Experimental Study On Stiffness Degradation Of Prestressed Concrete Beam Under Overload Fatigue

The prestressed concrete bridges are widespread and most constructed in China. As most prestressed concrete bridges on highways are under cycle loading of vehicle, stiffness degradation and deformation growth induced by fatigue and other factors is much more common than before. And problems such as excessive deflection and cracking are prominent. This paper is mainly studied on stiffness degradation and deflection evolution of prestressed concrete beam under overload fatigue, which is funded by the National High Technology Research and Development Program "Long term deformation and cracking control technology of large span concrete bridges"(Grant No.2008AA11Z102) and the National Natural Science Foundation "Research on durability and fatigue damage mechanism of large span prestressed concrete bridges over sea"(Grant No.51178042). The detailed contents and highlights are listed as following:(1) The development of prestressed concrete bridges is reviewed. Then, the causes and mechanism of excessive deflection and cracking is briefly analyzed, and the significance of study on fatigue of prestressed concrete bridges is introduced. The general analysis methods of fatigue as well as latest research on fatigue of prestressed concrete beams are summarized, and the significance and research basis of this paper is introduced.(2) Fatigue test of prestressed concrete beams under multilevel amplitude cycle loading of overload was carried out. The strain increasement of concrete or steel rebar, cracking development and deflection evolution of prestressed concrete test beam under fatigue loading was obtained. Results indicate that the fatigue strain of concrete or steel rebar presents a two-stage evolution under the fatigue loading level below cracking level or just overloading temperately, and the strain grows more rapidly with the increasing of the loading level and stress ratio. Even while the loading level is below cracking level, crack appears at the position of maximum bending moment on the prestressed concrete test beams, and the crack development indicates that it’s related to the fatigue loading level and stress ratio. The fatigue deflection evolution is somewhat like the development of the concrete strain, as it also performs a two-stage evolution and related to the loading level and stress ratio. The stiffness of the fatigue damaged test beams is about20%-30%lower in the elastic stage than those with no fatigue damage, and50%lower while near the ultimate load. And the ultimate bearing capacity is decreased as well.(3) Based on damage mechanics theories, a two-stage damage evolution model of concrete is derived, and the parameters in the model are fitted and verified by existing fatigue experiments. Results indicate that the model could well present the concrete damage evolution process in normal operating condition. Meanwhile, with regard to steel, the displacement field and stress field, which is near the crack tip region, is derived based on fracture mechanics theories. The mechanism of crack developing and the characteristics of the three stage of fatigue crack development are elaborated, and several crack propagation rate expressions are summarized. Besides, five typical fatigue cumulative damage theories and characteristics of which are discussed. And those are the theoretical basis of stiffness degradation analysis for prestressed concrete beam.(4) A fatigue stiffness degradation model for prestressed concrete beam is established based on section stiffness damage analysis, and damaged modulus of elasticity is applied to count concrete damage while damaged effective remaining area is applied to count steel damage. Then, a whole process analysis is carried out based on this model, and a program script is generated. The analysis results exhibit good agreements with the fatigue test results. The errors are generally within5%. And the model could well describe the stiffness degradation evolution of prestressed concrete beam under fatigue loading.(5) Based on the whole process analysis method of the fatigue stiffness degradation model, a method for fatigue deflection predicting analysis is presented. And two artificial neural network models are also established to gain an analysis compare. Results indicate that the results of the predicting method based on whole process analysis of fatigue stiffness degradation present good agreements with the test results, and the errors are generally stable and appropriately decrease with the increasing of sample size. The errors of artificial neural network models are exceeded when the sample size is not large enough, but errors would reduce while the sample size enlarges. However, the predicted fatigue deflection curve is influenced by the form of transfer functions and the number of hidden neurons. And, the stability of the predicting results is relatively poor. It is usually need a vast number of repeated simulating calculations to gain a preferable result.(6) Long term deflection leading by fatigue or concrete shrinkage and creep of a real prestressed concrete bridge of4x20m span was analyzed. Results indicate that during not a long time after opening to traffic, the deflection increasement leading by fatigue is larger than which leading by concrete shrinkage and creep. After then, the deflection increasement leading by fatigue grows steadly and slowly. However, the deflection growth leading by shrinkage and creep is still developing fast, and the shrinkage and creep deflection exceeds the fatigue deflection. As a result, it could be inferred that fatigue is a primary factor of deflection growth in the early time after opening to traffic, but deflection increasement leading by concrete shrinkage and creep is larger and larger and take the place of fatigue to be a primary factor. And overload lead to significant increasement on fatigue deflection.(7) Based on the fatigue stiffness degradation model, the performance function of fatigue deflection for prestressed concrete beam is obtained. The statistical properties of the variables are analyzed, and the time-variant reliability analysis of fatigue deflection for prestressed concrete beams is conducted. The relationship between reliability index decreasing and fatigue cycles is discussed.