Improvement effect of fiber alignment on resistance to elevated temperature of ultra-high performance concrete

This paper investigated mechanical properties of ultra-high performance concrete (UHPC) after exposure to elevated temperatures ranging from 200 °C to 800 °C. Two casting methods were applied to prepare UHPC specimens containing randomly distributed or aligned steel fibers. X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) were utilized to evaluate the microstructure degradation of UHPC with high temperatures. Experimental results indicated that compressive strength, flexural strength and toughness were significantly reduced by 83.4%, 82.5% and 84.6% for UHPC specimens with randomly distributed steel fibers after exposure to 800 °C. The alignment of steel fibers can positively alleviate the degradation of mechanical properties of UHPC, inducing an increase of 33.8%, 72.4%, 43.9% and 58.8% for the residual compressive strength, flexural strength, toughness and deflection at post-cracking peak point, respectively. According to microstructure measurements, cement hydration products were badly decomposed by high temperature exposures with the occurrence of coarsening pore structure in cement paste and worsening the interface between cement paste and steel fibers, subsequently causing the mechanical performance degradation of UHPC specimens. Although the tensile strength of fibers was notably reduced by high temperatures, the fibers still performed bridging effects on cracks in UHPC. This is the main reason that the fiber alignment can significantly improve the residue mechanical properties of UHPC after exposure to high temperatures. Therefore, the high temperature resistance of UHPC can be improved by optimizing the orientation of steel fibers.