Hygrothermal durability of fiber anchors for externally bonded CFRP strengthening of reinforced concrete members

Externally bonded carbon fiber-reinforced polymer (EB-CFRP) sheets are now widely used for strengthening and repair of reinforced concrete structures. To mitigate debonding failure, FRP sheets are often mechanically anchored to the substrate using fiber anchors, which have gained increased attention due to their material compatibility and ease of use with FRP strengthening systems. While the durability of FRP-concrete bond has been extensively studied, the durability of fiber anchors in typical bond-critical applications remains largely unexplored. To address this gap, this study investigated the durability of fiber anchors used in notched concrete beams strengthened with EB-CFRP sheets. The beam specimens, along with samples from constituent materials (CFRP, epoxy and fiber anchors) were subjected to hygrothermal conditioning (per ACI 440.9R-15), involving 3000 hours of water immersion at 50 ± 3 °C, and compared with a control group maintained under standard laboratory conditions (23 ± 3 °C and relative humidity of 50 ± 10%). The principal test variables were fiber anchor diameters of 6.4, 9.5, and 12.7 mm, CFRP bonded versus unbonded conditions, and the use of putty versus no putty. Results revealed that beams with 6.4-mm anchors retained 76% of their ultimate load capacity, whereas beams with 9.5- and 12.7-mm anchors exhibited no reduction in ultimate strength compared to the corresponding control beams. The CFRP coupons retained 84% of the original tensile strength, consistent with design guidelines, whereas no loss in interlaminar shear strength (ILSS) was observed for the cylindrical fiber anchor samples. The strength retention of anchored strengthened beams (e.g., 76% for 6.4 mm anchor) was significantly lower than that of CFRP sheet tensile strength retention (84%) and the ILSS of the fiber anchor samples (no loss), indicating that standard material characterization tests fail to capture the combined effects of complex multiaxial stress states and defect sensitivity present in beam-scale specimens.