments. Mild steel fixings mayonly be considered where conditions are permanentlydry, such as on the warm side of a vapour barrier. Insuch circumstances steel angles should be galvanisedand nuts and bolts sherardised for extra protection.

Design

Isometric view showing positions of loadbearing and restraint fixingson one side of a precast cladding panel

A precast balcony unit with projecting bars for stitching into thereinforcement of the main structural slab


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Loadbearing and restraint fixing to a concrete structure. A simpleconcrete bearing corbel with dowel restraint is the most economicaldetail but the height of the corbel may interfere with the raisedaccess floor detail

Loadbearing fixing to a steel structure. The corbel is dropped into apocket in the floor slab

Loadbearing fixing to a steel structure with composite floor slab, withconcrete bearing corbel

Loadbearing fixing to a steel structure; a support bracket bolted or welded to the steel column carries the weight of the cladding panel

Loadbearing fixing to a steel structure with composite floor slab, withsteel bearing angle

Loadbearing fixing to a concrete structure. A steel bearing angle takesup less height than a corbel, so is more suitable for use with raisedaccess floors

Loadbearing fixings


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Design

Restraint fixing to a concrete structure. A simple concrete bearingcorbel with dowel restraint is the most economical detail but theheight of the corbel may interfere with the raised access floor detail

Angle restraint fixing to a steel structure. See notes to angle restraint fixing above right.

Restraint fixing to a steel structure. The steel plate is welded tothe column at the fabricators works, and the detail requiresaccurate positioning of panels and structure.

Restraint fixing usingstainless steel threadeddowel pins between panels.

Restraint fixing usingstainless steel drop dowelpins between panels.Thepins are held inside stainlesssteel tubes until required.

Angle restraint fixing to a steel structure,with restraint socket cast into floor deck.

Detail (as shown above right) of threadedand drop dowel restraint fixing

Angle restraint fixing to a concrete structure. Slotted holes in theangles give tolerance. The stainless steel studs screwed into the socketin the panel have washers and nuts on both sides of the angle. Theseare used as a push-pull device to position the panel accurately.

Restraint fixings


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Joints and sealants

A precast concrete cladding panel can be considered asbeing practically impervious. But this serves littlepurpose if the weather can penetrate at the jointsbetween the units. It is essential to pay the closestattention to the specifying and treatment of joints, frominitial design to installation and maintenance of sealants.

As the maximum overall height that can betransported by road is about 4.9 metres, one dimensionof a panel should not exceed about 4 metres. However,since this is greater than the typical storey height of about 3.9metres, panels will usually be wide enough in

the other direction to match the column grid.

Width of joints

In order to remain attached to the two faces of a joint,the sealant has to be able to accommodate movementdue to thermal or other factors. This capacity iscommonly termed the movement accommodationfactor (MAF) and varies between different sealantmaterials. A typical sealant might be able to stretchabout 25% of its nominal width.Thus for an anticipatedmovement of say 4mm, a nominal joint width of 16mmis suggested.The minimum joint width is generally 10mmplus appropriate allowances for thermal and differentialmovement.

If units are stacked, all thermal movement has to beaccommodated at the top of the stack, and this couldmean a much wider joint than normal.

Even if panels and hence movements, are small, it isnot good practice for primary joints to be less than12mm wide - narrow joints make small dimensionalvariations apparent.

Joint profile

The simplest profile is a straight square joint. This hasseveral advantages: the panel is easier to cast; it allowsfull horizontal adjustment during erection; it gives fullspace for the sealant; it allows inspection of the innerseal; it does not interfere with other aspects.

A double seal is usually specified at the frontsurface. Occasionally a third inner seal of impregnatedfoam material is also used on the inner face.

Sometimes a joggle joint is specified. While thislooks a good solution in theory, in practice it should beavoided as it presents many problems. It is more difficultto cast; it restricts adjustment; it does not provideenough space for a double seal; it prohibits inspection; it

is interrupted by lifting devices; it could collect

moisture and cause problems with freezing.If moisture does get through the outer seal, it willeventually reach a vertical joint and descend. It is goodpractice to allow for drainage at the bottom of vertical

joints. This would also cater for any condensationforming on the inner face of cladding. A properlydesigned faade and insulation should deal withcondensation risks so that no moisture forms in the firstplace.

Shadow gaps and recesses

Recessing joints into the surface of the panel canenhance the simple profile advocated above. This willdisguise the actual line of the joint by making it adeliberate feature of the faade. It also provides anatural route for rainwater to flow down, enabling dirtto be concentrated in predefined areas.

Even within a shadow gap, the sealant should berecessed a further 3mm or so back from the front faceof the concrete. This minimises the risk of primergetting on to the front face and also helps ensure thatthe concrete against which the seal is acting is sound,especially if the surface has been textured.When sealingpanels with a front facing of stone or brick, the secondseal should be against the backing concrete and not thefacing material.

Priming

Concrete and stone should normally be primed beforeapplying the sealant to improve adhesion. Success islargely dependent on thorough preparation and carefulpriming.

Sealant materials

There are several types of sealant available on the market.Of these the most commonly used is the one-part, lowmodulus, silicone or ciloxane rubber sealant. These aregun-applied against a backing strip of foam polyethylene tofill the gap.The outer surface cures on exposure to air togive a smooth finish.

When seals are applied to a stone-faced panel, thereis an increased risk of staining of the stone by thesealant. Recently a range of more advanced hybridsealants has become available. All these materials comein a range of colours allowing a near match to manyconcrete and stone finishes.When properly installed byskilled operators, sealants should give a life in excess of


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25 years. If the seal is damaged, the simplest repairmethod is removal and replacement of the strip affected.

Mix and specificationThe mix for a precast element with a reconstructed stonefinish is complex. Compared to the mixes used forstandard in-situ concrete, it has a higher cement contentand low water/cement ratio, with minimal slump and theproportion and size of the aggregates are closely related tofinish and texture. ACA members have extensive samplelibraries, and will make samples to order before tendering.

Reconstructed stone mixes should be specified against asample approved at pre-tender stage, so that a truecomparison of colour and texture can be obtained frommanufacturers. Members of the ACA are prepared to agreea quality of specification with the client on every contract.Architects should be aware that not all manufacturers willbe capable of comparable quality management.

The design development and drawing processTo develop the design of a precast concrete faadeproject, co-operation is needed from all concerned inproviding information, in making decisions and inproviding approvals at the right time.

Initially the precaster needs basic dimensioneddrawings from the architect and engineer of elevations,plans and sections, together with access to the keyinterfacing specialist contractors.

The precasters drawing office will prepare initialgeneral arrangement drawings (GAs). These are issuedto the architect and engineer in the first instance andmeetings are held to discuss the development of design

principles and particular details.

A common system for indicating approval status is: -Status C Architects and engineers comments must beimplemented and the drawings re-issued.Status B The work can proceed taking account of allcomments.Status A Fully approved with no comments. This is aconstruction status drawing.

As this process proceeds every aspect of the faade isdefined and detailed in co-ordination with otherspecialists who may give and receive ideas and makedetailed improvements. A vital element of the precastersdesign development is the preparation of what is known

as builders work.This covers interface details with otherkey specialist contractors. On cladding contracts, a keyinterface is with the structural frame contractor and theopportunity to have support brackets, restraint fixingplates and holes formed during fabrication saves time andcost during site erection. Equally valuable interfacingshould take place with the window/curtain wallcontractor and the M & E specialist.

A series of detailed drawings is then prepared by theprecaster to communicate and agree the details.Obviously this process can only take place if the

interfacing specialist contractors have been appointed.Sequential procurement largely destroys the opportunityfor such valuable interfacing.

Ultimately the precast drawings become a definitivesource of information enabling the architect and, to a lesserextent, the engineer to review the full detail of the scheme.

Shop drawings

Part way through the GA approval process, i.e. whensufficient drawings are at B and A status, the precastersdrawing office begins to prepare shop drawings. Shopdrawings are only usually circulated within the precastfactory. These are created by extracting each faadepanel from the GAs and producing manufacturing detailsfrom which the mould is fabricated, the reinforcementcut, bent and assembled and the cast-in hardware e.g.lifting and fixing sockets, is positioned. Finishes andconcrete mix specification are defined.

GA preparation may involve many design meetings;provision of information and speedy decisions andapprovals are vital to the efficient progress of

manufacturing to enable the factory to manufacture inthe necessary sequence and volume to satisfy the siteconstruction programme. If the GAs become morecomplicated, the unit types are certain to increasewhich, in turn, generates more shop drawings. Thisresults in more moulds and more mould alterations.

The GAs are key drawings used by planners forproduction and construction programming and later bythe erection team to assemble the cladding panelscorrectly and accurately on the structure.

For the ACA member, every project is unique andprogramme times are planned carefully to suit a particularproject. However to give guidance on a typical 1 to 1.5million project the following periods are reasonable, as

Design


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shown in the typical programme opposite. Clearly the

precaster must have good, basic, thoroughly dimensioneddesign drawings from the architect and the engineer with theinstruction to proceed. A reasonable turnaround of comments on GAs and approvals is essential to start theshop drawing preparation; two weeks is the industrystandard. Changes of mind or last minute decision-makingcan result in lengthy delays and rapidly escalating costs.

Precast concrete finishesThe way in which a precast surface is finished will have adistinct effect on its appearance. Some techniques exposethe aggregate in its natural state; others physically changethe appearance by abrading or fracturing the surface.Within each technique the degree of exposure can bevaried, with the result that a considerable variety of effectscan be achieved.

Finishing processes can be divided into two basiccategories: wet and dry. Common techniques used tofinish precast concrete components are described below.

Wet techniques

Acid etching Acid etching is a method of removing the very thin layerof laitance, formed by fine aggregate and cementparticles from the concrete surface, exposing thetexture and colour of the matrix beneath. Hydrochloricacid in either diluted or gel form is used to etch thesurface.The depth of exposure is controlled by the levelof dilution and/or the length of time the acid remains incontact with the concrete before it is washed off withwater. Surfaces may also be etched more than once if agreater degree of exposure is required.

Care should be taken when acid etching verticalsurfaces to avoid streaking. Very light degrees of exposure should be avoided as this often fails to remove

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Top: polished columns on a grit blasted plinth.The corner panelsbehind were tooled to give horizontal rusticated bands.Below: an acid-etched surface is washed down with water

TYPICAL PROGRAMME

Weeks

Operation 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Precaster receives instruction to proceed

Design development inclusive of G.A. plans

Issue of builders work information

Shop drawings

Initial moulds

Manufacture

Site start


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Design

all the laitence; the residue may tend to craze in time asit is exposed to the weather.

Ideally etching should be undertaken three to fourdays after casting, when the concrete has attainedsufficient surface hardness but not to the extent that it

is difficult for the acid to penetrate.After etching, minorblowholes exposed by the removal of the laitance mayhave to be rubbed in.

Retarding

There are three main methods of retarding: a retardingagent is painted on to the formwork surfaces; retarderpaper is laid in the formwork; surface retarders areapplied after casting. In each case the retarder preventsthe surface of the concrete from hardening and allows itto be removed by either high pressure washing with wateror by brushing. The depth of retardation is controlled byusing different strengths of proprietary products.

Dry techniques

Grit blasting

Grit blasting can produce a finish similar to acid etching orit can be used as a more aggressive means of exposing thecoarse aggregates. Different grades of grit - from fine to

coarse will determine the depth of exposure revealed.In its most aggressive form grit blasting will physicallyabrade and fracture aggregate particles.The equipment isdriven by compressed air and the force at which the gritparticles hit the concrete surface is controlled byadjustment of the air pressure.

Tooling

Tooling is undertaken with a variety of pneumatic orelectric hand-held equipment,ranging from needle guns tobush hammers and chisel point tools.The points within aneedle gun may be varied in length depending upon thedepth of exposure required.This will also be influenced bythe pressure used and the duration of the treatment.The

A selection of grit blasted precast units


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same applies to the use of other tools.Tooled finishes givea more rustic appearance due to the aggressive nature of the finishing techniques.

Polishing

Precast panels can be polished to varying degrees of smoothness; from a honed matt finish to a high glosspolish which can resemble that of granite. Polishing of small components, or those with rebated surfaces, is bestcarried out by hand; large, flat precast units can bepolished mechanically.

A typical polishing machine will move over the precastunit with a preset polishing and pressure programme.Using a diamond-tipped plate, it will grind about 3mm off the face of the panel; this may reveal a coloured aggregate.A series of abrasives is then applied to the surface. Thefinal finish depends on the fineness of the polishing headsand the number of times they are passed over the panelsurface.

Brick and stone-faced precast concrete panelsBrick-faced precast panelsTraditional brickwork is a popular and successful facingmaterial, combining a traditional appearance with thequality, strength, speed and durability of precast concrete.

Brick: properties and selection

A brick with good uniformity of colour will minimise therisk of colour changes on different panels.The ideal typeis a brick perforated with three holes; a clean cut throughthe holes will provide a secure anchorage for fixing intothe panel. Solid bricks can be cut to give a dovetail anchor.

The tolerances set out in BS3921 are not really tightenough. A measured length of 24 bricks will vary from5235mm to 5085mm, or 3mm per brick. Most supplierswill improve on these figures by arrangement. Ideallybricks should be made with a tolerance of +1to 2mm.

Ideally a maximum water absorption figure of 12%should be specified.

A brick manufacturer should be chosen who is able tosupply all the bricks including any specials and who is ableto cut standard bricks.

Delivery period

Bricks with an extended delivery period for specialsshould be avoided; the production of a panel depends on

all the bricks for that panel being available.

Panel size and thickness

Panels should ideally be sized in normal brick modulesas with any wall.There is no practical limit to the size of panel other than that of transport.When designing thepanel to take loads, including self-weight, the thicknessof the bricks is not taken into account.

Brick layout

Any normal brick bond can be provided, although

excessive use of headers should be avoided. Edges of panels, particularly at returns and reveals, should be

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Top: a polishing machine in action Below: two examples of polishedfinishes


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Stone-faced precast concrete panelsThe methods of attaching natural stone or granite to aprecast concrete cladding panel were developed in theearly 1970s, in particular for the faade of the EMIcentre in Tottenham Court Road, London. It was abuoyant building period for the commercial sectorwhich highlighted the many advantages of off-siteprefabrication, such as the achievement of a speedyscaffold-free enclosure, using large components with anengineering quality of accuracy. Since then nearly allnatural stones have been used with precast systems,

including limestone (both hard and soft), sandstone,granite, slate and marble.The major challenge faced bythe precast concrete cladding industry and natural stonesuppliers was that the natural facings are fairly thincompared to the concrete panel and have different ratesof thermal movement. A method of attachment wasrequired which would not only support theconsiderable weight of the facing but also allowsufficient independent movement to eliminate thermalcracks or even worse, damage to the mechanical fixingsystem.

The following method of attachment has proved tobe so efficient and durable that, on more that oneoccasion after a terrorist bomb attack, the stone facingshave remained in position on the precast concretepanels even when the latter have been totally dislodged.

Preparation of stone facings

The stone supplier prepares the stone facings inaccordance with the concrete panel drawings or cuttingschedules. Tolerances are tight and it is common to

specify plus or minus 2mm in every direction. Codes of Practice state the minimum thickness for each type of stone; the supplier will slice the stone two millimetresthicker than the minimum requirement.

The stone supplier drills holes in the back face of the stone; these are to accommodate 6mm diameterstainless steel pins inclined at an angle of between 45 to60 degrees at a rate of approximately 11 to the squaremetre or whatever is demanded by other influences.Thepin will project approximately 60mm into the concretepanel and will be glued into the stone to a depth of atleast two thirds of its thickness. Neoprene grommets

approximately 15mm long are added to the pins to takeup differential movement between the two materials.

Laying the facings in the mould

The stone facing panels are positioned face-down in themould; this is normally a conventional timber mouldthough, in some cases, the stone facings may span an opengrid of support timbers as the seal is at the back face of the stone.

Joints

The joints between the stone facings are usually 6 to10mm wide to facilitate the application of either hard ormastic pointing after casting.The joints are then sealedwith a waterproof tape to eliminate ingress from thebacking concrete. Sometimes a polyethylene rod isplaced between the modules as an added precaution, forinstance when the rear face of the stone is uneven.

Movement

To allow for differential thermal movement, it isnecessary to provide a bond breaker between the stoneand the concrete, the most common form being asimple polythene sheet; liquid applied silicone or a PVAsolution has also been used to good effect.

Casting

The backing concrete (usually approximately 150mmthick) and reinforcement cage is put in place and themould is vibrated in the usual manner.Apart from pointingbetween the stone facings, very little attention is requiredto the unit, except for a clean down.

This is the only method of support for natural stoneand its success is endorsed by its inclusion in BS 8298.

The same process is adopted for attaching naturalstone to complex features such as arches; an example isshown in the case study of the new Merrill Lynchheadquarters in London. In most respects the designand manufacturing requirements of stone-facedconcrete panels are identical to any other category of precast concrete cladding. When produced in apunched panel arrangement, windows can be fixed atthe precasters factory and insulation can be provided asan integral part of the system.

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The construction processPrecast concrete panels are ordered before constructionstarts on site. Early agreement on panel design enables theprecaster to undertake manufacture, while on siteactivities begin in earnest; the two activities then continuein parallel. Precast manufacture allows a phased delivery,so units can be delivered to site to an achievable and

agreeable programme that is pre-arranged with the maincontractor or construction manager.

Panels are delivered in a near vertical position(depending on height) supported by A frames fixed to atrailer or, in the case of very large panels or small units,delivered flat on a trailer bed.

Once on site, panels are fixed by the manufacturersown team or a specialist precast erector. They are fittedwith cast-in lifting devices so that the panel can be lifted ina single crane movement from the delivery vehicle andplaced directly on to the building a form of just in timeconstruction. This makes for faster construction, avoidsdouble handling (taking up hook time on the site crane)and eliminates the need for storage on site. Erection takesplace immediately after the structural frame has beencompleted to give (together with the roof) aweatherproof envelope for following trades. Scaffolding isnot usually required other than when fixing panels on tothe face of plain walls, for example shear walls, where therear of the panel is inaccessible.

Pre-levelled shims support the panels in a level

position and restraint fixings secure them. When thepanels are safely in place, the crane hook is released asquickly as is safe to do so.After a group of panels has beenfixed, they are plumbed and lined in together. About 30panels can be fixed in a typical working week, whichincludes preparation (e.g. bolting on brackets) and finishingtasks such as applying fire-stopping. Very little exteriorfinishing work is required, apart from sealing jointsbetween panels (frequently done from mobile cradles).The external skin is completed with thermal insulationand dry lining.

Co-ordination with other tradesGreat saving in costs can be made by co-ordinating design

and detailing of the cladding with interface contractors structure, glazing and M&E.

Co-ordination of panels and glazingAlthough windows may be fitted on site, it is also possiblefor entire glazing units to be fitted at the precast factory.Factory-fixed window frames and glazing require the

window contractor to be appointed early enough forframes to be fabricated and supplied to the precaster forinstallation. More interest is being shown in the time andcost benefits that this brings.

Co-ordination of fixings and structureRestraint fixings to cladding panels generally consist of stainless steel angles and plates bolted to the structurewith stainless bolts (for details see Fixings section). Asingle panel might have over 100 worth of stainless steelhardware.Traditionally restraint fixings were applied afterthe panel had been positioned. But with early co-operation between the cladding precaster and thedesigner of the structure, it is frequently possible toarrange for fixings to be built into the structure. Socketsor channels can be cast into a concrete frame,avoiding theneed for site drilling;fabricated fixing plates can be weldedto a steel structure, with the additional advantage thatthey are then regarded as primary structure and can beformed of mild steel rather than stainless.

If mild steel is used, stainless steel securing bolts must

be suitably isolated to avoid bi-metal reaction, and thefixings would have to be positioned on the inside of theinsulation and vapour control layer.

If the fixing position is pre-determined, site erection of the panels is easier and therefore faster and considerablecost savings can be achieved.A pre-drilled fixing hole in asteel member costs a fraction of a site-drilled hole. Exactlythe same is true of casting fixings in to concrete frames.

The design of the superstructure should take accountof the need for consistency at all levels so that thepositions of fixing angles, plates, bolts and packers arerepeated. Edge details in particular should be as consistentas possible; this could mean, for instance, the use of anedge beam of similar dimensions on all levels, even though




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this is not justified by the span.The same principles apply to the design of fire stops;

and sealants should be straightforward and simple. Simpledetails ensure speed and quality.

Pre-fixing insulation to precast panelsBy fixing thermal insulation to the inside face of thecladding at the precast factory, site work is avoided and itis easy, rather than difficult, to ensure that the insulationpasses edge beams and columns. The use of insulatedsandwich panels (where insulation is included within theprecast cladding) further enables these tricky site

activities to be minimised (see JC Decaux case study).

Positive procurementAn increasing number of activities carried out traditionallyby following trades on site are being relocated to theprecast factory, where work (often multi-skilled) can becarried out in much more tightly controlled conditions. Inaddition, precast concrete offers other major benefitssuch as faster speed of construction on site, less wasteand better long-term building performance.The successfulrealisation of such benefits in practice depends on thewhole project team working together towards what the

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Top left: roof panels for Armagh arts centre were delivered flat on atrailer bed.Top right, bottom left and right: cladding panels are

delivered to site on a just in time basis so as to speed upconstruction and avoid double handling.


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cladding panels, taking a tricky task off site and reducingthe risk of misalignment between cladding and structure.The precast panels were also installed on site much fasterthan might otherwise have been the case.

Targeting the Egan agendaThe release of the Rethinking Construction report (bySir John Egan; DETR, 1998) created a very significant

change in the industry as a whole and it is still influencingdecision-making processes for building projects. Thereport established a series of Key Performance Indicators(KPIs) against which construction clients could measurethe success (or otherwise) of their projects coveringaspects such as construction time, cost, waste, defects,safety and client satisfaction.

It is now commonplace for construction clients (inparticular large, serial clients) to use KPIs not only as amethod of measuring on a building project but also as ameans of evaluating previous performance of designteams, contractors and suppliers. Project teams maychoose to emphasise particular KPIs on certain projects,but in all cases the early involvement of the precaster will

ensure the team has the best chance to achieve its targets.Precast concrete fares well against the Egan targets for anumber of reasons,many of which are associated with theremoval of risk by shifting activities off site; in fact, manypeople note how closely Egans agenda matches precast

concrete construction.Faster construction: manufacture in the controlledenvironment of the factory is not affected by weather andthe construction programme is accelerated by offeringproduction in parallel with site activities, just in timedelivery to a pre-agreed programme and the single cranemovement that takes the unit to its final position.

Zero defects: precast concrete is manufactured to highstandards under strictly controlled quality processes; itsuse can also eliminate doubts about the availability of wallconstruction materials or labour.The latter has become aparticular issue in the UK due to a decline in skilledbricklayers and masons many precasters offer brick-faced cladding as an alternative.

As a vehicle for publicising best practice within theUK construction industry, the Movement for InnovationsM4I Demonstration Projects have proved a remarkablysuccessful way of proving that the industry as a whole cancomply with Egans targets. Precast concrete cladding isfeatured in many Demonstration Projects (see St. GeorgeWharf case study).

New efforts under the Rethinking Construction:Accelerating Change banner emphasise the need for theconstruction industry at large to address client leadership,integrated teams and people issues, especially health andsafety, in its efforts to drive the Egan agenda forward.

Precast and sustainabilityLike the Egan agenda, government strategies forsustainable development and sustainable constructionreleased in 1998 and 2000 respectively have also affectedUK building design and construction. Sustainabilityrequires us to consider more fully the economic,environmental and social impacts of development toprevent compromising the quality of life of our


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Facing page left: glazing units can

be fitted at the precast factory,saving time and cost comparedto site fixing.Facing page right: brick facedprecast cladding eliminatesproblems of skilled labour shortage.

This page top and bottom:

precast concrete is used for itsfabric energy storage benefits atToyota headquarters and Armagharts centre.


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descendants.Although not yet obligatory, the construction

industry as creator of buildings has been stronglyencouraged to assess what it does with a view toimproving its performance against ten sustainability actionpoints that include reducing waste and increasingrecycling. The Movement for Innovation has produced aseries of environmental performance indicators (EPIs) forsustainable construction, another example of a tool withwhich clients and contractors can benchmark andmeasure their performance.

Environmental protection starts at the precast factorywhere water recycling, energy recovery from waste

formwork and the use of reinforcement that uses onlyrecycled steel are all commonplace. Indeed, mostprecasters have now introduced procedures fully in linewith ISO 14001. Materials are ordered from sustainablesources in strict quantities to minimise waste. Wastemanagement continues through the supply chain withformwork re-used as much as possible and precast unitstransported directly to site with no wasted journeys andno double handling. Pollution from dust and noise isconsequently minimised on site.

But it is in the application of precast concrete inbuildings that major environmental benefits can be seen.Precast concretes thermal mass acts as a control forbuilding temperatures, helping to iron out the peaks andtroughs. There are many examples of precast concretebeing used for its fabric energy storage benefits (seeToyota headquarters and Armagh arts centre casestudies). Using concrete means that, in buildings such asoffices, schools and theatres, air conditioning can beeliminated, saving equipment, energy and maintenancecosts for the client. In addition, the precise manufacture

and installation possible with precast concrete claddingensures a close-fitting building envelope that makes thestandards of airtightness required by UK BuildingRegulations easy to achieve.With buildings accounting for50% of UK energy use, it is clear why sustainableconstruction is becoming more important.

Health and safety issues: respect for peopleThe UKs Health and Safety Executive is keen to seeimprovements at all stages in the supply chain for buildingsand the manufacture and installation of precast concreteis no exception. Improved safety is part of the Egan agenda

and the care of people in the precast factory and on site

is of prime importance: the safety record of the precastindustry is very good.Maintaining the good health and safety of workers in

the clean, safe and weatherproof environment of a factoryis an inherent outcome of producing precast concrete.This is in addition to the fact that the planning that goesinto precast concrete projects means that the design teamcan work everything out on paper in the safety of theoffice, rather than risk working it out on site.

Every contract has a prepared method statement,agreed with the main contractor and other specialists as

appropriate, before work starts. During installation of precast concrete units, protection of workers is the firstconcern and it is customary to see safety netting systemsused on all sites. Small teams of on-site operatives arepermanently employed, fully trained and certified by theindustrys training council (RBPTC).

The British Precast Concrete Federation worksclosely with the HSE to ensure its members operate tothe highest possible standards and has introduced itsConcrete Targets campaign to further safeguard allworkers from the shop floor to sub-contractors.Members of the Architectural Cladding Association havecomprehensive procedures to comply with therequirements and spirit of the CDM regulations. Thesemay be summarised as:

consider safety at the design stage, to ensure that allrisks encountered during construction and in the life of the building are identified and appropriate measuresprovided

as far as is reasonably practicable, comply with the

rules of the Health and Safety Planprovide the principal contractor with any

information regarding any risks not included in theSafety Plan

report to the principal contractor allinjuries/dangerous occurrences, in accordance with theRIDDOR regulations 1995.

Further information can be found in the ACAs own publication,Guide to the safe erection of precast concrete cladding,British Precast Concrete Federation, Leicester, UK.



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Weathering

Top left: run off from in-situ concrete leadingto calcium carbonate streaks. The precastpanels, in contrast, have resisted staining.(Hayward Gallery, London)

Top right: diagrammatic sketch of soilingprocessBelow: strong washing and deposition of dirton St. Pauls cathedral. Unlike concrete,

Portland stone has rarely been criticised for the way it weathers


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Weathering

Top left: the weathering pattern is affected byheightTop right: careful detailing, including water

drainage, at the corner of a buildingBottom left: the effect of rain on surfaces setat different angles

Bottom right: projections create rain shadoweffects


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staining of the surface below. Details such as sills andcopings can influence the flow of water on the faade andcan help in shedding water. Forward sloping or protrudingsurfaces can also serve as a means of shedding water.Theproblem with any projection on a vertical surface is that itcreates a rain shadow, where little rain will fall andstreaking is likely. John Partridge of HKPA Architectsdesigned some of St. Annes and St. Antonys collegebuildings in Oxford with sculptured facades, using bothbackward and forward sloping surfaces to ensure thatweathering was negligible.

Collecting water from surfacesWater can be collected and moved away from thesurface in horizontal channels or gutters, and dischargedaway from the faade by pipes or systems of verticalchannels or grooves. The areas of a concrete buildingthat need special detailing are junctions, edges orparapets, windows or windowsills and any other positionon a wall where water tends to collect. Down-pipes andgutters collecting water may be visible on the elevationor concealed within or behind the cladding.Areas belowwindows always need special care, as they tend to

Top left: projecting sill detailAbove: St. Antonys College OxfordLeft: Detail from St. Antonys College, Oxford


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become streaked by water running down the glass.Horizontal surfaces normally acquire a lot of dirt.

When large amounts of water flow on to horizontalsurfaces, there is a danger that this dirt can be washed onto the faade, creating stains and streaks.It is important toavoid build-up of dirt and water on horizontal surfaces.

Examples of parapet details which successfully controlaccumulation of dirt and the flow of water on concretefacades, are shown on the left.

Designers should be aware of the conditionssurrounding the building and design accordingly. Rainwaterrun off will either clean the elevation positively or stain it.Water that is intercepted should be either thrown off thefacade or collected and directed away from it.

Consideration of the flow of water over facades,providing suitable details, surface finishes and specifyinggood quality materials, will lead to concrete buildings thatweather favourably and retain and reinforce theiraesthetic value within the environment.

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Weathering

Top: A positive use of weathering. The concrete end wall of Hertzog deMeurons factory in Mulhouse has been profiled to allow rainwater tocreate strong vertical patternsRight: water movement on a parapet


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Concrete was used extensively in Roman times but onlyemerged as a significant building material in the late 19thcentury with the invention of Portland cement.Originally it was seen as a substitute for natural stoneand was used extensively in precast reconstructed stoneblocks. Later, as the building industry became moremechanised, larger precast units were developed which

could be lifted by crane.Todays precast can combine thestructural properties of concrete with the appearanceof natural stone.

The beginnings of concreteConcrete based on Portland cement is a relativelyrecent innovation, but early forms of binding materialbased on lime date back to around 7000 BC. Hydratedlime was used for the construction of Babylon, and alime kiln dating from 2450 BC has been found. Thisprocess was known to the Egyptians it was illustratedin a mural from Thebes, of about 2000 BC.

It was the Romans who really developed concrete the very word comes from the Latin concretusmeaning grown together or compounded. Thedevelopment was largely based on the discovery, in thesecond century BC, of pozzolana, a fine volcanic ashcontaining silica and alumina which when mixed withlime resulted in a stronger material than anythingproduced previously.The result was used as a mass infillmaterial for stone and brick-faced walls and for

foundations, but also for daring and innovative structuralelements, particularly vaults and domes. One of thegreatest achievements in concrete construction was thePantheon in Rome, built in AD 127, whose dome, 43metres in diameter, was formed of lightweight pumiceaggregate concrete. These concrete domes and vaultswere monolithic and had no lateral thrust; they actedlike an inverted saucer, and supported their own deadweight, which was considerable as some were morethan 2 metres thick.

One of the earliest uses of precast concrete can betraced to Roman times; a breakwater made of concreteblocks which had been allowed to harden before usewas built at Naples in the reign of Caligula (AD 37- 41).

Pozzolanic cement was also combined with othermaterials to simulate stone one of the first examplesof this use is lintels cast from sandstone, aggregate andlime/pozzolana cement used in the repair of the Visigothwalls at Carcassonne, south-west France, in AD 1135.Although the Romans had introduced the art of concrete-making to Britain, and there is evidence that itwas re-introduced by the Normans, little concrete,apart from some burnt limestone products used in



Top: an Egyptian mural shows stages in the manufacture and use of mortar and concrete.Bottom: the Pantheon, Rome, has a domed roof of concrete withlightweight pumice aggregate


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foundations and wall cores, was used in medieval andrenaissance periods.

The search for a stone substituteInterest in stone substitutes revived in the eighteenthand early nineteenth centuries. One example, Coadestone, looks remarkably like stone and its provenancewas a mystery for many years. It was manufactured by

Eleanor Coade and her daughter from 1769 to about1840 in Lambeth, on the site of what is now the FestivalHall, and was used by many eminent architects (RobertAdam, Sir John Soane and James Wyatt). In fact it wasnot stone at all: Alison Kelly ( Mrs Coade's Stone, Self Publishing Association, 1990) has established that it wasa ceramic body, or type of stoneware. Its compositionincluded fine sand, flint, crushed glass and crushedstoneware or 'grog'. The latter, pre-fired clay, was thevital ingredient which reduced the shrinkage rate of thepieces on firing to just over 8 per cent. (Mrs Coade

advertised her product as Lithodipyra, 'stone twice fired'in Greek). Pieces were cast in plaster moulds and firedcontinuously for four days and nights in a 3metre longmuffle kiln. Coade stone was frost-resistant and had apleasant stone-like texture and buff or light grey colour.It was used to embellish London brick terraces withcrisp stone-like details. Sir John Soanes Portman Squarehouses (177376) have Coade stone plaques, pateraeand string courses. James Wyatt's Coade stone Ioniccapitals at Heaton Park, Manchester, are so crisp andwell-preserved that they were formerly thought to becast metal.The Coade stone factory did not long survivethe death of Miss Eleanor Coade in 1821. Its decline mayalso have been due to the parallel discovery of Portlandcement.

The discovery of Portland cementMany experiments were made to re-invent the bindingmaterial used by the Romans. In 1756 John Smeaton, aLeeds engineer, was commissioned to rebuild theEddystone Lighthouse, set on a rocky outcrop in the

English channel; previous timber structures had blownaway in gales. He chose to use stone blocks, and testedmany different limestone products in an attempt to finda mortar which might set underwater. The material heultimately used was a combination of burnt limestoneand Italian trass (a material similar to pozzolana) but hisresearch, published in A narrative of the EddystoneLighthouse,had much wider implications. In 1813 a copywas bought by young Leeds bricklayer, Joseph Aspdin. Itchanged his life. Inspired by Smeatons example hecontinued the research and in 1824 took out a patentfor the manufacture of 'Portland Cement' (so calledbecause it resembled Portland stone in colour). Aspdinsaw his invention as a method of producing a rendered

Above top: Coade stone details from the front entrance to a 18thcentury London terrace houseAbove below: Aspdins cement works at Gateshead, 1852, was thelargest in the world



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stone, the pioneers of the Modern Movement wereexploring the potential of concrete as a structuralmaterial.

A Newcastle builder, William Wilkinson, is creditedwith the invention of reinforced concrete; in 1854 hetook out a patent for embedding a network of iron barsin floors and beams, and seems to have been the firstperson to appreciate the composite nature of thematerial. Little interest was shown in his ideas, and itwas a Frenchman, Francois Hennebique, who developedreinforced concrete on a commercial scale. In 1898 the

first multi-storey reinforced concrete framed building inthe UK, Weavers Mill in Swansea, was built using theHennebique system.

Architects and engineers in the early years of the20th century soon recognised the potential of reinforced and prestressed concrete, though in mostcases the material was cast in-situ rather than precast.

Frank Lloyd Wright was one of the first architectsto experiment with precast in the form of hollowblocks made of a semi-dry mix cast in embossed woodmoulds. (Wright was also fond of adding earth and sandfound on site to the mix to give the blocks a naturalcolour). The Storer House and Millard House, LosAngeles, were built in 1923 using his Textile-blocksystem: the blocks were stacked up to form walls andcolumns which support timber roof beams. Wrighttextured the precast surface of the blocks: in his viewthis demonstrated the poured character of the materialcompared to wood and stone which, he suggested,should have plain surfaces to bring out the qualities of veining, grain and texture.The system did not flourish; it

was unable to compete economically with the timberplatform frame, the most popular method of houseconstruction in the US.

In 1930 Le Corbusier completed the Maison Suisse,a university building on the outskirts of Paris. It is arectangular four-storey block built on pilotis, with alightweight curtain wall assembly on the two longelevations and blank side-walls of precast concretepanels. This was probably the first use of precast on alarge scale and it was a design which had a greatinfluence on subsequent tall buildings.

In England, one of the first buildings to pioneer theuse of both structural concrete and cast stone wasFrank Broadhead's 1932 Viyella House, Nottingham.The

Facing page top:theCafe Royal has acast stone facadebuilt in 1924Facing page bottom:a contemporaryphoto of theinstallation of a richcast stone cornicein Hanover SquareThis page left:worms eyeaxonometric of Frank Lloyd WrightsStorer house, LosAngeles, showinghow hollowreconstructed stoneblocks were stackedinto colmns whichsupport the roof beamsBelow: LeCorbusiers MaisonSuisse was theprotoype of manylater precastbuildings


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structure consisted of mushroom-headed concretecolumns which reduced the thickness of concrete floorslabs. The exterior was clad in curtain walling, withstainless steel mullions, between lightly tooled semi-drymix cast stone spandrel panels which were produced bya local manufacturer, Trent Concrete, founded in 1917on the banks of the River Trent.

During the late 1940s and 1950s a significant changetook place. Increasing mechanisation of the buildingindustry, particularly the development of cranes, led tochanges in the construction process.Architects wanted

to maximise glazed areas of the faade to create deep-plan, column-free open-plan interiors for office use. Ademand was created for large precast cladding units toachieve economies of labour and equipment and tospeed erection. The increase in size put greater stresson the structural properties of precast and led to achange in the type of material used. Compared with thetraditional semi-dry mix cast stone, with its lowstrength and need for dry tamping, wet-cast, with itshigh strength and easy compaction in moulds bymechanical vibration, was a more suitable product forlarge panels. In addition, crushed natural stone of thetypes traditionally used in a semi-dry mix were rarelysuitable for high-strength concrete. Unlike semi-drymixes, the surfaces of these new wet-cast panels had tobe treated to remove surface laitance, either byexposing the aggregate granite, flint, river gravel orother hard material or by exposing only the finersands and aggregates to produce a finer texture, similarto traditional cast stone.

In contrast with the successful appearance of cast

stone buildings of the 1920s and 1930s, some buildingswith precast panels of this period, particularlyprefabricated tower blocks produced by industrialisedmethods, have suffered badly in appearance.The reasonsfor this are complex; sometimes an unfortunate choiceof cement and aggregate was to blame, sometimes a lack of awareness by the designer of how to design and detailwhat was then a relatively new building product. Butseveral buildings of the 1960s prove that careful designand detailing will produce cladding panels whichenhance a building and weather well. Skidmore Owing &Merrill's Heinz Research Centre at Hayes Park,Middlesex, 1965 has a two-storey colonnaded faade of cruciform structural columns and fascias in exposed

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An early modern concrete building, Viyella House, Nottingham

The Russell building, Wexham Springs, showed no signs of staining for 30 years


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Little weathering or streaking canbe seen on the Heinz ResearchCentre, designed by SOM in 1965


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Cornish granite aggregate. The colonnade shades adeeply recessed window, and equally importantly, thedesign avoids the condition which affected manycontemporary buildings water run-off from glazingstreaking the surface of cladding below it. Similarly, StAnne's College, Oxford, (1966, by HKPA Architects), hasa strongly modelled facade of exposed granite andDerbyshire spar aggregate panels and projectingwindow balcony units which show no sign of streaking.Even panels containing large-sized aggregate, aparticularly difficult medium to handle, can weather well.

The Russell Building, Wexham Springs, was designed byCasson and Conder as part of the headquarters of thethen Cement and Concrete Association; the boldfacades of precast panels showed no signs of staining for30 years (it has recently been demolished).

Recent developments in precast materialsMost architects today are aware of the differencebetween wet-cast reconstructed stone and semi-drycast stone. But there is also a difference in theperception of an architect who is looking for a materialto resemble natural stone reconstructed stone -andone who sees it as a heavyweight structural materialwith a variety of textures and finishes precastconcrete. In fact both wet-cast reconstructed stone andprecast concrete are terms which describe a material of exactly the same specification and appearance.

The boom in office building in the 1980s ledarchitects to re-examine the potential of precastcladding panels and, as many of the new buildings werein historic sites in London, the panels were required to

have a stone-like appearance. The material used was aprecast material produced by the wet-cast process andgenerally known as reconstructed stone. Reconstructedstone can be produced with mixes which give a closematch to natural stones such as Portland stone and Bathstone. Reconstructed stone cladding provides a faadeof solidity and gravitas which can relate to a historiccontext yet has all the advantages of a fast track construction product. Examples of its use are shown inthe accompanying case studies.

It was soon realised that a high-strength materialwith an attractive appearance was not only suitable forcladding panels, but could also be used for the structuralelements of a building. Michael Hopkins & Partners used

precast structural elements with exposed finishes inBracken House, London, 1992, Glyndebourne OperaHouse. Sussex, 1994 and the Inland Revenue offices inNottingham, 1996. The radiating precast beams atBracken House are exposed as part of the ceiling tomake maximum use of the restricted height of thebuilding.The auditorium at Glyndebourne Opera Houseis encircled with two horseshoe-shaped balconiescomposed of wedge-shaped precast panels which butttogether to form a curved front. The exposed precastfinish of the soffits enhances the acoustic and allows

integral lighting to be incorporated.MacCormac Jamieson Prichard has explored the use

of precast in the historic context of Oxford andCambridge colleges. Fitzwilliam College chapel,Cambridge and the Garden Quadrangle of St. Johnscollege, Oxford, are magical buildings of delicatelydetailed precast elements, as finely crafted as a piece of high quality furniture.

Precast can also be moulded into fluid forms andgiven strong colours and patterns. Blitzcrete, a mix of crushed brick aggregate and a coloured concretematrix, and Doodlecrete, coloured concrete inlaid withundercut spiral grooves filled with white grout, weredeveloped by the architect John Outram.They are usedas logs and saddles which support the cornice of the

Judge Institute, Cambridge.In the past few years perceptions about the design

of the workplace have changed to emphasise themanagement of energy, with the aims of energyefficiency, good environmental working conditions andlow running costs without the use of air-conditioning.A

heavyweight material such as precast concrete has theideal capacity for fabric energy storage (FES), in whichthe building fabric is used to attenuate and modify peak internal temperatures during the occupied period. Theundersides of precast floor slabs are particularly suitablebecause they form the largest surface area.An exampleof this is can be found in the case study of the Toyotaheadquarters in Epsom, by architect Sheppard Robson.

The discovery of concrete is a remarkable story. Inthe past decade the potential of precast concrete ascladding and structure has been taken further.The casestudies included in this book give some idea of thepossibilities of this unique material.



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Above: precast balcony units at Glydebourne opera house, by MichaelHopkins & Partners

Above and below: A giant order of columns runs across the facade of the Judge Institute surmounted by black precast capitals and anentablature of logs and saddles. The architect was John Outram

Above and right: Fitzwilliam College chapel, Cambridge by MacCormac Jamieson Prichard


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the raked floor of the 400-seat theatre follows the naturalfall of the ground to the east. Between the smaller spacesand the theatre runs the main concourse; it encompassesthree levels and opens out at its centre to a massivelyscaled entrance canopy with a grand flight of steps whichconnects Market Square to other parts of the city.

Apart from the fully glazed faade of the mainentrance, the walls of the centre are simple panels of polished white precast concrete cladding; in appearancethey reflect the austere ashlar stonework of locallimestone used on the adjacent buildings.The roof to thefoyer is also of polished precast units each of which

incorporates a row of precast louvres. Both wall and roof panels were precast by Histon Concrete Products using amix of Derbyshire limestone coarse aggregate, SpanishDolomite fines and white cement.

The interior of the building has a steel framestructure, with external walls of cavity construction; aninner leaf of blockwork and an outer leaf of 100mm thick precast panels stack-bonded with stainless steel bed-jointreinforcement and tied back to the inner leaf.

Left: visitors enter the building by a double flight of steps to aconcourse shaded with precast roof slabs.Above: the walls are washed with natural light from precast louvres


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Case study Armagh arts centreArchitect Glenn Howells Architects

Top: the arts centre has been inserted into the historic fabric of ArmaghAbove: exploded isometric of precast column and roof structureTop right: the pristine interior of the cafe


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The foyer roof comprises 22 polished white precastunits, each 7.2m x 3.6m x 350mm, weighing up to 12tonnes. They rest at their corners on 350mm diameterprecast columns,each up to 8.1metres high,and along their3.6metre long edges on steel beams set in the walls; the

internal roof panels are fixed to the steel frame structure.

The casting processRoof units were cast from a single steel-lined timbermould which was adapted as necessary to accommodatethe various unit types. Formers were used to create voidsfor downlighters and wall washer luminaires, rainwaterdisposal systems and fire detectors.

The precast louvres at the end of each roof unit werecast in advance as individual pieces and placed into themould before the main unit was cast. Two days aftercasting, the panel was ready to be lifted away from themould bed; it was then stored, covered and allowed tocure for 3-4 days before further handling. Soffits and

exposed vertical faces were finished to match the wallpanels; the soffits were polished with a floor grinder fittedwith diamond-impregnated abrasive pads; less accessiblesurfaces were hand-finished.

To erect the roof panels a mobile 24-tonne crane

lifted the panels directly from their low-loaders into theirlocations. When each had been aligned and levelledprecisely, the panels were grouted around the locatordowels.

The louvres are covered with pitched glazedrooflights.The roof units are covered in an insulated plydeck on treated timber spacers, 60mm insulation andprecast pavers on spacer pads. Rainwater is directedthrough insulated pipes running between the timberspacers to box gutters at the junction between roof unitsand to downpipes fixed in the walls.

CREDITSPRECASTER Histon Concrete Products

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sales, with consequential problems. Speed andbuildability were obviously essential. Access had to bearranged for both residents and construction sitepersonnel and plant, and the site had to be managed ina clean, tidy and quiet way that did not detract from the

first occupants enjoyment of their property or deterprospective purchasers from viewing. An in-situconcrete frame offered the advantage of short leadtimes and also good acoustic properties, of particularimportance in a prestige residential development. Theuse of precast concrete components, supplied by TheMarble Mosaic Company, was a further importantelement in the solution of these problems.

Precast concrete was used for the balconies, wherethe planners demanded a high standard of finish andwhere advantage could be taken of the repetitive design.Delivered on a just-in-time basis, they were craned intoplace and landed on table forms, and the projectingreinforcement was lapped in to the in-situ floor slabs.

Each balcony was delivered with cast-in spigots forinstalling balustrades and with hoppers for rainwatercollection. The top surface was waterproofed and tiledin the factory, minimising the need for finishing trades.

Reconstructed stone elements were also used in

external works such as the parapet cladding to theriverside walk.

Precast concrete was chosen for the external wallcladding for its well-known advantages of low-maintenance, high-quality finish and precision of construction. The size and proportion of the panelswere carefully detailed, in particular to provide drainageand run-off for rainwater, avoiding staining. The panelson the upper floors match the appearance of Portlandstone. On the two lowest floors the panels match theappearance of pink sandstone to give visual strength tothe base of the building and to echo the colour of thesmall-scale red-brick buildings in the vicinity.

The full-height panels were delivered with the

Case study St. George Wharf, LondonArchitect Broadway Malyan Architects


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The new seven-storey headquarters for Merrill Lynch in theCity of London has northern and internal courtyard faades of precast panels faced with stack-bonded brickwork and asouthern faade of precast panels faced with Portland stone.

The site of the new headquarters, close to St. Paulscathedral, is a historic one with archeological remains of

the Roman wall and bastion and part of a much laterdebtors prison. The height of the scheme wasdetermined by St. Pauls, and the basement depth by thepresence of Post Office underground railway tunnelsystems.The scheme consists of four main buildings; thecentral seven-storey building provides trading space atfirst and second floor levels for 2,400 traders, withfloors above for other departments and conferencerooms.

The building has a steel frame structure enclosedwith self-supporting facades of precast concrete columnand spandrel units. On the southern, public side theunits are faced with Portland stone; on the northernside they are faced with stack-bonded brickwork.

The stack-bonded brickwork facadeThe precast brick-faced panels are stack-bonded withstraight 3mm wide joints which proved difficult to achieve.

Traditional bricks are manufactured at up to +2mmtolerance, and when stack-bonded, any minor variation in

jointing stands out. Rubber bricks are specially fired sothat they can be easily shaved or rubbed into shape and

these were specified. Unlike conventional bricks, rubberbricks retain their durability and weather resistance if theformed or kiln face is removed.

Hence

Documents

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