Experimental study on dynamic characterisation of ultra-high performance concrete (UHPC) after cryogenic freeze-thaw cycles

The increasing demand for advanced construction materials capable of withstanding extreme environmental conditions has prompted extensive research into ultra-high performance concrete (UHPC). This study investigated the dynamic compressive properties of UHPC after cryogenic freeze-thaw (FT) cycles. UHPC specimens were exposed to 2, 4 and 8 FT cycles at -160 °C before being tested under dynamic loading conditions at the strain rate of 80, 130 and 180 s -1 by the use of a Split Hopkinson Pressure Bar (SHPB) device. The effects of strain rate and FT cycles on the compressive strength, energy absorption capacity, and microstructural changes of UHPC were examined. Results revealed that dynamic compressive strength increased with strain rate for all FT cycle conditions. The study also found that the dynamic increase factor (DIF) of UHPC was influenced by FT exposure, higher DIFs were observed after more FT cycles. The DIF after 4 and 8 FT cycles increased 1.31% and 2.61%, respectively, in comparison with 2 FT cycles at the strain rate of 130 s -1 . Repeated FT cycles led to progressive deterioration of the UHPC matrix and fibre-matrix interface, as evidenced by Scanning Electron Microscopy (SEM) analysis. After 8 FT cycles, the Calcium Silicate Hydrate (C-S-H) structure experienced further damage, with noticeable cracks forming between the steel fibres and matrix, indicating a weakening of the bond between these components. The behaviour and durability of UHPC under extreme environmental and dynamic loading conditions are better understood during this research.