Static And Dynamic Mechanical Behavior Of Concrete At Elevated Temperature

Concrete has become a widely used load-bearing construction material for civil engineering, industrial engineering and military projects. And concrete generally works under environmental activities such as static load, dynamic impact load and high temperature, etc. In order to design and analyze concrete structures better, the mechanical properties of concrete under abnormal conditions must be fully studied. Fully studying the mechanical properties of concrete under abnormal conditions is of critical importance in designing and analyzing concrete structures. But the experimental techniques and numerical simulation techniques about concrete with elevated temperature are not perfect, so the research and application of concrete are restricted. But the imperfect high-temperature experimental techniques and numerical simulation techniques lead to restrictions in understanding high-temperature mechanical behaviors of concrete. Around these goals, the following research on static and dynamic mechanical behaviors of concrete at elevated temperature has been conducted.?The experimental techniques for studying static and dynamic mechanical behaviors of concrete have been studied. First, A whole set of elevated temperature technique is put forward by the material testing machine and microwave heating technology. The temperature measuring equipment, insulation technology and temperature distribution of specimens are systematically analyzed and the validity is proved by the results of confirmatory tests. Last, the experimental techniques for studying dynamic mechanical behaviors of concrete with high temperature are studied by the split Hopkinson pressure bar (SHPB). After analyzing the impedance matching between concrete specimen and wave guide bar, the uniformity of specimen stress and the temperature changing process, the elevated temperature SHPB technique is established.?The static mechanical behaviors of concrete with high temperature have been studied. A series of compression tests with temperature changing from 20?to 600?have been completed. The laws about the compressive strength, the peak strain and the elastic modulus changing with temperature are analyzed, and the empirical formulas are also proposed. At the same time, the failure criterion of concrete with high temperature is developed. The stress-strain curves were analyzed by comparing with the empirical formula. Through comparative analysis of the stress-strain curve achieved from the experiment and the empirical formulas in the literature, the reliability of high- temperature static experimental techniques is demonstrated, which provides reference for future research.?The dynamic mechanical behaviors of concrete with high temperature have been studied. Through the result of SHPB tests with normal temperature, the effect of strain-rate is analyzed and the damage evolution rules of concrete under high strain rate are studied through the dynamic stress-strain curve. The dynamic destructional forms of concrete under different temperatures are observed during the high-temperature SHPB tests. The correlations among the compressive strength, the strain rate, the temperature and the loading rate are studied. The strengthening effect of strain-rate and the weakening effect of high temperature on concrete were putted forward. The coupling law of the strain-rate strengthening effect and high-temperature weakening effect on concrete is presented. The results show that the compressive strength reduces as the temperature increases under a constant strain rate, and the peak strain was more and more bigger. while the peak strain as well as the toughness accretes.?The static and dynamic constitutive model of concrete with high temperature has been studied. After comprehensive analysis of factors which influence the stress-strain curve, a unified damage constitutive model of concrete containing the strengthening effect of strain-rate and the weakening effect of high temperature is put forward. First, based on the Weibull's damage fracture probability density theory, a new static damage constitutive model of normal temperature concrete is established. Then the static constitutive model becomes a high-temperature damage model or a dynamic damage constitutive model after adding the effectors about the weakening effect of high temperature and the strengthening effect of strain-rate. Then after adding the high-temperature weakening factor and the strain-rate strengthening factor, respectively, the static constitutive model becomes a high-temperature static damage model and a normal-temperature dynamic damage model. Ultimately, a high-temperature dynamic damage constitutive model of concrete is obtained by adding both strengthening factor and weakening factor. And the effectiveness of the model is verified by comparing the theoretical formula with test data.?The numerical simulation method for concrete with high temperature has been established by the finite element software. The accurate parameters of HJC model, especially parameters affected by high temperature, are gotten. The temperature changing rules of concrete specimens during the entire test process are acquired by combination of the temperature measured in the experiment, and the static and dynamic mechanical behaviors of concrete at elevated temperature are also analyzed by numerical simulation, and the destructive phenomena of high-temperature concrete are represented. The result shows that the numerical simulation dovetails nicely with experimental phenomena.