Concrete-filled Fiber-reinforced Polymer Tubes Research Articles

Concrete-Filled FRP Tubes under Combined Torsion and Bending

Concrete-Filled FRP Tubes under Combined Torsion and Bending

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

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

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

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

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

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

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

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

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

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

EFFECT OF TRANSVERSE/AXIAL FIBER RATIOS ON THE FLEXURAL CAPACITY OF CONCRETE-FILLED FRP TUBE BEAMS

Fiber-reinforced polymer (FRP) composite materials have been used in the field of civil engineering constructions especially in corrosive environment. They can be used as internal reinforcement for beams, slabs, and pavements, or as external reinforcement for rehabilitation and strengthening different structures. One of their innovative applications is the concrete-filled FRP tubes (CFFTs) which are becoming an alternative for different structural members such as piles, columns, bridge girders, and bridge piers due to their high performance, durability and resistance to corrosion. In such integrated systems, the FRP tubes act as stay-in-place forms, protective jackets for the embedded concrete and steel, and as external reinforcement in the primary and secondary direction of the structural member [1,2,3]. This study investigates the flexural behaviour of square filament-wound FRP tubes filled with concrete, without any steel bars. The FRP tubes were fabricated by filament winding process and hand lay-up technique. Several test variables were chosen to investigate the effect of the fiber laminates structure, and the different ratios of axial-andtransverse fiber on the flexural behaviour of such CFFT beams. The beams were tested under four-point loading system. The results of the tested CFFT beams indicate significant gain in strength, stiffness, cracking moment and energy absorption with increasing the axial fiber percentage and by increasing the thickness of the FRP tube.

Compressive performance of concrete-filled steel tube columns with in-built seawater and sea sand concrete-filled FRP tubes

A structure termed as concrete-filled steel tube (CFST) columns with in-built seawater and sea sand concrete (SWSSC)-filled fiber reinforced polymer (FRP) tubes was investigated. The new composite columns are comprised of outer steel tubes, sandwich ordinary concrete, inner FRP tubes and core SWSSC. Thirty-six CFST columns with in-built SWSSC filled FRP tube and four CFST columns were manufactured, and the influences of the diameter of the FRP tube, FRP layers and the thickness of steel tube on compressive performance of the specimens were studied. The results indicated that the ultimate strength of the specimens was positively related to the FRP layers, diameter of the FRP tube and thickness of the steel tube. The stress–strain curve of new structure consists of four stages, and columns with larger FRP tube diameter and more FRP layers obviously underwent more stress-drop after the ultimate point. The installation of the FRP tube has a more significant strengthening effect on the columns with a thinner steel tube. Models to predict the ultimate stress, ultimate strain, peak stress and peak strain of new structure were proposed. Moreover, a simple model to predict the axial stress–strain response curve of CFST columns with in-built SWSSC-filled FRP tubes was suggested.

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.

Compressive behavior of rectangular concrete-filled fiber-reinforced polymer and steel composite tube columns with stress-release grooves

To investigate the behavior of rectangular concrete-filled fiber-reinforced polymer (FRP) and steel composite tube (CFCT) columns with stress-release grooves under axial compression, sixty specimens were fabricated and divided into five groups for axial compression testing. The main variables were the corner radius, the number of FRP layers, the steel tube thickness and the presence of stress-release grooves. The test results indicate that steel tubes and FRPs can complement each other and discontinuous steel tubes can provide an effective confinement force on in-filled concrete. The stress–strain responses of CFCT columns with stress-release grooves consist of a linear elastic stage, a nonlinear transition stage, a linear hardening stage and a residual stage. The ultimate strength of a CFCT column is in direct proportion to the number of FRP layers, the thickness of the steel tube and the value of the corner radius. Stress-release grooves only play a positive role in specimens with relatively large values of the corner radii (r ≥ 30 mm). A new model was suggested for calculating the ultimate stress of CFCT columns with stress-release grooves. The model is applicable for CFCT and concrete-filled steel tube (CFT) specimens with stress-release grooves; the predicted results agree well with the experimental statistics.

Behavior of FRP-confined FRP spiral reinforced concrete square columns (FCFRCs) under axial compression

Fiber-reinforced polymer (FRP) confinement has been demonstrated to be efficient in enhancing the deformation and strength capacities of concrete, while Polyethylene terephthalate (PET) FRP-confined concrete easily exhibits a strain-softening stress-strain behavior, especially for PET FRP-confined concrete in square columns. In this study, novel PET FRP-confined FRP spiral reinforced concrete square columns (FCFRSCs) are proposed. Capitalizing the benefits of internal FRP spiral reinforcement, the strain-softening behavior of PET FRP-confined concrete is expected to be alleviated, leading to excellent mechanical performance of the proposed square columns. The axial compressive behavior of FCFRSCs, as well as square concrete-filled PET FRP tube columns (CFFTs) and FRP reinforced concrete columns (FRCs) as reference columns, were investigated via experimentation. The experimental results show that the strain softening behavior of confined concrete in FCFRSCs has been substantially enhanced with the internal FRP reinforcement, confirming the viability of the system. The strain softening behavior of FRP-confined concrete after the first peak is prevented and a hardening behavior is achieved for concrete in FCFRSCs, especially when the GFRP spiral pitch is 30 mm.

Behaviour of square concrete filled FRP tube columns under concentric, uniaxial eccentric, biaxial eccentric and four-point bending loads

This study investigated the behaviour of short square concrete filled fibre reinforced polymer tube specimens under concentric, uniaxial eccentric, biaxial eccentric and four-point bending loads. Seven concrete filled glass fibre reinforced polymer (GFRP) tube specimens (GFRP-CFFT) and seven traditional steel bar reinforced concrete (steel-RC) specimens of 200 mm x 200 mm cross-section were tested. The GFRP-CFFT specimens obtained a larger P-M interaction surface (higher maximum axial loads and higher bending moments) than the steel-RC specimens. In addition, the GFRP-CFFT specimens exhibited a lower average reduction in ductility under eccentric axial loads than the steel-RC specimens.

Bending and Shear Behaviour of Waste Rubber Concrete-Filled FRP Tubes with External Flanges

An innovative beam concept made from hollow FRP tube with external flanges and filled with crumbed rubber concrete was investigated with respect to bending and shear. The performance of the rubberised-concrete-filled specimens was then compared with hollow and normal-concrete-filled tubes. A comparison between flanged and non-flanged hollow and concrete-filled tubes was also implemented. Moreover, finite element simulation was conducted to predict the fundamental behaviour of the beams. The results showed that concrete filling slightly improves bending performance but significantly enhances the shear properties of the beam. Adding 25% of crumb rubber in concrete marginally affects the bending and shear performance of the beam when compared with normal-concrete-filled tubes. Moreover, the stiffness-to-FRP weight ratio of a hollow externally flanged round tube is equivalent to that of a concrete-filled non-flanged round tube. The consideration of the pair-based contact surface between an FRP tube and infill concrete in linear finite element modelling predicted the failure loads within a 15% margin of difference.

Compressive behavior of ultra-high performance concrete confined with FRP

This paper presents an experimental study on the compressive behavior of ultra-high performance concrete (UHPC) confined with fiber-reinforced polymer (FRP) composites. A total of 18 cylindrical specimens were prepared from UHPC containing 2.5% of micro steel fibers by volume, among which 6 were unconfined and 12 were confined with carbon FRP (CFRP). They were tested to investigate the experimental compressive behavior of unconfined or CFRP-confined UHPC. The external confinement was provided by two to six plies of CFRP wraps with the confinement ratio ranging from 0.11 to 0.54. The test results obtained from this study were presented and compared with those from existing studies on unconfined and FRP-confined UHPC specimens in the form of concrete-filled FRP tubes or FRP-wrapped cylinders. Predictive equations were proposed for the compressive strength and ultimate strain of FRP-confined UHPC, based on the interpretation of test results from the present study and from the existing literature. An existing stress–strain model for FRP-confined conventional concrete has been recalibrated for FRP-confined UHPC. While the recalibrated model predicted the test results of FRP-confined UHPC reasonably well, the inadequacy of a parabolic curve for describing the stress–strain relationship of unconfined UHPC is noticed, as a result of the higher strength and more brittle behaviour of UHPC in compression.

Interfacial behavior of segmental concrete-filled Basalt FRP tube under compression

The joint in a segmental concrete-filled fiber-reinforced-polymer (FRP) tube deteriorates the confining strength at the interfacial section. To quantitatively investigate the interfacial load-carrying capacity and deformability of segmental concrete-filled FRP tube, this study experimentally investigated segmental concrete-filled Basalt-FRP tube (SCFBT) specimens under compression. The stirrup at the interface was considered as the main parameter and was expected to compensate for the confining strength. The SCFBT specimens underwent sudden brittle failure caused by the rapture of the Basalt FRP (BFRP) tube. The rapture was always initiated at the interface, where the confining stiffness and strength were reduced. The confined strength of the concrete in the SCFBT deteriorated owing to the discontinuous BFRP tube. However, an adequate amount of stirrup could compensate for and enhance the confining strength. With the interface, the deformability increased owing to the delayed rapture of the BFRP tube. Based on the test results, a modified confined SCFBT model was developed with consideration to supplementary stirrup.

Design equation for concrete-filled FRP tubes in flexure including damage effects

Nonlinear finite element models are developed to investigate the effect of tube damage on flexural strength of concrete filled fiber reinforced polymer (FRP) tubes (CFFTs). The tube, being essentially an external reinforcement, is vulnerable to accidental damage or vandalism. The tube damage is induced at the outermost tension side of the CFFT in the form of a linear narrow cut with a variable length. The model is verified using experimental results from the literature. A comprehensive parametric study, comprising 140 new models, was then performed to study the damage effects in tubes with diameter-to-thickness (D/t) ratios of 30 to 100, [±θ] angle-ply tubes with (θ) varying from 15° to 75° and cross-ply tubes with 25–75% longitudinal fiber fraction. Additionally, the effects of damage were examined for hollow FRP tubes and compared to CFFTs with the same damage. It was shown that for angle-ply tubes and cross-ply tubes with 25–50% longitudinal fiber fraction, a sharp decrease (30–50%) in flexural strength occurs at circumferential cuts of length-to-perimeter (α) ratio of 5%, at all (D/t) ratios. For larger cuts, the decrease in capacity continuous but at a reduced rate so the reduction increases to (60–75%) as α increases to 30%. For hollow tubes, the damage affects moment capacity, only when material failure governs (at small D/t), not when local buckling governs (at large D/t). Statistical analysis was used to assess results of the comprehensive parametric study. A simple empirical equation was then deduced for condition assessment purposes. It enables engineers to predict flexural strength of both intact and damaged CFFTs for any given α, D and t, for a wide range of FRP tube laminate structures.

Axial compressive behavior of ultra-high-strength steel fiber-reinforced concrete-filled fiber reinforced polymer (FRP) tube columns

This study presents the results of the first experimental research on the axial compressive behavior of ultra-high-strength steel (UHSS) fiber reinforced concrete-filled FRP tubes (UHSSFR-CFFT). 24 circular UHSSFR-CFFT specimens were prepared and tested under axial compression to study the influence of unconfined concrete strength and fiber type. CFFTs were manufactured with glass, carbon and basalt reinforced polymer (GFRP, CFRP and BFRP) tubes with concrete compressive strengths (f′co) ranging from 35 to 105 MPa. Axial and lateral behavior were examined closely, including variation of lateral behavior along height of specimen. The results showed that the influence of concrete compressive strength on axial compressive behavior of UHSSFR-CFFTs is dependent on fiber type. For a similar normalized lateral confining pressure of FRP jacket at ultimate (flu/f′co), axial stress enhancement ratios (k1) were observed to decrease with an increase in f′co for all three fiber types. On the other hand, the obtained axial strain enhancement ratios (k2) indicated that k2 is influenced more by flu/f′co than f′co. Comparison of the recorded values of k1 and k2 for UHSSFR-CFFT with FRP-confined plain concrete showed similar trends on the influence of (f′co) on k1, but an opposite trend was observed for k2 of BFRP-confined specimens of the current study, in which k2 increased with increasing f′co. The results showed that the shape of the lateral strain-to-axial strain curves for UHSSFR CFRP- and BFRP-confined concrete specimens were similar to those of FRP-confined plain concrete, and this relationship was not influenced noticeably by f′co. Conversely, the obtained shape of the lateral strain-to-axial strain curves for UHSSFR GFRP-confined specimens was noticeably different when compared to FRP-confined plain concrete, with this behavior influenced by f′co. Finally, the results indicated that the addition of UHSS fibers in CFRP-confined specimens did not alter the influence of f′co on strain reduction factor (kɛ) where a decrease was observed by increasing f′co. However, the converse outcome was observed for GFRP- and BFRP-confined specimens where an increase in kɛ was observed when increasing f′co.

Flexural Behavior of Post-Tensioned Normal- and High- Strength Concrete-Filled Fiber-Reinforced Polymer Tubes

Flexural Behavior of Post-Tensioned Normal- and High- Strength Concrete-Filled Fiber-Reinforced Polymer Tubes

A New Torsion Testing Apparatus for Circular Members

The concrete-filled fiber-reinforced polymer tube (CFFT) is a unique structural element that is well suited for reinforced concrete applications in harsh environments due to the corrosion-resistant reinforcement. It also benefits from the stay-in-place formwork capabilities of the reinforcement and the beneficial concrete confinement offered primarily in cases of axial compression. The strength and response of the CFFT system have been studied under axial, flexural, and combined loading conditions; however, its torsional behavior has not been evaluated. Some of the challenges have been the ability to provide successful gripping of the circular cross-section as well as avoiding the undesired second-order effects when pure torsion is investigated. This paper presents a novel apparatus developed to test circular members, including CFFTs, to failure under a variety of loading conditions including pure torsion, and combined torsion, bending, and axial compression. The apparatus can accommodate a complex set of geometric nonlinearity effects associated with the deformations arising from multiple degrees of freedom. The functionality of the testing system under a state of pure torsion and combined loading was verified. The results showed that the testing apparatus effectively simulated the intended loading condition. The samples experience relatively large and complex displacements that justified the details incorporated in the design of various components of the testing configuration.

Interfacial behavior of segmental concrete-filled Basalt FRP tube under cyclic loading

A segmental concrete-filled basalt fiber-reinforced polymer (BFRP) tube was proposed, whose lightweight characteristic promoted convenient bridge column transportation and construction. A special connecting component between the adjacent BFRP segments ensured effective transfer of sectional forces. Four specimens, including three segmental specimens and one comparative integral concrete-filled BFRP tube, were tested to investigate the mechanical performance of the BFRP tube under cyclic loading. Damage patterns, load-deformation response, and strain development were observed, showing that the segmental concrete-filled BFRP tube presented satisfactory load-carrying and deformation capacities. Further, the connecting component effectively guaranteed a satisfactory hysteretic performance. Based on the test results, the overall load-carrying capacity was mainly determined by the moment resistance of the interface. Furthermore, the segmental structure weakened the confining effect on the core concrete, though applying multiple stirrups could compensate for the reduced confining effect. Finally, design methods were proposed for the connecting component.