Outer Fiber-reinforced Polymer Tube Research Articles

Behavior of hybrid FRP-concrete-steel double-skin tubular member T-joints subjected to brace axial compression

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular members (DSTMs) are a novel form of structural members comprised of an outer FRP tube, an inner steel tube, and an annular concrete sandwiched between the two tubes. By leveraging the advantageous properties of each constituent material, DSTMs possess superior mechanical performance and exceptional durability, making them highly promising for applications in ocean structures. Despite extensive research highlighting the excellent performance of individual DSTMs such as beams and columns since their inception, investigations into the behavior of DSTM joints have been scarce. This critical knowledge gap presents a significant obstacle to the widespread practical application of DSTMs. To address this gap, the present study was proposed to investigate the behavior of circular DSTM T-joints subjected to brace axial compression. An experimental program was conducted to systematically examine the influence of key factors such as FRP/steel tube thickness, brace-to-chord diameter ratio, void ratio, and concrete strength on the performance of DSTM T-joints. The experimental results demonstrated that the DSTM T-joints exhibited a ductile behavior; FRP confinement was demonstrably effective in enhancing the joint bearing capacity; the brace-to-chord diameter ratio and void ratio were identified as two crucial factors influencing the joint bearing capacity. Finally, simple design equations capable of providing reasonably accurate and slightly conservative predictions for the bearing capacities of the test DSTM T-joint specimens were proposed.

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.

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.

Cyclic load tests and finite element modeling of self-centering hollow-core FRP-concrete-steel bridge columns

This paper presents experimental and numerical investigations of the seismic performance of a novel self-centering (SC) hollow-core (HC) fiber-reinforced polymer (FRP)-concrete-steel (SC-HC-FCS) bridge column. This new structure is fabricated by mounting external energy dissipators (ED) and applying unbonded post-tensioned (PT) basalt FRP (BFRP) tendon to the conventional HC-FCS column that consists of an outer FRP tube and an inner steel tube, with the space between filled with concrete. The SC-HC-FCS column combines the advantages of accelerated bridge construction and self-centering. The effects of the initial prestress force values and the configuration of the energy-dissipated aluminum bar on the column performance are studied. Based on the experimental and numerical results, it is found that the proposed SC-HC-FCS column shows adequate self-centering and energy dissipation capacities. However, it is required to properly select the configuration and material properties of the aluminum bar as energy dissipators to further refine the seismic resistance of the SC-HC-FCS column.

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.

Compressive behavior of PET FRP-confined concrete encased CFST columns

Environmental friendly construction materials such as polyethylene terephthalate (PET) fiber-reinforced polymer (FRP) and high-strength materials such as high-strength steel (HSS) are attracting wide attentions. In this study, a total of ten novel circular PET FRP-confined Concrete Encased Concrete-filled steel tube columns (PFCECs) with a Q690 steel tube and a PET FRP tube are tested to understand their axial compressive behavior, with the investigated variables covering the thickness of the outer PET FRP tube and the diameter ratio. The experimental results demonstrate that the load-carrying capacity of PFCECs with a given inner steel tube increase with an increase in the diameter ratio, but the ultimate axial strains and the residual axial loads do not necessarily increase with the increase of the diameter ratio. In the end, this paper compares the test results against the predictions from two existing models, and the comparisons show that the models give comparatively accurate predictions on the ultimate axial stresses/strains of the core concrete in PFCECs, but they are unable to capture complete axial stress-axial strain responses of the core concrete in PFCECs accurately.

Cyclic compressive behavior and load–strain model of FRP–concrete​ double tube composite columns

A novel fiber-reinforced polymer (FRP)–concrete double tube composite column, consisting of an outer filament winding FRP tube, an inner pultruded FRP tube and infilled core concrete and ring concrete, has been proposed and experimentally investigated under monotonic compression by the authors. It exhibited obviously improved deformability than the traditional FRP-confined concrete columns. In this study, cyclic compression tests were conducted on the composite column to examine its structural behavior under cyclic loadings. Effects of different outer and inner FRP tube thicknesses and ring concrete types were investigated. Failure modes, cyclic load–strain responses and hoop strain behavior were presented and analyzed. Cyclic load–strain model, including the envelope model, unloading and reloading models, plastic strain equation and stress deterioration equation, was proposed to predict the cyclic compressive behavior of the FRP–concrete double tube composite columns. The proposed model was verified against the test results and exhibited good performance.

Experimental study on the behavior of CFRP-high strength concrete-steel double skin tubular columns subjected to axial compression

This paper discusses the findings of an experimental investigation into the axial compressive behavior of fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) made using high-strength concrete. DSTCs are composite columns that consist of an inner steel tube, an outer FRP tube, and concrete in between. The diameter and wall thickness of the inner steel tube in DSTCs served as the test parameters for this investigation. 21 cylindrical specimens with a diameter of 152 mm and a height of 305 mm were fabricated from high-strength concrete with a compressive strength of 55.9 MPa and subjected to monotonic axial compression. The findings from this experimental investigation demonstrate that as the diameter of the inner steel tube grows the load capacity of the DSTCs decreases, with this impact being dependent on the thickness of the outer FRP tube. In addition, the load capacity of the DSTCs increases as the thickness of the inner steel tube increase, independent of the thickness of the outer FRP tube.

Hybrid FRP-concrete-steel double-skin tubular columns of varying slenderness ratios under eccentric compression

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) are an emerging form of hybrid column with excellent mechanical performance and durability. Such columns consist of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes. While extensive experimental and theoretical studies have been conducted on such columns, these studies have been focused on short specimens under concentric compression. This paper presents the results of a series of tests on hybrid DSTCs of varying slenderness ratios subjected to eccentric compression. The effects of FRP confinement stiffness, load eccentricity, and slenderness ratio on the behavior of hybrid DSTCs were examined. The test results showed that the test hybrid DSTCs exhibited excellent performance with great ductility. A theoretical model with incorporations of existing stress-strain models for FRP-confined concrete was then adapted to simulate the behavior of the test hybrid DSTCs. The comparisons showed that the direct use of a concentric loading stress-strain model led to underestimation of the deformation capacity of the test hybrid DSTCs. Compared with the other two models, the eccentric-loading stress-strain model previously developed by the authors' group provided more accurate predictions for the ultimate deformation of the test hybrid DSTCs.

Behavior of large-scale hybrid FRP–concrete–steel double-skin tubular columns under concentric compression

Hybrid fiber reinforced polymer (FRP)–concrete–steel double-skin tubular columns (hybrid DSTCs) are a novel form of columns with superior load-carrying capacity, ductility, and corrosion resistance. Such columns consist of an outer FRP tube, an inner steel tube, and the concrete filled between the two tubes. While they have attracted extensive research attention worldwide, most of the existing studies have been focused on small-scale specimens. In particular, the behavior of large-scale hybrid DSTCs with a filament-wound FRP tube and self-compacting concrete (SCC) has not been systematically investigated. Against this background, this paper presents a systematic experimental study on large-scale hybrid DSTCs under concentric compression with the following important issues being particularly investigated: the use of large-scale specimens, the use of prefabricated filament-wound FRP tubes compared with wet-layup FRP tubes, and the effect of shrinkage of SCC. The test results confirmed the excellent ductility of hybrid DSTCs and revealed that the types of concrete (SCC versus normal concrete) and FRP tube (wet-layup versus filament-wound tubes) significantly affected the compressive behavior of hybrid DSTCs. Finally, the predictions of an existing design-oriented stress–strain model for confined concrete in hybrid DSTCs, which was developed based on test results of small-scale hybrid DSTCs made with normal concrete and confined with a wet-layup FRP tube, are compared with the test results. It is shown that the model provides reasonably accurate predictions for confined concrete in the large-scale hybrid DSTCs with a typically bilinear shape, but it fails to predict the obvious axial stress drop after the first peak load caused by the large shrinkage of SCC.

Flexural behavior of hybrid FRP-recycled aggregate concrete-steel hollow beams

Hybrid fiber reinforced polymer (FRP)-concrete-steel (FCS) hollow members are a new form of structural members possessing superior mechanical behavior and durability. Such members consist of an outer FRP tube, one or more inner steel tubes, and the concrete filled between the tubes. Existing studies on such members have been mainly focused on their use as columns; limited studies have been conducted on their use as flexural members. Besides, the effect of shear connectors between the steel tube and the concrete on the behavior of hybrid FCS hollow flexural members has not been investigated in detail. This paper, therefore, presents the results of an experimental program on the flexural behavior of hybrid FCS hollow beams with a rectangular hollow cross section. Recycled aggregate concrete (RAC) was used for the purposes of addressing the serious issue of construction waste nowadays and capitalizing the advantage of FRP confinement in enhancing the performance of RAC. The test results showed that such hybrid FCS hollow beams are very ductile, and the shear connectors play a significant role on the behavior of such beams. Finally, a theoretical beam model based on conventional section analysis was proposed to predict the results of the test hybrid FCS hollow beams.

Axial compressive behavior and modeling of fiber-reinforced polymer-concrete-steel double-skin tubular stub columns with a rectangular outer tube and an elliptical inner tube

This paper presents the experimental results of monotonic axial compression tests on eighteen glass fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular stub columns (DSTCs) with a rectangular outer glass FRP tube and an elliptical/rectangular inner steel tube, as well as eight rectangular glass FRP-confined hollow stub columns (FCHCs) with an elliptical inner void. The effects of cross-sectional aspect ratio, FRP thickness, void area ratio and the shape of inner steel tube (i.e., elliptical and rectangular) on the axial compressive behavior of confined concrete core in rectangular DSTCs are considered and analyzed. The test results demonstrate that rectangular DSTCs possess excellent bearing capacity and ductility. Additionally, the axial stress-strain curves of confined concrete core in rectangular DSTCs are approximately trilinear and show obvious post-peak softening phenomenon because of the non-uniform and insufficient confinement. Furthermore, the larger aspect ratio, the thicker FRP tube, the higher void area ratio, as well as elliptical inner steel tube (compared with rectangular) have positive confinement effect on the axial compressive behavior of confined concrete core in rectangular DSTCs. Based on the analysis of experimental data, a design-oriented three-stage stress-strain model (considering post-peak softening behavior) of confined concrete in rectangular DSTCs with elliptical inner steel tube is proposed. Through the comparison, it is found that the predictions calculated by the proposed model accord well with the experimental results.

Axial compressive behavior of FRP-concrete-steel double skin tubular columns with a rib-stiffened Q690 steel tube and ultra-high strength concrete

Fiber-reinforced polymer (FRP)-concrete-steel double skin tubular columns (DSTCs), which possess an outer FRP tube, an inner steel tube and sandwiched concrete between both tubes, are a kind of hybrid column with a particular merit of lightweight. It is expected that using high-strength materials (i.e., ultra-high strength concrete (UHSC) and high-strength steel (HSS) tubes) in DSTCs magnifies the merit of DSTCs. This paper presents an experimental study on axial compression behavior of DSTCs with ultra-high strength concrete and a rib-stiffened Q690 steel tube. The rib-stiffened steel tubes, which are less susceptible to local buckling compared with those without stiffeners, were used in the present study. The parameters including the quantity of stiffeners, the length of stiffeners, the FRP tube thickness, the steel tube diameter and the concrete strength were carefully designed and investigated through the experimental study. The test results demonstrate that DSTCs with UHSC and HSS tubes exhibit a ductile behavior and the axial load-axial strain curves exhibit an ascending-descending-ascending-failure behavior. Additionally, the stiffeners are beneficial in enhancing the performance of DSTCs. The comparisons between the test results and theoretical results show that the existing model of Yu et al. is inaccurate in predicting the ultimate axial stress and the ultimate axial strain of HSC and UHSC in DSTCs.

Behavior and modelling of FRP-concrete-steel hybrid double-skin tubular columns under repeated unloading/reloading cycles

Fiber-reinforced polymer (FRP)-concrete-steel hybrid double-skin tubular columns (hybrid DSTCs) consist of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes. Extensive research has been conducted on the monotonic behavior of hybrid DSTCs, but only a limited number of studies have examined their behavior under cyclic axial compression. In particular, no systematic experimental study has been conducted on hybrid DSTCs under repeated unloading/reloading cycles. This paper first presents a systematic experimental study on hybrid DSTCs under two types of loading schemes: (a) a single unloading/reloading cycle to evaluate the relationship between the unloading strain and the plastic strain; (b) repeated unloading/reloading cycles to investigate the effect of loading history. In the present study, hybrid DSTCs were prepared using filament-wound FRP tubes of a relatively large scale (up to 300 mm in diameter), and the experimental program covered a wide range of concrete strengths (from 40.9 MPa to 104.4 MPa). The systematic experimental study with extensive instrumentation allowed the cyclic stress-strain behavior of the concrete in hybrid DSTCs to be examined in detail, clarifying the cumulative effect of the loading history on both the stress deterioration and the plastic strain of the concrete. This paper also presents a detailed comparison between the test results and the predictions of an existing cyclic stress-strain model for FRP-confined concrete in solid columns, in terms of the repeated unloading/reloading cycles. The cyclic stress-strain model is shown to provide reasonably accurate predictions, and thus can be used together with a monotonic stress-stress model for the concrete in hybrid DSTCs to predict the complete cyclic stress-strain curve of such concrete.

Confinement of concrete in double skin tubular members under axial compression loads

A hybrid double-skin tubular members (DSTM) consist of three composite materials in one section, include fiber-reinforced polymer (FRP) tube outward, steel tube inward and concrete is in between. These three materials give the member unique properties like protect member from harsh environment, reduce the amount of concrete by the presence of an inner steel tube that allowed the passage of services. Therefore, consider sustainable structure elements, as well as the rising strengthening of members and excellent corrosion resistance. This paper presents an experimental and analytical study of twenty-four circular columns with two types; concrete-filled FRP tube (CFFT) and double skin tubular column (DSTC), with a dimension of 100 × 310 mm which constructed and testing under an axial compressive load till failure, with four variables: numbers of layers of outer FRP tube, the compressive strength of concrete, thickness to diameter ratio and void ratio of inner steel tube. The experimental results showed that the concrete was effected confinement in hybrid DSTC led to very ductile behavior of stress–strain relationship and the number of outer FRP layers was the most effective parameters on DSTCs behavior. The nominal confinement ratio was affected by the thickness of outer FRP tube and compressive strength of concrete inside DSTCs.

Effective usage of high strength steel tubes: Axial compressive behavior of hybrid FRP-concrete-steel solid columns

The novel form of hybrid columns, being fiber-reinforced polymer (FRP)-concrete-steel solid columns (FCSSCs), involves the FRP tube and the steel tube as the confining system for the concrete. The buckling of the inner steel in FCSSCs is effectively delayed by the surrounding concrete while the strength of the steel is sufficiently utilized, with it being protected by the outer FRP tube against environmental attacks. A pilot experimental program is conducted to investigate the behavior of axially loaded FCSSCs with an outer polyethylene terephthalate (PET) FRP tube. The experimental results have demonstrated that the axial load carrying capacity of an FCSSC is much larger than the total resistances of the hollow steel tube and the concrete-filled FRP tube. The test FCSSCs possess a monotonically ascending load-strain response with an excellent ductility. The comparisons between the test results and the predictions from existing models demonstrate that one existing model gives satisfactory estimations on the ultimate axial stress and strain of the confined concrete in FCSSCs, while they are incapable to accurately predict stress-strain curves of the confined concrete in FCSSCs.

Behavior of hybrid FRP-concrete-steel multitube hollow columns under axial compression

Hybrid use of fiber reinforced polymer (FRP) composites with conventional construction materials offers an ample opportunity to develop economical and high-performance structures. A typical example is the hybrid FRP-concrete-steel double-skin tubular columns (DSTCs), which consist of an outer FRP tube and an inner steel tube, with the space between filled with concrete. This paper presents the conceptional development of a variation of the DSTC by replacing part of concrete sandwiched between the outer and inner tubes with a number of small-diameter concrete-infilled FRP tubes [referred to as “hybrid FRP-concrete-steel multitube hollow columns (MTHCs)”]. The small-diameter concrete infilled FRP tubes possess ultra-high strength and excellent deformability due to the effective confinement, thus enabling the novel form of columns to have an excellent structural performance. Results of tests on seven stub columns are presented in this paper, confirming some expected advantages of the proposed MTHCs. Test results also suggested that MTHCs usually fail by the hoop rupture of the outer FRP tube, and a large rupture strain FRP tube used as the outer tube could lead to a higher utilization rate of the tensile strength of inner CFPR tubes and thus an even better performance for MTHCs.

Experimental study of seismic performance of full-scale basalt FRP-recycled aggregate concrete-steel tubular columns

Abstract Hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) have received extensive attention in the research community over the past decade. Generally, a DSTC consists of an inner steel tube and an outer FRP tube, with the space between them filled with natural aggregate concrete (NAC). This paper studied a new form of hybrid FRP-recycled aggregate concrete (RAC)-steel tubular column (FRSTC) in which the outer FRP tube was made of environmental-friendly basalt fibers and the concrete was made of recycled coarse aggregates (RCAs). Quasi-static tests of five full-scale basalt FRSTCs were conducted to gain an improved understanding of the behavior of FRSTCs, and to enable further application potentiality of FRSTCs in earthquake-prone areas. Three key column parameters, including RCA replacement percentages, axial load ratios, and steel reinforcement ratios were carefully designed to clarify their influence on the performance of FRSTCs under combined axial compression and cyclic lateral loading. The test results, including the hysteretic loops, skeleton curves, curvature and strain distributions, ductility characterization, energy dissipation capacity, stiffness and strength degradation, and cumulative damage, are presented and discussed. The experimental results show that the RCA replacement percentage has little impact on the seismic performance of FRSTCs, while the effect of the axial load ratio is significant. The bearing capacity and energy dissipation of FRSTCs can be superior to those of DSTCs with NAC.

Behavior of circular ice-filled self-luminous FRP tubular stub columns under axial compression

This paper proposes an innovative ice-filled self-luminous fiber-reinforced polymer (FRP) tubular (IFSFT) column for temporary structures in cold regions. The proposed column possesses a good combination of the advantages of FRP tube and natural ice, as well as provides itself with aesthetic features or other service functions due to the self-luminous feature of the outer FRP tube in darkness. This paper presents an experimental study on tensile and luminance properties of FRP laminates modified with self-luminous powders, and axial compressive behavior of circular IFSFT stub columns and ice-filled FRP tubular (IFFT) stub columns. Experimental results indicate that the addition of self-luminous powders into FRP composites has little influence on their tensile properties as well as axial compressive behavior of IFSFT stub columns. Compressive strength, peak strain and elastic modulus of the confined ice cores in IFFT and IFSFT specimens have been significantly improved compared with those of unconfined plain ice. A simplified equation for evaluating axial bearing capacity of IFFT and IFSFT stub columns is proposed with a reasonable accuracy.