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Levelling compounds are usually mixed with water, screeded onto the concrete surface and trowelled smooth. It is essential to followthe manufacturers advice about the limitations of their products and their instructions for use.

A bonded topping is a layer of concrete that is placed on top of a pre-hardened structural slab or a suitably prepared hardened slab. Inthe first case the topping is placed while the slab is still in its plastic or workable state. This allows bonding of the two as they hardensimultaneously, effectively resulting in a monolithic unit. Generally, reinforcement is not required if the topping thickness is not greater than 50mm. Reinforcement can be used, however, to help control shrinkage cracking: F42 fabric is positioned (and supported by bar chairs) 20 mmbelow the top of the slab. Concrete can also be reinforced with steel fibres. Polypropylene fibres are also available and will help controlshrinkage cracking but do not increase the tensile strength of concrete as do steel bars, fabric or steel fibres.Unbonded toppings are usually adopted when the topping needs to be greater than 50 mm. They are prevented from bonding with the existingsubstrate by separating the two with a bond breaker, usually a plastic membrane. To help control shrinkage cracking, the topping is reinforcedwith F52 fabric positioned (and supported by bar chairs) 20 mm below the top of the slab.

Pre-hardened concrete should be screeded to an even surface, and allowed to stiffen so that it can carry foot traffic that makes slightimpressions in the surface but does not displace. Hardened concrete must be roughened, clean and structurally sound. Grease, oil or paint,which could affect the bond, must be removed. Old concrete surfaces that are disintegrating must be chipped back to a sound substrate.Smooth concrete should be scabbled backto reveal the coarse aggregateto give a key for the new topping. Finally hose and scrub thesurface to remove all dust, dirt and detritus before placing the topping.Unbonded toppings are placed on a smooth, structurally sound slab, and separated from same with a bondbreaker, usually a plasticmembrane.

The area of toppings should, generally be limited to 15-m2 sections. The length of a section should not exceed 1.5 times its width.Isolation joints should be formed where the topping abuts walls, penetrations (columns, pipework) or other structures, and allows the toppingand adjacent structure to move independently.Contraction joints are formed grooves or saw cuts in the surface, which create a line of weakness that controls the shrinkage cracking andminimises random cracking. When toppings are placed over existing hardened slabs it is crucial that new joints are aligned with those in theexisting slab to avoid uncontrolled cracking in those areas.If an area greater than 15 m2 is to be placed, divide the area with construction joints. Construction joints can be simple square finished buttjoints with a break to the full depth of the topping. If reinforcement is used it must continue across the joint.

HERZOG & de MEURON EXPOSEDCAST-IN STONEPOST AND BEAM BRAWNSPECIFYING JOINT SEALANTS FOR PRECASTMIX TIPS applying concrete toppings

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APPLYING CONCRETE TOPPINGS

Levelling compounds

Bonded topping slabs

Unbonded topping slabs

Preparationbonded toppings

Preparationunbonded toppings

JointsIsolation joints

Contraction joints

Construction joints

FIVE COMMON REASONS FOR APPLYING CONCRETE TOPPINGS:To apply a cost-effective veneer of decorative concrete, which may contain special aggregates and coloured pigments.To make flush uneven floor levels.To make a barrier against rising damp (a vapour barrier is placed between the existing slab and the topping).To cover an unsatisfactory, damaged, or worn surface.To contain in-slab heating systems, retrospectively installed.Purpose-made levelling compounds are typically less than 10 mm thick. The thickness of a topping can be up to 70 mm, however atopping containing a 10-mm aggregate would usually be in the range of 25-40 mm. A topping thicker than 70 mm should be consideredas an independent slab and designed and reinforced accordingly.

KEY POINTS TO CONSIDER:

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Locked Bag 2010ST LEONARDS NSW 1590Telephone 02 9437 9711Facsimile 02 9437 9470

348 Edward StreetBRISBANE QLD 4000Telephone 07 3831 3288Facsimile 07 3839 6005

1 Hobson StreetSOUTH YARRA VIC 3141Telephone 03 9825 0200Facsimile 03 9825 0222

PO Box 43WEST PERTH WA 6872Telephone 08 9214 3914Facsimile 08 9214 3998

Greenhill Executive Suites213 Greenhill RoadEASTWOOD SA 5063Telephone 08 8274 3758Facsimile 08 8373 7210

Locked Bag 2010ST LEONARDS NSW 1590Telephone 02 9903 7720Facsimile 02 9437 9473Email info@ccaa.com.auwww.concrete.net.au

The Association is a non-profit organisationsponsored by the cement industry in Australia toprovide information on the many uses of cement andconcrete. This Guide is distributed by the Associationfor that purpose. Since the information provided isintended for general guidance only and in no wayreplaces the services of professional consultants onparticular projects, no legal liability can be acceptedby the Association for its use.

PFAFFENHOLZ: Karl Millard concrete set retarders;

El Croquis p.93 Hisao Suzuki; Dean Boothroyd.

LA TERRAZZA: Trevor Mein; Rainer Strunz

FREEMASONS HALL: Fritz Kos from Constructional

Review, February 1969 pp.38-41.All images by C&CAA unless otherwise noted.

Levenspiel [lev@netspace.net.au]Vaughan PrintingABB GRAPHIX

C&CAA OFFICES:SYDNEY

BRISBANE

MELBOURNE

PERTH

ADELAIDE

LIBRARY

WEBSITE

DISCLAIMER

HOTOGRAPHY CREDITS

DESIGN/PRODUCTIONPRINTING

SCANNING

2

The concrete surface finish of Herzog & de Meurons Pfaffenholz Sports Centre (Basel,Switzerland) excited much interest in 1993 but remained veiled in secrecy until recently. Thetechnique makes use of concrete set retarders, widely used to produce exposed-aggregatefinishes. Set retarders are chemical agents that are applied to the surface of the formwork, orform liner, to slow the setting time of the surface of the concrete panels; and in the case ofpavements they are applied directly to the concrete surface. The strength of the retarder canbe varied to affect the surface of the concrete to different depths. Below the surface, theunaffected concrete hardens to its specified strength at the normal rate. The concrete panel islifted from the mould, usually within 24 hours. The cement paste under the influence of the setretarder is removed by water spraying and brushing to reveal underlying aggregates.

The following extracts from the August 1999 issue of the RIBA Journal (Concrete Couture byMiriam Cadji) outlines the technique. MIX Volume 2 (April 1999) reportedand illustratedsimilar experiments by Karl Millard.

The technique of etching images on to glass is well establishedits printing on to the concretethat is the magical part. In fact, there is no great mystery to the process. Simply put, it makesuse of a chemical concrete retarder, Serelith.

Herzog & de Meuron developed the concept with French construction chemicals manufacturerPieri, taking it a step further from its traditional use as a retarder and combining the techniquewith textile processes. We are interested in fashion, asserts Jacques Herzog. The artificialCement & Concrete Association of Australia

Leading Concrete Knowledge

GRANT ROBERTSEmail: grant@ccaa.com.au

EDITOR

HERZOG & de MEURON EXPOSED

PFAFFENHOLZ SPORTS CENTRE REVEALED

skin for the human body can be compared to a building.Rigorous experimentation with different screen-printed imagesresulted in the first printed precast concrete panels being used in1993 on the Pfaffenholz Sports Centre, for which Swiss precastconcrete manufacturer Brodtbeck carried out exhaustive paneltests. It was the first building executed with this technology,explains Erich Diserami of Herzog & de Meuron. It has beenwidely used in the past for decorative ornamentation, but onlywith simple geometric images, certainly nothing as complex as aphoto.

At Pfaffenholz, a photographic image of a cluster of pebbles wasenlarged to the point of near abstraction, and the finished effect isthat of patchwork camouflage.

The images are produced by selectively exposing the underlyingaggregates of the concrete panels. The process begins with amirror image of the selected drawing or photograph being screen-printed, using a reactive paint which acts as a retarder, on to asheet of extruded polystyrene (the image appears as tiny dots likethose in a newspaper). This sheet is then fixed to the bottom ofthe formwork into which the concrete is poured. After two days of

curing it is turned out and hosed down. Where the concrete has been in contact with theretarder dots it sets more slowly, as the chemicals delay the curing. Then the panel is washedclean, the image appears as the aggregates are exposed.

For best results, the original drawing, or black and white photograph must be a good qualitywith crisp detail and contrast, but the process allows for a subtle three-dimensional effectwhich can be achieved by varying the concentration of the retarder. The stronger the retarder,the deeper the etching. For this effect, correct concrete mix is essential and the grading of thesand and aggregate is crucial.

There are technical as well as aesthetic advantages to the process. Firstly, durability isimproved, as the exposed cement matrix surface is denser and stronger than the fair facesurface. Secondly, a rough surface allows less water ingress than a smooth one, as itencourages water dispersal, reducing cracks, joint leakage and discolouration. And thirdly,exposing the aggregate reduces the risk of attack from acid rain.

There are limitations to the technique, however. The size of the printed sheet cannot exceed1.0m x 1.2m, so for larger images, such as the Karl Mair department store where the image is3.0m x 3.5m, the photograph was divided into sections, and 12 individual sheets butted upagainst each other create the finished effect.

3

Examples of Karl Millards experiments with set retarders.

Finished precast panel

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Pfaffenholz

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PROJECT: La Terrazza ApartmentsLOCATION: Elgin Street, Carlton, VictoriaARCHITECT: Nation Fender KatsalidisSTRUCTURAL ENGINEERS: Barry Gale EngineersCONTRACTOR: LU SimonPRECAST CONCRETE CONTRACTOR: Eltrax Pty LtdMARBLE MASONARY: TTI Marble & GranitePROJECT COMPLETION: 1999

Construction photographs courtesy of Rainer StrunzAll other images by Trevor Mein

Typical anchor for marble veneer (left) and granite veneer (right). Source: Stone Veneer-faced Precast Concrete Panels,Prestressed Concrete Institute (of America) circa 1990.

L A T E R R A Z Z A A PA R T M E N T S

Marblepieces in

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Distinctive precast concrete panelslike giant marble mosaicsare thesignature of an apartment buildingdesigned by Architects Nation FenderKatsalidis. The development is aconversion of a light industrial buildingand comprises retail tenancies atground level and 48 single and multiplebedroom apartments in the four storeysabove. The four decorated boxescantilever over the footpath anddominate the solid and void modulationof the street elevation. They aresupported by the existing reinforcedconcrete slab at first floor level andrestrained by the new slab at the thirdfloor. Carrara marble was cut to irregularshapes and arranged face down in thecasting bed. The concrete mix wascoloured with a black mineral oxidepigment, which, together with acommon bluestone coarse aggregate,produced a dark grey mix in contrastwith the white marble. The concretewas placed over the backs of themarble pieces and carefully compactedby hand so the pieces were not moved.The panels were lifted onto their backsand the faces were polished with ahand-operated terrazzo grinder. Theresult is like a hybrid of Japanesecrackle-glazed ceramics and crazypaving. Ceramic tiles can also be fixed toconcrete with purpose made adhesivessuitable for the particular conditionsparticularly humidity. The tiles shouldhave no more than 3% moisturecontent.

FIXING STONE TO CONCRETEStone is fixed to concrete withmechanical anchors. The fixings shouldbe flexible to allow variations of in-planemovement of the concrete and thestone resulting from concrete shrinkageand temperature variations. They shouldalso be designed to deal with stressesfrom handling, transport and serviceloadsparticularly wind suction. The concrete and stone are separatedby a bond-breaker to prevent adhesion.The bond-breaker can be a polythenefilm or a spray on compound. This willalso act as a moisture barrier to impedethe migration of efflorescence to theface of the stone. Stresses resultingfrom shrinkage can be minimised bymanufacturing the precast backingpanels with low-shrinkage concrete.Those resulting from differences inthermal expansion can be minimised byusing aggregates in the mix with athermal coefficient that approximatesthe stone. Polishing stone will not affectits strength, however flamed or bush-hammered finishes will.

POST AND BEAM BRAWN

This is an abridged version of an article that appeared in National Precaster September1999the newsletter of the National Precast Concrete Association of Australia (NPCAA),and is published here with their permission. The NPCAA can be contacted on tel: 02 98908853.

Examine the manufacturers technical information and test certification to evaluate thesealants properties in relation to its in-service application. Seek advice from the suppliersand independent opinions to determine if the sealant is suitable for the application. Assessits performance in similar applicationspast and present. Look at current projects in thevicinity, which are subject to similar conditions.Consider the affect of in-service conditions on the life expectancy of the sealant andspecify accordingly for a reasonable service life.The size and geometry of the joint, and the sealant must deal with the dimensionalchanges of concrete panels (and consequently anticipated joint movement), which canoccur in more than one plane.

POLYSULPHIDE was for many years, used widely in masonry applicationsabove andbelow waterbut has largely been replaced by polyurethane. Unlike polyurethane, it isproduced only as two-part and non-fire rated sealants. Polysulphide needs a primer in allapplications, has a high initial cost and slower application time. POLYURETHANE is now preferred in all joint sealing applications in building construction,other than glazing. It is available in one and two-part products, with fast or standard curingtimes. It is certified variously for fire rating, acoustic rating, potable water and sewerageapplications. SILICONE-based sealants are used in glazing applications because of their UV lightresistance. They are not suitable for masonry construction because of surface stainingand poor long-term adhesion on porous surfaces.

ACRYLIC sealants are user-friendly because they are water-based. Their applicationsare, however, limited to internal low-movement joints because they suffer from dryingshrinkage and hardening in the long-term, which together diminishes their ability to dealwith movement in panels.BUTYL sealantsgun-applied and preformed extrusionsare used mainly in bedding orcompression seal applications.

The performance of a sealant depends on the spacing of joints, the joint design, surfacepreparation and application. It is rare that sealants will be applied in ideal weatherconditions, the substrates will be in perfect condition, and the as-built joints will match thedrawings. By following four rules, however, the performance of the sealant is optimisedand the life of the joint prolonged.The surface should be clean, dry and primed if necessary. Applying sealants to greenconcrete can result in bubbling and loss of adhesion. The surface temperature should beabove 5o.To behave correctly when panels move, sealants should adhere only to the sides of thejoint. Foam backing rods or bond-breaking tapes should be used to ensure this.For typical joints, the cross-sectional depth of the seal should be at least one half of thewidth, and never greater than the width. This may vary in narrow (10 mm) or wide (40 mm)joints.The setting time of the sealant determines when it can be brought into servicesubjectedto its in-service conditions. Polyurethane sealants with a standard curing period will cureat a rate of 2-3 mm of depth per day in temperatures over 5o. Fast curing and two-partproducts will be marginally faster. Acrylics and silicones skin quickly but have a slowercure-through-depth time.

The requirement for fire-rated sealants arose with the banning of mineral fibre woolproducts from Australian building sites in the mid-1980s. Polyurethane is used widely inall areas of external and internal fire-rated movement jointing because of its higher firerating, cost-effectiveness and ease of application (it does not need a primed surface).Acrylics are acceptable in areas of low movement and away from direct or constant UVlight attack.

Today there is a greater emphasis on reducing sound transmission in residential andcommercial properties. Sealants must have acoustic sound transmission class (STC)ratings. The STC rating is related to the specific density of the sealant in the cured state.It is generally accepted that the higher the density, the better the STC rating. Acrylicachieves on average 1.6, polyurethane approximately 1.5 and silicone 0.97. Althoughacrylic has a higher density, the acoustic integrity is compromised when the sealant losesits bond. One part fire-rated polyurethane is now preferred for acoustic joint sealing for avariety of materials. It has the fire rating and the long-term adhesion and flexibilitynecessary to maintain STC integrity.

Although there is some irony in building a Freemasons lodge of concreterather than stone, the off-form and sand-blasted finishes fit the brawnypragmatism of the post-and-beam structureand perhaps the publicperception of Freemasonry.

The Freemasons Hall in Perth is an example of modern regionalismarchitecture that appeared throughout the world at that time. Forbes &Fitzhardinge were the winners in a limited competition in 1966, for thedesign of the hall. It has the characteristics of the International Styleastructural grid, rectangular volumes, flat roof, hovering planes and stripwindowsand a whiff of Le Corbusiers potent buildings of the 1950s,namely Chandigarh and La Tourette, all brought together under a broadroof canopy to give shade from Perths fierce sun.

The building is raised off the ground and carried on squat columns and acantilevered ribbed floor, which pushes the structure to its practical limits.The walls are of load-bearing concrete masonry.

The open concrete screen at the entry depicting several Masonic symbolshas an austere and mysterious presence. By day it shines with the sunlightfrom an internal light well, by night it is backlit.

Today the increasing use of load-bearing precast panels throughoutAustralia would see such a post and beam expression of a reinforcedconcrete frame as the exception rather than the rule, although precastconcrete frames are widely used.

KEY SELECTION CRITERIAApplication

Life expectancy

Joint size

SEALANT TYPESHigh performance

sealants

Plastic or plasto-elastic sealants

JOINT DESIGN

Joint preparation

Backing systems

Joint geometry

Setting time

FIRE AND ACOUSTIC RATED SEALANTS

Fire-rated sealants

Acoustic sealants

FREEMASONS HALL 1967

76SPECIFICATION GUIDE

JOINT SEALANTS FOR FACE-SEALED PRECAST

PROJECT: Freemasons HallLOCATION: Terrace Road, East PerthCOMLETED: 1967ARCHITECT: Forbes & Fitzhardinge [Project Architect: Anthony Brand]STRUCTURAL ENGINEER: D.H. FraserBUILDER: Hutcherson Bros Pty LtdPHOTOGRAPHS: Black and white photographs by Fritz KosPublished in Constructional Review (Vol 42 No1), Feb 1969, pp.38-41

Concave sealantsurface Depth/width ratiocorrect. Backing rod makes aconcave curve at thebase of the sealincreasing its basedimension and itsability to respond tomovement.

No backing rodpoorgeometry to respondto movement. Cohesive tearsealant can onlycompensate formovement at thenarrowest point. Tearmay develop ifmovement exceedssealants limits at theneck of the joint.

CORRECT JOINT DESIGNGood joint width/depth profile

INCORRECT JOINT DESIGNJoint too deep

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