The Law of Combined Effect of Rate and Temperature on Compressive Strength of Concrete Materials

The dynamic compressive strength of concrete material not only has obvious strain rate strengthening (hardening) effect, but also has obvious temperature weakening (softening) effect. Under the combined condition of strain rate and temperature, there are not only clear strain rate inflection point in the process of compression strength changing with strain rate and temperature, the change of compression strength with strain rate is obviously different before and after the inflection point. Under the same condition, there also are significant differences in the strain rate values corresponding to the inflection points which are existed when the curve bends at different temperatures. Combined with theoretical analysis and references to the compression experimental data of concrete materials under the combined temperature and strain rate condition in recent years, the variation law of the joint effect factor K of concrete compressive strength under different temperatures (T) and different strain rates ($\dot {\varepsilon}$) is discussed. By fitting the experimental data, the prediction expressions of K(T)-$\dot{\varepsilon}$ at different strain rates and different temperatures were obtained, and the coupling effects of strain rate hardening and temperature softening on compression strength were determined. The relationship between the inflection point of strain rate and temperature is analyzed, and the combined rate-temperature boundary for strain rate sensitive and strain rate insensitive region is determined. The rate-temperature equivalent equation is established when the rate-temperature effect is equivalent (namely, K=1) and the rate-temperature equivalent parameters of concrete materials are determined.