Text of Surface delamination in slab on ground construction · Surface Delamination in Slab on Ground...
Surface delamination in slab on ground constructionA report based upon site experience &
observation in the Auckland region
Surface delamination in
slab on ground constructionA report based upon site experience & observation in the Auckland region
This technical report is jointly published by the Cement & Concrete Association of New Zealand and the Auckland Branch of the New Zealand Ready-Mix Concrete Association.
Support from the technical committee of the NZ Ready-Mix Concrete Association is gratefully acknowledged.
5 Mix design and production
5 Environmental considerations
6 The human element
7 Finishing processes
8 Conclusions / recommendations
10 Appendix 1. Recommendations for adding / dosing superplasticers at the site
11 Appendix 2. The ‘Window of Finishability’ , and on-site tests to determine it
12 Appendix 3. A discussion on the use of pan floats and ride-on floats
Readers please note:This document has been issued in the interests of activelypromoting industry understanding of the influences, both good andbad, that impact on slab on ground construction and in particular,with respect to the delamination of slab surfaces.
As such, it is seen by the promoting organisations as a ‘living’document and constructive comments are invited from individualsor organisations.
Comments, referencing this publication, should be directed to:
There is a perception, especially in Auckland, that recentindustrial floor slab installations are more prone todelamination of the top surface layer than they were inthe past. Although the incident level is not great, there isa high cost of remedial work to ready mix suppliers andcontractors. There is also the risk of the generaldissatisfaction of clients. All of these considerations haveprompted the preparation of this document.
This document is the culmination of nine meetingsbetween a group of readymix personnel, all of whomhave significant experience in the concrete constructionindustry. The points made in this report are acombination of perceptions based upon the groupsexperience, and literature searches.
The delamination, or blistering, of concrete floors isphysically represented by the separation of a thin layer ofmortar at the top surface of the concrete from the rest ofthe underlying concrete. The thickness of this layer canvary from paper thin up to 8mm thick, depending on themechanism involved. The size of the delaminated areacan vary from a couple of square centimetres to severalsquare metres.
The actual delamination can occur between one and twoweeks after the placing and finishing process iscompleted.
The delamination mechanism
Authoritative literature sources, such as the AmericanConcrete Institute explain the causes of delamination asbeing due to the formation of a closed surface layer ofpaste that seals the concrete prior to the completion ofbleeding. The surface layer is normally closed when it issteel trowelled. If this trowelling occurs too early, air andwater in the underlying concrete may continue to riseuntil it becomes trapped beneath the layer of densesurface paste. The trapped water or air can create a planeof weakness that may cause the top to delaminate due tosubsequent drying shrinkage or disturbance by traffic.
It is perceived that most cases of delamination are causedby trapping bleed water below the surface of the finishedslab, brought about by early finishing. Another possiblecause of delamination occurs when the finishing processis started too late in some areas. In these instances, thefinisher may attempt move the paste around the surfaceto achieve the desired finish. In isolated spots where thesurface has already set, this paste may simply sit on thesurface rather than form a monolithic mass.
A localised “drummy” area which will eventually
separate from the base concrete, due to wear and tear
from warehouse traffic.
Examples of “drummy” areas distributed within areas
of “crazed” surface cracking. The crazed areas, by
contrast, can provide acceptable long-term surface
durability. Note: The thickness of these surface
delaminations ranges between 1 to 2mm.
Example of a delamination repair. Although the repair
may have acceptable durability, surface discolouration
due to repair work can be an issue with warehouse
owners, and is an avoidable cost if good practice
guidelines are followed.
3Surface Delamination in Slab on Ground Construction
The prevalence of delamination seems to have increasedin recent years. The reasons behind the formation of alocalized and weakened top surface are many and varied.This report endeavors to provide guidance to helpreduce the risk of delamination occurring and coverssuch things as:• materials;• mix design;• placing and finishing techniques;• specification and supervision; and • environmental factors.
This document identifies risk factors thought to beassociated with delamination rather than identifyingabsolute causes, as these can be job-specific. The reportis concerned with the management of these risk factorsthrough the recommendations contained herein.
Although this technical report deals specifically withdelamination, there are a variety of other surface defectsassociated with fresh concrete characteristics and thefinishing process. Many of the causes identified as beingimplicated in the formation of the delaminationphenomenon for slabs can also be implicated in otherissues, such as:• crazed cracking;• plastic shrinkage cracking; and • dusting.
The intention of the report is to provide informationand guidance to interested industry parties in order tominimise the incidence of delamination. Commentariesand/or Discussion points have been added to facilitatebetter understanding between industry parties.
This publication is not intended to replace existing NewZealand Codes, but to be used in addition to thesedocuments and to be accepted as part of the resourceused to promote industry best practice. It is acompilation of information from several recognizedsources, but is not intended to be used as a design orspecification document, or as a tool to absolveresponsibility for the various parties involved in thecommercial process of designing, and contracting tosupply, place and cure a slab-on-ground.
Concrete is a material that is sometimes misused andabused though over-familiarity and a perception that itis a relatively simple material. There have also beensignificant changes to the expectations people have ofthe desirable properties of fresh and hardened concrete.These expectations are often driven by economicconsiderations. Many of the lessons learnt in the past canalso be overlooked or forgotten in the rush to embracenew technologies.
In this section, the constituent materials that make upconcrete (cement, aggregate, admixtures) are discussedwith reference to delamination. The following section,Mix Design and Production, explores how thecombination of these materials (the mix design) mayimpact on the probability of a surface delaminating.
Modern concrete now, almost invariably, consists of thetraditional components (cement and coarse and fineaggregates) together with admixtures and sometimes,mineral additives (used in conjunction with cement andoften referred to as ‘blended cements’. The cementchemistry of the two Type GP Portland Cement brandsmanufactured in N.Z. are not perceived to contribute tothe risk of delamination. However, admixtures caninfluence rates of bleeding and cement setting times. If aproblem exists, compare the setting times of differentcement/admixture type combinations with trial mixes.
New Zealand cement producers manufacture a GP gradecement product that targets a relatively high level of C3Sin the cement. The C3S content drives the early agestrength that is demanded by the industry. Additionally,an increase in the C2S content would require a higherfiring temperature in the kiln, thus increasingproduction costs and more significantly, increasing theCO2 emissions per tonne of cement manufactured.
Specific issues associated with cement that may lead to adelamination problem include:
• Concrete which uses blended cements that displaylittle or no bleed water and have extended setting mayhave an increased risk of premature finishing .Corrective measure: Be aware of the mix or environmentalconditions, which might extend setting times or affect theamount of bleed water on the concrete surface. HighEarly Strength (H.E.) cement, silica fume, GroundGranulated Blast Furnace Slag (GGBS) based cement andHuntly flyash tend to retard setting times. For concretewith little or no bleed water, protect the surface fromrapid drying by spraying an aliphatic alcohol film, knownin the industry as ‘antivap’ (see also Environment, page 6).
• Mix designs with higher cement contents (for highstrength floors) may produce a sticky or tacky mix,
admixtures to slabs prone to frost attack. There isevidence to suggest that this restriction minimizes theincidence of delamination failures in the USA. This viewis contested by some associated with the concreteindustry in New Zealand and is to be the subject ofongoing research driven by CCANZ.
Issues and corrective measures
Specific issues associated with admixtures that may leadto a delamination problem include:
• Chemical factors that retard concrete setting times, orimpede the progress of bleed water to the surface cancontribute to conditions that cause delaminations bymaking it appear that the concrete is ready to finish.Lower w/c ratios achieved with water reducers andsuperplasticisers will have less total free water and thusexhibit altered bleed and set characteristics.Corrective measures: Be aware of how the admixturesused in the mix will affect its bleed and setcharacteristics. Only use retarders where environmentalconditions make it necessary.
• Certain admixtures that can create excessive airentrainment. Examples are retarders, water reducersand superplasticisers. It is a recommendation of thisreport, that air entrainment not be used in mixes30MPa and above, where hard trowelling is required.High strength mixes will naturally have more paste,which provides for workability. Hence, there is noneed for artificial air entrainment. Be aware that25MPa (and below) mixes without air entrainmentmay make the mix too harsh for satisfactory placingand finishing to be achieved.Corrective measures: Admixture companies may overcomethe problem of excessive air entrainment problem byadding de-foaming agents, or the mix design engineermay compensate by reducing the dose of air entrainingagent. However, care in using these products is required,and air contents should be checked regularly both at theconcrete batch plant and on-site, (unless risk ofunintentional air entrainment is known to be negligible).
• High amounts of entrained air may reduce the bleed.Corrective measures: Take extra care when entrained air isnecessary, eg, freezer/cold rooms etc.
• Admixtures do not in themselves cause delaminationsbut can sometimes deceive concrete placers intocommencing finishing too soon, thereby trappingescaping air and bleed water below the surface.Corrective measures: Site-dosing and mixing of Mid-Range Water Reducers and High-Range Water Reducers(superplasticisers), are best avoided because the initialand final set times may vary from load to load. Avoidusing retarders unless there are long traveling timesbetween the concrete batch plant and the constructionsite.
which may have a tendency to crust prematurely,therefore increasing the risk of delamination (due tothe finishing operations commencing too early) orplastic cracking.Corrective measure: Protect concrete from the adverseeffects of sun and wind. Consideration should be given toprogramming pours for early or late in the day to avoidtemperature extremes. Ensure that the concrete hasstiffened uniformly and not just at the surface.
The use of good quality, well-graded, clean aggregates isperceived to reduce the risk of delamination occurrence.This approach ensures that the total water added to theconcrete mix is optimized to the absolute minimum,commensurate with optimum handling and placingrequirements. Lower water content in the mix meansthat there is less chance of the mix segregating orsettling, which in turn leads to the migration of water tothe surface and then becoming trapped by early finishingoperations.
Specific issues associated with aggregate type and qualitythat may lead to a delamination problem include:
• Poorly graded and/or dirty aggregates may createsome risk through a higher void content, higher waterdemand and potential settlement as a result of higheroverall water and void content.Corrective measures include: Always use well-gradedcoarse aggregates, without large gaps between sizefractions (uniform gradation). This strategy does notapply to mixes designed for the vacuum dewateringprocess, For fine aggregates, monitor sand equivalent ona regular basis to assure compliance with NZS 3121:1986.Specification for water and aggregate for concrete.
• Aggregates which are too fine (maximum size 13mmor less) and sands which have an excessive amount of‘fine fines’, ie, too much material passing 300 and 150micron sieves, can produce concrete with excessivepaste (sand/cement/water). This paste may come tothe surface during placing and finishing. It is perceivedthat this increases the risk of delamination.Corrective measure: Avoid using aggregates smaller than19mm (Max. size). Use mix designs with relatively lowsand percentages. Monitor sand equivalent on a regularbasis to assure compliance with NZS 3121:1986.
Admixtures are now used extensively in most concretesthat are used for slabs on grade. The most commonlyused admixtures are water reducers, retarders (forsummer use) and air entrainers. Specifications derivedfrom the USA strictly limit the use of air entraining
5Surface Delamination in Slab on Ground Construction
The environment can have a significant affect on correcttiming of the finishing operations of a slab. The timingissue for finishing operations to commence is recognisedas the key to providing a durable and long lastingsurface. Correct timing is critical to avoid delaminationproblems arising, particularly when bleed waterevaporation is to be considered. Traditionally, the loss ofsheen to the surface of the slab has been the acceptedcriteria to determine the commencement of the finishingoperation – this is no longer necessarily correct.
The most common problem encountered by finishers isthat the surface sheen has disappeared from the slabsurface, possibly due to it ‘crusting’ while the concretebelow the surface is still plastic and not yet sufficientlystiff to support a power float – referred to by some as the‘jelly effect’. To avoid the possibility of prematurefinishing operations commencing, consider the use of awalk behind power float for the first pass.
Specific issues associated with climatic conditions thatmay lead to a delamination problem include:
• Low relative humidity, high air temperatures withhigh concrete temperature, or any combination ofthese factors, may cause the rate of evaporation at thesurface to be greater than the bleed rate and causecrusting of the surface.Corrective measures: Graphs are available that indicate therelationship between wind speed, relative humidity andair temperature and how these factors influence the rateof evaporation. Some manufactures of aliphatic alcoholsprays suggest that a rate of surface evaporation higherthan 0.5kg/m2 to 1kg/m2 per hour causes excessivedrying stiffening of the surface (New Zealand Guide toConcrete Construction, 1999, fig 10.6). In the absence ofbleed water on the surface of the slab, precautions shouldbe taken immediately.Protect the surface particularly from high winds byerecting windscreens and using aliphatic alcohol sprays,to minimise surface drying. It is important to follow theapplication instructions from the suppliers in order forthese products to be effective.
• Direct sunlight increases surface temperature andcontributes to stiffening of the surface.Corrective measure: An internal environment is alwayspreferred, so schedule slab placing to occur once the roofand walls of the building are erected when ever possible
• Cold subgrades and vapour barriers contribute todifferential setting between the slab surface and theunderlying concrete.Corrective measure: Be aware that the concrete below thesurface will take longer to stiffen. Take precautions asabove to prevent premature finishing and to protect thesurface from drying out.
Mix design & production
Mix design (or the properties the mix design imparts tothe concrete) and production processes can have a stronginfluence on the tendency for a concrete mix to besusceptible to the delamination phenomenon. Carefulselection of well graded aggregates, admixtures, and theavoidance of over-sanded mixes and small aggregate sizesis believed to lower the risk of delamination occurring.
Specific issues associated with mix design andproduction processes that may lead to a delaminationproblem include:
• The effect a combination of admixtures may have onair entrainment and bleed characteristics.Corrective measure: Be aware of how the admixtures usedin the mix will affect the air content, and on how this caneffect bleed characteristics. Entrained air has a markedeffect on bleeding. Small air bubbles, deliberatelyintroduced into the mortar by an admixture have theeffect of reducing water demand; and settlement is alsoreduced. When bleeding is substantially reduced, concretecan be prone to premature drying and stiffening effectsThis is due to surface evaporation (when the rate ofevaporation exceeds the bleed rate) whilst the body ofconcrete below the surface behaves normally, stiffeninggradually due to chemical reaction of the cement.Bleeding and surface setting are therefore inter-related.
• Highly sanded/small aggregate sizes have higher fineaggregate content.Corrective measure: Avoid over-sanded mixes and smallaggregate sizes, eg, designed for ease of wide areafreehand screeding. Sand content should only be highenough to just provide a consistent, non-segregating mix,at target slumps.
• Match mix design to the method of placing and tofinishing equipment.Corrective measure: (see Placing and Finishing)
• Quality/consistency of concrete delivered to site.Corrective measure: Ensure the concrete is thoroughlymixed, with close control over the workability/consistency of successive loads as these affect bleed andsetting characteristics, particularly when more than onebatching plant is used to supply the site. Avoid over-wetmixes.
The human element
Performance-based contracts, commonly specified,unless properly supervised by the contractor/specifier,leave the sub-contractor to ‘get on with the job’. Thisproject management approach combined with lowestcost driven sub-contract price means that if a disputearises over issues of materials/quality of work, the causeof the dispute is difficult to determine after the event.
Specific issues associated with quality of work that maylead to a delamination problem-
• Concrete workers as subcontractors can influence thenature of the mix used.Corrective measure: To control this, advise and providetraining for all parties about the risks of higher sandcontents and higher admixture doses.
• Some concrete placers prefer very workable concrete,such as are found in some pump mixes, because theyhave a higher sand content and are often provided ata higher slump.Corrective measure: Balance any request for highlyworkable mixes with the other risks involved (see MixDesign)
• Higher slumps are achieved in part by using moresuperplasticiser or water reducer which may affectbleed rate and retardation, which in turn increase thelikelihood of the placer misreading the timing tobegin power-floating and trowelling Corrective measure: (see Finishing)
• Proper supervision of all aspects of placing andfinishing is often missing.Corrective measure: Adequate control/supervision of thepour and all sub-contract personnel by the maincontractor. It is important that a nominated principalplacer remain on site for the entire operation.
Correct placing techniques, used by experienced placingsub contractors, has the potential to overcome many ofthe problems seen in the industry today. The benefits ofa pre-pour meeting cannot be overemphasized. At thismeeting, all parties associated with the contract can meetto highlight any client expectations of the contractor andsub-contractors,and resolve any concrete supply andtechnical points. Issues such as placing/supply rates, thenumber of plants that may be batching concrete for thepour, commencement times etc can all be addressed,with problems subsequently avoided in the process.
Specific issues associated with placing that may lead to adelamination problem include:
• Insufficient concrete placers/finishers relative to thesize/area of the pourCorrective measure: The engineer, main contractor andplacer should establish the minimum labour resourceto be present for the whole pour. The number ofplacers and the minimum equipment needed will berelative to the area of pour and in accordance withindustry best practice or NZMCPA recommendations
• Lack of experience of concrete placersCorrective measure: The engineer and main contractorshould choose from a list of suitably qualified concreteplacers with a good record of producing quality slabs andfinishes for similar conditions – contact the NZMCPA.
• Selecting the correct screeding process.Several processes are available, namely free screedingtechniques and those using vibrating screed or beamsystems, on rails or with control by laser. Therelationship between the screeding method anddelamination is unclear though it is believed thatmethods and concrete mixes that create a thickersurface paste maybe more prone to delamination. Themethod of screeding will impact on the mix design:• Free screeded floors – require more labour to achieveflatness and levelness, so the concrete placer will prefera more workable mix. Concrete placers preferencesshould be balanced with factors that increase paste,retard mix, or slow bleed (see Mix design, page 5)• Floors placed with a vibrating screed – canaccommodate more coarse aggregate because using amechanical screed as well as a poker vibrator canconsolidate more effectively than using a pokervibrator on its own will do. Design and supply mixesappropriate to the placing and finishing techniques• Vibrating screed/beam vibrators – can ensure a highdegree of flatness and levelness before the finishingoperations begin. Therefore there is less reliance onputting so much work into achieving this with ride-onmachines. Use a vibrating screed/beam vibrator toachieve strict flatness tolerances rather than free-screeding and ride-on finishing.
7Surface Delamination in Slab on Ground Construction
prior to finishing operations beginning on any slab.This requires a regular walk over the floor todetermine how far along the pour a ride-on machinecan operate.If the floating operation is throwing paste wait untilthe surface has stiffened before resuming powerfloating. Alternatively, a walk behind machine may beused for the first pass of the finishing operation,followed by a ride-on machine.
• Trowelling. Trowelling is done at some time afterfloating with the delay to allow some stiffening of thesurface.Corrective measure: The first pass is done with theblades as flat as possible. Additional trowelling may beused to improve the density, smoothness or wearresistance of the surface. Successive trowelling shouldbe done with the blades tilted at an increasing angle tothe surface. If blisters form behind the trowel then theangle of the trowel is too great. These blisters shouldbe immediately pushed down with a magnesium floator a flat trowel and the angle of the blades reduced.Some time should be allowed between each pass toallow the water to evaporate that has been squeezed tothe surface.
• Trowelling does seal the surface so must only occurwhen all bleeding has stopped.Corrective measure: Ensure the concrete is ready tofinish. It is essential that the finishing process is to ‘hitthe gap’. Working concrete that is ready to finish willresult in reduced effort by the finisher to achieve thesame end result.
• Machine trowelling an air entrained slab can alsocause air to be squeezed out under the surface causingdelamination.Corrective measure: For mixes 30 MPa and greateravoid air entrainment in the mix when machinetrowelling.
As has already been stated, the simple rule has alwaysbeen: Do not start finishing until the bleedwater sheenhas gone from the surface and a footprint leaves anindentation no greater than 3-6mm in the surface. Thisdepth will vary depending on whether a walk behind orride-on power float is used.
The window of finish-ability varies for each concrete mixand can even vary from batch to batch on the sameproject. Therefore, there is no strict rule as to when tobegin the different finishing steps.
The concrete, the environment and the equipmentinfluence the window of finishability.
Specific issues associated with the finishing process thatmay lead to a delamination problem include:
• The type of finishing equipment has an effect onclosing the concrete surface.Corrective measure: Delay the floating and trowellingas long as possible to avoid closing the surface – this isthe key to avoiding the formation of blisters andsurface delamination.
• Be aware of the timing of the finishing operations inrelation to the rise of air and water through theconcrete.Corrective measure: There is a potential fordelamination where the bleed takes longer than theplacer expects and or the amount of bleed is maskedby reduced water content as in the case of asuperplasticised mix.
• Placers should have an evidential system to show theydid not get onto slabs too early.Corrective measure: See Appendix 2 for suggested QASystems to avoid this happening.
• Bull floating done incorrectly seals the surface tosome extent.Corrective measure: Bull floating should be completedprior to any bleedwater appearing. Care should betaken to avoid overworking the surface. Too muchpaste can lead to sealing.
• Floating. Floats can be full circular pans or blades,and ride-on or walk-behind. When large areas of slabare to be finished this will often require the use ofride-on floats and trowels.Corrective measure: Be wary of pan floating pastedepths greater than 6mm. These depths are likely to becaused by vibrating ‘soft’ mixes or shifting pastearound to fill low spots.The use of ride-on machines requires considerableexperience from the operator. There needs to be asystem for determining which sections of a floor areready to finish. Ensure the boot test is carried out
Conclusions & recommendations
The issue of the delamination of the wearing surface ofconcrete slabs and their causes, is a complex and at timescontentious topic. This document is the result ofindustry consultation and is to be viewed as a discussiondocument that represents the industry consensus view atthe time of publishing. Thus, it is subject to ongoingindustry review.
The conclusions and recommendations which follow arethe key points from many that have been covered in thispublication. Due to the at-times unpredictable nature ofslab delamination failures, readers are urged to considerall of the points covered in this report rather than to relyon the main points listed. These are listed to give readersan overview of the main issues only and are notpresented in any particular order of importance.
All American literature references air content as acontributing factor in delamination. Because of this, it issensible to suggest removing air from the mixes of30MPa and above. It is practical to retain air entrainmentin the lower strength mixes up to and including 25MPaas an aid to the cohesion of those mixes, to improve theirworkability and to reduce their water content.
Use of admixtures
Ensure that there are no unintended air entrainmentaffects when two or more admixtures are used in thesame mix. Be prepared to use ‘antivap’ surface films tocontrol the premature stiffening of the surface of theconcrete.
Finishing operations should be undertaken usingexperienced placers and finishers, who are familiar withrecognised best trade practice. (Placers who aremembers of the NZMCPA are encouraged to keep upwith latest trade practice.) Exercise caution in thedecision to commence finishing operations when usingpan and ride-on machines.
Concrete mix properties
Be aware that high cement content mixes and mixescontaining mineral additives such as fly ash or silicafume will be sticky to handle and may crust at thesurface, leading to premature finishing. Site observationsindicate that high sand content mixes may increase theoccurrence of delamination in slabs.
Site QA systems
Carry out on-site tests on the concrete to determine thebleed rate of the concrete. This will give a goodindication of the correct time to commence machinefinishing operations.
Consider obtaining sign-off from the main contractor,stating that all operations were undertaken in accordancewith good trade practice, to ensure that the sub-contractor is not pressured into actions which may, at alater date, rebound on him.
9Surface Delamination in Slab on Ground Construction
Aggregate, coarse: An aggregate predominantly retainedon a 4.75mm test sieve.
Air in concrete: There are two kinds: entrained air –fine spherical voids, of size less than1mm; and entrapped air – coarsevoids of irregular shape, size greaterthan 1mm, which should bedissipated through correctplacement, vibration andconsolidation of the freshly placedconcrete.
Blistering: The formation of blisters in theconcrete surface during trowellingcan be the result of entrained air orexcessive fines in the mix, of earlytrowelling or of an excessive angle ofthe trowel blades. If the air content isknown to lie within an acceptablerange, then blister formation is animmediate indication that the angleof the trowel blade is too great forthe surface in that area at thatparticular time for the concrete andcurrent job conditions.
BCITO: Building Construction IndustryTraining Organisation.
Concrete placers: Concrete workers who receive thefresh concrete from barrow, skip,pump, chute or conveyor and placethe concrete into position so that it isproperly consolidated and the slabsurface is left prepared to receive thefinishing process. Consolidation isachieved using (pencil) vibrators andvarious forms of vibrating screed.
Concrete finishers: Concrete workers who apply thefinishing process to the slab concretesurface. Work is begun after the bleedphase of the concrete setting processis complete. The finishing processconsists of floating followed bytrowelling, and may be executedusing walk behind or ride-on powertrowel machines.
GGBS: Ground granulated blastfurnace slag.
NZMCPA: New Zealand Master ConcretePlacers Association.
Paste: A thick mortar of sand, cement andwater, of creamy consistency.
SCMs: Supplementary cementitious materialssuch as pozzolans.
Admixture: A material other than water,aggregate and Portland Cement(including blended cement) that isused as an ingredient of concrete andis added to the batch in controlledamounts immediately before orduring its mixing, (unless statedotherwise by the manufacturer), toproduce some desired modificationof the properties of concrete.Common admixtures used are: air-entraining water reducing (seebelow), set retarding, set accelerating,combinations and high range waterreducing (superplasticiser).
Admixture, water reducingA water reducer can be defined as anadmixture that reduces the amountof mixing water in concrete for agiven workability. It improves theproperties of hardened concrete, andin particular, increases strength anddurability.Low range: The reduction in mixingwater is a minimum of 5% (NZS,ASTM, CAN) at the manufacturer’srecommended dosages, usually 0.2-0.3% by weight of cement.Mid range: The reduction in mixingwater is about 10% at themanufacturer’s recommendeddosages, usually 0.45-0.60% byweight of cement. Higher waterreduction can be achieved withhigher dosages (up to 1.0-1.2%).High range (superplasticisers): Thereduction in mixing water is about15% (but can be up to 30%) at themanufacturer’s recommendeddosages, usually 0.8-1.0% by weightof cement. High range waterreducers (HRWRs) are used toproduce concrete with a highworkability for easy placement, andhigh strength concrete with normalworkability but with lower watercontent. There are many HRWRswith different properties – selectingthe most suitable for a specificapplication requires a thoroughengineering process from mix designto lab trials to field tests.
Aggregate, fine: An aggregate predominantly passinga 4.75mm test sieve (sand).
Appendix 1: Recommendations for adding/dosing superplasticers at the site
Due to the mix specific nature of admixture modifiedconcrete mixes, site batching of admixtures should onlybe undertaken after preliminary site trials, undertakenby experienced operatives. In the following discussion itis assumed that the initial slump (before addition ofsuperplasticiser) is within the tolerances specified byNZS 3109: Concrete Construction.
Consultants and or specifications requiringsuperplasticiser addition on site usually want theconcrete to be a uniform slump after superplasticising.However, the concrete arriving on site will always vary inslump to some degree. The concrete is assumed to havebeen correctly batched but, for whatever reason, thebatcher has not been able to correctly estimate the sandmoisture content. A number of possibilities for dosingsuccessive loads of concrete arise. These are:
• Adding superplasticiser on site, varying the dosebetween loads: A ‘dry’ load (ie, drier than thespecified initial slump) will have less total bleed wateravailable. In an attempt to achieve the same finalslump, it will be given a higher dose of superplasticiserthan the wetter load placed next to it. This means thatin comparison to the load next to it, the dry load maybleed less, bleed slower and set slower due to theincreased dose. If finishing operations occur at thesame time for both loads (almost inevitable with ride-on power floats), then one or both loads will befinished at the wrong time. This greatly increases therisk of surface defects such as delamination.
• Add superplasticiser on site, using exactly the samedose of superplasticiser for each load: To achieve thespecified initial slump on site for dry loads, measuredquantities of water are added, and recorded, to bringthe load to the required slump. A measured quantityof superplasticiser is then added to achieve theincreased slump for placing. This quantity should bethe same for every load, regardless of their relativeslumps. The added water is simply correcting theinitial error in estimating the aggregate moisturecontents, so better uniformity of workability, bleedand set characteristics should result.
For ‘wet’ loads, no additional water needs to be added,simply add an identical quantity of superplasticiser asfor all the other loads. As long as the slump fallswithin the required tolerances, the strength anddurability of the concrete should be acceptable.
• Dosing with superplasticiser at the plant: In manysituations, adding superplasticiser at the plant can be abetter option. In any case, this is the usual method ofdosing; and it is certainly faster and more efficientthan site dosing. Better quality and consistency ofconcrete can be expected because the same person isusually involved in batching, dosing and mixing everyload. For central-mix plants in particular, thesuperplasticiser will receive better mixing through theconcrete than if it is truck-mixed on site. In addition,every load will automatically receive the same dose ofsuperplasticiser when the concrete is plant-dosed.
A cautionary note for specifiers
Specifiers should recognise that ready mix concretesuppliers work with only one brand of admixture on aplant-by-plant basis. As a consequence, the plantoperators, being batcher, production testing technicianand mix design engineer become very familiar with thecharacteristics of a particular brand of admixture and itsperformance when combined with local raw materialssupply, and further, with other same brand admixtureswithin the same mix.
Concrete suppliers trial new admixtures extensively fromlab to field trial and, for a given set of plant supplycharacteristics, will ask for adjustments to be made byadmixture suppliers to get the desired characteristicswhether this is for (excessive) air entrainment, settingtime, etc. Thus a long history of performance isestablished.
For a discussion on admixtures for concrete see Chapter5 in New Zealand Guide to Concrete Construction (1999),published by the Cement and Concrete Association ofNew Zealand.
11Surface Delamination in Slab on Ground Construction
On-site tests to determine the ‘window of
Photographic records and main contractor sign-off
Photographic documentation, or sign-off by thecontractor’s supervisor stating that the bleed water sheenhad gone and that the footprint test gave an indentationof no greater than:• 3mm indentation for a ride-on-float, or• 6mm indentation for a walk-behind-float,
is a method that can be used to acknowledge that bestindustry practice has been followed.
This approach is the simplest means that could be usedon site. It does not attempt to quantify the condition ofthe slab (other than by measuring slab indentation) anddepends upon the knowledge and co-operation of theparties involved for it to be successful as a QA tool.
Acceptance of this approach, and its contractual standingwould need to be agreed by all parties involved in thecontract before the work commences on site.
Bleed water measurement
This test is a method that attempts to duplicate theconditions of the freshly placed slab. The proposedmethod is a variation on the Suprenant and Malisch test(1998, p. 170).
The test procedure is suggested as follows:Samples of the wet slab concrete are placed in buckets ata depth equal to the finished slab depth using
Appendix 2: The ‘window of finishability’ – and on-site tests to determine it
consolidation similar to that achieved on the job. Thebuckets are then covered with a clear lid and slightlytilted in order to observe and to facilitate bleedwatercollection. The clear cover provides protection for thesample against any prevailing environmental conditions.The accumulating bleedwater is drawn off at intervalsuntil the operator is satisfied that bleed has effectivelystopped and this test may be combined with an top ofactual slab impression test as further verification that aparticular area of the slab is ready for the finishingprocedure to proceed.
As with all sampling procedures care is required toreplicate sample selection and actual placing andconcrete consolidation procedures. Where large slabpours are to be undertaken the concrete may be suppliedfrom several plants and the placing and finishing canoccur over a period of many hours. This means that thepotential for variations in the consistency of deliveredconcrete is high. In these circumstances the number ofsamples would need to be increased accordingly.
This QA system would provide the concrete placers andsuppliers with some protection if a slab delaminates.From the construction company or head contractor’spoint of view getting the timing right for slab finishingoperations is so important that they should requireevidence that not only was the timing correct but thattesting was continued throughout the duration of theconcrete pour.
Stiffening Bleedwater Sheen
1 2 3
Hitting the gap Late finishing(may not achievespecified surfaceflatness)
Premature finishing(may result in surfacedefects)
TIME in hours (rough guide only)
25mm 20mm 12mm 6mm 3mm Visible Not visibleDepth of footprint:
Appendix 3:A discussion on the use of pan floats and ride-on floats
There is some disagreement within the industry on thecorrect use of pan and ride-on floats. The two mostdisputed issues are discussed below.
1. Do pan floats seal or not?
Pan floats and subsequent power trowelling impart moreenergy to the concrete surface at an earlier time than dotraditional walk behind or ride-on machines equippedwith clip-on float shoes. Pan floats form a dark layer ofdense, almost air free, low w/c ratio paste at the surfaceof the slab. If the concrete below the finished top surface(to a depth of 5-10mm) is still somewhat plastic, anyvoids containing air or excess water are squeezed andelongated in the near surface zone before the concreteachieves final set.
The horizontal shearing action from the early use of panfloats causes near surface voids to come together. Underthe influence of drying shrinkage and floor traffic, theelongated voids interconnect and become subsurfacefractures that eventually delaminate.
An alternative view argues that the shearing action ofpan floats breaks open the surface much better than dofloat blades. Pan floats actually reduce the amount oftrapped bleedwater and air as long as they can breakopen the previously formed, densified top surface layer.
2. Ride-on machines exert less pressure on the slab,and thus encourage finishing operations to commenceprematurely.
It must be emphasised that huge areas of floor slabs havebeen finished with ride-on machines withoutdelamination of the top of the slab occurring. However,
there is an argument that was reinforced by theinvestigations for Project Slab undertaken by CCANZ,that the use of ride-on machines are contributing to anincrease in the incidence of delamination
Ride-on machines exert less pressure than that of thetraditional boot test. This enables the finisher to befloating the slab earlier, which if the views expressed inissue 1 above is true, can then lead to delamination ofthe surface of the slab.
The contact area of a pan float is greater than that ofclip-on or combination float blade. Therefore thefinisher can apply less pressure to float the slab and thustends to get on the slab sooner, especially if the ambientconditions or the setting characteristics of the concreteindicate that its starting to set or the surface paste iscrusting.
Walk behind and ride-on machines exert surfacepressures of 2.5 to 6.5kPa when equipped with blades.Pan floats decrease the pressure to 1.1 to 2.9kPa. Aconcrete finisher, walking on the slab, can apply 20 to40kPa depending on the individuals boot size andweight. Thus, it is much later in the window offinishability when a slab could support a finisher walkingbehind a power machine as opposed to when it couldsupport the mass and the surface pressure of a ride-onmachine with pan floats.
Users of ride-on machines state that the ridecharacteristics alter dramatically when the finisherencounters wetter concrete. Therefore, an experiencedoperator will not press on to finish areas that are notready.
New Zealand Guide to Concrete Construction. (1999).Wellington: Cement and Concrete Association of NewZealand.
Scali, Mauro., & Suprenant, Bruce. (2000, December).‘Do Pan Floats cause blisters or delaminations?’,.ConcreteConstruction [World of Concrete], 45 (12): 51-54.
Suprenant, Bruce A., & Malisch, Ward R. (1998,October) ‘The true window of finishability’. Aberdeen’sConcrete Construction, 43 (10): 859-863.
Suprenant, Bruce A., & Malisch, Ward R. (1999,September) ‘Sealing effects of finishing tools’. Aberdeen’sConcrete Construction, 44 (9): 39-43.
Suprenant, Bruce A., & Malisch, Ward R. (1998, January)‘Diagnosing slab delaminations: Part 1’. Aberdeen’sConcrete Construction, 43 (1): 29-35.
Suprenant, Bruce A., & Malisch, Ward R. (1998,February) ‘Diagnosing slab delaminations: Part 2’.Aberdeen’s Concrete Construction, 43 (2): 169-175.
Suprenant, Bruce A., & Malisch, Ward R. (1998, March)‘Diagnosing slab delaminations: Part 3’. Aberdeen’sConcrete Construction, 43 (3): 277-283.
Surface delamination in slab on ground constructionA report based upon site experience &