Concrete-filled Fiber-reinforced Polymer (FRP) Tubes Research Articles

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.

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.

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.

Experimental study on hybrid concrete-filled fiber reinforced polymer tube (HCFFTs) columns under simulated seismic loading

The concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system has been widely studied as a promising alternative to conventional reinforced concrete (RC) for the construction of bridge columns. While unreinforced CFFT columns are attractive for their ease of construction, their limited energy dissipation capabilities have restricted their application to seismic regions. The hybrid CFFT (HCFFT) system was introduced to address this shortcoming by incorporating steel reinforcement into the FRP shell in the form of 30-μm fibers. This system combines the ease of construction of conventional unreinforced CFFTs with the energy dissipation characteristics of lightly reinforced CFFT columns. This study reports the first experimental investigation on the seismic performance of HCFFT columns through large-scale testing of two columns under combined axial compression and cyclic lateral loading. Two shear loading conditions were studied. The flexural capacity, mode of failure, residual deformation, and energy dissipation capacity were evaluated. The low-shear specimen reached a drift ratio of 10.5% and the high-shear specimen reached a drift ratio of 6.5%. The specimens both reached displacement ductility demands above 5, demonstrating the system is appropriate for use as a bridge column in seismic regions. The curvature profiles for both specimens showed the hybrid shell prevented localized yielding of the steel fibers, which allowed for an efficient distribution of plasticity along the column height and enhanced displacement ductility. The results indicate that HCFFT columns can achieve high inelastic deformations under simulated seismic loading without substantial degradation of stiffness. It is anticipated that this large-scale experimental data will inform future research on the design and manufacturing process of HCFFT elements and encourage further experimental and analytical studies.

Behavior of concrete-filled FRP tube columns internally reinforced with FRP-steel composite bars under axial compression

Although concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) columns (CFFTs) have become increasingly popular, CFFTs has a weak bending performance as the orientation of the fibers in the FRP tube is generally close to the transverse direction. To this end, this paper proposes novel Concrete-Filled FRP Tube columns internally reinforced with FRP-steel Composite Bars (referred to as CFFT-CB columns) which consist of an external FRP tube, some longitudinal reinforcing FRP-steel composite bars (FSCBs), an FSCB spiral and concrete filled in the entire section. In this study, FSCBs are composed of separated FRP bars and steel bars due to fabrication difficulty. The axial compressive behavior CFFT-CB columns is studied experimentally in this paper. The effects of the spiral type, the spiral pitch, and the thickness of the FRP tube are carefully investigated and the test results confirm the effectiveness of the proposed columns. The ultimate condition of concrete in test columns are compared with four existing stress–strain models, with the confining stresses from both the FRP tube and the FSCB spiral being accounted for. The durable CFFT-CB columns are particularly suitable for marine and coastal structures constructed in harsh environment.

Efficient second-order nonlinear finite-element simulation of concrete-filled FRP tubular arches

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) are attractive as beams and arches in harsh environments due to their durability and ease of erection. A successful infrastructure application is the use of multiple, buried CFFT arches in short-span bridges. To effectively analyze and design CFFT arches under complex loadings, there is a need for numerical procedures that capture the structural behavior of CFFT arches including the effects of FRP confinement on the concrete core and second-order response. Here, a computationally efficient structural finite-element analysis technique for the second-order inelastic behavior of these CFFT arches is detailed. A curved, flexibility-based, layered-frame element is employed to handle geometric and material nonlinearities. A concrete constitutive model that explicitly considers the effect of dilation of the concrete core and confinement provided by the FRP tube is implemented. A bending test was performed on a CFFT arch, and experimental data compared with model predictions. Additional comparisons are made with CFFT arch and beam bending tests available in the literature. The model is shown to accurately predict the load-carrying capacity and ductility of CFFT members, and to capture arch collapse mechanisms arising from FRP rupture and concrete crushing.

Additive Fabrication of Large-Scale Customizable Formwork Using Robotic Fiber-Reinforced Polymer Winding.

Based on the established system of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) in civil engineering and construction industry, this research presents a novel fabrication method for freeform FRP formwork through an additive process of winding FRP fabric with industrial robots. Different from the filament winding or fused deposition modeling process in additive manufacture, large-scale formwork is fabricated with layered winding of FRP fabric and simultaneously applying fast cure epoxy resin in the proposed methods. It increases the fabrication speed and material efficiency compared with the typical fabrication process of FRP formworks, and achieved the geometry flexibility from the numerically controlled additive process. The fabrication methods are developed through a series of preliminary tests, exploring the appropriate fabrication parameters, such as the overlapping height of each layer, winding speed, and epoxy resin type. Two additional prototypes addressing geometrical flexibility are also fabricated. Based on the feasibility studies, the article discussed the potential application of this system on a double-skin tubular arch (DSTA) bridge and a tree-like topological optimized column as the future outlook of this method. As developed based on the established construction systems such as CFFTs and DSTAs, not only the proposed system is compatible with current structure and construction system, but it also benefits from combining an off-shelf material with a flexible and accurate programmable robotic process. This research contributes to the scope of additive manufacturing system by targeting the fabrication of nonuniform optimized large-scale structures.

Experimental investigation of a simple shear connection to concrete-filled FRP tube (CFFT) columns

The concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system has been widely studied as a robust alternative to conventional reinforced concrete (RC) for column construction. Despite the superior structural performance of this column system in terms of durability, strength and ductility, its application in building construction is limited due to a lack of viable designs for beams-to-column connections. To address this gap, this study proposes a simple connection for a circular CFFT column and a steel I-shape beam. The connection consists of a curved end plate with a shear tab that is attached to the finished CFFT using commercially available concrete anchors. In this study, three connection assemblies with adhesive and expansion anchors were tested under half-cyclic loading. The connection assemblies varied based on the type of the anchors and installation torque. The performance was evaluated in terms of ease of construction, shear capacity, moment-rotation relationship, and observable damage. The FRP shell eliminated concrete breakout failure and the anchors exceeded the theoretical pullout capacity noted by the manufacturer. A methodology for estimating the proof load of the proposed connection detail is presented. The proposed connection provides a viable option for simple connection of steel beams to circular CFFT columns. The findings may inform future research on beam-column connections for CFFTs and lead to an increased use of CFFT systems in building construction.

Compressive Strength Testing of Hybrid Concrete-Filled Fiber-Reinforced Plastic Tubes Confined by Filament Winding

In this study, an experiment on compressive strength of the hybrid concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) confined by filament winding was conducted to improve the longitudinal strength while considering the thickness of filament winding as a variable. A maximum error of 17% was observed when the results of performing the finite element analysis (FEA) by applying the mechanical properties of the fiber-reinforced polymer (FRP) materials suggested in previous studies were compared to those of the compressive strength experiment on the hybrid-CFFT. Moreover, a maximum error of 15% was exhibited when the results derived from the strength equation proposed by analyzing the compressive strength experiment were compared. Furthermore, the compressive strength of the hybrid-CFFT increased by up to 14% when the longitudinal compressive strength of the pre-tensioned spun high strength concrete (PHC) pile and concrete-filled tube (CFT) were compared.

Torsional Behavior of Circular Concrete-Filled FRP Tubes

Abstract Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been studied extensively under almost all loading conditions, except torsion. In this pioneering study, six 168-mm-diamete...

Responses of concrete-filled FRP tubular and concrete-filled FRP-steel double skin tubular columns under horizontal impact

This paper presented the experimental and analytical results of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) and concrete filled GFRP-steel double skin tubular columns (DSTCs) under horizontal impact loads. The influences of the thickness of FRP tubes and impact velocity were discussed. The thickness of the FRP tubes had insignificant influence for both the peak impact force and the maximum displacement. Under the same applied impact energy, the maximum displacement of concrete filled FRP-steel DSTC specimens was ~40% smaller than that of CFFT specimens. The impact velocity had more influence on the peak impact force than the duration. Three-dimensional finite-element (FE) models were developed to simulate the impact behavior of two types of composite columns and the numerical results are compared with the test data. Then, the verified FE model was used to conduct parametric study. Moreover, analytical solutions for lateral displacement of composite columns under impact were obtained, in which the effect of impact damage was considered by introducing reduction factors into the vibration equations. The comparison between analytical results and test results showed that the maximum displacement can be accurately predicted by the proposed theoretical model.

Modelling of concrete-filled filament-wound FRP confining tubes considering nonlinear biaxial tube behavior

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) are an attractive form of hybrid members, in which the FRP tube is typically manufactured by filament-winding with fibers suitably oriented for desired mechanical properties. A significant number of studies have been conducted on the behavior of CFFTs under axial compression, in which the FRP tube is commonly assumed to have linear-elastic behavior, and is often taken to be under a uniaxial stress state (i.e., hoop tension), especially when the fibers are oriented close to the hoop direction. However, in reality, FRP tubes in CFFTs are subjected to biaxial stresses (hoop tension in combination with axial compression) and may exhibit significant nonlinear behavior. This paper presents an improved model for the axial compressive behavior of CFFTs with the nonlinear biaxial behavior of the FRP tube duly taken into account. To verify the proposed model, seven circular CFFTs were tested under axial compression. Ancillary tests on bare FRP tubes were also conducted to determine the material properties of the FRP tubes. Comparison between predictions and test results indicates that the proposed model leads to more accurate predictions of the CFFT behavior than the existing ones in which the biaxial nonlinearity of the FRP tube is ignored.

Effect of concrete strength and tube thickness on the flexural behavior of prestressed rectangular concrete-filled FRP tubes beams

This paper presents the results of an experimental study aimed at evaluating the effect of important design parameters on the flexural behavior of unbonded post-tensioned rectangular concrete-filled fiber-reinforced-polymer (FRP) tubes (CFFTs) beams. Four full-scale beams were constructed and tested to failure in four-point bending over a simply supported span of 3,000 mm. The design parameters are the confinement effect of using FRP tube or steel stirrups, the concrete compressive strength, and the FRP tube thickness. The results showed that reinforced concrete (RC) rectangular CFFT beams prestressed with unbonded steel strands can achieve substantially higher flexural strength, inelastic flexural deformation, ductility, and energy absorption capacity than conventional prestressed RC beam without FRP tube. The ultimate strength, deformation capacity, ductility, and energy absorption were increased by 2.43, 1.89, 1.63 and 3.87 times than the prestressed RC beam. The results also showed that increasing the FRP tube thickness of the prestressed CFFT beams from 6.0 to 7.4 mm has led to the increase of the ultimate capacity, ductility, and energy absorption by 19%, 28% and 18%, respectively. Prestressed CFFT beams with high strength concrete (HSC, 70 MPa) enhanced the initial stiffness before cracking, increased the ultimate flexural moment capacity by 7%, with no significant change in the ultimate deflection, compared to normal strength concrete (NSC, 46 MPa). Based on the results of this study, the effect of increasing the FRP tube thickness is more effective in enhancing the flexural strength and stiffness of the prestressed CFFT beams than increasing the concrete strength. An analytical model based on plane sectional analysis using partially confined and unconfined concrete models as well as an empirical design equation are proposed to predict the flexural moment capacity of the tested beams. The proposed models using partially confined and unconfined concrete models and empirical equation successfully predict the flexural moment capacity with satisfactory accuracy on average of 1.06 ± 0.03, 1.23 ± 0.04 and 1.01 ± 0.08, respectively. It was also found that ignoring concrete confinement would highly underestimate the flexural strength. More investigations, however, are needed to assess the effect of a wide range of key influencing parameters to better model the flexural behavior of prestressed rectangular CFFT beams.

Flexural behavior of hybrid concrete-filled fiber reinforced polymer tube columns

The concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system has been widely studied as a durable alternative design for bridge columns. Research has shown that utilizing a small number of longitudinal steel bars in the concrete core provides the energy dissipation characteristics required for resisting extreme loads, such as seismic and blast. However, the construction complexity associated with the addition of rebar discourages the implementation of this system. In a novel approach, this study proposes and investigates the structural performance of CFFTs with a hybrid glass/steel fiber reinforced tube. This hybrid CFFT (HCFFT) system integrates the required longitudinal steel reinforcement into the FRP shell in the form of 30-µm fibers. This study details the experimental results from two CFFT and four HCFFT circular specimens tested under half-cyclic four-point bending. The effect of concrete slippage inside the tube was investigated. The flexural capacity, residual deformation, damage, and energy absorption capacity were experimentally evaluated. The HCFFT specimens exhibited higher energy absorption and lower degradations in stiffness compared to the CFFTs at comparable flexural deformations. Moment-curvature analysis was used to quantify the contribution of the concrete core when it is allowed to slide inside the tube.

Influence of fiber orientation and shell thickness on the axial compressive behavior of concrete-filled fiber-reinforced polymer tubes

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) present a viable alternative column system to conventional reinforced concrete for new construction. A CFFT system consists of two parts: the prefabricated FRP tube and the concrete core. The exterior shell provides longitudinal and transverse reinforcement for the concrete core. Many design variables of the FRP shell, including its thickness and winding angle, can be altered to achieve desirable confinement, axial capacity, and ductility in CFFT columns. This research proposes a methodology to determine the minimum fiber angle for filament wound FRP tubes to provide confinement to the concrete core in CFFTs. The proposed approach was used to calculate the threshold winding angle for glass FRPs, which was found to be ±51.5°. To evaluate the proposed fiber angle threshold, 11 circular specimens with a concrete core diameter of 152 mm, a height of 597 mm, and glass fibers at angles of ±45°, ±55°, or ±65° were tested under cyclic axial compression. The effect of fiber angle and thickness of the shell were studied. The experimental results show that the level of confinement increases with winding angle and number of layers of fiber reinforcing, and the specimens with wind angles below the threshold, i.e. ±45°, behave similarly to unconfined concrete.

Axial behavior of circular CFFT long columns internally reinforced with steel or carbon and glass FRP longitudinal bars

This paper presents the test results of an experimental study aimed at investigating the behavior of concrete-filled fiber-reinforced-polymer (FRP) tube (CFFT) long columns internally reinforced with longitudinal steel or carbon and glass FRP bars tested under axial compression loading. A total of ten reinforced concrete (RC) and CFFT columns measuring 1900-mm in height and 213-mm in diameter were constructed and tested until failure. The test parameters were: (1) internal reinforcement type and amount; (2) GFRP tube thicknesses; and (3) nature of axial loading (i.e. monotonic and cyclic). The experimental results showed that the GFRP-reinforced CFFT columns had comparable ultimate axial strength and strain capacities compare to their counterparts reinforced with steel bars. As expected, an increase in the FRP tube thickness (or stiffness) resulted in an increase in the strength and strain enhancement ratios. The results also indicated that the residual plastic strain of FRP-reinforced CFFT columns is linearly related to the envelope unloading strain, and this relationship is not influenced significantly by the FRP confinement level but strongly influenced by the internal reinforcement amount and type, particularly when the envelope unloading strain (>0.0035). The presented study showed the applicability of exclusively reinforcing the CFFT columns with FRP bars and subjected to axial compression load. However, further experimental investigations on the axial cyclic behavior of CFFT columns internally reinforced with FRP bars are required to establish such key relationships.

Flexural behavior of rectangular FRP-tubes filled with reinforced concrete: Experimental and theoretical studies

This paper presents experimental and theoretical investigations on the flexural behavior of rectangular concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) beams with steel rebar. Seven full-scale CFFT beams, 3200mm long and 305×406mm2 cross section, were tested under a four-point bending load and were compared to two control steel-reinforced concrete (RC) beams. The CFFT beams had the same flexural steel reinforcement, but they had different wall thicknesses of filament-wound FRP tubes. The experimental results indicate an outstanding performance of the CFFT beams in terms of strength and energy absorption compared to the RC beams since their flexural strength and energy attained values 344% and 1052% higher than that of the RC beams, respectively. A strain compatibility/equilibrium model was developed to predict the moment-curvature response of the CFFT section addressing the issue of confinement and tension stiffening of concrete. The analytical results match well the experimental results in terms of moments, curvature, strains, and neutral axis location. Based on the model, the deflection can be predicted by integrating the curvatures along the span of the flexural member.

Geopolymer concrete-filled FRP tubes: Behavior of circular and square columns under axial compression

This paper presents the results of an experimental study on the axial compressive behavior of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs), prepared using either ordinary Portland cement (OPC) concrete (OPCC) or fly ash-based geopolymer concrete (GPC). Thirty-six CFFTs were tested under axial compression. The effects of the type of the concrete and FRP tube material, number of FRP layers used in the FRP tube, and specimen cross-sectional shape were studied. The results suggest that the axial stress–strain behavior of CFFTs is affected by the type of concrete, with GPCFFTs developing a similar strength enhancement ratio to but a lower axial strain enhancement ratio than the companion OPCCFFTs. Owing to the higher shrinkage of the OPCC, OPCCFFTs exhibit a plateau at the transition region of their stress–strain curves, which is not seen in GPCFFTs. At a similar confinement ratio, OPCCFFTs confined with different fibers exhibit comparable strength and strain enhancement ratios, whereas slight differences are seen among GPCFFTs manufactured with different fibers. The strength and strain enhancement coefficients of both OPCC- and GPC-FFTs decrease with an increase in the thickness of FRP tube. An accurate model that was originally proposed to predict the ultimate conditions of FRP-confined conventional concrete was modified to enable the application of the model to circular and square GPCFFTs, and it is presented in the final part of the paper.

Corner strengthening of square and rectangular concrete-filled FRP tubes

This paper presents the results of an experimental study that investigated the application of a corner strengthening technique to square and rectangular concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs). Effects of the strengthening technique were investigated through axial compression tests undertaken on a series of CFFT specimens having different corner radii and sectional aspect ratios. The results indicate that the compressive behavior of square and rectangular CFFTs can be improved through corner strengthening, which significantly reduces or completely eliminates temporary post-peak strength softening observed in some CFFTs. The results also indicate that the premature FRP rupture at corners due to stress concentrations can be prevented through sufficient strengthening of the specimen corners. It is found that increasing the amount of corner strengthening leads to an increase in the confinement effectiveness of CFFTs and results in a more uniform confining pressure distribution along specimen cross-section. It is also found that corner strengthening provides a more significant improvement in the compressive behavior of CFFTs with lower original confinement effectiveness caused by a smaller corner radius or a larger aspect ratio.

Structural performance of new fully and partially concrete-filled rectangular FRP-tube beams

This research introduces rectangular partially concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) beams with inner voids and steel rebar. The beams contain an outer rectangular filament-wound FRP tube with an inner hollow circular or square FRP tube shifted toward the tension zone. The flexural behavior of the partially-CFFT beams was compared with a fully-CFFT beam, and another conventional steel-reinforced concrete (RC) beam having identical dimensions and flexural steel reinforcement. The results indicated that, the partially-CFFT beams had an overall strength-to-weight ratio 370% higher than that of the RC beam, while their weight was 30% lighter than the RC beam. In addition, the failure of the partially-CFFT beams was ductile in compression. The inner circular void indicated a slightly better performance than the square void in this partially-CFFT beam. Theoretical-section analysis has been conducted and compared with the experimental results.