Combining polypropylene and nylon fibers to optimize fiber addition for spalling protection of high-strength concrete

This study investigates experimentally the synergistic effect of a polypropylene (PP) and a nylon (NY) fiber combination to provide improved spalling protection of high-strength concrete subjected to fire. Optimum levels of fiber contents for spalling protection with a less adverse effect on workability are provided based on the results of the fire test by varying the NY and PP fiber content. The combination of NY and PP fibers shows a synergistic effect on spalling protection; that is, by using a combination of NY and PP fibers, the same level of spalling protection is achieved using half as much fiber content as that when PP or NY fiber is used alone. The combined fibers show no explosion occurring even at 0.05% of NY (0.025% out of 0.05%) and PP (0.025% out of 0.05%) fiber content. By combining both the comparably low melting point of PP fiber, which is useful at the early stage of a fire, and the larger number of fibers that can be used due to the thinner diameter of NY fiber, which is useful at the later stage of fire, the combination of the two fibers can improve spalling protection by providing connections between pores with low fiber content.