Experimental study on the thermal properties of a novel ultra-high performance concrete reinforced with multi-scale fibers at elevated temperatures

The fire resistance of a novel ultra-high performance concrete is critical to its application, while the evolution of its thermal properties with temperature plays a key role in its fire resistance. Therefore, this study presents an experimental investigation of the thermal performance of newly developed ultra-high performance concrete reinforced by cenosphere and multi-scale fibers (MSFUHPC) including carbon nanotubes, calcium sulfate whiskers, carbon fibers, steel fibers, and polyethylene fibers under high temperatures ranging from 25 °C to 800 °C. Meanwhile, scanning electron microscope, thermogravimetry analysis, and mercury intrusion porosimetry tests are also conducted to study the thermal effect on the microstructure. Accordingly, the effect of multi-scale fibers and cenospheres on the thermal conductivity, volumetric specific heat, and thermal expansion of MSFUHPC are discussed. The results show that adding carbon nanotubes, carbon fibers, and hybrid fibers (steel fibers and polyethylene fibers) can increase thermal conductivity, and adding calcium sulfate whiskers and cenospheres exhibits an inverse effect at ambient temperature. In addition, the thermal expansion can be enlarged by incorporating carbon nanotubes or steel fibers and polyethylene fibers and restrained by adding carbon fibers. The influence of calcium sulfate whiskers and cenospheres on thermal expansion depends on their content. At high temperatures, the thermal conductivity shows an overall decreasing trend and the thermal conductivity of MSFUHPC is higher than that of its matrix and traditional UHPC, HPC, and NSC due to the reduction in the generation and propagation of cracks. By contrast, the volumetric specific heat shows an increasing trend with temperature. Besides, the thermal expansion of MSFUHPC is lower than that of UHPC, HPC, and NSC. The formulas of thermal parameters are fitted based on experimental data by considering the multi-scale fibers and temperatures, which can be used as input parameters for the numerical simulation of MSFUHPC structure under fire.