FRP–Steel Composite Tube Confined Seawater–Sea-Sand Concrete Columns: State-of-the-Art Review

Abstract

With the depletion of river sand and the rapid expansion of marine infrastructure, seawater–sea-sand concrete (SSC) has attracted increasing attention due to its low cost and sustainability. However, the high chloride content in SSC accelerates steel corrosion. This significantly limits its use in conventional reinforced concrete structures. In recent years, the rise in FRP–steel composite confinement has offered a new solution to this durability bottleneck. Based on this background, scholars have proposed a new type of FRP–steel composite tube confined seawater–sea-sand concrete (FCTSSC) column. This paper reviews the research progress on SSC, CFST, FCFST, and FCTSSC. The latter systems are developed based on the former. The results show that advanced FCTSSC columns exhibit strong synergistic confinement between the FRP and the steel tube when compared with CFST and FCFST. This synergy enhances the bearing capacity, ductility, and post-peak behavior of SSC. Both external and internal FRP configurations can reduce the brittleness and expansion of SSC. They also effectively restrain local buckling of the steel tube. Existing studies mainly focus on short columns. Research on intermediate slender and slender columns remains limited. This includes structural behavior, rational design models, and long-term durability. Finally, future research directions are proposed to support the practical application of FCTSSC in marine engineering.

1. Introduction

With the rapid development of marine engineering, the demand for concrete in ports, seawalls, and sea-crossing bridges continues to increase. Meanwhile, freshwater and river sand are becoming scarce. Seawater–sea-sand concrete (SSC) has therefore been considered a sustainable alternative. Previous studies show that SSC exhibits good workability, early strength, and impermeability. However, its high chloride content can induce steel corrosion, which limits its application in conventional reinforced concrete structures [1,2,3].

Concrete-filled steel tubes (CFST) benefit from the confinement provided by steel tubes. They exhibit high load capacity and ductility and are widely used in practice. In marine environments, however, CFST still suffers from steel corrosion, local buckling, and durability degradation. To address these issues, fiber-reinforced polymer (FRP) has been externally applied to steel tubes due to its great performance [4,5,6], forming FRP-confined CFST (FCFST). External FRP confinement delays steel buckling and enhances strength and ductility. Nevertheless, ordinary concrete remains the core material, and steel corrosion in chloride-rich environments cannot be fully avoided.

To address these challenges, a novel FRP–steel composite tube confined seawater–sea-sand concrete (FCTSSC) column has been proposed, as shown in Figure 1. Unlike CFST and FCFST, FCTSSC innovatively incorporates FRP layers on both the inner and outer surfaces of the steel tube. This configuration effectively isolates SSC from steel, suppresses chloride ingress, and provides dual-confinement through the combined action of inner FRP, steel, and outer FRP. Existing studies indicate that FCTSSC columns outperform CFST and FCFST in axial capacity, ductility, and buckling resistance. However, current research mainly focuses on short columns under monotonic axial loading. The behavior of slender FCTSSC columns remains insufficiently studied, and their performance under cyclic and dynamic loading is still unclear. Moreover, the research parameters are limited. Few studies have examined components fabricated with GFRP fabrics, which are more economical for practical engineering.

Based on the above background, the main objective of this paper is to provide a systematic and in-depth review of current research on FCTSSC columns. This review is not a simple compilation of previous studies. Instead, it aims to clarify the evolutionary development from SSC to CFST and FCFST, and finally to the FCTSSC system. The review highlights the dual-confinement mechanism introduced by the inner and outer FRP layers. This mechanism distinguishes FCTSSC columns from conventional composite columns.

To achieve this objective, the review systematically: (i) summarizes the development of SSC, CFST, and FCFST, which form the technological foundation of FCTSSC; (ii) synthesizes the available experimental and numerical evidence on FCTSSC columns; (iii) classifies the key parameters and loading conditions investigated in the literature; and (iv) identifies critical research gaps related to slenderness effects, durability performance, and design modeling. The ultimate goal is to establish a clearer research framework and to provide actionable insights for the future design and application of FCTSSC columns in marine infrastructure.

2. Literature Search and Selection Methodology

To enhance transparency and reproducibility, we identified the literature through structured searches in the Web of Science Core Collection and Scopus. The search strings combined keywords related to seawater–sea-sand concrete and composite confinement systems. Terms included “seawater–sea-sand concrete or SSC,” “concrete-filled steel tubes or CFST,” “FRP-confined concrete-filled steel tubes or FCFST,” and “FRP–steel composite tubes or FCTSSC.” We also used “axial compression,” “slenderness ratio,” “buckling,” “cyclic loading,” “impact,” “durability,” and “finite element.” The primary window for SSC/CFST/FCFST studies was 1990–2025. For the emerging FCTSSC system, the window was 2019–2025.

Studies included in the core evidence synthesis were restricted to peer-reviewed journal articles. These studies were required to report: (i) member-level mechanical behavior, confinement mechanisms, and durability responses of SSC/CFST/FCFST/FCTSSC columns; or (ii) validated numerical or theoretical models. Exclusion criteria included non-peer-reviewed outputs, studies lacking sufficient experimental or numerical details for interpretation, duplicate publications, and studies focusing only on material-scale properties without addressing structural member behavior.

The screening procedure followed the PRISMA workflow. Records were exported from the two databases, merged, and duplicates were removed. Title and abstract screening was then conducted, followed by full-text eligibility assessment. The selected studies were subsequently classified according to structural system (SSC/CFST/FCFST/FCTSSC), loading type (monotonic loading, cyclic loading, impact loading, and durability testing), and methodological approach (experimental, numerical, and theoretical methods).

The statistical details of the PRISMA process are as follows. A total of 1660 records were identified from the databases, including 742 from Web of Science and 918 from Scopus. After removing 452 duplicate records, 1208 records remained for title and abstract screening. Subsequently, 915 records were excluded, as they were not directly related to SSC/CFST/FCFST/FCTSSC structural systems, did not involve member-level behavior, or fell outside the scope of structural engineering. Full-text assessment was conducted for 293 articles. Among them, 113 studies were excluded due to insufficient methodological information, focus on non-member-scale topics, duplicated datasets, or lack of peer review. Ultimately, 180 peer-reviewed journal articles were included in the core evidence synthesis. In addition, some standards or guidelines were retained as supplementary references to clarify the background of design provisions and engineering practice, but they were not included in the PRISMA core synthesis dataset.

3. Evolution of Composite Confinement Systems

3.1. SSC as a Sustainable Concrete Material

As shown in Figure 2, SSC is a new type of concrete. It uses seawater as mixing water and sea sand as fine aggregate, combined with coarse aggregate and cementitious materials. In the 1960s, British researchers pioneered a series of studies on the feasibility of using sea sand as a construction material [7,8,9]. Over time, research on SSC has focused on its workability, mechanical properties, and durability.

3.1.1. Workability

The workability of concrete mainly includes setting time and fluidity. It is commonly evaluated by the initial and final setting times and the slump. Existing studies generally show that SSC can accelerate cement hydration and shorten the setting time of concrete. Compared with conventional concrete, seawater concrete exhibits shorter initial and final setting times while maintaining similar consistency [10,11,12,13]. This acceleration is mainly attributed to the presence of salts in seawater. However, the effect depends on the salinity level. Moderate salt concentrations shorten the initial setting time. Excessively high salinity may partially offset this effect [12].

In contrast, the influence of sea sand on concrete fluidity remains inconsistent. Some studies report negligible effects or a slight increase in slump [14,15,16,17]. Other studies observe reduced workability with increasing sea-sand content [18]. These differences are mainly related to variations in sea-sand composition, particularly shell fragments and salt content. Shell particles can alter aggregate packing, whereas dissolved salts affect the rheological behavior of cement paste [19,20,21]. In addition, seawater and sea sand may improve pumpability and reduce autogenous shrinkage in high-performance marine concrete [22]. Overall, SSC generally accelerates the setting behavior. Their effects on workability are controlled by multiple factors, including salt concentration, shell content, and aggregate composition.

3.1.2. Mechanical Properties

The use of SSC in concrete production effectively alleviates the shortage of freshwater and natural river sand. However, the salinity of seawater and differences in aggregate properties may influence the mechanical performance of concrete. Many studies have shown that chlorides in seawater can accelerate cement hydration and enhance early compressive strength [17,23,24,25,26,27]. For example, seawater-mixed concrete usually exhibits higher strength at an early age, particularly at 7 days. This difference generally decreases with increasing curing time [17,25,26,27].

The development of compressive strength is also affected by curing conditions and salinity. Some studies indicate that seawater curing can accelerate early strength development [28,29]. However, excessively high salt concentrations may inhibit hydration and reduce long-term strength [12,19]. Similar trends have been reported in seawater coral concrete [30,31,32]. These findings suggest that salts in seawater can act as early-age activators and promote hydration reactions.

However, the long-term strength of SSC remains controversial. Some researchers reported that, with sufficient compaction and proper curing, the long-term strength of seawater-mixed concrete can be comparable to that of conventional concrete [17,33,34]. Other studies suggest that seawater curing or high salinity may reduce ultimate compressive strength or slow late-age strength development [12,26,28,35]. These discrepancies are mainly attributed to differences in mix design, concrete grade, and curing conditions [15,36]. In some cases, the long-term strength of seawater concrete is even slightly higher than that of ordinary concrete [37]. This phenomenon highlights the complex interactions between seawater chemistry and cement hydration.

3.1.3. Durability

Durability is another critical issue for SSC. It directly affects the long-term safety and service life of concrete structures. The durability of SSC is influenced by several factors. These include chloride ion penetration, carbonation resistance, permeability, freeze–thaw resistance, and environmental conditions.

Some studies report that SSC may exhibit a lower elastic modulus and reduced resistance to freeze–thaw cycles compared with conventional concrete. These drawbacks can be mitigated by incorporating supplementary cementitious materials, such as slag and pozzolans [38]. Chloride transport behavior has also been widely investigated. Some studies indicate that sea-sand concrete has higher chloride permeability [39]. In contrast, other studies report improved impermeability due to salt reactions and pore-filling effects that densify the matrix [13,19,40].

The carbonation resistance of SSC also remains controversial. Some researchers suggest that chloride ions refine the pore structure and enhance carbonation resistance [19,41,42]. Others report little difference between SSC and conventional concrete [12,15]. In addition, the interaction between chloride ions and carbonation introduces further durability challenges. Chloride ions may reduce porosity, whereas carbonation can release bound chlorides into free ionic forms, thereby accelerating steel corrosion [16].

Overall, SSC exhibits complex durability behavior. It may show advantages at early stages, but its long-term performance remains uncertain. These properties are strongly affected by salinity, aggregate impurities, mix design parameters, and curing conditions. Future studies should therefore clarify the coupled effects of chloride transport, carbonation, and environmental exposure. Such efforts will improve durability assessment and service-life prediction of SSC structures.

3.2. Development of CFST Column

As early as 1879, the United Kingdom had been in the actual engineering application of the CFST column for the abutments of the Steven Railway Bridge. In 1926, Lally [43] first gave the formula for calculating the bearing capacity of a round CFST column; the structure schematic diagram is shown in Figure 3. And only after entering the second half of the 20th century, various countries began to conduct extensive and systematic research on CFST [44].

3.2.1. Compressive Performance

Compressive performance is a fundamental property of CFST columns and has been widely investigated. The axial behavior of CFST members is mainly controlled by several key parameters. These include the strength of the core concrete, the diameter-to-thickness ratio of the steel tube, steel strength, slenderness ratio, loading eccentricity, and cross-sectional configuration.

Increasing the strength of the core concrete generally enhances axial load capacity. However, high-strength concrete may reduce ductility because of its brittle nature [45,46,47,48,49,50]. A smaller diameter-to-thickness ratio increases the contribution of the steel tube. Higher steel strength further improves confinement efficiency and load-bearing capacity [51,52,53,54].

The slenderness ratio strongly affects the failure mode. Short columns usually fail due to material strength limits. Slender columns are governed by global instability induced by second-order effects. Intermediate columns exhibit elastic–plastic buckling between these two modes [55,56]. Increasing slenderness weakens the confinement provided by the steel tube. As a result, the overall mechanical performance of CFST columns decreases [57,58,59,60]. Under eccentric loading, bending effects become pronounced. Local outward bulging may occur on the compression side. This phenomenon reduces the compressive capacity [61,62].

Various cross-sectional forms have been studied. These include circular, square, rectangular, elliptical, and polygonal sections. Circular sections provide the strongest confinement because of uniform stress distribution. Square and rectangular sections are also widely used due to their advantages in structural connections.

Beyond conventional solid CFST columns (Figure 4a–c), several advanced configurations have been developed to further enhance structural performance. These include double steel tube CFST columns, steel skeleton–steel tube CFST columns, internally reinforced CFST columns, hollow sandwich CFST columns, stiffened CFST columns, and composite CFST columns (Figure 4d–i). Systems with internal reinforcement can significantly increase load-bearing capacity [63,64,65,66,67,68], while hollow sandwich CFST columns exhibit higher bending stiffness and improved stability [69,70]. Stiffened CFST columns suppress local buckling of the steel tube through internal ribs [71,72,73], and composite CFST columns combine reinforced concrete with steel tube concrete to provide improved load capacity and protection against fire and corrosion [74,75].

3.2.2. Other Performance

In practical engineering structures, CFST members are often subjected to complex loading conditions. Therefore, their behavior under bending, shear, tension, torsion, and combined loads has been widely investigated.

During bending, the core concrete alters the failure mode of the steel tube. Wavy buckling usually appears in the compression zone, while multiple cracks form in the tension zone. Compared with conventional reinforced concrete members, CFST structures typically exhibit smaller crack spacing and narrower crack widths due to the confinement provided by the steel tube [76,77]. Under shear loading, the shear span-to-depth ratio governs the failure mechanism. A lower ratio leads to shear-dominated failure, whereas a higher ratio results in flexural behavior [78,79].

CFST members also show favorable tensile and torsional performance. Cracks in the core concrete are uniformly distributed, which promotes energy dissipation under tension [80]. Under torsion, the steel tube mainly resists diagonal tensile stresses, while the core concrete restrains local buckling of the steel tube [81].

Based on extensive experimental and analytical research, several international design standards have been developed to guide the application of CFST structures. These include ACI 318-19 [82], AISC 360-16 [83], Eurocode 4 [84], and the design recommendations of the Architectural Institute of Japan (AIJ) [85], as well as the Chinese national standard GB 50936-2014 [86]. These standards have played an important role in promoting the safe and standardized application of CFST structures.

Overall, CFST columns exhibit excellent load-carrying capacity, ductility, and structural stability under various loading conditions. However, steel tubes remain susceptible to corrosion in marine environments. This limitation has motivated the development of more durable composite material systems.

3.3. Emergence of FCFST Column

FRP materials emerged in the 1940s and were initially applied in aerospace and military fields due to their great performance. Since the 21st century, FRP has been used to strengthen CFSTs or combined with them to form innovative composite structures, prompting extensive investigations into their mechanical properties. Wang et al. [87] creatively proposed the idea of CFRP-confined CFST structure as shown in Figure 5, which is based on CFST and FRP cylinder filled with reinforced concrete. This demonstrates their feasibility and potential for civil engineering applications.

3.3.1. Fully Wrapped Structure

Fully wrapped FCFST columns are formed by externally wrapping CFST members with continuous FRP fabrics, as shown in Figure 5. This configuration provides additional confinement to the steel tube and the core concrete. Early studies by Xiao et al. [88,89,90] showed that CFRP confinement produces a bilinear load–displacement response under axial compression. It delays local buckling in potential plastic hinge regions. As a result, the load capacity, ductility, and hysteretic performance are significantly improved.

Subsequent studies further clarified the structural behavior of FCFST columns. Wang et al. [87,91,92,93,94,95,96,97,98] systematically investigated their responses under axial compression, bending, shear, and cyclic loading through experimental and numerical analyses. They also proposed design approaches for predicting the load capacity and restoring force of composite columns. Tao et al. [99,100,101,102,103] experimentally studied CFRP-confined circular and square CFST columns. The results showed that CFRP confinement significantly increases the load capacity of circular sections, while mainly enhancing the ductility of square sections. Their research also demonstrated that CFRP wrapping can effectively repair moderately damaged short columns after fire exposure.

Further studies by Lu et al. [104,105,106,107] and Teng et al. [108,109,110,111,112] extended the understanding of FCFST behavior under various loading conditions, including axial, cyclic axial, and lateral loads. The results confirmed that FRP confinement delays local buckling and significantly improves mechanical performance. These studies also established analytical models and stress–strain relationships for FCFST columns, providing an important basis for structural design.

3.3.2. Partially Wrapped Structure

In addition to the fully wrapped system, partially wrapped FCFST columns using discontinuous FRP strips have also been investigated, as shown in Figure 6. Sundarraja et al. [113] demonstrated that reducing the spacing of CFRP strips improves confinement efficiency and increases load capacity. Shen et al. [114,115,116] further reported that CFRP strips effectively confine short CFST columns, although local buckling is more likely to occur in the uncovered regions. For slender columns, global bending becomes dominant. This behavior reduces the effectiveness of external confinement.

A comparative study by Zeng et al. [117] showed that both CFRP and PET-FRP strips enhance yield strength and stiffness. However, PET-FRP provides greater deformation capacity because of its higher ultimate tensile strain. Recent studies have further extended FCFST applications to different material combinations. Al-Osta et al. [118,119] investigated CFRP-strengthened stainless-steel CFST columns. Their results showed that FRP wrapping effectively suppresses local buckling and improves both axial and flexural capacities. Dong et al. [120,121] examined CFRP-confined recycled-aggregate concrete-filled steel tubes. They reported that full wrapping generally provides higher strength than strip wrapping. Circular sections also perform better than square sections. Liu et al. [122] and Cao et al. [123] further explored the influence of high-strength and ultra-high-performance concrete. Their findings indicate that higher concrete strength increases peak capacity but reduces the marginal confinement effectiveness of external FRP.

Overall, FCFST columns rely on the combined confinement of the steel tube, concrete core, and FRP jacket. This composite action significantly enhances load capacity, ductility, and energy dissipation. As summarized in

Table 1. Comparison between fully and partially wrapped FCFST columns.

Comparison Factors	Fully Wrapped FCFST	Partially Wrapped FCFST
FRP arrangement style	Continuous wrapping along the full member length	Discrete strip wrapping with partial coverage
Confinement continuity	Continuous confinement, uniformly distributed along the axial direction	Discontinuous confinement, with localized effects
Core confinement mechanism	Stable and uniform triaxial confinement, with sufficient stress redistribution	Intermittent lateral confinement, with stress concentrated in wrapped zones
Strength and ductility enhancement	Significant and stable improvement, with a smoother post-peak response	Improvement achievable, but highly dependent on strip spacing and layout parameters
Local buckling suppression	Effectively delays or suppresses the local buckling of the steel tube	Local buckling often occurs in unwrapped regions
Stability of failure modes	Relatively stable failure, typically concrete crushing accompanied by FRP rupture	Nonuniform failure, often with localized and sudden collapse
Sensitivity to slenderness ratio	Effective for short and moderately slender members	Limited effectiveness for slender members, often governed by global buckling
FRP material efficiency	Higher FRP consumption and higher material cost	Lower FRP consumption and higher material efficiency
Construction complexity	Relatively complex procedures with strict quality control requirements	Simpler construction, suitable for strengthening and repair
Engineering economy	Higher initial cost but high-performance reliability	Lower cost but larger performance variability
Applicable scenarios	New structures, critical load-bearing members, and high-ductility demand projects	Strengthening of existing structures and cost-sensitive projects
Main limitations	High material cost and strict requirements on construction consistency	Discontinuous confinement and strong dependence on parameter design

, fully wrapped systems provide more uniform confinement and more stable failure modes. In contrast, partially wrapped systems offer advantages in material efficiency and construction economy. However, the effectiveness of FRP confinement decreases in slender members dominated by global buckling. These findings highlight the need for unified mechanical models and performance-based design approaches, particularly for marine environments where corrosion resistance and durability are critical.

4. State-of-the-Art of FCTSSC Column

4.1. Current Development on FCTSSC Column

FCTSSC columns have been proposed only in the past five years. Inspired by the replaceability of concrete types in FCFST columns, Wei et al. [124] used SSC and proposed a novel type of FCTSSC column. Unlike FCFST columns with single-layer FRP confinement, the inner FRP directly confines the lateral expansion of the SSC core. This promotes the early development of a triaxial compressive stress state. In contrast, the outer FRP mainly interacts with the steel tube and becomes effective after steel yielding. This sequential and complementary activation modifies the radial stress distribution. It reduces stress concentration at the steel–concrete interface. It also significantly delays the initiation and propagation of local steel buckling. Similar confinement mechanisms have been reported in past research [125]. Wei et al. [124] conducted axial compression tests on 36 FCTSSC columns. The experimental results indicated that under axial compression conditions, internal and outer FRP could synergistically enhance the structural bearing capacity and deformation capacity. Compared to the SFST column, the strength of FCTSSC columns could be increased by 11.7–66.5%. It can be observed that the essential difference between FCTSSC columns and FCFST columns lies in the presence of an inner FRP layer. This inner FRP enables an effective dual-confinement system. It significantly enhances the overall mechanical performance of the composite member. Meanwhile, the confinement mechanism is essentially similar to that of FCFST columns. Since FCFST and related composite members have been widely applied in practical engineering, such as buildings and bridges, the practical feasibility of FCTSSC columns can therefore be reasonably anticipated.

Based on that, several scholars investigated the mechanical properties of this column. Current research on FCTSSC columns primarily focuses on short columns with a slenderness ratio ranging from 5.6 to 8.4 [124,126,127,128,129,130,131,132], except for Refs. [1,2,3]. Experimental results show that these columns typically exhibited bulging or shear failure, accompanied by filament or banded rupture of the outer FRP fabrics. The number of FRP layers and the steel tube thickness have a significant positive effect on the ultimate stress of the specimens. Meanwhile, Wei et al. [126] proposed a classical model for predicting the ultimate bearing capacity of FCTSSC short columns. This model has been validated by a comprehensive database with favorable results. It can be observed that current research on FCTSSC columns is basically limited to short columns. There is a significant lack of studies on intermediate slender and slender columns, which are more applicable in actual engineering.

In addition, these studies utilize different composition materials to fabricate FCTSSC columns. In terms of the FRP fabrics, CFRP and BFRP are predominantly applied [2,3,124,126,127,128,129,130,131,133]. In contrast, studies on confinement using cost-effective GFRP materials remain very limited [1]. In some cases, the steel tube is replaced with steel wire mesh [129]. The core concrete includes common SSC [124,128,129,133], high-strength SSC [126,130], and coral aggregate SSC [127,131,132]. Compared with conventional SSC, coral aggregates typically exhibit higher porosity and greater brittleness. This degrades the strength of the aggregate skeleton. It also promotes microcrack initiation and crack propagation paths [134,135]. Such characteristics may influence the confinement efficiency by altering the lateral dilation behavior of the confined SCC core. Recent studies indicate that coral aggregate size significantly affects the uniformity of dilation and hoop strain. Existing stress–strain models developed for normal-aggregate concrete may not be directly applicable to coral aggregate concrete. This implies that the confinement efficiency requires recalibration [136]. In addition, the high porosity of coral aggregates accelerates the ingress and transport of aggressive ions. This leads to more pronounced durability degradation in marine environments. Therefore, for FCTSSC incorporating coral aggregates, these behaviors should be systematically investigated in future studies. In conclusion, the above results show that regardless of the FRP type, the FRP on both inner and outer surfaces works together effectively to provide confinement. CFCTSSC columns demonstrate higher ultimate strength, while BFCTSSC columns exhibit superior ultimate deformation capacity. For FCTSSC column specimens with higher concrete strength, the ultimate bearing capacity and its enhancement rate increase accordingly. Meanwhile, the steel wire mesh effectively reduced specimen damage and prevented sudden bearing capacity loss caused by FRP rupture.

Monotonic axial compression performance is fundamental to understanding the mechanical behavior of a kind of novel composite column structure. Current research on FCTSSC columns focuses primarily on this aspect [1,2,3,124,126,127,129,130,131,133]. Additionally, Zhang et al. [128] conducted cyclic axial compression tests on 12 FCTSSC columns. The results showed that FRP can effectively restrain the concrete expansion. Its strain recovery ability facilitates concrete shrinkage during unloading, and it slows down the development of plastic strain. Considering the different confinement mechanisms of the inner and outer FRP layers, a more rational cyclic stress–strain model is proposed.

As summarized in

Table 2. Summary of current research on FCTSSC columns.

References	Specimen Number	Slenderness Ratios	FRP Types	Outer FRP Layers	Steel Thickness (mm)	Key Findings	Limitations
[124]	36	8.4	BFRP, CFRP	1–3	4.5–7.0	The FCTSSC concept is innovatively proposed; significant improvements in axial strength and ductility are demonstrated.	The effects of slenderness ratio, number of inner FRP layers, steel tube strength, and concrete strength have not been considered. Mechanical performance under eccentric, cyclic loading and durability are also lacking.
[126]	36	8.4	BFRP, CFRP	1–3	4.5–7.0	The results show that high-strength SSC increases peak load capacity and reduces post-peak ductility. Confinement efficiency exhibits a nonlinear relationship with concrete strength.	The parameters of slenderness ratio, inner FRP layers, steel tube strength, and concrete strength are absent. Studies on mechanical behavior under eccentric, cyclic loads and durability are insufficient.
[127]	36	8.4	BFRP, CFRP	1–3	4.5–6.0	It is confirmed that coral-aggregate SSC alters expansion behavior and confinement efficiency due to its high porosity.	Key variables, including slenderness ratio, inner FRP layers, steel tube strength, and concrete strength, are missing. The mechanical response under eccentric, cyclic conditions and durability remains unexplored.
[128]	12	8.4	BFRP, CFRP	1–3	4.5	Cyclic axial tests show that double FRP layers mitigate stiffness degradation and cumulative damage.	The influences of slenderness ratio, steel tube thickness, inner FRP layers, steel tube strength, and concrete strength have not been addressed. Mechanical performance under eccentric loading and durability are not investigated.
[129]	18	5.6	BFRP, CFRP	2–3	-	It is verified that replacing steel tubes with wire mesh reduces the sudden capacity drop after FRP rupture.	The parameters of slenderness ratio, inner FRP layers, steel wire strength, mesh size of the steel wire, and concrete strength are not considered. Mechanical behavior under eccentric, cyclic loading and durability are not examined.
[130,131]	24	8.4	CFRP	1–3	4.5	The basic axial behavior and failure modes of CFRP–steel composite tubes confining SSC and coral-aggregate SSC are validated.	The effects of slenderness ratio, FRP type, inner FRP layers, steel tube strength, and steel tube thickness are not evaluated. Studies on mechanical properties under eccentric, cyclic loading and durability are lacking.
[132]	27	8.4	GFRP	2–4	4–6	The outer GFRP-tube thickness has a stronger influence on the compressive behavior of SSCAC-filled FCTSSC columns than the steel-tube thickness.	The parameters of slenderness ratio, FRP tube type, steel tube strength, and concrete strength are absent. The mechanical performance under eccentric, cyclic loads and durability remains unstudied.
[133]	18	11.2	CFRP	1–2	5	The ultimate capacity increases with the number of CFRP layers, and inner CFRP layers yield higher ultimate strength.	The influences of slenderness ratio, FRP type, steel tube thickness, steel tube strength, and concrete strength are not considered. Mechanical behavior under eccentric, cyclic loading and durability have not been investigated.
[1,2,3]	18	8.4–16.8	CFRP, GFRP	1–3	4.5	The GFRP system exhibits more progressive failure and better deformation compatibility than the CFRP system for the FCTSSC intermediate slender column.	The effects of FRP type, inner FRP layers, steel tube strength, steel tube thickness, and concrete strength are not addressed. Research on mechanical performance under eccentric, cyclic conditions and durability is insufficient.

, research over the past five years on FCTSSC columns has systematically clarified the mechanisms of strength enhancement, deformation improvement, and typical failure modes of short columns under monotonic and cyclic axial compression. A database-validated framework for load-bearing capacity prediction has also been established. However, existing studies are largely limited to short columns with small slenderness ratios. The mechanical behavior of intermediate slender and slender columns remains insufficiently understood, particularly under the coupled effects of second-order actions, global stability, and material nonlinearity. Research on damage evolution under cyclic or dynamic loading is also limited. In addition, systematic evaluations of cost-effective GFRP-confined FCTSSC and long-term durability performance are still lacking. Field applications and pilot projects of FCTSSC columns remain very limited. Future studies should expand the ranges of slenderness and key parameters. More comprehensive experimental databases are needed. Unified design and predictive models should be developed. These models should incorporate the differentiated confinement from inner and outer FRP layers and the associated stability effects. Such efforts will better support practical engineering and promote actual application.

4.2. Comparative Synthesis Analysis

4.2.1. Slenderness Ratio and Slenderness Ratio Limit

For CFST columns, studies [57,137,138,139,140,141,142] have investigated the effects of slenderness ratio on the mechanical performance of CFST columns with various concrete and steel materials. Specifically, Refs. [57,142] conducted experimental investigations on the monotonic axial compression and cyclic axial compression behavior of conventional CFST columns. It was found that specimens with larger slenderness ratios (20.1 to 61) failed due to global buckling. Specimens with smaller slenderness ratios (14.4 to 46) also experienced global buckling. However, thicker columns failed due to concrete crushing and steel yielding. For cyclic axial compression, the slenderness ratio (8.4 to 28) significantly influenced post-peak behavior, with strength degradation increasing markedly as the slenderness ratio rose. This transition in failure mode indicates that with increasing slenderness ratio, global buckling gradually governs structural response. In contrast, the influence of material strength and confinement gradually diminishes. This is consistent with the results reported in previous studies [143,144].

On the other hand, Fu et al. [137] performed axial compression tests on slender CFST columns filled with lightweight aggregate concrete. They found that the slenderness ratio (12 to 60.8) was the primary factor affecting the axial compression performance. Higher slenderness ratios resulted in lower ultimate bearing capacity and stability coefficients. These phenomena can be attributed to the amplified second-order effects induced by large lateral deflections in slender members. Such effects accelerate stiffness degradation and reduce axial load-carrying capacity [144]. Tokgoz et al. [138] used steel fiber concrete and stainless steel tubes to construct CFST columns and conducted bending and axial compression tests. Similarly, the slenderness ratio (12 to 20) significantly affected slender column behavior. Specimens with smaller slenderness ratios exhibited greater resistance and less buckling. Specifically, little to no local buckling was observed near the mid-height or at the mid-height of the columns. Huang et al. [139] investigated square CFST columns made with S960 steel tubes and high-strength concrete under axial compression. They observed that specimens with larger slenderness ratios exhibited noticeable lateral deformation and gradually developed global buckling before reaching the ultimate load. Ultimately, the slenderness ratio limit of CFST columns was determined, which failed in local buckling (L/D_s < 7), a combination of local and global buckling (13 < L/D_s < 19), or only global buckling (L/D_s > 24.5).

The above studies were conducted on defect-free CFST columns. However, Luo et al. [141] performed eccentric compression tests on slender CFST columns with localized corrosion. These specimens generally failed through overall compression-bending failure. When the corrosion volume loss was less than 10%, the failure mode was similar to that of non-corroded specimens, with the maximum lateral deflection occurring at mid-height. However, as the corrosion volume loss increased, the failure mode of columns with localized circumferential corrosion at one or both ends differed significantly from that of non-corroded columns. The peak deflection position gradually shifted from mid-height to the middle or vicinity of the corroded region at the ends. Multiple outward local buckling points appeared within the corroded areas due to stress concentration or interaction effects. These results indicate that initial imperfections and local damage can significantly modify the second-order response of slender CFST columns and alter their global instability path.

For FCFST columns, Gu et al. [145,146] found that compared to CFST columns, the load-bearing capacity enhancement of CFRP decreases as the slenderness ratio of specimens (12 to 90.2) increases. When the slenderness ratio reaches a certain threshold, the enhancement becomes negligible. Simultaneously, the confinement effect of the steel tube and CFRP on the core concrete diminishes. This is attributed to greater second-order effects at higher slenderness ratios, leading to early buckling failure. Past research also shows that although FRP confinement can effectively delay local buckling, global buckling governed by second-order effects becomes increasingly dominant as the slenderness ratio increases [147]. Consequently, the contribution of FRP confinement to overall buckling is limited. Subsequently, Wang et al. [148,149] conducted experimental studies on the axial and eccentric compression performance of FCFST columns. Their results revealed a significant influence of the slenderness ratio on ultimate bearing capacity. Specimens with low slenderness ratios (3 to 9) exhibited local surface bulging and strength failure, while those with higher slenderness ratios (13.5 to 22.5) experienced global buckling failure. This transition further confirms that the slenderness ratio controls the shift in FCFST columns from confinement-controlled behavior to instability-controlled behavior [150]. Additionally, Zeng et al. [151] fabricated FCFST specimens using PET-FRP and high-strength steel tubes for eccentric compression tests. They observed that as the slenderness ratio (9.5 to 28.5) increased, the load-bearing capacity decreased. Compared to the eccentricity ratio, the sensitivity of load degradation to the slenderness ratio was lower. Apart from circular FCFST columns, Li et al. [152] and Du et al. [153] investigated the axial compression performance of square and rectangular FCFST columns. They analyzed the effects of concrete strength, section aspect ratio, and steel tube thickness on the mechanical behavior of intermediate slender columns.

The studies mentioned above focused on fully wrapped FCFST columns. However, Shen et al. [114] examined the axial compression behavior and failure mechanisms of partially wrapped FCFST columns. Test results indicated that failure modes for partially wrapped FCFST columns include steel tube bulging, severe FRP rupture, concrete crushing, and adhesive delamination, with bulging primarily occurring in unwrapped regions. As the slenderness ratio (12.9 to 65.7) increased, the ultimate load-bearing capacity decreased. Moreover, the axial capacity of slender columns was insensitive to FRP strip spacing and the number of FRP layers. In summary, related studies on slenderness ratios and slenderness ratio limits are summarized in

Table 3. Summary of parameters investigated by past research.

Parameter Investigated	CFST	FCFST	FCTSSC
Slenderness ratio and slenderness ratio limit	[57,137,138,139,140,141,142]	[114,145,146,147,148,149,150,151,152,153]	[1,2]
Confinement materials	—	[112,114,154,155,156,157,158,159]	[124,126,127,128,129]
Confinement thickness	—	[108,114,145,146,149,154,155,156,157,158,159,160,161]	[1,2,3,124,126,127,128,129,130,131,132,133]
Finite element model	[139,140,162,163,164,165]	[114,152,157,166]	[133]
Empirical model	[50,140,141,142,162,163,164,167,168,169]	[113,114,146,153,155,157,159,160,161,166,170,171,172,173,174,175,176,177,178]	[126,128,133]

.

4.2.2. Confinement Materials and Thickness

Due to the susceptibility of CFST columns to outward buckling of the steel tube and environmental corrosion, FCFST columns were developed by externally confining CFST with FRP. This highlights the crucial role of FRP in enhancing the mechanical properties of FCFST columns. Wang and Gu [145,146,149] conducted pioneering studies on FCFST columns using CFRP fabrics (1 to 3 layers). They found that CFRP and steel tubes work synergistically to significantly improve the ultimate load capacity and stiffness of CFST columns. Subsequent studies [114,160,161] performed axial compression tests on FCFST columns using CFRP-confined steel tubes and various concrete materials. Results showed that FRP confinement increased the load capacity of CFST specimens by up to 71.35%, with load capacity exhibiting an approximately linear growth as the number of CFRP layers increased.

Following these studies, researchers began exploring different types of FRP for FCFST columns to compare their mechanical performance. Hu et al. [108] investigated FCFST columns confined with GFRP fabrics (1 to 4 layers) through axial compression tests. Their results indicated that GFRP significantly delayed or even suppressed the local buckling of steel tubes. GFRP confinement also notably enhanced both the load capacity and ductility of CFST columns. Other studies [112,154,155,156,157] used both CFRP (1 to 3 layers) and GFRP (1 to 3 layers) to fabricate FCFST columns, examining their seismic, axial compression, cyclic loading, and lateral impact performance. The findings revealed that increasing the number of FRP layers, whether CFRP or GFRP, improved the mechanical properties. However, CFRP exhibited superior enhancement in seismic and axial compression performance, whereas GFRP provided better ductility. Under lateral impact, CFRP laminates showed brittleness, while GFRP wrapping significantly reduced FRP damage severity. In addition, Yang et al. [158] and Liu et al. [159] conducted axial compression tests on FCFST columns confined with BFRP and CFRP. Compared to CFST specimens, FCFST columns demonstrated significant improvements in ultimate load capacity and ductility. Moreover, CFRP exhibited superior enhancement in ultimate load capacity and post-yield stiffness compared to BFRP.

For FCTSSC columns, an internal FRP layer is added to FCFST to prevent chloride-induced corrosion in SSC. Wei et al. [124,126,127,128,129,130,131] investigated FCTSSC short columns confined with CFRP fabrics (1 to 3 layers) and BFRP (1 to 3 layers), using SSC types such as ordinary SSC, high-strength SSC, and coral aggregate SSC. Test results showed that the ultimate load capacity of FCTSSC columns increased by up to 66.5% compared to CFST columns. The ultimate load capacity also increased with additional FRP layers, consistent with the findings in Ref. [133]. Overall, CFRP-confined specimens exhibited superior enhancement in ultimate load capacity compared to BFRP-confined specimens, while the opposite trend was observed for ductility. Additionally, Sun et al. [132] replaced FRP fabrics with GFRP tubes to fabricate FCTSSC columns. Axial compression tests revealed that the thickness of the external GFRP tube had a more significant impact on compressive performance than the steel tube thickness. For specimens with the same steel tube wall thickness, the ultimate compressive load capacity increased by 66.5–131.9% compared to CFST columns. Moreover, the external GFRP tube provided significantly better confinement than the internal GFRP tube. In summary, related studies on confinement materials and thickness are summarized in Table 3.

4.2.3. Model Development

For finite element numerical models, the accuracy and effectiveness of finite element models for CFST columns have been developed and validated in the studies [139,140,162,163,164,165]. These studies employed appropriate material constitutive models, element types, interaction definitions, meshing strategies, and boundary conditions to realistically simulate the mechanical behavior of specimens. Various parameters influencing the compressive, flexural, seismic, impact, and durability performance of CFST columns were investigated. These parameters include steel yield strength, steel tube width-to-thickness ratio, concrete strength, slenderness ratio, eccentricity, and impact velocity. On the other hand, based on experimental studies, reasonable and effective finite element models for FCFST columns have also been developed [114,152,157,166]. These models examined the effects of FRP sectional ratio, steel tube wall thickness, and concrete strength on the axial compression behavior of FCFST specimens.

The development of these finite element models effectively captured the failure modes and stress distributions of the specimens. They allowed extending parameter ranges beyond experimental conditions, offering a more comprehensive understanding of the specimens’ mechanical behavior. In contrast, no finite element models have been developed for FCTSSC columns to date, excluding Ref. [133]. This gap highlights a major deficiency in current research on FCTSSC columns. The modeling of this novel structural system faces several challenges as follows: (1) The complex interactions among SSC, the steel tube, and the inner and outer FRP layers must be captured. (2) An appropriate constitutive model for concrete confined by FRP and steel is required. To address these issues, future model development for FCTSSC columns may adapt the following standardized frameworks: (1) Perfect bonding among FRP, steel, and concrete can be assumed, with interfacial slip neglected. (2) A validated concrete damage plasticity model, or an equivalent constitutive model calibrated specifically for confined SSC, can be adopted. (3) A benchmark experimental database should be established to support model validation and enable comparisons across different studies.

For bearing capacity prediction models, researchers have proposed models for CFST columns based on experimental studies and theoretical analyses, achieving good agreement with test results [50,141,162,163,164,167,168]. Some classical capacity and stress–strain models have been derived and validated through integrating CFST experimental databases and theoretical deductions [140,142,169]. Validation results show that these models exhibit high accuracy. For FCFST columns, experimental studies have examined monotonic axial compression, cyclic axial compression, and durability performance [111,114,146,153,155,157,159,160,161,170,171,172,173]. This leads to the development of corresponding bearing capacity models. Over time, these models have been continuously validated and updated, contributing to experimental databases and forming more comprehensive and reliable classical models for analysis and design [166,174,175,176,177,178].

However, for FCTSSC columns, only Wei et al. [126,128] and Chen et al. [133] have proposed capacity prediction models for short columns based on experiments. Due to the limited research on FCTSSC columns, the development of mature and accurate classical models remains a significant challenge. In summary, related studies to model development are summarized in Table 3.

4.3. Summary

In summary, the current development of related research and the research and tests conducted by past research are illustrated in

Table 4. Research and tests conducted by past research.

Type	References	Axial- Compressed	Eccentric- Compressed	Seismic	Impact	Durability
SSC	[7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]	✓	✗	✗	✗	✓
CFST	[43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,137,138,139,140,141,142,162,163,164,165,167,168,169]	✓	✓	✓	✓	✓
FCFST	[87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,166,170,171,172,173,174,175,176,177,178]	✓	✓	✓	✓	✓
FCTSSC	[1,2,3,124,126,127,128,129,130,131,132,133]	✓	✗	✗	✗	✗

and Figure 7, respectively. Durability research refers to the evolution of the structural performance of FCTSSC members under environmental exposure and subsequent loading. It is evident that SSC exhibits mechanical properties similar to those of conventional concrete, and its applicability in practical engineering has been demonstrated. Although CFST columns exhibit good mechanical performance, they suffer from local buckling and steel tube corrosion. FCFST columns address these issues. However, current research indicates that they are predominantly filled with ordinary concrete. When combined with SSC for practical engineering applications, the inner walls of the steel tubes still experience corrosion. This will reduce the strength and durability of the components. FCTSSC columns can effectively overcome these drawbacks. They offer superior mechanical properties compared to the aforementioned composite columns. Meanwhile, they can prevent corrosion of the steel tubes by external environments and internal seawater–sand concrete. This makes them highly suitable for marine engineering applications.

Based on Table 3 and Table 4, it can be seen that the CFST columns are a mature system with a wealth of experimental evidence, widely accepted theoretical models, and mature design specifications. Building on CFST columns, FCFST columns have been studied in considerable depth. Multiple design-oriented approaches have been proposed. However, broader standardization and unified codes remain limited. This is mainly due to the diversity of FRP configurations and failure mechanisms. In contrast, FCTSSC columns are still at an early stage of development. Existing evidence is limited in terms of specimen databases, loading conditions, slenderness ratio ranges, and durability verification. System-level numerical frameworks and design codes have not yet been standardized. Therefore, substantial advances are required before FCTSSC columns can be widely adopted in practical engineering applications.

5. Sustainability and Engineering Implications

5.1. Techno-Economic Feasibility, Sustainability Trade-Offs, and Scalability

For the practical adoption of emerging FCTSSC columns, techno-economic feasibility, sustainability trade-offs, and scalability must be carefully planned. Regarding the FRP materials used in FCTSSC columns, GFRP is generally considered more cost-effective than CFRP and BFRP. However, the economic feasibility of FCTSSC columns should be assessed from a life-cycle perspective rather than by initial cost alone. Compared with conventional corrosion-protection strategies for steel–concrete systems (e.g., coatings, cathodic protection, or stainless steel), the dual-layer FRP configuration in FCTSSC introduces additional upfront costs. These include FRP materials, fabrication of inner liners and outer wraps, and quality control during installation. Concrete materials with superior structural performance, such as engineered cementitious composites (ECC), and monitoring techniques required for post-service structural maintenance should be considered [179,180,181,182]. Recent studies on FRP structural systems indicate that the higher initial investment can be partially or fully offset by reduced maintenance, delayed corrosion initiation, and extended service life in aggressive marine environments. This highlights the necessity of life-cycle cost assessment rather than simple material cost comparison [183,184].

From an environmental perspective, the sustainability advantages of FCTSSC arise from the combined use of SSC and corrosion-resistant FRP confinement. Previous life-cycle assessment studies consistently show that SSC significantly reduces freshwater consumption and alleviates pressure on natural river sand resources. This is particularly beneficial in coastal and island regions [185]. However, FRP production, especially for carbon-fiber composites, is associated with relatively high embodied energy and environmental burdens. These burdens mainly stem from fiber manufacturing and resin synthesis processes [184,186]. Therefore, the net environmental benefit of FCTSSC depends on the balance among three factors: (1) the environmental benefits provided by SSC; (2) the embodied impacts of FRP materials; and (3) the extent to which FRP confinement extends service life and reduces repair or replacement frequency. Future studies should thus adopt consistent life-cycle assessment frameworks to evaluate the sustainability of FCTSSC at the structural system level, rather than at the material level alone.

Beyond economic and environmental considerations, the scalability and constructability of FCTSSC medium-to-long columns also pose practical challenges. The fabrication of long FCTSSC members requires stringent quality control of multi-interface systems. These include the SSC–inner FRP interface, the inner FRP–steel tube interface, and the steel tube–outer FRP interface. Potential defects, such as voids, debonding, or resin-rich zones, may impair confinement efficiency and durability. Transportation constraints often limit the feasible length of prefabricated members. As a result, segmented fabrication and on-site assembly represent a practical solution for future applications. In this context, the mechanical performance and durability of segmental joints, end anchorage of FRP layers, and continuity of confinement become critical issues for future design and construction.

5.2. Ethical and Regulatory Considerations

Beyond mechanical performance and durability, the application of FCTSSC columns in marine infrastructure requires careful consideration of ethical, regulatory, and environmental issues. From a structural safety perspective, current international design standards do not provide unified provisions for composite columns integrating SSC, steel tubes, and dual-layer FRP confinement. A pragmatic compliance pathway can be established by adopting Eurocode 4 [84] as the primary framework for steel–concrete composite action and stability design, while using ACI 318-19 [82] as a reference for fundamental concrete structural safety principles. The performance and long-term stability of FRP components can be guided by existing FRP-specific provisions, such as ACI 440.11-22 [187] for GFRP-reinforced concrete and ACI 440.1R-15 [188], particularly with respect to durability reduction factors and material safety considerations.

From an environmental and ethical standpoint, potential concerns must be addressed regarding the release of FRP-related debris or microplastics into marine ecosystems. In FCTSSC columns, FRP is typically embedded as inner liners and external wraps around steel tubes, rather than being directly exposed to freely flowing seawater. However, recent marine pollution studies indicate that GFRP fragments can be a source of marine debris and microplastic pollution under conditions of abrasion, accidental damage, or end-of-life disposal [189,190,191]. These findings suggest that ecological risks are primarily associated with damage scenarios (abrasion and impact), construction waste (cutting and grinding), and end-of-life disposal processes, rather than with normal and intact service conditions during routine operation.

Accordingly, responsible engineering practice for FCTSSC columns should incorporate both preventive and management measures. These include surface protection and inspection to minimize wear-induced damage, strict control and recycling of FRP waste during manufacturing and construction, and clearly defined end-of-life management strategies. By explicitly aligning emerging composite systems such as FCTSSC with existing structural codes, environmental regulations, and engineering ethics principles, future applications in marine infrastructure can better balance innovation, safety, and environmental protection.

6. Conclusions

This paper reviews the current research progress on SSC, CFST, FCFST, and FCTSSC structures. It focuses on their mechanical behavior, confinement mechanisms, material influencing factors, and existing key issues. The main conclusions are as follows:

(1) SSC offers advantages in workability and early-age strength. However, chloride ions in seawater and sea sand induce steel corrosion. This significantly reduces durability. Thus, the direct use of SSC in conventional reinforced concrete structures remains limited.

(2) CFST improves concrete strength and ductility through external steel confinement. Yet local buckling and corrosion of the steel tube remain major weaknesses. For CFST structures in marine environments, durability concerns are particularly critical and require further improvement.

(3) FCFST effectively delays steel-tube buckling and enhances overall behavior under axial, flexural, and cyclic loads. CFRP markedly improves load capacity. GFRP and BFRP provide better ductility. FRP thickness shows an approximately linear effect on strength enhancement. However, when SSC is used as infill, internal corrosion of the steel tube remains, limiting long-term use in marine engineering.

(4) FCTSSC applies inner and outer FRP confinement. This dual-layer system blocks chloride ingress into the steel tube and enables the feasible use of SSC in concrete-filled steel tube structures. Tests show that its load capacity exceeds that of CFST. Different FRP types also meet varied engineering demands. Expansion, shrinkage, and brittle behavior of SSC are significantly restrained under dual confinement.

(5) Current research on FCTSSC focuses mainly on short columns. Studies on global buckling of medium- and long-slenderness columns, second-order effects, and FRP–steel interaction mechanisms remain insufficient. Unified design theories and accurate nonlinear FE models are still lacking. Research on durability is very limited, especially regarding chloride transport, multi-factor coupling, and long-term behavior.

Overall, FCTSSC integrates resource sustainability with superior structural performance. It represents a promising system for future marine infrastructure. However, further studies are needed to establish a complete theoretical and design framework. Future research directions include:

(1) Investigating the mechanical behavior of medium- and long-slenderness columns under axial, flexural, and cyclic loading, and clarifying buckling evolution under dual confinement.

(2) Developing a comprehensive FRP–steel–SSC interaction model and corresponding design methods.

(3) Conducting systematic durability studies, including chloride transport based on rapid chloride migration tests and long-term natural diffusion tests, and freeze–thaw cycles based on cyclic freeze–thaw tests.

(4) Establishing high-accuracy FE frameworks and computational models for engineering optimization.

(5) Advancing engineering demonstrations and practical applications of FCTSSC in marine engineering, offshore wind power, and port structures.