Effects of alternating positive and negative temperature curing on the mechanical properties of ultra-high performance concrete

The Tibetan region is characterized by long winters and large diurnal temperature variations. Using prefabricated bridges combined with a small amount of cast-in-place concrete joints can address the challenging construction environment. To extend the bridge construction window for concrete in the joint, the performance of Ultra-High Performance Concrete (UHPC) under alternating positive and negative temperature environments was investigated. The effects of curing conditions (-10°C to 10°C and natural curing), different proportions of sulfoaluminate cement (0% / 6%), different concrete materials (UHPC/NC), and the presence of antifreeze admixtures on the macro-strength and late-stage microstructure of UHPC were explored. The results indicated that the slump expansion of the prepared UHPC in this study exceeds 600 mm. Alternating temperature environments significantly degrade the mechanical properties of UHPC without antifreeze admixtures. However, for self-made UHPC with antifreeze admixtures, the formed early structure can prevent freeze-thaw damage as long as the critical antifreeze strength is reached. After the temperature rebounded, the later-stage strength of UHPC can continue to develop and reach the compressive strength of samples that had not been exposed to low-temperature curing. The appropriate addition of sulfate aluminate cement positively affected the early compressive strength (1 day) and elastic modulus of UHPC. It also reduces the drying shrinkage of UHPC.