Effect of vehicle-induced vibration on the strength, nano-mechanical properties, and microstructural characteristics of ultra-high-performance concrete during hardening process

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 (eta theta). 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.