Effect of steel fiber on the compressive performance and microstructure of ultra-high performance concrete at elevated temperatures

Studies on ultra-high performance concrete (UHPC) have primarily focused on its preparation methods and mechanical properties at ambient temperature. In order to understand the degradation mechanism of UHPC at elevated temperatures, an experimental study was conducted to investigate the effect of steel fiber on the compressive performance and microstructure of UHPC specimens at temperatures ranging from 20 °C to 1000 °C. The results showed that the critical content of steel fiber to meet the workability requirement of UHPC specimen was 6 %. The steel fiber had significant influence on the appearance damage and mass loss of UHPC specimens at elevated temperatures. With the exposure temperatures increasing, the failure mode of UHPC without steel fibers transited from brittleness to ductility, and the residual compressive strength initially increased and then decreased, achieving the peak at 200 °C. Moreover, the high-temperature stability of the steel fiber had a significant effect on the residual compressive strength enhancement rate of UHPC specimens. When the temperature was below 600 °C, the residual compressive strength enhancement rate of UHPC specimens increased with the increasing temperature, obtaining the maximum of 26 % at 600 °C by 3 % steel fiber content. However, steel fibers had a negative effect on residual compressive strength enhancement rate of UHPC due to excessive oxidation at temperatures above 600 °C. According to regression analysis of the test results, the calculation models of UHPC compressive strength at ambient temperature and elevated temperatures were proposed, respectively.