Interface bonding characteristics of 3D printed ultra-high performance concrete after elevated temperatures

This study explored the influence of elevated-temperature exposure and interlayer time intervals on the interface bonding strength of hybrid-fibre 3D printed ultra-high-performance concrete (3DP-UHPC), and analysed the potential mechanisms driving these observed results. A relationship model for the bonding strength of hybrid fibre 3DP-UHPC in elevated-temperature environments was proposed. Results revealed that at 800°C, localized damage occurred in 3DP-UHPC, but the addition of 0.5% polypropylene fibres delayed the occurrence of spalling behaviour and enhanced its elevated-temperature resistance. Furthermore, as the time interval increased, the bonding strength of 3DP-UHPC gradually decreased, particularly at temperatures above 400°C, where the melting and volatilization of polypropylene fibres negatively affected the bonding strength. The study suggested that polypropylene fibres inhibited spalling behaviour of 3DP-UHPC after elevated temperatures through moisture loss and thermal stability. However, they may also lead to interface weakening, resulting in a decrease in bonding strength. These findings provide important guidance for further development and design of 3DP-UHPC structures in elevated-temperature environments.