Long‐Term Durability Modeling of GFRP Rebar Under Natural and Accelerated Alkaline Conditioning Using Logarithmic Regression and Time–Temperature Superposition

ABSTRACT The long‐term durability of glass fiber reinforced polymer (GFRP) rebars was evaluated through prediction modeling based on tensile strength and tensile modulus retention under alkaline conditioning representative of concrete environments. GFRP rebars were fully immersed in alkaline solution under two exposure regimes: outdoor ambient conditioning at approximately 14°C for up to 10 years and accelerated conditioning at 60°C for up to 24 months. Tensile strength retention showed an approximately linear reduction with log(time), indicating progressive degradation under alkaline exposure. Time–temperature superposition (TTSP) analysis indicated that degradation at 60°C proceeded approximately 2–3 times faster than at 14°C, depending on the selected retention criterion. Using TTSP‐based modeling, service‐life projections at 27°C predicted tensile strength retention of approximately 62%–63% after 50 years, and 58% after 100 years. In contrast, tensile modulus retention remained largely stable, indicating limited loss of elastic stiffness. EDX analysis showed only minor surface compositional changes in the fibers, while T g results confirmed substantial retention of the resin properties. The combined mechanical, thermal, and microstructural results suggest that the observed tensile strength reduction is associated predominantly with progressive matrix and fiber–matrix interphase deterioration, while substantial degradation of the load‐bearing glass fibers was not evident under the investigated conditions.