Durability and Mechanical Behaviour of Recycled PET-Modified Polyester Resin and GFRP Bars under Aggressive Environments

The development and experimental evaluation of low-environmental-impact Glass Fiber Reinforced Polymer (GFRP) bars manufactured using an orthophthalic polyester resin modified with 20% by weight recycled polyethylene terephthalate (rPET) are presented. The combination of conventional orthophthalic polyester with recycled PET offers a sustainable and effective alternative to traditional vinylester and epoxy systems for structural applications. The experimental programme comprised three complementary investigations. First, tensile tests were performed on resin dog-bone specimens after 1000 hours of conditioning in distilled water, alkaline solution, and saline solution to evaluate the residual mechanical properties of five thermosetting resin systems. Second, tensile tests were carried out on unconditioned pultruded GFRP bars to determine their mechanical properties. Finally, three-point bending tests were performed on reference and environmentally conditioned GFRP bars coated with orthophthalic polyester (OP) + rPET, vinylester, and epoxy resin systems to increase the environmental resistance of the modified polyester matrix. The OP+rPET resin achieved a tensile strength of 48.7 MPa, an elastic modulus of 3.4 GPa, and an ultimate strain of 2.86% in the unconditioned state. The corresponding GFRP bars exhibited a tensile strength of 689 MPa and an elastic modulus of 62.2 GPa, while the OP+rPET-coated bars showed reductions in flexural strength of 13.6% and 27.1% after alkaline and saline conditioning, respectively. These results confirm the potential of the OP+rPET system as a sustainable alternative for GFRP reinforcement in concrete structures.