Journal of Composites for Construction

The axial and flexure combined loading performance of a slender partially-composite concrete insulated wall design constructed using basalt fiber-reinforced polymer (BFRP) shear connectors and longitudinal BFRP reinforcement is investigated. The load-bearing wall panel sections, 600 mm wide and 2,700 mm long, were first subjected to axial loads ranging from 0 to 900 kN applied to the structural wythe only. The panels were then loaded to failure in four-point bending to develop the axial load-bending moment interaction diagram of the partially-composite system, which was compared to theoretical fully-composite and noncomposite curves. As axial load increased, failure under transverse loading changed from shear-compression of the structural wythe to diagonal splitting of insulation foam and rupture of BFRP connectors, then to crushing of the structural wythe. Composite action by the strength criterion reduced from 47 to 3% as axial load increased from 150 to 900 kN; while that by the stiffness criterion was much lower, 8–2.5%. Relative to a similar steel-reinforced wall panel with the same reinforcement ratio, the tested walls were 60–89% the strength; and the percentage increased with axial load. A method is introduced to estimate the effective centroid of the partially-composite wall in order to calculate the moment from the eccentric axial load applied to this load-bearing system.

This study reports the findings of an experimental investigation into the behavior of an inclined shear plane in reinforced concrete, such as a diagonal crack in the web of a beam, strengthened with externally bonded carbon fiber–reinforced polymer (CFRP) fabric. A modified push-off test of novel geometry was developed for this study. This test generates a diagonal failure plane subject to combined shear and tension. Both unwrapped and wrapped tests were conducted, allowing the load sharing and load-displacement behavior of the reinforced concrete, and the reinforced concrete with externally bonded CFRP fabric, to be investigated. Fully wrapped and U-wrapped CFRP fabric configurations were tested. Results indicate that for the arrangement tested, concrete, steel, and CFRP contributions to resistance are not independent, and that effective anchorage lengths given in the United Kingdom and United States guidance for U-wrapped CFRP may not be adequate in some cases.

This paper presents the seismic performance of concrete bridge columns reinforced with both steel and fiber-reinforced polymer (FRP). A bond-based parametric experimental study was conducted on five FRP steel–reinforced concrete (FSRC) bridge columns—using basalt FRP (BFRP) bars and BFRP sheets—and two steel-reinforced concrete (SRC) bridge columns served as references to investigate the fundamental characteristics of the proposed reinforcement. The investigated bond parameters included the texture of the FRP bars (smooth and ribbed), diameter of the FRP bars, location of the FRP bars with respect to the steel bars, and application of external FRP confinement. All columns were tested under the combined effect of constant axial load and reversed cyclic loading. The experimental results showed that the FSRC bridge column could realize the existence of a stable postyield stiffness (hardening behavior) as well as a reasonable displacement ductility of up to 10 before encountering strength degradation. Moreover, the FRP bars added for column longitudinal reinforcement did not have a substantial impact on the column elastic stiffness. The bond condition of the FRP bars to the surrounding concrete had pronounced effects on the column failure mode, postyield stiffness, residual displacement, and ductility; thus, it could be adopted as a design parameter.

Fabric-reinforced cementitious matrix (FRCM) composites made of dry-fiber fabric embedded in an inorganic matrix are advanced cement-based materials designed for retrofitting masonry or concrete structures. Characterization of the tensile behavior of FRCM composites provides the parameters needed for the design of the structural reinforcement and has given rise to numerous research studies on the aspects that influence its mechanical properties. To obtain the tensile behavior characteristics of this composite under different boundary conditions, two test setups were investigated. A clevis grip (pin action) was used to reproduce field boundary conditions from typical installation and to obtain design parameters. A clamping grip was used to obtain a complete characterization of the composite by inducing a tensile failure of each constituent material. Several FRCM systems made with different fabrics were used for the investigation: polyparaphenylene benzobisoxazole (PBO), carbon (C), and glass (G), plus carbon and glass with a special protective coating. This paper offers a critical analysis of the experimental results and provides recommendations for the tensile characterization of FRCM materials.

This study investigated the efficiency of externally bonded L-shaped fiber-reinforced polymer (FRP) laminates in strengthening seismically damaged reinforced concrete (RC) interior joints. Ten ½-scale interior beam-column joints, including two reference specimens, were constructed and then damaged under simulated cyclic lateral loads. In addition to repairing visual cracks with epoxy injections, four of eight specimens were strengthened with externally bonded L-shaped carbon fiber–reinforced polymer (CFRP) laminates and the rest were strengthened with basalt fiber–reinforced polymer (BFRP) laminates. Retesting after retrofitting showed that the average peak strength of the CFRP-strengthened and BFRP-strengthened specimens increased by approximately 20 and 10%, respectively. The two strengthening systems also clearly enhanced the deformability of the specimens. Further analysis indicated strength increase was controlled by the end debonding of FRP laminates near critical sections of the specimens, thus the bending strengthening formulas, which were extensively used in practice, overestimated the strengthening efficiency. To solve this problem, alternative formulas accounting for the end debonding of FRP were proposed to predict the peak strength and FRP strain of the specimens. The predictions showed acceptable agreement with the test results, demonstrating the proposed formulas were applicable to predict the efficiency of externally bonded L-shaped FRPs in strengthening damaged interior framed joints.

Deficient concrete-bridge columns can be effectively upgraded by bonding on the exterior surface fiber-reinforced polymer composite sheets with the fibers oriented in the columns’ circumferential direction, thus providing additional confinement. The efficiency of this rehabilitation method has been proven by the results of a number of cyclic loading tests performed on column specimens. The number of these tests, however, is much less than similar ones performed on reinforced-concrete columns without fiber-reinforced polymer confinement. This paper presents cyclic flexural test on reinforced-concrete columns confined with conventional circular hoops and carbon fiber–reinforced polymer and subjected to different axial-load levels. In addition to enriching the available database, the tests in this research program were programmed such that failure would occur in the composite sheets to validate an innovative stress-strain model that considers the passive confinement provided by both transverse-steel reinforcement and fiber-reinforced polymer wrapping. This experimental program includes eight circular reinforced-concrete columns having a 305 mm diameter and a height of 2 m. The parameters studied in this research program include the spacing of the hoops (75 or 150 mm), the intensity of the constant axial load (10 or 35% of the gross section capacity), and the presence of one or no carbon fiber–reinforced (CFRP) layer. The test results show that the addition of CFRP sheets enhances the behavior of the columns in terms of ductility and energy dissipation. The enhancement is more pronounced for the specimens with larger transverse steel–reinforcement spacing and lower axial-load level. The predictions of the moment-curvature show very good correlation with the measured responses.

This study investigates the bond characteristics and degradation under fatigue loading of near-surface mounted (NSM) carbon-fiber-reinforced polymer (CFRP) rods and strips embedded in concrete. Thirty-six concrete block specimens ($350\times 300\times 150\mathrm{mm}$) were tested under single shear. Sinusoidal waveform fatigue loading with a range of 10–50% of the corresponding static bond strength was applied with the number of cycles varying from 0 to 10,000, which was then followed by static pullout test to determine the local bond-slip relationship between the NSM reinforcement and concrete. Two separate sets of specimens were also subjected to the same fatigue loading up to failure to obtain the corresponding fatigue life. It was found that specimens strengthened with NSM rods mostly failed due to the breakage of concrete and epoxy, while those with NSM strips all failed due to slippage of NSM reinforcements at a lower pullout strength after the fatigue cycles. For both types of reinforcement, as the number of fatigue cycles increased, the average bond strength showed little decrease but the local bond strength decreased substantially. Empirical degradation equations were proposed for bond strength prediction with respect to the number of fatigue cycles. The adopted analytical approach provided satisfactory prediction of the fatigue bond characteristics.

Shear span-to-effective depth ratio ($a_v/d$) is known to affect the shear behavior of RC members. However, this effect has not been appropriately addressed in existing design guidelines for the fiber-reinforced polymer (FRP) strengthening of RC structures. This paper focuses on the effect of $a_v/d$ on the behaviors of RC beams shear strengthened with full-wrapping FRP strips. A total of six strengthened beams and six normal beams are tested, with $a_v/d$ ratios ranging from 1.0 to 3.5. The experimental results indicate that the FRP shear contribution increases initially with the $a_v/d$ ratio, but decreases when the ratio is beyond 2.5. The FRP strain along the critical shear crack distributes in different manners for different $a_v/d$ ratios. The authors’ experimental results and the test results from the literature are compared with predictions from available design guidelines. Although some recent design guidelines always provide a safe value, it can be overconservative. Conversely, other recent design guidelines may overestimate the shear-strengthening effectiveness at low span/depth ratios so they have to be used with caution.

A model to predict the corrosion rate of steel reinforcing bars in fiber-reinforced polymer (FRP) wrapped concrete columns is presented. A model proposed by the author to predict the reinforcement corrosion rate and cracking of the concrete cover due to diffusion of chloride ions and oxygen in concrete members is modified to predict the corrosion rate of reinforcing bars inside FRP-wrapped columns. As corrosion progresses and rust layers build up, oxygen has to diffuse through three different layers to be consumed in the cathodic reaction: the FRP wrap, the concrete cover, and the rust layer. As the rust layer thickness increases, the corrosion rate decreases until it reaches a very slow steady-state descent. The predicted corrosion rate is compared with experimental results reported in the literature. Initially, the model proposed previously by the author was used to predict the corrosion rate of an unwrapped pile to calibrate parameters such as the diffusion coefficient of oxygen with experimental data reported in the literature. Subsequently, the modified model was used to predict the corrosion rate of reinforcing steel in an FRP-wrapped column. The model yields very good agreement with experimental results reported in the literature.

The flexural failure mode of concrete beams reinforced with glass fiber-reinforced polymer (GFRP) bars changes from GFRP rupture to concrete crushing as the reinforcement ratio increases. Due to the uncertainties of material strengths, assumptions made in analysis, and variations in locations of reinforcements and dimensions of sections, there is a transition region where both flexural failure modes are possible. An iterative procedure is required when GFRP rupture governs the design. To avoid this iteration, the current American standard adopts a simplified but conservative procedure. In this study, the upper bound of the reinforcement ratio for beams in the transition region is revised. Moreover, a simplified yet rational design equation for calculating the flexural capacity of under-reinforced beams is proposed based on rigorous sectional analyses. Also, alternative design equations based on regression analyses are developed to predict the flexural capacity of beams in the transition region and over-reinforced beams, respectively. Moreover, the performance of the proposed equations is compared to that of design equations of recent standards by comparing their predictions with experimental results of 173 GFRP reinforced concrete beams collected from the available literature.

This study presents an analysis of carbon fiber reinforced polymers (CFRP)-to-parent material interfaces based on 40 single-lap shear tests intended to highlight the strength of the interfaces under fracture mode II. Three different substrates are analyzed: timber; concrete, and steel, using the same CFRP laminates and adhesive agent. The externally bonded reinforcement (EBR) technique was used throughout the study. The results show that the CFRP-to-timber interfaces had the highest strength but also showed that these interfaces need a longer bonded length in order to reach maximum strength, i.e., CFRP-to-timber interfaces had the longest effective bond length. The local nonlinear bond-slip curve of CFRP-to-concrete can be approximated to exponential curves, whereas the CFRP-to-timber or steel interfaces showed trilinear and bilinear bond-slip relations, respectively. Also, the CFRP-to-timber interfaces revealed the highest fracture energy.

Axially-loaded concrete columns with postpeak strain-softening suffer large inelastic deformation only in a localized failure zone, leading to nonuniform deformation along the height of the columns. Therefore, calculation of axial deformation of such a column is more complicated and involves the concept of compressive fracture energy. No model of compressive fracture energy and the relevant stress–strain relationship has been developed in the literature for fiber-reinforced polymer (FRP) confined concrete columns. Different definitions of the fracture energy have been adopted in extant literature, which has caused inconsistency and difficulty in application of the models. This analytical study involves the development of a more rational and general framework for stress–strain modeling of axially-loaded concrete columns involving failure localization. Modeling of the postpeak inelastic deformation and its associated compressive fracture energy is studied and its parameters are scrutinized and evaluated. A unified relationship is formed to relate different definitions of the fracture energy so that they can be compared with each other. The new stress–strain model shows good agreement with experimental results, and can be used for design of axially-loaded FRP-confined concrete columns.

Externally bonded fiber-reinforced polymer (FRP) composites are becoming a method of choice for the repair/strengthening of civil structures, some of which are situated in environments detrimental to FRP-concrete bond durability. Based on analysis of an extensive database of durability test results from notched-beam three-point bending tests, a bond durability factor (BDF) of 0.60 was identified as an appropriate estimate of bond durability for wet-layup carbon fiber-reinforced polymer (CFRP). This factor was compared to a normalized database of durability data from the literature, and it was found that a 0.60 BDF agrees well with these data. A modified procedure for determining the ultimate design strain primarily in bond-critical applications (such as external flexural and shear FRP reinforcement), is proposed. This procedure makes a distinction between durability properties associated with the FRP rupture and debonding failure modes. When used for practically feasible strengthening ratios and in conjunction with BDF of 0.60, the proposed procedure yields overall ultimate flexural-member design strengths that are 0–15% lower than those determined using current design procedures.

This paper offers an innovative technique for rehabilitation of severely corroded reinforced concrete (RC) T-beams using fabric-reinforced cementitious matrix (FRCM). Eight RC T-beam specimens were constructed and tested to failure under four-point load configuration. One beam was neither corroded nor repaired to act as a benchmark. Seven beams were presubjected to accelerated corrosion for 5 months that corresponded to an average tensile steel mass loss of 22%. Corrosion was restricted to the tensile steel located in the middle third of the beam span. Six corroded beams were repaired with either carbon or basalt FRCM system whereas one corroded beam was left unrepaired. The fabrics were internally embedded within the clear cover of the corroded-repaired region and/or externally bonded along the beam span. Corrosion damage significantly reduced the flexural capacity and ductility of the unrepaired beam. The basalt FRCM system could not restore the original flexural capacity of the beam whereas the carbon FRCM system fully restored the capacity. Doubling the amount of the internally embedded carbon FRCM layers slightly increased the strength gain but restored only 90% of the original beam ductility. The use of a combination of internally embedded and externally bonded carbon FRCM layers was more effective in improving the flexural response than the use of same amount of FRCM layers internally embedded within the corroded-repaired region.

This paper presents an assessment of the seismic performance of concrete columns internally reinforced with glass-fiber-reinforced polymers (GFRP) bars. Eight full-scale column prototypes with 1,650-mm shear span and 350-mm square cross-section were constructed and tested under combined lateral cyclic quasi-static and constant axial loading. The test specimens represent the lower portion of first-story columns between the footing and the contra-flexure point. The test parameters included longitudinal reinforcement type and ratio, level of axial load, and stirrup spacing. Test results showed that the drift capacity of GFRP-reinforced concrete (RC) rectangular columns at failure ranged between 8.5 and 12.5%, which exceeds the limitations of North American building codes. This indicates that the deformability of GFRP-RC column prototypes successfully replaced the ductility in steel–RC columns in dissipating the seismic energy in the presence of constant axial load. Furthermore, there was insignificant strength degradation before failure due to the well-confined concrete core.

The shear behavior of fiber-reinforced-polymer–strengthened reinforced concrete (FRP-strengthened RC) members is not fully developed and accurately predicted because of the lack of accurate constitutive laws for the components of the composite members. This paper presents experimental and analytical investigations of tensile stress-strain relationships of concrete and steel in FRP-strengthened RC members. These stress-strain relationships are required in formulations of softened truss models to predict the shear behavior of the FRP-strengthened RC element. Thirteen full-scale FRP-strengthened RC prismatic specimens with different FRP reinforcement ratios, steel reinforcement ratios, and FRP wrapping schemes were tested under uniaxial tension loading. The results show that the tensile behavior of the concrete and steel is altered because of the externally bonded FRP sheets. Modified constitutive laws are proposed and incorporated in the softened membrane model (SMM) to demonstrate through two tests the behavior of FRP-strengthened RC element subjected to pure shear. Moreover, crack spacing and crack width were studied and compared with existing code provisions.

Two carbon-fiber-reinforced polymer (CFRP) systems developed as possible means to strengthen existing concrete structures have been applied to a bridge at Kiruna (Sweden). The bridge was no longer in service and was thus used in extensive tests to failure. It was a posttensioned five-span bridge consisting of three longitudinal main girders topped with a concrete slab. The program reported here consisted of a series of three tests of the girders in the second span: loading each girder up to 2.0 MN before strengthening, loading each girder up to 2.0 MN after strengthening, and loading to failure after strengthening. One of the girders was strengthened by installing three $10\times 10{\mathrm{mm}}^2$ near-surface mounted (NSM) CFRP bars on the soffits, and another by installing three prestressed $1.4\times 80{\mathrm{mm}}^2$ CFRP laminates. This paper focuses on the NSM-strengthened girder, which failed at a maximum load of 6.1 MN, with a corresponding midspan deflection of 159 mm. The failure was a combination of flexure and shear, and the strain in the NSM bars at the ULS was 1.2%.

In the present study, a new method is introduced for confining square concrete columns in which fiber-reinforced polymer (FRP) strips are used at corners and FRP battens on column sides. In this method, FRP battens confining the section do not undergo any curvature but similar to what happens in flat coupons, they are stretched as completely flat strips. To compare the proposed method with the intermittent wrap technique, 25 square reinforced-concrete (RC) columns with cross-sections of $133\times 133\mathrm{mm}$ and heights of 500 mm were subjected to load eccentricities of 0, 30, 60, 90, and 120 mm. The test parameters included the technique employed for confining columns, flexural strengthening technique, and applied load eccentricity. The experimental results demonstrated that confining battens in the new method of corner strip-batten are uniformly stretched under the tension stresses of confinement; therefore, more-uniform distribution of confining pressure on section occurs and stress concentration at corners is eliminated. Thus, compared to both unconfined and intermittently-wrapped RC columns, square RC columns confined through the corner strip-batten technique exhibit improved performance in terms of enhanced load-carrying capacity and ductility under eccentric loading. Finally, axial loading–bending moment (P–M) interaction diagrams were drawn for the strengthened columns. Comparison of the experimental results with those obtained from the expressions suggested in different codes revealed that the estimates of the codes were conservative when compared to experimental measurements.

Using carbon-fiber-reinforced polymer (CFRP) materials can extend the fatigue life of damaged steel structures. However, several issues still need to be studied. In this paper, experimental and numerical studies were conducted on the fatigue behavior of cracked steel plates strengthened with CFRP materials. A total of 10 specimens were designed, including six central-cracked and four edge-cracked specimens. The repair effectiveness of the different CFRP systems and CFRP configurations were compared under the equivalent CFRP tensile stiffness. The test results indicate that the CFRP can effectively decrease the crack growth rate and prolong the fatigue life of cracked steel plates. For the central-cracked specimens, the fatigue life can be extended by 3.5–4.9 times that of the unstrengthened specimens. For the edge-cracked specimens, the extension ratio of the fatigue life was 6.9–11.3. Under the equivalent CFRP tensile stiffness, the repair effect of the CFRP plates was better than that of the CFRP sheets, and the different CFRP configurations had an effect on the fatigue repair effectiveness of the specimens. A finite element (FE) model was developed to evaluate the stress intensity factor of the specimens. Based on the FE results and the linear elastic fracture mechanics theory, the fatigue life and the crack propagation curve were predicted. The numerical and experimental results were in good agreement.

This paper studies the fatigue behavior of high-strength basalt fiber-reinforced polymer (BFRP) tendons for potential applications in prestressed structures. An effective anchoring method of winding fiber sheets was first developed for fatigue testing of BFRP tendons to avoid premature failure at the anchorage. The fatigue stress ranges from 0.05 to $0.14f_u$ ($f_u$ = ultimate tensile strength) and maximum stresses from 0.6 to $0.8f_u$ were determined, whereas the elastic modulus of BFRP tendons during the fatigue tests was measured. The fatigue failure mechanism was analyzed and elaborated at both the macro- and microscopic levels. The appropriate fatigue stress range and maximum stress level were predicted from both experimental fitting and reliability analysis. The results show that the fatigue failure of a BFRP tendon is mainly induced by the debonding among fiber-matrix interfaces at the outer layer of the tendon. The fatigue stress range greatly affects the fatigue life of BFRP tendons. The BFRP tendons can sustain 2 million cyclic loadings under a stress range of $0.05f_u$ (85 MPa) and maximum stress of $0.6f_u$ (1,018 MPa). Furthermore, the elastic modulus of BFRP tendons before failure remains constant regardless of the number of cycles. Prediction of fatigue strength (limit of stress range and maximum stress to sustain 2 million cyclic loadings) based on experimental fitting shows a satisfactory consistency with the previously mentioned experimental results (0.05 and $0.6f_u$). A recommendation of a stress range of $0.04f_u$ (68 MPa) and maximum stress of $0.53f_u$ (899 MPa) are proposed with 95% reliability for BFRP tendons in prestressing applications.

The use of bolted sleeve joints has been proposed for assembling pultruded glass fiber-reinforced polymer (GFRP) tubular profiles into space lattice shell structures. Such joint configurations may introduce a semi-rigid end condition and further affect the capacity of the connecting members in compression. Three batches of specimens assembled with pultruded GFRP profiles of different lengths and bolted sleeve joints at both ends were prepared and tested under static axial compression. A detailed three-dimensional finite element model considering bolt geometry, contact behavior, bolt pretension, initial geometric imperfection, and failure criterion for fiber-reinforced polymer (FRP) composites was developed and validated with experimental results showing good comparisons. It was found that the bolted sleeve joints exhibited semi-rigid behavior and that the failure modes and the effective length factor were dependent on member slenderness. The relationship between effective length factor and member slenderness was derived based on finite element analysis and then used in conjunction with existing FRP column design equations to predict the member capacity for structural members with bolted sleeve joint end conditions.

This paper investigates the bond characteristics between the concrete substrate and fiber-reinforced polymer (FRP) material processed with advanced manufacturing techniques. Tests provide further understanding regarding the interface behavior of the joints manufactured with a heated vacuum bag only (HVBO), vacuum-assisted resin infusion (VARI), wet lay-up, and pultrusion techniques. A specific criterion, the equalization of the processing techniques, is proposed to compare the characteristics of the joints. Based on the outcomes, the advanced processing techniques such as HVBO and VARI can be successfully applied to achieve a high-quality bond in strengthening of the reinforced concrete structures. Furthermore, a new analytical approach is presented to determine the interface behavior, which can be applied to any type of the FRP processing technique.