Ground Bearing Concrete Slabs - Chapter 2

2.1 IntroductionA correctly designed and constructed ground bearing concrete slab combines theadvantages of hard wear, long life and the ability to carry heavy loads at low costs. Thepurpose of a ground bearing slab will vary according to application and each projectrequires its own individual characteristics, including strength, abrasion resistance,flatness and aesthetics. An important factor commonly taken into consideration is speedof construction and the savings that accrue from fast-track construction.

While this chapter deals with some general construction matters, Chapter 5 presentsten case studies that illustrate project-specific information in more detail. They are:

Section 5.3 A single pour industrial floorSection 5.4 A long strip floorSection 5.5 A long strip external hardstandingSection 5.6 An unreinforced concrete roadSection 5.7 Differential settlement of a single pour floorSection 5.8 Repairs to a cracked external hardstandingSection 5.9 Construction of a floor perimeter beamSection 5.10 Traditional industrial floor installationSection 5.11 Construction defects in an industrial roadSection 5.12 Overlaying a cracked external hardstanding

Note that Section 5.12 includes a specification for the overlay of a cracked concreteexternal hardstanding using concrete block paving as the new surfacing material.

2.2 Principal issuesThe important issues in ground bearing concrete are:

• level tolerances• cracking — load induced, moisture loss induced and temperature profile induced• joint performance/load transfer• skidding and/or abrasion resistance• foundation

2 Construction

• ride quality• surfacetextureanddrainage• loadingregime.

Oneof the uniquefeaturesin groundbearingslabsis the relationshipbetweendesign,constructionandperformance.All of theaboveissuesareinfluencedby bothdesignandconstruction.

2.3 Traditional construction methods2.12.3.1 Long strip constructionFor both internal and external concrete slabs, long strip construction has beenestablishedastheconventionalway of constructionsincetheearly1970s.Theconcreteis laid in a seriesof stripsup to 6m wide usingtimberor steelformwork.Everysecondstrip is concretedinitially, leaving infill unconcretedstrips. The infill strips areconcretedseveraldayslater usingthe originally laid stripsasthe formwork. Stripscanbe up to 60m or more in length.Slabsare often thicker than 150mm andsteelmeshreinforcementmay be provided.The concretemay be placedand compactedin twolayers,the upperlayer being placedwhile the lower is still plastic.Compactiontakesplaceusingeither internal pokervibratorsor twin-beamvibrating compactorsrunningon the formwork or on the previously cast slabs.Care has to be exercisedin themanagementof concretedeliveries— if thereis a delaybetweendeliveries,theearlierconcretemay begin to setprior to the next delivery beingplaced,resultingin a ‘coldjoint’. This might havestructuralimplications.Sections5.4 and5.5 illustratelong stripconstructionfor a floor andan externalhardstanding,respectively.

2.3.2 Wide strip constructionWide strip constructionwasintroducedin theUK following thedevelopmentof a widespancompactingbeamknown as the ‘Razorback’ (a spaceframe compactingbeamshownin Fig. 2.1) in theUS.Razorbacksenableslabsto belaid in stripsof width up to25m althoughit is more commonto work in the range9–15m. Laying the concretefollows thesameprinciplesasfor thelong strip method.Theconcreteis placedbetweenformwork, levelled, screeded,compactedand left to cure. In the caseof roadsandexternalhardstandings,a curingcompoundis applied,but in thecaseof floors, finishingtechniquesprecludethis andprotectionfrom sunandrain is achievedby thebuilding inwhich the floor is cast. An industrial floor should be installed when the building’scladdinganddoorsare installed.

Often, the use of two-layer construction to incorporate mesh reinforcementisuneconomicalsostoolsor largediametercircular fabric supportsareusedto enablethemeshto be positionedprior to the placingof the concrete.

The advantageof wide spanconstructionover long strip constructionis that greaterdaily outputcanbe achievedwith a similar sizedlabour force. As largerareascanbeconstructed,more skilled finishersare required,and in the caseof floor construction,multi-headedpowerfloatsarecommonlyused,so enablingonelayer compaction.

36 GROUND BEA RI NG CONCRETE SL A BS

2.4 Large bay construction2.1,2.2Frequently,concreteslabs,particularlyfloors,arelaid usinglargebayor largepourmethodsso as to increasethe daily outputand to speedup the project.The first UK methodwasdevelopedby A. Monk & Co. Limited working with Silidur SA of Belgium anddifferedfrom conventionalgroundfloor constructionin thatno sideformswererequired,excepttocontaina day’spour.High workability concretehasa slumpin excessof 150mm which ismadepossibleby theadditionof super-plasticizers.Thehighslumpvalueallowsconcretetobepoureddirectly from a truck mixer andto bespreadmanually.Theconcretealmostself-levelssuchthatasatisfactorylevel canbeachievedby undertakingfinal adjustmentsbasedupon levels provided by laser transmitters.Any discrepanciesfrom true level can becorrectedusingtimberscreedboardsto bringthesurfaceto thecorrectlevel.Compactionisachievedby lightweight screedbeamsor vibrating pokers.Theprincipal disadvantagesofthe methodare in the segregationof the aggregate,with the larger particlessometimessinkingto thebottomof theslabandthepoorsurfaceregularityoftenachieved.Segregationcan lead to a high concentrationof fines at the surfaceand consequentloss of abrasionresistance.For this reason,suchfloors areoften treatedwith abrasionenhancingtoppings,for exampleAmorex.2.3Thistechniqueallowstheuseof steelmeshreinforcement,which isusuallylaid out a dayaheadof the placingof the concrete.

Laserguidedscreedingmachinescanbeconfiguredto install concreteto a fall soasto allow externalslabsto be installed.They havebeenusedto install externalconcreteslabsandcementstabilisedroadbases.

2.5 Laser screed slab installation2.2,2.3Hughes Group and John Kelly (Lasers) Ltd2.3 introduced laser guided screedingmachinesto EuropeandScandinaviain the1980s.PrecisionConcreteFloorsLtd2.3 werethefirst UK companyto adoptlaserscreeding.By 2002,therewereover25 laserguidedscreedingmachinesconstructingconcretefloors in the UK. Section5.3 illustratestheinstallationof a floor usinga laserguidedscreedingmachine.

Fig. 2.1. Useof `Razorback' enablesgreaterwidthsof concreteto be placed

CONSTRUCTI ON 37

2.5.1 The Somero S240 Laser Screed2.2

The machinedescribedin this sectionis oneof severaltypesof laserguidedscreedingmachines.Laser guided screedingmachinescombine state-of-the-artlaser controlsystemswith conventionalmechanicalscreedmechanisms.The machineshave fourwheeldrive, four wheelsteer,including‘crab’ steerfor awkwardareas,andareoperatedby onepersonseatedat a point of maximumvisibility. SeeFig. 2.2.

Mounted on the twin axles, a circular fully slewing turntable carries a counterbalancedtelescopicboom,typically havinga6m reachon theendof which is attacheda3–4m wide screedcarriageassemblywhich comprisesa plough,anaugerto spreadtheconcreteaccuratelyanda vibrating beamfor compaction.Testresultshaveshownthatsuchmachinescancompactconcreteto depthsin excessof 300mm.

A self-levellinglasertransmitteris fixed at a visible point closeto thework soastoproject a 360º rotating beam acrossthe working area. Dependingon the type oftransmitter,variousinclinationsof floors can be achieved,including level, single anddual grades.The level of the laser screedingmachineis controlled by a laser beamwhich activatesreceiversmountedon the screedcarriageassembly.During concretingthe signals are relayed continuouslyto an on-boardcontrol box that automatically

Fig. 2.2. Laserguidedscreedingmachine.Thecarriage to the left is pulled towardsthe remainderof the machineand therebyplaces,compactsand levelsthe concreteinoneoperation.Thelevelsare controlledautomaticallyso the operatorcan focusuponmovingthe concrete.2000m2 can be installeddaily usingthis technique


controlsthe level of the working screedheadby direct interventionon the machine’shydraulicsystem.Thelasertransmitterrotatesat300rpmsothattheheightof thescreedcarriageis adjustedfive timesper second.

2.5.2 Laser screed operationLaserguidedscreedingmachinesareusedin conjunctionwith mixer truckswhich placeconcrete25–35mm abovethe finishedfloor level. The positioningof the laserscreedingmachine, mixer trucks, sli p membrane, joints and reinforcement requires carefulorganisationso asto attainmaximumoutput.Oncethe concretehasbeendepositedfromthemixer, theboomof thescreedingmachineis extendedoverthefreshlypouredconcreteandthescreedcarriageis lowereduntil thereceiverslock ontothesignalgeneratedby thelaser transmitter.The boom is then retracted,drawing the screedcarriagetowardstheoperatoracrossthe concrete,placing,compactingandscreedingsimultaneously.

In onepassa laserscreedingmachinecanplace,compactandscreed20m3 concretein undertwo minutes.Becauseof thegeometryof thehorizontalauger,screedingtakesplacefrom left to right with anoverlapbetweensequentialscreedingrunsof 300mm soas to ensureoptimum level and surfaceregularity acrossthe entire floor slab.As thepour is containedby the perimeterof a building, or by kerbs,no formwork is requiredexceptto allow for doorwaysand drains,or to containa day’s pour for slabswhichcannotbe finishedin oneday.

Outputdependson site-specificdetails,type of reinforcementused(fabric canposeseveralproblemsas the laser screedingmachinetends to lift the mesh out of theconcrete)andthe speedat which the concretesuppliercandeliver the concreteto site.Outputsof between2000m2/day and 3000m2/day are normal, and 5000m2/day hasbeenreported.

2.5.3 Laser screeding level controlThescreedinglevel of themachine’sscreedcarriageis maintainedby anautomaticlasercontrolledsystem.Laserreceiversmountedat eachendof thescreedcarriagedetectthereferencedatumplaneemittedby a laserlevel transmittersituatednearthe work area.Theon-boardcontrolbox checksandadjuststhescreedcarriagelevel in relationto thelaserplanefive timesper second.

2.5.4 Advantages associated with laser screedingmachinesThosecontractorsusinglaserscreedingmachineshavereportedthe following benefitsascomparedwith traditional long strip construction.

• Higher strength,denserandmoredurablefloors.• Wide bay constructionwith maintainedtolerances.• Flatter floors (seeFig. 2.3).• Working methodensureshigh productivity as it eliminatesmanualscreeding.• Ensuresconstructionprogrammesarekeptto time,enablingpossibilitiesof earlier

useof facilities.• Damage-proneconstructionjoints arekept to a minimum.

CONSTRUCTI ON 39

• Choiceof meshor fibre reinforcement.• Largerareasof floor canbeplaced,screeded,vibrated,compactedandleft to cure

in a singleday.5000m2/day hasbeenreportedbut outputsof 2000–3000m2/dayaremorecommon.

2.6 Finishing and curing processesOncethe slab hasbeenlaid and screededthe final stagesof finishing and curing areimportantto achievea flat smoothdurablefloor or externalhardstanding.

2.6.1 Floor finishingFinishing is a critical and skilful operationthat takesplace after the slab has beenscreeded.Preliminary finishing involves the use of a lightweight alloy straight edgewhich is drawn acrossa freshly laid surfaceto producea flatter surfacethan thatscreeded.In thecaseof a floor slab,oncetheconcretehasgainedsufficient strengthtosupporta person,powerfloatingtakesplace using walk-behindmono powerfloatsormulti-headedride-onpowertrowelsin orderto achievea harddenseandflat surface,asshownin Section5.3. On floors wherehigh surfacetolerancesarerequired,suchasinwarehouserackingbuildings,a combinationof powertrowelsandstraight-edgesmaybeused.Oncefinishing hastakenplacethe concreteis left to cure.

In thecaseof externalhardstandings,drainageandskidresitanceneedto beachieved.A brushedfinish is often applied,as shownin Section5.5. Oncethe correct surfacetextureis achieved,a curingcompoundis sprayedover thesurfaceto preventthe rapidevaporationof the concrete’sfree water— seeSection2.6.2.

2.6.2 CuringCuring is an essentialpart of the constructionprocess.Someconsiderthat the labourintensivetraditionalmethodof coveringtheslabwith wet hessianfor at leastsevendays

+1·000

–1·000

–1·000

0

+1·000

0

(a)

(b)

Fig. 2.3. Comparisonof floor profiles achievedwith: (a) manualscreeding;and (b)laser screeding


is themosteffectivecuringmethod.If waterescapestoo earlytheupperpartof theslabwill dry out quickly with slowerratesof curing occurringbelow.This canleadto lowabrasionresistanceand slab curling. Curing time, and hence concretestrength, isaffectedby temperature,wind, rain and sun. Shadingmay be sufficient to allow theconcreteto cure.Alternatively, a spraycuremembranesuchasProseal,manufacturedby Armorex,2.3 may be appliedimmediatelyfollowing powerfloating.

2.7 Slip membranes2.1,2.22.7.1 Purpose of slip membranesThe purposeof slip membranesis to reduce the coefficient of friction betweenaconcreteslabandits sub-base.Theyarenot intendedto actasa damp-proofmembrane(DPM) althoughthematerialdoesallow theretentionof somemoisture.If a DPM wereto beusedthenno moisturewouldescapedownwardsfrom theslab.Perforationscannotbeentirelyavoidedandsoanamountof moisturemayescapeinto thesub-base.Thesliplayerreducesthecoefficientof friction soallowing theconcreteto movemoreeasilyasit shrinksduring the curing process,hencereducingstressesin the slab,decreasingthepossibility of crackingandassistingin the developmentof inducedjoints.

2.7.2 Slip membrane materialsAt presentthereis no British Standardgoverningthe useand type of slip membranealthoughpolyethylenesheetsarecommonlyusedwith thicknessesof 1000gauge(250microns)and1200gauge(300 microns).

2.7.3 Importance of sub-base with regard to slip membranesSlip membranesare installed betweenthe slab and sub-baseimmediatelybefore theconcreteis placed.Theyhaveto bestrongenoughto withstandconstructiontraffic, suchastruck mixers.It is importantto overlapthesheetsby at least200mm andusetapetoensureadjacentsheetsdo not moveor separate.Caremustbe takento avoid wrinklesandrips, which may inducecracksin the strengtheningslab.

If ruts arepresentin the sub-basesurfaceand the slip membranesheetsareplacedover them,voids may form beneaththe slabwhenthe concreteis placed.Weakspotswill developleading to a possibility of cracking when load is applied. If the cross-section of the sub-baseconsistsof rises and falls or peaks and troughs, the slipmembranemight fail to do its taskastheshapeof thesub-basemaycauseinterlockandthereforelimit thehorizontalmovementof theslabasit shrinks.Stressescouldincreaseandcrackingensue.It is importantto ensurethat thesub-baseis asflat aspossibleandthat careis takenin laying the slip membranesheets.

2.7.4 Use of slip membranes in post-tensioned slabsA post-tensionedslabhastendonsrunningthroughits length.Oncethe concretehasgained enoughstrength,the tendonsare tensionedso compressingthe slab andincreasingthe strengthof the floor by preventingtensilestressfrom developinginthe concrete.Two layersof slip membranecanbe usedto reducethe coefficientoffriction betweenthe slab and the sub-base.This may lead to a reduction in the

CONSTRUCTI ON 41

amount of reinforcement required within the slab and also to a reduction in thenumberof contractionjoints. When two layersof slip membraneareusedfor theirslip characteristics,as opposed to their strength, thickness may be reduced.Typically, a sheet used in two layer construction might be of 500 gauge (125microns).Greatercaremustbe takenin the handlingandplacingof thin membranesheets.

2.7.5 Considerations in the design and use of slip membranesTherearenospecificguidelinesregardingtheuseof slip membranesandthedesignerhasthechoiceof whetheror not to usea slip membrane.Thechoicewill bebasedonconsiderationsof shrinkage,strength,curlingandjoint details.Thepurposeof theslipmembraneis to allow the concreteslab to shrink freely andso reducethe levelsofstressdevelopedby restraint to movement.Allowing movementreducesthe numberof contraction joints required,but the joints which areprovidedmaybemoreactive.A consequenceof eliminating the slip membraneis the developmentof interlockbetweenthe sub-baseand the slab. Slab/sub-base friction is enhancedand thecoefficientof friction increasesfrom 0�2 to 0�7 soinhibiting themovementof theslabandrequiringmorecontractionjoints, eachmovingby less.For this reason,omittingthe slip membranemight be beneficial in warehouseracking buildingswherelargejoint movementscould rendera floor unsuitablefor pallet andfork lift trucksowingto the possibility of suchequipmentbecomingunsteadywhen carrying load acrossjoints.

Curling is causedby differential moisturelossbetweenthe surfaceandbaseof aconcreteslab.Whena slip membraneis used,themajority of moistureescapesfromtheslabthroughits uppersurfacewhich thenshrinksandcuresfasterresultingin theslabattemptingto curl up at its edges.Eliminating a slip membraneallowsmoistureto escapethroughthebaseof theslab,which leadsto moreevencuringof theslabandsoreducescurling. If too muchwaterescapesthroughthebaseof theslab,insteadofcurling upwards,theslabmaydevelophoggingwith theedgesof a slabattempting tocurl downwards. Blinding (i.e.providingfine materialat thesurface)thesub-basecanreducehoggingby allowing moistureto escapefrom theslabwhile at thesametimepreventingit from flowing away through the sub-base.The ideal slip membranewould:

• allow theslabto moverelativeto thesub-baseasit shrinksduringcuring,enablingfewer joints to be constructed

• be easyto handleandstrongenoughnot to tear• be perforatedso as to allow a controlledamountof moistureto escapeand so

eliminatecurling.

2.8 Joint detailsIn theearlylife of aconcreteslab,it will contractandis in dangerof crackingandthis iscontrolledby anarrangementsof joints. In slabswhereoperatingconditionscanpermitthe presenceof cracks,for examplein slabswhere hygieneand dust control are of


secondaryimportance,thenumberof jointscanbereducedandadditionalreinforcementprovided.Particularattentionshouldbepaidto thealignment,settingandcompactionofconcreteat joints.

2.8.1 Movement jointsMovement joints are provided to ensureminimum restraint to movementcausedbymoistureand thermalchangesin the slab.Movementjoints are designedto allow theslabto contract.Expansionjoints areusedin regionswheretemperaturechangescanbesubstantialin a shortperiodof time andarerare in the UK.

2.8.1.1 Formed Doweled Contraction (FDC) joint. This type of joint (Fig. 2.4) isprovidedat the endsandsidesof a constructionbay. It includesdebondeddowel bars.To ensurethedowelbarandsleevearrangementcanmove,thebarsaredebondedusingoneof threetechniquesillustratedin Fig. 2.5.

120 mm gap

30 mm × 5 mm formed surfacegroove sealed later Dowel bars, end in

2nd bay debonded

Polythene slip membrane

1st bay 2nd bay

50–7

5 m

m

Fig. 2.4. A FormedDoweledContraction(FDC) joint

Debonding compound

Tight fitting rigid PVC tube

Cork

PVC or polythene flexible sleeve

Fig. 2.5. Threemethodsof debondingdoweledjoints

CONSTRUCTI ON 43

2.8.1.2 Induced Doweled Contraction (IDC) joint. This type of joint is designedtoallow movementsas the concreteshrinks. It is usedin constructionbays which aredesignatedlarge (any area greater than 1000m2) and which has been poured in acontinuousoperation.This typeof joint (Fig. 2.6) is formedby crackinduction(Section2.8.3).Thedowelbarsenablevertical load transferandarefixed prior to theplacingofthe concrete.

2.8.1.3 Induced Contraction (IC) joint. Figure 2.7 shows the type of inducedcontraction joint used in large constructionbays. It reducesthe cost of joints byeliminating dowel bars and reducesthe risk of joint breakdown owing to poorworkmanshipin theplacingof thedowelbarsbeforeconcreting.Loadtransferdependson aggregateinterlock acrossthe induction groove.Floors incorporatingthis type ofjoint aresuitablefor lightly loadedapplicationswherereinforcementrequirementsareminimal.

2.8.1.4 Isolation joints. Isolation joints or full movementjoints are usedto permitmovementsaroundthe fixed partsof buildingssuchasrecesses,walls (Fig. 2.8),drains

120 mm gap

5 mm saw cut or crack inducerboth 50 mm deep to be sealedlater

Dowel bars, 2/3length debonded

Polythene slip membrane

50–7

5 m

m

Bars tied toreinforcement stools

Induced crack

Fig. 2.6. InducedDoweledContraction(IDC) joint

Polythene slip membrane Induced crack

Crack inducing groove

Fig. 2.7. InducedContraction(IC) joint


and columns.They keepthe concreteslab separatefrom fixed elements.At columnsthere are two different methodsof isolation — diamondor circular (Fig. 2.9). Thecircular joint is often preferredas it usesless expensiveformers and has no stressinducingsharpcorners.

2.8.2 Tied jointsTied joints hold two constructionbaystogetherusingtie barsandareprimarily usedtorestrainmovementand contractionin the horizontalplane.The tie barsalso act as aform of stressrelief astheyarebondedwithin theconcreteandprovidea largeamountof strengthacrossthejoints.Tie barsareinsertedin pre-drilledholesin theformworkasthe concreteis placed.

Sealing groove

Compressible filler

Slip membrane

Sub-base

Fig. 2.8. Isolation joint at a wall

100 mm min.cover to stanchion

Isolationjoint

Fig. 2.9. Circular and diamondisolation joints at stanchions(plan view)

CONSTRUCTI ON 45

2.8.2.1 Formed Tied (FT) joint. This type of tied joint (Fig. 2.10) is providedaroundthe edgesof a constructionbay or at a stop-endcommonlywhenconstructingconcretefloors usingthelong strip methodandhasprovedsuccessfulin controllingmovementatjoints.A grooveis providedto allow thejoint to besealed(Section2.8.5)andis formedby a strip placedon the edgeof the first bay cast.

2.8.2.2 InducedTied (IT) joint. Long strip andlargebaypoursareproneto cracking.Usinginducedtied joints (Fig. 2.11),constructionbaysarereducedin sizeto reducethepossibility of cracking. As the floor attemptsto shrink it is restrainedby the ties.Crackingmayoccuranda crackinduceris placedin thesurfaceof theslabencouragingthe slabto crackat the joint.

2.8.3 Crack inductionmethods2.8.3.1 Sawn joints. The sawcut actsasa line of weaknesswhich is incorporatedinto theslabat thepositionof thejoint suchthattheslabwill crackat thatpoint owing

Slip membrane Tie bars

Optional groove formed by strip stuck to edge offirst bay cast

60 mm

Fig. 2.10. FormedTied (FT) joint

120 mm gap in fabric reinforcement

Crack inducer (or saw cut)

Fabric reinforcement (as in top of slab)supported to give 50 mm bottom cover

Note: main wires of longitudinal fabrics mustbe parallel to side forms

Fig. 2.11. InducedTied (IT) joint


to an increasein the tensilestressin theremainingdepthof slab.Sawcutsareformedwhentheconcretehasgainedsufficient strengthto withstandtheeffectsof a concretesawbut not somuchthat theeffectof sawingwould damagethefloor. Sawnjoints areparticularlydurable.Theyareexpensiveandcancostasmuchastentimesasmuchaswet formedjoints.Thesawcut breakstheupperlayerof reinforcementandthegroovehasa depthof 40mm or 50mm (Fig. 2.12).As well asthejoint beingthemostdurablecrackinductionform, a sawnjoint is alsovery serviceablewith no differencein levelat eachside of the cut. This aids flatness,which is important on heavily traffickedfloors.

2.8.3.2 The timingof forming sawn joints. Jointsshouldbecut whentheconcretehasgainedsufficient strengthto supportthe weight of the cutting equipmentbut beforeithasgainedsufficient strengththat sawingmight loosenor pull out aggregatesandfibrereinforcement.A suggestedsawingtime-scaleis between24 and48 hoursafter initialconcreteset.This leavesa time window asshownin Fig. 2.13assumingthe following.

• Concreteis mixed 1 hour beforeplacing.• First concreteis mixed at 7.00 a.m.• Last concreteis mixed at 4.00p.m.• Concreteis placedbetween8.00a.m.and5.00p.m. on day 1.• Assumeconcretetakes6 hoursto reachinitial set.• Therefore,earliestinitial set= 1.00p.m.; latestinitial set= 10.00p.m.

If theconcreteis to besawnbetween24and48hoursafterinitial set,thefirst cut canbeperformed24 hoursafter the latestinitial setsoasto ensurethatall of theconcretehasgainedsufficient strength.All sawcutting mustbe finished48 hoursafter the earliestinitial set.In this example,sawcuttingcancommenceat 10.00p.m.on day2 andmustbe finishedby 1.00p.m. on day 3. This gives15 hourssawingtime.

120 mm gap in fabric

Saw cut, 5 mm × 50 mmat 8 to 48 hours

Note: 50 mm suitable for slabs up to 200 mm thick. For greater depths feature must be at least 1/4 depth

Fig. 2.12. Crack induction joints usingthe sawcut technique

CONSTRUCTI ON 47

2.8.3.3 `Soff-cut' sawnjoints.2.4 Soff-cut is a way of forming crackinductionjointswithin a slabprior to theconcretedevelopingsignificantstrength.Thesawcut is madeimmediatelythepowerfloatsor powertrowelshavefinished,savingtime andforming aneconomicaljoint. TheSoff-cutsystemis usedon US highwaypavementsanda Soff-cutjoint servesthe samepurposeasthe moreconventionalsawnjoint.

2.8.3.4 Othermethodsofcrack induction. Figure2.14showstraditionalcrackinducersinvolving the useof metalor plasticstripswhich areinsertedinto the wet concreteafterplacing.Theuppersection of thestrip is removedoncetheconcretehashardenedleavingthe lower sectionin theslabto form the crack inducer.Themetal insert(Fig. 2.14(b))isshapedso asto not disruptthe concrete surfacewheninserted.It is strongenoughto beableto bepushedinto theconcretevertically.Thezip-strip is usedmorecommonlyin theUS andcomprisesa plasticextrusionmadefrom two identicalparts to form a T-shape.The rigidity of the plastic section allows the vertical section to be pushedinto theconcreteand the top sectionis removedfor reuseprior to powerfloating.Immediatelyfollowing concreting,a grooveis formedusing a bricklayers’trowel againsta string lineto ensureaccuracy.Oncetheinducingstrip is in placefurthercompaction andvibration isnecessaryto ensurethat no air hasbeenintroducedinto the concrete.

2.8.4 Position of joints2.5

Table2.1 showsjoint spacingsfor varioustypesof concrete.The joint spacingsin thetable have been used successfullyfor many years in the UK. Some consider thatspacingscan be greaterthan 12m for steel fibre reinforcedfloors. While 14m joint

Set 24 h curing

Set 24 h curing

Set 48 h curing

Set 48 h curing

Delivery

Delivery

Midnight Noon Midnight Noon Midnight Noon Midnight

6 a.m. 6 p.m. 6 a.m. 6 p.m. 6 a.m. 6 p.m.

Time

15 h saw cut window

Day 1 Day 2 Day 3

Fig. 2.13. Thetiming of forming sawnjoints


spacings will probably be acceptable for most applications, the additional movementthat would occur at joints might lead to loss of aggregate interlock and joint degradation.Joints spacings are frequently designed to coincide with column spacings or otherfeatures such as recesses or floor width changes. In any project, it is necessary todevelop a joint layout identifying all practical issues prior to the placing of concrete.

(a) Traditional crack inducer (b) Metal insert

Two sectionsassembled

(c) Zip strip

Fig. 2.14. Traditional crack inducers

Table2.1. Commonconcretesand suggestedjoint spacings

Concretetype Joint spacing:m

Plain C30 concrete 6Micro-silica C30 concrete 620kg/m3 ZC 60/1�00 steelfibre reinforcementC30 concrete 630kg/m3 ZC 60/1�00 steelfibre reinforcementC30 concrete 840kg/m3 ZC 60/1�00 steelfibre reinforcementC30 concrete 10Plain C40 concrete 6Micro-silica C40 concrete 620kg/m3 ZC 60/1�00 steelfibre reinforcementC40 concrete 630kg/m3 ZC 60/1�00 steelfibre reinforcementC40 concrete 1040kg/m3 ZC 60/1�00 steelfibre reinforcementC40 concrete 12

CONSTRUCTI ON 49

2.8.5 Joint sealingThepurposeof sealingjoints is to preventdust,waterandotherdebrisenteringthejoint.Thesealantmustbeableto withstandthestrainof theopeningof thejoint resultingfromcontractionof theslabandremainfixed to thefacesof thegroovesides(Fig. 2.15).Forfloors which are to be trafficked by rigid-tyred vehiclesand where joint widths aregreaterthan 5mm, it may be necessaryto use a strong semi-rigid sealant,such aspouringgradeepoxyor grout, in orderto providesupportto theedgesof the joint. Thelow elasticityof suchsealantsmight causethemto fail whenappliedto anactivejoint.For this reason,they are applied at a later date when shrinkagemovementshaveoccurred.Alternatively, more flexible mastic basedmaterialscan be used. In slabswhere joint spacingsexceed10m, it is particularly important to carry out the jointsealing.With deepcrackinducinggrooves,apolyethylenebackingrodmaybeplacedinthe lower part of the sealinggrooveto reducethe amountof sealantrequired.

Durability can be improvedwith polysulphidesealantsand when appearanceis ofconcern,gun-gradematerials can be used,especially for narrow grooves.Sealantsshouldbe inspectedandmaintainedregularly.

2.9 Floor construction case study2.9.1 Details of case studyFigure2.16showsa planof anindustrialbuilding which is to includea singlepourfloorthat will be subjectedto patch loadsfrom vehicles,uniformly distributedloadsfrom

Sealant

Polyethylenebacking rod

Fig. 2.15. Details of joint sealinggroove


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CONSTRUCTI ON 51

materialsstorageandpoint loadsfrom thesupportingcolumnsof amezzaninefloor. Thedetailedcalculationsfor the floor are presentedin Chapter5. The building includesseveralroomswhich areto beconstructeddirectly off the floor. Chapter5 showsthata225mm thick floor on 250mm thicknessof granularsub-basematerial is suitable.AC40 concretereinforcedwith 20kg/m3 of steel fibre is to be usedfor the floor slab.Figure 2.16 shows the grid lines which align with structural column centres.Thebuilding uponwhich this exampleis basedwasconstructedin the UK in 1994.

Figure 2.17 shows the proposedarrangementof joints. The joint spacingsaredeterminedin Chapter5. A perimeterbeamcircumscribesthefloor soasto facilitatetheuseof a laserscreedingmachine.Theperimeterbeamis 700mm wideandis widenedatexternal column locations. The perimeter beam is seperatedfrom the floor by anisolationjoint which allowsthe floor andtheperimeterbeamto moveindependentlyofeachother.Eachof the threeinternalcolumnsis surroundedby a diamondshapedpadthat is separatedfrom the floor by an isolation joint. The mezzaninefloor supportingcolumnsareto be boltedonto the floor anddo not haveindependentfoundations.Themain structural columns and the building perimeter wall are to have independentfoundations.

2.9.2 Setting outFigure2.18showsthegrid linesthatareestablishedon thesiteprior to anyconstructionwork. Pegsshouldbelocatedin thegroundusingaccuratesurveyinginstrumentsat eachendof eachgrid line. Temporarybenchmarksshouldalsobeestablished.At this stage,it may becomeevidentthat the proposedlevels of the building are inappropriate.Forexample,it mayberealisedthattheproposedlevelswill leadto excessivecutor fill. It isoftenthecasethata revisionin levelsis possibleat this stage,althoughcareneedsto beexercisedin relationto drainagefalls andhighwaygradients.Notethat,at this stage,noattemptis madeto locatethe positionof the floor.

2.9.3 Site stripFigure2.19showshowthetopsoilandunderlyingmaterialis removeddownto thelevelof the undersideof the sub-base.Usually, this involves removing material approxi-mately1m beyondthe extremegrid lines.Careneedsto be exercisedin the protectionof the underlying material from weatherand constructiontraffic, especiallyas theexposedsurfacewill be flat andthereforesubjectto ponding.Many typesof soil in theUK will be significantly weakenedif trafficked in a saturatedcondition and onceweakenedmay neverrecovertheir undisturbedstrength.The floor designerwill haverelied upon a soil strengthin his structuralcalculationsand failure to ensurethat thestrengthis maintainedmay lead to the floor failing. Should the excavationbecomewaterlogged,it may be necessaryto suspendconstruction.

2.9.4 Excavate and construct pad footings for steel columnsPadfootingsof plandimensions1�5m � 1�5m areto beconstructedby excavatingbelowthesitestrip level asshownin Fig. 2.20.If thesoil remainsstablewith a verticaledge,noformwork is requiredandthepadfootingscanbeconstructeddirectly againstthesoil. Wet


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CONSTRUCTI ON 53

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CONSTRUCTI ON 55

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blinding concreteis first pouredinto eachexcavation,steelmeshand/orbarsarethenfixedandtheconcreteis poured.Socketsareleft in the surfaceof the padfootingsto allow thefixing of thesteelcolumns.Theupperlevel of the foundationshasto besufficiently deepto permitboththefloor andthefoundationto thebuilding perimeterwall to beconstructed.In this case,the foundationsare750mm thick andtheir uppersurfaceis 300mm beneaththe lower surfaceof the floor sub-base.After casting,a checkshouldbe madethat eachbasealignsaccuratelywith thegrid. In this case,eachbaserequires1�68m3 of concretesooneconcretedelivery of 5m3 will be sufficient for threebases.

2.9.5 Install underground servicesServicesneedto beinstalledandtakenthroughthesubgrade,thenvertically upwardstothe level of the floor. Backfilling of all trenchesmustbeundertakendiligently to avoidlatersettlementwhich would inducestressin thefloor slab.All waterandgaspiperunsshouldbe pressuretestedprior to any trenchreinstatement.The positionof all servicesshouldbe recordedto minimisedisruptionin caseof future repairs— seeFig. 2.21.

2.9.6 Erect structural steelworkThe structuralsteelworkand the cladding must be installed before the floor can beconstructed— seeFig. 2.22.Usually,thestructurecompriseshot rolled mild steelportaland gableframessupportingcold rolled galvanisedsteelpurlins and side rails. Windbracing may be installed in one or more baysand the column baseplatesare fixedpermanentlyto their padfooting foundationbases.

2.9.7 Fix roof and side claddingIt is commonfor industrial buildings to include corrugatedsteel sheetsas the roofcladdingand as claddingfor the upperpartsof the sides,as shownin Fig. 2.23. Thecladdingmay include an inner lining material to provide thermal insulationand rooflights, and windows may be included.More sophisticatedcladding systemsmay beinstalledwhereendusageis predominantlyoffice accomodationor high-techworkshopfacilities. The claddingmustpreventsunshineandwind from interferingwith the floorconstructionoperation.

2.9.8 Excavate and construct external wall foundations, then externalwallsMost industrial buildings include a height of masonryor similar durable claddingmaterialasthe lower partof thebuilding walls.This is partly to improvedurability andpartly to improveappearance.Two leavesareusuallyprovided,aninnerleafof concreteblocks constructedin discretebays betweenthe flangesof the columns(sometimesincluding a box aroundthe columns)andan externalleaf of brickwork sailing pastthecolumnsexternallyto createan uninterruptedeffect.The masonryis constructedon itsown foundationcomprisinga strip footing constructedabovethe columnpadfootings.The concreteblock leaf is 100mm thick, the brickwork leaf is 105mm thick and thecavity betweenthemis 50mm thick, so the total wall thicknessis 255mm. The innerfacemay be plasteredor dry lined with plasterboard,adding10mm — seeFig. 2.24.

CONSTRUCTI ON 57

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CONSTRUCTI ON 59

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CONSTRUCTI ON 61

2.9.9 Construct sub-baseThe crushedrock or cementstabilisedsub-baseis now installed by spreadingthematerial and compactingit with a vibrating roller — seeFig. 2.25. The material isinstalledright upto themasonrywallsandaplatevibratoris neededto compactadjacentto the walls to avoid damage.The platevibrator is alsorequiredaroundserviceducts.Compactedcrushedrock materialshave a variety of surfacetextures.In the caseofharderor coarsergrainedmaterials,the surfacemay be opentexturedor ‘hungry’, inwhich caseit shouldbe blinded with limestonedust to createa closedsurface.In thecaseof cementstabilisedsub-bases,curingwill berequiredcomprisinganimpermeablespray.Although the sub-basesurfacecanbe usedby constructiontraffic, caremustbetakento avoid rutting the surfaceof the material.This can occur in the caseof somesofterlimestonecrushedrock materials,for exampledolomitewon from someDurhamquarries.

2.9.10 Construct perimeter strip and isolation pads around internalcolumnsThe perimeterstrip is approximately700mm wide andthe samedepthasthe floor —225mm in this case.It is separatedfrom the masonrywalls andthe columnsby a fullmovementjoint. It can be constructedfrom the samefibre reinforcedconcreteas themain floor or can be reinforced with longitudinal bars. Joints should be providedaccordingto the reinforcementpercentageas for the main floor. The perimeterstripjoints neednot align with the main floor joints, indeed,they shouldnot be at columnpositions, otherwise points of weaknesswill occur. Figure 2.26 shows diamondsurroundsto the columns.Alternatively, circular surroundsmay be provided.A fullmovementjoint shouldbe providedaroundeachisolationpadandaroundthe insideofthe perimeterstrip. This joint shouldcomprisea 12mm wide piece of compressiblematerialfixed thefull depthof theperimeterstrip.A sacrificial timberor syntheticstripis installed along the upper 15mm of the perimeterbeam,to be removedlater andreplacedwith a permanentsealer.

2.9.11 Install concrete floor slabIf a slip membranehasbeenspecified,it shouldbe installedfirst, carebeing takentoavoid folds or ripplesthatmight weakentheconcreteslab.Theconcreteis thenspread,compactedand finished by laserscreed,with power trowelling taking placeafter theconcretehasset but before it hasgainedstrength.Often, the power trowelling takesplacebetween8 hoursand16 hoursafter theconcreteis mixed.Fibrescanbeaddedtothe concreteat the mixing plant or alternativelyat the site by emptyingfibre bagsintothereadymixtruck andmixing for two minutes.Usually,thereadymixtrucksdischargeat the intendedpositionwheretheconcretewill form thefloor. If accessis not possible,the concretecanbe pumpedup to 100m — seeFig. 2.27.

2.9.12 Cut 40mm deep grooves to induce jointsGroovesmust be cut as soon as the concretewill allow the operationto take placewithout forming a ragged edge to the cuts. Usually, the cutting can commence


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CONSTRUCTI ON 65

approximately24 hoursafter mixing theconcrete.Cuttingshouldbecompletewithin48 hoursof mixing in order to avoid the risk of uncontrolledcracking.A 3mm widevertically sidedgroove is formed using a grinding wheel. The joints are cleanedoutusinga high pressureair line anda sealerwith anexpansioncapabilityof 30%or moreis appliedto the joints, including the full movementjoints at the perimeterandaroundthe internal columns — the sealer replacesthe sacrificial strip placed above thecompressiblematerial— seeFig. 2.28.

2.9.13 Construct internal walls andmezzanine floor support columnsBecausethe internal walls andmezzaninefloor supportcolumnsare constructedontothesurfaceof thefloor, theyareleft until last.Thewalls areconstructedconventionally,with a layerof polyethelenebeingplacedbetweenthefloor andtheundersideof thewallto act asa moisturebarrier.The mezzaninefloor columnsareboltedto the floor usingexpandingbolts. Shimsmay be neededto ensurethat the columnsarevertical — seeFig. 2.29.

2.10 Drainage for external hardstandingsHardstandingdrainageneedsto beconsideredalongsidethedesignof theareasincebaysizes,slopes(falls) and joint detailsmay all influencethe drainagesystem.The firstissueto consideris wherethe precipitationfalling on the hardstandingwill go whenitleavesthe site. The designerwill needto liaise with the local drainageauthority toestablishwhetherthe downstreamdrainagesystemhassufficient existing capacitytoacceptthedrainagefrom theproposedsite.Also, theauthoritymayrequirethata petrolinterceptorbe installedto removeany fuel or other light contaminentswhich becomemixed with the surfacedrainage.In caseswhere there is insufficient downstreamcapacity,it maybenecessaryto introducea detentionsystemthatwill hold stormwateruntil sufficientcapacitybecomesavailable.Largediameterpipesoftenconstitutea cost-effectivedetentionsystem.In this case,the diameteris designedto achievethe storagevolumerequiredratherthanthe flow rate.

Surfacedrainagecanbeby gulleysor by a lineardrainagesystem,whichmayinclude‘built-in’ falls. Calculationsare require for the number of dischargepoints but acommonly adoptedrule of thumb is that a gulley is required for each 300m2 ofhardstandingwith 150mm diameterpipesleadingto progressivelyhighercapacitypipesasthe waterprogressestowardsthe existingdrainagesystem.

The surfacefalls are often specifiedas a compromisebetweenthe ideal 1 in 40(2�5%), which ensuresrapid removal of surface water, and the flat horizontalsurface,which often suitsoperationsbest.Falls of 1 in 80 to 1 in 100 arecommon.Figure2.30showsa typical drainagearrangementfor a mediumsizedhardstanding.The gulleys have beenlocatedin the slab corners.Care needsto be taken in thedetailing of the adjacentconcreteand somedesignersprefer to locategulleys nearthe centreof slabs.Wherevera gulley is locatedtherewill be a reductionin slabstrength.They shouldbe locatedawayfrom the zonesof repeatedheavyloadingorcontainercornercastings.


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CONSTRUCTI ON 69

2.11 Hardstanding construction case studyFigure2.31illustratesa typical long strip hardstandingshowingthe joints, the falls andthedrainage.If theoperationalcharacteristicsof theareaaredeterminedat designtime,thegulleyscanbelocatedawayfrom thepredominantloading.Thecontractormight optto constructalternatestrips, each with formwork at both sides, then construct theintermediatestrips,usingthe existingconcreteasformwork.

Figure2.32showstheway in which the individual sheetsof 2�4m� 4�8m mesharelocatedwithin eachbay.In this case,eachsheetwill haveto beshortenedfrom 4�8m tojust lessthan4m to fit the4m widestrips.Figure2.33 showsthatif A393meshis used,the distancebetween free joints can be up to 40m. This means that all of thelongitudinaljointscanbetiedandonly onefull movementtransversejoint is required—the remainingtransversejoints canbe tied with a mid-depthsheetof mesh,or theycanbe eliminated.

Figure 2.34 showsthe samehardstandingconstructedusing a laser guidedscreedmachine. By including steel fibres in the concrete,the mesh can be omitted, sofacilitating construction.It is usual to use the samejoint spacingin both orthogonaldirections.Increasingtheweightof steelfibresincreasesthejoint spacinganddecreasesthe slab thicknessrequired. The most cost effective solution is not obvious. Theadditionalcostof fibrescanbeoffsetby a reductionin joint thicknessandin a reductionin joint constructioncosts.

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Fig. 2.34. Typical hardstandingshowingrelationship betweendrainage and joints.This is an exampleof a singlepour hardstandingin which the joints havebeenformedby sawinggrooves

CONSTRUCTI ON 71

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