Study On Fatigue Behavior Of Partial Prestressed Concrete Beams With Corroded Strands

Partially prestressed concrete(PPC)structure is one of the most common structural forms of concrete bridge.At present,it is usually considered that the fatigue failure of PPC structure is due to the fatigue fracture of the tensile reinforcement.However,after cracking by fatigue loading,the steel strands in PPC structure will be corroded by the corrosion factors,such as:gas,water,chloride ions and so on.As a result,the failure of the PPC structure will be caused by fatigue rapture of the corroded strands prematurely.Consequently,the study on the fatigue behavior of PPC beams with corroded strands is very important for the structural safety.Therefore,based on the National High Technology Research and Development Program of China(863 Program)"Design Method of Concrete Beam Structure Based on Fatigue and Life"(Grant No.2007AA11Z133),the experimental and theoretical analysis of chloride diffusion in compressed concrete and fatigue performance of PPC beams with corroded strands are carried out.The main contents and analysis results are summarized as follows.(1)Experiments of chloride diffusion of concrete under compression loading are carried out.The calculation indicator is deduced by compressive strain for characterizing the internal micro-cracks in concrete,and then the relationship between the chloride diffusion and specific crack area is analysed.The experimental results show that there is a linear relationship between the chloride diffusion coefficient and residual specific crack area,and the effect of the increase of the chloride diffusion coefficient by compressive loading is more obvious in OC than HPC.However,compared with OC,the chloride diffusion coefficient of HPC is more sensitive to microcracking in the lateral surface.(2)After several cycles of fatigue loading applied on the partially prestressed concrete beams,artificial accelerated corrosion is applied on steel strands.And then,the fatigue flexural tests of partially prestressed concrete beams with corroded strands are carried out.Experimental results show that the initiation of fatigue failure of experimental beams is owing to the fracture of the corroded stress strands and the increase of corrosion rate has a significant effect on the fatigue performance and fatigue life of experimental beams.(3)Considered fatigue flexural compressive deformation modulus of concrete and effective area as the damage parameters for concrete and reinforcement respectively,a full-range analysis method of experiment beams that can account for the effect of steel stress redistribution and the residual strain of reinforcement is proposed,based on the stress analysis of cross-section.Because of the low stress of cross section on experiment beams under high cycle fatigue loading,the decrease of section stiffness is not obvious and the depth of neutral axis is stably while the material of the beams is still in elastic deformation stage.When the reduction factor of fatigue strength caused by nonuniformed corrosion in steel strands is 1.3,the prediction fatigue life of experiment beams is proved to be generally coincided with the experimental results.(4)Based on the static analysis of experimental beams by ANSYS software,the fatigue process is simulated by FE-SAFE finite element software and the fatigue life is calculated.Comparisons of the prediction results with the experimental results,the feasibility and validity of the simulation method are verified.(5)Based on the fatigue damage cumulative model,the reliability of experiment beam is calculated,and the time-dependent reliability of PPC beams with corroded strands in practical projects is analysed by considering the randomness and the time-variation of fatigue loading and material performance paramerters.The calculated reliability results show that the corrosion rate of steel strand has a significant effect on fatigue damage of experiment beam.After steel strands corroding by chloride attack,the fatigue reliability has an obvious reduction.So it is important for bridge inspection and repair schedule for ensuring enough reliability to avoiding the corrosion on the steel strands.