Critical Fiber Dimesions for Preventing Spalling of Ultra-high Performance Concrete at High Temperature

This paper presents an experimental study that investigates the effect of fiber dimension on preventing spalling of ultra-high-performance concrete (UHPC). UHPCs with different dimensions of polypropylene (PP) fibers were prepared for permeability test at different temperatures and spalling test under ISO834 fire curve. Scanning electron microscope (SEM) observation was also conducted focusing on the fiber of a UHPC sample at the same location after it was subjected to elevated temperatures. The results of permeability tests and spalling tests showed that longer fibers of 12 mm and 18 mm and finer fibers of 28 μm and 33 μm were more beneficial for increasing the permeability of UHPC at high temperature, resulting in higher effectiveness on spalling prevention. Permeability measurements indicated a significant increase occurred at 150°C regardless of fiber dimensions; this increase plays an important role in spalling prevention. SEM results further testify that this increase can be attributed to the formation of microcracks caused by expansion of PP fibers. An aspect ratio of 300 is recommend for spalling prevention of UHPC, when 3 kg/m³ of PP fibers are used.