Structural Behavior and Design of Recycled LDPE-FRP Hybrid Beams for Bridge Collision Protection

The paper summarizes results of a pilot study that evaluates both the service and ultimate short-term behavior of hybrid beams made of recycled low-density polyethylene (LDPE) plastic matrix and reinforced with fiber-reinforced polymer (FRP) composite rebars. The primary objective of this study is to explore the feasibility of utilizing such hybrid beams for construction applications, including collision protection systems for bridge piers, bridge pier collision protection, and other secondary structural uses. The experimental program comprised two phases. The first phase focused on materials’ characterization of both the LDPE matrix and FRP reinforcement material. The second phase of the experimental program involved full-scale flexural testing of different LDPE-FRP hybrid beams. Two groups of hybrid beams, each with a different FRP reinforcement configuration, were evaluated experimentally. For each group, three identical hybrid beans were tested. Two beams within each group were loaded to their maximum flexural capacity, controlled by midspan deflection, while the others were subjected to multiple loading/unloading cycles followed by an increasing monotonic loading regime until the maximum capacity was reached. Experimental results indicated that the hybrid beams exhibited linear behavior up to 80% of the ultimate load, followed by a nonlinear response to maximum load. For all beam specimens, no cracks or failures within the LDPE matrix were observed. However, slippage of the FRP rebars, particularly in the top compressive reinforcements, was the dominant mode of local failure that caused a significant reduction in stiffness. In addition, a closed-form analytical model, based on strain compatibility and force equilibrium, was developed to predict the flexural behavior of such hybrid beams. Using the proposed model, a good agreement between the experimental and theoretical results was achieved.