experimental investigating to increases the fire resistance of the ultra-high-performance concrete by using hybrid fibers

Nowadays fire has become one of the large prominent threats to buildings and concrete structures in the world. in this research, an experimental study was performed to examine the spalling phenomenon and residual mechanical properties of fiber toughened ultra-high-performance concrete (polypropylene (ppf) and steel fibers (sf)). moreover, the effect of high temperatures, namely, 250 ºc and 500 ºc for 2.5 hours and 5 hours has been studied for each mix of samples. this research discussed the results of compressive strength, flexural tensile strength, and splitting strength. weight loss of the specimens and the effect of hybrid fibers incorporation (ppf and sf) behavior ultra-high-performance concrete at high temperature were studied. the study concludes that the residual resistance of uhpc decreases as the temperature increases. also, increasing the heating time resulted in lowering the residual concrete strength. the addition of the optimum percentage of ppf (0.8%) results in a remarkable effect on decreasing the risk of spalling in the uhpfrc. polypropylene fibers provide channels in the concrete for this reduced pore pressures and the risk of spalling. incorporating hybrid fiber seems to enhance the resistance of uhpfrc to explosive spalling due to the significant increase of permeability in uhpfrc. in addition to that, the steel fibers will increase the ductility of the uhpfrc and render it more able to withstand the high internal pressures which were experimentally confirmed by this work.