1CIVL3012 Prestressed Concrete StructuresIntroduction to Prestressed Concrete StructuresEarly attempts in prestressing were unsuccessful because low-strength steel was used long-term effects of creep and shrinkage reduced the prestress force very muchFor example, Mild steelShrinkage strainEffective prestressLossUse of Prestressing6310 100010 200200 == ~ssEfc610 600 ~1000600 1000200 ~% 10020080 200~fy= 250 N/mm2; FOS = 1.25 fs= 200 N/mm2(1000 microstrains)= 80 N/mm2= 60% Modern high strength steelShrinkage strain of 60010-6is not significant (around 10% loss) Applications of prestressed concrete: bridges, piles, buildings, marine structures, etc. Big names in prestressed concrete:French engineer Eugne FreyssinetMr. Prestressed Concrete (USA) T.Y. Lin ()Use of Prestressingfs= 1300 N/mm26310 650010 2001300 == ~ssEfc (6500 microstrains)Can high strength steel be used as reinforcement in R.C. structures?RC structures PC structuresUse of High Strength Steel LL c ? High yield steel High strength steel o Prestressed c High strength steel o LL Barrel staves compressed with hoopsMetal hoopHeated iron tyreWooden rim of wheelSpokeCartwheel compressed by contracting tyreExamples of precast beamsDouble Tee BeamExamples of standard bridge beamsVoided SlabU-Beam M-Beam Box Beam21. Prestress2. Live load++-+A concentrically prestressed concrete section An eccentrically prestressed concrete sectionPrestressPrestress + LL++-+-+Definitions PrestressingAtechniqueforintroducingstressesinstructuresduring the process of construction, so that a more favourable state of stress will prevail under service loads. Prestressed ConcreteConcreteinwhicheffectiveinternalstressesareinduced artificially,usuallybymeansoftensionedsteel,priortoloading the structure. TendonA stretched element in a concrete member or structure to impart prestress to the concrete (usually made of steel), different from reinforcement which is without initial prestress. Method of Prestressing1. External Prestressing In the past, external prestressing is prestressing in the form of concrete strain induced by a device external to the concrete structure. Now external prestressing refers to cases in which the structure is prestressed by tendons placed alongside it (though it was classified as internal prestressing in the past). This type of external prestressing takes the form of post-tensioning only. External Prestressing (old usage, obsolete now) Flat jack Figure 21.Flat jackInlet Vent External Prestressing (modern usage)Eg Tsing Yi South Bridge (Remedial works)Duplicate Tsing Yi South BridgeRambler Channel Bridge Tsing Yi South Bridge (Schematic diagram)3Method of Prestressing2. Internal PrestressingInternal prestressing is prestressing produced by tendons placed inside the structure.Internal prestressing may take the form of pre-tensioning or post-tensioning. (a) Pre-tensioning The tendons are tensioned before concrete is placed. In this method, the tendons, usually wires or strands, are stretched between fixed bulkheads or abutments and the concrete is poured around them. When the concrete has hardened and reached the required strength, the tendons are released from the anchorages and the prestress is transferred to the concrete by bond. This is particularly suitable for repetitive units. Members cast around tendons JackPrestressing bed Abutment with anchorages Tendons stretched between abutmentsTendons cropped Figure 1.Pretensioning Figure 2.Pretensioning with deflected tendons Figure 3.Post-tensioning End anchorageJack Method of Prestressing2. (b) Post-tensioning This is a method of prestressing by which tendons are tensioned after the concrete has hardened. In this method, the concrete is cast, incorporating sheaths for the tendons to pass. When the concrete has gained the required strength, the tendons are tensioned against the steel bearing plate cast into the ends of the member.The prestress is transferred to the concrete by mechanical anchorage. Method of Prestressing2. (b) Post-tensioning The space between the tendons and the sheaths is usually grouted on completion of the tensioning operation.The grout not only prevents corrosion of the tendons, but it also improves the bonding between the tendons, the sheath and the concrete. The tendons may be bonded or unbonded. 5 tons each Post-tensioning:5-ton jack 4 times Pre-tensioning:Bulkhead (at least 20 ton strength);5-ton jack 4 Railway sleeperTransmission mastExamples of pretensioned members4Prestressed member vs. columnCurved prestressed memberAdvantages of Prestressed Concrete over Reinforced Concrete1. Free from cracks under service loads (except for Class 3 structures) and hence better appearance and durability. 2. Greater shear resistance, and hence thinner web possible; reduces dead weight, especially in long span structures. 3. Greater rigidity due to effectiveness of the whole uncracked section, hence smaller deflection. Reinforced concrete Prestressed concreteAdvantages of Prestressed Concrete over Reinforced Concrete4. Lighter structure: hence saving in headroom and smaller load on foundation, usually suitable for long-span structures. 5. Test-proofing of materials: For many structures, during prestressing, both the steel and the concrete are subjected to the highest stresses that will exist in the structures during their life of service.Hence, if the materials can withstand prestressing, they are likely to possess sufficient strength for the service load. Materials for Prestressed ConcreteSteelPrestressing steel should be one of the following types: 1. High tensile steel wire or strand complying with BS 5896:1980 Specification for high tensile steel wire strand for the prestressing of concrete. 2. Alloy steel bars complying with BS 4486:1980 Specification for hot rolled and hot rolled and processed high tensile alloy steel bars for the prestressing of concrete. WireStrandBar Materials for Prestressed ConcreteConcrete1. The cube strength of concrete specified for prestressedconcrete work normally ranges from 30 to 60 N/mm2.To achieve the high strength, the water/cement ratio is in the range of 0.35 to 0.45.Usually a plasticizer (or waterreducingagent)isaddedtoimprovethe workability of concrete.2. Designofprestressed concretemembersisusually basedonthecharacteristicstrength(at28days),any furtherincreaseofstrengthafter28daysbeing ignored. 5Materials for Prestressed ConcreteConcrete3. Thestrengthattransfer(stageatwhichprestress is transmittedtoconcrete)shouldbenotlessthan25 N/mm2andisreachedinabout7days.Rapid hardeningcementmaybeusedtoobtainearly strength.Aminimumstrengthattransferof40 N/mm2is often specified.4. Inprecastfactory,steamcuring(atatmospheric pressure)isusuallyusedtospeedupproduction.Withsteamcuring,therequiredstrengthisreached within24hourssothatadailyturnoverof prestressing bed is possible.Loss of Prestress Losses of prestress are due to the immediate and deferred strains in the structural materials and to various factors, which are inherent in the stressing operation.Other Useful Terms Abutment.An end support of a bridge. Anchorage.Adevice,frequentlypatented,forpermanentlyanchoringthe tendons at the end of a post-tensioned beam, or for temporarily anchoringpretensioned tendonswhiletheconcretegains strength. Bearing.The support of a beam. Bonded and Unbonded Tendons.Tendonbonded(notbonded)throughoutitslengthtothe surrounding concrete.Abutment Bearing Deck Figure 23.A typical stressing anchorage CablesA group of tendons. CamberThe upward deflection of a prestressed concrete beam when it is prestressed. Circular and Linear PrestressingCircularprestressing referstoprestressing inroundstructures like tanks and pipes; prestressing in all other members is termed linear. Composite constructionDifferentmaterialsusedtogether,suchassteelbeamsin reinforced concrete floors; precast with insitu concrete; etc.Other Useful TermsCircular prestressing CreepTime-dependentdeformationduetoload.Inconcrete,a sustainedloadsqueezeswaterfromthecementgelatordinary temperatures, and this produces deformation, which may be two to three times as great as the elastic deformation. Creep deflectionThe deflection of a beam due to creep.Elastic deflection occurs instantly, while creep deflection requires time to develop, and it may be several months before it becomes noticeable. Other Useful Terms End BlockTheportionoftheprestressed concretebeamsurrounding the anchorages of post-tensioned tendons. Within the length oftheendblock,prestress istransferredfrommoreorless concentrated areas to the entire cross section. There are high burstingstressesalongtheaxisofthetendonashort distance inside the end block and high spalling stresses near the end face. Full and Partial PrestressingDegree of prestress applied to concrete in which no tension (some tension and usually some flexural cracking) is permitted in the concrete under the working loads. Other Useful Terms PSplitting 6 Line of PressureThe locus of concrete force along a member. PierAwidecolumnorshortwallofmasonryorplain orreinforced concrete for carrying heavy loads, such as a support for a bridge. Precast concreteConcrete members which are cast and partly matured on site or in a factory before being lifted into their position in a structure.Other Useful Terms Prestressed and Nonprestressed ReinforcementReinforcement in prestressed concrete members, which are elongated(not elongated) with respect to the surrounding concrete. ShrinkageContractionofconcreteandtimberduetodryingofthe materials. SubstructureThatpartofanystructurewhichisbelowroadlevel,in particularthefoundationsandpiersofabridge.The substructure of a suspension bridge can include its towers. SuperstructureThe visible part of a structure; that part above the substructure.Other Useful Terms TransferThe transferring of prestress to the concrete.For pre-tensioned members, transfer takes place at the release of prestress from the bulkheads; for post-tensioned members it takes place after the completion of the tensioning process.Other Useful TermsP PConcrete member P PCable Figure 4.Free bodies of concrete and curved tendon P P e1 e1 e2 Elevation of member with varying section -Pe2 -Pe1 -Pe1 Moment due to prestressing Figure 5.Member with varying section Figure 6.Loss of prestress due to frictionP-APPConcrete member AP Cable T-ATTAT Live end or stressing endDead end7 Class 1: full prestressing Class 2: limited prestressing Class 3: partial prestressing Figure 7.Classes of prestressed concrete memberUnder most unfavourable design loading:Eccentric Prestressing(Assuming weightless beam and negligible loss)wP PleeAt mid-span, 82wl M =Section moduli:ZtZbEccentric Prestressing-A PbZ PetZ Pe bZ M tZ Mct tfZMZPeAPs + tb bfZMZPeAP > ++-++--++++ = orStresses due to P Stresses due to MtcbfZPeAPs +tttfZPeAP > ++At transferorAt serviceUsual Sign Convention for Prestressed Concrete:Compressive stress +ve; tensile stress veNote:The case of minimum load is important as net tension may occur at the top at transfer.TransferTensionCompressionEccentric PrestressingInternal equilibrium (simplified treatment)wP PLine ofpressurex A Bw kN/m wl/2 kN wl/2 kN x l m Simply supported beam under UDLMx ) (2x lwxMx =Eccentric PrestressingInternal equilibrium (simplified treatment) w x P e P Concrete At transfer PeP Tendon Pe P V z R=wl/2Pz=Mx=wx(l-x)/2 PeP At service A w wl/2 Mx Vx x ) (2x lwxMx =Note: Minor variation of P is ignored.Eccentric PrestressingInternal equilibrium (simplified treatment) w P PLine of pressure xMx/ P8Pressure LineThe pressure line in a statically determinate prestressedconcrete member with a straight / deflected / curved tendon, and with no external applied load, is located at the position of the steel tendon for any section along the member. No reaction Pressure LineNot applicable!Pressure line Non-zero support reactionsRough Comparison between Reinforced Concrete and Prestressed ConcreteA simply supported beam with negligible weight is to resist a working moment of 200 kNm at mid-span that is equivalent to an ultimate moment of 320 kNm (~ 1.6 200 kNm).It is designed as a reinforced concrete beam and a prestressedconcrete beam for comparison. Reinforced ConcreteThe design of the reinforced concrete section is mostly governed by the ultimate limit state.M = 320 kNm (at ultimate limit state)fcu= 30 N/mm2fy= 460 N/mm2Assume that the singly reinforced section is acting at its limiting moment of resistance, i.e. 2bd f K Mcu' =mm 523250 30 156 . 010 3206= ='=b f KMdcumm 581 9 . 0 523 9 . 0 = = ~ d D

250 D d ~ 0.9 D For this case, d z 775 . 0 =mm 405 523 775 . 0 = = z26mm 974 , 1405 460 87 . 010 32087 . 0= = =z fMAysArea of concrete = 145,250 mm2Area of steel = 1,974 mm2Reinforced Concrete Fcc 0.67fcu /m ~ 0.45 fcu Fst s/2 z s = 0.9x For balanced section, z = d 0.45 d /2 = 0.775 d The design of the prestressed concrete section is mostly governed by the serviceability limit state.The effects of loss of prestress are ignored for the time being. 250 De = D/6 -P/bDP/bD P/bDStresses at transfer + ++-+2P/bD =ct cwf f D/6+ 92 bD f Pcw=cwcw cwf bMD fbDMZMf6 62= = =pucwpucwpusfbD ffbD ffPA4 . 1 7 . 0 2 7 . 0== =2mm N 8 . 19 60 33 . 0 = =cwf(Grade 60 concrete, cube strength at transfer 40 N/mm2)2mm N 767 , 1 =puf (Super strand) -P/bD P/bDP/bD Stresses at transfer ++ + - + 2P/bD =ct cwf f