Microstructure and mechanical behaviour of 3D printed ultra-high performance concrete after elevated temperatures

This study investigated the characteristics of 3D printed ultra-high performance concrete (3DP-UHPC) after elevated temperatures. The effects of the bonding strip, steel fibre, specimen preparation method, loading direction and temperature on the fire resistance of 3DP-UHPC were analysed. The variations in microstructure and mineral composition of 3DP-UHPC after different temperatures were examined using scanning electron microscopy (SEM) and energy spectrum analyser (EDS). The strength degradation mechanism of 3DP-UHPC after the elevated temperatures was revealed in terms of the macro and micro levels. Meanwhile, the compressive strength of 3DP-UHPC after the elevated temperatures was measured, and its corresponding compressive constitutive model was proposed. The experimental results indicated that 3DP-UHPC had certain fire resistance, and the addition of steel fibre and the preparation method improved its fire resistance. The expansion of the crack at the junction of the steel fibre and matrix, as well as the oxidation and decarburization of steel fibre, affected the compressive strength of 3DP-UHPC after 400 ℃. During heating, water vapour escaped from the weak interface of the bonding strip endowed 3DP-UHPC with slightly better elevated-temperature burst resistance as compared to mould-casting ultra-high performance concrete (MC-UHPC). The compressive strength of 3DP-UHPC was the highest after 300 ℃ for the target temperatures set in this study, but the temperature had little effect on the strength difference between each direction of 3DP-UHPC. The compressive constitutive model of 3DP-UHPC after the elevated temperatures was developed, facilitating its engineering application in the field of fire safety.