GFRP-Reinforced Concrete Columns: State-of-the-Art, Behavior, and Research Needs

Abstract

This comprehensive review paper delves into the utilization of Glass Fiber-Reinforced Polymer (GFRP) composites within the realm of concrete column reinforcement, spotlighting the surge in structural engineering applications that leverage GFRP instead of traditional steel to circumvent the latter’s corrosion issues. Despite a significant corpus of research on GFRP-reinforced structural members, questions about their compression behavior persist, making it a focal area of this review. This study evaluates the properties of GFRP bars and their impact on the structural behavior of concrete columns, addressing variables such as concrete type and strength, cross-sectional geometry, slenderness ratio, and reinforcement specifics under varied loading protocols. With a dataset spanning over 250 publications from 1988 to 2024, our findings reveal a marked increase in research interest, particularly in regions like China, Canada, and the United States, highlighting GFRP’s potential as a cost-effective and durable alternative to steel. However, gaps in current knowledge, especially concerning Ultra-High-Performance Concrete (UHPC) reinforced with GFRP, underscore the necessity for targeted research. Additionally, the contribution of GFRP rebars to compressive column capacity ranges from 5% to 40%, but current design codes and standards underestimate this, necessitating new models and design provisions that accurately reflect GFRP’s compressive behavior. Moreover, this review identifies other critical areas for future exploration, including the influence of cross-sectional geometry on structural behavior, the application of GFRP in seismic resistance, and the evaluation of the size effect on column strength. Furthermore, the paper calls for advanced studies on the long-term durability of GFRP-reinforced structures under various environmental conditions, environmental and economic impacts of GFRP usage, and the potential of Artificial Intelligence (AI) and Machine Learning (ML) in predicting the performance of GFRP-reinforced columns. Addressing these research gaps is crucial for developing more resilient and sustainable concrete structures, particularly in seismic zones and harsh environmental conditions, and fostering advancements in structural engineering through the adoption of innovative, efficient construction practices.

1. Introduction

Reinforced Concrete (RC) columns are widely implemented in contemporary construction due to their strength, durability, and cost-effectiveness. However, conventional steel reinforcement used in RC columns has several limitations, such as susceptibility to corrosion and high density [1,2,3,4]. The superior mechanical and environmental resistance properties of fiber-reinforced polymer (FRP) composite bars have made them a promising alternative to conventional steel reinforcement [5,6,7,8]. Hence, FRP reinforcement has sparked considerable research interest [9,10,11,12,13,14,15,16,17,18]. The main components of FRP composites are the fibers, resin, and interface. Glass, basalt, aramid, and carbon fibers are usually utilized to make up the FRP reinforcement, i.e., Glass FRP (GFRP), Basalt FRP (BRFP), Aramid FRP (AFRP), and Carbon FRP (CFRP) [19]. Thermosetting and thermoplastic polymers are popular resin types, in which thermosetting polymers are more common for forming FRP bars [20]. Thermosetting resins can have four main groups: vinyl ester, Bisphenol-A fumarate polyester, Terephthalic polyester, and Isophthalic polyester; each has certain functionality appropriate to the intended application [21].

Due to the minimal cost of glass fibers, GFRP has been the most popular in construction applications than other FRPs [20,22,23,24]. Moreover, GFRP possesses several excellent properties. GFRP has high strength and stiffness, is lightweight, corrosion-resistant, non-conductive, and highly resistant to environmental degradation [25,26]. These properties make GFRP ideal for applications where weight reduction and high strength are essential, such as aerospace, automotive, and marine industries [27,28,29,30]. Also, the versatility of GFRP allows for the tuning of the properties to meet specific requirements [31,32,33,34]. Unlike other reinforcement materials, GFRP is exceptionally formable and can strengthen curved and unbalanced geometric structures [35]. Accordingly, GFRP is widely used for repairing and strengthening structures [36,37,38,39,40,41,42,43]. Moreover, due to the environmental resistance of GFRP, GFRP-reinforced structures usually have a prolonged life, especially in harsh environments [44]. Nonetheless, the properties and performance of the GFRP depend on several factors, such as the fiber volume fraction, length, orientation, and dispersion [45].

GFRP has been successful in reinforcing the structural elements in concrete structures, i.e., beams [46,47,48,49], slabs [50,51,52], and shear walls [53,54,55]. Recently, GFRP bars have also been employed as internal reinforcement (longitudinal and transverse) bars in concrete columns [49,56,57,58,59,60,61,62,63,64,65,66], as they can resist the axial and lateral loads until failure without excessive reduction in stiffness [67].

Recent advancements in GFRP-reinforced columns research have focused on enhancing their durability [37,68,69], stiffness [70], and seismic performance [56,58,71,72]. Regarding durability, GFRP reinforcement has demonstrated outstanding potential for application in offshore structures and artificial islands, presenting substantial environmental and economic benefits. Moreover, GFRP bars have been proven to steadily provide significant stiffness at significant strains, a vital aspect for columns [67]. On the other hand, GFRP may not be an excellent substitute for steel in earthquake-resistant columns due to its brittleness, as the inelastic behavior of RC structures is a significant contributing factor in absorbing seismic energy [58]. Recent studies have aimed to develop innovative connections between GFRP columns and other structural elements, such as beams [73] and foundations [74], to improve their seismic performance.

In this comprehensive review of GFRP-reinforced elements such as beams, slabs, shear walls, and columns, the database was meticulously curated from various scientific publications indexed in leading databases like Scopus. The selection criteria for including journals in this review were multifaceted, emphasizing not only the impact factor and disciplinary relevance but also the rigor of peer review processes to ensure the inclusion of high-quality, credible research. Priority was given to journals renowned for their contributions to civil engineering and materials science, ensuring that the selected studies are at the forefront of GFRP research. This methodological approach aimed to construct a robust database that reflects the breadth and depth of global advancements in the application of GFRP in construction over the 21st century, as summarized in Figure 1. The most significant number of publications was conveyed for GFRP-reinforced beams from 2001 to September 2024, followed by GFRP-reinforced columns. Moreover, the number of publications on GFRP-reinforced beams and columns has shown an overall increasing trend up to 2022, which is expected to be maintained in the future. A 28% and 10% increase in the publications for beams and columns, respectively, is achieved between 2021 and 2022. Conversely, the number of publications concerning GFRP-reinforced slabs and shear walls has almost been stable over the same period. These observations indicate the growing research interest in implementing GFRP reinforcement in concrete structural members, especially beams and columns. A breakdown of the number of publications reported for GFRP-reinforced columns by country and territory since the beginning of the century is shown in Figure 2, as GFRP-reinforced columns are the focus of this study. It should be noted that only the countries/territories that produced more than 10 publications are displayed in Figure 2. The analysis of the results indicates that most research efforts on GFRP-reinforced columns have been allocated in China, Canada, and the United States. Figure 3 further displays the breakdown of the publications counted for GFRP-reinforced columns by publication type. Articles, conference papers, and conference reviews have been the most publishable items on GFRP-reinforced columns, making up approximately 69, 24, and 5% of the total number of GFRP-reinforced columns’ publications, respectively. The illustrated results were acquired through the Scopus database [75]. In short, as observed, GFRP-reinforced columns have been the subject of numerous studies worldwide. Nevertheless, limited review papers have been published regarding GFRP-reinforced columns, e.g., [76].

In the evolving landscape of construction practices, this review positions itself at the intersection of established standards and pioneering research, focusing on the innovative application of GFRP in structural components. While codes such as ACI 440 provide foundational frameworks for the utilization of GFRP in elements like slabs, beams, columns, and walls, this examination ventures further by exploring the complex behaviors of GFRP-reinforced columns under a variety of conditions. The main objective is to assess the behavior of GFRP-reinforced columns under various conditions while identifying key research gaps, such as the performance of UHPC when reinforced with GFRP, the influence of cross-sectional geometry on structural behavior, the application of GFRP in seismic resistance, and the impact of column size on strength, areas not fully addressed by current standards. Additionally, long-term durability in varying environmental conditions and the environmental and economic implications of GFRP usage are identified as underexplored, while the potential for Artificial Intelligence (AI) and Machine Learning (ML) to predict GFRP-reinforced system performance is suggested as a future research direction. By methodically cataloging recent advancements and evaluating critical design factors, this review provides a deep understanding necessary for assessing and optimizing GFRP-reinforced structures. Ultimately, this work contributes to the sustainable advancement of construction methodologies by promoting GFRP as a key material for developing durable, cost-efficient, and eco-friendly structural solutions, offering updated insights to augment existing frameworks like ACI 440 and guiding researchers and practitioners toward adopting cutting-edge design practices.

2. Research Limitations

The limitations of this review can be summarized in several key areas. First, there is potential for selection bias due to the focus on Scopus-indexed publications, which may have excluded relevant studies from other databases or non-English literature. Additionally, the temporal range of studies (1988–2024) might not capture the most recent advancements or ongoing research. Geographically, most of the reviewed publications come from China, Canada, and the United States, potentially limiting the generalizability of findings to other regions. Moreover, while various variables like concrete type, geometry, and loading protocols are covered, complex interactions between these factors were not fully explored, reducing the depth of analysis. Another significant limitation lies in methodological variability among studies, with differing experimental methods potentially affecting the reliability of the conclusions. The reliance on secondary data without primary experimental research limits the review’s ability to offer novel insights or validate existing findings. Additionally, the review lacks a quantitative meta-analysis, relying instead on qualitative assessments, which constrains the ability to quantify the influence of different variables. Lastly, the review does not extensively examine the environmental and economic implications of GFRP use, focusing more on technical performance than on sustainability across the material’s life cycle. These limitations reflect the boundaries of the study and suggest areas for future investigation.

3. GFRP Rebar Properties

Several types are available for glass fibers, including Alkali glass (A-glass), Alkali Resistant glass (AR-glass), Chemical glass (C-glass), Dielectric glass (D-glass), Electrical glass (E-glass), Structural glass (S-glass), S-2-glass, and Reinforcement glass (R-glass). When it comes to the performance of GFRP in concrete reinforcement, the type of glass fiber used plays a crucial role in determining both the mechanical properties and durability of the composite [77]. In particular, A-glass fibers offer good chemical durability but are not specifically designed for high pH environments, making them less effective in concrete applications due to the risk of degradation from alkaline exposure. In contrast, AR-glass fibers are tailored to resist such conditions as a result of the addition of zirconium dioxide, which provides enhanced protection against alkaline attack. This makes AR-glass fibers particularly suitable for GFRP-reinforced concrete in chemically aggressive environments, ensuring long-term durability. C-glass fibers are known for their excellent resistance to chemical corrosion, especially in acidic environments. However, their relatively lower tensile strength compared to other glass types makes them less ideal for concrete reinforcement where high mechanical strength is a priority. D-glass fibers, on the other hand, are valued for their superior electrical insulation properties, making them more appropriate for applications that require both electrical insulation and moderate mechanical reinforcement, though they are rarely used in structural concrete reinforcement. E-glass fibers are the most widely used in GFRP due to their excellent balance of tensile strength, chemical resistance, and cost-effectiveness. However, they may be prone to degradation in highly alkaline environments unless treated for protection, though they remain highly versatile in various structural applications. For high-performance applications, S-glass fibers offer significantly higher tensile strength and modulus, making them more suitable for load-bearing GFRP-reinforced concrete structures, though they are more expensive than E-glass. A variant of S-glass, S-2 glass fibers provide even greater mechanical properties, including superior tensile strength and impact resistance, making them ideal for GFRP in high-load applications, such as seismic zones or other high-stress environments. Finally, R-glass fibers are specifically designed for reinforcement purposes, offering a balanced combination of mechanical strength and chemical resistance, making them suitable for both general and specialized structural applications.

Table 1. Physical, mechanical, thermal, and electrical properties of various GFRPs.

Glass Type	A	AR	C	D	E	S	S-2	R
Density (g/cm3)	2.44	2.70	2.54	2.12	2.56	2.48	2.46	2.54
Tensile Strength (GPa)	3.13	3.24	3.31	2.42	2.45	4.58	4.89	4.14
Modulus of Elasticity (GPa)	68.90	73.10	68.90	51.70	72.30	66.20	86.90	85.50
Elongation Break (%)	4.80	4.40	4.80	4.60	4.80	5.40	5.70	4.80
Refractive Index	1.54	1.56	1.55	1.47	1.56	1.53	1.52	1.55
Specific Heat Capacity (J/g$°$C)	0.80	-	0.79	0.73	0.81	0.74	0.74	0.73
Linear Temperature Expansion Coefficient ($\mathsf{\mu }\mathrm{m}/\mathrm{m}\mathrm{℃}$)	9.00	6.50	6.30	2.50	5.00	5.40	1.60	3.30
Thermal Expansion Coefficient	73.00	65.00	63.00	25.00	54.00	-	16.00	33.00
Dielectric Constant	6.20	8.10	6.90	3.80	6.20	5.22	5.30	6.40

presents the various mechanical, thermal, physical, and durability properties of different GFRPs. Figure 4 summarizes the key characteristics of GFRP materials. The following subsections discuss the physical, mechanical, thermal, and durability properties of GFRP composite bars.

3.1. Physical Properties

The use of GFRP is remarkably growing because they are made of naturally available materials and engineered materials produced from constituent substances possessing various physical properties such as density, rigidity, stiffness, and strength-to-weight ratio. GFRPs are great candidates for structural applications, as they possess low density and high strength [78]. Compared to steel, GFRP fibers are 80% lighter and boast a 3–7 times higher strength-to-weight ratio significantly reducing the weight of concrete columns [79]. This reduction in weight offers numerous practical advantages, including decreased transportation and lifting costs during construction, which can lead to overall project cost savings. Additionally, lighter columns exert less force on the foundation, allowing for less expensive and less complex foundation systems to be designed. This is particularly advantageous in areas with poor soil conditions or retrofitting and upgrading existing structures, where minimizing additional loads is crucial. Furthermore, the reduced weight without compromising structural integrity facilitates the construction of taller buildings and longer spans with reduced material use, contributing to more sustainable construction practices by minimizing resource consumption and environmental impact. The density of GFRP composites can be calculated from Equation (1).

$${\rho }_c=V_f{\rho }_f+V_m{\rho }_m$$

(1)

where ${\rho }_f$ and ${\rho }_m$ are the fiber’s density and the matrix’s density, respectively, and $V_f$ and $V_m$ are the fiber’s volume fraction and the matrix’s volume fraction, respectively.

Additionally, regarding the GFRP’s physical properties, [80] declared that the fiber content for GFRP bars was 74 to 76% to comply with the minimum requirements of ASTM D7957 [81] and AC454 [82]. Arslan [83] studied the effect of adding glass fibers to concrete to form Glass Fiber-Reinforced Concrete (GFRC). It was reported that adding 1.0 kg/m³ of glass fibers increased the fracture energy by 35% compared to the control plain concrete specimen.

3.2. Mechanical Properties

The mechanical properties of GFRP are typically evaluated using various testing methods, such as tensile, compressive, shear, and flexural testing [84,85]. The results of these tests are employed to determine the modulus of elasticity, ultimate tensile strength, and yield strength of the material, among other properties. Advanced methods such as acoustic emission, digital image correlation, and non-destructive testing offer significant advantages in evaluating GFRP’s mechanical behavior [86]. Acoustic emission enables real-time monitoring of stress and potential micro-cracks within the material. Digital image correlation allows precise tracking of surface deformations, providing detailed strain distribution data under load. Non-destructive testing methods, on the other hand, allow for the inspection of internal defects without damaging the material. Together, these methods provide early detection of failure mechanisms, helping predict long-term performance and enhancing safety assessments.

GFRP reinforcement has a significantly higher tensile strength and stiffness-to-weight ratio than traditional steel reinforcement, making it an ideal choice for concrete structures that require high strength and durability [87]. GFRP, with a tensile strength approximately 5–6 times higher than steel [20], positions itself as an exceptional material for reinforcing concrete, particularly in scenarios where tensile stresses dominate. This significant tensile strength enhancement enables columns reinforced with GFRP to exhibit a marked increase in resistance to cracking, thereby ensuring superior structural performance under loads. The material’s intrinsic ability to bear higher tensile stresses without yielding contributes directly to its crack resistance, delaying the onset of crack propagation and reducing crack widths compared to traditional steel reinforcement. However, it is essential to consider that GFRP’s lower modulus of elasticity, compared to steel, implies a different deformation behavior under load. While this lower modulus results in higher flexibility, which contributes to the material’s capacity to absorb energy and resist crack formation, it also suggests a potential for increased buckling susceptibility under compressive forces [88]. This characteristic necessitates careful design considerations to optimize the benefits of GFRP’s high tensile strength while mitigating the challenges posed by its lower stiffness, ensuring the structural integrity and longevity of GFRP-reinforced concrete elements. Also, GFRP is less ductile than steel due to its elastic behavior. This means that GFRP, unlike steel, does not deform plastically until failure. Although carbon fibers have superior mechanical characteristics, glass fibers have been more commonly used because of their reasonable cost [89]. The low compressive strength of GFRP compared to steel is a key reason why it is less commonly used in compression zones. Standards and codes like CSA S806-12 [90] and ACI 440.11-22 [91] exclude GFRP’s contribution under compression because GFRP’s fiber structure, optimized for tension, does not perform well under compressive loads. In structural design, this exclusion impacts the reinforcement strategy, often requiring the use of steel or other materials in compression zones to ensure adequate load-bearing capacity and stability, particularly in columns under axial loads. This limits GFRP’s application primarily to tensile reinforcement.

Also, GFRP has low compressive strength relative to steel. Hence, CSA S806-12 [90] guidelines do not account for FRP contribution for reinforcing the compression zone of structural members under flexural or axial compression loading. Also, ACI 440.11-22 [91] neglects the contribution of GFRP rebars under compression.

Numerous studies have been conducted to study the influence of GFRP material on the mechanical aspects of concrete structural members. Dehghan et al. [92] acknowledged that adding recycled GFRP fibers to concrete increased the splitting tensile strength, with almost all the substitution percentages by weight of the coarse aggregates, compared to the control mix. Moreover, several studies revealed that tensile strength improvement is present regardless of the fiber length [93,94]. However, longer fibers improved tensile strength than short fibers [95].

Furthermore, recent advancements in the application of GFRP for reinforcing and retrofitting concrete structures have demonstrated significant improvements in structural performance. Lamanna et al. [96] stated that the ultimate load capacity of RC beams was enhanced by 103% when a U-wrapped composite of GFRP layers was added. Sharafeddin et al. [97] asserted that wrapping concrete with GFRP sheets of various thicknesses (20, 30, and 50 mm) resulted in a bond strength increase between 96% and 134%. Moreover, the type of glass fibers and composites in directly made specimens significantly affect the flexural properties. In a study by Shaw et al. [98], an RC bridge suffering from cracking damage was repaired with mortar and retrofitted using GFRP laminates. The peak load of the damaged concrete bridge was around 73%, whereas repaired bridges with GFRP laminates showed a peak load of 102% relative to the control.

Additionally, Xiong et al. [99] found that strengthening beams with a hybrid combination of GFRP and CFRP increased the deflection ductility by 89.7% compared to beams strengthened only with CFRP. Similarly, a study by Hawileh et al. [100] revealed that beams strengthened by a combination of GFRP and CFRP sheets showed an increase in load-bearing capacity in the 30–98% range compared to the control unstrengthened beam. Baggio et al. [101] declared that partial depth GFRP sheets for strengthening shear deficient beams increased the shear capacity by 52 and 36% in the case of using and not using GFRP anchors compared to the control unstrengthened beam. Ritchie et al. [102] developed an analytical computer program to predict the behavior of beams that are adhesively bonded with glass, aramid, and carbon FRP plates. The results showed a 17 to 95% increase in stiffness and a 40 to 97% increase in ultimate strength with FRP plate bonding. Attari et al. [103] confirmed the more significant ductility and lower cost for glass than carbon fibers, making them more appropriate for strengthening and repairing applications. Li et al. [104] conducted a novel study on seismic-resilient precast segmental columns of CFRP tendons instead of steel tendons with damage-controllable and repairable designs. The special designs limited the damage of the column within a predetermined plastic hinge zone while keeping the remaining parts of the column damage-free. The repairable mechanism involved wrapping the repaired region with a GFRP sheet for better confinement. It was demonstrated that the proposed seismic resilient designs could be used in earthquake-prone regions to ensure that the precast segmental column would remain functional even after strong earthquakes.

Table 2. Mechanical properties of GFRP.

Structure	Tensile Strength (GPa)	Elastic Modulus (GPa)	Reference
GFRP rebar	0.48–1.60	35.00–51.00	[91]
GFRP sheet	3.24	72.40	[105]
GFRP sheet	1.70	71.00	[41]
GFRP sheet	-	72.50	[106]
GFRP sheet	-	73.00	[107]
GFRP sheet	0.79	34.10	[100]
GFRP fiber	0.27	9.73	[108]
GFRP fiber	0.26	8.66	[109]
GFRP fiber	0.78	28.65	[110]
GFRP Woven	0.35	43.70	[111]
GFRP Woven	0.28	4.89	[112]

summarizes the mechanical properties of different types of GFRP, while

Table 3. Comparative ranking between the properties of steel and GFRP.

Property	Steel	GFRP
Strength	High	Very High
Weight	Low	Very High
Corrosion resistance	Medium	High
Fire resistance	High	Medium
Handling	Medium	Very High
Maintenance	Medium	Very High
Toughness	High	High

shows a brief comparative ranking between the properties of GFRP and steel.

3.3. Thermal Properties

The thermal properties of GFRP composites are essential because they can affect the behavior of RC structures under various temperature conditions. The most critical thermal properties of GFRP composites include thermal expansion and thermal conductivity. The coefficient of thermal expansion is important because GFRP can expand or contract differently from concrete, potentially causing internal stresses or cracks in reinforced concrete structures during temperature fluctuations. Thermal conductivity is also vital as it affects the material’s ability to transfer heat, which can influence structural behavior under fire or extreme temperature conditions. These properties must be carefully considered in the design of GFRP-reinforced structures. Specifically, GFRP is characterized by its inherently low thermal conductivity compared to traditional construction materials such as metals. As detailed by Berardi et al. [113], GFRP’s low thermal conductivity makes it an effective insulating material, reducing the risk of thermal expansion-induced stress in RC structures. This characteristic directly contributes to mitigating the thermal effects that structures might endure, particularly thermal expansion and contraction. Thermal expansion can be defined as the length change per temperature unit rises [113,114]. Traditional materials like steel and concrete, with higher thermal conductivities, can undergo significant dimensional changes in response to temperature variations, potentially leading to stress concentrations, cracking, and other structural damage. In contrast, GFRP’s lower thermal conductivity can reduce thermal expansion and contraction, thereby lessening the likelihood of such damage. It is crucial to note, however, that while GFRP’s thermal properties can offer advantages in terms of reduced thermal stress, the overall impact on a structure also depends on design considerations, the composite matrix, and the interaction with other materials used in construction, requiring a holistic approach to leverage these benefits effectively.

Sim et al. [115] investigated the performance of carbon, glass, and basalt fibers after exposure to high temperatures. The fibers’ tensile strength did not vary up to 200 °C, while a decrease in strength was noticeable beyond 200 °C for 2 h. The basalt fibers maintained 90% of tensile strength at average temperatures up to 600 °C, while the tensile strength excessively deteriorated for carbon and glass fibers after 2 h. Furthermore, basalt fibers could maintain their volumetric integrity at such high temperatures. It was further found that the glass fibers melted partially, the carbon fibers melted utterly, and the basalt fibers showed thermal stability up to 1200 °C. Wang et al. [44] studied the influence of different glass fibers percentage replacement of 0.5, 1.0, and 1.5% by weight on the thermal conductivity of concrete at high temperatures of 440, 500, 580, 800, and 1000 °C for 1 h exposure. It was observed that the thermal conductivity decreased with the percentage replacement of glass fibers. Also, temperatures in the range of 400 to 500 °C had the most noticeable influence on the thermal conductivity of concrete. The thermal conductivity of GFRP depends on the fiber’s type, volume fraction, and characteristics [116]. Additionally, GFRP is characterized by its low electrical conductivity; hence, GFRP has been used as an insulating rod [117], as stated earlier.

Fire resistance is one of the critical issues in FRP-RC structures due to the ineffective behavior of FRP reinforcement when subjected to fire. Remarkably, the heat and smoke result in polymer matrix breakup. Moreover, when strengthening concrete members with GFRP, the bond is affected adversely when the epoxy’s glass transition temperature is exceeded [118]. Nonetheless, GFRP can contribute to passive protection against fire [113,119]. Petersen et al. [120] utilized Alumina Tri-Hydrate (ATH), a fire-resistant filler, to improve the fire resistance of GFRP. The mechanical properties of GFRP with ATH filler were evaluated at three different amounts by weight: 0, 25, and 50%.

3.4. Durability Properties

Durability is the most vital property of RC structures to withstand long-term exposure to harsh environmental conditions with minimal degradation, which can compromise the RC elements’ structural integrity. For example, concrete members in contact with harsh environmental conditions demonstrate a high possibility of cracking due to drying shrinkage [121,122]. Unlike steel and various other types of reinforcement, structures reinforced with GFRP are less likely to suffer from corrosion-related issues such as cracking or spalling. This property can significantly increase the life span of GFRP-reinforced columns, reducing the need for maintenance and repair.

Although GFRP has been considered corrosion-resistant [80,123,124], GFRP materials are not recommended in highly corrosive environments beyond concentrations of 50% of corrosion resistance [37]. FRP decks, for instance, are typically implemented in harsh changeable temperature and humidity conditions. Such conditions, known as hot/wet cycles, are extremely harsh conditions for polymetric materials [125,126,127,128,129,130,131,132,133], as they deteriorate the performance and service life of the FRP decks [125]. This is because of the relaxation swelling and concentration gradient that cause diffusion in the FRP/GFRP composite; hence, absorption usually occurs.

In addition, numerous factors control the mechanical durability of FRP composites, such as fibers and resin materials, freeze-thaw fluctuations, production methods, and external loads [134]. Tests reported that the resin properties are responsible for controlling the durability of FRP materials, especially GFRP. The epoxy in GFRP composites made of recycled fiberglass absorbs moisture in the range of 1–7% by weight [135]. In contrast to glass fibers, carbon fibers are resilient to organic, alkali, and acid solvents since they do not absorb liquids [135,136]. Also, carbon fibers can withstand decay in salty water [135]. However, glass fibers are susceptible to the aforementioned factors: salt, moisture, and stress corrosion/creep rupture. It was found that the flaws in glass fibers foster stress corrosion cracking; thus, stress corrosion failure happens in GFRP composites [137]. In any case, environmental conditions are better sustained by mineral fibers than other types, such as vegetal fibers [138]. Araújo et al. [139] investigated the water absorption behavior of fiberglass/polyester composites of various volume fractions of recycled fiberglass. The results showed that the water sorption decreased as the fiber volume fraction increased by 20 to 40% by weight.

The study of fatigue behavior is vital for an element repeatedly subject to loading cycles. FRP significantly impacts resistance [140] and fatigue endurance [134]. The increase in the external work consumes the FRP composite’s internal strain energy when subjected to fatigue loading. The damage or consumption of internal strain energy occurs just over 90% of the loading cycles to failure due to fiber breakage, fiber delamination, and matrix cracking. The discussed fatigue behavior mainly corresponds to GFRP composites [134,141]. Wang et al. [142] investigated the fatigue performance of GFRP/epoxy composites embedded with Shape Memory Alloy (SMA) wires. It was observed that GFRP/epoxy composites with two SMA wires had superior fatigue performance compared to GFRP/epoxy composites, as SMA controlled the cracking behavior and slowed down the cracking rate of the matrix. Shan et al. [143] found that incorporating carbon fibers in glass fibers composite to form glass-carbon fiber reinforced epoxy matrix composites exhibited better fatigue performance than unidirectional glass fiber counterparts, which was up to 107 cycles of tension-tension fatigue tests.

4. GFRP-Reinforced Columns Behavior

This section provides a comprehensive review of most available studies on the behavior of GFRP-reinforced columns, focusing on key aspects like failure modes, load-carrying capacity, load-deflection behavior, confinement, and ductility. The response of these columns is influenced by various factors such as concrete type and strength, cross-sectional geometry, slenderness ratio, reinforcement details, and loading protocol. This review discusses these factors, providing insights into their overall structural performance under different conditions.

4.1. Concrete Type and Strength

Concrete is widely utilized in the construction industry due to its availability, affordability, strength, and ease of use. Accordingly, extensive research has been conducted into the behavior of concrete columns reinforced with GFRP. Various types of concrete have been examined in the literature, including Normal Strength Concrete (NSC), High Strength Concrete (HSC), Ultra-High-Performance Concrete (UHPC), Fiber-Reinforced Concrete (FRC), Recycled Coarse Aggregate Concrete (RCAC), and Geopolymer Concrete (GPC). Each type of concrete listed has a unique influence on the performance of GFRP-reinforced columns. In particular, NSC provides a baseline, with moderate strength and durability, but may limit the full potential of GFRP. HSC enhances the load-carrying capacity, complementing GFRP’s tensile strength. UHPC, with its superior strength and durability, pairs well with GFRP to resist extreme loads and environmental conditions. FRC improves crack resistance, further enhancing GFRP’s durability. Additionally, using sustainable alternatives like RCAC and GPC in combination with GFRP reinforcement represents an evolving frontier in eco-friendly construction practices, offering insights into the recyclability and reduced carbon footprint of construction materials. RCAC and GPC reduce the carbon footprint of construction by utilizing recycled materials and industrial by-products, aligning with sustainability goals and potentially lowering material costs. Specifically, RCAC supports sustainability efforts but may present challenges in consistency, affecting overall performance. In contrast, GPC, a sustainable alternative, works well with GFRP in terms of durability but may require additional studies to fully assess its long-term structural performance. Therefore, examining these diverse types of concrete is not merely an academic exercise but a meaningful endeavor to advance our understanding of their practical applications and performance in GFRP-reinforced structures, aiming to optimize construction methods for enhanced durability, sustainability, and structural efficiency.

Table 4. A Summary of reviewed publications for GFRP-reinforced columns of various concrete types.

Type of Concrete	Publication
Conventional Normal Strength Concrete (NSC)	No. of publications = 44 [49,56,58,62,63,64,67,69,76,88,123,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177]
High Strength Concrete (HSC)	No. of publications = 19 [36,56,63,67,70,76,123,144,145,152,155,157,165,166,167,172,178,179,180,181]
Ultra-High-Performance Concrete (UHPC)	No. of publications = 0
Fiber-Reinforced Concrete (FRC)	No. of publications = 4 [162,171,174,182]
Recycled Coarse Aggregate Concrete (RCAC)	No. of publications = 2 [57,162]
Geopolymer Concrete (GPC)	No. of publications = 9 [57,60,61,62,65,66,164,183,184]

surveys the reviewed publications for GFRP-reinforced columns made of different concrete types in this paper. The majority of reviewed studies focus on NSC with 44 publications, indicating it is the most commonly researched material for GFRP reinforcement. HSC follows with 19 publications, highlighting its growing interest. Other concrete types like FRC, RCAC, and GPC have fewer studies, with UHPC notably lacking any reviewed publications, suggesting a research gap in this area. It should be noted that some publications are repeated under more than one category in Table 4. Moreover, some references, e.g., [144], define concrete of a compressive strength below 30 MPa as Low Strength Concrete (LSC). Other references, e.g., [145], consider concrete below 30 MPa as NSC. In Table 4, for simplicity, the NSC category involves columns of LSC. This section reviews the valuable and latest studies existing in the literature on GFRP-reinforced columns of different concrete materials.

To date, researchers have investigated the effects of NSC and HSC on the performance of GFRP-reinforced columns with longitudinal and transverse reinforcements, alone or in combination. Hasan et al. [63] evaluated the compressive behavior of GFRP-reinforced columns made of NSC and HSC of 45 and 90 MPa, respectively. It was stated that the contribution of GFRP rebars in RC columns was almost half that of steel rebars in NSC and HSC. Moreover, the NSC columns exhibited higher ductility using GFRP rebars, while HSC columns showed higher ductility using steel rebars. Experimental tests were performed on HSC non-slender and slender columns reinforced with GFRP [165]. It was asserted that the GFRP rebars and spirals are effective types of reinforcement in slender and short columns. It was also claimed that the hoop strain in GFRP spirals at failure was lower for the HSC columns in the study than in the NSC columns in the literature. This implies that GFRP spirals are more effective at resisting lateral deformation in HSC than in NSC. This reduced strain indicates that HSC columns require less confinement to maintain structural integrity. However, it also means that the GFRP spirals may need to be designed differently for HSC to optimize confinement. Further studies are needed to explore the specific confinement behavior of GFRP in HSC columns under various loading conditions, as well as to develop design guidelines that ensure optimal use of GFRP reinforcement in high-strength applications, particularly in slender column configurations.

AlAjarmeh et al. [145] conducted an experimental study on Hollow Concrete Columns (HCCs) with GFRP rebars and spirals. The compressive strength for the NSC making the HCCs was 21.2, 26.8, 36.8, and 44.0 MPa. It was implied that the axial load capacity increased; however, the confinement efficacy and ductility decreased in the HCCs with higher concrete compressive strength. Prajapati et al. [64] conducted a full-scale experimental study to assess the seismic performance, including stiffness degradation, energy dissipation, and ductility of normal-weight concrete columns of 30 MPa. Specifically, the behavior of rectangular concrete bridge columns with GFRP rebars, GFRP spirals and cross ties under simulated seismic loading was examined. The study’s results could offer valuable insight into the behavior of GFRP-reinforced columns under seismic loading.

Almomani et al. [153] assessed existing confinement models based on experimental tests collected from the literature for circular GFRP-reinforced NSC and HSC columns transversely reinforced with GFRP spirals or hoops. Also, a model was proposed to predict the confined compressive strength of GFRP-reinforced columns, which showed an excellent correlation to the experimental results. Hadhood et al. [181] evaluated the applicability of several Equivalent Rectangular Stress Block (ERSB) models experimentally and analytically for HSC structural members subject to combined compressive and flexural loads and reinforced with FRP reinforcement, such as GFRP. An experimental database of 92 specimens was assembled to assess the accuracy and conservatism of the ERSB models. It was found that the concrete compressive strength highly influenced the estimations of the ERSB models for the database’s specimens.

UHPC is an auspicious material for future-oriented applications due to its remarkable mechanical properties, such as high strength, durability, and ductility [185]. To the authors’ knowledge, only a few studies exist in the literature about UHPC columns with FRP reinforcement. Moreover, the authors found no publications to review about UHPC columns with GFRP reinforcement. Previous investigations by Zeng et al. [186] and Ma et al. [187] have focused on the axial compressive behavior of UHPC-filled tubes. In the former study, the columns were made of an outer GFRP tube with a steel core in some specimens, while in the latter study, the columns were made of an outer steel tube with an inner-filled GFRP core. These studies provided valuable information on composite materials’ failure modes and load-deformation relationships, shedding light on the interaction mechanism between the different components. Moreover, Zeng et al. [188] and Zeng and Long [189] presented and discussed the results for CFRP grid-UHPC tubular columns under axial compression that have great potential in pipelines and formworks applications. Meng and Khayat [190] simulated UHPC elements reinforced with GFRP grids subject to internal pressure due to concrete casting and gravity loads in ABAQUS. The proposed elements proved efficient for developing a Stay-In-Place (SIP) formwork system for columns.

Fibers such as steel and polypropylene have been added to concrete for improved ductility, resulting in a more ductile failure for columns reinforced with GFRP [174]. The use of fibers has evidenced enhancement in strength, ductility, energy absorption, and a decrease in cracking in FRC columns subject to compression loading [174,191,192,193]. Raza et al. [174] studied the experimental behavior of hybrid FRC columns with steel and GFRP rebars and spirals. The columns with GFRP reinforcement demonstrated lower axial strength by 8.68% than their steel-reinforced counterparts. Nonetheless, the ductility was higher by 19.71% for GFRP-reinforced columns. Patil and Prakash [171] evaluated the compressive behavior of GFRP-reinforced columns made of macro synthetic polyolefin fibers, hybrid steel, and macro synthetic polyolefin fibers. It was found that including fibers enhanced the peak load, post-peak behavior, and pseudo ductility of columns subject to concentric compression load. Furthermore, more considerable energy absorption and pseudo ductility were determined for columns with a hybrid than macro synthetic polyolefin fibers. Ali et al. [182] performed an experimental and numerical study on circular GFRP-reinforced columns made of hybrid polypropylene and polyvinyl alcohol fibers. The experimental and numerical studies indicated a 2.4 and 3.7% difference between axial strength and deflection at the ultimate axial strength. Thus, the effectiveness of the developed numerical models was confirmed for predicting the performance of hybrid FRC columns with GFRP rebars and spirals.

Additionally, the continuous production of concrete and waste has encouraged the use of RCAC, leading to greater sustainability in construction. Raza et al. [57] tested polypropylene macro synthetic reinforced RCAC columns with GFRP rebars. A finite element model was developed using a modified damaging plastic model for the polypropylene macro-synthetic reinforced RCAC that estimated the efficiency of specimens with significant accuracy.

GPC has gained significant attention in the construction industry due to its environmentally friendly composition. GPC utilizes industrial by-products such as Ground Granulated Blast-furnace Slag (GGBS) and Fly Ash (FA), resulting in lower CO2 emissions than OPC-based concrete [194,195,196] as these emissions are a concern for sustainable development [197]. GPC columns with GFRP bars have been investigated experimentally [57,60,65], numerically [60], and analytically [61,62,66] in the literature. Maranan et al. [65] studied the behavior of circular GPC columns made of GGBS and Class F FA and reinforced with GFRP rebars and spirals under concentric compression loading. It was observed that GFRP-reinforced GPC columns exhibited more significant compression strength (97.3%) compared to their OPC-based concrete counterparts (88.3%). That was because GPC had an elastic modulus of 33 GPa, while OPC-based concrete had an elastic modulus of 29 GPa for the same compressive strength of 38 MPa. Hadi et al. [198] conducted experimental and theoretical studies on circular GPC columns reinforced with GFRP rebars and spirals until failure under concentric, eccentric, and flexural loading. The columns were wrapped with CFRP sheets at both ends to prevent premature failure. It was stated that considering the contribution of GFRP rebars and confinement of GFRP spirals allowed the reasonable prediction of the axial load-carrying capacity through the theoretical equations.

4.2. Cross-Sectional Geometry

The configuration of RC columns is assumed vital in determining their behavior and strength. Circular columns generally provide more efficient confinement and perform better under axial loads due to uniform stress distribution, making them ideal for seismic resistance. Square or rectangular columns are more commonly used in construction because they are easier to form and connect with beams, but they may require additional confinement reinforcement to achieve the same performance as circular columns under similar loading conditions.

Research studies have been conducted to date on various cross-sectional shapes for columns, such as circular [49,56,59,61,65,74,153,183,199,200], square, and rectangular [58,60] configurations. Despite the significance of this aspect of research, relatively limited research has been conducted on GFRP-reinforced columns. Raval and Dave [201] declared that the cross-sectional shape considerably impacts the strength, stiffness, and deformation characteristics of the columns wrapped with GFRP sheets. GFRP-wrapped circular columns exhibited higher axial load-carrying capacity than rectangular and square RC columns. Specifically, circular columns experienced even confining pressure, whereas rectangular and square ones had a maximum pressure at the corners and a minimum pressure between the ends. However, the effect of the cross-sectional shape on the axial load-carrying capacity of unwrapped RC columns was insignificant. Prachasaree et al. [173] evaluated the compressive behavior of GFRP-reinforced columns of rectangular and circular cross-sections. It was determined that the concrete cover increased the axial load capacity insignificantly (0.68–1.35%) for rectangular columns and significantly (8.7%) for circular columns. The study also concluded that GFRP lateral reinforcement significantly enhances concrete strength and deformability by increasing confining pressure. The confinement effectiveness coefficient ranged from 3.0 to 7.0, with deformability factors of 4.2 for spirals and 2.8 for ties. Overall, lateral reinforcement had a stronger impact on improving deformability than on increasing the column’s strength.

In addition, hollow GFRP-reinforced columns demonstrated better behavior, i.e., higher axial load and bending moment, than solid GFRP-reinforced columns under concentric and eccentric loading [49]. This behavior could be attributed to the theory of change in the position of the neutral axis, with the increase in eccentricity resulting in minimal effect and contribution of the concrete cross-section. In other words, as the eccentricity increases, the neutral axis shifts, reducing the effectiveness of the concrete in resisting compressive forces. This shift is less pronounced in hollow columns, which helps maintain higher load-bearing capacity. Furthermore, Anwar et al. [179] evaluated the effect of various geometrical shapes of GFRP-reinforced columns on the axial capacity based on Data Envelopment Analysis (DEA) and Artificial Neural Networks (ANNs) approaches. DEA helps identify the most efficient column geometries in terms of material use and load-bearing capacity, while ANNs provide predictive modeling to forecast column behavior under different conditions. Together, these tools enable engineers to optimize column geometry for maximum axial strength, leading to more material-efficient and cost-effective designs in real-world applications.

An excellent match was demonstrated between the models and the 266 GFRP-reinforced columns from the literature. It was observed that an increase in the geometrical area of the columns increased the axial strength of the GFRP rebars in the columns. Also, Cakiroglu et al. [157] employed Machine Learning (ML) algorithms, including Gradient Boosting Machine, Random Forest, and XGBoost, to predict the axial capacity of FRP RC columns, including GFRP and CFRP. The input variables were the type of cross-section, gross cross-sectional area, the type of concrete, compressive strength of concrete, material type of longitudinal reinforcement, material type of transverse reinforcement, the ratio of longitudinal reinforcement, the configuration and spacing of transverse reinforcement, elasticity modulus and ultimate strength of the longitudinal reinforcement, and slenderness ratio. The variables that had the most significant impact on the output were in the following descending order: gross cross-sectional area, concrete compressive strength, spacing of transverse reinforcement, and slenderness ratio. The best-performing model achieved an R² score of 0.978, with the final equation providing the most accurate predictions. The authors recommend further experimental and numerical research efforts to better comprehend the direct influence of column cross-sectional shape on longitudinally and transversely GFRP-reinforced columns, providing additional insight into their behavior. Indeed, understanding the impact of column shape on GFRP-reinforced columns is essential for developing more effective design and construction practices to ensure these structures’ longevity and durability.

4.3. Slenderness Ratio

In structural engineering, the design of columns is a crucial aspect that demands careful consideration of whether the column is categorized as short or long. This categorization heavily influences the overall approach selected to achieve optimal structural performance. Design typically involves two stages: first, assessing the influence of second-order effects on the structure’s behavior and deformation; second, an appropriate section must be developed to resist applied loads. If the column is slender, a second-order elastic analysis or moment magnification procedure is performed. Alternatively, if the column is non-slender (short), a first-order elastic analysis is carried out [202].

The slenderness limit for steel-reinforced compression members is ignored in braced and unbraced systems if Equations (2) and (3) are satisfied per ACI 318-19 [203].

(a)

Not braced against side-sway

$${\displaystyle \frac{k{l}_u}{r}}\le 22$$

(2)

(b)

Braced against side-sway

$${\displaystyle \frac{k{l}_u}{r}}\le 34+12\left({\displaystyle \frac{M_1}{M_2}}\right)$$

(3)

$${\displaystyle \frac{k{l}_u}{r}}\le 40$$

(4)

where $M_1/M_2$ is negative when the column bends in single curvature and positive when it bends in double curvature. $M_1$ is the lesser factored end moment on the compression member, $M_2$ is the greater factored end moment, $k$ is the effective length factor, $l_u$ is the unbraced length, $r$ is the radius of gyration given as the square root of $I_g$ by $A_g$, where $I_g$ is the moment of inertia of the gross cross-section about the centroidal axis and $A_g$ is the gross area of the concrete cross-section. The equation remains the same in the current version, i.e., ACI 318.11-22 [204].

Additionally, the slenderness ratio limit was reduced from 22 (Equation (2)) to 17 for GFRP compared to steel-reinforced members. The slenderness ratio limit of 17 was proposed for GFRP-reinforced columns in side-sway frames in a study conducted by [205], who examined more than 11,000 columns of varying parameters. It was indicated that the more negligible stiffness of GFRP rebars makes the columns of greater vulnerability to slenderness. Zadeh and Nanni [206] proposed replacing the slenderness ratio limit value of 22 by 17 for the GFRP-reinforced columns; hence, the slenderness limit ratio cap of 40 (in Equation (4)) was replaced by 35, as shown in Equations (5)–(7). These equations are adopted in ACI 440’s current version, i.e., ACI 440.11-22 [91]. The slenderness ratio limit allows for an increase of 5% in the moments due to slenderness, which is lower than steel-reinforced concrete in ACI 318-19 [203] according to Zadeh and Nanni [206,207].

(a)

Not braced against side-sway

$${\displaystyle \frac{k{l}_u}{r}}\le 17$$

(5)

(b)

Braced against side-sway

$${\displaystyle \frac{k{l}_u}{r}}\le 29+12\left({\displaystyle \frac{M_1}{M_2}}\right)$$

(6)

$${\displaystyle \frac{k{l}_u}{r}}\le 35$$

(7)

Notably, the ACI 440 committee modified ACI 318-19’s equation, assuming a slenderness ratio limit cap of 30 instead of 40, which is more conservative [123]. Zadeh and Nanni [207] proposed a slenderness limit compatible with a drop of 5% in the second-order compared to the first-order capacity or a moment magnification factor of 1.14 for GFRP-reinforced columns subject to low-eccentricity; the proposed limit is given in Equation (8).

$${\displaystyle \frac{k{l}_u}{r}}\le 28+14\left({\displaystyle \frac{M_1}{M_2}}\right)\le 35$$

(8)

In the same study by Zadeh and Nanni [207], for GFRP-reinforced columns of 70 MPa concrete, a cap of 31 was proposed for the slenderness limit. Abdelazim et al. [146] employed an experimental database and analytical method with a 5% drop in capacity for proposing a slenderness limit for sway and non-sway frames. A slenderness limit of 18 was advocated for sway frames and its equivalent in non-sway frames, as given in Equation (9).

$${\displaystyle \frac{k{l}_u}{r}}\le 30+12\left({\displaystyle \frac{M_1}{M_2}}\right)\le 36$$

(9)

It should be noted that a low slenderness limit leads to overdesigning columns, while a high slenderness limit leads to less reliable design of columns. Furthermore, although the 5% drop in capacity has been widely accepted and implemented in the design guidelines, the safety corresponding to the slenderness limits based on the 5% drop has not been quantified. Further details about the slenderness limits can be found in [123].

To achieve a balance between cost and safety in GFRP-reinforced columns, quantifying safety concerning the slenderness limit is critical. This can be done by setting an acceptable reliability index, which ensures structural integrity while minimizing costs. However, design codes such as ACI 318 and ACI 440 use deterministic slenderness limits, which lack reliability-based approaches [123].

To address this gap, Khorramian et al. [123] developed a novel reliability-based approach for quantifying safety related to the slenderness limit of GFRP-reinforced columns. The analysis, employing ANNs and Monte Carlo simulations, found reliability indices ranging from 3.99 to 4.53 for the existing ACI 440 expressions. To improve the design, four alternative equations were proposed and optimized to achieve a target reliability index between 4.0 and 4.5, ensuring a balance between safety and economy.

Almomani et al. [152] utilized Genetic Expression Programming (GEP) models based on experimental results of FRP RC columns subject to concentric and eccentric loading to predict the axial capacity. Different types of reinforcement, GFRP, and CFRP, were considered in the 247 short and slender columns database. It was determined that the axial capacity decreased as the slenderness ratio increased.

Numerous studies on short and slender GFRP-reinforced columns are available in the literature. Some of the most noticeable are the experimental, analytical, and numerical studies conducted by [160,161,162,167] on short columns and [146,147,149,167,178] on slender columns.

4.4. Longitudinal Reinforcement Type and Ratio

As stated earlier, GFRP bars are known to have more than two times the tensile strength of steel. Hence, the GFRP-reinforced member can withstand substantially higher levels of tensile forces than their steel-reinforced counterparts. This property of GFRP encourages using GFRP bars as longitudinal reinforcement, as at the bottom of a simply supported beam. However, GFRP as a material is inferior to steel in terms of non-yielding, compressive strength, and modulus of elasticity. These characteristics impact the load percentage carried by longitudinal bars in GFRP-reinforced columns. Afifi et al. [150] asserted that longitudinal steel bars carried significantly higher loads than longitudinal GFRP bars, attributed to the less significant modulus of elasticity of GFRP bars. Moreover, Hassan et al. [63] inferred that the contribution of longitudinal steel bars was almost two times that of GFRP bars for both NSC and HSC. Studies showed that the contribution of GFRP longitudinal rebars is within 15% of the capacity of the column [49,63,65,150]. Nonetheless, this is inconsistent with the findings of De Luca et al. [208], which stated that the contribution of GFRP bars to the capacity of the column is less than 5%.

Table 5. Surveyed database for the contribution of GFRP bars.

Research	No. of Columns Tested *	Longitudinal Reinforcement	Transverse Reinforcement	Load Contribution of GFRP Rebar	Contribution of GFRP Bars to the Capacity of a Column
[150]	12	Steel, GFRP bars	GFRP spirals	5.0–10.0%	-
[208]	5	Steel, GFRP bars	Steel, GFRP spirals	Less than 5.0%	-
[49]	10	GFRP bars	GFRP spirals	13.0–15.0%	-
[63]	12	Steel, GFRP bars	Steel helices	10.0–40.0%	Contribution of GFRP bars was 50% of that of steel bars in the axial load-carrying capacity of the column.
[65]	8	GFRP bars	GFRP spirals and hoops	6.5–12.0%	-
[176]	8	GFRP bars	GFRP spirals and hoops	3.0–7.0%	-

* Total number of columns tested in the publication.

summarizes the findings of the literature on the contribution of GFRP rebars in concrete columns.

The performance of GFRP bars as longitudinal reinforcement, specifically as tension members, has wide applications. However, the performance of GFRP bars in compression has been debated ever since. Therefore, the international codes and standards, i.e., ACI 440.11-22 and CSA S806-12, ignore the contribution of FRP/GFRP bars in compression, as stated earlier. The literature has been enriched with many studies conducted to assess the contribution of GFRP in compression. Hasan et al. [63] declared that the contribution of longitudinal FRP bars in columns’ axial load-carrying capacity should not be neglected. However, the steel bars should not be directly substituted with GFRP bars owing to the relatively lower contribution of GFRP. The contribution of GFRP rebars following the reviewed publications in this section is summarized in Table 5. Furthermore, according to Tu et al. [176], the results of several experimental investigations showed that the compressive strength of GFRP bars ranges between 20% and 70% of the tensile strength [176,209,210,211,212]. This encourages the acceptance of GFRP as a compression reinforcement material and evidence that neglecting the role of compression GFRP rebars is unreasonable. Hence, the assumption of ACI [204] and CSA S806-12 [90] can be deemed inaccurate in underestimating the contribution of the compression GFRP rebars.

Additionally, the amount of longitudinal reinforcement, quantified as reinforcement ratio, significantly impacts the columns’ structural behavior, such as ductility and failure mode. A direct relation between the reinforcement ratio and ductility index of GFRP-reinforced columns was evidenced by [63,176]. That was owing to the additional confinement by the longitudinal rebars [63]. Gouda et al. [49] found that increasing the longitudinal reinforcement ratio from 2.5 to 3.8% in HCCs did not affect the behavior as much as the eccentricity increase from 0 to 65.6% did. Moreover, the increase in the longitudinal reinforcement ratio increased the ductility, as the columns could dissipate higher energy levels. Prajapati et al. [64] stated that the increased longitudinal reinforcement ratio through the diameter of the rebar governed the GFRP-reinforced and steel-reinforced column’s lateral load and drift capacities in advantageous and disadvantageous manners, respectively.

4.5. Transverse Reinforcement Type and Ratio

Transverse reinforcement like spirals, hoops, and cross ties significantly enhance the performance of RC columns by confining the concrete core and preventing the longitudinal bars from buckling. Accordingly, transverse reinforcement prevents the member from exhibiting a sudden and brittle failure by simultaneous concrete crushing and rebar buckling. One crucial parameter affecting longitudinal bars’ lateral stability and resistance to buckling is the spacing of transverse reinforcement. According to Maranan et al. [65], the unbraced/unsupported length of the GFRP longitudinal rebars influences the stability. The study investigated the behavior of circular concrete columns reinforced with a constant ratio of longitudinal GFRP reinforcement and different types of transverse GFRP reinforcement, like hoops and spirals. It was found that unconfined specimens, i.e., specimens with no transverse reinforcement, displayed a brittle behavior and failed suddenly, showcasing no post-peak behavior. This emphasized the significance of appropriate lateral confinement. Furthermore, a more closely spaced system enhanced the concrete core’s confinement efficiency, which is given by fcc’/fco’, where fcc’ is the confined-column compressive strength and fco’ is the unconfined-column compressive strength [65,88,150,173]. Confinement efficiency quantifies the strength enhancement of the concrete core [150]. Moreover, the concrete core’s confinement determines the columns’ deformability [213]. The confinement efficiency also depends on the transverse reinforcement ratio and the even distribution or configuration of longitudinal bars around the core [175].

Maranan et al. [65] contended that among the types of transverse reinforcements, spiral-confined concrete columns showed a more ductile behavior than hoop-confined columns; that is, spiral confinement offered a more uniform confinement pressure than hoops. Similarly, Prachasaree et al. [173] compared the effect of two types of GFRP transverse reinforcement, spirals and ties, and concluded that the confinement efficiency of spirals was superior to that of ties. Elchalakani et al. [88] tested circular concrete columns with GFRP rebars and spirals under concentric, eccentric, and flexural loading. As the spiral pitch decreased from 80 to 40 mm, the load-carrying capacity and ductility increased, on average, by 10 and 38%, respectively. Moreover, GFRP-reinforced columns had 11.7% higher ductility but 13.3% lower capacity than steel-reinforced columns with spirals of 80- and 120-mm pitch. However, for well-confined columns of 40 mm spiral pitch, the ductility was higher by 14.5%, but the capacity was lower by 12.5% for GFRP-reinforced columns. Afifi et al. [150] demonstrated that spirals of small-rebar diameter and close spacing complemented the column’s post-peak behavior, and such specimens showcased a relatively gradual failure pattern.

As the name suggests, the volumetric ratio quantifies the volume of transverse reinforcement with respect to the core volume of the structural member [175]. The volumetric ratio plays a vital role in predicting the failure mode of the column [150,214,215]. Afifi et al. [150] proclaimed that small volumetric reinforcement ratios of poor confinement (i.e., 0.70% or spiral spacing of 120 and 145 mm) resulted in longitudinal rebar buckling failure. In contrast, high volumetric ratios of a well-confining reinforcement (i.e., 3.00% or small to moderate spiral spacing of 40 to 80 mm) resulted in a desired failure mode initiated by crushing the concrete core and rupturing of spirals. It was further indicated that an increase in the volumetric ratio had a proportional increase in lateral confining pressure. Similarly, Afifi et al. [215] declared that the volumetric ratio impacted the failure mode and the post-peak strength decay of the GFRP-reinforced columns. A small volumetric ratio (i.e., 0.70%) caused a faster rate of strength decay in the post-peak stage. The low volumetric ratio also restricted the development of the post-peak curve, unlike the enhanced post-peak curve of higher volumetric ratios (i.e., 2.70 and 3.00%), which was attributed to the efficient confining of the concrete core through GFRP spirals. The reviewed publications in this and previous sections are summarized in

Table 6. Surveyed database for the longitudinal and transverse reinforcement.

Publication	Shape	Longitudinal Reinforcement Ratio *	Transverse Reinforcement Rebar Diameter *	Transverse Reinforcement Configuration	Transverse Reinforcement Spacing	Ductility Index	Maximum Axial Load	Confinement Efficiency	Failure Mode
[150]	Circular	2.20%	6.4 mm	Spirals	35 mm	2.85	2951 kN	1.76	Ductile behavior in the post-peak stage
		1.10, 1.70, 3.20%	9.5 mm		40 mm	1.13–4.75	2804–3019 kN	1.35–1.93	Less ductile behavior
					80 mm
					120 mm
		2.20%	12.7 mm		145 mm	1.19	2865 kN	1.29	Failed in a brittle and explosive manner
[208]	Square	1.00%	12.7 mm	Ties	76, 305, 406 mm	-	2417–3425 kN	-	Concrete core was crushed
[88]	Circular	0.55, 0.73, 0.86, 0.92%	7.2 mm	Spirals	40, 80, 120 mm	Ductility improved by 10% and 38% with the increase in reinforcement ratio.	1376 kN	-	Ductile behavior enhanced with an increase in transverse reinforcement ratio
[49]	Circular Hollow	2.50%	9.5 mm (#3)	Spirals	80 mm	1.80–4.80	330–2380 kN	1.80–4.80	Ductile behavior in the post-peak stage
		3.80%		Spirals	80 mm	2.30–6.90	420–2500 kN	2.30–6.90
[63]	Circular	2.40%	6 mm	Steel helices	40 mm	5.66	1203 kN	-	Ductility of GFRP-reinforced columns was higher than the hybrid counterparts
		3.20%				5.96	1464 kN	-
		4.30%				5.94	1396 kN	-
[65]	Circular	2.43%	-	-	-	-	1772 kN	-	Failed suddenly, buckling of GFRP bars. Showed no post-peak behavior
			9.5 mm (#3)	Hoops	50 mm	2.08	1872 kN	1.84	More of a ductile behavior
					100 mm	1.32	1981 kN	1.74
					200 mm	-	1988 kN	-	At peak load, the crushing of concrete and buckling of GFRP bars occurred
				Spirals	50 mm	2.99	2160 kN	2.13	Failed suddenly, buckling of GFRP bars. Showed no post-peak behavior
					100 mm	1.79	1208–2063 kN	1.67	More of a ductile behavior
[173]	Square	1.42, 2.05%	Steel 10 mm	Spirals	50 mm and 250 mm in the middle zone	-	250–390 kN	1.64–2.09	Concrete core was crushed, and longitudinal reinforcements buckled
	Circle	1.91, 2.63%				-	250–370 kN	1.89–2.43
	Square	1.42, 2.05%		Ties		-	270–375 kN	1.64–2.10
[64]	Square	1.48, 2.14%	No. 4	Spirals and cross-ties	100 mm	7.56 (deformability index for GFRP columns)	196–251 kN	-	GFRP-reinforced columns experienced a more gradual failure than hybrid columns
					120 mm	4.60 (deformability index for GFRP columns)		-
					150 mm	5.17 (deformability index for GFRP columns)		-
		Steel 1.48, 2.14%			100 mm	9.89 (displacement ductility index for hybrid columns)	192–250 kN	-	Hybrid RC columns dissipated more energy and exhibited superior ductility than GFRP-reinforced columns
					120 mm	6.22 (displacement ductility index for hybrid columns)		-
					150 mm	4.01 (displacement ductility index for hybrid columns)		-
[175]	Square	GFRP 8 No. 6 12 No. 5 4 No. 4 + 3 No. 5 8 No. 4	GFRP No. 4	Square spirals with c-shaped or closed ties	120, 80 mm	Acceptable ductile behavior for columns with steel rebars and FRP spirals	3900–5159 kN	Acceptable ductile behavior for columns with steel rebars and FRP spirals	Confinement efficiency is higher for columns with closed than c-shaped ties
		CFRP 2 × 8 No. 4	CFRP No. 3		67 mm
		Steel 4 M15 + 4 M10 12 M10	GFRP No. 4		120, 80 mm
			CFRP No. 3		60, 80, 120 mm
[176]	Square	0.8, 1.1, 1.5%	10, 12, 14 mm	Hoops	30, 50, 80 mm	Ductility of the column increased with the reinforcement ratio	937–982 kN	Columns with large stirrup spacing failed in a brittle and explosive manner Columns with small stirrups failed in a ductile manner.	The confinement efficiency was increased by reducing the stirrup spacing
				Spirals			928–954 kN

* The reinforcement material is GFRP if no material type is mentioned.

and

Table 7. Surveyed database for the volumetric ratio.

Research	Specimen Description	Volumetric Ratio	Failure Pattern
[150]	Circular concrete columns with GFRP bars and spirals subjected to concentric load	0.70%	Small spiral diameter and 0.7% volumetric ratio specimen failed in a brittle manner.
		1.00, 1.50%	Specimens with a volumetric ratio smaller than 1.5% failed explosively.
		2.70%	A high volumetric ratio with closer spacing controlled the longitudinal rebar buckling.
[215]	Circular concrete columns with GFRP bars and spirals subjected to concentric load	0.70, 1.00, 1.50, 2.70, 3.00%	Post-peak curve limited for low volumetric ratios; enhanced post-peak behavior for higher volumetric ratios
[214]	ECC-GFRP spiral confined concrete cylinder subjected to axial compression and lateral cyclic tests	2.00%	Cracks were more pronounced for highly confined specimens with large volumetric ratios, i.e., 6.30%, than poorly confined specimens.
		4.00%
		6.30%
[175]	Square concrete columns reinforced with steel, CFRP and GFRP bars, and CFRP and GFRP spirals subjected to concentric load	0.96 to 6.08%	The spacing of transverse reinforcement governed the buckling of longitudinal bars.

, where only columns with GFRP reinforcement are considered.

It is demonstrated that there is a shortage of detailed investigations of the effects of varying the volumetric reinforcement ratio in the available literature. Given that this is a governing parameter in the performance of GFRP-reinforced columns, it is recommended that further research be carried out in that regard.

4.6. Loading Protocol

Research studies have been carried out by subjecting GFRP-reinforced columns to different types of loading, such as monotonic (concentric and eccentric), impact and lateral impact, and cyclic loading. Examples of such studies are [63,69,145,151,171,173,175,176,177,180,198,215] on monotonic axial compression, [49,123,174,180,182,198] on eccentric axial compression and flexural, [172] on impact and lateral impact, and [64,154,159] on cyclic loading to study their behavior and characteristics. The type of load, along with other parameters, dictates the failure mechanism of the column [164].

Hadhood et al. [180] perceived that a typical axial compression failure due to crushing of the concrete core occurred for GFRP-reinforced HSC columns subjected to concentric and low eccentricity to diameter ratio (e/d) loading of 8.2 and 16.4%. Nonetheless, flexural tension failure due to the excessive cracks and lateral deformation on the tension side and secondary failure on the compression side occurred for the columns subject to high e/d of 32.8 and 65.6% without any GFRP rebar rupture. The failure modes were consistent with that of Gouda et al. [49], who further recognized that the failure mechanism of eccentrically loaded GFRP-reinforced HCCs was governed by the extent of eccentricity rather than the longitudinal reinforcement ratio. Likewise, when Gouda et al. [49] tested GFRP-reinforced columns under different levels of eccentricities—low, medium, high, and extreme eccentricity (8.2, 16.4, 32.8, 65.6%)—it was noticed that the cracking behavior of columns was governed by eccentricity. Moreover, for the low and medium eccentricity-loaded columns, vertical cracks were first noticed on the compression side at mid-height in contrast to the crack development in high and extreme eccentricity-loaded columns, which developed lateral tension cracks that propagated through the height of the column.

Pham et al. [164] admitted that GFRP-reinforced HSC columns sustained equal pure concentric axial loads compared to steel-reinforced HSC columns. Moreover, the ductility was lower by 30%, replacing steel with GFRP in concentrically loaded columns. However, when eccentricity comes into play, GFRP-reinforced columns were noted to show a lower GFRP rebar efficiency in carrying the axial load by 12% as the loading condition changed from concentric to eccentric loading at 50 mm from the center of the column. Similarly, test results of past research indicated that the axial load capacity of the column reduced with an increase in eccentricity [67,158]. Nevertheless, the ductile behavior of the GFRP-reinforced columns was enhanced with increased eccentricity [49].

Pham et al. [172] experimentally investigated the behavior of solid square NSC and HSC columns to lateral impact loading at different GFRP longitudinal reinforcement ratios. The experiment set-up involved an impact pendulum weighing 300 kg released at different impact angles ranging between 10° and 40°. It was observed that the extent of damage and failure pattern depended on the impact angle, the velocity of impact, and the longitudinal reinforcement ratio. Moreover, the impact behavior relied on the longitudinal reinforcement and velocity of impact. The impact behavior was in two phases, local and global. The local behavior phase involved only the side of the column that sustained direct impact while the rest stayed intact. The entire column suffered a significant displacement when the impact ceased at the global behavior phase.

Elshamandy et al. [159] reported and discussed the results of GFRP-reinforced columns subject to cyclic loading. The columns exhibited significant deformations with no degradation in strength and satisfied the building’s codes specified drift limit. Furthermore, adequate ductility and energy dissipation were accomplished for GFRP-reinforced columns compared to their steel-reinforced counterparts. The results evidenced the potential for using GFRP-reinforced columns to construct lateral-force resisting systems, like RC frames, in low-to-moderate seismic zones. However, further efforts are needed to develop design guidelines and recommendations appropriate for such GFRP-reinforced structural members. This section’s reviewed publications are summarized in

Table 8. Surveyed database for the loading protocol.

Research	Loading Protocol	Column Type	Reinforcement Ratio ρl	Reinforcement Material	Load Parameters	Peak Load Pmax	Failure of Specimens
[158]	Concentric axial compression	Solid rectangular	2.48%	GFRP-reinforced columns with steel ties	e = 0 mm	1046 kN	All GFRP columns failed in compression due to concrete crushing. Load carrying capacity of the column increased with eccentricity
	Eccentric axial compression				e = 40 mm	585 kN
					e = 80 mm	364 kN
[159]	Quasi-static lateral cyclic	Solid square	0.63%	All GFRP-reinforced columns	e = 0 mm	166–167 kN	All GFRP columns failed in compression due to concrete crushing.
			0.95%			160–214 kN
			2.14%			282 KN
[49]	Concentric axial compression	Hollow circular	2.50%	All GFRP-reinforced columns	e = 0 mm	2380 kN	Typical axial compression failure
	Eccentric axial compression				e = 25 mm	1950 kN	A more ductile failure mode was observed with an increase in eccentricity
					e = 50 mm	1550 kN
					e = 100 mm	770 kN
					e = 200 mm	330 kN
	Concentric axial compression		3.80%		e = 0 mm	2500 kN	Typical axial compression failure
	Eccentric axial compression				e = 25 mm	2000 kN	A more ductile failure mode was observed with an increase in eccentricity
					e = 50 mm	1550 kN
					e = 100 mm	930 kN
					e = 200 mm	420 kN
[180]	Concentric axial compression	Solid circular	2.18%	All GFRP-reinforced columns	e = 0 mm	4709 kN	Typical axial compression failure for columns subject to concentric and low eccentric loading. Flexural tension failure for columns subject to high eccentric loading.
	Eccentric axial compression				e = 25 mm	3309 kN
					e = 50 mm	2380 kN
					e = 100 mm	1112 kN
					e = 200 mm	497 kN
	Concentric axial compression		3.27%		e = 0 mm	4716 kN
	Eccentric axial compression				e = 25 mm	3380 kN
					e = 50 mm	2339 kN
					e = 100 mm	1135 kN
					e = 200 mm	513 kN
[67]	Concentric axial compression	Solid circular	2.20%	All GFRP-reinforced columns	e = 0 mm	2564 kN	Typical axial compression failure
	Eccentric axial compression				e = 25 mm	2060 kN	Failure was less brittle and more ductile with the increase in eccentricity.
					e = 50 mm	1511 kN
					e = 100 mm	776 kN
					e = 200 mm	366 kN
[164]	Concentric axial compression	Solid circular	2.30%	All GFRP-reinforced columns	e = 0 mm	1425–2041 kN	An increase in the initial eccentricity decreased the axial load capacity and ductility of GFRP-reinforced columns.
	Eccentric axial compression				e = 25 mm	781–1003 kN
					e = 50 mm	494–592 kN
	Flexural				e = ∞ mm	268–452 kN
[63]	Concentric axial compression	Solid circular	2.40, 2.70, 3.20, 4.30%	GFRP-reinforced columns with steel helices	e = 0 mm	600–857 kN (NSC)	Typical axial compression failure
					e = 0 mm	1630–1828 kN (HSC)
[65]	Concentric axial compression	Solid circular	2.43%	All GFRP-reinforced columns	e = 0 mm	1208–2160 kN	Typical axial compression failure
[173]	Concentric axial compression	Solid square and circular	1.42, 1.91, 2.05, 2.63%	GFRP longitudinal reinforcement and steel spiral and ties	e = 0 mm	240–390 kN	Typical axial compression failure
[64]	Quasi-static lateral cyclic	Solid square	1.48, 2.14%	All GFRP-reinforced columns with spirals and ties	Load value = 20% of column capacity + 2 drift cycles @ rate of 0.03Hz	196–251 kN	Crushing of longitudinal GFRP bars
				Steel-reinforced columns with GFRP spirals and ties		192–250 kN	Failure was more rapid in steel-reinforced columns than in GFRP-reinforced columns. Longitudinal steel rebar buckling and rupture
[172]	Lateral Impact	Solid square	0.64, 1.23, 2.02, 2.89%	All GFRP-reinforced columns	A pendulum impactor weighing 300 kg dropped at different release angles of 3°, 10°, 20°, 30°, and 40°	725 kN (NSC) 700–1171 kN (HSC)	Failure modes were governed by the impact angles, their velocities, and longitudinal reinforcement.

for columns with GFRP reinforcement only.

5. Analytical Models

The study of GFRP-reinforced concrete columns requires the development and application of robust analytical models to accurately predict their behavior under various loading and environmental conditions. These models help engineers simulate the performance of structures without the need for extensive experimental work, facilitating the design and optimization of GFRP-reinforced systems. This section delves into the major categories of analytical models used in the field, including finite element (FE) models, machine learning (ML) approaches, and simplified design formulations.

5.1. Finite Element Models

FE modeling is one of the most commonly employed methods to analyze the complex behavior of GFRP-reinforced concrete columns. FE models offer a detailed, computational approach to solving structural problems by discretizing the member into small elements and solving the governing equations for each element [216,217,218]. Several studies have used FE models to simulate the interaction between GFRP reinforcement and concrete under axial, flexural, and combined loading conditions. These models are particularly effective in capturing the nonlinear behavior of materials and the progressive failure mechanisms in GFRP-reinforced columns. In particular, Awera et al. developed FE models to predict the compressive behavior of GFRP-reinforced concrete columns under axial loads. These models show that increasing the reinforcement ratio enhances both ductility and peak load capacity [219]. Additionally, the thickness of the GFRP tube significantly impacts the axial compressive bearing capacity and deformation capacity, as mentioned by Yang et al. [220]. The performance of GFRP-reinforced columns under eccentric loading has been investigated, revealing that load eccentricity reduces the load-carrying capacity and overall performance of the columns [221]. The eccentric distance notably affects the bending stiffness and ultimate bearing capacity, with greater eccentricity leading to a significant reduction in these properties [222]. Studies have also explored the behavior of GFRP-reinforced columns under combined loading conditions. For instance, the axial strength of GFRP-reinforced columns was found to be 91.32% of that of steel-reinforced columns, but with higher ductility [223]. This suggests that while GFRP may offer slightly lower strength, it provides better ductility, which is beneficial under combined loading scenarios. Non-linear FE models have been used to simulate the seismic behavior of GFRP-reinforced columns. These models accurately replicate hysteresis behavior, crack patterns, and ultimate loads, showing that increasing concrete compressive strength improves lateral load capacity [58,224].

Accurately simulating the interaction between GFRP reinforcement and concrete in FEMs presents several challenges. One of the primary challenges is accurately modeling the bond-slip relationship between GFRP bars and concrete. This interaction is crucial for predicting the structural behavior under various loading conditions. Several studies emphasize the importance of incorporating bond-slip laws to simulate the GFRP-concrete interaction accurately [58,224,225,226]. GFRP has different mechanical properties compared to traditional steel reinforcement, such as lower compressive strength and elastic modulus. These differences necessitate specialized constitutive models for GFRP and concrete to capture their nonlinear behavior accurately [227]. The accuracy of FEMs can be highly sensitive to mesh size and the chosen material models. Proper calibration against experimental data is essential to ensure the reliability of the simulations [228,229]. Simulating the dynamic response of GFRP-reinforced concrete under impact loads involves additional complexities, such as strain rate effects and the interaction between different types of reinforcement [230,231]. In summary, the main challenges in simulating GFRP-concrete interaction in FEMs include accurately modeling bond-slip relationships, accounting for the unique material properties of GFRP, considering temperature effects, ensuring mesh sensitivity and model calibration, and addressing the complexities of shear, flexural, and dynamic behaviors.

5.2. Machine Learning Models

In recent years, ML techniques have gained popularity for predicting the behavior of GFRP-reinforced concrete columns. ML models can analyze large datasets and recognize patterns that may not be evident from traditional approaches, making them useful for modeling complex structural systems. ANNs, Support Vector Machines (SVMs), and Genetic Algorithms (GAs) are among the most common ML techniques used to model the performance of GFRP-reinforced structures. These models are typically trained using experimental data or numerical simulations and can predict key structural parameters such as load-bearing capacity, failure modes (buckling, cracking, etc.), strain distribution, and time-dependent behaviors such as creep and shrinkage. Furthermore, ML models, such as Back Propagation Neural Networks (BPNNs), have shown high accuracy and robustness in predicting the compressive strength of GFRP-confined reinforced concrete columns. The BPNN model achieved a coefficient of variation of only 14.22% and goodness of fit above 0.9 for both training and testing sets [232]. Hybrid models like Genetic Algorithm–Artificial Neural Network (GA-ANN) have been developed to predict the axial load-carrying capacity of FRP-reinforced concrete columns. The GA-ANN model outperformed traditional empirical formulas, demonstrating very high R² values (0.993) and low root mean squared error (RMSE) [233]. For UHPC beams reinforced with GFRP bars, models such as eXtreme Gradient Boosting (XGBoost) have been effective in predicting shear strength. The XGBoost model showed the highest predictive performance among various ML models tested [234]. Automatic machine learning (AutoML) and other ML methods have been used to predict the ultimate displacement and bearing capacity of GFRP columns. These models help in understanding the influence of parameters like cross-section shape, concrete strength, and GFRP wall thickness on column performance [235].

ML models can be refined continuously as more experimental data becomes available, improving their accuracy over time. Nevertheless, the reliability of ML models heavily depends on the quality and diversity of the training data, and these models can sometimes behave unpredictably outside the range of the data used to train them. In particular, applying ML to predict the behavior of GFRP-reinforced concrete columns presents several challenges. One of the primary challenges is the limited availability of experimental data, which is crucial for training accurate ML models. Additionally, insufficient data can lead to models that do not generalize well to new scenarios [232,236]. The behavior of GFRP-reinforced concrete columns is influenced by numerous parameters, such as reinforcement configurations, material properties, and loading conditions. Capturing these complex, nonlinear relationships accurately is difficult for traditional models and requires sophisticated ML techniques [232,237,238]. Moreover, identifying and understanding the sensitivity of various parameters, such as concrete strength and FRP thickness, is essential. Different parameters can have varying levels of impact on the model’s predictions, and this sensitivity needs to be accurately captured and interpreted [232,237]. High computational costs associated with traditional finite element and finite difference methods necessitate the development of efficient ML models. However, ensuring these models are both accurate and computationally efficient remains a challenge. Furthermore, ensuring the stability and robustness of ML models is critical. Models like Support Vector Regression (SVR) have shown lower volatility and higher prediction stability compared to others, but achieving consistent performance across different datasets and conditions is still challenging [236,239]. The brittle nature of GFRP bars under certain loading conditions adds another layer of complexity. This behavior must be accurately modeled to predict failure modes and overall structural performance [240,241].

5.3. Simplified Design Models

For practical engineering applications, simplified design models are often preferred due to their ease of use and ability to provide quick estimates. Several international codes and standards have developed simplified models specifically for GFRP-reinforced concrete members, including design recommendations for axial capacity, flexural strength, and shear capacity. Several studies propose design-oriented models to predict the behavior of GFRP-reinforced concrete columns. These models consider critical design parameters such as reinforcement ratios, concrete compressive strength, and column geometry to accurately describe load-strain behavior [148,149,242,243,244]. Comprehensive models address both ultimate limit state (ULS) and serviceability limit state (SLS), providing equations for confined concrete strength and axial strain and simplifying the design procedure for columns under combined compression and bending [244]. Simplified models also incorporate safety factors and stability considerations, such as the stiffness reduction factor and permissible tensile design strains, to ensure conservative and safe design outcomes [147,149,244].

These models typically employ safety factors to account for uncertainties in material behavior and load conditions. While simplified models are useful for routine design tasks, they may not fully capture the complex interactions between GFRP and concrete, especially under non-standard loading conditions or for advanced materials like UHPC. Simplified models like the moment magnification method are often used to avoid the high computational costs associated with complex nonlinear analyses. However, these simplified methods can compromise accuracy, especially for slender columns with significant material and geometric nonlinearities [147]. GFRP bars exhibit brittle behavior, which complicates the prediction of failure modes and mechanical properties under different loading conditions, such as seismic actions. This brittleness necessitates detailed investigations into the mechanical properties and failure modes of GFRP-reinforced structures [241]. Current design codes do not adequately account for the compressive contribution of GFRP bars, leading to potential underestimations of load-bearing capacities. This gap necessitates the development of new models and design provisions that consider the unique properties of GFRP materials [245]. There is a need for comprehensive experimental investigations to validate the effectiveness of GFRP bars in concrete columns, particularly under compression and bonding conditions. This validation is crucial for developing reliable design models [246].

A variety of analytical models are employed to predict the behavior of GFRP-reinforced concrete columns. While finite element models offer detailed and accurate simulations, machine learning techniques are rapidly emerging as powerful tools for modeling complex behaviors with minimal computational effort. Empirical and simplified models remain essential for practical design applications, while long-term performance models address the durability of GFRP in real-world environments. As GFRP becomes more widely adopted in structural engineering, the continued refinement and integration of these analytical models will play a critical role in advancing the design and application of GFRP-reinforced systems.

6. Current Research Needs

The state-of-the-art review has identified a significant gap in research concerning UHPC columns reinforced with GFRP despite UHPC’s known benefits and applications in structural engineering. To date, studies specifically addressing GFRP-reinforced UHPC columns are virtually non-existent, underscoring a critical need for focused research in this area. The combination of UHPC’s exceptional mechanical properties and GFRP’s corrosion resistance, tensile strength, and lightweight offers a promising avenue for enhancing structural efficiency, durability, and sustainability. The lack of exploration into how these materials interact, especially in column applications, presents a unique opportunity to advance structural design and construction technology. Addressing this research void is essential for bridging the existing knowledge gap and exploring new frontiers in construction that could lead to buildings and infrastructure capable of enduring extreme environmental stresses and loads. This research direction is pivotal for meeting modern engineering challenges, pushing the boundaries of sustainability, and enhancing the resilience of future constructions.

In addition, further efforts are needed to investigate the effect of cross-section shape and geometry on GFRP-reinforced columns’ behavior, as most such studies focused on FRP-strengthened (wrapped) RC columns. Moreover, few research studies discussed the influence of varying the GFRP transverse reinforcement volumetric ratio as a dominant parameter in column behavior.

Additionally, there is a lack of comprehensive research studies incorporating more than a few variables and specimens; hence, existing provisions for GFRP-reinforced columns are still behind. The Canadian bridge standard CSA S6-14 [247] includes provisions for FRP-reinforced decks; however, no specific provisions are available for FRP-reinforced columns in the standard. On the other hand, CSA S806-12 [90] addresses the design and construction of building structures using FRP, offering guidelines for the use of FRP materials in various structural applications. Despite this, there remains a gap in providing comprehensive design standards for FRP-reinforced columns in critical infrastructure, underscoring the need for further research and code development. Also, the 2018 AASHTO [248], which involves the provisions for GFRP-reinforced columns, does not consider columns under seismic loading [168]. Furthermore, the authors found relatively limited research studies about GFRP-reinforced columns subjected to cyclic loading, although many research studies in the literature have been conducted on RC structures under seismic loading, e.g., [249,250,251,252,253,254,255,256,257,258]. Thus, more efforts are needed to substantiate the use of GFRP-reinforced columns in structures subject to seismic loading conditions. ACI 440.11-22 [91] has recently been published for FRP-reinforced structural concrete members. The code involves the design requirements for members designated to Seismic Design Category (SDC) A, and the members are not part of the seismic force-resisting systems in SDC B and C. However, the code does not cover SDC D, E, and F requirements.

Current design codes and standards fail to fully account for the compressive contribution of GFRP bars (such as ACI 440.11-22 [91] and CSA S806-12 [90]), leading to conservative estimates of load-bearing capacities in GFRP-reinforced concrete columns. To address this gap, there is a critical need for new models and design provisions that reflect the unique behavior of GFRP under compression. Comprehensive experimental research is essential to validate the performance of GFRP bars, particularly regarding their compressive strength and bonding with concrete. This validation will enable the development of more accurate design models, which can be integrated into updated design codes to optimize the use of GFRP in structural applications, enhancing safety and efficiency.

Technological advancement has enabled experimental investigation of specimens of actual/real-life dimensions instead of prototypes. Literature and previous studies lack evidence on research that has considered the impact of size effect on the behavior of the column. Although many studies have considered full-scale and small-scale columns, none, to the best of the authors’ knowledge, have specifically investigated the effect of their size on the structural behavior. However, in the case of columns, it is by virtue a known fact that the column strength is reduced with the increase in size and slenderness [208]. It can be assumed that as technological advancement enables the investigation of real-life scenarios, small-scale specimens are no longer studied unless otherwise for specific design demands.

Conducting comprehensive long-term durability studies on GFRP-reinforced concrete structures across a spectrum of environmental conditions is imperative. This research should encompass assessments under extreme temperatures, varying humidity degrees, and prolonged exposure to corrosive agents like chlorides and carbon dioxide. The aim is to elucidate the detailed mechanisms of deterioration and quantify these environmental factors’ impact on the structural integrity and performance of GFRP-reinforced concrete over time. Special emphasis should be placed on evaluating the effects of cyclic loading and environmental stressors on the bond strength between GFRP and concrete and the GFRP itself. Understanding these interactions is crucial for predicting the life span, formulating effective maintenance strategies, and establishing reliable prediction models for the long-term behavior of these structures. The outcomes of such studies are expected to provide vital insights for developing design guidelines, improving material formulations, and proposing protective measures that enhance the durability and sustainability of GFRP-reinforced concrete structures, ensuring their safety and functionality over their intended service life.

Initiate detailed studies to assess the environmental footprint and sustainability merits of incorporating GFRP in construction projects. This involves conducting rigorous life cycle analyses (LCA) to systematically evaluate the environmental impacts of all stages of GFRP’s life span—from raw material extraction to manufacturing, transportation, installation, maintenance, and end-of-life disposal or recycling. Additionally, precise carbon footprint evaluations are essential to quantify the greenhouse gas emissions of GFRP-reinforced structures throughout their life cycles, comparing these impacts with those of traditional construction materials like steel and concrete. The objective is to unearth insights into how GFRP application in construction aligns with or enhances green building standards and certifications, such as LEED or BREEAM. Identifying areas where GFRP significantly reduces environmental impacts can bolster its adoption in eco-conscious construction practices. Moreover, these studies should aim to uncover opportunities for optimizing the production and application processes of GFRP to minimize ecological footprints further, contributing to the development of more sustainable, resource-efficient, and environmentally friendly construction methodologies. The findings are anticipated to guide policymakers, designers, and builders in making informed decisions that support the broader adoption of sustainable practices within the construction industry, ultimately fostering a shift towards more sustainable and resilient built environments.

Undertake in-depth cost-benefit analyses to evaluate the economic viability and long-term financial advantages of GFRP-reinforced structures compared to conventional steel-reinforced alternatives. This research should meticulously account for a wide array of cost factors, including but not limited to initial material and labor costs, frequency and cost of maintenance, life cycle longevity, and eventual decommissioning or recycling expenses. Additionally, it is crucial to integrate the potential financial impacts of durability and environmental resilience into these analyses, considering the reduced degradation and lower repair needs of GFRP-reinforced structures in aggressive environments or under extreme weather conditions. The analysis should also explore the economic implications of the lighter weight of GFRP on transportation and installation costs and the potential for reduced foundational support requirements due to the decreased load. By quantifying these aspects, the research aims to provide a holistic understanding of the total economic impact of adopting GFRP in construction projects, offering critical insights into direct cost savings and the added value brought by enhanced durability, sustainability, and resilience. The findings of this comprehensive cost-benefit analysis will serve as a valuable resource for architects, engineers, builders, and policymakers, aiding in the strategic decision-making process by highlighting the long-term economic and environmental benefits of integrating GFRP into modern construction practices.

Embarking on rigorous investigations to assess the resilience of GFRP-reinforced structures against seismic events, blasts, and high-impact loads is paramount to ensuring their effectiveness in disaster-prone areas. This research will delve into the dynamic response of GFRP reinforcements, comparing their performance with traditional materials under extreme conditions to understand their potential to enhance structural integrity and safety. Through experimental analyses and computational modeling, the goal is to uncover GFRP’s energy absorption capabilities, its behavior under sudden loads, and the overall impact on structural durability. The investigation aims to validate the use of GFRP in mitigating disaster-related damages and to pioneer design innovations that optimize these materials for shock absorption and energy dissipation. Ultimately, the outcomes will inform the development of advanced engineering standards and promote the adoption of GFRP-reinforced structures as a viable solution for building resilient infrastructure, contributing significantly to the safety and sustainability of communities in high-risk regions.

The employment of the power of Artificial Intelligence (AI) as a general modeling technique [259], such as ML and ANN, appears to have great potential in GFRP-reinforced columns’ research. That is due to its time, cost, and effort efficiency compared to other methods of investigations. Moreover, using such techniques has been motivated by the availability of databases [260]. Furthermore, ANNs have received significant research attention because of their excellence in nonlinear modeling [261]. However, very few studies have been published based on GFRP-reinforced columns’ investigations conducted through AI. That could be attributed to the limitations of the developed ANN and ML models. An instance of such a limitation is the applicability of the models to values within the ranges of the input and outputs [155,157]. In this case, additional research is needed to improve the covered range and precision of the models [155]. Moreover, research efforts are needed to address the limitations of using AI techniques in GFRP-reinforced columns research.

Filling these research gaps will have significant practical implications, advancing construction practices, and shaping future design codes and material standards. By exploring areas such as GFRP-reinforced UHPC columns, seismic performance, and durability under environmental stressors, this research can lead to more resilient, sustainable, and cost-efficient structures. Incorporating findings into building codes like ACI and CSA could provide more comprehensive design guidelines, especially for seismic applications and long-term durability. Ultimately, this research will drive innovation, optimize material use, and promote the wider adoption of GFRP in modern construction.

7. Concluding Remarks

This paper provides a state-of-the-art review of GFRP composites and their use in concrete columns of various parameters. Publications on the physical, mechanical, thermal, and durability properties of GFRP were initially reviewed. Moreover, publications on GFRP-reinforced columns behavior were more extensively investigated. The review covered the impact of varying the concrete type and strength, cross-sectional geometry, slenderness ratio, longitudinal reinforcement type and ratio, transverse reinforcement type and ratio, and loading protocol on the behavior of GFRP-reinforced columns. This critical state-of-the-art review provided insight into the existing research needs, where further efforts are recommended. Based on the review of more than 250 publications from 1988 until 2024, the following observations are made:

• The analysis of the Scopus database revealed that the number of publications on GFRP-reinforced columns has been increasing throughout the 21st century. Furthermore, the research on GFRP-reinforced columns has been prolific in China, Canada, and the United States. Out of 1534 of the Scopus-indexed publications on GFRP-reinforced columns, 69% of the publications were journal articles.
• The mechanical properties of GFRP, tensile strength, and stiffness-to-weight ratio make it an ideal alternative to conventional steel reinforcement in concrete structures that require significant strength and durability. However, GFRP reinforcement is not recommended in corrosive environments of greater than 50% concentration, although GFRP is non-corrosive, and GFRP-reinforced structures have a lower probability of suffering from corrosion-related issues.
• The analysis of publications data revealed that GFRP is most commonly implemented with conventional Normal Strength Concrete (NSC) structures. As such, further efforts are still required to promote other types of concrete with GFRP reinforcement.
• The compression GFRP rebars’ contribution to the columns’ capacity ranged between 5% and 40%. However, current design codes underestimate the compressive contribution of GFRP bars in concrete columns, leading to conservative load-bearing capacity estimates. To bridge this gap, new models and design provisions that reflect the unique behavior of GFRP under compression are urgently needed.
• AI and ML techniques, such as ANNs, demonstrate outstanding potential in the GFRP-reinforced columns research field. However, a limited number of publications were produced employing such techniques.
• This study has meticulously identified and underscored a broad spectrum of critical research needs within the realm of GFRP-reinforced concrete structures, marking a pivotal step toward advancing structural engineering and construction technology. From exploring the synergistic potential of GFRP with UHPC to delving into the effects of cross-sectional shapes, seismic loading conditions, and the environmental sustainability of GFRP applications, this research agenda is poised to fill existing knowledge gaps. Furthermore, it emphasizes the need for rigorous cost-benefit analyses and resilience studies under extreme conditions, alongside leveraging AI and ML technologies for enhanced predictive modeling. By addressing these diversified research needs, this study not only aims to foster significant technological innovations and sustainability in construction but also seeks to enhance future infrastructure’s safety, efficiency, and environmental footprint, thereby contributing to the enduring resilience and advancement of built environments worldwide.

In conclusion, while GFRP-reinforced concrete columns are gaining attention for their potential in structural engineering, several research gaps must be prioritized to drive advancements in the field. The most urgent need is investigating the seismic performance of GFRP-reinforced columns, as it could lead to critical updates in building codes and enhance safety in earthquake-prone regions. Additionally, exploring GFRP’s integration with UHPC offers promising avenues for improving structural efficiency and sustainability. Addressing the durability under environmental stressors, cross-sectional geometry impacts, and cost-benefit analyses will further refine construction practices and standards. By resolving these gaps, GFRP-reinforced columns could become a standard in modern, resilient, and sustainable construction practices. This paper showcased that GFRP-reinforced concrete columns are a highly promising technique that is increasingly attracting the attention of the structural and construction engineering community. Nonetheless, additional extensive research efforts are still needed to address the identified research needs.