Substitutive effect of nano-SiO2 for silica fume in ultra-high-performance concrete on fiber pull-out behavior

This study investigated the effect of substituting nano-SiO2 for silica fume on the fiber-matrix interfacial bond performance of ultra-high-performance concrete (UHPC). In this study, silica fume was substituted by nano-SiO2 in the weight range of 0–50%. The degree of pozzolanic reaction of binder materials was evaluated using the thermogravimetric analysis (TGA) and compressive strength measurement. The single fiber pull-out test was conducted along with a measurement of autogenous shrinkage to evaluate the interfacial bond. The degree of pozzolanic reaction of nano-SiO2 was found to be higher than that of other binder materials. Although the packing density was predicted to increase continuously up to a substitution ratio of 50%, the highest compressive strength was obtained when 10% of silica fume was replaced by nano-SiO2, which improved the compressive strength by 5.9% compared to that of the plain sample. The autogenous shrinkage increased with an increasing content of nano-SiO2 up to 30%; however, it remained similar beyond the nano-SiO2 content of 30%. The best pull-out performance was obtained when 20% of silica fume was replaced by nano-SiO2, in which the average bond strength and pull-out energy were improved by approximately 21 and 68%, respectively. Therefore, substitution of 10–20% of silica fume by nano-SiO2 was recommended as an optimal amount considering the improvements of the compressive strength and fiber-matrix bond performance of UHPC.