Ultra-high-performance concrete (UHPC) has been rapidly accepted in accelerating bridge construction attributed to its superior workability and mechanical performance. However, the unavoidable vehicle-induced vibration in construction site may cause the performance degradation of UHPC, and the influencing mechanisms have not been well-documented. In this study, the effects of various vibration parameters (amplitudes, frequencies, and phases) on the macroscopic properties, nano-mechanical properties and microstructure evolution of unhardened UHPC were investigated. Results demonstrated that the microstructural evolution and strength development of UHPC differed at different introduced vibration energy, although the vibration during the hardening process can disperse the unhydrated cement particles, decrease the Ca/Si values and form additional C-S-H gels. The low-energy vibration (low-amplitude vibration and vibration before initial setting) refined the pore structure and optimized the fiber orientation in UHPC, leading to a volume increase of gel nano-pores (<10 nm) and a higher fiber orientation coefficient ([[EQUATION]]). This improvement results in an increase in its compressive and flexural strength by 0.6% - 3.8% and 6.8% - 24.5%, respectively. In contrast, however, the high-energy vibration (high-amplitude vibration and vibration after initial setting) coarsened the large capillary pores (100–5000 nm) and macro-pores macro-pores (>5000 nm). Furthermore, the interaction (adhesion and friction) between the aggregate and matrix in UHPC was also disrupted under this condition, as confirmed by SEM. This leads to the evolution of interfacial transition zone (ITZ) in UHPC from a dense framework to a framework with microcracks.
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.
