Experimental assessment and modeling of shear capacity in salvaged pretensioned hollow-core slabs

Pretensioned hollow-core slab beam bridges (PHSBBs) are widely utilized across China’s highway networks. These bridges are prone to developing shear cracks under long-term traffic loads, raising concerns regarding their shear capacity. To address this issue, an experimental program was carried out on seven full-scale hollow-core slab beams salvaged from decommissioned PHSBBs after approximately 20 years of service, all with existing diagonal cracks near the supports. Shear tests with different shear span-to-depth ratios were conducted to investigate the shear behavior. Crack development and strand slip were monitored during loading. It was observed that the principal diagonal crack in most cases initiated at a certain distance from the support rather than at the support section. A four‑phase process for strand slip was identified: no slip, progressive slow slip, bond failure with sudden slip, and post‑failure slip. Based on test observations, a linear model is proposed for estimating the horizontal projection length of the principal diagonal crack. Furthermore, a mechanics-based model is developed to predict the shear capacity of cracked hollow-core slab beams. The model incorporates the shear-compression strength criterion of concrete and is founded on equilibrium of axial force, shear force and bending moment in the cracked end free body, with the compression zone depth derived dynamically as an output of the equilibrium solution. The proposed model shows good agreement with test results, achieving a mean experimental-to-predicted capacity ratio of 1.01 and an RMSE of 0.13, and generalizes well to independent test data from the literature with a mean ratio of 0.95 and an RMSE of 0.25. This study provides insights into the shear failure mechanism and offers a practical tool for estimating the ultimate shear capacity of PHSBBs.