This study investigates the development of sustainable polymer composites for structural strengthening by incorporating waste glass fibers and natural fibers (flax and hemp) into an epoxy matrix, in response to the growing environmental concerns. Mechanical, thermal, and durability-related properties were evaluated through tensile testing, dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), water absorption, and water immersion aging tests. Results showed that incorporating waste glass fibers enhanced the tensile strength and thermal decomposition temperature by 88% and 5.4%, respectively, compared to composites reinforced with solely natural fibers. Water absorption tests indicated that waste glass fiber-reinforced hybrid composites exhibited lower water uptake than flax and hemp fiber-reinforced composites. After water immersion, the tensile strength loss was recorded as 22, 25, and 8.5% for the composites reinforced with hemp, flax, and waste glass fiber, respectively. The findings confirm that incorporating waste glass fibers into natural fiber composites effectively mitigates moisture sensitivity and improves mechanical performance. Hybridizing flax and hemp fibers with waste glass fibers provides a practical and sustainable approach to enhancing composite performance, making them a viable alternative for strengthening reinforced concrete structures requiring long-term resistance. The recycled waste glass fibers employed in this study offered comparable mechanical performance while drastically lowering raw material consumption and environmental impact, in contrast to virgin glass fibers frequently used in earlier investigations. This demonstrates how recycling-oriented composite design can provide both sustainability and performance benefits.

Strengthening deficient structural reinforced concrete (RC) elements by using externally bonded carbon fiber-reinforced polymer (CFRP) has become a popular method because of CFRP’s superior mechanical properties. However, previous studies have shown that sudden failure due to debonding is the major problem for CFRP-strengthened RC members, especially for the deficient beams. Debonding of CFRP results in sudden fracture of the structural element, and most studies show that it is not easy to predict debonding of CFRP from strain values. In cases that whole surface of the RC member is covered with CFRP strips and U-jacketing is preferred, it is impossible to see the cracks on the surface that may lead to failure of the RC member. Acoustic emission (AE) is known to be a useful method for crack monitoring of an element under load. In this study, in order to understand the behavior of a CFRP U-plate-strengthened RC beam, a specimen strengthened with CFRP U-plate against shear was tested under cyclic loa...