Concrete structures have several advantages, including high strength, good corrosion resistance, and high resistance to deformation, especially suitable for high-rise buildings as it facilitates reduced column size [28–33]. Due to concerns about brittleness, these structures’ applications in building structures are still limited, but their ductility can be greatly improved by implementing sufficient lateral confinement. While a high-tensile strength steel tube can be used to confine the core concrete, the diameter-to-thickness ratio must be kept as small as possible to avoid a local buckling problem. In aggregate, this results in material waste, a relatively heavy structural weight, and a high cost [34–39]. The CFRP (carbon fiber reinforced polymer) material has a high strength-to-weight ratio, and has excellent durability and fatigue resistance, making it an effective material for enhancing the strength and deformability of core concrete [40–44]. However, sudden rupture failure is frequently observed, resulting in brittle failure. As a result, the CFRP sheets and steel tubes may complement one another and offer sufficient confinement for the FRP concrete structure.

Chen et al. [22] conducted axial compression tests on cylinders made of carbon fiber-reinforced polymer (CFRP) and recycled aggregate/natural aggregate concrete at three different sizes with diameters ranging from 150 to 300 mm. A potential size effect in CFRP-confined RAC was investigated, and it was concluded that the compressive behaviour of CFRP-confined RAC and NAC is similar compared across all specimen sizes analyzed, with the stiffness of the FRP confinement having a similar role in both types of concrete. In general, CFRP-confined RAC has a lower compressive strength and a greater ultimate axial strain than CFRP-confined NAC. This discrepancy does not imply that the use of recycled aggregate has a substantial influence on the efficacy of FRP confinement since the enhancement ratios for compressive strength and ultimate axial strain of the two types of concrete were fairly similar under the same confinement ratio. Rather than that, this discrepancy is considered to be mostly due to the RAC’s reduced strength and axial stiffness. Islam et al. [26] studied the lateral dilatation of CFRP constrained square concrete columns of 10 and 20 mm, constructed using RBA and RCA. Natural aggregates were used to replace recycled aggregates at various replacement rates, such as 15%, 30%, 60%, and 100%. One, two, and three layers of CFRP were used to reinforce the concrete. The maximum strength and compressibility of concrete built using RBA and RCA were both altered and in comparison to other replacement rates, such as 15%, 30%, and 60%, the CFRP confinement efficacy for concrete built with 100% RBA and RCA was determined to be lower.

A CFRP confined recycled brick aggregate (RBA) concrete was examined by Jiang et al. [27] where the replacement rate of RBA with natural aggregates was 15%, 30%, 60%, and 100%. It was concluded that CFRPs are useful to enhance the ultimate strength of the concrete when fabricated using unidirectional carbon fabric layers of 1, 2, and 3 were employed with epoxy resin. The effectiveness of the carbon FRP confinement was determined to be lower with the increase in replacement rates. Fig. 2 represents the different failure modes observed in the experiment where the different size of aggregate was used by different authors.

Gao et al. [45] conducted a systematic investigation of the axial compressive behaviour of glass and carbon fiber-reinforced polymer tubes encased in recycled aggregate concrete using recycled clay brick aggregate. Glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) tubes were fabricated utilizing unidirectional glass and carbon fabrics reinforced in epoxy resin with PVC pipes as moulds. The tubes were constructed using two, four, and six layers of GFRP and CFRP. The brick aggregates were replaced with natural aggregates at various replacement rates, including 50%, 70%, and 100%. The results indicate that the introduction of GFRP and CFRP tubes is favorable since the confinement of RAC with recycled clay brick aggregates (RCBA) enhanced the compressive strength almost 5.5 times while utilizing 6-layer GFRP tube. However, Fig. 3 represents a typical stress-strain curve for GFRP and CFRP composites and from the experimental results, it was determined that compared to GFRP tube concrete, CFRP tube confined concrete had a lower axial strain and a greater compressive strength. Fig. 4 shows the failed specimens of neat, CFRP and GFRP confined concretes. In the case of GFRP confined concretes, the wrapping failed at half the lateral strain obtained by the wrapped pre-tensioned jacket with three layers, signifying 100% fiber utilization whereas for CFRP confined concretes, the lateral and longitudinal stresses were about 10 times higher than in unconfined concrete [41].

FRP jackets allow concrete to achieve higher compressive strength and ultimate axial and lateral stresses, which helps prolong concrete crack propagation and avoid displacement of disaggregated concrete particles. Tensile tension needs to be established in the FRP confined concrete materials to maintain a high degree of confinement. This is why most studies have tried to incorporate the majority of fibres along the column member’s transverse direction to create tensile stress in the FRP material, which produces lateral confining pressure.

Chaiyasarn et al. [53] developed a novel, low-cost, high-performance hybrid FRP composite using basalt fabrics and E-glass fibers with resin to test its ability to increase the RAC’s strength and stiffness with CBA. Under axial compression tests on a set of 36 concrete cylinders, it was determined that recycling aggregate concrete using clay brick aggregates may be made stronger and more ductile using hybrid basalt/E-glass FRP composites. For recycled aggregate concrete containing clay brick aggregates, hybrid basalt/E-glass FRP composites are more effective at increasing ultimate strength and strain when using low-strength RAC than when using high-strength RAC. Using a filament winding procedure, Xia et al. [54] investigated unidirectional flax–basalt hybrid fiber reinforced polymer plates and tubes for their mechanical and compressive characteristics. In comparison to flax fiber-reinforced polymers, hybrid fiber–reinforced polymers demonstrated significantly improved tensile characteristics. The lateral–axial interaction between the hybrid fiber-reinforced polymer–confined cylinders is determined by the fiber layer configuration.

Fanggi et al. [55] investigated the effects of the compressive behaviour of concrete-filled double-skin tubular columns and two concrete-filled FRP tubes that were manufactured with S-Glass FRP tubes. The FRP tubes of a diameter of 152.5 mm and a height of 305 mm were made using S-Glass fiber layup process in the hoop direction and the filler’s strength in the unconfined concrete ranged from 82.4 to 96.2 MPa. Due to the dual impacts of FRP and steel confinement on the concrete, it was observed that the specimen with FRP and steel reinforcement demonstrate a higher degree of improvement than those confined by FRP tubes alone. Thus, researchers have established that specimens constructed of dual grade concrete display superior compressive strength than those constructed of single grade concrete [56].

Zhang et al. [57] conducted an experimental study on hybrid double-skin tubular columns filled with HSC that were subjected to axial compression and cyclic lateral loading over a relatively large column section to allow for reliable experimental modelling of real columns. However, when high strength concrete with a cylinder compressive strength of approximately 120 MPa is employed, hybrid DSTCs show good ductility, seismic resistance and higher moment capacity but a smaller deformation capacity. Cao et al. [58] reported the findings of an experimental investigation of the behaviour of circular concrete-filled steel tubes enclosed in FRP under axial compression, intending to develop a thorough knowledge of structural behaviour and valid theoretical models. The results of the tests indicated that the FRP wrap can significantly defer, the development of local buckling deformation in the steel tube. As a result, FRP confinement may significantly improve the load-carrying capacity and ductility of concrete-filled thin steel tubes. Also, the number of FRP layers and steel tube diameter are critical elements affecting the prestress of FRP concrete columns [59,60].

To evaluate the validity and performance of concrete confined with the hybridization of jute and polyester (FRP) composite sheets to utilize its superior properties, Wahab et al. [29] presented experimental and analytical results of square concrete columns confined with jute–polyester hybrid FRP composites. 32 plain, square concrete specimens were tested under monotonic axial compressive loads, all of which were wrapped with an external layer of a jute–polyester FRP composite. The results indicate that jute-based FRP confinement enhanced the strength, strain, and ductility of the specimen, whereas wrapping had a substantial influence on the low-strength specimens with a circular cross-section. While due to the substantial rupture strain of the outer polyester jacket on the high strength specimens, the outer polyester jacket dominated the post-peak stress-strain behaviour.

Yu et al. [61] presented a combined experimental and theoretical study on the behaviour of FRP-confined concrete-encased steel I-section columns. From the standard coupon tests, the elastic modulus, yield stress, and tensile strength average values are 218.1 GPa, 321.7 MPa, and 447.0 MPa, respectively. Under both concentric and eccentric stress, the steel I-section buckling as well restricted, and the concrete was well contained, resulting in a very ductile reaction. Experimental and computational analyses of axially loaded steel-reinforced concrete-filled GFRP tubular columns are presented by Xue et al. [62]. The specimens fail by the rupture of the GFRP tubes under hoop tension and only outward buckling was observed because of the presence of the concrete core and steel section. The numerical analysis was conducted, and two failure modes considered for the study were equivalent stress-strain relationship for the concrete core and bilinear stress-strain response for the FRP-confined concrete model.

It is known that the ultimate tensile strain of conventional FRP jackets is lower than that observed in tensile tests of laminates of non-hybrid FRP of the same material. Thus, there is scope to further enhance the performance of FRP-confined concrete [63]. Ribeiro et al. [30] concentrated on pure compression testing on small-scale plain concrete columns confining with 16 unidirectional interlayer hybrid composite combinations, making use of the hybrid effect and pseudo-ductility of this confining material. The FRP tensile strain at failure was determined to be inaccessible in situ with FRP jackets. However, hybridization showed that this drop in efficiency may be mitigated, if not removed. In fact, for a substantial number of hybrid combinations, the strain reduction factor was found to be more than one [31].

It is determined from the literature that, concrete-filled steel tubes are frequently utilized as columns in many structural applications, and a prevalent form of failure for such tubular columns is inelastic outward local buckling near the column end. The use of FRP in these steel structures as jackets/wraps to minimize such local buckling has been proposed by many researchers and they demonstrated to have promising outcomes in both retrofit/strengthening and new construction. As hybrid FRP columns are constantly changing through the use of fillers, steel tubes, and steel I sections, their primary objective for development is to offer lightweight structures. Table 2 gives a review of the existing hybrid FRP combinations from the literature.

Chakrabarti et al. [40] developed a nonlinear FE model for the study of plain and reinforced concrete column sections with circular and square cross-sections that are confined by FRP sheets. The proposed FE model was used to analyze the behaviour of small and large-sized FRP-wrapped concrete columns subjected to uniaxial compression loading. Following validation, a comprehensive parametric investigation was performed to quantify the influence of the FRP sheets’ thickness, stiffness, and fibre orientation, as well as the unconfined concrete strength, on the confinement and stiffness of the FRP reinforced columns. Additionally, the study findings provide an appropriate stress distribution in concrete and FRP, which assists in the understanding of the concrete columns’ confinement mechanism. Similar studies were carried out using CFRP-wrapped concrete cylinders [35]. The numerical analysis findings indicated that the compressive strength, stiffness, and ductility of the CFRP-wrapped concrete cylinders were significantly increased when compared to the unconfined concrete cylinders.

Zakir et al. [64] conducted the first experimental and analytical study to assess the effect of stiffening steel tubes on the confined concrete behaviour of square-shaped double-skin tubular columns. A total of 18 specimens were tested by varying steel stiffener properties such as quantity, configuration for a similar total stiffener-rib area, effective placement, and rib cross-section dimensions and shape, and it was discovered that stiffened steel tubes specimens had a significant increase in axial load capacity of about 52% with enhanced ductility compared to unstiffened specimens. Ibrahim et al. [65] developed a sophisticated three-dimensional finite element model to assess the structural performance of FRP-steel RC columns, taking into account the influence of various bond conditions between FRP bars and concrete. Since ANSYS does not directly model steel bar buckling, the solution was terminated after the lateral displacement reached a value equivalent to the maximum displacement attained during the experimental test before failure.

Under axial concentric and eccentric loadings, the structural behaviour of fiber-reinforced polymer (FRP) reinforced concrete columns confined by FRP tubes was investigated by Raza et al. [36]. A commercially available FE tool ABAQUS was used to construct FEA model of FRP-tube confined concrete columns with longitudinal reinforcements of CFRP and/or GFRP bars. It was determined that increasing the diameter of the column has a stronger influence on improving the ultimate strength of columns. When the diameter was increased to 350 mm and the GFRP reinforcement was increased from 1.2% to 2.2%, the ultimate strength increased up to 2157.44%. It can be concluded that, by using the 3D finite element model for FRP-confined concrete in compression, a very good agreement between experimental, analytical and numerical compressive stress-strain and lateral strain-axial strain curves can be reached [66–68].

Mohammadi et al. [69] studied large rupture strain FRPs which enhance the strength and ductility of concrete columns with the same mechanism as conventional FRPs by rendering passive lateral confinement pressure. The concrete damage plasticity module (CDPM) was calibrated in the FE model using the confinement pressure and stiffness approach where the active confinement pressure technique demonstrates that the secondary stiffness of the large rupture strain FRP jacket dominates the concrete plastic flow of FRP-confined concrete. Results demonstrated that the plastic dilation angle of the proposed FRP confined concrete changed about 3 times faster than that of conventional FRP confined concrete. The geometric factors are included in the estimation of plastic dilation angle, allowing for the development of a novel plastic dilation angle model for the FEA of non-circular FRP-confined concrete columns.

Incorrect strength prediction can be caused by inaccurate modelling of FRP reinforced concrete behavior [70]. As a result, sophisticated constitutive models capable of modelling well-suited stress-strain relation of FRP reinforced concrete with accurate failure models are required. The stress-strain model of FRPs-confined concrete was used by Ismail et al. [67] to estimate the lateral response of RC columns under axial and lateral loads. The adopted model was used to simulate the seismic response of RC-circular columns retrofitted with FRP jackets, but the simulation results showed that all stress-strain models could not accurately predict the columns’ ultimate lateral displacements. The stress-strain model used was altered to take into account the eccentricity ratio, and it was determined that it had a significant impact on the modelling of the seismic response of RC columns under combined bending and axial loadings.

Piscesa et al. [68] presented 3D nonlinear FEM for conventional steel stirrups and FRP-wrapped concrete columns. To solve the equilibrium forces in the global solution, the FEA employed the initial stiffness approach with “process modification”. The constitutive model is based on the plasticity formulation, which captures the constraining concrete specimen’s effective lateral modulus. This lateral modulus is calculated by measuring the primary incremental stresses and strains at the gauss points of each element. It was determined that the boundary condition, the dilatant behaviour of the constitutive model, and the Poisson’s ratio of the external confining device had a significant impact on the lateral modulus. Fig. 5 depicts the meshes of the finite element models for the specimens with circular ties as transverse reinforcement and spirals as transverse reinforcement.

Lo et al. [71] applied 2-D FE methodologies on the cross-sections of axially loaded FRP-confined rectangular concrete columns with no assumptions or constraints on the non-uniform and the failure under triaxial compression was defined by Eq. (1)(1) F(ξ,ρ,θ)=(1.5ρfc)2+m[ρ6fcr(θ,e)+ξ3fc]−c=0 where ξ represents hydrostatic length, ρ represents the deviatoric length and θ represents lode angle. The numerical findings for determining the effects of structural characteristics revealed that aspect ratio and corner roundness have little effect on yield strength and yield strain, but they may have a considerable effect on post-yield behaviour.

Strengthening reinforced concrete (RC) columns with external confining devices such as FRP wraps or steel tubes is widely used in construction. By using external confining devices, both the strength and ductility of RC columns are significantly improved. However, numerical modelling to predict the capacity of strengthened RC columns is limited and often oversimplified. Thus, the available FE models have been reviewed and the development of a sophisticated FE model to simulate accurate FRP and concrete interaction has been determined. An overview of various FE models developed has been presented in Table 3.