Mechanisms of Mechanical Degradation and Optimization Design of Fiber-Reinforced Polymer (FRP) Formwork Incorporating Recycled Construction Waste Under Complex Constraints

FRP formwork containing recycled construction waste may reduce material consumption, but recycled particles can introduce interfacial defects, weaken interlaminar load transfer, and accelerate stiffness loss under humid-heat exposure, eccentric loading, and variable boundary constraints. This study clarifies the degradation mechanism and develops an optimization design method for recycled-particle-modified FRP formwork. A coupled framework was established by linking recycled aggregate replacement ratio, particle size, water absorption, fiber volume fraction, number of plies, formwork thickness, adhesive-layer thickness, humid-heat cycles, eccentricity, distributed pressure, and boundary stiffness. Orthogonal experiments with 162 valid specimens were used to identify degradation states, calibrate damage evolution parameters, and validate a constraint-driven multi-objective optimization algorithm. The results show that increasing the recycled aggregate replacement ratio and humid-heat cycles jointly reduced elastic modulus retention, interlaminar shear resistance, and bending stiffness. When the replacement ratio exceeded 20% and humid-heat cycles exceeded 200, elastic modulus retention dropped below 0.65, indicating a critical transition in interfacial damage accumulation. After optimization, the replacement ratio was adjusted from 20% to 15.2%, the fiber volume fraction increased from 50% to 56.8%, and the ply number increased from 6 to 8. The optimized formwork achieved a 19.6% increase in bending stiffness, a 13.6% reduction in structural mass, a 16.7% reduction in interfacial shear stress, and a 20.5% reduction in maximum deflection. The main innovation lies in connecting recycled-particle-induced interface degradation with constrained structural optimization, thereby shifting recycled FRP formwork design from material feasibility assessment to service-reliability-oriented parameter control.