Concrete-filled Fiber-reinforced Polymer Tubes Research Articles

Flexural behavior evaluation of concrete filled small-angle winded GFRP tube columns

In harsh environments such as ocean engineering, reinforced concrete structures suffer from severe corrosion, and concrete-filled fiber-reinforced polymer tubes (CFFT) have been widely used as a substitute for them. In this paper, extremely small-angle GFRP tubes are manufactured using the winding process and the flexural behavior of ±55° and ±15° angle-ply GFRP tube columns is investigated. The effects of the winding angle on the flexural behavior of CFFT were studied by experiment. The experimental results showed that the failure modes of CFFT were dominated by fiber rupture, slip between concrete and GFRP tube, and concrete crushing. Reducing the winding angle and filling concrete can effectively improve the ultimate flexural load by 117.25%, and 182.01%, respectively. Furthermore, a numerical model was established to predict the flexural behavior of CFFT, the Concrete Damaged Plasticity model was used to simulate the behavior of the concrete, and the Hashin failure criterion was used for the GFRP tube. The predicted failure modes and load-displacement curves were in good agreement with the experimental results. Parametric analysis showed that increasing the GFRP tube thickness can improve the ultimate flexural load by 62.72% and increasing concrete strength changes the failure mode of CFFT from GFRP tube rupture to concrete damage.

Repair of concrete-filled FRP tube columns with damaged tubes

The effectiveness of repairing damaged Concrete-Filled Fiber Reinforced Polymer (FRP) Tube (CFFT) axial members is studied. The damage was introduced in the form of linear cuts through the full tube thickness. The effectiveness of adhesively-bonded FRP patches that may or may not include mechanical fasteners is investigated. The study varied the cut angle, the bond length of the patch, the fiber type being glass or carbon, the number and pattern of mechanical fasteners, the patch extension beyond cut tips, and compared wet layup glass-FRP (GFRP) patches to prefabricated GFRP shells cut from a similar tube. It was shown that the vertical cut reduced axial compressive strength significantly. As the bond length of the GFRP repair patch increased the axial strength recovery increased, but not fully. When the ends of the GFRP patch were overlapped, a full strength recovery was achieved. The prefabricated GFRP shell patch (cut from similar tube) provided the highest strength recovery, compared to FRP wet layup. Adding mechanical fasteners enhanced strength recovery further, to a certain point. A simple empirical design model is proposed and a design example is presented.

Finite Element Model of Concrete-Filled, Fiber-Reinforced Polymer Tubes for Small-Scale Wind Turbine Towers

The finite element method was used to study the feasibility of concrete-filled, fiber-reinforced polymer tubes (CFFTs) for small-scale wind turbine towers in remote areas. Although CFFTs have been successfully employed for a variety of structural applications, their use for wind turbine towers is novel and has yet to be investigated in detail. The objective of the study was to identify, for the first time, the most important parameters for design and compare the behavior of CFFT towers versus conventional steel and concrete towers. The model was first validated using experimental results reported in the literature followed by a series of parametric studies to evaluate the importance of several key parameters. In the first phase, the effect of different geometric properties (taper and concrete filling ratio) and reinforcement configurations (FRP laminate configuration, steel reinforcement ratio, and prestressing level) were investigated for cantilever tower models with concentrated lateral loads. A 10 m high CFFT wind turbine tower model was subsequently modeled and studied under different loading configurations. The influence of the height-to-diameter (h/D) ratio on cantilever CFFT models was also studied and a conservative preliminary design that can be refined for specific turbine systems and wind conditions was adopted using the h/D ratio. The CFFT tower model was compared to concrete and steel tubular models with similar geometry to study the advantages of CFFT towers and showed that CFFTs can be an efficient alternative.

Elliptical concrete-filled FRP tubes with an embedded H-shaped steel under axial compression and cyclic lateral loading: Experimental study and modelling

Concrete-filled FRP tubes (CFFTs) have found increasing applications due to their superior ductility and remarkable corrosion resistance. In published literature, circular CFFTs and rectangular CFFTs have been investigated extensively, while there are relatively few studies on elliptical CFFTs. There is no research about elliptical CFFTs with an embedded H-shaped steel (i.e., HS-ECFFTs) under axial compression and cyclic lateral loading. Against this background, this paper investigated the seismic behaviour of HS-ECFFTs experimentally and numerically. Test results indicated that: (1) all HS-ECFFTs performed well with remarkable energy dissipation ability; (2) due to the effective FRP confinement, the local buckling of the H-shaped steel was prohibited; (3) the elliptical aspect ratio had limited influences on the ductility, the stiffness degradation and the energy dissipation; (4) the FRP thickness, after a certain threshold, showed beneficial but limited influences on the peak load, the stiffness degradation and the energy dissipation; (5) the HS-ECFFT specimen performed much better in terms of the peak load, the ductility, the stiffness degradation and the energy dissipation when it was bending around the strong axis. The proposed numerical model established on the OpenSees platform could generate reasonably accurate numerical results for all HS-ECFFTs.

Axial strength prediction of seawater sea sand concrete-filled circular FRP tubes under alkaline environment based on ensemble learning algorithms

The rapid development of marine and urban infrastructure led to the extensive studies on seawater sea sand concrete (SWSSC) filled fiber reinforced polymer (FRP) / steel tubes. The material property of FRP is a function of fiber orientations, and the confinement of SWSSC by FRP tubes enhances its axial load carrying capacity. And therefore, its strength prediction is very challenging because of various FRP layouts, surrounding harsh environment and complicated failure modes. The existing empirical models do not consider effects of surrounding seawater environment under elevated temperature. Therefore, this study concentrates on evaluating the axial capacity of two types of physical models: (1) SWSSC filled circular FRP tubes immersed in seawater environment (Pu1), and (2) SWSSC filled FRP-steel-FRP circular tubes (Pu2), with the help of machine learning (ML) algorithms namely extreme gradient boosting (XGBoost), support vector machine (SVM) and artificial neural networks (ANN). The experimental results of 138 and 120 tested specimens were used for the training and testing of ML models. The models were trained with the best hyperparameters based on grid search and cross validation approach. The models were evaluated with number of statistical indices i.e., R2, MAE and RMSE accompanied with visual comparison of trend line between experimental and predicted values and predicted to experimental ratios. The ML models were graded on the basis of performance as XGBoost > ANN > SVM for the Model 1, whereas for the Model 2, the order was observed as ANN > XGBoost > SVM. Further, SHAP analysis was conducted based on XGBoost to see the influence of input attributes on ML models.

Seismic performance of circular concrete-filled FRP tubes consisting of H-steel with shear studs: Experimental study and numerical modelling

Existing studies about concrete-filled FRP tubes (CFFTs) are generally focused on their behaviour under axial load, which have demonstrated their excellent compressive ductility. For their applications in regions with seismic risk, longitudinal reinforcements should be provided for CFFTs in order to ensure their ability to resist bending moments. In published literatures, longitudinal reinforcements for CFFTs were mostly steel rebars or FRP rebars, while only limited studies were about CFFTs reinforced with profiled-steel. Against this background, 6 circular large-scale CFFTs consisting of H-steel with shear studs (HS-CFFTs) were investigated experimentally under combined axial load and lateral cyclic load to study their seismic behaviour. These HS-CFFTs were designed with a total height of 2150 mm and a concrete diameter of 300 mm, and the testing parameters included the FRP thickness (tfrp = 4.0 mm ∼ 6.0 mm), the axial load ratio (n = 0.2 ∼ 0.33), as well as the area ratio of H-steel (ρ = 4.54% ∼ 5.67%). Test results indicated that, (1) HS-CFFTs exhibited rounded hysteretic curves with ample energy dissipation capacity; (2) the H-steel experienced no local buckling due to the effective concrete support and the sufficient FRP confinement; (3) the FRP tube thickness was positively correlated with the peak lateral load and the ductility of HS-CFFTs; compared with HS-CFFTs with tfrp = 4 mm, HS-CFFTs with tfrp = 6 mm had a higher peak load FP (7.3% ∼ 7.4% higher) and a larger ductility index μδ (3.4% ∼ 11.7% higher); (4) increasing the H-steel ratio could effectively improve the seismic performance of HS-CFFTs; HS-CFFTs with ρ = 5.67% had higher peak load (2.5% ∼ 10.0% higher) compared with those with ρ = 4.54%; (5) the ductility of HS-CFFTs would decrease obviously as the axial load ratio increased; the ductility index μδ for HS-CFFTs with n = 0.33 was smaller (14.1% ∼ 25.0% lower) than the counterparts with n = 0.2. A numerical model incorporating with “NonlinearBeamColumn” elements as well as specific constitutive models for the FRP tube, the H-steel and the concrete core was established on the OpenSees platform. The proposed numerical model provided a simple, computationally efficient and reasonably accurate approach for simulating the hysteretic behaviour of HS-CFFTs.

EXPERIMENTAL INVESTIGATION OF THE COMPRESSIVE STRENGTH OF CONCRETE-FILLED FRP TUBE CYLINDERS SUBJECTED TO FIRE

Concrete-filled fiber reinforced polymer tubes (CFFTs) are becoming an attractive system for many different structural applications, such as piles, columns, and piers. However, many engineers are hesitant to use FRPs in structure because of the lack of knowledge of their behavior when they are subjected to fire. This study presents test results of thirteen CFFT cylinders with tube thickness ranges from 1.9 to 4.3 mm subjected to direct fire for periods ranges from 2 to 15 minutes and compared to reference confined and unconfined concrete cylinder. The tubes are composed of filament wound E-class fiber immersed in polyester resin. The test results show almost no losses in compressive strength for 2 minutes burning period for all tested tube thicknesses. By increasing the burning period to 15 minutes, the residual compressive strength significantly deteriorated to 44%, 56%, and 74% of the referenced strength for the unburned tubes with thickness 1.9, 3.7, 4.3 mm, respectively. Further investigation and observations were recorded for the surface condition after exposure to fire.

Combined torsion and axial compression loading of concrete-filled FRP tubes

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFT) were studied under combined torsion (T) and axial compression (P). Tests were performed on 166 mm diameter CFFTs with 30 MPa unconfined concrete strength (f’c) and a nearly cross-ply FRP tube. Three stub column samples were tested in axial compression and five samples were tested in torsion while subjected to varying levels of effective axial compressive stresses, namely zero, 0.22, 0.45, 0.65, and 0.85f’c. The study showed that the P-T interaction curve was characterized by an increasing trend of T with small-to-moderate levels of P, up to a ‘balanced’ point where concrete damage mode changes, followed by a reduction in T with P thereafter. Effective axial stresses of 0.45f’c and 0.65f’c increased maximum T by 21% and 29%, respectively, while at 0.85f’c the maximum T reduced to a value similar to pure T. The maximum increase in T occurs at the balanced point where P restrains diagonal tension crack opening but does not cause concrete shear–compression damage and sliding at cracks. Failures occurred from sudden rupture of the FRP tube near a concrete crack location at high levels of twist, with a pseudo-ductile response from partial concrete confinement and FRP non-linearity. The torque contribution of the concrete core (Tc) and FRP tube (Tfrp) were determined using experimental data and Classical Lamination Theory (CLT). The total torque was predominantly from Tc (66%–76%); however, this significant concrete contribution was only enabled by the FRP tube.

Experimental study on concrete-filled steel tube columns with inner distributed seawater and sea sand concrete-filled fiber-reinforced polymer tubes under axial compression

To effectively solve the problem of the corrosion of undisturbed seawater and sea sand concrete (SSC) in practical applications, a new structure of concrete-filled steel tube (CFT) columns with inner distributed SSC-filled fiber-reinforced polymer (FRP) tubes was proposed. A total of 40 specimens were tested to investigate the axial compression behavior according to three parameters. The results indicated that the ultimate strength of CFT columns with inner distributed SSC-filled FRP tubes increased with the increase of FRP tube thickness, FRP tube diameter and steel tube thickness. The arrangement of inner FRP tubes effectively improved the axial compressive behavior and ductility of CFT columns. The ultimate strength of the CFT columns improved by 6.23 %–91.14 % due to the inner FRP tube. The failure of the specimens was always accompanied by fracture of one of the inner FRP tubes, while the remaining inner FRP tubes continued providing confinement. Lastly, two models based on different theories were proposed and evaluated to predict the ultimate load of this structure. The evaluation results indicated that both models provided accurate and reasonable predictions for the design of this structure.

Seismic behavior and modeling of elliptical concrete-filled FRP tubes with longitudinal steel rebars under lateral cyclic load and vertical constant load

Concrete-filled FRP tubes (CFFTs), which consist of a confining tube made of FRP, the internal concrete core and optional longitudinal reinforcements made of FRP or steel, are a novel type of structural member particularly attractive for new civil and industrial structures. Existing studies on CFFTs are mainly about specimens fabricated with a circular, a square or a rectangular FRP tube, while there are rather limited studies on CFFTs with an elliptical FRP tube. Similar to circular CFFTs, elliptical CFFTs have a cross-section with a continuous curved circumference, leading to effective confinement to the internal concrete. Elliptical CFFTs are attractive for engineering applications due to their appealing aesthetic appearance as well as different bending capacities around two symmetrical axes. This paper systematically investigated elliptical CFFTs reinforced with longitudinal steel rebars (ER-CFFTs) under combined lateral cyclic load and vertical constant load with special research attentions on the influences of elliptical aspect ratio (i.e., the ratio of the sectional major axis over the sectional minor axis, which was ranging from 1.0 to 2.5 with an interval of 0.5). The effects of the FRP thickness and the bending direction of lateral cyclic load were also investigated. Test results indicated that, the elliptical FRP tube had been effectively utilized to provide confinement during the cyclic loading process; ER-CFFTs exhibited generally rounded hysteretic curves with limited pinching, even for the specimen with a large elliptical aspect ratio of 2.5; the ductility of ER-CFFTs showed a general tendency to increase with the elliptical aspect ratio except the specimen with an elliptical aspect ratio of 2.5. A numerical model was established to simulate the seismic performance of ER-CFFTs, which was able to yield predictions with reasonable accuracy. In practical engineering, the elliptical aspect ratio for ER-CFFTs should be optimized according to the bending demand around two symmetrical axes.

Compressive Behavior of Concrete-Filled Filament-Wound FRP Tubes with Local Tube Damage

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) are an emerging and attractive form of column for new constructions. Such a column consists of outer filament-wound FRP tubes that are filled with plain or steel-reinforced concrete. The fibers in the filament-wound FRP tube are predominantly in the hoop direction to confine the inner concrete, which leads to significantly enhanced strength and ductility in the confined concrete. Extensive studies have been conducted on the behavior of CFFTs that were subjected to various loading conditions, which confirmed the excellent performance of such members. As CFFTs become increasingly used in practice, there is a concern about the performance of CFFTs when the FRP tube is subjected to local damage that is caused by accidents, vandalism, or designed holes or cuts to accommodate connections with other structural components. Some studies have been carried out on the performance of CFFTs as flexural members with a locally damaged filament-wound FRP tube; however, the research on such CFFTs as columns remains limited. This paper presented the results of a comprehensive experimental program on the axial compressive behavior of CFFTs with a filament-wound FRP tube that was subjected to local tube damage. Two types of damage (i.e., holes and cuts) with different parameters were investigated. The test results showed that the compressive strength [i.e., peak stress (fcc′)] and the corresponding axial strain (ɛcc) of the damaged CFFTs were significantly reduced due to the local tube damage (e.g., fcc′ reduced from 12.2% to 64.8% and the corresponding ɛcc reduced from 35.2% to 77.2%). Finally, an existing model for FRP-confined concrete that considered the local FRP damage was evaluated which suggested the need for the recalibration of the model or the development of a new model for CFFTs with a locally damaged filament-wound FRP tube.

Behavior and modeling of large-scale concrete-filled FRP tubes with longitudinal steel rebars under lateral impact loading

Fiber-reinforced polymer (FRP) has been utilized extensively as a confining material for concrete in civil engineering. Concrete-filled FRP tubes (CFFTs), consisting of a FRP tube, a concrete core and optional longitudinal rebars, are particularly attractive to be used as bridge piers in harsh environments. Most studies on CFFTs have been focused on their behaviors under static loading conditions. To extend existing studies, three identical large-scale CFFTs with longitudinal steel rebars (SR-CFFTs) were investigated experimentally, two of which were tested under lateral impact loading and one was under lateral static loading. To compare the confining behavior between FRP tube and steel stirrups, three identical large-scale reinforced concrete (RC) columns with approximately the same hoop confinement stiffness as SR-CFFTs (i.e., steel stirrups for RC columns, FRP tube for SR-CFFTs) were tested as companion specimens. The test results indicated that, (1) under lateral static loading, the RC column failed with a shear-failure manner, while the SR-CFFT specimen failed with combined bending and shear failure characteristics; (2) under lateral impact loading, the shear-dominant failure mechanism was evident for RC columns, while the failure mechanism of SR-CFFTs was flexural-dominant owing to the effective hoop confinement provided by FRP; (3) under the same impacting velocity, the SR-CFFT specimen had much less damage and much less lateral displacement than the corresponding RC column. The established FE models could provide reasonably accurate predictions for the dynamic behaviors, the impact force–time history curves and the column deformations for both RC columns and SR-CFFTs. Parameter studies were conducted to analyze the effects of the concrete strength, the longitudinal steel ratio, the impact velocity and the impact mass.

Compressive Behavior of GFRP Tubes Filled with Self-Compacting Concrete

Self-compacting concrete (SCC) features excellent flow ability without the need of vibration for compaction during casting. This makes it particularly suitable for application in hybrid fiber-reinforced polymer (FRP)-concrete–steel double-skin tubular columns (DSTCs), a high-performance structural form evolved from concrete-filled FRP tubes (CFFTs). In a DSTC, the space between the inner steel tube and the outer FRP tube for filling concrete may be narrow and the application of vibration may be difficult. However, because of the large shrinkage of SCC compared to traditional concrete of normal flow ability, the integrity and composite action of DSTCs with SCC become a major concern. To clarify this issue, an understanding of the behavior of CFFTs filled with SCC is an essential prerequisite. While a large number of studies have been published on CFFTs with normal concrete (NC), only limited studies have been carried out on CFFTs with SCC. This paper presents a comprehensive investigation on the effect of SCC on the behavior of large-scale CFFTs. Axial compression tests were carried out on concrete-filled glass FRP (GFRP) tubes with diameters of 150, 200, 300, and 400 mm. The compressive behavior of the CFFT specimens filled with NC, a nonexpansive SCC, and an expansive SCC (ESCC) was compared and discussed. The results showed that the specimens with nonexpansive SCC behaved differently compared to those with NC as a result of larger shrinkage, especially for large specimens for which the confinement level was lower. This situation, however, improved in specimens where ESCC was used. Finally, the test results were further analyzed using a theoretical model recently proposed by the authors’ group.

New Analytical Analysis-Oriented Stress-Strain Model for FRP-and-Steel Confined Concrete

Application of fiber-reinforced polymer (FRP) composites in the strengthening of reinforced concrete (RC) structures has become an increasingly accepted engineering practice. In particular, the use of externally bonded FRP wraps as a confining material for concrete can enhance both the compressive strength and the ultimate strain of concrete, making it suitable for strengthening and/or seismic retrofit of existing RC columns. This paper focuses on a recently developed analysis-oriented iterative FRP-and-steel confinement model for concrete and proposes a new optimization procedure to obtain an analytical expression for the corresponding monotonic envelope, reduce the associated computational cost, and increase the corresponding numerical robustness. Several analytical functions were evaluated in terms of their capability to fit the iteratively-generated stress-strain monotonic envelope for confined concrete. The newly proposed analytical formulation of the confined concrete stress-strain model was compared with the original iterative formulation in terms of computational cost for two examples of nonlinear seismic response analyses for: (1) an experimentally-tested concrete-filled FRP tube bridge column of a two-column bridge pier; and (2) a five-span bridge structure with FRP-retrofitted RC piers. It is found that the use of the newly proposed optimization-based analytical monotonic envelope can reduce by more than 30% the computational time associated with the original iteration-based monotonic envelope with negligible changes in the structural response prediction at both global and local levels.

Concrete-Filled FRP Tubes under Combined Torsion and Bending

Concrete-Filled FRP Tubes under Combined Torsion and Bending

Experimental investigation of axial strength of damaged concrete-filled FRP tubes

Axial compressive strength of circular concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) benefits greatly from a significant concrete confinement by the tube. Accidentally or deliberately induced damage to the tube would undermine the CFFT strength. In this study 33 CFFT short columns of 144–168 mm (5.67–6.61 in.) outer diameter (D) and 420–460 mm (16.53–18.11 in) height (H) (i.e. H/D = 2.74–2.92) were tested under concentric loading to failure. Tube damage was artificially induced in the form of through-thickness linear cuts at mid-height. The cut length (x) and orientation (θ) with respect to the longitudinal axis were varied. Two types of GFRP tubes were studied, namely a nominal [±55o] angle-ply tube with diameter-to-thickness ratio (D/t) = 35 and a [86°/−12°] near cross-ply with D/t = 48, to investigate two commonly available but distinctly different tubes. The effect of cuts in the two tubes studied was very similar. A nonlinear descending trend can be seen in axial strength ratio (Pn/Pno), relative to the intact CFFT strength (Pno), with x/H ratio. Initially, a rapid decent from 1.0 to 0.6 occurs in (Pn/Pno) (i.e. reduction in confinement effectiveness fcc’/fc’ from 2.0 to 1.2), as x/H ratio increases from zero to 20 % (i.e.x/D = zero to 60 %) followed by a lower rate decline to 0.52 (fcc’/fc’=1.0) at x/H = 60 % (x/D = 175 %). Vertical and diagonal cuts significantly affected strength, whereas horizontal cuts had insignificant effect on strength. Strain measurements revealed stress concentrations at the tip of the cut, where crack propagations occurred in the tube. A design-oriented simple model is presented to capture the strength reduction of the damaged CFFT column.

Strain localization analysis using digital image correlation for PVA fiber-reinforced concrete-filled FRP tubes under compressive loading

This study presents the outcomes of the first experimental study on the strain localization of polyvinyl alcohol (PVA) fiber-reinforced concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs). 24 CFFT specimens were prepared and tested with various instrumentations, such as linear variable differential transformer (LVDT) and strain gauges (SGs), to record required full-field strains, i.e., axial, lateral and Von Mises strains. The strain evolutions over the surface of the specimens were recorded during axial compression loading. The FRPs were prepared with carbon, S-glass or basalt fiber to examine the influence of the fiber type and confinement stiffness on the behavior and strain localization of CFFTs. The evolution of Von Mises strains was determined and studied under axial compression. The obtained results show that although the general pattern of localization for both CFFTs without and with PVA fiber is nearly similar, the characteristics of the localization are different. The expansion of shear zone for insufficiently basalt FRP (BFRP)-confined concrete is more localized compared to the more homogenized behavior of sufficiently FRP-confined concrete. However, insufficiently BFRP-confined PVA fiber-reinforced concrete exhibits less localized behavior compared to insufficiently BFRP-confined plain concrete, which is due to the influence of the internal fibers on the concrete cracking behavior. At FRP rupture, hoop strains vary along the height and around the perimeter of the specimens, which results in the inability of SGs to consistently capture the actual hoop rupture strain.

Flexural strength of post-tensioned concrete-filled fiber-reinforced polymer rectangular tube beams

This study extends an extensive research program carried out at the University of Sherbrooke to design and assess the potential use of rectangular, concrete-filled fiber-reinforced-polymer tube (CFFT) beams post-ten­sioned with steel tendons in bridge applications. This paper describes research to enhance the flexural perfor­mance of post-tensioned CFFT beams. Five rectangular post-tensioned CFFT beams were tested up to failure, and the effects of attaching a thin carbon-fiber-rein­forced polymer (CFRP) laminate embedded in tension flange and its total reinforcement ratio as well as tube structure fiber laminate were investigated. Last, a simpli­fied design approach is proposed based on strain com­binability and force equilibrium to estimate the flexural moment capacity of the tested beams. The specimens with two inclined fiber patterns in the hoop direction or added CFRP laminate strips embedded in the bottom flange of the tubes exhibited substantially greater flexur­al strength, absorbed energy, and serviceability perfor­mance than the control specimens. The ductility index and energy ratio ranged from 8.3 to 10.6 and from 82% to 87%, respectively, which indicates ductile behavior. Also, adding CFRP laminate strips embedded in the bottom flange of the tubes enhanced the flexural strength by 17% on average compared with post-tensioned CFFT without CFRP laminate. The specimen with the CFRP laminates in the bottom flange of the tube achieved flexural strength and energy absorption that was com­parable to the flexural strength and energy absorption of the specimen with two layers of inclined fiber patterns. The findings suggest that the design can be optimized to achieve more efficient post-tensioned CFFT structural members. The proposed design approach successfully predicts the flexural strength of the tested beams with an average of 1.05 ± 0.05 for the partially confined concrete model and an average of 1.11 ± 0.07 for the unconfined concrete model.

Fibre-Reinforced Polymer Reinforced Concrete Members under Elevated Temperatures: A Review on Structural Performance.

Several experimental and numerical studies have been conducted to address the structural performance of FRP-reinforced/strengthened concrete structures under and after exposure to elevated temperatures. The present paper reviews over 100 research studies focused on the structural responses of different FRP-reinforced/strengthened concrete structures after exposure to elevated temperatures, ranging from ambient temperatures to flame. Different structural systems were considered, including FRP laminate bonded to concrete, FRP-reinforced concrete, FRP-wrapped concrete, and concrete-filled FRP tubes. According to the reported data, it is generally accepted that, in the case of insignificant resin in the post curing process, as the temperature increases, the ultimate strength, bond strength, and structure stiffness reduce, especially when the glass transition temperature Tg of the resin is approached and exceeded. However, in the case of post curing, resin appears to preserve its mechanical properties at high temperatures, which results in the appropriate structural performance of FRP-reinforced/strengthened members at high temperatures that are below the resin decomposition temperature Td. Given the research gaps, recommendations for future studies have been presented. The discussions, findings, and comparisons presented in this review paper will help designers and researchers to better understand the performance of concrete structures that are reinforced/strengthened with FRPs under elevated temperatures and consider appropriate approaches when designing such structures.

Compressive Behavior and Analytical Model of Ultra-Early Strength Concrete-Filled FRP Tube With Zero Curing Time

Fiber-reinforced polymer (FRP) has been widely used in civil engineering due to its light weight, high strength, convenient construction, and strong corrosion resistance. One of the important applications of FRP composites is the concrete-filled FRP tube (CFFT), which can greatly improve the compressive strength and ductility of concrete as well as facilitate construction. In this article, the compressive performances of a normal concrete-filled FRP tube (N-CFFT) column with 5-hour curing time and an ultra-early strength concrete-filled FRP tube (UES–CFFT) column with zero curing time were studied by considering the characteristics of rapid early strength improvement of ultra-early strength concrete and the confinement effect of the FRP tube. Monotonic axial compression tests were carried out on 3 empty FRP tubes (FTs) without an internal filler and 6 CFFT (3 N-CFFTs and 3 UES-CFFTs) specimens. All specimens were cylinders of 200 mm in diameter and 600 mm in height, confined by glass fiber–reinforced polymer (GFRP). Test results indicated that the compressive bearing capacity of the specimens increased significantly by adopting the ultra-early strength concrete as the core concrete of the CFFT, although the curing time was zero. It was also shown that the compressive behavior of the UES–CFFT specimens with zero curing time increased significantly than that of the N-CFFT specimens with 5-hour curing time because the former was able to achieve rapid strength enhancement in a very short time than the latter. The ultimate compressive strength of UES–CFFT specimens with zero curing time reached 78.3 MPa, which was 66.2 and 97.2% higher than that of N-CFFT with 5-hour curing time and FT specimens, respectively. In addition, a simple confinement model to predict the strength of UES–CFFT with zero curing time in ultimate condition was introduced. Compared with the existing models, the proposed model could predict the ultimate strength of UES–CFFT specimens with zero curing time with better accuracy.