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Medium span rcc flooring systems

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Rishikesh Kalal

This document discusses various reinforced concrete floor systems for medium and long span structures. It describes objectives like minimizing floor-to-floor height, maximizing column-free spaces, minimizing weight and construction time. Systems covered include flat slabs with drop panels, flat plates, ribbed slabs, waffle slabs, and band beams & slabs. Each system is summarized briefly, including advantages like simple formwork, long spans, and material savings, as well as disadvantages like depth controlling fire rating and need for specialized formwork. Reinforcement details to improve slab punching shear resistance are also mentioned.

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RCC Floor Systems for Medium & long span Structures Introduction: Reinforcedconcrete floorsysteminthe RCCbuildinginfluencesalmostall the majordesign and planningissues like cost,speedof construction, optimizationof space usage, ease toaccommodate services,etc. The overall objectivesinbuildingdesigninrelationtofloorsystemandthere benefits along withpossible meansof achievingthe objectives, inthe buildingprojectare listedbelow. Sr. no Objective Benefit for the project Possible Meanstoachieve objectives 1 Smallestpossible floorto floorheight A) Lesserbuildingheightforthe same numberof storeys B) Savingonvertical structural members,claddingetc. C) Savingonmechanical risers, lifts,staircasesetc. D) Savingonthe air conditioning by reducingthe overall floor volume. A) Reducingover-all depthof the floorsystem byeliminating beamsor reducingitsdepth. 2 Largestpossible columnfree spaces, i.e.longspan A) Flexibilityinuse B) Maximumcarpetarea A)Constructingstifferfloorsystem 3 Lowestpossible weightof the floor A) Savingonthe vertical structural membersand foundation. B) In seismicareas,savingon lateral loadresistingsystem. A)Usinglightweightmaterial. B)Eliminatingmaterial fromthe systemwhichisnot structurally requiredi.e.Introducingvoidsin the non performingareasof the floorcross section etc 4 Highrepeatability fromfloorto floor A) Improvementof constructabilityandthussaving of time A) Simple standardiseddetailsfor reinforcement B)Simple standardiseddetailsfor formwork . 5 Quickestpossible floorcycle A) Savingof time B) Avoidance of clashes betweendifferenttrades. C) Reductioninthe required numberof form-worksets A)Highearlystrengthconcrete B)Simple reinforcementandform work C)Simple detailswithhigh repeatability 6 No.Back Propping whereverpossible A) Directsavingof formwork installationtime. B) Savingof time by allowing buildingfitouttostart earlierdue to earlyaccessto the floor. A)Use of self-supportingformwork requiringsupportnearvertical structural elementsi.e.columns, structural wall etc. B)Highearlystrengthconcrete All the variationsof RCCfloorsystemsdiscussedbelow are withthe considerationtosatisfyone or numerousabove statedobjectives. Actual floorsystemsinbuildingscome inall sizes,shapes,andforms.There are somanyvariablestoany floorsystemsuchas differentspans,offsetspans,cantilevers,andthe extentof continuity,the effectsof
beams,columnsandwallsonthe slabsystem, etcthat itis difficulttocoverall situations withone specific floorsystem.The floorsystemsemployedinthe actual buildingdesignwill alwaysbe acombinationof several basicsystems. The RCC flooringsystemscanbe; 1. FlatPlate 2. FlatSlab 3. RibbedSlab 4. Waffle Slab 5. Band beam& Slab 6. Pre-stressedSlab Flat Slab: A flatslabis a reinforcedconcrete slabsupporteddirectlybyconcrete columnswithoutthe use of beams.Itmay have a columnheadand/or a drop panel totake care of shear.Whena droppanel or column headisnot provideditisalsoreferredtoas flatplate. A flatslabis a one-wayortwo-waysystemwiththickeningsinthe slabat the columnsandload bearingwalls called'drop panels’.Droppanelsactas T-beamsoverthe supports.Theyincrease the shearcapacityandthe stiffnessof the floorsystemundervertical loads,thusincreasingthe economicalspanrange.The plan dimensionsof the droppanelsare a minimumof 1/3 of the span inthe directionunderconsideration,usually roundedto the nearest100 mm.The overall depthof the droppanel istypicallytakenas1.75 to 2 timesthe depthof the slab,again roundedtosuittimbersizesorthe nearest25 mm.The principal featuresof aflat slabfloorare a flatsoffit,simpleformworkand easyconstruction.The economicalspan'L' of a reinforced concrete flatslabis approximatelyDx 28 for simplysupported,Dx 32 for an endspan andD x 36 foran interiorspan.Pre-stressingthe slabincreasesthe economical spantoDx 35, D x 40 and D x 45 respectively, where D isthe depthof the slabexcludingthe droppanel. Usesof columnheads:•increase shearstrengthof slab •reduce the momentinthe slabby reducingthe clear or effectivespan Usesof drop panels : •increase shearstrengthof slab •increase negative momentcapacityof slab •stiffenthe slaband hence reduce deflection
FLAT SLAB WITH DROP PANEL
Advantages:  Simple formwork allowsfasterconstruction  ReducedservicesandcladdingcostsFlatslabconstructionplacesnorestrictionsonthe positioning of horizontal servicesandpartitionsandcanminimizefloor-to-floorheightswhenthere isno requirementfora deepfalse ceiling.Thiscanhave knock-onbenefitsintermsof lowerbuilding height,reducedcladdingcostsandprefabricatedservices.Minimumstructural depth will resultin Lowerstoreyheightwhichinturnwill reduce buildingweightdue tolowerpartitionsandcladdingto façade,approx.saves10% invertical members,reduce foundationload.  Usuallydoesnotrequire shearreinforcementatthe columns due to droppanel. 1 REINFORCED 2 PRESTRESSED
Disadvantages:  Mediumspans  Generallynotsuitable forsupportingbrittle(masonry)partitions  Drop panelsmayinterfere withlargermechanical ducting  Vertical penetrationsneedtoavoidareaaroundcolumns  For reinforcedflatslabs,deflectionatthe middle stripmaybe critical. Flat Plate: A flatplate isa one-or two-waysystemusuallysupporteddirectlyon columnsorload bearingwalls.Itisone of the mostcommonformsof constructionof floorsinbuildings.The principalfeature of the flatplate floor isa uniformornear-uniformthicknesswithaflatsoffitwhichrequiresonlysimple formworkandiseasyto construct.The floorallowsgreatflexibilityforlocatinghorizontalservicesabove asuspendedceilingorina bulkhead.The economical spanof aflat plate forlow to mediumloadsisusuallylimitedbythe needto control long-termdeflectionand mayneedtobe sensiblypre-cambered(notoverdone)orpre-stressed.An economical spanfora reinforcedflatplate isof the orderof 6 to 8 m and forpre-stressedflatplatesisinthe range of 8 to 12 m.The span'L' of a reinforcedconcrete flat-plateisapproximatelyDx 28 for simply supported,Dx 30 foran endspanof a continuoussystem, toDx 32 for internal continuousspans.The economical spanof a flatplate can be extendedbypre-stressingtoapproximatelyDx 30, D x 37 and D x 40 respectively,where Disthe depthof slab.
1 REINFORCED 2 PRESTRESSED Advantages: Simple formworkandsuitablefordirectfix orsprayedceiling No beams—simplifyingunder-floorservices Minimumstructural depthandreducedfloor-tofloorheight. Disadvantages: Mediumspans Limitedlateral loadcapacityaspart of a momentframe May needshearheadsorshearreinforcementatthe columnsor largercolumnsforshear Long-termdeflectionmaybe controllingfactor May notbe suitable forsupportingbrittle(masonry)partitions May notbe suitable forheavyloads.
Reinforcement:Inflatslaba slab-columnconnection’sresistance topunchingshearmaybe enhanced adoptingsome actions,asthe increase of the columnsection,orthe slabthickness,orthe flexural reinforcementratio,orthe compressive strengthof concrete,orbyusingdrop panelsandcolumncapitals. However,the increase of the columnsectionorthe use droppanelsandcapitalsusuallygenerateproblems fromthe architectural pointof view. The increase of the slabthicknessmaymeana substantial elevationof the structure and foundationcosts. Finally,increasingeitherthe flexural reinforcementratioorthe compressive strengthof concrete wouldhave poorefficiency. Thus,whenitisdesirable toincrease the punchingresistance,one of the mostpracticable solutionsmaybe the use of shearreinforcementin additiontothe flexural reinforcement,locallyinthe shearzone aroundthe vertical support (columnor wall). Stirrups,studsandstructural sectionsare the alternative typesof reinforcement,thatare employed to countershearnear the column. Please refertothe appendix fordetailsof the same. OPTIMUM SINGLE AND MULTISPAN FOR FLAT PLATE
Ribbedand waffle slab:Ribbedslabsare made upof wide bandbeamsrunningbet weencolumnswithnarrowribsspanningthe orthogonal direction.Normallythe ribsandthe beamsare the same depth.A thintoppingslabcompletesthe system. Theyare eitherone-wayspanningsystemsknownasribbedslabora two-wayribbedsystemknownasa waffle slab.A ribthicknessof greaterthan125 mm isusuallyrequiredtoaccommodate tensile andshear reinforcement.Ribbedslabsare suitable formediumtoheavyloads,canspanreasonable distances,are very stiff andparticularlysuitablewhere the soffitisexposed.Slabdepthstypicallyvaryfrom75 to 125 mm and rib widthsfrom125 to 200 mm.Rib spacingof 600 to1500 mmcan be used.The overall depthof the floor typicallyvariesfrom300 to 600 mmwith overall spansof up to 15 m if reinforced,longerif post-tensioned. The use of ribsto the soffitof the slabreducesthe quantity of concrete andreinforcementandalsothe weightof the floor.The savingof materialswillbe offsetbythe complicationinformworkandplacingof reinforcement.However,formworkcomplicationisminimisedbyuse of standard,modular,reusable formwork,usuallymade frompolypropylene orfibreglassandwithtaperedsidestoallow stripping.Forribs at 1200-mm centres (tosuitstandardforms)the economical reinforcedconcrete floorspan'L' is approximatelyDx 15 fora single spanandD x 22 fora multi-span,where Disthe overall floordepth.The one-wayribsare typicallydesignedasT-beams,oftenspanninginthe longdirection.A soliddroppanel is requiredatthe columnsandloadbearingwallsforshearandmomentresistance. Advantages:  Savingsonweightandmaterials  Long spans  Attractive soffitappearance if exposed  Economical whenreusable formworkpansused RIBBED SLAB
 Vertical penetrationsbetweenribsare easy. Disadvantages:  Depthof slabbetweenthe ribsmaycontrol the fire rating  Requiresspecial orproprietaryformwork  Greaterfloor-to-floorheight  Large vertical penetrationsare more difficulttohandle. Waffle slab
Ribbed Slab Band Beam: This systemconsistsof aseriesof parallel,wide,shallow beams (knownasbandbeams or thickenedslabbands) withthe floorslabspanningtransverselybetweenthe bands. The floorslabis designedasacontinuousslab,withthe shallow bandbeamscarryingall loadsfromthe slab. Bandbeamsor thickenedslabbandsare a two-wayslabsystem. Band beamsare commonlyusedforlongerspanstructures oftenwiththe bandspost-tensionedandthe slabsreinforced. Sometimes,compositeconcrete/metal deckingisusedforthe slabs,providedthe slabspansare not toolarge. The bandbeam hasa relatively wide,shallow crosssectionwhichreducesthe overall depthof floorwhile permittinglongerspanssimilarto the traditional concrete beam. The concrete sectionsimplifiesboththe formworkandserviceswhichcan pass underthe beams. Advantages:  Relativelysimple formwork  Shallowbeamstoallowservicestorununderthe floor  Minimumstructural depthandreducedfloor-tofloorheight  Long spans
 Good cost/time solution  Allowsthe use of flyingforms. Disadvantages:  Long-termdeflectionmaybe controllingfactorandpost-tensioningmaybe required  May needservice penetrationsthroughbeamswhichare difficulttohandle. In a single-spanfloor,the spacingof the bandbeamsmaycoincide withthe columns,orthe bandsmay be more closelyspacedtoreduce the thicknessof the slabspanningbetweenwallsorbeams. For single span reinforcedconcrete floorsthe economical span 'L' of the bandbeamis D x 20 to D x 22 dependingon the widthandspacingof the band beam,where Dis the depthof the slabplusbandbeam. Pre-stressing the bandbeamgiveseconomical band-beamspans inthe range of D x 24 to D x 28. In a multi-spanfloor,the spacingof the bandbeamsisfixedbythe transverse spacingof the columns. For initial sizingof the slab,the span-to-depthratiosfromSection6.3 can be used. Forinternal spansthe slab thicknessisbasedonthe clearspan betweenbandbeams,andforanexternal bayisfromthe edge of bandto the columnline of the external band. The depthof the bandis typically1.5 to 2 timesthe depth of the slaband the minimumeconomical spanfora bandbeamis about7–8 m. In multiple spansusing reinforcedconcrete,the economical slabof the bandbeam'L' is approximatelyDx 22 for 1200-mm-wide bandbeamsand D x 26 for a 2400-mm-wide beamsat 8400-mm centres. Pre-stressingincreasesthe economical span'L' to D x 24 to D x 28 forsimilarbeamwidths. D is the depthof slabplusbandbeam in each case. The maximumspanforreinforced concrete bandsshouldnotnormally exceed12m. Above this span,bandsshouldbe pre-stressed. The slabbandwidthshouldbe betweenband-spacing/3toband- spacing/4 and,where possible,shouldbe basedonamodule of a standard sheetof plyof 2.4 m x 1.2 m. Vertical sidesshouldbe used if possible tosimplifyformwork. Slopingsidesare sometimesusedwhere bandsare exposedtovieworwhere the effective spanof the slab needstobe reduced.
SINGLE SPANBAND BEAM
MULTI SPAN BAND BEAM
Medium span rcc flooring systems
Medium span rcc flooring systems
Prestressedand post-tensioned RCC Slabs: Review of Reinforced Concrete: Reinforcement is Passive.It crosses the crack but does not prevent it. How to notallowthe concrete belowneutral zone togointensile stressbeyondthe tensile strengthof concrete which leadstoexcessivedeflectionandcracking? Option-1-Increase the depthof the beam. Disadvantage-Increaseinself-weightandmaterial wastage. Option-2-Provide additional steel alongthe vertical face of steel. Disadvantage-Increaseuse of steel. Option-3-Introduce compressionstressestocounterthe tensile stress. Advantage:Noextramaterial isrequired. Disadvantage:Use of sophisticatedequipmentandrequirementof specialisedskilledlabour.
As the spansincreasesorloadingincreasesthere isanecessitytoenhance the strengthof the concrete structure (beams,columnsandslab)byusinghighstrengthmaterial and/orincreasingthe dimensionsof the structural members.Beyondaparticularlimitincreaseinthe dimensionof structural membersbecomes unviable andthere isalimitationtoenhancingthe materialstrength.The mostviable optionisof introducingcompressive stresstocounterthe tensile stressoccurringdue toself-weightandimposedload. Tension+ compression = Netzerostress Principle ofpre-stressing: The prestressinformof compressive stressescanbe introducedinthe member withoutthe use of steel bymeansof external jackatthe end.Howeveritisrarelypractical.The ulternative methodis stretchingor tensioninghightensilesteelbarsorwireswhichare thenboundor anchoredto concrete member.Inthe processof steel’stendency toreleasethe tension bycontractingtothe original unstretched length,acompressive force isappliedtothe concrete. The distributionof the pre stressacross the sectiondependsuponthe pointof applicationof the pressure.Itwillbe seenthatbyapplyingthe pressure atsome pointwithinthe the lowerthird,compressivestressesare inducedinthe bottomportion and the smallertensile stressesinthe topportionof the beam.Byselectingappropriatemagnitude of tensionandappropriate pointof application,the stressesacrossthe sectionmaybe soapportioned that whenactingtogetherwiththose setupbythe deadloadof the beamthe resultingstressatthe topis zero while the stressatthe bottomrepresentsthe maximumpermissiblecompressivestressof the concrete. Since the forcesdue to prestressingand the deadloadactssimultaneouslythe upperfibresof he concrete are notsubjectedtotensile stresses due toprestressing,northe lowerfibrestoexcessive compression. Whenthe live loadisappliedadditional compressive stressesare setupinthe topand additional tensile stressesinthe bottomfibres,andthese forces,actingtogetherwiththe residual forcesfromthe combinationof deadandprestressingloads,resultinacompressive stressinthe topfibre anda small compressionorzerostressin the bottom.Greatesteconomyisobtainedif the maximumcompressivestress inthe bottomfibres,due todeadandprestressingload,isequal tothe maximumtensile stresssetupbythe live load,thusproducingzerostressatthe bottomand maximumpermissible compressive stressatthe top whenbeamisunderload.
Behaviour of pre-stresses Concrete member Compressive stresscanbe introducedbefore orafterthe concrete memberiscastand are termedpre- tensioningandpost-tensioningrespectively. Reasons for Prestressing: Prestressedconcrete hasbeendevelopedtoovercome some of the limitationsof reinforced concrete,namely: 1) In flexure of reinforcedconcretemember,concrete iscrackedandfunctionsonlytoholdthe reinforcingbarsinplace andprotect themfromcorrosion,therebygivingexcessweight withoutstructural action. 2) Deflectionof amemberisinverselyproportionaltothe momentof inertiaof itssection – cracking lowersthe momentof inertiaof the section,therebyincreasing deflection. 4) Eliminate crackingatservice loadingconditions 5) Improve shearandtorsionstrengths 6) Addprotectiontothe steel DefinitionofPre-stress: Pre-stressisdefinedasa methodof applyingpre-compressiontocontrol the stresses resultingdue to external loadsbelowthe neutral axisof the beamtensiondevelopeddue toexternal loadwhichismore than the permissible limitsof the plainconcrete. The pre-compressionapplied (maybe axial oreccentric) will induce the compressivestressbelow the neutral axisorasa whole of the beamc/s. Resultingeitherno tensionorcompression.
Pre-stressedconcrete isbasicallyconcrete inwhichinternal stressesof asuitable magnitude and distributionare introducedsothatthe stressesresultingfromthe external loadsare counteractedtoa desireddegree. Pre-stress concrete requires concrete, which has a high compressive strength reasonably rapid hardening withcomparativelyhighertensilestrength than ordinary concrete. Higher the grade of concrete higher the bond strength which is vital in pre-tensioned concrete, Also higher bearing strength which is vital in post- tensioned concrete. Further creep & shrinkage losses are minimum with high-grade concrete. GenerallyminimumM30 grade concrete isusedfor post-tensioned&M40 grade concrete isusedfor pre- tensionedmembers. 20 mm coveris requiredforpre-tensionedmembers. The prestressing of concrete has several advantages as compared to traditional reinforced concrete (RC) without prestressing. A fully prestressed concrete member is usually subjected to compression during service life. This rectifies several deficiencies of concrete. The followingtextbroadlymentionsthe advantagesof aprestressedconcrete memberwithanequivalent RC member.Foreacheffect,the benefitsare listed. Sectionremains un-cracked underservice loads Reductionof steel corrosion Increase indurability. Full sectionisutilized Higher momentof inertia(higherstiffness) Lessdeformations (improvedserviceability). IndianIIncrease inshearcapacity. Suitable foruse inpressure vessels,liquidretainingstructures. Improvedperformance (resilience)underdynamicandfatigue loading. High span-to-depthratios Larger spanspossible withprestressing (bridges,buildingswithlarge column-free spaces) Typical valuesof span-to-depthratiosinslabs are givenbelow.Non-prestressedslab 28:1 Prestressedslab 45:1 For the same span,lessdepthcomparedtoRC member. • Reductioninself weight • More aestheticappeal due toslendersections • More economical sections. Advantage of PrestressedConcrete
1. The use of high strength concrete and steel in prestressed members results in lighter and slender members than is possible with RC members. 2. In fully prestressed members the member is free from tensile stresses under working loads, thus whole of the section is effective. 3. In prestressed members, dead loads may be counter-balanced by eccentric prestressing. 4. Prestressedconcrete memberpossesbetterresistance to shear forces due to effect of compressive stresses presence or eccentric cable profile. 5. Use of high strength concrete and freedom from cracks, contribute to improve durability under aggressive environmental conditions. 6. Long span structures are possible so that saving in weight is significant & thus it will be economic. 7. Factory products are possible. 8. Prestressed members are tested before use. 9. Prestressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before collapse. 10. Fatigue strengthisbetterdue tosmall variationsinprestressingsteel,recommendedtodynamically loaded structures. Disadvantages of PrestressedConcrete 1. The availabilityof experiencedbuildersisscanty. 2. Initial equipmentcostisveryhigh. 3. Availabilityof experiencedengineersisscanty. 4. Prestressedsectionsare brittle 5. Prestressedconcrete sectionsare lessfire resistant. ClassificationsandTypes Prestressing of concrete can be classified in several ways. The following classifications are discussed.
1. Source of prestressing force Thisclassificationisbasedonthe methodbywhichthe prestressingforce isgenerated.There are four sourcesof prestressingforce:Mechanical,hydraulic, andelectrical.HydraulicPrestressing Thisis the simplesttype of prestressing,producinglarge prestressingforces.The hydraulicjackusedforthe tensioningof tendons,comprisesof calibratedpressure gaugeswhichdirectlyindicatethe magnitudeof force developedduringthe tensioning.Mostlyusedforposttensioninginbuildings. a) Mechanical Prestressing In thistype of prestressing,the devicesincludesweightswithorwithoutlever transmission,geared transmissioninconjunctionwithpulleyblocks,screw jackswithorwithoutgeardrivesandwire-winding machines.Thistype of prestressingisadoptedformassscale production. b) Electrical Prestressing In thistype of prestressing, the steel wiresare electricallyheatedandanchoredbeforeplacingconcrete in the moulds.Thistype of prestressingisalsoknownasthermo-electricprestressing. 2. External or internal prestressing Thisclassificationisbasedonthe locationof the prestressingtendonwithrespecttothe concrete section. a) External Prestressing Whenthe prestressingisachievedbyelementslocatedoutside the concrete,itiscalledexternal prestressing.The tendonscanlie outsidethe member (forexampleinI-girdersorwalls)orinsidethe hollow space of a box girder.Thistechnique isadoptedinbridgesandstrengtheningof buildings. b) Internal Prestressing Whenthe prestressingisachievedbyelementslocatedinside the concrete member (commonly,by embeddedtendons),itiscalledinternal prestressing.Mostof the applicationsof prestressingare internal prestressing. 3. Pre-tensioning or post-tensioning This is the most important classification and is based on the sequence of casting the concrete and applying tension to the tendons. a) Pre-tensioning: In whichthe tendonsare tensionedbeforethe concrete isplaced,tendonsare temporarilyanchoredandtensionedandthe prestressistransferredtothe concrete afteritis hardened.The pre-compressionistransmittedfromsteeltoconcrete throughbondoverthe transmissionlength . . Post-tensioning: In which the tendon is tensioned after concrete has hardened. Tendons are placed in sheathing at suitable places in the member before casting and later after hardening of concrete. The pre- compression is transmitted from steel to concrete by the anchorage device (at the end blocks). 4. Linear or circular prestressing Thisclassificationisbasedonthe shape of the memberprestressed. a) Linear Prestressing Whenthe prestressedmembersare straightorflat,inthe directionof prestressing,the prestressingiscalled linearprestressing.Forexample,prestressingof beams,piles,polesandslabs.The profile of the prestressing tendonmay be curved. b) Circular Prestressing
Whenthe prestressedmembersare curved,inthe directionof prestressing,the prestressingiscalled circularprestressing.Forexample,circumferential prestressingof tanks,silos,pipesandsimilarstructures. 5. Full, limited or partial prestressing Basedon the amountof prestressingforce,threetypesof prestressingare defined. a) Full Prestressing Whenthe level of prestressingissuchthatno tensile stressisallowedinconcrete underservice loads,itis calledFull Prestressing. b) Limited Prestressing Whenthe level of prestressingissuchthatthe tensile stressunderservice loadsiswithinthe crackingstress of concrete,itiscalledLimitedPrestressing. c) Partial Prestressing When the level of prestressing is such that under tensile stresses due to service loads, the crack width is within the allowable limit, it is called Partial Prestressing 6. Uniaxial, biaxial or multi-axial prestressing As the namessuggest,the classificationisbasedonthe directions of prestressingamember. a) Uniaxial Prestressing Whenthe prestressingtendonsare parallel toone axis,itiscalledUniaxial Prestressing.Forexample, longitudinalprestressingof beams. b) Biaxial Prestressing Whenthere are prestressingtendons paralleltotwoaxes,itiscalledBiaxial Prestressing.Forexample prestressingof slabonbothsides. c) Multiaxial Prestressing When the prestressing tendons are parallel to more than two axes, it is called Multiaxial Prestressing. For example, prestressing of domes. The most widelyusedmethodforprestressingof structural concrete elementsislongitudinal tensioningof steel bydifferenttensioningdevices. Pretensioning:Steel tensionedbefore casting concrete In pretensioningthe prestressingtendons (wires,strands)are stretchedtoa predetermined tensionandanchoredtofixedbulkheads or molds. The concrete is poured around the tendons, cured, and upon hardening the tendons are released. As the bond between the tendons and the concrete resists the shorteningof the tendons,the concrete iscompressed.Pretensioningisthe methodmostoftenusedfor the productionof precast prestressed concrete elements, because it offers great potential for mechani zation. pre-stressingplusself weightand live load.
(2) Post-tensioning: In post-tensioningthe tendonsare stressedandanchoredatthe endsof the concrete memberafterthe memberhasbeencast andattainedsufficientstrength.Commonly,amortar-tightmetal pipe orduct (also calledsheath)isplacedalongthe memberbefore concrete casting.The tendonscouldbe preplacedloose inside the sheathpriortocastingor couldbe placedafterhardeningof concrete.Afterthe concrete has attainedthe requiredstrengththe tendonsare stressedusing prestressingjacksatthe endsof the concrete member(fromone or bothendssimultaneously)andanchored.Afterstressingandanchoring,the void betweeneachtendonanditsductisfilledwithamortargrout whichsubsequentlyhardens.Grouting ensures bondingof the tendontothe surroundingconcrete,improvesthe resistanceof the memberto cracking andreducesthe risksof corrosionfor the steel tendons Tendonsare made of;  Wires  Strands(individualorgroup)  Bars (tensionedone atatime) Bondedtendons:Use grout as explainedabove. Unbondedtendons:Use grease or bituminousmaterialinsteadof grout,orput outside the RCsection. Thistechnique iswidelyusedinslabsystemsof residentialandparkingstructureswithseveralbays(upto10 bays),because of itsefficiencyaneconomy.
Post-tensioningisprimarilyan in situ operation -usedinlarge projectssuchas continuouslongspanslabsof buildingsorbridges etc. Processof post tension - Casta duct,containingthe desirednumber of strands,inthe concrete at locationwhere prestressingsteel isrequired - Use metal sheathto forma duct or alternativelyuse aplasticductsince steel tendstocorrode - Locate duct in formworkbyattachingto stirrupsandlongitudinalnonprestressed reinforcement - It isimportanttoensure that the duct isproperlysecuredtoavoidshiftinginpositionduringcastingof concrete - Duct shouldbe watertighttoavoidleakage of wetconcrete intothe ductandpluggingit - Afterconcrete attains requiredstrengthanchortendonatone endusinga mechanical anchor - Atotherendof tendonattach a prestressingjacktothe tendon,tensionitandthenanchorit.
To avoidtension,itisnecessarytoreduce the eccentricitysothatthe centroidof the prestressingsteel atthe endof the beamis withinthe middlethirdforarectangularsection. Thisis achievedbyusing harpedor blanketedstrandsinpretensionedbeams,and drapedtendonsinpost- tensionedbeamstomaintain emax atmid-span,while havingasmallereccentricityatthe ends. In post-tensioningwe use a small number oflarge tendonsas opposedto a large number of individual strands in pretensioning. Reasons for thisare: 1. In pretensioningwe relyonbondbetweenconcreteandsteel, andthuswe wishto maximize bondsurface,whereasinpost-tensioningwhere we relyonmechanical anchorage at endof a tendonbondisnot an issue. 2. Fewerlargertendonsresultsinlesslabour,andthuslesscost,intermsof the stressing Operation. In post-tensionedconcrete,steel tendonsare usuallygrouted after anchoring to preventcorrosion.The prestressingsteel isundera relativelyhighlevel ofstress and is susceptible tostresscorrosion. Cement(or epoxy)grout > bondedmember Grease (or no grout) > unbondedmember Grout ispumpedintoduct underpressure toensure thatductis completelyfilledwithgrout. The behaviourof bondedand unbondedmembersis the same as longas concrete is not cracked - once cracking occurs there is a distinctdifference inbehaviour. Pretensioning>Factory operation - precast industry Post-tensioning>Insitu construction mainly
 Steel heldat length longerthan what it wants to be—Tension  Concrete compressedshorter than what it wants to be—Compression Differences of Pre-stressed Concrete Over Reinforced Concrete: 1. In pre-stressconcrete membersteel playsactive role. The stressinsteel prevails whether external load isthere or not. But in R.C.C., steel plays a passive role. The stress in steel in R.C.C members depends upon the external loads. i.e., no external load, no stress in steel. 2. In pre-stress concrete the stresses in steel is almost constant where as in R.C.C the stress in steel is variable with the lever arm. 3. Pre-stress concrete has more shear resistance, where as shear resistance of R.C.C is less. 4. In pre-stressconcrete members,deflectionsare lessbecause the eccentricpre-stressingforce will induce couple which will cause upward deflections, where as in R.C.C., deflections are more. 5. In pre-stress concrete fatigue resistance is more compare to R.C.C. because in R.C.C. stress in steel is external load dependent where as in P.S.C member it is load independent. 6. Pre-stress concrete is more durable as high grade of concrete is used which are more dense in nature. R.C.C. is less durable. 7. In pre-stressconcrete dimensionsare lessbecause external stresses are counterbalance by the internal stressinducedbypre-stress. Therefore reactionsoncolumn&footing are lessasa whole the quantityof concrete is reduced by 30% and steel reduced by about 60 to 70%. R.C.C. is uneconomical for long span because in R.C.C. dimension of sections are large requiring more concrete & steel. Moreover as self-weight increases more reactions acted on columns & footings, which requires higher sizes.
Comparative Study: Pre-tension Vs Post-tensioned Member Pre-tensionmember Post-tensionedmember 1. In pre-tensioned pre-stress concrete, steel is tensionedpriortothat of concrete. It is released once the concrete is placed and hardened. The stresses are transferred all along the wire by means of bond. 1. Concreting is done first then wires are tensioned and anchored at ends. The stress transfer is by end bearing not by bond. 2. Suitable for short span and precast products like sleepers, electric poles on mass production. 2. Suitable forlongspanbridges. 3. In pre-tensioning the cables are basically straightand horizontal. Placingthemin curved or inclined position is difficult. However the wire’s can be kept with eccentrically. Since cables cannot be aligned similar to B.M.D. structural advantages are less compare to that of post- tensioned. 3. The post tensioning cables can be aligned in any mannerto suitthe B.M.D due to external load system. Therefore it is more economical particularly for long span bridges. The curved or inclined cables can have vertical component at ends. These components will reduce the design shear force. Hence post-tensioned beams are superior to pre-tensioned beams both from flexural and shear resistances point. 4. Pre-stress losses are more compare to that of post-tensioned concrete. 4. Losses are less compare to pre-tensioned concrete.

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Medium span rcc flooring systems

  • 1. RCC Floor Systems for Medium & long span Structures Introduction: Reinforcedconcrete floorsysteminthe RCCbuildinginfluencesalmostall the majordesign and planningissues like cost,speedof construction, optimizationof space usage, ease toaccommodate services,etc. The overall objectivesinbuildingdesigninrelationtofloorsystemandthere benefits along withpossible meansof achievingthe objectives, inthe buildingprojectare listedbelow. Sr. no Objective Benefit for the project Possible Meanstoachieve objectives 1 Smallestpossible floorto floorheight A) Lesserbuildingheightforthe same numberof storeys B) Savingonvertical structural members,claddingetc. C) Savingonmechanical risers, lifts,staircasesetc. D) Savingonthe air conditioning by reducingthe overall floor volume. A) Reducingover-all depthof the floorsystem byeliminating beamsor reducingitsdepth. 2 Largestpossible columnfree spaces, i.e.longspan A) Flexibilityinuse B) Maximumcarpetarea A)Constructingstifferfloorsystem 3 Lowestpossible weightof the floor A) Savingonthe vertical structural membersand foundation. B) In seismicareas,savingon lateral loadresistingsystem. A)Usinglightweightmaterial. B)Eliminatingmaterial fromthe systemwhichisnot structurally requiredi.e.Introducingvoidsin the non performingareasof the floorcross section etc 4 Highrepeatability fromfloorto floor A) Improvementof constructabilityandthussaving of time A) Simple standardiseddetailsfor reinforcement B)Simple standardiseddetailsfor formwork . 5 Quickestpossible floorcycle A) Savingof time B) Avoidance of clashes betweendifferenttrades. C) Reductioninthe required numberof form-worksets A)Highearlystrengthconcrete B)Simple reinforcementandform work C)Simple detailswithhigh repeatability 6 No.Back Propping whereverpossible A) Directsavingof formwork installationtime. B) Savingof time by allowing buildingfitouttostart earlierdue to earlyaccessto the floor. A)Use of self-supportingformwork requiringsupportnearvertical structural elementsi.e.columns, structural wall etc. B)Highearlystrengthconcrete All the variationsof RCCfloorsystemsdiscussedbelow are withthe considerationtosatisfyone or numerousabove statedobjectives. Actual floorsystemsinbuildingscome inall sizes,shapes,andforms.There are somanyvariablestoany floorsystemsuchas differentspans,offsetspans,cantilevers,andthe extentof continuity,the effectsof
  • 2. beams,columnsandwallsonthe slabsystem, etcthat itis difficulttocoverall situations withone specific floorsystem.The floorsystemsemployedinthe actual buildingdesignwill alwaysbe acombinationof several basicsystems. The RCC flooringsystemscanbe; 1. FlatPlate 2. FlatSlab 3. RibbedSlab 4. Waffle Slab 5. Band beam& Slab 6. Pre-stressedSlab Flat Slab: A flatslabis a reinforcedconcrete slabsupporteddirectlybyconcrete columnswithoutthe use of beams.Itmay have a columnheadand/or a drop panel totake care of shear.Whena droppanel or column headisnot provideditisalsoreferredtoas flatplate. A flatslabis a one-wayortwo-waysystemwiththickeningsinthe slabat the columnsandload bearingwalls called'drop panels’.Droppanelsactas T-beamsoverthe supports.Theyincrease the shearcapacityandthe stiffnessof the floorsystemundervertical loads,thusincreasingthe economicalspanrange.The plan dimensionsof the droppanelsare a minimumof 1/3 of the span inthe directionunderconsideration,usually roundedto the nearest100 mm.The overall depthof the droppanel istypicallytakenas1.75 to 2 timesthe depthof the slab,again roundedtosuittimbersizesorthe nearest25 mm.The principal featuresof aflat slabfloorare a flatsoffit,simpleformworkand easyconstruction.The economicalspan'L' of a reinforced concrete flatslabis approximatelyDx 28 for simplysupported,Dx 32 for an endspan andD x 36 foran interiorspan.Pre-stressingthe slabincreasesthe economical spantoDx 35, D x 40 and D x 45 respectively, where D isthe depthof the slabexcludingthe droppanel. Usesof columnheads:•increase shearstrengthof slab •reduce the momentinthe slabby reducingthe clear or effectivespan Usesof drop panels : •increase shearstrengthof slab •increase negative momentcapacityof slab •stiffenthe slaband hence reduce deflection
  • 3. FLAT SLAB WITH DROP PANEL
  • 4. Advantages:  Simple formwork allowsfasterconstruction  ReducedservicesandcladdingcostsFlatslabconstructionplacesnorestrictionsonthe positioning of horizontal servicesandpartitionsandcanminimizefloor-to-floorheightswhenthere isno requirementfora deepfalse ceiling.Thiscanhave knock-onbenefitsintermsof lowerbuilding height,reducedcladdingcostsandprefabricatedservices.Minimumstructural depth will resultin Lowerstoreyheightwhichinturnwill reduce buildingweightdue tolowerpartitionsandcladdingto façade,approx.saves10% invertical members,reduce foundationload.  Usuallydoesnotrequire shearreinforcementatthe columns due to droppanel. 1 REINFORCED 2 PRESTRESSED
  • 5. Disadvantages:  Mediumspans  Generallynotsuitable forsupportingbrittle(masonry)partitions  Drop panelsmayinterfere withlargermechanical ducting  Vertical penetrationsneedtoavoidareaaroundcolumns  For reinforcedflatslabs,deflectionatthe middle stripmaybe critical. Flat Plate: A flatplate isa one-or two-waysystemusuallysupporteddirectlyon columnsorload bearingwalls.Itisone of the mostcommonformsof constructionof floorsinbuildings.The principalfeature of the flatplate floor isa uniformornear-uniformthicknesswithaflatsoffitwhichrequiresonlysimple formworkandiseasyto construct.The floorallowsgreatflexibilityforlocatinghorizontalservicesabove asuspendedceilingorina bulkhead.The economical spanof aflat plate forlow to mediumloadsisusuallylimitedbythe needto control long-termdeflectionand mayneedtobe sensiblypre-cambered(notoverdone)orpre-stressed.An economical spanfora reinforcedflatplate isof the orderof 6 to 8 m and forpre-stressedflatplatesisinthe range of 8 to 12 m.The span'L' of a reinforcedconcrete flat-plateisapproximatelyDx 28 for simply supported,Dx 30 foran endspanof a continuoussystem, toDx 32 for internal continuousspans.The economical spanof a flatplate can be extendedbypre-stressingtoapproximatelyDx 30, D x 37 and D x 40 respectively,where Disthe depthof slab.
  • 6. 1 REINFORCED 2 PRESTRESSED Advantages: Simple formworkandsuitablefordirectfix orsprayedceiling No beams—simplifyingunder-floorservices Minimumstructural depthandreducedfloor-tofloorheight. Disadvantages: Mediumspans Limitedlateral loadcapacityaspart of a momentframe May needshearheadsorshearreinforcementatthe columnsor largercolumnsforshear Long-termdeflectionmaybe controllingfactor May notbe suitable forsupportingbrittle(masonry)partitions May notbe suitable forheavyloads.
  • 7. Reinforcement:Inflatslaba slab-columnconnection’sresistance topunchingshearmaybe enhanced adoptingsome actions,asthe increase of the columnsection,orthe slabthickness,orthe flexural reinforcementratio,orthe compressive strengthof concrete,orbyusingdrop panelsandcolumncapitals. However,the increase of the columnsectionorthe use droppanelsandcapitalsusuallygenerateproblems fromthe architectural pointof view. The increase of the slabthicknessmaymeana substantial elevationof the structure and foundationcosts. Finally,increasingeitherthe flexural reinforcementratioorthe compressive strengthof concrete wouldhave poorefficiency. Thus,whenitisdesirable toincrease the punchingresistance,one of the mostpracticable solutionsmaybe the use of shearreinforcementin additiontothe flexural reinforcement,locallyinthe shearzone aroundthe vertical support (columnor wall). Stirrups,studsandstructural sectionsare the alternative typesof reinforcement,thatare employed to countershearnear the column. Please refertothe appendix fordetailsof the same. OPTIMUM SINGLE AND MULTISPAN FOR FLAT PLATE
  • 8. Ribbedand waffle slab:Ribbedslabsare made upof wide bandbeamsrunningbet weencolumnswithnarrowribsspanningthe orthogonal direction.Normallythe ribsandthe beamsare the same depth.A thintoppingslabcompletesthe system. Theyare eitherone-wayspanningsystemsknownasribbedslabora two-wayribbedsystemknownasa waffle slab.A ribthicknessof greaterthan125 mm isusuallyrequiredtoaccommodate tensile andshear reinforcement.Ribbedslabsare suitable formediumtoheavyloads,canspanreasonable distances,are very stiff andparticularlysuitablewhere the soffitisexposed.Slabdepthstypicallyvaryfrom75 to 125 mm and rib widthsfrom125 to 200 mm.Rib spacingof 600 to1500 mmcan be used.The overall depthof the floor typicallyvariesfrom300 to 600 mmwith overall spansof up to 15 m if reinforced,longerif post-tensioned. The use of ribsto the soffitof the slabreducesthe quantity of concrete andreinforcementandalsothe weightof the floor.The savingof materialswillbe offsetbythe complicationinformworkandplacingof reinforcement.However,formworkcomplicationisminimisedbyuse of standard,modular,reusable formwork,usuallymade frompolypropylene orfibreglassandwithtaperedsidestoallow stripping.Forribs at 1200-mm centres (tosuitstandardforms)the economical reinforcedconcrete floorspan'L' is approximatelyDx 15 fora single spanandD x 22 fora multi-span,where Disthe overall floordepth.The one-wayribsare typicallydesignedasT-beams,oftenspanninginthe longdirection.A soliddroppanel is requiredatthe columnsandloadbearingwallsforshearandmomentresistance. Advantages:  Savingsonweightandmaterials  Long spans  Attractive soffitappearance if exposed  Economical whenreusable formworkpansused RIBBED SLAB
  • 9.  Vertical penetrationsbetweenribsare easy. Disadvantages:  Depthof slabbetweenthe ribsmaycontrol the fire rating  Requiresspecial orproprietaryformwork  Greaterfloor-to-floorheight  Large vertical penetrationsare more difficulttohandle. Waffle slab
  • 10. Ribbed Slab Band Beam: This systemconsistsof aseriesof parallel,wide,shallow beams (knownasbandbeams or thickenedslabbands) withthe floorslabspanningtransverselybetweenthe bands. The floorslabis designedasacontinuousslab,withthe shallow bandbeamscarryingall loadsfromthe slab. Bandbeamsor thickenedslabbandsare a two-wayslabsystem. Band beamsare commonlyusedforlongerspanstructures oftenwiththe bandspost-tensionedandthe slabsreinforced. Sometimes,compositeconcrete/metal deckingisusedforthe slabs,providedthe slabspansare not toolarge. The bandbeam hasa relatively wide,shallow crosssectionwhichreducesthe overall depthof floorwhile permittinglongerspanssimilarto the traditional concrete beam. The concrete sectionsimplifiesboththe formworkandserviceswhichcan pass underthe beams. Advantages:  Relativelysimple formwork  Shallowbeamstoallowservicestorununderthe floor  Minimumstructural depthandreducedfloor-tofloorheight  Long spans
  • 11.  Good cost/time solution  Allowsthe use of flyingforms. Disadvantages:  Long-termdeflectionmaybe controllingfactorandpost-tensioningmaybe required  May needservice penetrationsthroughbeamswhichare difficulttohandle. In a single-spanfloor,the spacingof the bandbeamsmaycoincide withthe columns,orthe bandsmay be more closelyspacedtoreduce the thicknessof the slabspanningbetweenwallsorbeams. For single span reinforcedconcrete floorsthe economical span 'L' of the bandbeamis D x 20 to D x 22 dependingon the widthandspacingof the band beam,where Dis the depthof the slabplusbandbeam. Pre-stressing the bandbeamgiveseconomical band-beamspans inthe range of D x 24 to D x 28. In a multi-spanfloor,the spacingof the bandbeamsisfixedbythe transverse spacingof the columns. For initial sizingof the slab,the span-to-depthratiosfromSection6.3 can be used. Forinternal spansthe slab thicknessisbasedonthe clearspan betweenbandbeams,andforanexternal bayisfromthe edge of bandto the columnline of the external band. The depthof the bandis typically1.5 to 2 timesthe depth of the slaband the minimumeconomical spanfora bandbeamis about7–8 m. In multiple spansusing reinforcedconcrete,the economical slabof the bandbeam'L' is approximatelyDx 22 for 1200-mm-wide bandbeamsand D x 26 for a 2400-mm-wide beamsat 8400-mm centres. Pre-stressingincreasesthe economical span'L' to D x 24 to D x 28 forsimilarbeamwidths. D is the depthof slabplusbandbeam in each case. The maximumspanforreinforced concrete bandsshouldnotnormally exceed12m. Above this span,bandsshouldbe pre-stressed. The slabbandwidthshouldbe betweenband-spacing/3toband- spacing/4 and,where possible,shouldbe basedonamodule of a standard sheetof plyof 2.4 m x 1.2 m. Vertical sidesshouldbe used if possible tosimplifyformwork. Slopingsidesare sometimesusedwhere bandsare exposedtovieworwhere the effective spanof the slab needstobe reduced.
  • 16. Prestressedand post-tensioned RCC Slabs: Review of Reinforced Concrete: Reinforcement is Passive.It crosses the crack but does not prevent it. How to notallowthe concrete belowneutral zone togointensile stressbeyondthe tensile strengthof concrete which leadstoexcessivedeflectionandcracking? Option-1-Increase the depthof the beam. Disadvantage-Increaseinself-weightandmaterial wastage. Option-2-Provide additional steel alongthe vertical face of steel. Disadvantage-Increaseuse of steel. Option-3-Introduce compressionstressestocounterthe tensile stress. Advantage:Noextramaterial isrequired. Disadvantage:Use of sophisticatedequipmentandrequirementof specialisedskilledlabour.
  • 17. As the spansincreasesorloadingincreasesthere isanecessitytoenhance the strengthof the concrete structure (beams,columnsandslab)byusinghighstrengthmaterial and/orincreasingthe dimensionsof the structural members.Beyondaparticularlimitincreaseinthe dimensionof structural membersbecomes unviable andthere isalimitationtoenhancingthe materialstrength.The mostviable optionisof introducingcompressive stresstocounterthe tensile stressoccurringdue toself-weightandimposedload. Tension+ compression = Netzerostress Principle ofpre-stressing: The prestressinformof compressive stressescanbe introducedinthe member withoutthe use of steel bymeansof external jackatthe end.Howeveritisrarelypractical.The ulternative methodis stretchingor tensioninghightensilesteelbarsorwireswhichare thenboundor anchoredto concrete member.Inthe processof steel’stendency toreleasethe tension bycontractingtothe original unstretched length,acompressive force isappliedtothe concrete. The distributionof the pre stressacross the sectiondependsuponthe pointof applicationof the pressure.Itwillbe seenthatbyapplyingthe pressure atsome pointwithinthe the lowerthird,compressivestressesare inducedinthe bottomportion and the smallertensile stressesinthe topportionof the beam.Byselectingappropriatemagnitude of tensionandappropriate pointof application,the stressesacrossthe sectionmaybe soapportioned that whenactingtogetherwiththose setupbythe deadloadof the beamthe resultingstressatthe topis zero while the stressatthe bottomrepresentsthe maximumpermissiblecompressivestressof the concrete. Since the forcesdue to prestressingand the deadloadactssimultaneouslythe upperfibresof he concrete are notsubjectedtotensile stresses due toprestressing,northe lowerfibrestoexcessive compression. Whenthe live loadisappliedadditional compressive stressesare setupinthe topand additional tensile stressesinthe bottomfibres,andthese forces,actingtogetherwiththe residual forcesfromthe combinationof deadandprestressingloads,resultinacompressive stressinthe topfibre anda small compressionorzerostressin the bottom.Greatesteconomyisobtainedif the maximumcompressivestress inthe bottomfibres,due todeadandprestressingload,isequal tothe maximumtensile stresssetupbythe live load,thusproducingzerostressatthe bottomand maximumpermissible compressive stressatthe top whenbeamisunderload.
  • 18. Behaviour of pre-stresses Concrete member Compressive stresscanbe introducedbefore orafterthe concrete memberiscastand are termedpre- tensioningandpost-tensioningrespectively. Reasons for Prestressing: Prestressedconcrete hasbeendevelopedtoovercome some of the limitationsof reinforced concrete,namely: 1) In flexure of reinforcedconcretemember,concrete iscrackedandfunctionsonlytoholdthe reinforcingbarsinplace andprotect themfromcorrosion,therebygivingexcessweight withoutstructural action. 2) Deflectionof amemberisinverselyproportionaltothe momentof inertiaof itssection – cracking lowersthe momentof inertiaof the section,therebyincreasing deflection. 4) Eliminate crackingatservice loadingconditions 5) Improve shearandtorsionstrengths 6) Addprotectiontothe steel DefinitionofPre-stress: Pre-stressisdefinedasa methodof applyingpre-compressiontocontrol the stresses resultingdue to external loadsbelowthe neutral axisof the beamtensiondevelopeddue toexternal loadwhichismore than the permissible limitsof the plainconcrete. The pre-compressionapplied (maybe axial oreccentric) will induce the compressivestressbelow the neutral axisorasa whole of the beamc/s. Resultingeitherno tensionorcompression.
  • 19. Pre-stressedconcrete isbasicallyconcrete inwhichinternal stressesof asuitable magnitude and distributionare introducedsothatthe stressesresultingfromthe external loadsare counteractedtoa desireddegree. Pre-stress concrete requires concrete, which has a high compressive strength reasonably rapid hardening withcomparativelyhighertensilestrength than ordinary concrete. Higher the grade of concrete higher the bond strength which is vital in pre-tensioned concrete, Also higher bearing strength which is vital in post- tensioned concrete. Further creep & shrinkage losses are minimum with high-grade concrete. GenerallyminimumM30 grade concrete isusedfor post-tensioned&M40 grade concrete isusedfor pre- tensionedmembers. 20 mm coveris requiredforpre-tensionedmembers. The prestressing of concrete has several advantages as compared to traditional reinforced concrete (RC) without prestressing. A fully prestressed concrete member is usually subjected to compression during service life. This rectifies several deficiencies of concrete. The followingtextbroadlymentionsthe advantagesof aprestressedconcrete memberwithanequivalent RC member.Foreacheffect,the benefitsare listed. Sectionremains un-cracked underservice loads Reductionof steel corrosion Increase indurability. Full sectionisutilized Higher momentof inertia(higherstiffness) Lessdeformations (improvedserviceability). IndianIIncrease inshearcapacity. Suitable foruse inpressure vessels,liquidretainingstructures. Improvedperformance (resilience)underdynamicandfatigue loading. High span-to-depthratios Larger spanspossible withprestressing (bridges,buildingswithlarge column-free spaces) Typical valuesof span-to-depthratiosinslabs are givenbelow.Non-prestressedslab 28:1 Prestressedslab 45:1 For the same span,lessdepthcomparedtoRC member. • Reductioninself weight • More aestheticappeal due toslendersections • More economical sections. Advantage of PrestressedConcrete
  • 20. 1. The use of high strength concrete and steel in prestressed members results in lighter and slender members than is possible with RC members. 2. In fully prestressed members the member is free from tensile stresses under working loads, thus whole of the section is effective. 3. In prestressed members, dead loads may be counter-balanced by eccentric prestressing. 4. Prestressedconcrete memberpossesbetterresistance to shear forces due to effect of compressive stresses presence or eccentric cable profile. 5. Use of high strength concrete and freedom from cracks, contribute to improve durability under aggressive environmental conditions. 6. Long span structures are possible so that saving in weight is significant & thus it will be economic. 7. Factory products are possible. 8. Prestressed members are tested before use. 9. Prestressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before collapse. 10. Fatigue strengthisbetterdue tosmall variationsinprestressingsteel,recommendedtodynamically loaded structures. Disadvantages of PrestressedConcrete 1. The availabilityof experiencedbuildersisscanty. 2. Initial equipmentcostisveryhigh. 3. Availabilityof experiencedengineersisscanty. 4. Prestressedsectionsare brittle 5. Prestressedconcrete sectionsare lessfire resistant. ClassificationsandTypes Prestressing of concrete can be classified in several ways. The following classifications are discussed.
  • 21. 1. Source of prestressing force Thisclassificationisbasedonthe methodbywhichthe prestressingforce isgenerated.There are four sourcesof prestressingforce:Mechanical,hydraulic, andelectrical.HydraulicPrestressing Thisis the simplesttype of prestressing,producinglarge prestressingforces.The hydraulicjackusedforthe tensioningof tendons,comprisesof calibratedpressure gaugeswhichdirectlyindicatethe magnitudeof force developedduringthe tensioning.Mostlyusedforposttensioninginbuildings. a) Mechanical Prestressing In thistype of prestressing,the devicesincludesweightswithorwithoutlever transmission,geared transmissioninconjunctionwithpulleyblocks,screw jackswithorwithoutgeardrivesandwire-winding machines.Thistype of prestressingisadoptedformassscale production. b) Electrical Prestressing In thistype of prestressing, the steel wiresare electricallyheatedandanchoredbeforeplacingconcrete in the moulds.Thistype of prestressingisalsoknownasthermo-electricprestressing. 2. External or internal prestressing Thisclassificationisbasedonthe locationof the prestressingtendonwithrespecttothe concrete section. a) External Prestressing Whenthe prestressingisachievedbyelementslocatedoutside the concrete,itiscalledexternal prestressing.The tendonscanlie outsidethe member (forexampleinI-girdersorwalls)orinsidethe hollow space of a box girder.Thistechnique isadoptedinbridgesandstrengtheningof buildings. b) Internal Prestressing Whenthe prestressingisachievedbyelementslocatedinside the concrete member (commonly,by embeddedtendons),itiscalledinternal prestressing.Mostof the applicationsof prestressingare internal prestressing. 3. Pre-tensioning or post-tensioning This is the most important classification and is based on the sequence of casting the concrete and applying tension to the tendons. a) Pre-tensioning: In whichthe tendonsare tensionedbeforethe concrete isplaced,tendonsare temporarilyanchoredandtensionedandthe prestressistransferredtothe concrete afteritis hardened.The pre-compressionistransmittedfromsteeltoconcrete throughbondoverthe transmissionlength . . Post-tensioning: In which the tendon is tensioned after concrete has hardened. Tendons are placed in sheathing at suitable places in the member before casting and later after hardening of concrete. The pre- compression is transmitted from steel to concrete by the anchorage device (at the end blocks). 4. Linear or circular prestressing Thisclassificationisbasedonthe shape of the memberprestressed. a) Linear Prestressing Whenthe prestressedmembersare straightorflat,inthe directionof prestressing,the prestressingiscalled linearprestressing.Forexample,prestressingof beams,piles,polesandslabs.The profile of the prestressing tendonmay be curved. b) Circular Prestressing
  • 22. Whenthe prestressedmembersare curved,inthe directionof prestressing,the prestressingiscalled circularprestressing.Forexample,circumferential prestressingof tanks,silos,pipesandsimilarstructures. 5. Full, limited or partial prestressing Basedon the amountof prestressingforce,threetypesof prestressingare defined. a) Full Prestressing Whenthe level of prestressingissuchthatno tensile stressisallowedinconcrete underservice loads,itis calledFull Prestressing. b) Limited Prestressing Whenthe level of prestressingissuchthatthe tensile stressunderservice loadsiswithinthe crackingstress of concrete,itiscalledLimitedPrestressing. c) Partial Prestressing When the level of prestressing is such that under tensile stresses due to service loads, the crack width is within the allowable limit, it is called Partial Prestressing 6. Uniaxial, biaxial or multi-axial prestressing As the namessuggest,the classificationisbasedonthe directions of prestressingamember. a) Uniaxial Prestressing Whenthe prestressingtendonsare parallel toone axis,itiscalledUniaxial Prestressing.Forexample, longitudinalprestressingof beams. b) Biaxial Prestressing Whenthere are prestressingtendons paralleltotwoaxes,itiscalledBiaxial Prestressing.Forexample prestressingof slabonbothsides. c) Multiaxial Prestressing When the prestressing tendons are parallel to more than two axes, it is called Multiaxial Prestressing. For example, prestressing of domes. The most widelyusedmethodforprestressingof structural concrete elementsislongitudinal tensioningof steel bydifferenttensioningdevices. Pretensioning:Steel tensionedbefore casting concrete In pretensioningthe prestressingtendons (wires,strands)are stretchedtoa predetermined tensionandanchoredtofixedbulkheads or molds. The concrete is poured around the tendons, cured, and upon hardening the tendons are released. As the bond between the tendons and the concrete resists the shorteningof the tendons,the concrete iscompressed.Pretensioningisthe methodmostoftenusedfor the productionof precast prestressed concrete elements, because it offers great potential for mechani zation. pre-stressingplusself weightand live load.
  • 23. (2) Post-tensioning: In post-tensioningthe tendonsare stressedandanchoredatthe endsof the concrete memberafterthe memberhasbeencast andattainedsufficientstrength.Commonly,amortar-tightmetal pipe orduct (also calledsheath)isplacedalongthe memberbefore concrete casting.The tendonscouldbe preplacedloose inside the sheathpriortocastingor couldbe placedafterhardeningof concrete.Afterthe concrete has attainedthe requiredstrengththe tendonsare stressedusing prestressingjacksatthe endsof the concrete member(fromone or bothendssimultaneously)andanchored.Afterstressingandanchoring,the void betweeneachtendonanditsductisfilledwithamortargrout whichsubsequentlyhardens.Grouting ensures bondingof the tendontothe surroundingconcrete,improvesthe resistanceof the memberto cracking andreducesthe risksof corrosionfor the steel tendons Tendonsare made of;  Wires  Strands(individualorgroup)  Bars (tensionedone atatime) Bondedtendons:Use grout as explainedabove. Unbondedtendons:Use grease or bituminousmaterialinsteadof grout,orput outside the RCsection. Thistechnique iswidelyusedinslabsystemsof residentialandparkingstructureswithseveralbays(upto10 bays),because of itsefficiencyaneconomy.
  • 24. Post-tensioningisprimarilyan in situ operation -usedinlarge projectssuchas continuouslongspanslabsof buildingsorbridges etc. Processof post tension - Casta duct,containingthe desirednumber of strands,inthe concrete at locationwhere prestressingsteel isrequired - Use metal sheathto forma duct or alternativelyuse aplasticductsince steel tendstocorrode - Locate duct in formworkbyattachingto stirrupsandlongitudinalnonprestressed reinforcement - It isimportanttoensure that the duct isproperlysecuredtoavoidshiftinginpositionduringcastingof concrete - Duct shouldbe watertighttoavoidleakage of wetconcrete intothe ductandpluggingit - Afterconcrete attains requiredstrengthanchortendonatone endusinga mechanical anchor - Atotherendof tendonattach a prestressingjacktothe tendon,tensionitandthenanchorit.
  • 25. To avoidtension,itisnecessarytoreduce the eccentricitysothatthe centroidof the prestressingsteel atthe endof the beamis withinthe middlethirdforarectangularsection. Thisis achievedbyusing harpedor blanketedstrandsinpretensionedbeams,and drapedtendonsinpost- tensionedbeamstomaintain emax atmid-span,while havingasmallereccentricityatthe ends. In post-tensioningwe use a small number oflarge tendonsas opposedto a large number of individual strands in pretensioning. Reasons for thisare: 1. In pretensioningwe relyonbondbetweenconcreteandsteel, andthuswe wishto maximize bondsurface,whereasinpost-tensioningwhere we relyonmechanical anchorage at endof a tendonbondisnot an issue. 2. Fewerlargertendonsresultsinlesslabour,andthuslesscost,intermsof the stressing Operation. In post-tensionedconcrete,steel tendonsare usuallygrouted after anchoring to preventcorrosion.The prestressingsteel isundera relativelyhighlevel ofstress and is susceptible tostresscorrosion. Cement(or epoxy)grout > bondedmember Grease (or no grout) > unbondedmember Grout ispumpedintoduct underpressure toensure thatductis completelyfilledwithgrout. The behaviourof bondedand unbondedmembersis the same as longas concrete is not cracked - once cracking occurs there is a distinctdifference inbehaviour. Pretensioning>Factory operation - precast industry Post-tensioning>Insitu construction mainly
  • 26.  Steel heldat length longerthan what it wants to be—Tension  Concrete compressedshorter than what it wants to be—Compression Differences of Pre-stressed Concrete Over Reinforced Concrete: 1. In pre-stressconcrete membersteel playsactive role. The stressinsteel prevails whether external load isthere or not. But in R.C.C., steel plays a passive role. The stress in steel in R.C.C members depends upon the external loads. i.e., no external load, no stress in steel. 2. In pre-stress concrete the stresses in steel is almost constant where as in R.C.C the stress in steel is variable with the lever arm. 3. Pre-stress concrete has more shear resistance, where as shear resistance of R.C.C is less. 4. In pre-stressconcrete members,deflectionsare lessbecause the eccentricpre-stressingforce will induce couple which will cause upward deflections, where as in R.C.C., deflections are more. 5. In pre-stress concrete fatigue resistance is more compare to R.C.C. because in R.C.C. stress in steel is external load dependent where as in P.S.C member it is load independent. 6. Pre-stress concrete is more durable as high grade of concrete is used which are more dense in nature. R.C.C. is less durable. 7. In pre-stressconcrete dimensionsare lessbecause external stresses are counterbalance by the internal stressinducedbypre-stress. Therefore reactionsoncolumn&footing are lessasa whole the quantityof concrete is reduced by 30% and steel reduced by about 60 to 70%. R.C.C. is uneconomical for long span because in R.C.C. dimension of sections are large requiring more concrete & steel. Moreover as self-weight increases more reactions acted on columns & footings, which requires higher sizes.
  • 27. Comparative Study: Pre-tension Vs Post-tensioned Member Pre-tensionmember Post-tensionedmember 1. In pre-tensioned pre-stress concrete, steel is tensionedpriortothat of concrete. It is released once the concrete is placed and hardened. The stresses are transferred all along the wire by means of bond. 1. Concreting is done first then wires are tensioned and anchored at ends. The stress transfer is by end bearing not by bond. 2. Suitable for short span and precast products like sleepers, electric poles on mass production. 2. Suitable forlongspanbridges. 3. In pre-tensioning the cables are basically straightand horizontal. Placingthemin curved or inclined position is difficult. However the wire’s can be kept with eccentrically. Since cables cannot be aligned similar to B.M.D. structural advantages are less compare to that of post- tensioned. 3. The post tensioning cables can be aligned in any mannerto suitthe B.M.D due to external load system. Therefore it is more economical particularly for long span bridges. The curved or inclined cables can have vertical component at ends. These components will reduce the design shear force. Hence post-tensioned beams are superior to pre-tensioned beams both from flexural and shear resistances point. 4. Pre-stress losses are more compare to that of post-tensioned concrete. 4. Losses are less compare to pre-tensioned concrete.