Journal of Composites for Construction

This paper discusses the potential ingress and effects of simulated concrete pore solution species on glass fiber-reinforced polymer (GFRP) rebars. Knowing that diffusion of moisture into the fiber-matrix interphase in a composite could cause fiber-matrix debonding and that the presence of alkalis at the location of the glass surface would lead to fiber degradation, particular attention was devoted to investigating whether GFRP rebars allow both species to penetrate or allow only water, while blocking the alkalis. To investigate this scenario, GFRP rebars were immersed in five types of simulated concrete pore solutions at elevated temperatures. Penetration of alkalis was assessed using X-ray mapping of backscattered electron images (BEI) and crosschecked by line and point energy dispersive spectroscopy (EDS) techniques. The effects of diffusion at the fiber-matrix interphase were observed by investigating the degradation of constituents at the interphase. Degradation of polymer matrix was assessed by a Fourier transform infrared (FTIR) spectroscopy. Degradation of fiber was investigated by observing highly magnified scanning electron microscopy (SEM) images and point EDS analyses close to the interphase. Debonding of fibers from the matrix was investigated using a SEM technique. Fiber-matrix debonding took place in a few samples, despite the fact that the glass fibers and polymer matrix remained essentially intact and that no penetration of alkalis into the GFRP rebars was observed. This debonding, which occurred only for samples exposed to 75°C, is believed to be caused by hydrolysis of fiber sizing at high temperatures. Finally, this study shows that the vinyl ester (VE) polymer matrix used acts as an effective semipermeable membrane by allowing the penetration of water, while blocking alkali ions.

This paper investigates the behavior of concrete beams prestressed with carbon fiber-reinforced polymer (CFRP) rods under high-cycle fatigue at low temperature. Seven precast T-beams were tested, including five beams prestressed to various levels with the CFRP rods and two beams with conventional steel strands. All beams had a history of sustained loading. Some beams were directly loaded monotonically to failure as control specimens. Other beams were subjected to three million cycles of flexural loading, either at room temperature or at $-28°\mathrm{C}$, prior to being monotonically loaded to failure at the same temperature. All CFRP-prestressed beams survived the three million cycles, whereas the steel-prestressed beam did not. It was shown, however, that the bond between CFRP rods and concrete could be weakened because of cyclic loading, low temperature during loading, or high prestress level. This resulted in a premature bond failure at 70 to 90% of the full flexural strength in subsequent monotonic loading. Also, stiffness and camber gradually decreased during cyclic loading.

Deflection of reinforced concrete is typically computed with an effective moment of inertia $I_e$ that accounts for nonlinear behavior after the concrete cracks. Existing expressions for $I_e$ tend to overpredict the member stiffness of concrete reinforced with fiber-reinforced polymer (FRP) bars, and an alternative expression is used as the basis for developing a practical design approach to compute deflection. The proposed expression has a rational basis that incorporates basic concepts of tension stiffening to provide a reasonable estimate of deflection for both steel and FRP-reinforced concrete without the need for empirically derived correction factors. Calculation of deflection with the proposed expression for $I_e$ is recommended using the code value for the elastic modulus $E_c$ of concrete because computed values of deflection are relatively insensitive to variations in $E_c$, and shrinkage restraint is taken into account by using a reduced cracking moment less than the code-based value of the cracking moment $M_{cr}$. $I_e$ is conservatively based on the moment at the critical section (where the member stiffness is lowest), unless more accuracy is required with an integration-based expression that gives an equivalent moment of inertia $I_e^{\prime }$ to account for the variation in stiffness along the member length. Recommendations are validated by comparison with a database of deflection test results for FRP-reinforced concrete.

This study is concerned with the determination of the effects of shear span-to-depth ratio ($a/d$) and beam depth, or size, on the concrete contribution to the shear resistance of beams longitudinally reinforced with carbon fiber-reinforced polymer (CFRP) bars. One of the distinguishing features of the study is the unsymmetrical nature of the applied load, which creates two distinct $a/d$ ratios in the same beam and allows the effect of the $a/d$ ratio on shear strength to be clearly seen. Six simply supported large size CFRP reinforced concrete beams without shear reinforcement were tested, each under a single concentrated load. The test variables were the $a/d$ ratio, varying from 1.0–11.5, and the beam depth varying from 200–500 mm. All the beams failed in shear, but the failure load and location for some of these beams could not be predicted by the shear design recommendations of American Concrete Institute (ACI) Committee 440. The reason is that these recommendations do not account for the effects of $a/d$ and beam size on shear strength. Suggestions are made for the inclusion of these parameters in the shear design equations.

Several independent studies have confirmed that fiber-reinforced polymers (FRP) used for repairing corrosion-damaged concrete structures slow down the corrosion rate. This suggests that in this application, FRP serves as a barrier to the ingress of moisture and oxygen that are critically important for sustaining electrochemical corrosion of steel in concrete. Because oxygen molecules are smaller than both water and chloride molecules, they diffuse faster. Therefore, their permeation through FRP is more critical. This paper presents results from an experimental study that determined the oxygen permeability of FRP laminates. Four different commercially available carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) systems were investigated, and four different fiber orientations were evaluated for one-layer and two-layer configurations. The results showed that the oxygen permeability of FRP was somewhat poorer than the epoxy used for its fabrication. Single-layer FRP laminates were less permeable than two-layer laminates, a finding that had previously been reported but considered anomalous. Scanning electron micrographs indicated that this could be attributed to voids between the layers. The nonzero oxygen permeability of FRP explains why it can slow down but cannot completely stop chloride-induced corrosion of concrete.

In this paper, the efficiency and effectiveness of carbon-fiber-reinforced polymer (CFRP) sheets for upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint were studied and compared with an American Concrete Institute (ACI)-based design joint specimen. One as-built joint specimen, representing the preseismic code design and construction practice for joints and one ACI-based design joint specimen, satisfying the seismic design requirements of the current code of practice were cast. The as-built specimen was used as baseline (control) specimen. These two specimens (i.e., the as-built control and the ACI-based specimens) were subjected to cyclic lateral load histories to induce damage equivalent to damage expected from a severe earthquake. The damaged control specimen was then repaired by filling its cracks with epoxy and externally bonding CFRP sheets to the joint, the beam, and part of the column regions. This specimen was identified as the repaired specimen. The repaired specimen was subjected to a similar cyclic lateral load history, and its response history was recorded. The response histories of the as-built control, the repaired, and the ACI-based design specimen were then compared. The test results demonstrated that externally bonded CFRP sheets can effectively improve both the shear strength and the deformation capacity of seismically deficient and damaged beam-column joints to a state comparable to the ACI-based design joint.

Most previous studies on concrete short columns confined with fiber-reinforced polymer (FRP) composites were based on small-scale testing, and size effect of the columns still has not been studied thoroughly. In this study, 99 confined concrete short columns wrapped with aramid FRP (AFRP) jackets and 36 unconfined concrete short columns with circular and square cross sections were tested under axial compressive loading. The circular specimens were divided into six groups, and the square specimens were divided into five groups, with each group containing different levels of the AFRP’s confinement. In each group, the specimens were geometrically similar to one another and had three different scaling dimensions. Statistical analyses were used to evaluate the size and interaction effects between the specimen size and the AFRP’s confinement, and a size-dependent model for predicting the strength of the columns was developed by modifying Bazănt’s size-effect law. The experimental results showed that the size of a specimen had a significant effect on the strength of AFRP-confined concrete short columns, lesser effect on the axial stress-strain curves, and slight effect on the failure modes. The modified Bazănt model was in good agreement with the experimental data.

Reinforced concrete (RC) hollow piers in bridges withstand high moment and shear demands ensured with reduced mass and lower stress on foundations compared with solid piers. Failure of hollow columns is typically affected by premature buckling of reinforcing bars and concrete cover spalling. At present, no guidelines are available for the design of their upgrade, and few research investigations can be found on hollow columns strengthened by using fiber-reinforced polymer (FRP) materials. This paper discusses an experimental program carried out on purely compressed RC hollow columns externally wrapped with glass-fiber-reinforced polymer (GFRP). Three specimens were tested: one specimen was unstrengthened and used as the benchmark; the other two specimens were GFRP-wrapped with different confining reinforcement ratios. Each specimen was designed according to dated codes (i.e., prior to 1970) accounting only for gravity loads. In particular, steel longitudinal bars cross section and steel tie-spacing were designed with the minimum amount of longitudinal reinforcement and minimum tie area at maximum spacing. Tests results highlight that the GFRP-jacket mainly provided ductility increases before low strength increments could be obtained. Refined and simplified numerical models for hollow square RC columns, previously proposed by the authors, herein extend to hollow rectangular members. Comparisons of experimental results and theoretical predictions on the basis of both refined and simplified confinement models were performed and showed good agreement. In the case of the simplified model, a value for the effective ultimate FRP strain was suggested.

A detailed investigation on the fatigue performance of concrete beams strengthened with glass-fiber composite (GFC) is performed in this study. Cyclic load tests were conducted on reinforced concrete beam specimens strengthened with two layers of GFC bonded to the beams’ bottom surface using a special epoxy resin. Midspan-deflection and cracks were measured at different numbers of load cycles and varying fatigue loading levels during the tests. Investigated parameters include total midspan-deflection, residual midspan-deflection after unloading, crack width, crack length, and crack distribution at different loading stages. The fatigue performance of concrete beams strengthened with GFC was evaluated by comparing the deflections, crack sizes, and crack distributions with unstrengthened beams. The concrete beams strengthened with GFC investigated in this study showed significant improvement on fatigue performance.

The opportunities provided by the use of modern repair schemes for the seismic retrofit of existing RC structures were assessed on a comparative experimental study of carbon fiber-reinforced polymer (CFRP) and more-conventional seismic retrofitting techniques for the repair of reinforced concrete members and masonry walls of bare and infilled RC frames, respectively, damaged because of cyclic loading. Four 1-story, one-bay, one-third-scale frame specimens are tested under cyclic horizontal loading up to a drift level of 4%—two bare frames with spirals or stirrups as shear reinforcement, respectively, and two infilled frames with weak infills and spirals or stirrups as shear reinforcement, respectively. The applied repair techniques are mainly based on the use of thin epoxy resin infused under pressure into the crack system of the damaged RC joint bodies or on the additional use of CFRP plates to the surfaces of the damaged structural RC members as external reinforcement and the use of a polymer modified cement mortar or two-sided diagonal CFRP fabrics for the damaged infill masonry walls. After repair, specimens were retested in the same way. Conclusions concerning the comparison of the effectiveness between conventional and CFRP seismic retrofitting applied techniques on the basis of maximum cycles load, loading stiffness, and hysteretic energy absorption capabilities of the tested specimens are drawn.

A full-scale unreinforced masonry (URM) wall with an opening was tested under in-plane lateral loading. The wall was first subjected to monotonically increasing displacements until a moderate damage level was reached. The damaged specimen was then cyclically tested up to almost the same maximum drift attained during the monotonic test to investigate the effects of previous damage on its nonlinear response. Finally, the masonry wall was repaired with inorganic matrix-grid (IMG) composites and subjected to a cyclic displacement-controlled test up to a near-collapse state. Most of the observed damage developed in the spandrel panel affecting both lateral resistance and strength degradation. Rocking of piers governed lateral stiffness and hysteretic response, which was characterized by low residual displacements and recentering behavior. The comparison between the experimental force-displacement curves demonstrated that the IMG strengthening system was able to provide energy dissipation capacity to the spandrel panel, restoring load-bearing capacity of the as-built wall, and delaying strength degradation that was indeed observed at larger displacements. Bilinear idealizations of force-displacement curves allowed the identification of displacement ductility, global overstrength, and strength reduction factor of the tested wall systems.

A fiber-reinforced polymer (FRP) shear bolt system has been recently developed at the University of Waterloo in Canada. The system is used to protect previously built reinforced concrete (RC) slabs against brittle punching shear failure. The system requires drilling small holes in a RC slab around the perimeter of a column, inserting bolts into the holes, and anchoring the bolts at both external surfaces of the slab. Many existing RC slabs have been built without any shear reinforcement. Also, many of these slabs are in corrosive environments, e.g., parking garages, where the use of deicing salts accelerates reinforcement corrosion and concrete deterioration. Therefore, FRPs are ideal materials to be used for such retrofit. The challenge, however, is the development of mechanical end anchorages for FRP rods that are efficient, aesthetic, cost effective, and that can be installed on site. The research presented in this paper includes development of FRP bolts with mechanical anchorages and the results of testing done using the developed systems. A new anchorage technique for the FRP rods based on crimping the rod ends with the aluminum fittings was developed. The testing was done on isolated slab-column specimens representing interior slab-column connections in a continuous flat plate system. The specimens were subjected to simulated gravity loading. The developed FRP bolts worked very well in improving the performance of the slab-column connections and showing the benefits of using FRP in punching shear retrofit of reinforced concrete slabs in corrosive environments.

This paper aims to examine the effectiveness of near-surface-mounted (NSM) glass fiber-reinforced polymer (GFRP) composite rebars in combination with external confinement with carbon fiber-reinforced polymer (CFRP) composite sheets to repair and strengthen reinforced concrete (RC) columns exposed to axial load and biaxial bending. Nine columns with a square cross section of $150\times 150\mathrm{mm}$ were constructed and tested under biaxial eccentric loading with equal eccentricity along each principal axis. Test parameters included load eccentricity, concrete grade, and level of the CFRP confinement used in combination with the NSM-GFRP reinforcement. The effectiveness of the NSM-GFRP reinforcement was greatly affected by the CFRP-confinement level and the load eccentricity. For columns with a high level of CFRP confinement, the gain in the load capacity attributable to the NSM-GFRP reinforcement was higher at a lower eccentricity. For columns with a low level of CFRP confinement, the gain in the load capacity attributable to the NSM-GFRP reinforcement was higher at a higher eccentricity. The enhancement in the load capacity was more pronounced in the columns with a lower concrete grade. An analytical model for predicting the load capacity of RC columns strengthened with NSM-GFRP rebars in combination with CFRP confinement under axial load and biaxial bending is introduced. The model accounts for the nonlinear behavior of materials and the change in geometry under biaxial eccentric loading. The model accuracy is demonstrated by comparing the model predictions with the experimental results.

The near-surface-mounted (NSM) method has proved to be a reliable alternative to the existing externally bonded (EB) method for the repair and strengthening of concrete structures using fiber-reinforced polymer (FRP) composites. This technique is especially advantageous in bridge deck upgrades for larger barriers and slab overhang strengthening. This paper presents the results of a comparison of the flexural behavior of bridge slab overhangs strengthened in negative bending moment regions with various types of NSM reinforcement that differ in surface condition (e.g., textured and sand coated), cross-sectional shape (e.g., round and square), material type (carbon and glass), and prestressing effect. Eleven full-scale overhang specimens (1.524 m long in overhang and 0.914 m wide) were tested under a cantilever condition. Test results showed that the FRP NSM reinforcements were effective in increasing both yield and ultimate strength of predamaged slab overhangs. All surface treatments were more beneficial than the smooth condition, and the square-shaped reinforcement displayed better performance than the round shape. The prestressing unit developed in this study is simple to apply and could be further explored for field applications.

A commonly observed failure mode in laboratory tests involving surface bonded fiber-reinforced polymer (FRP) laminates or near-surface-mounted (NSM) bars is premature delamination, that is, the separation of the FRP from the substrate well before the FRP reaches its ultimate strain capacity. To delay the onset of delamination and to ensure that the NSM FRP reinforcement continues to contribute to member strength after partial delamination, a new self-anchored carbon fiber-reinforced polymer (CFRP) bar was developed and tested for this investigation. This bar is made with a series of monolithic spikes that can be anchored deep inside the concrete. In addition to cutting grooves into the concrete cover for the placement of the primary reinforcing bar, holes are drilled deep into the concrete to insert the spikes. To test the performance of this bar, six large, simply supported, reinforced, concrete beams were retrofitted with NSM bars and tested in four-point bending. Two beams were strengthened with NSM bars without anchors or spikes but were otherwise similar to the self-anchored bar and served as control specimens (Series B1). Two beams were strengthened in flexure with the new self-anchored NSM bars (Series B2), and the remaining two beams (Series B3) were strengthened in flexure and shear by using the self-anchored NSM bars as partial shear reinforcement. The effect of the proposed strengthening system on the beams’ strength, failure mode, deformability, and ductility are discussed on the basis of the experimental results. The anchors delayed delamination and enabled the NSM bar to experience at least a 77% higher strain at failure than the companion bar without anchors. The anchors also increased beam displacement ductility and energy ductility at a 20% strength degradation by at least 34% and 42%, respectively.

Externally bonded fiber-reinforced polymer (FRP) has been successfully implemented to repair existing RC structures. Previous studies on the development of a reliability-based design method for externally bonded FRP have considered the additional uncertainties introduced by the FRP, but have relied on descriptions of the existing concrete and reinforcing steel representative of new construction. These descriptions are unlikely to be appropriate for an in-service structure in need of repair. This paper investigates the effects of different levels of uncertainty in existing RC structures and FRP materials on the reliability of repairs designed with ACI 440.2R-08 recommendations. Repairs to restore flexural strength for each beam in an inventory of rectangular and $t$-shaped beams were designed. The reliability index of these repair designs was evaluated to identify the most influential variables on the reliability of the FRP repairs. When the effects of the existing structure were compared with those of the FRP material, uncertainties related to the existing structure had a much greater impact on the reliability of repair designs. The most significant uncertainties were related to the area of the remaining reinforcing steel and the existing concrete strength.

A significant proportion of damage to buildings in hurricanes occurs owing to weak roof-to-wall connections. The objective of this study was to develop an innovative, efficient, and nonintrusive roof-to-wall connection for wood-frame structures by using high-performance fiber reinforced polymer (FRP) composites. Several connection configurations were developed and tested at the component level under uplift loading. The most feasible configuration was selected and further tested at the component level under lateral loadings. The selected FRP tie system was then tested at a full-scale model designed to represent conventional wood-frame buildings. The objective was to assess the connection’s in situ performance under simulated uplift forces. The results of the full-scale tests were in close agreement with those obtained from the component-level tests. Control tests were also performed to evaluate the load capacity of a typical commercial metallic hurricane clip to facilitate comparison of its results with that of the newly developed FRP tie. The FRP tie system described in this study offers an easy-to-apply, nonintrusive, and viable alternative to existing metal hurricane clips for both new construction and existing structures.

This study explores a new moment connection of concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) to concrete footing. The tube is tightly fitted and adhesively bonded to a short reinforced concrete stub protruding from the footing, which facilitates concrete filling of the tube without the need for shoring. To establish the critical stub length ($X_{cr}$), specimens with heavily steel-reinforced stubs varying in length from $0.5D$ to $2.0D$, where $D$ = diameter of the CFFT, were fabricated and tested in flexure by using a cantilever setup. The $X_{cr}$ required to achieve flexural failure of the CFFT was $1.05D$. Additional specimens with a sufficient stub length of $1.5D$ were then fabricated to examine the effect on strength and ductility of the steel reinforcement ratio (${\rho }_s$) in the stub. The optimal ${\rho }_s$ of the stub required for the CFFT to reach flexural failure was 3.2%. Finally, the effect of low-cycle reversed bending fatigue was studied with and without an axial compression load. Remarkable ductility associated with the formation of a plastic hinge was observed at ${\rho }_s$ of 2%. An analytical model capable of predicting (1) moment capacity of the connection, (2) whether failure is governed by flexure or bond, and (3) $X_{cr}$ was developed and validated. It was then used in a parametric study to explore the effects of CFFT mechanical and geometric properties on $X_{cr}$.