Journal of Composites for Construction

Increasing interest in the use of fiber-reinforced polymer (FRP) reinforcement for reinforced concrete structures has made it clear that insufficient information about the shear performance of such members is currently available to practicing engineers. This paper summarizes the results of 11 large shear tests of reinforced concrete beams with glass FRP (GFRP) longitudinal reinforcement and with or without GFRP stirrups. Test variables were the member depth, the member flexural reinforcement ratio, and the amount of shear reinforcement provided. Results showed that the equations of the Canadian CSA shear provisions provide conservative estimates of the shear strength of FRP-reinforced members. Recommendations are given along with a worked example on how to apply these provisions including to members with FRP stirrups. It was found that members with multiple layers of longitudinal bars appear to perform better than those with a single layer of longitudinal reinforcing bars. Overall, it was concluded that the fundamental shear behavior of FRP-reinforced beams is similar to that of steel-reinforced beams despite the brittle nature of the reinforcement.

This paper presents the flexure of concrete beams prestressed with aramid fiber-reinforced polymer (AFRP) tendons. Three-dimensional nonlinear finite-element analysis and iterative sectional analysis are conducted to predict the behavior of AFRP-prestressed members, including experimental validation. The beams are simply supported and monotonically loaded until failure occurs. The sectional properties of the beams include a reinforcement ratio of 0.15% to 0.36% and an $I_g/I_{\text{cr}}$ ratio of 25 to 77, where $I_g$ and $I_{\text{cr}}$ are the gross and cracked moment of inertia, respectively. Various prestressing levels are applied to the beams to evaluate the load versus displacement response, variation of neutral axis depth, effective moment of inertia, and deformability of the beams. The applicability of code provisions and existing predictive equations are examined. The prestress level in the AFRP tendons significantly influences the flexural behavior of the beams, namely, cracking load, strain development of AFRP, and neutral axis depth. Sectional properties such as the $I_g/I_{\text{cr}}$ ratio are an important parameter that affects the deflection characteristics of AFRP-prestressed beams, including a deformability index. Recommendations to improve the current design provisions are addressed.

The development/splice strength and the pullout local bond stress-slip response of glass fiber-reinforced polymer (GFRP) bars in tension were experimentally investigated using beam specimens and pullout specimens, respectively. Two types of 12-mm (0.47-in.)-diameter GFRP bars were evaluated, namely, thread wrapped and ribbed. The test parameters included the concrete cover, the splice length, and the area of steel confinement for the beam specimens, and the concrete compressive strength for the pullout specimens. Companion steel reinforced beams were also tested for comparison. All beam specimens reinforced with thread-wrapped GFRP bars experienced pullout mode of bond failure, while all specimens reinforced with ribbed GFRP bars or steel bars experienced splitting mode of bond failure. It was found that the bond strength of FRP bars is largely dependent on the surface conditions of the bars. The pullout local bond stress-slip response of ribbed GFRP bars is intrinsically similar to that of steel bars reported in the literature. The bond strength of both types of GFRP bars investigated was about two to three times lower than that of steel bars. Predictions of the development/splice strength of GFRP bars in accordance with the ACI Committee 440 guidelines were unconservative in comparison with the test data. Also, in contradiction with the current ACI 440 report, the use of transverse confining reinforcement increased the bond strength by a sizable 15–30%.

Continuous concrete beams are commonly used elements in structures such as parking garages and overpasses, which might be exposed to extreme weather conditions and the application of deicing salts. The use of the fiber-reinforced polymers (FRP) bars having no expansive corrosion product in these types of structures has become a viable alternative to steel bars to overcome the steel-corrosion problems. However, the ability of FRP materials to redistribute loads and moments in continuous beams is questionable due to the linear-elastic behavior of such materials up to failure. This paper presents the experimental results of four reinforced concrete beams with rectangular cross section of $200\times 300\text{mm}$ continuous over two spans of 2,800 mm each. The material and the amount of longitudinal reinforcement were the main investigated parameters in this study. Two beams were reinforced with glass FRP (GFRP) bars in to different configurations while one beam was reinforced with carbon FRP bars. A steel-reinforced continuous concrete beam was also tested to compare the results. The experimental results showed that moment redistribution in FRP-reinforced continuous concrete beams is possible if the reinforcement configuration is chosen properly. Increasing the GFRP reinforcement at the midspan section compared to middle support section had positive effects on reducing midspan deflections and improving load capacity. The test results were compared to the available design models and FRP codes. It was concluded that the Canadian Standards Association Code (CSA/S806-02) could reasonably predict the failure load of the tested beams; however, it fails to predict the failure location.

In this paper, efficiency and effectiveness of carbon fiber reinforced polymers (CFRPs) in upgrading the shear strength and ductility of seismically deficient corner or knee reinforced concrete beam-column joints have been studied. For this purpose, four as-built corner/knee joints were constructed with no transverse reinforcement, representing extreme case of preseismic code design construction practice of joints and encompassing many existing beam-column corner joints. Out of these four as-built specimens, two specimens were used as baseline specimens (control specimens) and other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epoxy bonded to joint, beams, and part of the column regions. In the second scheme, however, sheets were epoxy bonded to joint region only but they were effectively prevented against any possible debonding through mechanical anchorages. All these four subassemblages were subjected to cyclic lateral load histories to simulate loading due to earthquake and provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks through epoxy and externally bonding them with CFRP sheets under the same above two schemes. These repaired specimens were subjected to the similar cyclic lateral load history and their response histories were obtained. Response histories of control, repaired, and strengthened specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, column profiles, ductility, and stiffness degradation. The comparison shows that CFRP sheets are very effective in improving shear resistance and deformation capacity of the corner beam-column joints and delaying their stiffness degradation. Shear capacities of control, repaired, and strengthened specimens were also predicted using writers’ published formulation. The predicted shear capacities were in a good agreement with the experimental values.

The paper aims to contribute to a better understanding of the behavior of reinforced concrete columns confined with fiber-reinforced polymer (FRP) sheets. In particular, some new insights on interaction mechanisms between internal steel reinforcement and external FRP strengthening and their influence on efficiency of FRP confinement technique are given. In this context a procedure to generate the complete stress-strain response including new analytical proposals for (1) effective confinement pressure at failure; (2) peak stress; (3) ultimate stress; (4) ultimate axial strain; and (5) axial strain corresponding to peak stress for FRP confined elements with circular and rectangular cross sections, with and without internal steel reinforcement, is presented. Interaction mechanisms between internal steel reinforcement and external FRP strengthening, shown by some experimental results obtained at the University of Padova with accurate measurements, are taken into account in the analytical model. Four experimental databases regarding FRP confined concrete columns, with circular and rectangular cross section with and without steel reinforcement, are gathered for the assessment of some of the confinement models shown in literature and the new proposed model. The proposed model shows a good performance and analytical stress-strain curves approximate some available test results quite well.

In this paper the failure of a three layer system comprising a concrete column, an intermediate epoxy layer, and fiber-reinforced polymer (FRP) confinement is investigated. We perform a series of numerical experiments to investigate how the failure loads and ultimate strains of axially loaded plain cement concrete (PCC) and reinforced cement concrete (RCC) columns change with the type of the bond between concrete and epoxy and between epoxy and FRP. Three types of interfacial behavior are considered: rigid, cohesive compliant, and unbonded contact. An idealized spring model for the resultant confinement stiffness is used to explain the effect the nature of the bond has on the results. It is found that the type of bond has a significant effect on the ultimate strength of PCC columns. The results also indicate that the presence of longitudinal and hoop steel reinforcement allows use of comparatively less stiff FRP sheets as confinement material for RCC columns.

In this paper, a nonlinear finite-element model is developed for the analysis of plane stress members, such as RC beams and walls, strengthened either unidirectionally or bidirectionally with fiber-reinforced polymer (FRP) composites and subjected to either monotonic or cyclic loading. The model takes into account the effects of the bonded interface between the FRP and concrete while allowing slippage in each direction. A two-dimensional membrane contact element is developed to model the effects of local bond-slip with debonding failure between the FRP and concrete capable of being captured. The model has been incorporated into a finite-element program for the analysis of RC members subject to plane stress with verification against test data of FRP-strengthened RC joints, beams, and walls. The numerical results show good agreement with the experimental data for both load-displacement responses and for the overall failure mechanisms.

This paper presents the development of a numerical model for evaluating the performance of fiber-reinforced polymer (FRP)-strengthened RC beams under fire conditions. The model is based on a macroscopic finite-element approach and utilizes moment-curvature relationships to trace the response of insulated FRP-strengthened RC beams from linear elastic stage to collapse under any given fire exposure and loading scenarios. In the analysis, high temperature properties of constitutive materials, load and restraint conditions, material and geometric nonlinearity are accounted for, and a realistic failure criterion is applied to evaluate the failure of the beams. The model is validated against fire test data on FRP-strengthened RC beams and is applied to study the effect of FRP-strengthening, insulation scheme, and failure criterion on the fire response of FRP-strengthened RC beams. Results from the analysis indicate that FRP-strengthened RC beams should be protected with supplemental fire insulation to satisfy fire resistance requirements. A case study is presented to illustrate the application of the model for optimizing the fire insulation scheme to achieve required fire resistance in FRP-strengthened concrete beams.

In the last two decades, several seismic retrofitting techniques for masonry structures have been developed and practiced and fiber-reinforced polymer (FRP) material has been increasingly used owing to its high strength/stiffness to mass ratio and easy application. Although much research has been carried out on FRP strengthening of unreinforced masonry (URM) structures, most of it has been experimental studies to investigate the effectiveness of retrofitting techniques rather than the development of a rational design model. In addition, more research has been conducted on FRP-retrofitted URM walls under out-of-plane loads where flexural behavior dominates, the research on the shear strength of FRP-retrofitted URM walls has been limited. This paper presents a review of research in this area. Existing retrofitting techniques are overviewed, followed by a detailed discussion of experimental results of failure modes as they are directly related to the design model. The available design models are then assessed based on a test database collected from the available literature. Limitations of each model are addressed.

An experimental investigation was conducted to study the in-plane shear behavior of masonry panels strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer strips (CFRP). As part of the study four unreinforced masonry panels and seven strengthened panels were tested in diagonal tension/shear. Different reinforcement orientations were used including vertical, horizontal, and a combination of both. The effect of nonsymmetric reinforcement was also studied. The results of these tests are presented in this paper, and include the load-displacement behaviors, crack patterns, failure modes, and FRP strains. The results showed that the vertically aligned reinforcement was the most effective, with significant increases in strength and ductility observed. The dowel strength of the vertical reinforcement did not likely contribute significantly to the shear resistance of the masonry. Instead, it was likely that the vertical reinforcement acted in tension to restrain shear induced dilation and restrain sliding. In some panels cracking adjacent to the FRP strip, through the panel thickness was observed. This type of cracking reduced the bond between one side of the FRP strip and the masonry, and led to premature debonding. A comparison of the test results with the results of other tests from the literature is also presented in this paper.

A combined experimental and numerical program was conducted to study the in-plane shear behavior of clay brick masonry walls strengthened with near surface mounting carbon-fiber-reinforced polymer (CFRP) strips. This paper is focused on the numerical program. A two-dimensional finite-element (FE) model was used to simulate the behavior of FRP-strengthened wall tests. The masonry was modeled using the micromodeling approach. The FRP was attached to the masonry mesh using the shear bond-slip relationships determined from experimental pull tests. The model was designed in a way so that FRP crossing a sliding crack (perpendicularly) would prevent crack opening, normal to the direction of sliding (dilation), and increase sliding resistance. This sliding resisting mechanism was observed in the experimental tests. The FE model reproduced the key behaviors observed in the experiments, including the load-displacement response, crack development, and FRP reinforcement contribution. The FE model did not include masonry cracking adjacent to the FRP and through the wall thickness (as observed in some experiments). This type of cracking resulted in premature FRP debonding in the experiments. Debonding did not occur in the FE model because this type of masonry cracking was not modeled.

Impact tests using drop-weight pendulum on nine 1.2-m-high full-scale concrete masonry block walls were conducted to investigate the out-of-plane impact behavior of unreinforced masonry (URM) walls externally strengthened with carbon-fiber-reinforced polymer (CFRP) composites. Three strengthening schemes on one side of the wall were studied: continuous unidirectional and continuous woven sheets, discrete strips in a vertical pattern, and discrete strips in orthogonal and diagonal patterns. All walls were vertically positioned resting on a knife-edge support with one face leaning against two steel rollers close to the upper and lower edges of the wall. The impact load was applied at the wall center through a drop-weight pendulum impact tester with various drop heights. Test results revealed that using composite laminates or strips could significantly improve the impact performance of URM walls. The wall strengthened with continuous woven sheets performed better than the one with unidirectional sheet. With the same amount of fiber-reinforced polymer strip material, the wall with narrower but more closely spaced strips performed slightly better than the one with wider strips.

This paper presents the negative bending of reinforced concrete slabs strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. Six slab specimens, three of which are strengthened with CFRP strips, are tested in static and fatigue loads. A wheel-running fatigue test machine is used to simulate vehicular loads on a bridge deck. The effectiveness of CFRP strengthening for bridge decks in cantilever and pseudonegative bending is examined based on moment-carrying capacity and cyclic behavior under the wheel-running fatigue loads, including crack patterns and damage accumulation. The moment-carrying capacity (static) of the cantilever slab strengthened with the NSM CFRP strips is improved by 68.4% when compared to that of an unstrengthened slab. The damage accumulation rate of the strengthened cantilever slab owing to the fatigue load is significantly lower than that of the unstrengthened slab. The damage accumulation of the strengthened slab gradually increases and is irreversible when the fatigue cycles increase. The fatigue-induced flexural cracks of the slabs develop along the wheel-running direction. A simple predictive model is presented to estimate the fatigue life of the test slabs.

Composite fiber patching techniques have been considered as alternatives to traditional methods of strengthening and fatigue crack repair in steel structures. It is known that the fatigue strength of a cracked steel element depends on the stress intensity factor (SIF) at the crack tip which is a function of the stress/strain distribution of the plate. This paper presents an experimental study of tension tests of cracked steel plates repaired by single-side carbon fiber-reinforced polymer (CFRP) patching in investigating the strain distribution in the vicinity of the cracked region. The test parameters included patch length, patch width, tapered end, and axial stiffness ratio of adherend. It is shown from the test results that the single-side CFRP patching applied onto the cracked steel plate decreased the crack tip strains significantly in the patched face and increased the strains in the unpatched face. Finite-element analyses of the specimens using both the three layers model proposed by previous researchers and a modified three layers model proposed in this study were conducted to examine the strain distributions in the vicinity of the crack. In general, the strain distributions of the specimens were predicted well by the finite-element analyses using either model. The finite-element results showed that the SIF at the crack tip through the plate thickness was significantly reduced except on the unpatched side and the modified three layers model was able to capture the nonlinear SIF variation through the thickness of a cracked steel plate with single-side patching. Meanwhile, the three layers model overestimated the SIF on the patched side and underestimated the SIF on the unpatched side by about 10% on average compared to those of the modified three layers model. Based on the finite-element analysis results of the modified three layers model, it is shown that the width and the length of patching had only a marginal effect on the SIF. On the other hand, the effect of the number of layers of patching on the reduction of SIF was more pronounced.

Structural repair and strengthening have long been dynamic and challenging activities in construction work. One of the most commonly used methods for such repairs is the application of fiber-reinforced polymer (FRP) sheets to strengthen RC or even steel structure members. A major issue of concern in flexural strengthening of RC beams with FRP laminates is the debonding of the concrete substrate, which leads to premature failure of the structural member thus strengthened. One reason for this premature rupture may be the lack of proper preparation of the concrete surface in contact with the FRP sheet. Surface preparation is typically associated with such constraints as adverse environmental impacts, economic losses due to stoppage of activities, repair costs, or even inaccessibility of the member(s) to be strengthened. This study aims to investigate surface preparation for application of FRP sheets in an attempt to develop substitute methods for conventional surface preparation methods. The experimental specimens used for the purposes of this study included a minimum of 100 prism specimens of dimensions $100\times 100\times 500\text{mm}$ subjected to four-point flexural loading. The specimens were divided into the two control and experimental groups. The control group lacked FRP sheets, while the experimental one had FRP sheets tested for their ultimate failure strength as a result of both surface preparation and transverse, longitudinal, and diagonal grooves as substitutes for surface preparation. The results indicated that surface preparation prior to bonding of FRP sheets increased ultimate rupture strength. It was also found that the substitute preparation methods greatly compensated for the lack of conventional surface preparation such that they changed, in some cases, the ultimate failure behavior of the member.

Numerical analysis using a finite-element model was performed to simulate and investigate the long-term behavior of two RC beams with similar steel reinforcement, cast from the same batch of concrete. One beam was a plain RC beam and the other beam was strengthened using carbon fiber-reinforced polymer (FRP) strips. The deflections of both beams have been monitored for 5 years after loading. The finite-element model included both creep of concrete and viscoelasticity of the epoxy adhesive at the concrete-carbon FRP (CFRP) interface. The results of the finite-element analysis are compared to experimental observations of the two beams. The finite-element analysis was found to be able to simulate the long-term behavior of the CFRP-strengthened beam and help us understand the complex changes in the stress state that occur over time.

The assessment of durability of fiber-reinforced polymers (FRPs), used to strengthen masonry or concrete structures, if subjected to weathering is a knotty problem. Environmental factors can have a significant effect on their performance in service. In order to investigate on this aspect, the mechanical behavior of two commercial composites, reinforced with unidirectional carbon and glass fibers, was analyzed after a long-term immersion in distilled water. For comparison purposes, three different commercial epoxy resins, used as primer, putty, or adhesive to manufacture and apply the composites through the wet lay-up technique to the surfaces to strengthen, were subjected to the same treatment. In order to take into account the peculiarities of the three cold-curing epoxy resins, a novel procedure to dry the specimens before the immersion treatment was used. The mechanical tests, performed on the composites before and after their immersion in water, evidenced that this agent has a limited effect only on the in-plane tensile strength of wet lay-up manufactured glass FRP, while the mechanical properties of carbon FRP are substantially unaffected by water. On the other hand, the effect of water on the thermal and mechanical characteristics of the three epoxy resins is quite severe, with significant reductions in the glass transition temperature, stiffness, and strength.

The effects of moisture on the initial and long-term bonding behavior of fiber reinforced polymer (FRP) sheets to concrete interfaces have been investigated by means of a two-year experimental exposure program. The research is focused on the effects of (1) moisture at the time of FRP installation, in this paper termed “construction moisture,” consisting of concrete substratum surface moisture and external air moisture; and (2) moisture, in this paper termed “service moisture,” which normally varies throughout the service life of concrete. Concrete beams with FRP bonded to their soffits were prepared. Before bonding, concrete substrates were preconditioned with different moisture contents and treated with different primers. The FRP bonded concrete beams were then cured under different humidity conditions before being subjected to combined wet/dry (WD) and thermal cycling regimes to accelerate the exposure effects. Adhesives with different elastic moduli were used to investigate the long-term durability of each adhesive when subjected to accelerated WD cycling. Pull-off tests and bending tests were conducted at the beginning of the cycling and then again after 8 months, 14 months, and 2 years of exposure so as to evaluate the tensile and shear performance of the FRP-to-concrete interfaces. It was found that the effect of the concrete substrate moisture content on short-term interfacial bond performance could be eliminated if an appropriate primer was used. All FRP-to-concrete bonded joints failed at the interface between the primer and concrete after exposure while those not exposed usually failed within the concrete substrate. After exposure to an environment of accelerated WD cycles, it was also found that the interfacial tensile bond strength degraded asymptotically with the exposure time while the flexural capacity of the FRP sheet bonded plain concrete beams even increased. The mechanism behind the above, which is an apparently contradictory phenomenon, is discussed.

In this study, numerical models of fiber-reinforced polymer (FRP)-strengthened beams were developed using nonlinear fracture mechanics for the modeling of the concrete-FRP (longitudinal and U-wrap) interfaces. Mode 1, Mode 2, and mixed-mode interfacial behaviors were considered. Results from the finite-element models were compared with experimental tests of large-scale strengthened beams using FRP U-wraps as anchors. The numerical program assessed the effect of the interfacial modeling in the global and local responses. A parametric study was conducted to determine the effect of additional longitudinal FRP sheets in strengthened beams with and without FRP U-wraps. Results from this study indicate that the use of a mixed-mode concrete-FRP interface is a robust numerical approach for the prediction of the global and local responses of large-scale FRP-strengthened beams. The parametric study shows that the use of FRP U-wraps could improve the strength and ductility of the FRP-strengthened beams by changing their failure mode and deflection response. Appropriate modeling of the concrete-FRP interfaces is needed to successfully predict these effects.

The design and construction of bridge systems with long-term durability and low maintenance requirements is a significant challenge for bridge engineers. One possible solution to this challenge could be through the use of new materials, e.g., fiber-reinforced polymer (FRP) composites, with traditional materials that are arranged as an innovative hybrid structural system where the FRP serves as a load-carrying constituent and a protective cover for the concrete. This paper presents the results of an experimental investigation designed to evaluate the performance of a 3/4 scale hybrid FRP-concrete (HFRPC) bridge deck and composite connection under sustained and repeated (fatigue) loading. In addition, following the sustained-load and fatigue portions of the experimental study, destructive testing was performed to determine the first strength-based limit state of the hybrid deck. Results from the sustained-load and fatigue testing suggest that the HFRPC deck system might be a viable alternative to traditional cast-in-place reinforced concrete decks showing no global creep behavior and no degradation in stiffness or composite action between the deck and steel girders after 2 million cycles of dynamic loading with a peak load of 1.26 times the scaled tandem load (TL). Furthermore, the ultimate strength test showed that the deck failed prior to the global superstructure at a load approximately six times the scaled TL.

This study was undertaken to address the effect of the main steel corrosion on the structural performance of RC slabs strengthened with carbon-fiber-reinforced polymer (CFRP) strips and exposed to a corrosive environment. A total of eight specimens $\left(500\times 100\times 1,500\text{mm}\right)$ were constructed and tested under monotonic static loading. Three specimens were CFRP-strengthened and corroded, three specimens were CFRP-strengthened and kept at room temperature, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Three different strengthening schemes were applied: (1) externally bonded CFRP strips; (2) externally bonded CFRP strips provided with CFRP anchors; and (3) near surface mounted (NSM) CFRP strips. During the corrosion process, the specimens were placed in a small tank filled with sodium chloride (NaCl) solution concentration (3%) which covered only the slabs’ bottom third, and corrosion was induced by means of an impressed current. The corrosion process lasted for 20 days, and the average mass loss of the main steel reinforcement due to corrosion was 9%. Following corrosion, the specimens were tested under four-point bending. The experimental results showed that the increase in flexural capacity achieved using the three strengthening schemes were significantly reduced due to corrosion of the main steel. The recorded reductions in flexural strength gains for the CFRP-strengthened corroded slabs relative to the gains for the strengthened uncorroded slabs were about 55, 38, and 41% for the externally bonded CFRP system without anchors, externally bonded CFRP with anchors, and NSM-CFRP system, respectively.