Hybrid effects of carbon nanotubes and steel fiber on dynamic mechanical properties of ultra-high performance concrete

This study aims to investigate the hybrid effects of carbon nanotubes (CNTs) and steel fiber (SF) on dynamic mechanical behavior of ultra-high performance concrete (UHPC) under the high-speed dynamic loading effects. The functionalized CNTs content varied from 0.0% to 0.20%, and SF volume content was added from 0.5% to 2.0%. The UHPC specimens were tested at strain rates ranging from about 60 s(-1) to 240 s(-1) using the split Hopkinson pressure bar. The dynamic behavior of specimens was evaluated in terms of stress-strain relationships, energy absorption capacity and failure patterns. Test results show that dynamic compressive properties of UHPC are strongly sensitive to strain rates. The individual addition of CNTs obviously improves the dynamic performance of plain UHPC, and there exists an optimal CNTs content of about 0.10% to achieve better improvement effects. The dynamic increase factor (DIF) is almost unrelated to CNTs content but closely correlated to strain rate variations, which can be well described by a modified empirical formula. Furthermore, it is found that the hybrid CNTs and SF display more significant reinforcing effects on dynamic properties of UHPC, as the UHPC specimen with 0.10% CNTs and 2.0% SF exhibits the highest dynamic compressive strength and toughness. Meanwhile, UHPC reinforced by hybrid fibers is less sensitive to high strain rates compared to plain UHPC. From microscopic observations, it is reasonably believed CNTs and SF exert synergistic reinforcing effects on UHPC matrix in view of their respective characteristics, which are more significant in improving the interfacial transition zone (ITZ) structure between SF and the surrounding matrix. This work promotes a fundamental understanding for the reinforcing effects of CNTs on UHPC, and provides an effective strategy to reinforce UHPC from multiscale perspectives.