PRECASTCONCRETEA SELECTIONGUIDE FOR

SURFACE FINISHES

Athol GudgeonManager, NSW

EPM Concrete Pty Ltd

Reprinted from Constructional Review Vol.60, No.4, November 1987

Decorative finishes for precastconcrete products combine thetraditional skills of the concreteartisan, the design skills of theprofessional engineer and the practi-cal experience of the precaster.

Knowing that he has these skills athis disposal, the architect can directhis efforts towards combiningshapes, textures and colours toachieve the desired effect on thestructure and its surrounds, andaccordingly, on the environment.

The increasing requirement toprovide fire rating (2, 3 or 4 hours) toindustrial developments has generat-ed the incentive for clients anddevelopers to select the cost-effective option of providing hollow-core cladding with exposedaggregate finishes. The result is a sig-nificant improvement in the lightindustrial and commercial environ-ment. The upgrading of such estab-lishments has been achieved withonly a small percentage increase incost.

A similar trend is apparent in themulti-storey commercial buildingsector. Greater emphasis is nowbeing placed on achievement of thearchitect’s concept, self-cleaningfacades energy conservation andmaximum quality rather than mini-mum cost.

The following finishes for precastconcrete have been selected on thebasis of:• practical precasting;• architectural appeal;• application to precast units which

will provide structural solutions toproject requirements.Finishes considered include:

• off-form finishes;• water-washed exposed aggregate;• chemically retarded exposed

aggregate;• sandblasted exposed aggregate;• acid-etched finishes;• polished reconstructed stone;• honed or polished and acid-

etched;• polished and sandblasted finish.

These finishes have been selectedas the most useful group of finishesthat the precaster can offer to archi-tects and clients. The list is obvious-ly not intended to cover all finishesnor is it intended to suggest limita-tions in developing new finishes.

Mix design, careful selection ofmaterials and the need for practicaland innovative mould design are ofvital importance in achieving pleas-ing results for the architect, satisfac-tory structural solutions and a prof-itable result for the precaster.

A number of factors require con-sideration in the preparation of mixdesigns and trial mixes for allfinishes.

These include:• the finish required;• the shape and size of the units;

PRECASTCONCRETEA SELECTIONGUIDE FOR

SURFACE FINISHESAthol GudgeonManager, NSW

EPM Concrete Pty Ltd

• the workability required to allowthe mix to be placed and consoli-dated in difficult areas;

• the required compressive strength(usually governed by the precast-er’s requirement for a 24-hourcasting cycle);

• the durability requirements (waterabsorption) freeze thaw condi-tions, exposure to salt water spray,etc;

• trial mixes should be carried outusing techniques and finishingmethods that follow proposed pro-duction methods as closely as pos-sible.The concrete produced must pro-

vide the required surface finishesand comply with the appropriatestandards and project specification.

All materials should be tested forcompliance with the appropriatecodes and for any inclusions thatcould cause long-term staining.Stockpiling aggregates is recom-mended to ensure uniformity.Off-Form Finishes

Whilst a smooth off-form finishmay be one of the more economicalfinishes, the production of off-formconcrete to a consistent colour andstandard of finish will require a highdegree of production control if prob-lems are to be avoided.

Cement and sand colour to a less-er extent will control the final colourof the unit, with the fine aggregatecontributing to some extent. Thecolour of the coarse aggregate maynot be significant unless the particu-lar unit requires extremely heavyvibration, in which case some aggre-gate transparency may occur, caus-ing a blotchy and irregular appear-ance. Grey cements are more likelyto cause colour irregularity than areoff- white or white cements.

Moulds need to be carefullydesigned to ensure against any leak-age whatsoever. Wherever possibletapers should be built in so that themould can be permanently sealed.After each casting, meticulous clean-ing will be necessary, followed byregular burnishing with steel wool(assuming a steel mould). A very thincoating of mould oil, usually sprayedon and then wiped with clean rags,will ensure high quality castingsfrom steel moulds.

Off-form units usually will havesome surface imperfections. Joints insteel plates, minor variations in thesurface texture of a steel castingtable, voids caused by air trapped inthe vertical mould surfaces, etc, willbe obvious on this finish. As with allother finishes, a sample programmeshould be undertaken before castingcommences to ensure that the archi-tect is aware of the surface finish thatwill be finally achieved.

Off-form units produced withconcretes with high cement contentmay exhibit some surface crazingafter curing. This is a surface defect

which is usually only visible on care-ful close examination. However, ifexposed to the atmosphere, accumu-lation of atmospheric grime willaccentuate these cracks. Other thanaffecting the appearance, these~cracks do not affect the structuralproperties of the unit nor its longterm durability

Uniformity of manufacturingprocedures is most important.Procedures that ensure consistenttechniques for cleaning the mould,application of mould oil, uniformityof concrete quality, consistent curingprocesses and careful storage proce-dures will all contribute to produce aquality off-form product.

Handling procedures and mate-rials should be designed to minimisechipping and other damage. Asmooth off-form finish is probablythe most difficult of all precast fin-ishes to repair.

It is recommended that whereverpossible, off-form surfaces should bemodulated by grooving, sculpturingor profiling. Any introduction ofshapes to provide shadow effectswill enhance the final result.

Facing page – polished reconstructed-granite-aggregate panels on the CMLBuilding, Brisbane.Top – OTC House Sydney - a combina-tion of polished and sandblastedfinishes.Bottom – detail of OTC House showingrecessed polished borders contrastingwith the sandblasted central area of thepanels.

Materials other than steel may beused for the moulds. Plastic linerscan be used to provide a variety ofsurface textures and shapes. TImberand plywood also may be used.When considering materials otherthan steel for off-form finishes, it isalso necessary to consider the effectsof the curing process on these alter-natives. Steam curing will adverselyaffect timber and plywood mouldsurfaces.

A plaster-of-paris mould sealedand then treated with a wax will pro-vide an excellent off-form surface forsculptures.

Finishes such as hammered nibfollow on from off-form casting tech-niques. As with other off-form finish-es, consistency is of vital impor-tance. Hammered nib finishes areproduced by breaking the nibs on anoff-form unit in a consistent and uni-form manner.

Trials to establish the mix andaggregates which provide therequired appearance on the brokensurface are essential.

A similar technique is a rope fin-ish formed by fixing ropes over a ply-wood or timber mould, moisteningthem before casting and then aftercasting, the ropes are pulled out ofthe hardened concrete by mechani-cal means. As in all other finishes fullscale trials are essential to achieve asatisfactory result.Water-washed ExposedAggregate

A dense, round or crushedaggregate water-washed finish is use-ful and attractive. Aggregates shouldbe selected for colour and consistentgrading. Aggregates that crush toflaky angular shapes may not give aconsistent appearance. Round riveraggregates are ideal and generally agap graded mix should be consid-ered. The washing process willremove intermediate sized aggregatefrom a fully graded mix thus reduc-ing the density of the finished sur-face.

The ideal maximum aggregatesize for a given project will dependon a number of factors:• The washing process will remove

a large volume of matrix. The larg-er the aggregate, the greater thedepth of matrix to be removed.

• The washing process should leaveabout two thirds of the aggregateembedded.

• Depth of washing, ie the maxi-mum size of aggregate, affects theunit thickness as the requiredcover for reinforcement may needto be increased to include thedepth of aggregate exposure.It is desirable to vary the matrix

colour wherever possible to matchor blend in with the colour of theaggregate. This match can beachieved by careful selection of

cement and sand colours. Goodmatrix to aggregate match will prevent “patchy” effects which some-times show up only after erection onsite.

Water-washing is suitable for useon all flat architectural units whichcan be cast face up, from 600 x 600mm paving slabs to longline pre-stressed wall panel systems. Withcareful mould design heavily pro-filed window units, etc, are feasibleand economical.

Mould design of such units shouldincorporate tapers to allow mouldsections to be “slid” off the precastunit rather than lifted off. The “slid-ing” process will minimise slumpingand aggregate dislodgement prob-lems.

Plant requirements include anadequate water supply and waterrecycling facilities. The washingprocess creates a large volume ofslurry for disposal.

Water-washed exposed aggregateis the most economical finish avail-able and, in some circumstances, isless costly than trowelled finishes.

Heat-accelerated curing methodsare suitable. Condensation stainsfrom steam curing are readily remov

able with a light hydrochloric acidwash. Regardless of the curingmethod, a light acid wash is neces-sary to thoroughly clean the aggre-gate and exposed matrix. The unitshould be thoroughly wetted withwater before applying acid and, afterallowing a short period for acidcleaning, all traces of acid should beremoved with high pressure water.Chemically Retarded ExposedAggregate

This process requires the applica-tion of a chemical retarder to theface of the mould. A curing period isrequired to allow the retarder to setprior to casting the concrete. Theselection of the retarder will dependupon the depth of exposed aggregaterequired. Retarders are available forlight, medium and heavy surface fin-ishes. The effect of the retarder is todelay, but not to prevent, the set ofthe cement matrix so that after theconcrete has been cured usuallyovernight the cement matrix may beremoved from around the aggre-gate by brushing and/or the use ofhigh-pressure water jetting.

The selection of the appropriateretarder to produce the depth of cutrequired will depend upon the size

of the aggregate to be exposed. Afterthe selection of the aggregate andthe design of the mix (usually gapgraded) a number of sample panelsshould be prepared to evaluate theability of the particular retarder toprovide the depth of exposurerequired. The manufacture of thesamples should follow as closely aspossible the proposed manufactur-ing procedure. Variations in acceler-ated curing procedures will affectthe ability of the retarder to maintainsufficient cement matrix in a plasticstate to allow its removal withoutexcessive difficulty and labour costs.

As retarders are normally used forthose exposed aggregate surfaceswith considerable architecturaldetail, the ability of the retarder toresist abrasion on vertical and slop-ing faces during the concrete placingprocess, and still maintain an evenretardation, will be critical.

Samples, wherever possible,should include the more detailedportions of the proposed product. Ifthe concrete during placement isable to pick up retarder and cause itto accumulate in corners, groovesand rebates, the effect will be heavi-ly exposed details and under-exposed adjacent surfaces. Thepreparation of a series of samplescomplying with the retarder-manu-facturer’s recommendations and theprecaster’s requirements, willensure that production problems areminimised.

Facing page: top – example of verticalcasting to obtain a smooth off-formfinish on both sides; middle – aggregateexposed by water-washing in thecasting yard; bottom – water-washedaggregate finish.

Top left – application of retarder tomould face; bottom – panels cast facedown with aggregate exposed bychemical retardation; top right –cement matrix washed off by high-pressure water jet.

In production, the retarder may beapplied by roller, brush or sprayConsiderable care is needed toensure that the coatings are even tominimise variable depth of expo-sure. When production commencesit will be necessary to maintain areasonably uniform time delaybetween placing the retarder, allow-ing it to dry and then placing theconcrete. Concrete should be placedwith great care to minimise flowunder vibration so as to prevent, asfar as possible, the concrete causingan accumulation of retarder. Clothsand paper impregnated withretarders are also available.

After curing, the cement matrixmay be removed by brushing andwashing by hand, or by the use of ahigh pressure water jet combinedwith some brushing, to achieve uni-formity Again this process shouldtake place immediately after strip-ping and if possible at a predeter-mined time after casting.

The production of good retardedfinishes requires consistent and high-quality concrete, uniform curingprocedures and maximum adher-ence to a uniform time cycle.

As with other exposed aggregatefinishes, the finished unit should becleaned with a dilute solution ofhydrochloric acid after wettingdown with water. The acid should beremoved using a pressure spray andsufficient water to ensure that alltraces of acid have been removed.

A retarded surface can also beproduced by casting the productface up and then spraying a surfaceretarder to achieve the same result asdescribed for the face down process.

This process has only limitedapplications because the same resultcan be achieved by water washing atsignificantly less cost.Sandblasted Exposed Aggregate

Sandblasting using water as thecarrier provides a dense even “sand-stone” textured precast panel. As thefinished surface comes “off theform”, profiled shapes, ribs, reveals,etc are all possible, thus giving thearchitect great freedom in the designof the exposed face.

The sandblasting process requiresa great deal of operator skill. The

areabeing cut by the sand is hidden witha film of water and can be assessedfor the required depth only when thecutting stream of sand and water ismoved, usually in a circular motion,to the next area. The cutting streamof sand and water must be continu-ally moved to ensure even cutting.Angle of cutting can also be impor-tant to ensure heavily profiled unitsare cut evenly on all exposed sur-faces.

Sandblasting using air as the car-rier allows better operator visibilitybut may be restricted because of pol-lution effects (dust) or Governmentregulations. Cutting materials thatcould cause “silicosis” must beavoided. Copper slag is a suitablealternative.

Depth of cut is usually compara-tively light varying from a minimum“wash” blast, which is only sufficientto remove the cement skin from theoff-form surface, to a “deep” cut

removing some cement and sand toreveal some aggregate colour.

Cement and sand colour shouldbe chosen to blend with the slightly“bruised” colour of the sandblastedaggregate as the cement sand matrixcolour will predominate.

Casting is generally economicalprovided reasonable repetition isavailable.

Concrete needs to be carefullyplaced in the moulds to ensure full

mould coverage before vibration.Slurry lines exposed during blastingcan be caused by uneven placing.Moulds need to be sufficiently rigidto prevent local “hot spots” develop-ing, as these areas will lack surfaceaggregate density after sandblasting.Units should be crack-free at thetime of sandblasting as cracks,including very fine shrinkage andstructural cracks, are opened out andaccentuated by sandblasting.

Heat accelerated curing methodsare suitable.

A light hydrochloric acid wash(after wetting down) is necessaryafter sandblasting to expose the nat-ural colour of the sand and aggre-gate. Finally clean with high pres-sure water.

Plant facilities required include asemi-enclosed blasting area andadequate cranage. Product needs tobe:• lifted from casting bed to storage

area;• moved to sandblasting;• possibly rotated;• then lifted to storage.

Sandblasting equipment shouldbe selected to provide an adequatecutting rate within the operator’scontrol. Overpowered equipmentcan lead to excessive cutting whichmay not be repairable, thus causingunit rejection.

Facing page: top left – Wet sandblastingof striated finish; bottom – spandrelswith striated finish; top right – offices,Mary Street, Brisbane - sunhoods castface down, then sandblasted.

Top left – light acid-etched finish usedat Parliament House ACT; top right –etching with phosphoric acid; bottomleft – CML Building, Brisbane - polishedreconstructed-granite-aggregate panels;bottom right – polishing a panel using atwin-head Hensel machine.

Acid-Etched FinishesThis finish is produced by cutting

the cement matrix from the face ofthe product by etching withhydrochloric acid.

Acid etching carries with it a sig-nificant risk of reinforcement corro-sion because of residual chlorides.Accordingly it should be used onlyafter the engineer has consideredand specified the additional cover toreinforcement required to ensureadequate protection.

Acid etching is a fine finish - thematerials exposed are primarily thecement and sand with only a smallpercentage of coarse aggregatesbeing visible. Consequently, stock-piling of sand is important to ensureuniformity as variation in sandcolour will show on the finishedproduct.

The etching process is executedby applying acid, allowing a reac-tion period and then removing alltraces of acid by high pressure waterspray The product should be thor-oughly wet before the application ofacid. Dipping is NOT recommend-ed.

The introduction of a small per-centage, eg 1-2%, of marble dustwill accelerate the process andensure that all of the acid appliedwill be neutralised.

The acid should not be allowed tolie on the surface but should bescrubbed or brushed as the forma-tion of silica ‘gel” will slow downacid attack and if allowed to dry outwill be most difficult to remove.Polished Reconstructed Stone

Polished reconstructed graniteand stone finishes comparefavourably with polished naturalstone facings giving the architect the

freedom to design a building facadeusing the full structural capability ofprecast panels.

This finish is produced by grindingapproximately 3mm off the off- formface of the precast components.

Degrees of surface finish may beproduced depending on the grade ofthe last grinding stones used:• No.3 stone = coarse hone• No.4 stone = fine hone• Felt pad and polishing powder =

full polish.• Honed finishes do not bring out

the full colour potential of thestone; however, they provide anattractive finish suitable for floorsor paving.A fully graded mix is preferred,

carefully designed to provide maxi-mum aggregate density on the sur-face to be polished. Even placementof the concrete over the full face areafollowed by heavy vibration will pro-vide the necessary maximumaggregate - minimum matrix surfacerequired for this finish.

Surfaces cast vertically willrequire special attention to ensurethat slurry lines do not showbetween concrete placements.

The use of “poker” vibrators willresult in vibration patterns and slurryconcentrations and are therefore notrecommended.

It is possible to hand float a sur-face to be polished; however, carefulattention will be required to preparethe dense aggregate surface.Additional grinding will be neces-sary to match the off-form surfacesafter polishing.

Careful detailing to maximise theuse of automatic polishing equip-ment and minimise hand polishing

will ensure minimum cost. Productdimensions should provide forgrinding allowances. Moulds needto be sufficiently rigid to transfervibration to the concrete withoutcreating vibration “hot spots”.

Hand polishing of arrises, returns,etc not accessible with automaticequipment, is slow and costly andshould be designed out where possi-ble.

Components requiring uniformfinish on all surfaces will need spe-cial attention and care to matchhand finished faces with verticalfaces and horizontal surfaces.

As with other finishes, finalappearance and uniformity will ben-efit if it is possible to match or blendmatrix colour with aggregate colour.

Heat-accelerated curing methodsare suitable.Combined Finishes

Honed and Acid Etched – thecombination of a machine finished,polished or honed surface, and acidetching provides a surface whichexposes a very high percentage ofstone. After the grinding process, theacid then removes the cement matrixand fines from between the largeraggregate particles. This surface ishighly resistant to weathering and isself-cleaning to a high degree.

This combination is very suitablefor paving. The recesses created bythe acid etching provide a surfacethat is reasonably non-slip. Provideda high degree of aggregate density isachieved on the surface this finishcan be wear-resistant and retain itsappearance for many years.

The comments on polished recon-structed stone finishes and acid etch-ing apply However, in this instance,because of the effect of the etching,

aggregate colour predominates.Unusual effects can be achieved byusing round river pebbles as theaggregate. As the external surface ofthe aggregate is exposed by the acidetching and the internal surface bythe grinding process, interesting andattractive results can be achieved.

Additional cover to reinforcementshould be allowed to provide forp05sible effects of residual chloridesfrom the acid etching process.

Sealing of the mould to preventany leakage of cement matrix undervibration is essential to prevent dis-colouration at the point of leakage.

All of the cautions previouslygiven for honed, polished and acidetched finishes should be con-sidered.

Surfaces cast vertically tend toshow a finer aggregate configurationthan horizontal surfaces cast againsta mould face. Hand-finished (hor-izontal) surfaces which are intend-ed to match vertical and off- formsurfaces after finishing, are difficultto achieve and should be avoidedwherever possible.

The structural design of the prod-uct should provide for crack- freestripping and handling of the prod-uct. Acid etching of fine, bending orshrinkage cracking will dramatically emphasise the cracks.Rectification of these faults can be

most difficult. After stripping, thesurfaces to be finished should bestored on “dipple pad spacers”.These spacers will allow air circula-tion so ensuring that the contract sur-face at the point of support will cureto the same degree as the remainderof the unit. Storage of unfinishedproduct on timber; plastic or anyother material which prevents air cir-culation will result in a “curingmark” which can take a very consid-erable period to weather out.

All exposed metal work, fittings,threaded inserts and projecting rein-forcement should be properly pro-tected from acid attack.

In the interests of economy, theprecast units should be designed toallow maximum use of automaticpolishing equipment.

Polished and Sandblasted Fin-ishes – whilst the polished/acid-etch-ing combination provides a surfacefinish in which the colour of theaggregate predominates, the pol-ished/sandblasted finish provides adramatic contrast between the pol-ished aggregate and the sand- blast-ed matrix of the concrete.

The possible use of this combina-tion requires the designer-architectto take an early decision to ensurethat the overall concept makesallowance for the change in colourand texture of the two finishes andthat a suitable demarcation isdetailed to separate them - usually aV groove. The exact shape of this Vgroove will require consultation withand trials by the precaster to ensurethat the angle within the Vgroove is sufficiently Hat to preventchipping the edge in the grinding

process and causing an unsightlyline on one side of the groove.

The designer will also need toconsider that the polishing processwill create a step in the surfacebetween the polished and the sand-blasted finish of approximately 3mmwhich can cause problems whenaligning precast units on site. Thiscan be overcome during the designprocess. Casting a series of trial pan-els is strongly recommended to pro-vide full knowledge of the combinedeffects.

It is necessary to mask the pol-ished finish during the sandblastingprocess. The polishing process iscompleted before sandblasting. Thenormal light hydrochloric acid washto the sandblasted finish is appliedwith care to ensure that any acid thatmay be applied to the polished finishis removed immediately before itcan attack the matrix.Summary

Surface finishes offering architec-tural and structural solutions consid-ered from a precaster 5 viewpoint.Finishes considered include:• Off-form: usually regarded as the

least expensive finish but prob-lems can occur with colour unifor-mity

• Water-washed Exposed Aggregate:a low cost solution that has beentested over many years.

• Chemically Retarded ExposedAggregate: an excellent solutionwhen water-washing isimpractical.

• Sandblasted Exposed Aggregate:offers good fine finishes for pro-filed shapes.

• Acid Etching: an attractive finishwith some possible long-termdurability risks.

• Polished Reconstructed Finishes:prestige appearance and excellentself-cleansing properties.

• Combination Finishes: offer excel-lent possibilities for architecturaluse of tones and textures in facadetreatments

BibliographyRecommended Practice –

Design and Detailing of PrecastConcrete, Concrete Institute ofAustralia.

Hanson, J A. and Jenny D P,Precast Concrete Panels:Materials and Tests, AmericanConcrete Institute.

Gutmann, Phillip W., PrecastConcrete Wall Panels:Manufacturing Processes.

Manual for Quality Control forPlants and Production ofArchitectural Precast ConcreteProducts, Prestressed ConcreteInstitute (USA).

The author wishes to acknowledgethe assistance of Boral Ltd in provid-ing photographs.

Facing page – State Parliament Offices,Sydney, where yellow-brown quartziteaggregate was exposed by honing andacid-etching; top – panel shape requir-ing some costly hand-finishing;bottom – finish of precast panels, StateParliament Offices, Sydney