FRP-reinforced Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) permanent formwork with post-cast joint for reinforced concrete members

This study investigates the flexural behavior of reinforced concrete (RC) beams incorporating post-cast-jointed FRP-reinforced ultra-high-strength engineered cementitious composites (UHS-ECC) as stay-in-place permanent formwork. A post-cast UHS-ECC joint was introduced into the bottom formwork to represent a realistic segmented construction scenario. Four-point bending tests were conducted on two composite beam specimens (UE50-J), and the results were compared with those of the conventional RC control beams (CON). Digital image correlation was employed to capture strain-field evolution and crack localization during loading. A refined three-dimensional finite element model was developed and validated against the experimental results, and a modified sectional analysis method was proposed to evaluate the flexural capacity of the jointed composite beams. The experimental results show that the post-cast joint has limited influence on the cracking load but substantially reduces the post-cracking and yield stiffness. Nevertheless, the jointed composite beam retains a comparable ultimate load to the reference beam. The UHS-ECC formwork remains effective in crack control despite the joint-induced discontinuity, maintaining smaller maximum crack widths and a more uniform crack distribution throughout loading. The finite element analysis reveals that the post-cast joint interrupts uniform tensile stress transfer along the bottom formwork, yet does not cause abrupt loss of composite action. The modified sectional analysis method, which accounts for joint-induced nonuniform FRP stress development, yields predictions in reasonable agreement with the test results. These findings demonstrate the feasibility of employing jointed FRP-reinforced UHS-ECC permanent formwork in segmented construction and provide a practical basis for its flexural design.