TYPES OF SLABS DESCRIPTION

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.

12. SINGLE SPANBAND BEAM

13. MULTI SPAN BAND BEAM

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.