Fiber Reinforced Polymer Tubes Research Articles

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.

Chord axial compressive behavior of hybrid FRP-concrete-steel double-skin tubular member T-joints

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular members (DSTMs) are a promising form of structural members with superior mechanical performance and durability, which have great potential for application in ocean structures. Such hybrid DSTMs consist of three components: an inner steel tube, an outer FRP tube, and concrete filled between the two tubes. Despite a significant number of studies demonstrating the excellent performance of hybrid DSTMs, no studies have been concerned with the joints of these members. The lack of a reliable design method for DSTM joints is certainly a huge obstacle to their wide application in practice. Against the above background, a research project has been proposed to understand the static behavior of circular DSTM T-joints through a combined experimental, modeling, and theoretical study. This paper presents the results of an experimental program on the axial compressive behavior of the chord in DSTM T-joints. The effects of various important factors, such as FRP tube thickness, steel tube thickness, brace-to-chord diameter ratio, void ratio, and concrete strength, on the performance of the DSTM T-joints were examined in detail. The test results demonstrated that the DSTM T-joints exhibited a ductile behavior provided that the joint region was appropriately strengthened and the presence of a brace did not significantly affect the compressive behavior of the chord in the joints with a brace-to-chord diameter ratio of 0.5. Finally, a simple method was proposed to predict the axial load-axial strain behavior of the DSTM T-joints.

Durability assessment of hybrid double-skin tubular columns (DSTCs) under simulated marine environments

Fiber-reinforced polymer (FRP)-concrete-steel hybrid double-skin tubular columns (hybrid DSTCs), consisting of an outer FRP tube, an inner steel tube, and concrete infill between them, are a relatively new structural system. Despite DSTCs being proven to have enhanced structural capabilities over traditional concrete columns, their durability in aggressive environments remains unclear. This study presents a durability assessment of hybrid DSTCs with a glass fiber-reinforced polymer (GFRP) tube. Hybrid DSTCs were immersed in artificial seawater at different temperatures (i.e., room temperature, 40 °C, and 60 °C) for a duration of up to 540 days. The time-dependent behaviors, including axial load-strain (axial and hoop) curves, compressive strength, and ultimate axial and hoop strain, are systematically investigated at different aging times. Test results reveal that the compressive strength of DSTCs exhibits a slight reduction at room temperature and 40 °C, while an obvious reduction is observed at 60 °C. After 540 days, the compressive strengths for hybrid DSTCs with 3-mm-thick and 6-mm-thick GFRP tubes reduce to 79% and 93%, respectively. A decrease in strength is notable for DSTCs with a 3-mm-thick GFRP tube at 60 ℃. The degradation of GFRP tubes is also observed via scanning electron microscope (SEM) analysis, with greater degradation observed in 3 mm-thick GFRP tubes compared to the 6 mm-thick counterparts.

Novel hybrid tube consists of GFRP-PU-graphite powder for lightweight structure applications

This research successfully introduced an innovative lightweight tube made from lightweight PU Foam material and reinforced with glass fiber. The key findings from the tests are as follows: Glass fiber-reinforced polymer (GFRP) tubes without polyurethane (PU) foam withstood a pressure of 4.03 kN in compression testing. The specimen with 5 % graphite filler had the highest density at 0.262 g/mm3, while the specimen with 10 % graphite filler had the lowest density. The specimen with 5 % graphite filler also exhibited the highest hardness (19.5 Shore Hardness D), whereas the specimen with 20 % graphite filler had the lowest hardness (13.2 Shore Hardness D). The contraction of PU foam significantly influenced the material strength. GFRP tubes reinforced with PU foam offer advantages in terms of material lightness and improved material strength. These tubes can be applied in structural, automotive, and aerospace applications.

Mesoscale study on the mechanical properties and energy absorption characteristics of aluminum foam-filled CFRP tubes under axial compression

This study investigates the axial compression mechanics and energy absorption characteristics of aluminum foam-filled carbon fiber-reinforced polymer (CFRP) tubes from both experimental and simulation perspectives. Quasi-static compression experiments of aluminum foam specimens, single CFRP tubes and aluminum foam-filled CFRP tubes are carried out. Based on the experimental results, compression simulations are conducted to investigate the structural deformation and the energy absorption characteristics of aluminum foam-filled CFRP tubes at the mesoscale. The effects of structural parameters on the compression mechanics and energy absorption characteristics of aluminum foam-filled CFRP tubes are explored.

Influence of cyclic loading on lightweight self-compacting concrete double-skin tubular columns

Self-compacting concrete (SCC) has gained prominence in recent decades due to its inherent qualities, particularly its ability to compact autonomously. SCC proves invaluable in scenarios where traditional compaction is challenging due to placement conditions or the presence of close space reinforcement. This study focuses on evaluating the structural performance of lightweight self-compacting concrete (LWSCC) columns confined with glass fiber-reinforced polymer (GFRP) and steel tube (ST) subjected to cyclic loading. The LWSCC columns, denoted as LWSTC, are designed with inner steel tubes and outer GFRP tubes, imparting corrosion resistance and reduced weight alongside enhanced ductility. Through quasi-static loading, five fabricated LWSTC columns were subjected to assess their structural response under dynamic effects. The investigation delves into three key variables: axial load ratio, reinforcement ratio, and the replacement ratio of coarse lightweight expanded clay aggregate (LECA) in the fabricated LWSTC columns. A comprehensive analysis of various parameters, including skeleton curves, ductility, hysteretic cycles, damage modes, dissipation of energy capacity, stiffness behavior, strain spreading, and strength reduction, was conducted on the LWSTC columns. The findings reveal that the replacement of LECA with natural aggregates has an insignificant impact, whereas varying the axial load ratio significantly influences the structural behavior of LWSTC. Notably, LWSTC exhibits a remarkable 121% higher horizontal load capacity compared to counterparts using natural aggregate. Furthermore, LWSTC demonstrates superior axial strength and enhanced capability for energy dissipation.

Stability analysis of pultruded basalt fiber-reinforced polymer (BFRP) tube under axial compression

This study analyzed mechanical properties and stability of pultruded basalt fiber-reinforced polymer (BFRP) tubes under compressive axial loading for large-span space and truss structures applications. This study assessed the compressive strength of BFRP tubes with three different cross-sectional shapes and investigated the failure modes under compression with slenderness ratios ranging from 20 to 150. The results demonstrated that short BFRP tubes can achieve a compressive strength of up to 122.36 MPa with a compressive elastic modulus of 40.39 GPa. Three types of compressive failure modes were observed in the BFRP tubes, including local material, critical, and overall buckling failures. Furthermore, based on the analysis of experiment results, two design-oriented three-stage theoretical models were proposed for BFRP tubes with three different cross-sectional shapes. The proposed models were able to predict both the stress–strain and load-lateral deflection curves by taking into account the post-peak softening behavior of the stress–strain curve. In addition, a stability equation was also derived for predicting the compressive strength of slender BFRP tubes and was validated by experiments. The predictions derived from proposed models were consistent with experiment results.

Axial compressive behavior and load carrying capacity prediction of prefabricated UHPC tube filled with seawater sea-sand coral aggregate concrete

To reduce construction costs, seawater, sea sand, and dead coral are often used in concrete buildings on distant islands. Such seawater-sea sand coral aggregate concrete (SSCAC) typically has insufficient mechanical properties and high chloride ion content. In this paper, a novel composite column, named SSCAC-filled ultra-high performance concrete (UHPC) tube (SFCT), is proposed and studied. The SFCT column combines a prefabricated fiber reinforced polymer (FRP) hoop reinforced thin-walled UHPC tube with the infill SSCAC. A total of 36 relative large-scale columns were conducted for axial compression tests. The SFCT columns showed a multi-crack failure mode on the surface of the tube without obvious peeling-off of the UHPC cover in the whole loading process. The test results reveal that the UHPC tube in the composite system effectively takes on the additional load released by the inner SSCAC during compacting due to the porous nature and low strength of coral aggregates. Therefore, the compressive behavior of the pre-peak stage of SFCT column is effectively improved compared with existing SSCAC confined by FRP tube or hoops, especially important is increased the axial stiffness. The proposed composite system effectively combines the ultra-high compressive performance of UHPC and the lateral confining effect from FRP hoops. A model was also proposed for predicting the axial load carrying capacity of SFCT columns. The analysis results indicate that selecting relatively thinner UHPC tube in SFCT composite system and using relatively flexible and low-cost basalt FRP (BFRP) and glass FRP (GFRP) hoops instead of carbon FRP (CFRP) hoops as the lateral confining component are economical and could be reasonable measures for manufacturing the composite columns.

Axial compression behavior of double-skin FRP-concrete-steel tubular columns: Experimental and analytical investigations

Double-skin tubular columns (DSTCs) with fiber-reinforced polymer (FRP) confined concrete have shown promising load-bearing capacity and ductile behavior under axial loading. A new variation of DSTC with concrete sandwiched between an outer FRP tube and a stiffened inner steel tube can provide additional benefits than the traditional unstiffened column form. The first objective of this study was to experimentally investigate the behavior of square-square (SS) shaped stiffened DSTCs with concrete sandwiched between outer FRP-tube and an inner steel tube. Experimental results of 22-test specimens are discussed and then analysis-oriented stress-strain model is proposed for the FRP confined concrete, quantifying the influence of stiffened steel tubes, stiffener characteristics and unconfined concrete strength used in the tests. The proposed confined-concrete model was then used to perform the finite element (FE)-based calibration study against test results for stiffened DSTCs with square-square (SS) cross-sectional shape. The stiffener properties varied in both experimental and analytical studies include quantity, layout or configuration, and cross-sectional dimensions. Based on the proposed confined concrete model, the FE-based calibration demonstrated superior agreement with the experimental results (2 to 5% average difference at critical stages of loading, i.e. “elastic”, “peak” and “ultimate” axial loads). The experimental and numerical based results were finally used to propose a simple equation for accurately predicting the axial load capacity of stiffened SS-shaped DSTCs.

Recycled aggregate concrete-filled steel tube columns with small-diameter circular FRP-confined sea-sand concrete cores: Conceptual and experimental investigations

Recycled aggregate concrete (RAC) prepared from waste concrete can achieve the green recycling of resources; however, its inferior material properties hinder its applications. Sea-sand concrete (SSC) can help alleviate the resource shortage of fresh water and river sands but is prone to steel corrosion. To address these challenges, this study developed a new RAC-filled steel tube column with a small-diameter circular fibre-reinforced polymer (FRP)-confined SSC core. This paper presents and discusses the results of the axial compression tests conducted on RAC-filled steel tube columns, with and without small-diameter circular FRP-confined SSC cores. The study investigated the effects of the configuration of the FRP-confined SSC core, diameter-to-thickness ratio of the FRP tubes, compressive strength of RAC and SSC, and cross-sectional shape of the column. The results revealed that using small-diameter circular FRP-confined SSC cores to partially replace RAC was effective in improving the compressive performance of RAC-filled steel tube columns. The increase in the number of FRP tubes, in-filled concrete strength, and confinement stiffness of the steel and FRP tubes contributed to improvements in the structural strength and ductility, resulting in progressive ladder-shaped declines in the post-peak curve. Finally, a reasonable load–capacity model is presented for the proposed columns.

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.

Compressive behavior of FRP-confined cruciform steel-reinforced normal- and high-strength concrete columns

Fiber-reinforced polymer (FRP)-confined steel-reinforced concrete column (FCSRC), which consists of an external FRP tube and a steel-reinforced concrete (SRC) core, have been proposed to avoid the brittleness of SRCs. Capitalizing on the benefits of FRP-confine concrete, it is expected that full strength capacities of high-strength materials (i.e., high-strength concrete and high-strength steel) in FCSRCs can be exploited. Axial compression tests on FCSRCs with different strengths of concrete and steel were conducted in this study. A cruciform steel was adopted in FCSRCs. The parameters investigated in this study included the concrete strength, the steel strength and the FRP tube thickness. The test results demonstrated that the excellent performance of FCSRCs. Compared with normal-strength cross-shaped steel, the high-strength cruciform steel delays the development of hoop strains in the FRP tube for FCSRCs with normal-strength concrete. Additionally, the ultimate axial load of FCSRCs is generally larger than the direct summation of those of the steel section and FRP-confined concrete section (i.e., CFFT), demonstrating that the interaction between the three components (concrete, steel profile and FRP tube) in FCSRCs are in a benificial manner.

Axial compression behavior of FRP-confined steel-reinforced columns with a flange-broadened cross-shaped steel

Due to the hybrid confinement effect and reforming effective confinement area, the composite action of square columns is enhanced by utilizing cross-shaped steel on fiber-reinforced polymer (FRP) tube-confined steel-reinforced columns. These novel hybrid columns are FRP-confined concrete-encased cross-shaped steel columns (FCCSCs). This study adopted flange-broadened cross-shaped steel to develop the potential advantages of FCCSCs. Twenty-two specimens were tested under monotonic and cyclic axial compression, and the test parameters were column form, FRP thickness, cross-shaped steel dimension, and slenderness ratio. FCCSCs with broad steel flanges exhibited high axial load capacity and axial deformability. In addition, increasing the thickness of the flange, web, or FRP tube resulted in stronger hybrid confinement and outperformed strength behavior in confined concrete. Slender FCCSCs presented lower axial capacity and higher lateral deflection. Finally, the calculation method for compressive load capacities of slender FCCSCs is proposed.

Eccentric Compression Properties of FRP-Concrete-Steel Double-Skin Square Tubular Columns.

FRP (fiber-reinforced polymer)-concrete-steel double-skin square tubular (FCSST) columns are composed of an outside FRP tube, an inside steel tube and the concrete filled between them. Under the continuous constraint of the outside and inside tube, the strain, strength and ductility of concrete are improved significantly compared with those of traditionally reinforced concrete without lateral restraint. Additionally, the outside and inside tube not only function as the permanent formwork in casting but improve the bending and shear resistance of composite columns. Meanwhile, the hollow core also reduces the weight of the structure. Through the compressive testing of 19 FCSST columns subjected to eccentric load, this study focuses on the influence of eccentricity and layers of axial FRP cloth (away from the loading point) on the evolution of axial strain along the cross-section, axial bearing capacity, axial load-lateral deflection curve and other eccentric properties. The results can provide basis and reference for the design and construction of FCSST columns and are of great theoretical significance and practical value for the application of composite columns in the engineering of structures in a corrosive environment and other harsh conditions.

Cyclic Compressive Behavior and Load-Strain Model of FRP-ECC-HSC Composite Columns

An innovative composite column, which consists of a high strength concrete (HSC) core, engineered cementitious composites (ECC) ring, and fiber-reinforced polymer (FRP) tube, has recently been developed and subjected to monotonic axial compression testing by the authors. In this study, the cyclic compressive behavior of this proposed FRP-ECC-HSC composite column was examined. Test parameters, including HSC core strength, FRP tube thickness, and ECC ring thickness, were investigated. Typical failure modes, dilation behavior, and axial load versus axial strain behavior were discussed and analyzed. It was found that the FRP-ECC-HSC composite columns exhibited improved deformability, compared with the counterpart traditional FRP-confined HSC columns, with the ultimate axial compressive strain increased by 0.7%–69.1% for the tested specimens. Meanwhile, the ultimate axial strain for cyclically loaded specimens is larger than that for monotonically loaded specimens in general, indicating a delayed column failure. Cyclic axial load-axial strain models, including the envelope model, unloading and reloading models, plastic strain equation, and stress deterioration equation, were proposed to predict the cyclic compressive behavior of the tested specimens. The proposed model was verified with the test results and exhibited good performance.

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.

Bond behaviour between CFRP, GFRP, and hybrid C-GFRP tubes and seawater sea sand concrete after exposure to elevated temperatures

The paper provides an overview of the current research on bond performance between fibre-reinforced polymer (FRP) tubes and SWSSC and investigates the bond strength of SWSSC-filled FRP tubes after exposure to different temperatures using push-out tests. A total of 27 samples, including glass FRP (GFRP), carbon FRP (CFRP), and glass-carbon hybrid FRP (HFRP) tubes, were tested to determine the effect of elevated up to 250 °C temperatures on the bond performance of the samples. It was observed that exposure to elevated temperatures increased the bond strength of all the tubes. The GFRP and CFRP tubes exhibited the strongest and weakest bond strengths, respectively. Since the inner layer of the HFRP tube was composed of CFRP, the bond strength of HFRP was found to be more similar to that of CFRP samples than GFRP samples. The study concluded that the bond strength of all the samples was predominantly influenced by the surface roughness of the inner tube rather than the fibre type.

Cyclic behavior of recycled-lump-concrete-filled GFRP tubular columns: Experimental investigation and numerical modeling

A fiber-reinforced polymer (FRP) tube offers the advantages of lightweight formwork, external confinement, and non-corrosive reinforcement for recycled lump concrete (RLC), creating an attractive opportunity for structural applications of this green material. While prior research has predominantly focused on the axial compressive behavior of RLC-filled FRP tubular columns, this study addresses the implications of using such members in seismic areas. Six large-scale RLC-filled Glass FRP (GFRP) tubular columns were tested under constant axial compression and quasi-static reversed lateral loading to examine the impact of crucial parameters on their cyclic behavior. Additionally, the study discusses the numerical modeling of such columns using a fiber beam element approach. The findings indicate that the replacement rate of recycled lumps (<33%) has limited impact on the cyclic behavior of the columns. Reducing the GFRP tube thickness and simultaneously adding a minimum amount of longitudinal steel reinforcement can improve the hysteretic response. An off-axis tube with a fiber orientation close to 50° appears more suitable for structural use compared to 80°. Moreover, even with a tube diameter-to-thickness ratio of 100, the columns still meet the code requirement for lateral drift limit of flexural-dominated members. Overall, the study contributes to instilling confidence in the scaling-up applications of RLC-filled GFRP tubular columns in earthquake-prone areas.