## 1 Introduction  
Recently, in densely populated earthquake regions, advanced FRPs are being increasingly used for the urgent retrofitting, rehabilitation and strengthening of damaged structures because of the advantages of increasing the ductility and strength of reinforced concrete (RC) columns wrapped with externally FRPs confinements. The increase in axial strength and strain of RC columns is the main reason for the attraction of the use of fiber-reinforced polymers (FRP) confinements. The repairing, strengthening and ductility enhancement of RC compression members is one of the most popular applications of the FRP composites in both corrosive and seismic environments [1]. The advantages of the FRP composites include corrosion resistance, lightweight, high strength to weight ratio, durability, chemical resistance, dimensional stability and electromagnetically neutral [2]. The factors affecting the FRP confinement are elastic modulus of FRP (\(E_{f}\)), thickness of FRPs (\(nt\)), angle of orientation of FRP wraps, and unconfined strength of concrete material (\(f^{\prime}_{co}\)) [3]. Nowadays, FRP confinement has become an advanced technology for increasing the efficiency and strength of structural concrete [4]. FRPs are preferred over conventional steel jacketing due to their easy handling, slight disturbance of concrete members, easy installation and less time for construction [5]. The strengthening technique, which is influenced by the orientation, geometry, and shape of the concrete member, is also very important for increasing the capacity of structural members [6; 7]. The strength of confined concrete is reduced when the fibers are used at an angle between the longitudinal and transverse directions of concrete members [8]. The damaged concrete structures due to earthquakes lose their serviceability and strength and thus, they need retrofitting and rehabilitation to enhance strength and ductility [9]. The high corrosion resistance properties of FRPs make them suitable for being used in marine and coastal structures [10]. The applications of FRPs are not limited to concrete building members but also in the bridge priors to enhance their strength by lateral confinement [11; 12; 13]. In fact, the increase in strength and ductility of concrete members after FRPs wrapping is due to the confinement and prevention of the lateral expansion of concrete material. This effect of confinement is more dominant in case of triaxial state because the concrete would be subjected to multiaxial loads when FRP wrapping resists lateral expansion [14]. Various researches have been performed in the literature for predicting the axial strain (\(e^{\prime}_{cc}\)) of externally confined concrete members based on different techniques [15; 16; 17; 18; 19; 20; 21; 22]. Based on the concept of the linear relationship between the confinement pressure and strain, the strain models were proposed [17; 22]. [PERSON] and [PERSON] [20] proposed the strain model based on the concept of dilation behavior and volume strain of concrete. In addition, the strain model proposed by [PERSON] et al. [16] uses the concept of the energy balance method according to which the additional strain energy of confined concrete is equal to energy required to break the FRP hoops. It was observed from the literature that most of the studies focused on the predictions of axial compressive strength of confined concrete members but, fewer models were proposed for axial compressive strain. For a designer, the enhancement of axial compressive strain is very important for concrete structural elements after addition of the FRPs. The previously proposed empirical models poorly predict the strains of confined concrete members as compared with the experimental predictions [23]. Moreover, the mean absolute error (MAE) of prediction of confined concrete strength is less than 20% while, that for confined concrete strains ranges from 35% to 250%. Thus, the literature shows excellent performance for the strength models but not good for the predictions of corresponding strains.