Abstract: This study investigated the effects of the amplitude and frequency of vehicle-bridge coupled vibrations on the mechanical properties, homogeneity, and microstructure of widening concrete in bridge reconstruction and expansion projects. The layered mechanical properties and aggregate distribution of widening concrete before and after vibration were evaluated using compressive strength tests and computer vision technology. The changes in the microstructure of the concrete layers were analyzed using XRD and TG-DTG techniques. The results show that vibrations significantly exacerbate the layering phenomenon of widening concrete. Vibrations have a notable impact on the early strength of concrete, with the 3 d compressive strength reduced to 80% of the non-vibrated state. At 28 d, the compressive strength difference between the upper and lower layers further increases with the rise of amplitude and frequency. Higher amplitudes intensify the accumulation of coarse aggregates in the lower layer, reducing the spacing between aggregates and leading to a significant decline in overall homogeneity. Under high-frequency vibrations, hydration products such as calcium silicate hydrate (C-S-H) and ettringite tend to accumulate in the upper layer, while increased amplitude inhibits the hydration reaction in the lower layer. This results in an increased residual content of unhydrated tricalcium silicate (C₃S), reduced production of hydration products, and a distinct difference in the macro and micro structural characteristics between the upper and lower concrete layers.

Key words: vehicle-bridge coupled vibration, microstructure, mechanical property, computer vision, homogeneity, aggregate distribution