PSI - Issue 83

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

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1. Introduction Fiber-Reinforced Polymers, commonly abbreviated as FRP, has been revealed as a potential alternate material to steels in concrete structures, as it has outstanding tensile strength, lightness, as well as resistance to corrosion. Despite the availability of this outstanding property, there has been one major issue in the widespread use of this material in concrete structures of buildings, which includes the inability of the material to withstand extreme temperatures. The polymer material composition in the FRP starts melting at lower temperatures compared to the melting temperature of steels, thereby reducing the adhesion between the concrete structures and the FRP material. On the onset of the influence of fire, there has been a progressive but drastic reduction in the key mechanical properties in the FRP concrete structures. Verification has confirmed that beyond the glass transition temperature (Tg) of the resin material, there has been a drastic reduction in the tensile strength, modulus of elasticity, as well as stiffness. The reduction in properties isn't linear but normally takes place following the drastic reduction in the early stages after surpassing the temperature Tg, thereby resulting in the loss of strength at temperatures above 400°C.What's most important, the interfacial strength between the concrete and the FRP, which helps create the composite, can degrade by as much as 90% when the resin becomes soft. This paper brings together the evidence from current research into the post-fire response of concrete reinforced with FRP materials. The process of thermal degradation, the progressive loss of property over time, as well as the current level of effectiveness in existing countermeasures, such as concrete cover, can now be evaluated. The facts show that, while it might be possible to calculate the fire resistance period of FRP concrete, the long-term integrity of structures could be significantly threatened as a result of a fire, as the structure’s ultimate strength could well be reduced as a consequence. 2. Progressive Reduction of Mechanical Properties (Stiffness, Modulus, and Strength) One of the most important issues in evaluating post-fire performance is the decrease in mechanical characteristics of FRP reinforcement at high temperatures. When CFRP rebars are exposed to temperatures over around 300 °C, they drastically lose their tensile strength and elastic modulus, as shown by (Yun et al., 2025)(Figure 1)n (Table 1). There have been reports of stiffness decreases happening concurrently with strength declines of up to 50%. The polymer matrix's glass transition (Tg) and decomposition (Td) temperatures, where fiber oxidation, resin breakdown, and fiber– matrix debonding accelerate, were associated with the most noticeable degradation. The residual load-carrying capability of FRP-reinforced parts is directly compromised by this loss of modulus and strength.

Fig. 1. Experimental results versus predicted results for retention tensile strength ratio (Yun et al., 2025)