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The objective of this research was to evaluate the feasibility of producing structural lightweight concrete pre-stressed girders with load-bearing capacities similar to conventional concrete girders. The methodology con-sisted of comparing the results of the production and monitoring of scaled prestressed concrete beams at aprecasting plant and the subsequent flexural testing. The beamswere produced using two types of lightweightconcrete and a conventional reference concretewith similar compressive strengths to allowfor a comparison ofthe structural performance of the beams. The goalwas to estimate an upper bound for the development lengthof fully developed prestressing strands with a diameter of 15.2 mm(0.600). The load-bearing capacity and duc-tility of the beams were determined by flexural tests. All lightweight and conventional concrete beamsexceeded their nominal flexural strengths, which were calculated without accounting for the splitting cracksthat all manufactured lightweight concrete beams developed after the prestress release on their lower side.Based on these tests,33655
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upper boundswere determined for the development length of the prestressed strandswitha diameter of 15.2 mm for the three types of concrete tested. Subsequently, the ductility factors of the beamstested under flexure were determined. Despite all of the tested beams were able to develop the full strengthof the prestressed strands and showed a remarkably ductile behavior, it was concluded that the use of theselightweight concretes is not recommended in the fabrication of prestressed concrete bridge girders becauseof the potential reduction in their durability and bearing capacity due to stress corrosion, which is a result ofthe splitting cracks detected in all of the produced prestressed lightweight concrete beams. 1. IntroductionIn prestressed concrete structures, the prestressing force istransmitted by bond, a property that manifests itself differentlyin the two parts into which the development length of the pre-stressing steel (either strands or wires) is pided [1,2]: The transfer length, lt, is the length of embedded pre-ten-sioned strand required to transfer the effective prestressingstress,
fpe, to the concrete, where fpi is the prestressing steelstress immediately before prestress release. The flexural bond length, lfb, is the distance in addition tothe transfer length that is required for the bonded pre-stressing steel to develop the stress associated with the nominal strength of the member, fps. This study focuseson fully developed strands; consequently, fps is equal tothe yield strength of the strand, fpy.The development length, ld, is the sum of the transfer length andthe flexural bond length, as shown in Fig. 1. In this figure, the pre-stressing steel stress rp(x) is plotted versus the distance x from theend of the structural element.Because of the tensioning of the strand in the pre-tensioningbed, a reduction in the prestressing steel diameter occurs due toPoisson’s effect. Conversely, prestressing steel exhibits a recoveryin its diameter due to Poisson’s effect because of the instantaneousprestress loss experienced after prestress release. The maximumrecovery of the diameter occurs at the end of the structure wherethe prestress force is zero after release, and the minimum recoveryoccurs in the interior of the central zone where there is a perfectbond. This wedging effect caused by the lateral expansion of theprestressing steel, called the Hoyer effect, results in improved bondperformance along the transfer length. However, circumferentialtensile stresses are generated in the concrete that surrounds the prestressing steel, which are greater at the ends of the structuralmember. These stresses are called splitting stresses [3,4].In the central area of the beam, i.e., the zone sufficiently farfrom the ends, in the absence of external applied loads (includ-ing self-weight), the prestressing strand stress reaches the valuefpe, which corresponds to the specific time of the service life ofthe structure. This value is determined by the instantaneous pre-stress loss and the time-dependent prestress loss resulting fromthe combined shrinkage and creep of the concrete and the relax-ation of the prestressing steel.