New insights into the evolution mechanism of mechanical and shrinkage properties of UHPC under sustained sub-elevated temperatures

In this paper, the mechanical properties, damage pattern, mass loss, and drying shrinkage of ultra-high performance concrete (UHPC) under sustained sub-elevated temperatures (60 ℃-200 ℃) are studied. The effect of UHPC mixture, temperature levels and durations are analyzed. The performance evolution mechanism is revealed based on the analysis of hydration products and microstructures. Results show that at 150 ℃, the water loss of C-S-H gel is continuous (>90d). After sustained action at 100 ℃-200 ℃ for 3d or 7d, UHPC shows obvious expansion, which is attributed to the interlayer water evaporation of gel phase causing spacing between the gel sheets. At 60 ℃ and 100 ℃, the strength incresear and the mean chain length (MCL) decrease are associated with the increase in the SiO4 tetrahedron production in the C-S-H gel phase. At 150 °C and 200 °C, the increase of C-S-H gel’s MCL, gel phase stiffening, and pore structure refinement are important reasons for the increase in compressive strength of UHPC. • The water loss of C-S-H gel is continuous at 150 ℃. • The microstructure of UHPC samples changed from flocculent to "bulk" after sustained sub-elevated temperatures. • The increase of C-S-H gel MCL at 150 ℃ and 200 ℃. • The volume of UHPC samples increase due to the expansion of interlayer water vapor of C-S-H gel phase at 150 ℃ and 200 ℃ for 3d or 7d. • The pore structure of UHPC pastes is refined by sustained sub-elevated temperatures.