Draft
Chapter 12
PRESTRESSED CONCRETE
12.1 Introduction.
1 Beams withlonger spansarearchitecturallymoreappealingthan thosewithshort ones.
However,fora reinforcedconcretebeamto spanlongdistances,it wouldhave to have to be
relativelydeep(andat somepoint theselfweight may becometoo largerelative to thelive
load),or highergradesteelandconcretemustbe used.
2 However,if we wereto usea steelwithfymuch higherthan 60 ksiin reinforcedconcrete
(R/C),thento take fulladvantageof thishigheryieldstresswhilemaintainingfullbondbetween
concreteandsteel,willresultin unacceptablywidecrack widths. Largecrack widthswillin
turnresultin corrosionof therebarsandpoor protectionagainst re.
3 Oneway to controltheconcretecrackingandreducethetensilestressesin a beam is to
prestressthebeamby applyinganinitialstateof stresswhich is oppositeto theonewhich will
be inducedby theload.
4 For a simplysupportedbeam,we wouldthenseekto applyaninitialtensilestressat the
topandcompressive stressat thebottom.In prestressedconcrete(P/C)thiscanbe achieved
throughprestressingof a tendonplacedbelow theelasticneutralaxis.
5 Mainadvantagesof P/C:Economy, de
ection& crack control,durability, fatiguestrength,
longerspans.
6 Theretwo type of PrestressedConcretebeams:
Pretensioning:Steelis rststressed,concreteis thenpouredaroundthestressedbars.When
enoughconcretestrengthhasbeenreachedthesteelrestraints arereleased,Fig.12.1.
Postensioning:Concreteis rstpoured,thenwhenenoughstrengthhasbeenreacheda steel
cableis passedthrua hollow coreinsideandstressed,Fig.12.2.
12.1.1 Materials
7 P/Cbeamsusuallyhave highercompressive strengththanR/C.Prestressedbeamscanhave
f
0
cas highas 8,000psi.
8 Theimportanceof highyieldstressforthesteelis illustratedby thefollowingsimpleexample.