FRPs have exceptional properties, including light weight, high strength, electrical insulation, low thermal conductivity, impact resistance, dimensional stability, corrosion resistance, and they are non-magnetic [5,6,7,8]. Due to the significant advantages of FRPs over conventional construction materials, including steel and concrete, for retrofitting and strengthening concrete structures, they have gained attention as viable alternatives for reinforcing and retrofitting concrete structures [8,9,10]. These composites are typically employed as “externally bonded” systems to increase the axial sectional, flexural, torsion, and shear capacities of the structural elements of the reinforced concrete, increase the structural members’ stability and serviceability, and provide additional confinement [11,12,13]. Two distinct types of strengthening methods for FRP-reinforced concrete (FRP-RC) are presented: the first makes use of FRP sheets and/or plates and the second uses near-surface mounted (NSM) bars [14]. To prepare the external surface of the concrete for FRP plates and sheets, either high-pressure jet washing or sandblasting is used. Following that, FRP products are used on the concrete surface [15,16,17,18]. This form of external reinforcement is simple and quick to implement. Three types of FRP reinforcements are available for new structures: (1) internal reinforcement with FRP bars; (2) FRP formwork for RC members that stays in place; and (3) FRP tendons for prestressed concrete (PC) components [19,20,21].

Due to concerns about the performance of FRPs at high temperatures, the widespread application of FRP-RC in structures has been limited [22,23,24]. In general, prolonged contact with temperatures around and above the glass transition temperature (T_g) of the resin degrades the mechanical properties of FRP materials [25]. T_g is normally represented by a single T_g (often between 50 and 120 °C for resins cured at ambient temperatures), which can be evaluated experimentally using differential scanning calorimetry (DSC) or determined by dynamic mechanical analyses (DMA) [17,26]. In design decision-making, T_g is frequently used as a “critical temperature”, although mechanical performance degrades prior to reaching T_g [27]. When subjected to elevated temperatures (usually greater than 300–400 °C), the thermal decomposition of the FRP organic matrix occurs, potentially emitting smoke, soot, toxic/combustible volatiles, and heat [7,28,29,30]. Organic fibres (e.g., biofibres, PBO, and aramid) employed to strengthen some polymer composites may also degrade and form smoke, fumes, and heat [31,32]. These decomposition processes often result in the further deterioration of the physical and mechanical properties of FRPs due to the degradation of the matrix and, in certain circumstances, the fibres [6,33,34]. Deuring [35] is one of the leading researchers conducting fire tests on externally reinforced concrete beams. According to Deuring’s report, unprotected beams that were strengthened with FRP could withstand a fire for 81 min. In comparison, a similar beam with protected FRP systems could withstand a fire for 146 min. Williams et al. [36] conducted a more recent investigation, in which they tested the performance of CFRP-strengthened RC T-beams under normal fire conditions. The beams were insulated with vermiculite gypsum (VG) insulation. The findings of this experiment revealed that FRP and reinforcing steel components can be kept at a temperature below the critical value necessary to retain their structural integrity by using a suitably insulated system.

2. Mechanical Properties of Individual Components at Elevated Temperature

2.1. FRP

FRPs exhibit significantly different behaviours from steel or concrete at high temperatures. When exposed to a substantial amount of heat, all polymer matrix composites will burn. Additionally, matrix elements, such as epoxy, vinylester, and polyester, not only facilitate burning but also produce huge amounts of dense black smoke [37]. Furthermore, FRPs degrade in terms of stiffness, strength, and bond characteristics when exposed to even mildly elevated temperatures [38,39]. Numerous research investigations focusing on the mechanical characteristics of FRPs and their constituents at elevated temperatures have been published in the literature [40,41,42,43,44].

When elevated temperatures below T_g are applied to the resin matrix, the resin matrix remains relatively unaffected (i.e., some microcracks may form) and the surface of the resin matrix remains rough and similar to that of the unconditioned sample [45]. In this situation, no significant changes occur in the strength or stiffness of the FRP composites. When FRP composites reach their T_g, the resin undergoes a phase transition from glassy to rubbery. In this instance, the FRP materials soften and creep, resulting in a significant loss of stiffness and strength [46]. It has been observed that when FRP materials are subjected to temperatures that are near the resin’s decomposition temperature, their organic matrix decomposes, which emits heat, soot, smoke, and hazardous volatiles. Exposure to such high temperatures (e.g., 300–500 °C) causes the breakage of the modular chains of the resin, chemical bonds, and bonds between the fibres [47,48]. At higher temperatures, the composites ignite and burn. The critical temperature (i.e., the temperature at which 50% of the strength is lost) was reported to be typically between 87–90 °C for pultruded GFRP profiles in compression, 300–330 °C for FRP reinforcing bars in tension, 180–250 °C for laminates in bending, and 200–300 °C for laminates in tension [49]. Figure 1 shows the reported critical temperatures in the literature (i.e., the temperatures that are equivalent to an approximately 50% reduction in mechanical properties) for different FRP composites under different loading conditions.

Figure 1. The FRP strength retention versus critical temperature as reported in the literature [50]: (a) FRP bars; (b) FRP laminates; (c) FRP laminates; and (d) pultruded FRP profiles.

The compression and interlaminar shear failure of FRP composites occurs at substantially lower loads and temperatures than flexure and tension [50]. Elevated temperatures have a lesser effect on the elastic modulus of FRP composites than on the corresponding strength values. This is mostly due to the fact that the elastic modulus of FRP composites is more closely related to the elastic modulus of the fibres than to the elastic modulus of the resin [49].

A few of the parameters that influence the properties of FRPs are the configuration of fibres and resin, the production technique, and the quality control of the final products. The stiffness and strength properties of FRPs decrease with increasing temperature, although there are considerable variations in the results due to the wide variety of fibre volume fractions, formulations of the matrix, and fibre orientations represented in the data [33,51].

2.2. Concrete

While concrete generally has a high resistance to fire, its mechanical characteristics, such as elastic modulus and strength, degrade when exposed to high temperatures. At elevated temperatures, the failure is mostly due to the creation of cracks that are parallel to the heated surface, changes in the chemistry, and an increase in pore pressure owing to water evaporation. At elevated temperatures, concrete undergoes a variety of physical (vapor diffusion, evaporation, phase expansion, and condensation), chemical (dehydration and thermo-chemical damage), and mechanical (cracking, spalling, and thermo-mechanical damage) phenomena that degrade its qualities [52,53]. The water on the surface of the concrete and capillary water evaporates as the temperature rises and this process is hastened by the reduced cohesive interactions between the water molecules, which is caused by water expansion. At 105 °C, the free water begins to evaporate rapidly. The dehydration of ettringite occurs between 80 and 150 °C, followed by gypsum decomposition between 150 and 170 °C. When the temperature approaches 300 °C, the evaporation of the chemically bound water begins, which reduces the compressive strength of concrete [31,36,54,55,56,57]. Portlandite decomposes between 400 and 540 °C as the temperature increases further. When the temperature of the concrete exceeds 400 °C, the strength of the concrete deteriorates more rapidly due to the breakdown of calcium–silica–hydrate (C–S–H). Between 600 and 800 °C, the second phase of the C–S–H decomposes to create β-dicalcium silicate (β-C₂S). At 900 °C, the C–S–H fully degrades. As a result, the critical temperature range for concrete is around 400–900 °C and concrete loses the majority of its strength within this range [58].