FRP Tube Research Articles

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.

Numerical investigation on bending behaviour of hollow precast slab with FRP tubes using Ansys

Hollow-core precast concrete slabs are frequently employed in modular building construction to reduce concrete volume and weight. Nevertheless, the presence of the hollow-core increases susceptibility to failure. To address this issue, a composite reinforcing system (CRS) has been developed to support the void in precast slabs. This research aims to investigate the bending behavior of a composite slab comprising precast concrete, steel, and the composite reinforcement system. Bending tests will be conducted on precast slabs with various CRS configurations (including changes in tube material properties and tube cross-section) to assess the structural performance of this innovative construction system. Additionally, a non-linear finite element modeling was employed to gain a comprehensive understanding of the behavior of the internal components of the slab. The findings of this study will offer valuable insights for designing more reliable and cost-effective hollow-core precast concrete slabs.

Axial compressive behavior of GFRP tube-reinforced concrete-steel double skin tubular columns

Hybrid FRP-reinforced concrete-steel double-skin tubular columns (DSTCs) exhibit the benefits of high bearing capacity, sufficient ductility and excellent durability. However, there are fewer studies on reinforced DSTCs under axial compression compared to unreinforced DSTCs. Eleven circular cross-section columns were tested to study the compressive behavior of GFRP-reinforced concrete-DSTCs. The main parameters considered were the thicknesses of GFRP tubes, longitudinal reinforcement ratio, hollow ratio and concrete strength. The test results showed that reinforced DSTCs had higher bearing capacity and better ductility than traditional DSTCs, and their bearing capacity could be increased by 36%–48%. The bearing capacity and initial stiffness increased with the increase of longitudinal reinforcement ratio, thicknesses of GFRP tubes and concrete strength, while they decreased with the increase of hollow ratio. When the load was within the range of 55%Pu, the restriction of the GFRP tube to the concrete was not obvious. Furthermore, an analytical model was established to predict the full load-strain curves of reinforced DSTCs, which considered the restraining effect of stirrups and the bi-axial stress state of FRP tube. The established model was in good agreement with most of the test results.

Behaviour of corroded circular steel tube strengthened with external FRP tube grouting under eccentric loading: Numerical study

This paper uses ABAQUS software to perform finite element simulation on eccentric compression member of corroded circular steel tubes strengthened by FRP tube grouting. The working mechanisms, failure mode and the lifting of mechanical properties are investigated. The results show that the load–displacement curve of the member represents both bilinear and trilinear models. The former is in the overall buckling failure mode, while the latter is in the strength failure mode of the circular steel tube with bare ends. After strengthening, the bearing capacity increases by up to 114.99%. Moreover, the increase positively correlates with corrosion rate, grouting thickness, and FRP tube thickness. It first decreases before increasing as the slenderness ratio changes, while the concrete strength has little effect. At the same time, the bound flexural stiffness ratio is derived and provides reference suggestions for the practical engineering application of FRP tube grouting to strengthen corroded steel structures.

A refined finite element model for UHPC-filled FRP tube column

A sound finite element model is essential for the nonlinear structural analysis of FRP tube-confined UHPC columns, wherein an accurate constitutive model to characterize the mechanical behavior of UHPC under passive constraints is of utmost importance. This study presents a refined finite element modeling methodology for the UHPC-filled FRP tube column. The constitutive model for UHPC under passive constraint is developed, in which the yield criterion, hardening/softening law, plastic flow rule, and damage evolution law are determined based on the experimental results of UHPC under multiaxial loading conditions. Furthermore, a solid element model of FRP tube with definable layer information is utilized to simulate the behavior of FRP tubes under complex stress states. The proposed finite element model is implemented in ABAQUS and verified through available experimental results. The results indicate that the proposed model can accurately predict the stress–strain behavior of confined UHPC with varying fiber and coarse aggregate contents, and describe the damage progression of UHPC and FRP tubes. Particularly, the interaction process between FRP and UHPC can be well reflected. The proposed finite element model serves as a paradigm for numerical analysis of UHPC-filled FRP tube elements and provides a reference for simulations of UHPC structures.

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.

Seismic performance of elliptical FRP-concrete-steel tubular columns under combined axial load and reversed lateral load

By combining FRP composites with both steel and concrete, double-skin tubular columns (i.e., DSTCs) with a hollow core are a novel structural member to be used as bridge piers or other columns in harsh environments. Up to now, DSTCs with circular or square sections (i.e., C-DSTCs or S-DSTCs) have been investigated extensively, while DSTCs with an elliptical section (i.e., E-DSTCs) are rarely studied. E-DSTCs are composed of two concentric elliptical tubes (i.e., the external FRP tube and the internal steel tube) and a sandwiched concrete layer. E-DSTCs can be designed with two distinct bending stiffness with respect to the two axes of symmetry. Similar to C-DSTCs, E-DSTCs have an FRP tube with a continuously curved circumference, thus effectively confining the concrete layer. To extend the existing research, this paper tested six large E-DSTCs under combined axial load and reversed lateral load to investigate the effect of elliptical aspect ratio. The testing parameters also included the direction of reversed lateral load and the FRP thickness. Experimental results showed that, E-DSTCs performed with rounded hysteresis loops and remarkable ductility even for the specimen with a large aspect ratio (i.e., for the elliptical aspect ratio from 1.0 to 2.0, the ductility index increased monotonically from 6.50 to 7.75); a thicker FRP tube led to marginally higher peak load but noticeably better ductility (i.e., the specimen with 2.10 mm FRP had an average peak load of 77.65 kN and a ductility index of 7.14, while the corresponding specimen with 1.05 mm had an average peak load of 77.35 kN and a ductility index of 6.29); as expected, the specimen bending around its strong axis had much superior ductility than the companion specimen bending around its weak axis. A numerical column model was established based on OpenSees to simulate the hysteretic behavior of E-DSTCs, and a parametric study was conducted to assess the influences of FRP thickness, concrete strength, axial load ratio, steel tube thickness, void ratio as well as elliptical aspect ratio. In practical engineering design, it was recommended that: (a) the elliptical aspect ratio as well as other key parameters (e.g., the FRP thickness, the steel tube thickness, the area void ratio) should be optimized based on specific loading conditions; (b) to make more effective usage of FRP composites, the FRP tube thickness should be optimized for different height regions of the column according to the moment gradient caused by the reversed lateral load.

Experimental and Numerical Studies on Sea Sand Concrete Filled Stainless Steel Tube with Inner FRP Tube Subjected to Axial Compression

Experimental and Numerical Studies on Sea Sand Concrete Filled Stainless Steel Tube with Inner FRP Tube Subjected to Axial Compression

Residual compressive strength of seawater sea sand concrete filled hybrid carbon-glass fibre reinforced polymer tubes under seawater: Effects of fibre type and orientation

Despite the superior mechanical and durability properties, the higher cost of carbon, compared to other fibre options, remains a major concern in the widespread use of carbon fibre-reinforced polymer (CFRP) composites in construction applications. The hybrid formation of carbon fibres with other fibres could help reduce the cost while remaining appropriate structural properties. The present study examines the durability performance of Seawater Sea Sand Concrete (SWSSC) filled hybrid carbon-glass fibre-reinforced polymer (HFRP) tubes after exposure to seawater. Ninety-six samples filled with an alkaline solution simulating SWSSC were exposed to seawater for different durations and at different temperatures. The compressive mechanical properties of the conditioned samples were compared to the unconditioned reference samples to determine the level of mechanical degradation of the tubes. Based on Arrhenius's theory, long-term performance was predicted using the collected experimental data from short-term performance. Fibre type (i.e. carbon, glass, and hybrid carbon-glass) and fibre orientation (cross-ply and hoop) were considered as the material-based variables. When exposed to the most severe condition of 5 months at 60 °C, CFRP showed the highest average strength retention of 86.01%. The hybrid cross-ply tubes exhibited better strength retention than GFRP with strength retention of 64.84% for HFRP compared to 57.23% for GFRP. However, over the long term, the difference in performance between the two types of FRP tubes decreased and became more comparable. The onset of damage in GFRP tubes started sooner than that in HFRP. However, the long-term ultimate strength reduction of HFRP tubes was found similar to the weaker fibre type (i.e. GFRP tubes).

Shear behaviour of FRP-confined concrete/UHPC core-encased rebar: Experimental investigation and design method

FRP-confined concrete/UHPC (ultra-high-performance concrete) core-encased rebar (FCCC-R) is a recently developed novel structural component for FRP-concrete composite structures. FCCC-R utilization can improve both the strength and ductility of composite structural members under both axial compression and combined bending-compression loading. When implemented in vertical structural members such as columns and shear walls with a low shear-span ratio, FCCC-Rs are subjected to combined axial compression and shear. In this study, the shear behaviour of FCCC-Rs is experimentally investigated. A total of 28 FCCC-R specimens are tested. The failure mechanism and modes, the load–deflection curves and the ultimate strength were obtained from the experiments. The FCCC-Rs exhibited higher load capacity and deformability. It could be concluded that the combination of the FRP tube, concrete and rebar can achieve the effect of “1+1+1>3”. Furthermore, the influence of the shear-span ratio, concrete strength, tube thickness and fibre orientation are examined. Finally, design equations for the shear strength of FCCC-Rs are proposed, which attain a good agreement with the test data.

Compressive behavior of circular GFRP tube-confined UHPC-filled steel-encased stub columns

Using ultra-high-performance concrete with coarse aggregate (UHPC-CA) instead of plain concrete, the mechanical performance of FRP tube-confined concrete-filled steel-encased columns (FUSCs) can be significantly improved with controllable cost. In this study, the axial compressive behavior of FUSCs was investigated, in which the thickness and fiber winding angle of FRP tube, section size of profile steel, and substitution rate of coarse aggregate were the variables considered. The failure mode and the load-deformation relation were analyzed, upon which the interaction mechanism between the profile steel, UHPC, and FRP tube was uncovered. The results showed that FUSCs mainly failed in two distinct failure modes, i.e., shear failure and crush failure respectively, depending on the magnitude of FRP confinement and the size of profile steel. Meanwhile, the incorporation of coarse aggregate could enhance the constraint effect of FRP tube, resulting in an increase in the strength index and peak strain up to 16.2% and 38.2%, respectively. With extra restraint after the peak load, the profile steel provided a significant contribution to the FUSCs’ ductility, leading to a transition of FRP confinement effect from non-uniform to uniform. The research outcome serves as a reference for design of FUSCs and promotes the application of FUSC structure.

Concrete-Filled FRP Tubes under Combined Torsion and Bending

Concrete-Filled FRP Tubes under Combined Torsion and Bending

Concrete-Filled Multicellular Steel-Tube Shear Walls with FRP Tubes Embedded in CFST: Concept and Seismic Performance

Concrete-Filled Multicellular Steel-Tube Shear Walls with FRP Tubes Embedded in CFST: Concept and Seismic Performance

Behavior of large-scale hybrid FRP-concrete-steel double-skin tubular columns subjected to eccentric compression

Hybrid FRP-concrete-steel double-skin tubular columns (DSTCs), consisting of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes, are a novel form of hybrid columns which possess excellent mechanical performance and durability. Extensive studies have been conducted on such columns under concentric axial compression. However, limited research has been focused on the behavior of hybrid DSTCs subjected to eccentric compression which is a common loading case in practice. Therefore, an experimental program on nine large-scale hybrid DSTCs subjected to concentric or eccentric compression was conducted in the present study. The load eccentricity and the thickness of the filament wound FRP tube were the main test variables. The test results demonstrated that the hybrid DSTCs exhibited a highly ductile behavior under eccentric compression. A theoretical column model was then adapted by incorporating three stress–strain models for FRP-confined concrete for predicting the behavior of test DSTCs under eccentric compression. It was found that all the three stress–strain models provided reasonably accurate predictions for the axial load-carrying capacities of the test columns. However, the concentric loading stress–strain model led to significant underestimation in the deformation capacities of the test columns due to the neglect of the effect of eccentric compression.

Residual compressive section capacity of filament wound carbon, glass, and basalt fibre-reinforced polymer tubes: Influence of elevated temperatures

This study investigates the residual compressive properties of various filament wound FRP tubes after exposure to elevated temperatures. Exposure temperature and fibre type were considered as the test variables. The experimental program included compression tests for BFRP, GFRP, and CFRP tubes at room temperature and after exposure to elevated temperatures up to 350 °C. A total number of 72 specimens were tested in compression. In addition, Differential Scanning Calorimetry, and Scanning Electron Microscopy were conducted on selected samples to investigate the thermal and microstructural properties of the tubes. The compressive strength of tubes, regardless of the fibre type, seemed to be affected by the competing effects of post-curing and resin matrix degradation. With respect to fibre type, for temperatures below 350 °C, BFRP exhibited the highest compression strength retention, whereas CFRP tubes showed the lowest strength retention. Temperature of 350 °C was found to be the critical temperature (the temperature that FRP loses 50 % of its load carrying capacity) for CFRP tubes, while the critical temperature for GFRP and BFRP tubes were respectively, somewhere lower than 350 °C. The maximum compressive strength reductions for BFRP, GFRP, and CFRP were 94 %, 63 % and 50 % respectively after exposure to 350 °C.

Cyclic axial compressive performance of the RC columns reinforced with FRP confined concrete core encased rebar

During an earthquake, the collapse of a structure is generally controlled by the failure or damage of its columns. It is important to enhance the seismic performance of reinforced concrete (RC) columns, including load capacity, ductility, and energy dissipation capacity. Column configurations with several small confined concrete components, namely, the confined concrete cores in this paper, are regarded as an effective way to improve the mechanical performance of square RC columns based on previous studies. The external confining material of a confined concrete core can be steel spirals, steel tubes, or FRP tubes. In this study, a new component, namely, FRP confined concrete core encased rebar (FCCC-R), and a new member, i.e., an RC column reinforced with FCCC-R were investigated. Eight RC columns with/without FCCC-R were cast and tested under cyclic axial compressive loading. The test result indicates that the RC columns reinforced with FCCC-R generally have a much higher axial load capacity. The axial load-displacement curves of all the tested specimens are compared, and the influences of the steel hoops, FCCC-R, and center FCCC are discussed. Moreover, the behaviors of different parts in a hybrid column, including steel hoops, and FRP tubes, are analyzed according to the strain development during the test. Finally, a section superposition method is proposed to predict the axial load-axial deformation behavior, whose results agree well with the experimental results.

Experimental investigation on eccentric compression behavior of FRP-confined concrete-encased cross-shaped steel columns

FRP-confined concrete-encased cross-shaped steel column (FCCSC), composed of an outer square glass-fiber-reinforced plastic (GFRP) tube, the internal cross-shaped steel, and the filled concrete, is a novel type of composite column. Eleven square FCCSCs were tested subject to eccentric axial compression load. The influence of eccentricity and slenderness ratio was evaluated and investigated. The test results showed that both the axial load-bearing capacity and lateral confinement efficiency of FRP tube reduced with the increase of eccentricity and slenderness ratio. The maximum reduction in axial load-bearing capacity was up to 50.7 % and 71.1 %, with the eccentricity varying from 0 mm to 50 mm and 100 mm, while the maximum reduction was about 18.1 % and 27.5 % when slenderness ratios increased from 10.44 to 20.87 and 34.79, respectively. The ductility of eccentric compressed FCCSCs can be evaluated by combined strain. It is indicated that short columns exhibited the best ductility, and the ductility increased with the increase of eccentricity. However, the ductility of slender columns reduced with the increase of eccentricity. Based on the test results, the experimental axial force (N)-moment (M) interaction curve of FCCSCs was obtained, and the balanced failure between compression and tension failure mode was recommended.

Compressive behaviour of elliptical FRP tube-confined concrete columns

Elliptical FRP tube-confined concrete columns (EFCCCs) are an extension of circular FRP tube-confined concrete columns recently proposed at The Hong Kong Polytechnic University. Like their circular counterparts, EFCCCs are corrosion-resistant and can be used in situations where a column is subjected to substantially different loads in the two orthogonal lateral directions. The elliptical section of EFCCCs leads to more effective confinement of the concrete and an easier filament-winding fabrication process for the FRP tube compared with their rectangular counterparts. Existing studies on FRP-confined concrete in elliptical columns have been few. This paper first presents a systematic experimental study on the axial compressive behaviour of EFCCCs, with a focus on the stress–strain behaviour of the confined concrete. The test variables included the concrete strength, the thickness and fibre orientations of the FRP tube, and the aspect ratio of the cross-section. The test results showed that the confinement effectiveness of FRP tubes decreases with the aspect ratio and the concrete strength but increases with the FRP tube thickness. The orientations of fibres in the FRP tube were found to have a significant effect on its confinement effectiveness and the failure mode of EFCCCs. On the basis of the test results, existing stress–strain models for FRP-confined concrete in elliptical columns are evaluated. Furthermore, a new stress–strain model for the concrete in EFCCCs is proposed, which includes a more rigorous treatment of the confinement stiffness of EFCCCs and can provide more accurate predictions for the whole stress–strain curve than the existing models.

A theoretical axial stress–strain model for gangue ceramsite concrete confined with FRP tube

• The strength and ductility of GAC are both significantly enhanced under FRP confinement. • The confinement efficiency of FRP-confined concrete is highly dependent on aggregate characteristics. • The proposed model provide a more accurate prediction with various types of concrete confined with FRP. A total of 48 cylinders prepared using gangue ceramsite concrete (GAC) confined with fiber reinforced polymer (FRP) were tested under axial compression, and the effects of concrete type, concrete strength and FRP confinement level were analyzed. The results show that the porosity and low strength of aggregate lead to brittle failure of concrete, whereas both the strength and ductility of GAC have been significantly enhanced with the confinement provided by an external FRP tube. By using the microscopic analysis to reasonably consider the effect of aggregate characteristics on FRP-confined concrete, the proposed model can more accurately predict the stress–strain behavior of FRP-confined GAC.

How Concrete Filling Fundamentally Changes Stress–Strain Curve of Angle-Ply FRP Tubes in Tension

How Concrete Filling Fundamentally Changes Stress–Strain Curve of Angle-Ply FRP Tubes in Tension