PSI - Issue 83

Wijdane Kachach et al. / Procedia Structural Integrity 83 (2026) 154–161

159

reduction of strength were mirrored in the reduction in stiffness/modulus in both cases, thereby again illustrating that this particular degradation process occurs in all FRP materials. The bending strength and rigidity of FRP-strengthened beams decrease proportionately depending on the temperature and duration of exposure, as indicated in the analysis made by Saafi in (Saafi, 2002). The decrease occurs much faster in the case of FRP compared to the usual steel reinforcement. Based on the research by (Protchenko, 2022) the tensile strength of the FRP bars has been seen to reduce drastically by as much as 30-80% when the temperature range goes between 400-500°C. The glass transition temperature, denoted as Tg, which falls in the range of 100-180°C, was reported in research to influence the important threshold value that, when exceeded, translates to the degradation of the structure due to the softening process of the resin material. This behavior was supported by experimental data by (Sharifianjazi et al., 2022), which showed that the residual bond strength between FRP and concrete dropped to just 17% of its ambient value at temperatures slightly over Tg (around 150 °C). The composite interaction between FRP and concrete was shown to completely disappear with increased heating or longer exposure times. Additionally, fire severely damages compressive qualities. According to (Demir et al., n.d.), fire-treated FRP-confined concrete demonstrated deterioration in both modulus and compressive strength, as well as increased slippage and early, brittle failure modes. These results demonstrate that the anticipated ductility and load-sustaining benefits of FRP confinement are undermined by the quick decline in confinement benefits at fire-level temperatures. Crucially, mechanical deterioration does not follow a straight path. A rapid initial decrease immediately above Tg, a plateau phase with modest property loss, and a near-complete collapse of strength and stiffness at higher temperatures (>400°C) are the usual stages of the process, as noted by (Protchenko, 2022). Although they cannot completely stop it, protective measures like thicker concrete coverings can slow this trajectory. For example, GFRP-reinforced beams were able to withstand about 90 minutes of ASTM E119 fire exposure when the concrete cover was increased to 70 mm; yet, throughout this time, significant losses in stiffness and flexural strength still transpired. This means that the bond strength can decrease by as much as 90 percent as soon as the temperature of the resin exceeds the glass transition temperature by a significant amount, as this can cause a quick delamination of the composite along with the loss of composite activity, as stated in research by (Ahmed & Kodur, 2011). The limit state resulting from this breakdown process due to the bond has proven to be the most important one when the composite material has been exposed to the temperature conditions. The damage to the residual properties still continues even when the fire has been extinguished. Despite the provision of beneficial protective conditions, like increased concrete cover or the provision of insulation, the mechanical as well as the bonding properties were still found to decrease significantly from the original values, as revealed by (Protchenko, 2022).

Fig. 3. Variation in the fire resistance with respect to the concrete cover thickness (Van Doan & Van Cao, 2025)