Rate-sensitive pullout resistance of smooth-steel fibers embedded in ultra-high-performance concrete containing nanoparticles

Abstract

Single-fiber pullout tests for smooth-steel fibers embedded in ultra-high-performance concrete (UHPC) containing nanoparticles (NPs) were conducted at various pullout speeds (0.0167–500 mm/s). Three types of NPs, namely, nano-CaCO3, nano-SiO2, and nano-CNT, were added to UHPC at dosages between 1 and 3 wt % of cement. The addition of NPs to UHPC notably increased the rate sensitivity of fiber pullout resistance. Among the NPs, the addition of nano-CaCO3 (3 wt %) produced the highest rate sensitivities for the dynamic increase factors (DIFs) of both τeq (1.95) and τpeak (2.06), whereas UHPC without NPs obtained the lowest rate sensitivity of τeq (DIF = 1.18), and UHPC containing 3 wt % nano-CNT had the lowest rate sensitivity of τpeak (DIF = 1.62). Nanoindentation, scanning electron microscopy, and energy-dispersive X-ray spectroscopy tests revealed that the higher amount of C–S–H at the fiber–matrix zone produced higher pullout resistance and rate sensitivity.