1Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarLecture - 07Introduction to Prestressed ConcreteBy: Prof Dr. Qaisar AliCivil Engineering DepartmentUET Peshawarwww.drqaisarali.com1Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarTopics Introduction Principle of Prestressing Advantages of Prestressed Concrete Prestressing Steel High Strength Concrete used for Prestressed Construction Methods of Prestressing22Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarIntroduction Background Concrete is basically a compressive material, with its strength intension being relatively low. Prestressing applies a precompression to the member that reducesor eliminates undesirable tensile stresses that would otherwise bepresent.3Reinforced concrete cracked under load. Post-tensioned concrete before loading.Post-tensioned concrete after loading.Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology Peshawar4Principle of Prestressing3Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology Peshawar5Principle of PrestressingProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology Peshawar Some important conclusions can be drawn fromprevious simple examples: Prestressing can control or even eliminate concrete tensilestress for specified loads. Eccentric prestress is usually much more efficient thanconcentric prestress. Variable eccentricity is usually preferable to constanteccentricity, from the viewpoints of both stress control anddeflection control.6Principle of Prestressing4Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology Peshawar7Advantages of Prestressing Prestressing results in the overall improvement in performanceof structural concrete used for ordinary loads and spans. Prestressing extends the range of application far beyond thelimits for ordinary reinforced concrete, leading not only to muchlonger spans with economical member cross sections thanpreviously thought possible, but permitting innovative newstructural forms to be employed.Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology Peshawar8Advantages of Prestressing Objectionable deflection and cracking, which would otherwisebe associated with the use of non prestressed reinforcedconcrete members at high stress, are easily controlled byprestressing.5Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarImportance of High Strength Steel9Stress variation with strainEffect of shrinkage and creep of concrete in reducing prestress force for axially prestressed beamPrestressing SteelProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarImportance of High Strength Steel The initial strain in the steel is:si= fsi/Es= 30/29000 = 1.03 x 10-3 And the steel elongation is:s= sils= 1.03 10-3ls The sum of shrinkage and creep strain in the concrete isabout 0.90 10-3, and the corresponding length change is:(sh+ cu)lc= 0.90 10-3lc10Prestressing Steel6Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarImportance of High Strength Steel Since lsand lcare nearly the same for fst= 30 ksi, whichmeans that the combined effects of shrinkage and creep ofthe concrete is almost a complete loss of the stress in steel. The effective steel stress remaining after time-dependenteffects would be fse= (1.03 0.90) 10-3 29 103 4 ksi11Prestressing SteelProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarImportance of High Strength Steel For high strength steel at an initial stress of 150 ksi, theinitial strain would be:si= 150/29000 = 5.17 10-3 sils= 5.17 10-3ls The effective steel stressfseafter losses due to shrinkageand creep would be fse= (5.17 0.90) 10-3 29 103=124 ksi The loss is about 17 percent of the initial steel stress in thiscase compared with 87 percent loss when mild steel wasused.12Prestressing Steel7Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarTypes of High Strength Steel Round wires Wires are normally bundled in groups of up to about 50individual wires to produce prestressing tendons of the requiredstrength. Stranded cable Stranded cable, more common than wire in U.S. practice, isfabricated with six wires wound around a seventh of slightlylarger diameter. Alloy steel bars Available in the form of deformed bar similar to reinforcing bar13Prestressing SteelProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarTypes of High Strength Steel Sizes14Steel Diameter range, inch (mm)Prestressing Wire 0.192 to 0.276 (5 to 7 mm)Strand 0.250 to 0.600 (6 to 15 mm) Alloy Steel Bars0.750 to 1.375 (19 to 35 mm)(as plain round bars)0.625 to 2.50 (16 to 63 mm)(as deformed bars)Prestressing Steel8Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarTypes of High Strength Steel Tensile Yield Strength15Steel Grades (ksi)Round wires 235, 240, 250Strands (7-wired)250 (fpu= 250 ksi)270 (most often used)300 (not recognized by ASTM A421)Alloy steel bars150 (regular grade, most common)160 (special grade, may be ordered)Prestressing SteelProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarStress-Strain Curves16The yield strength is somewhatarbitrarily defined as the stress at a totalelongation of 1 percent for strand andwire and at 0.7 percent for alloy steelbars.Prestressing Steel9Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarStress-Strain Curves17The prestressing steels havesignificantly less ductility.Prestressing SteelProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarElastic Modulus for various High Strength Steels18Steel Es (psi)Un-bonded strand 26,000,000 psiBonded strand 27,000,000 psiSmooth round wires 29,000,000 psi (same as for reinforcing bar)Alloy steel bars 27,000,000 psiPrestressing Steel10Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarRelaxation When prestressing steel is stressed to the levels that arecustomary during initial tensioning and at service loads, itexhibits a property known as relaxation. Relaxation is defined as the loss of stress in stressedmaterial held at constant length. The same basic phenomenon is known as creep whendefined in terms of change in strain of a material underconstant stress.19Prestressing SteelProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarRelaxation To be specific, if a length of prestressing steel is stressed toa sizable fraction of its yield strengthfpy(say 80 to 90percent) and held at a constant strain between fixed pointssuch as the ends of a beam, the steel stress fpwill graduallydecrease from its initial value fpi.20Prestressing Steel11Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarRelaxation A satisfactory estimate for ordinary stress relieved strandand wires can be obtained from eq. (1), which was derivedfrom more than 400 relaxation tests of up to 9 yearsduration:Where, fp= final stress after t hours, fpi= initial stress, andfpy=nominal yield stress. For low-relaxation strand, eq. (1) is replaced by:21Prestressing SteelProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarHigh Strength Concrete Used for Prestressed Construction22Definition Although the exact definition is arbitrary, the term generallyrefers to concrete having uniaxial compressive strength inthe range of about 8000 to 15,000 psi or higher. Such concretes can be made using carefully selected butwidely available cements, sands, and stone; certainadmixtures including high-range water-reducing superplasticizers, fly ash, and silica fume; plus very careful qualitycontrol during production.12Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarHigh Strength Concrete Used for Prestressed Construction23Advantages High strength concrete has a higher elastic modulus than thelow strength concrete, so that loss of prestress forceresulting from elastic shortening of the concrete is reduced. Creep and shrinkage losses are also low. High bearing stresses in the vicinity of tendon anchoragesfor post tensioned members are more easily accommodated. In the case of pretensioned elements, higher bond strengthresults in a reduction in the development length required totransfer prestress force from the cables to the concrete.Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarHigh Strength Concrete Used for Prestressed Construction24Advantages Finally, concrete of higher compressive strength also has ahigher tensile strength so that the formation of flexural anddiagonal tension crack is delayed.13Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarHigh Strength Concrete Used for Prestressed Construction Stress-Strain Curves25Typical set of compressive stressstrain curve for normal densityconcrete, obtained from uniaxialcompressive test performed atnormal, moderate testing speedson concretes that are 28 days oldHigh strength concrete are morebrittle.Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarHigh Strength Concrete Used for Prestressed Construction Stress-Strain Curves26In present practice, compressive strengthbetween 4000 and 8000 psi (28 and 55MPa) is commonly specified forprestressed concrete members, althoughstrengths as high as 12000 psi (83 MPa)have been used.14Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarHigh Strength Concrete Used for Prestressed Construction Stress-Strain Curves27It should be emphasized, however, thatthe concrete strength assumed in thedesign calculations and specified mustbe attained with certainty, because thecalculated high stresses resulting fromprestress force really do occur.Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarMethods of Prestressing Although many methods have been used to produce the desiredstate of precompression in concrete members, all pre-stressedconcrete members can be placed in one of two categories: Pre-tensioned, Post-tensioned.2815Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPre-tensioning (Procedure) The strands are tensioned over the full length of the castingbed at one time, after which a number of individual membersare cast along the stressed tendon. When the jacking force is released, the prestress force istransferred to each member by bond, and the strands are cutfree between members.29Methods of PrestressingProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPre-tensioning (Procedure) In present practice anchorage and jacking abutments maybe as much as 800 ft apart. Cable depressors are often used with long-line prestressing,just as with individual members.30Methods of Prestressing16Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPre-tensioning31Methods of PrestressingProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPre-tensioning32Methods of Prestressing17Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPre-tensioning (Advantages) Well suited to the mass production of beams using the long-line method of prestressing. Pretensioning is a particularly economical method ofprestressing, not only because the standardization of designpermits reusable steel or fiberglass forms, but also becausethe simultaneous prestressing of many members at onceresults in great saving of labor. In addition, expensive end-anchorage hardware is eliminated.33Methods of PrestressingProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPost-tensioning Usually hollow conduits containing the unstressed tendonsare placed in the beam forms, to the desired profile, beforepouring the concrete. The conduit is wired to auxiliary beam reinforcement(unstressed stirrups) to prevent accidental displacement,and the concrete is poured. When it has gained sufficient strength, the concrete beamitself is used to provide the reaction for the stressing jack.34Methods of Prestressing18Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPost-tensioning With the tendon anchored by special fittings at the far end ofthe member, it is stretched, and then anchored at the jackingend by similar fittings, and the jack removed. The tension is gauged by measuring both the jackingpressure and the elongation of the steel. The tendons are normally tensioned one at a time, althougheach tendon may consist of many strands or wires.35Methods of PrestressingProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPost-tensioning36Methods of Prestressing19Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarPost-tensioning (Advantage) A significant advantage of all post-tensioning schemes is theease with which the tendon eccentricity can be varied alongthe span to provide the desired counter moment.37Anchor blocks and wedges Post tensioning under progressConduitMethods of PrestressingProf. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology Peshawar Design of Concrete Structures 13 Ed. by Nilson, Darwin andDolan.38References20Prof. Dr. Qaisar AliReinforced Concrete Design IIDepartment of Civil Engineering, University of Engineering and Technology PeshawarThe End39