Effect Of Corrosion On Prestressing Transfer In Pretensioned Prestressed Concrete Members

The prestressed concrete bridges have been widely used in bridge engineering due to its high resistance in concrete cracking and high spanning capacity.Among them,the pretensioned prestresded concrete members are key components of prestressed concrete structures dut to its simple construction and constitution.However,under the coupling effects of poor construction quality,continuous environmental attack and high stress,some serious durability problems have been observed in pretensioned prestressed concrete bridges.Corrosion has been reporeted as a main cause of safety deterioration in bridges.For the pretensioned prestressed concrete structures,the bond performance between prestressing strand and concrete determines the overall structural performance.Thus,in order to analyze accurately the residual structural capacities of pretensioned prestressed concrete structures,it,s critical to have a good knowledge of the bond performance of corroded prestressing strand and transmition of prestressing stress during corrosion process.Thus,the present study focuses on the bond performance of corroded prestressing strand and the corresponding assessment method for prestressing stress transfer after corrosion.The main research works are arranged as follows:1.The effects of corrosion-induced crack on the bond between strand and concrete are investigated experimentally in the present section.The confinement role of stirrups on the deterioration of bond behavior affected by concrete cracking is addressed.Twenty pull-out specimens with and without stirrups were designed and accelerated to induce concrete cracking by strand corrosion.Data on force-slip response,bond strength,and bond stress dist:ribution of the specimens with different corrosion-induced crack widths are presented.2.An analytical model,which considers the helical structural features of prestressing strand,is proposed to study the bond strength for the pull-out failure case accompanied with strand rotation.The force equilibrium equation between strand and surrounding confined concrete is established to develop the relationship between bond strength and radial compressive stress at interface.Consequently,the radial compressive force at interface are obtained for the two kinds of failure modes,namely concrete splitting failure and pull-out failure accompanied with strand rotation.After that,the surrounding concrete is regarded as a thick-walled cylinder model to analyse the development of corrosion-induced stress and confining stress.Three cracking stages of concrete cover during strand corrosion process,namely uncracked,partly cracked and fully cracked stages,are introduced-During the analysis process,the degradation of concrete tensile stress is considered by introducing a degradation model of cracked concrete.Finally,the three components of bond strengths,which are contributed by corrosion-induced stress,concrete confining stress and adhesion stress,are obtained for various corrosion levels.Thus,an analytical model is established for the corroded strand and concrete by taking helical characters of strand into account.3.Bond behavior between corroded strand and concrete in pre-tensioned prestressed concrete beams is investigated experimentally in the present study.The prestressing strand in pretensioned prestressed concret emembers is corroded more easily than posttensioned ones due to the protection of corrugated pipe.The corroded prestressing strand reduces the cross sectional area and ultimate tensil stress.In addition to that,the bond behavior between prestressing strand and concrete is affected inevitably.Thus,nine pre-tensioned beams with different corrosion levels were tested with two-points symmetric loading to study corrosion’s effects on global force-slip response,bond strength and failure mode.4.An analytical method is developed to deternine the transfer length of corroded pretensioned prestressed concrete members.At first,three parameters related to Hoyer’s Effect,which include radial stress at the interface between strand and concrete,the radial displacement and radial strains,are analyzed during the releasing process.Consequently,the radius of crack front along the transfer length,which is caused by the releasing of prestressing stress,is obtained for the uncorroded prestressing strand.After that,the effect of strand corrosion is considered to analyze the evolution of the radius of crack front,and the bond stress distribution along the transfer length is obtained for various corrosion levels.Finally,the transfer length of corroded pretensioned concrete members is determined by analyzing the variation of strains for prestressing strand and its surrounding concrete.5.An analytical method is proposed to determine the local bond stress-slip relationship for corroded prestressing strand.At first,the variations of strains for prestressing strand and concrete subjected to external flexural and shear loads are analyzed to obtain a second order ordinary nonlinear diffential equation for the slips between prestressing strand and concrete.After that,consider the boundary conditions during the loading process,the energy method is used to deduce the relationship between pull-out force and slips at free and loaded ends.On the basis of this relationship,the Runge-Kutta method is applied to solve the second order ordinary nonlinear diffential equations.Consequently,the relationship between the pull-out force and slip is obtained.By minizuing the diffence between computational and experimental results,the local bond stress-slip relationships for various corrosion levels are thus determined.