Advanced ECC strengthening techniques for D-region flexural enhancement in two-level RC slabs

Two-level one-way reinforced concrete (RC) slabs are widely adopted in multi-story buildings, industrial facilities, and infrastructure projects where elevation discontinuities are required for architectural and functional purposes. These structural systems inherently contain discontinuity regions ( D -regions), in which the stress fields deviate from classical Bernoulli–Euler assumptions, leading to localized cracking, complex nonlinear stress transfer, and reduced flexural capacity. Addressing this critical challenge, the present study experimentally and numerically investigates the efficacy of engineered cementitious composite (ECC)-based overlay systems in enhancing the flexural performance of two-level one-way RC slabs. The experimental program comprised nine full-scale slab specimens: one control slab in the as-cast condition and eight strengthened slabs with ECC overlays (20 mm thick) reinforced using welded steel mesh, expanded steel mesh, and glass fiber mesh, with and without mechanical anchorage. Additionally, hybrid systems incorporating ECC overlays, glass fiber reinforcement, anchors, and near-surface-mounted (NSM) steel bars (10 mm and 12 mm diameters) were tested. Results demonstrated substantial improvements in both load-carrying capacity and ductility. The maximum gain was achieved in the hybrid system (ECC + glass fiber mesh + anchors + 12 mm NSM bars), which attained an ultimate capacity of 120.51 kN, representing a 156% increase over the control specimen (47.08 kN). A finite element (FE) model based on the Concrete Damaged Plasticity (CDP) framework was developed and validated against the experimental results, capturing both stiffness and ductility with an average predictive accuracy exceeding 93%. The verified FE model was further employed to conduct a parametric study assessing the effects of ECC layer thickness and multiple strengthening layers on slab performance.