Post-fire residual structural capacity of cement- and geopolymer-based UHPC columns

Ultra-high-performance concrete (UHPC) has been increasingly adopted in structural applications; however, its residual mechanical and structural performance after fire exposure remains insufficiently understood, particularly for geopolymer-based UHPC. In this study, comprehensive experiments were conducted on cement-based UHPC (C-UHPC) and geopolymer-based UHPC (G-UHPC) subjected to elevated temperatures. At the material level, after fire exposure, cubic specimens were subjected to uniaxial compression tests to evaluate the temperature dependent degradation in compressive strength and elastic modulus. At the structural level, full-scale rectangular UHPC columns were exposed to 800 °C and 1000 °C and subsequently tested under axial compression to examine load–axial strain behaviour, failure modes, and residual load-bearing capacity. The experimental results show that elevated temperature leads to pronounced stiffness and strength degradation at the material scale. Although G-UHPC exhibits superior fire resistance at the material level, the residual structural capacity of UHPC columns after fire exposure is governed primarily by stiffness degradation and global instability, particularly for high-slenderness members. These material deteriorations translate into severe reductions in structural axial capacity and altered post-peak behaviour of UHPC columns after fire. Drawing on the experimental results, an analytical framework is proposed to predict the residual structural capacity over a wide temperature range (20–1000 °C) by incorporating degraded material properties, reinforcement reduction, and stability effects. The proposed approach provides a practical basis for post-fire assessment and design-oriented evaluation of UHPC columns.