Experimental Study On Fatigue Behavior Of The Partially Prestressed Concrete Beams

Partially prestressed concrete (PPC) structure is one of the most common structural forms of bridge, in which cracking is allowed at levels below the full service loading. This structure is subjected not only to static loading, but also to fatigue loading caused by moving vehicle, wind and wave, etc. Consequently, fatigue behavior is an important research field in the durability of the structure. Presently, most experimental studies are focused on the overall performance, such as the crack, deflection, stiffness and fatigue life. However, in practice, the initiation of fatigue failure of PPC structure is usually due to the fracture of the tensile reinforcement. This means that the stress state of the tensile reinforcement will have an important effect on the fatigue behavior. Therefore, based on the National High Technology Research and Development Program of China (863Program)(Contract Number:2007AA11Z133) and Scientific Research Fund of Liaoning Provincial Education Department (Contract Number:2008T231), fatigue tests are conducted for fourteen PPC beams. Failure mode of the test beams and stress state of the tensile reinforcement in beams are examined and analyzed under fatigue loading. On this base, some methods for calculation of fatigue life, residual strain in the non-prestressed reinforcement and crack width in PPC beams are developed, respectively. The main research contents and findings are summarized as follows.(1) Tests are carried out for PPC beams under constant-amplitude fatigue loading. The strains in the tensile reinforcement and concrete are monitored by using Fiber Bragg Grating (FBG) sensors. The experimental results show that fatigue failure of PPC beams is initiated due to the fatigue fracture of the non-prestressed reinforcement. Moreover, by comparing the measured data from the resistance strain gauges, it is verified that FBG sensor is more appropriate for monitoring the strains of the tensile reinforcement in PPC beams under fatigue loading.(2) The stress state in the non-prestressed reinforcement and the stress redistribution between prestressed and non-prestressed reinforcement are analyzed. The development trends of non-prestressed steel strain and the ratio of strain range between prestressed and non-prestressed reinforcement during fatigue loading are described. On the other hand, based on the existing cracked section methods of PPC beams, a modified cracked section method for calculation of the steel stresses in PPC beams is presented by introducing the steel stress distribution coefficient, which accounts for the effect of steel stress redistribution. (3) The causes of residual strain of the non-prestressed reinforcement in beams under fatigue loading are analyzed. Based on the monitored results from FBG sensors, the development trend of the residual strain of the non-prestressed reinforcement is investigated. Meanwhile, an analytical model for estimation of this residual strain is presented. Comparisons of the calculated results with the experimental results indicate a good agreement.(4) On the base of the bond-slip relationship under monotonic loading, a fatigue bond-slip relationship of the non-prestressed reinforcement is established. Secondly, a method is presented to calculate the stress of the non-prestressed reinforcement in the cracked section under fatigue loading, which takes into account the effect of the steel stress redistribution, the residual strains in concrete and non-prestressed reinforcement, and the area loss of the tensile reinforcement. On this base, a model is proposed to estimate fatigue crack width in PPC beams. The calculated results are found to be in good agreement with the experimental results.(5) Based on the local stress-strain method, a model for prediction of fatigue life of PPC beams is developed. The model takes into consideration the effect of stress concentration in the non-prestressed reinforcement, and its validity is verified by the experimental results.(6) Tests are carried out for PPC beams with corroded steel strands under constant-amplitude fatigue loading. The strain in the steel strands is measured by using FBG sensors. The experimental results show that the development of the maximum strain and residual strain in the corroded steel strands can be divided into three stages. In addition, the coupling action of corrosion fatigue has a significant effect on fatigue life of PPC beams.