Effect of nano-SiO2 on fiber-matrix bond in ultra-high-performance concrete as partial substitution of silica flour

This study investigated the effect of nano-SiO2, which partially substituted silica flour, on the rate-dependent bond behavior of straight steel fibers in ultra-high-performance concrete (UHPC). As much as 0%-50% by weight of the silica flour was replaced with the nano-SiO2. The packing density of the granular particles increased with the nano-SiO2 content up to 15%. However, upon increasing the nano-SiO2 content further, the packing density gradually decreased. A higher packing density was obtained at a replacement ratio of 15% than those of conventional UHPC. The static bond performance of steel fiber in UHPC was improved by adding up to 40% of nano-SiO2. Moreover, the best pullout resistance was observed in UHPC containing 10%-15% nano-SiO2 because of its filling effect and secondary pozzolanic reaction. The equivalent bond strength obtained on adding proper amounts of nano-SiO2 was thus nearly twice that of the plain UHPC. The impact pullout resistance for the UHPC matrix incorporating up to 30% of nano-SiO2 was higher than that for the plain UHPC; however, the best impact pullout resistance was observed at 10%-20% nano-SiO2 contents. A remarkable rate sensitivity of the bond strengths of straight steel fibers in the UHPC matrices was observed. The rate sensitivity of the average bond strength in the plain UHPC decreased on adding nano-SiO2. However, the rate sensitivity of the equivalent bond strength was insignificantly affected by the nano-SiO2 content. Overall, a 10%-15% nano-SiO2 content was considered to be the optimal in terms of the improved static and dynamic bond performances.