# Making a Case for Hybrid GFRP-Steel Reinforcement System in Concrete Beams: An Overview
## 2 Background
### Fibre Reinforced Polymers (FRP) as Alternative Reinforcement in RC Beams  
The established corrosion protection methods, such as epoxy-coating, galvanic coating, and anodic protection that are currently used in the industry only act as temporary solutions and sometimes can be expensive and impractical. A more durable approach may be to use Fibre Reinforced Polymer (FRP) instead of steel. The earliest use of FRP composite materials in the construction industry dates back to the 1970s when they were used to rehabilitate a bridge girder in Japan [55]. It was followed by many rehabilitations as well as projects where FRP bars in RC structures were successfully implemented [56; 57; 58]. Several studies [59; 60; 61] are currently in progress to further the development of FRP applications in RC beams. These studies focus on advancing the utilisation of FRP for retrofitting and new build purposes. The early use of Carbon Fibre Reinforced Polymers (CFRP) demonstrated its potential and became prevalent in the retrofitting industry [62; 63]. The success fuelled the demand for CFRP, but due to the limited supply, the commercial price of CFRPs increased substantially. As a result, its use in a large volume was no longer economically feasible. The strength of carbon fibres and other engineering fibres for FRPs are shown in Figure 2. The high strength of carbon fibres as depicted in Figure 2 made it the material of choice in FRP design for structural applications, and CFRP is generally a highly valued engineering composite. However, the relatively high price of carbon fibres (Figure 2) compared to the other types of engineering fibres for FRP made CFRP too costly to be viable as a construction material. Figure 2 shows the relative costs of engineering fibres and thus indicates that CFPRs could be 10 to 30 times more expensive than GFRPs another lower-priced fibres with comparative tensile strength. Apart from carbon fibres, alternative FRPs are made from glass- (GFRP), aramid-(AFRP), and basalt (BFRP) fibres. The composite property of FRP is inherently dependent on the intrinsic properties of the reinforcing fibres. The primary mechanical properties of strength and modulus versus density are shown in Figures 3 and 4. It is shown that Aramid (Kevlar) fibres have the best tensile strength to the density ratio and show good resistance to most types of chemicals. However, they are sensitive to humidity and UV light and  
Figure 1: Schematic illustration of corrosion attack path in RC [48]. Reproduced with permission from Elsevier 2023.  
degrade when exposed to several acids and alkalis [64]. Their highly variable value of Young's modulus (Figure 4) made it not suitable for FRP in construction materials. Basalt fibres, manufactured by melting weathered volcanic lava, are characterised by high strength and high values of Young's modulus and have the highest thermal resistance amongst the engineering fibres, with a melting temperature of 1450 \({}^{\circ}\)C. On the other hand, basalt fibres are more vulnerable to alkaline effects and are rarely used in practical engineering FRPs [65]. Figure 3: Tensile strength versus density (Generated by CES EduPack, 2015). Figure 2: Tensile Strength versus Price (Generated by CES EduPack, 2015). Glass fibres with high strength and modest Young's modulus are the most commonly used fibres for commercial construction amongst the FRP family. They can be produced cheaply and are applicable in engineering applications where the high stiffness of carbon fibres is not required [66]. Therefore, applying GFRP bars in construction is the most economically feasible. It is now receiving more attention as it represents a viable alternative to steel reinforcement in order to address corrosion-related durability issues in conventional concrete structures [67]. Table 1 gives an overview of the market's performance and appraisal of the commercially available FRPs (GFRP, CFRP and BFRP) variants. Several studies [68, 69, 70, 71, 72] have been conducted to study the durability of GFRP bars in harsh conditions.