Spalling resistance and mechanical properties of ultra-high performance concrete reinforced with multi-scale basalt fibers and hybrid fibers under elevated temperature

The weak spalling resistance of ultra-high performance concrete (UHPC) at elevated temperature restricts the further application of UHPC and urgently needs to be improved. Macro basalt fiber (MBF), a kind of minibar produced by basalt fiber reinforced polymers (BFRP), was innovatively used to improve the spalling resistance of UHPC in this study as mono fiber or one of hybrid fibers. The effect of MBFs and hybrid fibers on the spalling resistance and mechanical properties of UHPC under elevated temperature were systematically tested and evaluated. The mechanism was comprehensively revealed by analyzing the permeability, porosity, microstructures of UHPC, and thermal analysis of fibers. According to test results, compared to UHPC with steel fibers, UHPC with MBFs showed better spalling resistance and higher mechanical properties at 400 and 500 °C, as well as the close mechanical properties and higher flowability at normal condition. The reason for higher spalling resistance was that more vapor released from channels formed by resin volatilization which was a part of MBF, thereby decreasing pore pressure. Although the obvious synergistic effects of hybrid micro and macro basalt fibers on the mechanical properties of UHPC were observed at normal condition, the spalling resistance was significantly reduced compared to UHPC with MBFs at elevated temperature. It was because that microcracks were restricted by micro basalt fibers with superb thermal stability, resulting in relatively isolated channels and decrease in vapor release. In brief, this work proposed a novel method to improve the spalling resistance and mechanical properties of UHPC under elevated temperature by using MBFs, the method which slightly reduced the mechanical properties and improved the flowability of UHPC at normal condition compared to the UHPC with steel fibers.