Experimental And Theoretical Research On Residual Strength Of Concrete Under Fatigue Loading

Many reinforced and prestressed concrete structures are widely used. These civil engineering structures may be subjected to fatigue loading. It is widely concerned in fatigue of concrete structures. Concrete in such structures works in the complex states. Attenuation of residual strength is the first cause of the failure of concrete structure under fatigue loading. Literature on the experiments and the theories of residual strength of concrete under fatigue loadings is extremely scarce, which is the drawback on the design and analysis of these structures. So the study of residual strength is necessary. The thesis is also an important part of the project of National Natural Science Foundation-Failure criterion of concrete under multi-axial variable-amplitude fatigue loading (No. 50078010), the main contents are summarized as follows:(1) An experimental research program is carried out with the aim of investigating the residual strength of concrete under fatigue compression and tension. The experimentation method designed by the paper could research the attenuation rule of residual strength. When the specimens are on the verge of failure, the residual strength decreases immediately, the "Sudden Death" is seen distinctly.(2) The probabilistic distribution of the fatigue residual tensile and compressive strength is found out, the P-R-N ( P for probability of failure, R for residual strength, N for number of cycles) curves are obtained based on test data with reliability. Based on the P-R-N curves for P = 0.50 , the residual fatigue life of plain concrete under variable-amplitude fatigue loading can be predicted. The result is more precise than Miner's rule.(3) The experimental results show good relationship between residual tensile or compressive strength and residual secant elastic modulus. The residual secant elastic modulus declines with the residual strength in the structure under fatigue loading. Under fatigue loading the residual secant elastic modulus decreases linearly with logarithm of number of cycles.(4) The damage variable based on normalized maximum strain in vertical direction is introduced into the analysis of the fatigue damage for concrete. The damage variable has a specific physical meaning and is fathomable. The damage and the residual strength have the similar evolution rule. The damage in concrete results in deterioration of residual strength.