Characterizing and analyzing the residual interfacial behavior of steel fibers embedded into cement-based matrices after exposure to high temperatures

Abstract

Fiber reinforced cementitious materials, when exposed to a fire, can have better residual mechanical properties than plain cementitious materials, particularly the tensile property. One critical factor governing the residual bridging effect is the fiber-matrix bond performance. However, the effect of temperature on the residual interfacial bond performance remains poorly understood. Here, based on the pull-out tests, we investigate the residual interfacial behavior of aligned straight and hooked-end steel fibers embedded into nine groups of matrices exposed to temperature levels from 20 °C to 600 °C with an interval level of 100 °C. Specifically, cement paste, mortar, and concrete with three w/c ratios (0.3, 0.4, and 0.5) are considered. Meanwhile, based on thermoelastic mechanics and the pulley frictional model, we quantitatively analyze the effect of thermal shrinkage, thermal incompatibility, deformation of hook end, and fiber and matrix degradation on residual interfacial bond performance. By combining the analysis and test results, we show that the thermal shrinkage can enhance the fiber-cement paste bond performance. The thermal incompatibility can impose negative effects upon the residual bond performance of all the fiber-matrix systems. Interestingly, we find that the decrease of the angle of the hook end can to some extent improve the peak pull-out force of the hooked-end fibers. Moreover, we also find that the degradation of fiber yielding strength and non-straightening hook end are the main causes of the reduction in the residual peak pull-out force of hooked-end steel fibers.