# Making a Case for Hybrid GFRP-Steel Reinforcement System in Concrete Beams: An Overview
## 1 Introduction  
The durability of a reinforced concrete (RC) structure is associated with many factors [1; 2; 3; 4], and most of them are related to the corrosion of steel reinforcement bars. This is a general problem that can affect the integrity of assets built to last for extended service life periods. Studies on the durability of steel embedded in concrete structures now generate a lot of interest, as exemplified in recent publications [5; 6; 7; 8; 9; 10]. Numerous structures have been examined for their structural integrities, and corrosion is identified as a significant threat affecting these assets' durability [11; 12; 13; 14].  
Glass Fibre Reinforced Polymer (GFRP) bars as an alternative to steel reinforcement in concrete structures have been receiving increased attention over the past two decades [15; 16; 17; 18; 19]. Due to its inherent corrosion resistance, GFRP has gained a reputation [20; 21; 22] as an alternative to steel in RCs to meet the requirements of durability in aggressive environments. GFRP is known to exhibit high tensile strength, typically about 1100 MPa, compared to steel, generally considered to be about 400 MPa. However, GFRP is not known to exhibit the classic plastic yielding known in steel before failing. Concrete structures reinforced with GFRPs may show little evidence of overload-cracking-development and excessive deformation until beyond their ultimate limit state due to the lack of plasticity in the reinforcement elements [23; 24; 25]. The established design philosophy in RC structures relied on steel reinforcement to provide sufficient ductility to prevent concrete elements from unpredictable failure by leveraging on the elastic deformation and the plasticity of steel. On the other hand, the lower elasticity of GFRP allows for larger elastic deformation but offers no protection from brittle failure. The limited intrinsic ductility in GFRP leads to designing flexure beams with over-reinforced character and conservative safety coefficients.  
Though the latest design standards, such as ACI-440.11-22 put forward methods to design GFRP-only reinforced structures, the lack of discernible plasticity in GFRP itself remains a concern on the suitability of GFRP rebars as a complete replacement for the steel rebars in concrete. The designers' reticence to specify over-reinforced structures still places steel as favourable to GFRP [26; 27; 28]. Hybrid reinforcement concepts of GFRP and steel reinforcement where steel confers ductility and GFRP provide strength is the concept being proposed [29; 30; 31] to address the possibility of brittle failures in RCs with complete replacement of steel with GFRP. In this arrangement, GFRP takes the primary reinforcement role and governs the ultimate state whilst the steel reinforcement controls the failure mode [32; 33; 34; 35; 36; 37; 38]. Furthermore, the steel bars are located more interiorly in the RC element to ensure sufficient concrete cover protection without increasing the total depth of the beam whilst sufficiently governing the required ductile failure mode.  
Although the hybrid GFRP-steel RC beams have many advantages, their reinforcement arrangements must be optimal to leverage strength and ductility. Their structural behaviour also needs to be understood to achieve a safe and accurate design for construction. Thus, many investigations [39; 40; 41; 42; 43] have considered the various aspects of hybrid GFRP-steel RC beams and proposed different design models to identify optimal reinforcement arrangements. This paper reviews the related analytical models that assess the behaviour of hybrid beams under various mechanical and physical effects. The review outlines the challenges and the need for further research for the safe and reliable application of hybrid GFRP-steel RC beams.