Study On Mechanical Properties Of Concrete Under Axial Tension-Compression Fatigue Loading

The situation of fatigue failure on bridge often occurred. One of reasons is that the discount coefficient of concrete fatigue strength is higher than the real one when the ratio of stress is negative. The stress ratio of prestressed concrete bridges is often negative. It is said that the bottom of bridge is tension when there are vehicles on the bridge, but it would be compresstion when there are not vehicles on the bridge. Some of concrete tests show that the fatigue strength of concrete would be decrease or the fatigue life would be reduce when the stress ratio is negative. Studying the tests is necessary when the stress ratio of reinforcment and concrete is negative. The thesis is an important part of the project of National High Technology Research and Development Plan (863 Plan) - " The design method of concrete bridge structures based on fatigue and life ", and it studys mechanical properties of prestressed concrete under axial tension-compression fatigue load, the contents of which includes five chapters as follows:The first chapter is preface. The significance of this study and the relevant literatures are expounded. Meanwhile the historical and present background of the research on expriment and theroy of plain concrete is introduced systematically. Based on the information, the objectives of this paper are put forward. the fatigue tests of P.P.C beams are studyed, which includes constant-amplitude, two-stage variable-amplitude and random-amplitude fatigue loading. The propagation rule of maximum crack widths of P.P.C members and the stress growth law of prestressed reinforcement and ordinary steel reinforcement are obtained. In addition, fatigue behavior of retard-bonded partially prestressed concrete beams is also explored.The second chapter described general situation of the test. The test system, preparation of the speciments and experimental ways are introduced.The third chapter carries out the axial tension fatigue experiments on tapered prism specimens subjected to constant-amplitude fatigue loading. The damaged features of speciments are explored. A unified S-N curve equation to calculate tension fatigue ultimate strength of concrete is established. In addion, the deformation characteristics including the development of the cyclic stress-strain curve are obtained.In the forth chapter, the axial tension-compression fatigue experiments on tapered prism specimens subjected to constant-amplitude fatigue loading are carried out. The damaged features of speciments are explored. A unified S-N curve equation to calculate tension-compression fatigue ultimate strength of concrete is established. In addion, the deformation characteristics including the development of the cyclic stress-strain curve, the longitudinal strains, the secondary stage strain rate, the deformation modulus of concrete in terms of cyclic number are obtained.In the fifth chapter, the main achievements of this paper are summarized and the further research of this field is pointed out.