Insights and theoretical model of thermal conductivity of thermally damaged hybrid steel-fine polypropylene fiber-reinforced concrete

The use of hybrid steel-fine polypropylene fiber-reinforced concrete (HFRC) may be an effective strategy for avoiding concrete thermal spalling under fire loading. The thermal conductivity of HFRC is one key parameter for the fire resistance assessing. In this research, the thermal conduction behavior of HFRC is experimentally and numerically studied to obtain a thorough understanding on its thermal degradations and their mechanisms after high-temperature exposure. New mechanisms are discovered to explain the remarkable reduction of thermal conductivity. In addition to the effect of moisture loss and porosity increase, the combined effect of heat bridge and interfacial thermal resistance (ITR) is found to be the dominant mechanisms. Moreover, the evaporation of polypropylene fibers adhering to the coarse aggregates enhances the ITR effect. Based on these mechanisms, a multi-scale homogenization method is also proposed to evaluate the thermal conductivity of thermally damaged HFRC, whose accuracy are demonstrated by the excellent agreements between the experimental data and the numerical results.