Constitutive model of interfacial bond-slip between steel fibers and concrete matrix after exposing to high temperature

In the current study, the bonding properties between two kinds of steel fiber with different shapes and concrete matrix after exposing to high temperatures are investigated experimentally and theoretically. A total of 55 samples were prepared and divided into five groups, subjected to different temperatures ranging from 25 °C to 800 °C, then the residual fiber pull-out strength was measured after naturally cooling down to room temperature. The results of the fibers pulling-out experiment and microstructure analysis show that the bond-slip property of hooked-end steel fiber is better than that of straight steel fiber at any temperature. The influence of temperature on the fiber-pulling performance is mainly due to the change of microstructure in the cement matrix. With the increase of temperature, the cracks of the matrix gradually increase, and the fiber will degrade after 800 °C. According to experimental analysis, a new elastic-plastic constitutive model of bond-slip is developed within the thermodynamic framework. A unified yield function is proposed to describe the pull-out process of fibers of different shapes, which only needs to determine the hardening characteristics. Combined with the principle of free energy dissipation, a damage evolution model of the interface after exposing to high temperature is proposed. The model requires several parameters with definite physical meanings to be obtained from the experiment. Compared with the experimental results, the model proposed in this paper is reliable and provides a basis for further research on the mechanical properties of fiber-reinforced concrete after high temperatures.