Journal of Composites for Construction

This paper reports on the shear behavior of concrete beams reinforced with basalt fiber-reinforced polymer (BFRP) bars. In this study, 10 reinforced concrete beams with no transverse reinforcement, including 2 beams reinforced with longitudinal steel bars, were constructed and tested until failure. The test variables were the longitudinal reinforcement ratio and the span-to-depth ratio of the beams. The test results were compared with predictions of different available codes and design guidelines. Shear design equations of the CAN/CSA-806-12 code and the JSCE-97 guidelines provided accurate predictions. However, the predictions of CAN/CSA-806-12 were not conservative in some cases, whereas those of the ACI-440.1R-15 and the CAN/CSA-S6-10 were conservative. The test results were combined with the experimental results of a large database, including 75 beams and one-way slabs reinforced with carbon, glass, aramid, and BFRP bars. The behavior of beams reinforced with BFRP bars was quite similar to that of beams reinforced with glass and aramid fiber-reinforced polymer (FRP) bars. The test results demonstrate the potential of using BFRP bars as alternative reinforcement in concrete members.

This paper examines the shear strength and behavior of concrete beams reinforced with basalt fiber-reinforced polymer (basalt FRP) bars with and without shear reinforcements. Six $200\times 300\mathrm{mm}$ ($8\times 12\mathrm{in.}$) and six $300\times 200\mathrm{mm}$ ($12\times 8\mathrm{in.}$) concrete beams were, respectively, made with and without basalt FRP shear reinforcements. The flexural reinforcement ratios (${\rho }_f$) ranged from 2.69 to 14.8 times the balanced ratio (${\rho }_{fb}$) for nonshear reinforced (NSR) beams and 1.69 to 6.88 for the shear reinforced (SR) concrete beams. Two different shear span-to-depth ($a/d$) ratios were considered for the NSR-beams (5.65 and 7.0) and three different $a/d$ for the SR-beams (1.5, 2.5, and 3.5). The test results are presented in terms of crack patterns, failure modes, load-deflection, load-strain behavior, and shear capacity. It was observed that for both SR- and NSR-beams, the shear capacity increased when the area of basalt FRP reinforcement increased for the same span to depth ratio ($a/d$), whereas the shear capacity decreased when the span to depth ratio ($a/d$) increased. In light of the experimental results, prediction models and design code equations were evaluated to test their accuracy in predicting the shear strength of basalt FRP RC-beams. Standard provisions predictions were both conservative and nonconservative. The predictions based on the modified compression field theory for the nonshear reinforced beams were the closest to the experimental.

Extensive research has been conducted on fiber-reinforced polymer (FRP)-confined plain and RC columns, leading to a large number of stress–strain models. Most of these models have been developed for FRP-confined plain concrete and are thus applicable only to concrete in FRP-confined RC columns with a negligible amount of transverse steel reinforcement. The few models that have been developed for concrete under the combined confinement of FRP and transverse steel reinforcement are either inaccurate or too complex for direct use in design. This paper presents an accurate design-oriented stress–strain model for concrete under combined FRP-steel confinement in FRP-confined circular RC columns. The proposed model is formulated on the basis of extensive numerical results generated using an analysis-oriented stress–strain model recently proposed by the authors and properly captures the key characteristics of FRP-steel-confined concrete as revealed by existing test results. The model strikes a good balance between accuracy of prediction and simplicity of form and is shown to provide close predictions of test results and perform significantly better than existing stress–strain models of the same type.

Experimental observations were made for the shear capacity of reinforced concrete beams with carbon fiber-reinforced polymer rectangular stirrups (CFRPRS). In total, 12 concrete beams were tested under three-point loading. Each beam measured 1,400 mm long, 150 mm wide, and 250 mm deep. For comparative studies, different parameters were considered: different sectional shapes (circular and rectangular CFRP stirrups), material types (steel and CFRP), and reinforcement ratios of stirrups (0.0038 and 0.0096). All beams were subjected to three-point loading. Test results indicated that concrete beams reinforced with CFRPRS stirrups could maintain shear behavior comparable to that of the concrete beam reinforced with steel stirrups. Compared with the beams with conventional CFRP stirrups with circular sections, superior shear behavior was observed for the beams reinforced with CFRPRS stirrups in terms of shear strength and crack width.

Concrete beams reinforced with a combination of steel and fiber-reinforced polymer (FRP) bars can provide increased strength, serviceability, and durability. However, the amounts of FRP and steel necessary to ensure sufficient strength and ductility are unclear. Because of the linear elastic behavior of FRP bars before failure, the deformation characteristics of hybrid reinforced concrete beams differ from those of conventional steel reinforced concrete beams and pure FRP reinforced concrete beams. Conventional ductility indices are not suitable for concrete beams in hybrid reinforcement approaches. To ensure the ductile failure of beams, proper reinforcement ratio limits are proposed. In addition, a new ductility index is defined in terms of deformability and energy absorption capacity. Various comparisons between experimental results and theoretical predictions show that the developed models can accurately predict the load capacity and ductility. In addition, the influences of various parameters on ductility are discussed. Based on various requirements for ductility, reasonable ratios of FRP to steel bars are found.

Large and small cylindrical shells have long been used as tanks and silos to store materials such as oil and its derivatives. The radius-to-thickness ratio of these shells is $500:2000$; as a result, buckling collapse of these thin-walled structures is of major concern for designers. The present study examined inelastic buckling behavior of cylindrical shells near the base, known as elephant foot buckling. This form of buckling occurs under high internal pressure exerted simultaneously with axial compression. The buckling of cylindrical shells subjected to combined axial loads and internal pressure was experimentally studied and tested and a new method of strengthening steel cylindrical shells using fiber-reinforced polymer (FRP) composite materials is presented. The proposed method was studied by numerical methods using a nonlinear algorithm, and the results were evaluated for resistance to buckling of cylindrical shells. The results provide effective and useful information for use in retrofitting cylindrical shell tanks.

The loss of composite action at the negative moment region for a continuous composite girder reduces the girder’s strength and stiffness. This paper presents an experimental investigation into the use of carbon fiber–reinforced polymer (CFRP) to maintain the composite action at the negative moment region of continuous composite girders. This is achieved by bonding CFRP sheets to the top of a concrete slab at the negative moment region. Six two-span continuous composite girders were tested. CFRP sheet thickness was varied to assess its effect on girder behavior. The girders were designed to have full composite action between the concrete slab and the steel girder. Moment capacity at the positive and negative moment regions was evaluated experimentally and theoretically. A plastic analysis was conducted to evaluate the ultimate capacity of the girders. Finite-element modeling evaluated girder performance numerically. The experimental results confirmed the effectiveness of CFRP sheets in maintaining composite action at the negative moment region and in preventing crack initiation in a concrete slab under service load. The use of CFRP improved the strength and stiffness of the continuous composite girders. The plastic analysis safely estimated the girders’ ultimate capacity. The developed finite-element model yielded satisfactory results.

Damage of reinforced-concrete (RC) structures commonly occurs during major earthquakes, and becomes worse because of aftershocks. For these reasons, repair technology for seismic damage is needed to prevent possible collapse of damaged RC structures before aftershocks. In this paper, a seismic retrofitting device is proposed to improve the seismic performance of damaged RC columns using glass fiber–reinforced polymer (GFRP), which can be easy to install and quick to repair. The proposed GFRP-strip device consists of a GFRP-composite strip and aluminum clip connectors, used to increase the shear strength of the damaged column. Three RC specimens were constructed with reference to existing RC structures. Two of the specimens were damaged before installing the GFRP-strip devices. After the pretest damage was done, the GFRP-strip devices were installed along the plastic-hinge regions on both ends of the test columns. A comparison of the experimental results indicates that the seismic performance of the damaged RC columns with the GFRP-strip device was improved, and that the shear-failure mode of the RC columns was changed to ductile-flexural-failure mode. According to these results, the maximum strength, accumulated-dissipated energy, and displacement ductility of the retrofitted specimens was increased. Therefore, the proposed GFRP-strip device can be used to improve the seismic performance of damaged RC columns.

This paper presents an analysis of the seismic performance of square RC bridge columns retrofitted with near-surface-mounted (NSM) basalt fiber-reinforced polymer (BFRP) bars and/or BFRP sheet confinement based on fiber element modeling. An axial stress versus loaded-end displacement model of NSM FRP bars, including both elastic elongation and bond-slip effects, is proposed to account for the significant slippage of FRP bars in the plastic hinge region of strengthened columns. The simplified FRP bar model is then converted into the equivalent stress–strain relationship for easy implementation in fiber-based analysis. Moment-curvature analysis is performed based on the proposed FRP bar model, and the analytical moment versus fixed-end rotation relationship of a strengthened column is calculated and assigned to the rotational spring in the fiber element model. The proposed method avoids the nested iterations in sectional analysis that require force equilibrium and deformation compatibility. Comparisons between the numerical simulations and experimental results indicate that the proposed method is appropriate for predicting pushover curves and the hysteretic response of strengthened columns. Furthermore, the mechanism of the hybrid effect of combining NSM FRP bars with externally bonded FRP sheets can be interpreted after the analytical study; that is, confining the column with FRP sheets improves the bond conditions for NSM FRP bars, while the bond-slip effect mitigates the premature fracturing of the FRP bars, leading to more effective utilization of the NSM reinforcement and a more ductile column behavior.

Bonding a composite patch to a cracked steel section is an efficient technique to reinforce cracked members or to delay fatigue crack growth in the structural elements. In this paper, a numerical study was performed to highlight the effect of bonded prestressed composite patches on the fracture parameters, such as crack tip opening displacement and the plastic zone. A three-dimensional finite element model of the double sided strengthened specimen is used to study the fracture behavior of an inclined edge crack under different combinations of modes I and II loading conditions. The influence of the pretension level, patch stiffness ratio, and fiber orientation on the crack tip opening displacement and plastic zone are also investigated. The introduction of a compressive stress by pretensioning of the composite patch prior to bonding produces a significant reduction in the crack tip opening displacement and plastic zone. This pretensioning reduces the crack tip driving force and subsequently reduces the fatigue crack growth rate, which increases the fatigue life of the structural member. Furthermore, the increase in the patch stiffness ratio reduces the amount of plastic zone and opening displacement at the crack tip. The effectiveness of the fiber orientation in the composite patch depends primarily on the loading directions and crack inclination angle.

In this study, 38 push-through tests were performed on a precast concrete insulated sandwich panel design using combined angled and horizontal connectors. Basalt fiber-reinforced polymer (BFRP) and steel connectors were tested and compared. The key parameters were various inclination angles and diameters of connectors; orientation of the diagonal connector relative to loading (i.e., tested in tension or compression); and panels with or without an active foam-to-concrete bond. Steel connectors failed by yielding in tension and inelastic buckling in compression. Larger-diameter BFRP connectors usually pulled out under tension and crushed in compression. Smaller-diameter BFRP connectors ruptured in tension and buckled in compression. Strength and stiffness increased with the connector angle and diameter. The insulation foam bond was found to contribute similarly regardless of connector material. An independent theoretical model accounting for material, bond, and stability failure modes, as well as geometric distortion attributable to relative slip, was developed. It provided an average test-to-predicted value of 1.15 with a standard deviation of 0.25 for the strength of the shear connection system.

Fiber-reinforced polymer (FRP) material is very interesting for its use as the sole material in the construction of structures because of its remarkable lightness, which reduces seismic forces; its ease of assembly; and its durability. All-FRP structures use bolted connections to form nodes between columns and beams, particularly steel bolts. This research focuses on the use of FRP pultruded plates instead of web and flange cleats for bolted beam-to-column joints. Currently, angles and bolts between columns and beams are widely employed, whereas bolted plates are still a novelty. Both experimental and numerical studies of two different configurations of bolted connections made by pultruded plates and C-shape FRP profiles demonstrated that the plates have considerable influence on the moment-rotation response of joints so that the joint’s failure mode involves mainly the bolted plate. The experimental program in this paper determined the initial stiffness of each connection, the ultimate moment capacity, and the rotation associated with the ultimate strength. These results were then compared against numerical predictions obtained using a commercial code for finite-element analysis.

Six full-scale reinforced-concrete (RC) interior slab-column connections of glass fiber–reinforced polymer (GFRP) were constructed and tested to failure. The specimen consisted of a square slab with a 2,800-mm side length and a 200-mm thickness in addition to a 300-mm-square column stub extended for 1,000 mm above and below the slab. The test specimens were subjected to vertical shear forces and unbalanced moments. The test variables included the moment-to-shear ratio, GFRP double-headed shear studs ratio, and the type of GFRP bar surface texture (ribbed or sand-coated). The test results revealed that increasing the moment-to-shear ratio reduced the vertical shear capacity and increased the deflections and the strains at failure. Moreover, the presence of the GFRP shear studs enhanced the slab capacity but was not able to change the punching shear mode of failure. Furthermore, the used two types of GFRP bars showed comparable behavior. The results were compared with the predictions of the available fiber-reinforced polymer (FRP) design provisions such as those from Canada, the United States, and Japan. Finally, a new method is proposed to predict the punching shear capacity for the slabs with FRP shear studs.

The mechanical performance of four types of interfaces between fiber-reinforced polymer (FRP) plate and concrete was compared by double-lap-shear test. The study found that the failure characteristics of the four interfaces are obviously distinct. Among them, the shear capacity load of the shear key combined with wet-bond (SK-WB) interface was the highest, followed by the dry-bond and the coarse-aggregate-coating (CAC) interfaces; the wet-bond interface had the lowest shear load bearing capacity. The type of the resin used for the wet-bond adhesive has a slight influence on the bond strength, whereas the type of the resin used for the shear key bonding has a much more significant effect on the shear capacity load of these interfaces. The thickness of the wet-bond epoxy resin layer also has a significant impact on the bond performance of the wet-bond interface. The shear key forms has less influence when the flexural stiffness of the shear keys itself is sufficient; the load capacity of the SK-WB interface is often dependent on the bonding area between the shear key and the concrete. The concept of bond zone was put out when creating interface model of the SK-WB interface. Finally, unified form formulas of the shear capacity and the interface fracture energy are provided for the different interfaces, and the values of them are estimated.

This paper presents test results of six full-scale glass fiber-reinforced polymer (GFRP) reinforced concrete slab–column edge connections. The test connections included two series with three connections each. The connections of Series 1 investigated the effect of a new type of GFRP shear reinforcement on the punching shear behavior of slab-column edge connections, while those of Series 2 investigated the effect of moment-to-shear ratio. For Series 1, one connection did not have shear reinforcement, while the other two connections had ribbed-deformed GFRP shear studs with headed ends at different radial spacing (120 and 80 mm). For Series 2, three different moment-to-shear ratios were applied to the connections (0.2, 0.4, and 0.6 m). The typical dimensions of the slabs were $\mathrm{2,800}\times \mathrm{1,550}\times 200\mathrm{mm}$ with a 300-mm square column extending above and below the slab. The headed ends of the studs provided adequate anchorage, which allowed the studs to control the propagation of shear cracks and to increase the deformability and capacity of the connections. On the other hand, increasing the moment-to-shear ratio significantly decreased the deformability and shear force capacity of the connections.

This paper presents a comprehensive two-dimensional (2D) nonlinear finite-element (FE) analysis of reinforced concrete (RC) beams strengthened with prestressed or non-prestressed near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) reinforcement. In this model, the bond-slip relationship between the steel reinforcement and surrounding concrete as well as that between the NSM CFRP reinforcement and concrete substrate are precisely modeled with cohesive interface elements. Based on the smeared crack approach, the tensile and shear behavior for concrete cracking are considered using appropriate models. The prestressing effect was enforced in the NSM CFRP reinforcement using the equivalent temperature method, and the anchoring effect of U-shaped steel end anchors on the NSM CFRP reinforcement is properly implemented in the model. Moreover, the radial stresses induced by the tensile steel bars on the surrounding concrete are properly considered, and the bar-end concrete cover separation failure mode is successfully captured. Comparisons between the numerical and experimental results confirm that the proposed FE method is appropriate for predicting the load-deflection curves, failure modes, and cracking patterns of the strengthened beams. Based on this study, the 2D FE model is utilized to conduct a parametric study and can be used to aid in realistic structural applications and optimization of the use of prestressed/non-prestressed NSM reinforcement for strengthening RC beams.

Glass-fiber-reinforced polymer (GFRP) bar has emerged as a preferable alternative to steel bar in reinforced concrete (RC) members in harsh, corrosive, coastal environments in order to eliminate corrosion problems. However, only limited experimental studies are available on the performance and behavior of concrete columns reinforced with GFRP bars under different loading conditions. This study investigates the use of GFRP bars and GFRP helices (spirals) as longitudinal and transversal reinforcement, respectively, in RC columns. A total of 12 circular concrete specimens with 205-mm diameter and 800-mm height were cast and tested under different loading conditions. The effect of replacing steel with GFRP reinforcement and changing the spacing of the GFRP helices on the behavior of the specimens was investigated. The experimental results show that the axial load and bending moment capacity of the GFRP-RC columns are smaller than those of the conventional steel-RC columns. However, the ductility of the GFRP-RC columns was very close to the ductility of the steel-RC columns. It is concluded that ignoring the contribution of the GFRP bars in compression leads to a considerable difference between analytical and experimental results.

This paper presents results of a series of single shear tests of carbon fiber–reinforced polymer (CFRP) bonded concrete prisms exposed to three environmental conditions, namely, temperature cycles, wet-dry cycles, and outdoor environment, in terms of shear stress-slip relationships and fracture energies. Temperature cycles and wet-dry cycles were chosen in a manner to study the sole effects of temperature cycles and wet-dry cycles. The maximum temperature of the temperature cycles was intentionally kept below the glass transition temperature of epoxy resin. In the wet-dry cycles, specimens were exposed to varying humidity while temperatures close to ambient were maintained. Also, outdoor environmental exposure was applied to address the lack of test data on natural aging of FRP-concrete bond system. All the environmental conditions were applied for extended durations (the maximum duration of 18 months). Single shear tests (pullout test) were conducted to investigate maximum normal stress developed in CFRP (pullout strengths), strain distribution along the bond line and failure modes of control (unexposed) and exposed specimens. Local shear stresses and slips at the debonding tip were obtained from the strain profiles and CFRP stiffness values for both unexposed (control) and exposed conditions. Subsequently, local shear stresses at the debonding tips were fitted to corresponding slips and fracture energies were determined from the numerical integration of the shear stress-slip curves. Fracture properties, namely, peak shear stresses and fracture energies, of exposed specimens were compared with the control specimens. In addition, the change of material properties of concrete and CFRP were investigated and the effect of the changing material properties on the fracture properties of bond are presented in this paper. Based on the results, the most significant degradation of fracture energy was observed for wet-dry cycles, whereas the outdoor environment caused only initial deterioration. The temperature cycles, however, did not cause any negative effect on the fracture energy during the one year exposure.

This study investigates the impact behavior of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) beams and FRP contribution to shear strength. Thirteen RC beams ($\mathrm{1,500}\times 150\times 250\mathrm{mm}$) were tested against static and impact loads. The beams were strengthened with FRP U-wraps and 45°-angle wraps to investigate the effectiveness of different wrapping schemes. The experimental results show that the debonding strain of FRP under impact loads is slightly smaller than that under static loads. The FRP contribution to shear strength is discussed and verified against a design code. In terms of using the same amount of FRP, 45°-angle FRP wraps provide better performance than the FRP U-wraps in terms of both the load capacity and displacement. Fully wrapping a RC beam with FRP is more efficient than wrapping distributed FRP strips. The impact behavior and failure modes of FRP strengthened RC beams are also discussed.

While the majority of U-shape wrapping (Uwrap) studies have focused on the mechanism of the shear-strengthening effect, very few studies have been devoted to the use of Uwraps as anchorage for longitudinal fiber-reinforced polymer (FRP) sheets or plates. This investigation therefore focuses on the anchor effect of the carbon-fiber-reinforced polymer (CFRP) Uwrap on externally bonded FRP-reinforced concrete elements. Based on selected parameters from the preliminary study, a parametric study was conducted on concrete blocks with an attached FRP sheet with and without Uwraps. Pull-out tests were conducted on these specimens. Slips along the length of the FRP sheet and debonding propagation in the region under the Uwrap were observed using a linear variable differential transformer (LVDT) and strain gauges. Among the tested specimens, the FRP Uwraps with a 45° angle without an edge distance maximizes the benefit of the anchoring effect. Two factors were found to control the Uwrap performance: the stress concentration at the corner region of the Uwrap, and the frictional behavior between the debonded surfaces of the concrete and the FRP sheet to determine the behavior of the Uwrap and its anchor capacity.

This work presents the results of an experimental investigation on the behavior of 42 solid clay brick masonry columns internally strengthened by fiber-reinforced polymer (FRP) fabrics or stainless steel grids placed in horizontal mortar joints. In some cases, every course, and in other cases, alternate courses were reinforced with steel grids, carbon, and basalt FRP fabrics. Monotonic compressive loading tests were carried out under concentric and eccentric loads. Eccentric tests were carried out by loading the specimens on a reduced area with respect to the entire cross section, producing a disturbed region (D-region). A detailed characterization of the constituent materials was made experimentally. The compression tests show the effectiveness of the proposed strengthening techniques to produce increases in strength and the energy required for collapse of brick columns under concentric loads. Under eccentric loads, the reduction in the load-carrying capacity owing to the reduced loaded area is attenuated by the presence of the transverse reinforcements. An analytical expression is proposed that gives the strength of clay brick masonry columns considering that the biaxial strength domain of the bricks is modified because of the tensile contribution provided by the presence of transverse reinforcements. Moreover, a strut-and-tie model was utilized to calculate the load-carrying capacity in the D-region and was verified against experimental data, with good agreement.