Effects of combined nanoparticles on dynamic strength of ultra-high-performance fiber-reinforced concrete

This study explored the effects of nanoparticle (NP) combinations on the dynamic strength of ultra-high-performance fiber-reinforced concrete (UHPFRC). The dosages of NPs, based on the cement weight, were 3 wt% for nano-CaCO3, 3 , 1 wt% for nano-SiO2, 2 , and 1 wt% for nano-carbon nanotube (CNT). The stress-strain responses of the UHPFRCs at high strain rates (86.62-158.02 s- 1 ) were investigated under compression and tension. Additionally, the single- fiber pullout resistance of the smooth steel fibers was evaluated at high-rate pullout loads (372-495 mm/s). The results indicate that the UHPFRCs containing both nano-CaCO3 3 and nanoSiO2 2 exhibited significant synergies in strength at high strain rates, whereas those containing the other NPs did not exhibit any discernible synergy. At high-rate loading, the synergies in compressive-, bond-, and tensile strengths of the UHPFRCs containing both nano-CaCO3 3 and nano-SiO2 2 were 11.49 %, 13.97 %, and 10.19 %, respectively; those of the UHPFRC containing nano-CaCO3 3 and nano-CNT were-27.02 %,-17.69 %, and-36.35 %, respectively; and those of the UHPFRC containing nano-SiO2 2 and nano-CNT were-23.74 %,-11.49 %, and-34.62 %, respectively. Furthermore, all matrices containing the combined NPs exhibited rate-sensitive mechanical responses to the applied loading. The dynamic increase factors of the matrices were between 1.04 and 1.43 for compressive strength, between 1.72 and 2.11 for peak bond strength, and between 2.50 and 3.12 for post-cracking strength. The use of nano-CaCO3 3 and nanoSiO2 2 increased the calcium-silicate-hydrate content of the UHPFRC, thereby enhancing its dynamic strength. The reduction in the dynamic strengths of the UHPFRCs incorporating nano-CNT is attributed to the agglomeration of excessive NPs, such as nano-CNT combined with nano-CaCO3 3 or nano-SiO2. 2 .