Study On Creep And Creep Recovery Of Concrete Members Under Axial Compression

Concrete creep has been studied for more than a hundred years. It is a time-dependent characteristic of concrete as a viscoelastic material under loading. Due to the chronicity of creep and the complexity of its mechanism and influence factors, it is difficult to accurately calculate the creep effect of concrete members or structures. Although there are many prediction models of concrete creep, with the wide application of high strength and high performance concrete, because mineral admixtures in the concrete influence the microstructure of hydration products and also will change the creep properties, the applicability of the creep models for high strength concrete need to be improved. Compared with concrete creep, there is less study on concrete creep recovery of concrete members or structures after unloading. In practice, if the structures or members involve stress reduction or unloading, creep recovery will appear. The recovery has much important significance on the structures and members under complex stress.For creep and creep recovery problems of common concrete and high strength concrete, this thesis summarizes the creep theory of concrete and uses a large number of test data in the Bazant-Li-2008database and the B3variation coefficient method to compare the accuracy of existing creep models. Then, on the basis of analyzing the creep characteristics of high strength concrete, the applicability of existing creep models for high strength concrete is analyzed. According to test data and the MC2010model, a high strength concrete creep prediction model with flexibility is proposed.Through collecting the recovery test data of concrete members under axial compression from our country and abroad, analyzing the recovery development with time and making the reasonable hypothesis according to conclusions of related research, and considering the influence of loading age, unloading age and load time, a creep recovery calculation model of common concrete is established and by introducing the corrected coefficient of concrete strength, a high strength concrete creep recovery calculation model is also presented. The calculation results of the two proposed models agree well with the experimental data and existing conclusions.Finally, through analyzing the shortage of the existing creep calculation theory in the reducing stress situation and considering creep and creep recovery effect, based on the proposed creep recovery model, the creep calculation method for the concrete structure or member under reducing stress is presented. Taking concrete filled steel tube as an example, the proposed method is compared with the Elastic Continuation and Plastic Flow theory, the age adjusted effective modulus method and experimental results. It is verified that the proposed method has a certain feasibility.