Journal of Composites for Construction

Concrete cover separation is a common failure mode of RC beams strengthened with a fiber-reinforced polymer (FRP) plate externally bonded to the tension face (FRP-plated RC beams). This failure mode initiates at the critical plate end and then propagates at the level of steel tension reinforcement in the direction of increasing moment. Plate-end anchorage by FRP U-jackets has been specified in some design guidelines as a mitigation measure to delay or suppress concrete cover separation, although its effectiveness is far from clear. This mitigation method is more attractive than other options because the same strengthening material is used and the installation procedure is simple. This paper presents the first systematic experimental study on the use of FRP U-jackets for mitigating plate-end concrete cover separation failure, with particular attention to the effect of inclination angle between the U-jacket and the beam axis. A total of 10 full-scale FRP-plated RC beams were tested. The test results show that both the load-carrying capacity and the ductility of an FRP-plated RC beam can be significantly enhanced by FRP U-jacketing at the critical plate end. Inclined U-jacketing at 45° to the beam axis was found to be much more effective than vertical U-jacketing (i.e., at 90° to the beam axis) in mitigating plate-end cover separation failure.

Integrating fiber-reinforced polymer (FRP) reinforcement into high-strength concrete (HSC) would effectively contribute to enhancing the stiffness of cracked concrete sections when the FRP bars are undergoing high strain and stress levels. Nonetheless, neither the concentric nor the eccentric behavior of HSC columns reinforced with FRP reinforcement has yet been investigated. This paper presents the results of an experimental investigation on the concentric and eccentric behavior of full-scale circular HSC columns reinforced with glass FRP (GFRP) bars and spirals. A total of 10 columns were tested under monotonic loading with different eccentricities. The test variables were the eccentricity-to-diameter ratio and the longitudinal-reinforcement ratio. Compression failure in the concrete controlled the ultimate capacity of specimens tested under small eccentric loading. A flexural-tension failure initiated in specimens tested under high eccentric loading, however, resulted from excessive axial and lateral deformations and cracks on the tension side until a secondary compression and stability failure occurred owing to the strain limitations in the concrete. The axial force–moment interaction diagrams were predicted based on the principles of strain compatibility and internal force equilibrium in accordance with the recommendations in the available design standards.

Innovative materials and technologies have been developed to limit the effects of earthquakes on structures, and the use of composite materials has been shown to be effective in existing buildings. In view of this background, experimental tests can provide a contribution to the interpretation of available strengthening interventions. The results of experiments with an innovative strengthening system are presented herein. The strengthening technique used is based on a textile-reinforced mortar (TRM) system coupled with traditional strengthening approaches, and was applied to a type of full-scale masonry vault that is typically found in the roofs of religious buildings. The experiments consisted of several shaking table tests, both before and after the application of the strengthening system. The seismic behavior of the vault after strengthening was significantly improved. The instrumental response of the vault started to change before the initial visible damage, which only occurred when the peak ground acceleration (PGA) was almost doubled. The seismic capacity of the unreinforced vault was more than doubled when the strengthening was applied, with the vulnerability moving from the curved element to the masonry abutments. As a consequence, additional interventions should focus on lateral abutments. There was no debonding between the TRM and the masonry substrate, and the strengthening strategy (combination of innovative and traditional approaches) was effective in preventing the failure of the vault.

Concrete prestressed with carbon fiber–reinforced polymer (CFRP) tendons beneficially utilizes the strain capacity and durability characteristics of the CFRP material. However, changes to CFRP tendon material formulations or production processes present a challenge when building up a body of knowledge to inform the long-term behavior of a prestressed concrete system. Initial qualification tests can help identify the potential for fiber–reinforced polymer (FRP) tendons to meet long-term mechanical and durability requirements. Protocols that can be undertaken on commercially produced cylindrical FRP pultrusions are particularly advantageous because the constituent materials and manufacturing processes are representative of the actual product. Selected mechanical and durability characterization approaches, and their suitability for adaptation for wet specimens with cylindrical geometries, were critically assessed. A series of qualification tests were then conducted on CFRP tendons with three different resin systems. Tendon samples were exposed to wet environments to evaluate the longer-term solution uptake and associated mechanical durability implications. While characterization measures such as the glass transition temperature, optical imaging, and moisture uptake provided comparative results, the correlation with mechanical properties obtained from uniaxial tension, double notch shear, and torsion tests was unclear. Using a subset of the mechanical test protocols, a retrospective analysis of CFRP tendons extracted from a prestressed concrete lighting pole under sustained load for 16.5 years was also reported.

This paper presents the results of a collaborative research project between Quebec’s Ministry of Transportation and Ontario’s Ministry of Transportation, which aimed at characterizing a new type of headed glass fiber–reinforced polymer (GFRP) reinforcing bar and evaluating its suitability as internal reinforcement for concrete structures. To achieve these objectives, the project was implemented in three stages: (1) evaluation of the physical and mechanical properties; (2) determination of the pullout behavior in concrete; and (3) characterization of the long-term durability of the headed GFRP bars. A total of 57 specimens embedded in a 200-mm concrete cube were tested with the direct pullout test to investigate the effect of confinement, bar size, concrete compressive strength, and exposure conditions on the pullout behavior of the headed GFRP bars. Simultaneously, microstructural analyses and measurements of the physicochemical and mechanical properties were carried out on conditioned and unconditioned headed GFRP bars. The results show that the materials, geometry, and interface configuration of the head provided very good mechanical interlocking to the GFRP bars. Up to 63 and 53% of the guaranteed tensile strength of the straight GFRP bars were achieved for 15.9- and 19-mm diameter bars with headed ends, respectively. Scanning electron microscopy and differential scanning calorimetry showed no material changes in the head and bars after exposure to alkaline solution and freeze–thaw cycling. Exposure to the alkaline solution under sustained loading had the most detrimental effect, with the bar retaining 79.4% of its pullout strength. The results indicate that the tested headed GFRP bar has suitable mechanical and durability properties for use as reinforcement in concrete bridge components.

Bond strength durability of carbon fiber–reinforced polymer (CFRP) to concrete is investigated for two bio-based resins and compared with conventional epoxy (E). The first is an epoxidized pine oil blend (EP) and was used for wet layup installation. The second is a furfuryl alcohol resin derived from corn cobs and sugar cane (FA), and was used to prefabricate CFRP plates, which were installed using an epoxy paste. A total of 137 concrete prisms with a midspan half-depth saw cut were tested. Specimens were conditioned in a 3.5% saline solution at 23, 40, or 50°C for up to 240 days before testing for bond strength. Short-term bond strengths were 3.9, 4.3, and 5.5 MPa for the E, EP, and FA resins, respectively, using a 33 MPa concrete. Reductions in bond strength over the 240 days of conditioning did not exceed 15%. Bond failures of E-CFRP (EC) and EP-CFRP (EPC) were predominantly adhesive-based, where a thin layer of cement paste remained attached to CFRP, while that of FA-CFRP (FAC) was cohesive-based, where there was a thicker layer of concrete remaining on the CFRP. This suggests that bond between the furfuryl alcohol resin and epoxy paste is excellent and remained intact. ANOVA showed that the difference between EPC and EC strengths were insignificant, while that between FAC and EC were statistically significant. Only FAC at 40°C after 240 days showed a statistically significant difference from its unconditioned counterpart. Finally, the 40 and 50°C conditioning was not statistically different from the 23°C.

This paper presents probabilistic models for estimating shear capacity of concrete members reinforced internally with fiber-reinforced polymer (FRP). The application of FRP in concrete structures has steadily gained popularity since the late 1980s; however, the wide range of mechanical properties and various manufacturing processes have made it challenging to unify the design formulation for engineers and code developers. Consequently, the conservative approach has been used to predict structural performance, particularly shear capacity, which has limited its application and resulted in excessive use of materials. In this paper, a robust prediction model is proposed to capture the variety of characteristics of FRPs based on an available database. The database includes various FRPs used as flexural reinforcement for beam members. The model was formulated using the Bayesian parameter estimation method, which takes into account the uncertainties of the parameters that are considered the potential predictors in the model. This paper presents the overall development of the model and provides comparisons with existing models and design equations. The model was formulated for shear capacities without FRP shear reinforcement (flexural reinforcement only). The results indicated that the performance of the proposed model was superior to comparable models.

This paper reports on the efficiency of fabric-reinforced cementitious matrix (FRCM) in enhancing the flexural capacity and deformational characteristics of RC beams. In the main experimental part of the paper, 12 RC beams, 2,500 mm long, 150 mm wide, and 260 mm deep, were fabricated. The beams had two different steel reinforcement ratios, namely, ${\rho }_s^{D12}=0.72\%$ and ${\rho }_s^{D16}=1.27\%$, representing typical underreinforced beam sections. The strengthened beams utilized two FRCM types, carbon and polyparaphenylene benzobisoxazole (PBO) FRCM systems. In the second part of the work, tensile material characterization tests were performed on the FRCM coupons to determine the tensile characteristics of the FRCM composites. The beams were tested in flexure under four-point loading until failure. Two beams without FRCM strengthening were used as a benchmark. Six beams were externally strengthened using one, two, and three layers of carbon FRCM system. Four beams were strengthened with one and two layers of PBO FRCM system. Test results showed that the flexural capacity of the strengthened beams increased by 77% for the carbon FRCM system (three layers) and to 27% for the PBO FRCM system (two layers) over that of their benchmark (unstrengthened) beams.

In this paper, test results of six large-scale glass fiber–reinforced polymer (GFRP) RC continuous T-beams are presented. The test specimens include one steel RC beam to serve as reference, one GFRP RC beam designed to meet the serviceability criteria at the service load level calculated for the reference beam, and four GFRP RC beams designed to achieve the same theoretical ultimate load of the reference beam. The test variables included the assumed percentage of moment redistribution, the spacing of lateral reinforcement in flange, and the arrangement of shear reinforcement. The test results showed that moment redistribution from the hogging to the sagging moment region occurred in GFRP RC beams with T-sections and that a small spacing of stirrups enhanced the moment redistribution percentage. In addition, decreasing the spacing of lateral reinforcement in the flange improved the moment redistribution through enhancing the stiffness of the sagging moment region.

An experimental investigation was undertaken into the effectiveness of unanchored and anchored externally bonded (EB) U-wrapped carbon fiber–reinforced polymer (CFRP) shear strengthening for reinforced concrete T-beams at a range of realistic sizes. The T-beam sizes, geometry, and reinforcement were chosen to reflect existing slab-on-beam structures with low levels of transverse steel shear reinforcement. Geometrically similar reinforced concrete T-beams were tested across three sizes ranging from 360 to 720 mm in depth and with different amounts of EB CFRP shear reinforcement. The beams were subjected to three-point bending with a span-to-depth ratio of 3.5. All the beams failed in diagonal shear. The experimental results indicate significant variability in the capacity of unstrengthened control beams, and a number of these control beams showed greater shear capacity than their EB CFRP-strengthened counterparts. Greater thicknesses of CFRP reinforcement did not lead to increased shear capacity compared with less thicknesses of unanchored or anchored EB CFRP, but anchored EB CFRP did lead to moderate increases in shear capacity compared with both control and unanchored EB CFRP-strengthened beams.

Carbon fiber–reinforced polymer/plastic (CFRP) used in bridge cables may be subjected to lateral impacts caused by vehicle collisions when in service. However, to date, few studies have been conducted on the impact behavior of CFRP cables and their anchor systems. In this paper, the impact behavior of CFRP cables was investigated through drop-weight tests of eight CFRP cable specimens. A seven-wire CFRP strand was used as the tendon, and reactive powder concrete (RPC) was used as the filling grout for anchors at both ends of each specimen. The results show that a bond length of no less than 36 times the diameter of the wire of the CFRP strand in 128 MPa RPC grout can provide a CFRP strand with reliable anchorage under impact. The average transverse impact resistance of the specimens was approximately 16 kN and was not significantly influenced by pretension. The specimens fractured at a cable tension of 30% of the static tensile breaking load under transverse impact. In addition, the energy dissipation capacities for specimens with pretension to static tensile breaking load ratios of approximately 10, 21, and 26% were 632, 727, and 902 J, respectively. Finally, a normalized energy profile diagram (EPD) was established for the design and damage evaluation of a CFRP cable in the case of an impact event.

This study investigates the impact resistance of fiber-reinforced polymer (FRP) confined concrete. Concrete cylinders were wrapped with carbon FRP (CFRP) or glass FRP (GFRP) with a varied number of layers and wrapping schemes. The impact tests were conducted by using drop-weight apparatus at different impact velocities. Dynamic behavior of the specimens has been investigated. The experimental results have shown that the failure modes are very different than those from static tests. Identical specimens experienced different damage as the impact velocities changed. The dynamic rupture strain of FRP was found to be substantially lower compared with that under static loads. As a result, the FRP efficiency factors were 0.17 and 0.56 for CFRP and GFRP, respectively. Interestingly, although GFRP has lower tensile strength and elastic modulus, it showed much better performance against impact compared with CFRP in terms of both the strength and ductility. The higher rupture strain of GFRP compared with CFRP results in higher confinement efficiency of GFRP under impact loads. A confinement model is proposed to predict the confined concrete strength under impact.

Concrete with reinforcing steel bars across a shear plane was strengthened using carbon fiber–reinforced polymer (CFRP) strips externally bonded to resist shear transfer. Specimens were designed and tested to give insight on shear transfer based on a component model. In this model, five components, i.e., the friction, aggregate interlock, adhesion, shear dilation, and dowel action, contributed to the overall shear resistance. Results indicated that the ultimate shear transfer capacities improved by 6 to 50% for specimens with steel reinforcement ratios varying from 0.46 to 1.20% and CFRP reinforcement ratios varying from 0.224 to 0.298%. The mechanism of shear resistance increase was due to the additional clamping force provided by CFRP strips, which led to a significant increase in the shear component of the adhesion. A best-fitting expression was presented to predict the ultimate shear transfer capacities of CFRP-strengthened concrete with satisfactory accuracy by comparing the calculated results with test data in this study and in literature. A general concept of equivalent reinforcement ratio was proposed to clarify the limitation of strengthening effect for shear transfer.

Considerable research in recent years has exclusively considered concrete members with rectangular cross sections in developing fiber-reinforced polymer (FRP) shear design provisions. Although there is no evidence that these provisions do not apply equally well to nonrectangular sections, the behavior of circular sections has yet to be confirmed with experimental results. This paper reports experimental data about the shear strength of circular concrete beams reinforced with carbon-FRP (CFRP) bars and spirals. A total of five full-scale concrete beams with a total length of 3,000 mm and diameter of 500 mm were constructed and tested up to failure. The test parameters included the type of reinforcement (CFRP versus steel) and the ratio of shear reinforcement (spiral spacing). The investigation revealed that the beams reinforced with CFRP bars and spirals exhibited high load-carrying capacity and performance comparable with that of the control beam reinforced with steel. The experimental shear strengths of the CFRP-reinforced concrete beams were compared with theoretical predictions provided by current codes and design guidelines. The comparison indicated that some of the available design methods provide reasonable predictions. However, a more precise formula for the shear strength $V_{sf}$ was proposed to account for the mechanical properties and geometry of CFRP spirals. The proposed equation provided more reasonably accurate and conservative predictions.

This paper presents an experimental study of concrete columns reinforced by steel basalt-fiber composite bars (SBFCBs) and steel basalt-fiber hybrid stirrups (SBFHSs) as innovative hybrid reinforcements. Four RC columns were cast; one column, which was reinforced longitudinally and transversely with traditional steel bars, served as a reference. Three columns were constructed with varying amounts of a basalt fiber–reinforced polymer (BFRP) material in the SBFCBs; the postyield stiffness ratio of the hybrid reinforcement was 0.1, 0.3, or 0.5. All the columns were subjected to a constant axial load and lateral cyclic loading. Several parameters affecting the columns’ responses, such as the BFRP reinforcement ratio and the ratio between the postyield and elastic stiffness of the longitudinal bars, were examined. The experimental results showed that the failures of all four columns were flexural, and the postyield stiffness of the longitudinal reinforcement had a significant effect on the stiffness, bearing capacity, residual deformation, and amount of energy dissipated by the test columns.

North America’s current design codes and guidelines allow the use of fiber–reinforced polymer (FRP) bars as the primary reinforcement in concrete structures and provide design recommendations for using these bars. Because of a lack of experimental data, however, FRP bars have not been recommended for resisting compression stresses as longitudinal reinforcement in columns or compression reinforcement in flexural elements. This paper presents test results of an experimental program to investigate the structural performance of 10 full-scale circular concrete columns reinforced with carbon fiber–reinforced polymer (CFRP) bars and spirals subjected to combined axial compression loads and bending moments. The test variables include different eccentricity-to-diameter ratios and two types of reinforcement (CFRP and steel). The test results show that the CFRP- and steel-reinforced concrete columns behaved similarly up to their peak loads. The failure of the test specimens under different levels of eccentricity was not triggered by rupture of the CFRP bars in the tension side, but rather it was attributed to gradual concrete crushing, followed by bar crushing on the compression side. Based on the test results, a detailed sectional analysis and plane section analysis were then conducted. Furthermore, a comprehensive parametric investigation was performed to generate numerous nominal axial force–bending moment ($P\text{-}M$) interaction diagrams. The experimental and analytical results are discussed and compared.

This paper presents an experimental study on the fatigue behavior of cracked steel beams strengthened using different patch systems and high-strength materials. These materials included normal modulus carbon fiber–reinforced polymer (CFRP) laminate, high-strength steel (HSS) plate, and SafStrip (SAF) plate. Adhesive bonding and mechanical anchorage were selected to attach these overlays. A digital image correlation (DIC) system was adopted to detect stress distribution at the vicinity of the crack front. Different failure modes were observed for specimens with different retrofitting schemes. Test results showed that, in comparison with control specimens without strengthening, application of these retrofitting materials significantly retarded crack propagation and extended fatigue life of defected steel beams. The stiffness decay and crack mouth opening displacement (CMOD) were also reduced in repaired cases. Based on the fatigue cycles when the crack propagated to half-height of the steel beam, CFRP laminate with structural adhesive Araldite 420 led to the best strengthening performance. This study extends the understanding of fatigue repair for steel beams and provides some useful suggestions for the strengthening method.

Tests of deep concrete beams with internal fiber-reinforced polymer (FRP) reinforcement have shown that such members can exhibit lower shear strength than members with conventional steel reinforcement. To model this effect, the current paper proposes an approach based on a two-parameter kinematic theory (2PKT) for conventional deep beams. The 2PKT is built on a kinematic model with two degrees of freedom that describes the deformation patterns of cracked beams. Using this theory shows that large strains in FRP longitudinal reinforcement result in reduced shear resistance of the critical loading zones (CLZ) of deep beams. The original 2PKT is therefore modified by introducing a reduction factor for the shear carried by the CLZ. The extended 2PKT approach is then applied to a database of 39 tests of FRP-reinforced deep beams from the literature, resulting in an average shear strength experimental-to-predicted ratio of 1.06 and a coefficient of variation of 18.3%. The results show that the 2PKT adequately captures the effects of the stiffness of the reinforcement, section depth, concrete strength, and shear-span-to-depth ratio on the shear strength of FRP-reinforced deep beams.

In this paper, accelerated tests are conducted on the bond performance of steel fiber–reinforced polymer composite bars (SFCBs) with sea sand concrete and the flexural performance of SFCB-reinforced sea sand concrete beams after conditioning in a simulated seawater wet–dry cycling environment for 30, 60, and 90 days. The beams are coupled with a sustained load during conditioning. The steel bar pullout specimens and steel bar–reinforced sea sand concrete beams are conditioned in the same environment for comparison. Overall, 24 direct pullout specimens and eight beams are tested. The test results indicate that the maximum bond stresses of the SFCBs improve after wet–dry cycling, whereas the average bond stresses at 0.05, 0.10, and 0.25 mm of free-end slippage decrease slightly. Both the maximum bond stresses and the average bond stresses at 0.05, 0.10, and 0.25 mm of free-end slippage of the steel bars decrease. No significant changes occur in the characteristic loads of the steel bar beams during the aging period, whereas the characteristic loads of the SFCB beams decrease. Residual deflections after unloading increase the flexural stiffness and energy ductility of all the conditioned beams. The crack widths of the steel bar beams significantly increase after conditioning, but these increases are not obvious for the SFCB beams.