Journal of Composites for Construction

Several codes and design guidelines are now available for the design of concrete structures reinforced with fiber-reinforced polymer (FRP) bars under flexural and shear loads. Yet, because of a lack of research, North American codes and design guidelines do not recommend using FRP bars as longitudinal reinforcement in columns to resist compressive stresses. This paper reports on 12 full-scale circular reinforced concrete (RC) columns that were tested under concentric axial loads. The columns were reinforced with longitudinal glass FRP (GFRP) bars and newly developed GFRP spirals. The 300-mm diameter columns were designed according to code requirements. The test parameters included reinforcement type (GFRP versus steel); longitudinal FRP reinforcement ratio; and the volumetric ratios, diameters, and spacing of spiral reinforcement. The test results indicated that the GFRP and steel RC columns behaved in a similar manner. The average load carried by the longitudinal GFRP bars ranged between 5% and 10% of the maximum load. The ductility and confinement efficiency can be better improved by using small GFRP spirals with closer spacing rather than larger diameters with greater spacing. Ignoring the contribution of GFRP bars in the design equation underestimated the maximum capacity of the tested specimens.

This paper investigates the shear behavior of continuous concrete beams reinforced with glass fiber–reinforced polymer (GFRP) bars and stirrups. A total of six beams, with $200\times 300\mathrm{mm}$ rectangular cross section and continuous over two spans of 2,800 mm each, were constructed and tested. Three main variables, namely, concrete strength, longitudinal reinforcement ratio, and transverse shear-reinforcement ratio, were investigated. Two beams were without transverse reinforcement to evaluate the concrete contribution to the shear strength and four beams had GFRP stirrups to examine the minimum shear-reinforcement contribution to the shear strength. All test specimens were designed to satisfy an assumed 20% moment redistribution. It was observed that all test specimens failed in shear near the interior support after significant moment redistribution from hogging moment to sagging moment regions. In addition, the test results were compared with the predictions of the CSA-S806-12, CSA-S6-06, and ACI 440.1R-06.

The paper uses the membrane hypothesis to formulate the confining behavior of fiber-reinforced polymer (FRP) confined rectangular columns. A model was developed to calculate the strength of FRP confined rectangular concrete columns. The model was verified using a database of 190 FRP confined rectangular concrete columns. The database covers unconfined concrete strength between 18.3 and 55.2 MPa, and specimens with dimensions ranging from 79–305 mm and 100–305 mm for short and long sides, respectively. The performance of the proposed model shows a very good correlation with the experimental results. In addition, the strain distribution of FRP around the circumference of the rectangular sections was examined to propose an equation for predicting the actual rupture strain of FRP. The minimum corner radius of the sections is also recommended to achieve sufficient confinement.

This paper aims to investigate the structural response of concrete corbels reinforced internally with steel rebars and externally with carbon fiber-reinforced polymer (CFRP) composite sheets. Nine specimens were constructed and tested. Test parameters included the amount of internal longitudinal steel rebars and configuration of the external composite sheets. Nine two-dimensional finite-element (FE) models were developed, assuming a perfect bond between the CFRP and concrete. Three additional FE models were developed in which an interfacial bond stress-slip model was adopted between the diagonal CFRP reinforcement and the concrete. The external CFRP composite reinforcement resulted in up to 40% increase in the load capacity. The contribution of the external CFRP reinforcement to the load capacity decreased with an increased amount of internal steel rebars. The addition of primary longitudinal CFRP sheets in a direction parallel to the primary steel rebars reduced the steel strains and increased the yield and ultimate loads. The inclusion of secondary CFRP longitudinal reinforcement at the midheight of the corbels did not result in additional strength gain. The diagonal CFRP reinforcement restricted growth and widening of the shear cracks, and hence, increased the gain in the load capacity. The numerical load capacity was in the range of 10% error band. The numerical crack pattern, deflection response, and steel and CFRP strain responses were in good agreement with those measured experimentally. The integration of the interfacial bond stress-slip model in the FE analysis between the diagonal CFRP reinforcement and concrete resulted in more conservative/accurate predictions for the structural response.

The present investigation deals with carbon-fiber-reinforced polymer (CFRP) rebar manufacturing, experimental flexural and shear response of near-surface-mounted (NSM) CFRP-strengthened reinforced concrete (RC) beams, and detailed parametric study of NSM-strengthened RC beams using Abaqus. The CFRP bars were manufactured locally using the twisting technique. Parametric study was performed by considering various key parameters which affect the strengthening of the RC beams such as a CFRP bar’s orientation, geometry, intergroove separation, type of groove, cross-section area of bars, and material strength. The parametric study is based on the nonlinear finite-element modeling using Abaqus for examining the flexural and shear behavior. All beams were tested under a two-point loading system. The numerical results obtained using Abaqus are validated with the experimental results. The present study results show a significant increase in the strength of RC beams strengthened with NSM CFRP bars. Furthermore, based on this study, guidelines for realistic structural applications and optimization of the use of NSM CFRP bars are proposed.

Complete closed-type carbon fiber-reinforced polymer stirrups with a rectangular section (CFRPRS) were developed and tested in the research reported in this paper. The use of a CFRPRS is intended to relieve stress concentrations and reduce the number of kinked fibers at the bent portion. A total of 16 B.5 specimens were tested with consideration of the bend strength of the CFRPRS as well as carbon fiber-reinforced polymer (CFRP) stirrups with circular sections. Observations were made regarding the bend strength and failure modes. Three different types of CFRPRS were tested with different width-thickness ratios (34.4 for a sectional area of $27.9{\mathrm{mm}}^2$ and 4.5 or 8.0 for $72{\mathrm{mm}}^2$) and three different ratios of the bend radius to the area-equivalent bar diameter (1.5, 3.0, and 4.5). The test results indicated that CFRPRSs exhibited improved bend strength as compared to its counterpart, the conventional CFRP stirrups with circular sections. CFRPRSs possessing a larger stirrup width-thickness ratio provided more strength for a given sectional area. The best correlation between the test results and predictions of the CFRPRS bend strength was observed when sections of the CFRPRS were modeled as a collection of transformed individual circular sections.

The epoxy-bonded fiber-reinforced polymer (FRP) technique has been attracting more attention for repairing steel structures. In this paper, cracked steel plates repaired with FRP laminates were investigated with the finite element (FE) method. Three different FRP configurations designed with equivalent tensile stiffnesses were employed to repair cracked steel plates to determine the best FRP configuration for extending the crack growth life. The stress intensity factor ($K$) and the crack growth life of FRP-repaired specimens were compared, and the parameters influencing the repair effectiveness were analyzed. The results showed that FRP configurations have an obvious effect on $K$ and the crack growth life. Configuration 1 was more effective than Configuration 2. The superiority of Configuration 1 over Configuration 2 was more evident with an increase of FRP thickness and/or initial crack length, but the superiority decreased with a thicker adhesive thickness and/or a larger local debond size. However, a comparison between Configuration 1 and Configuration 3 was highly dependent on FRP thickness, initial crack length, FRP width arranged in Configuration 3, and local debond size. Based on limited analyses of the three FRP configurations, FRP Configuration 1 is recommended for practical engineering applications when the fatigue cracks of steel members are similar to the center crack. Finally, two additional investigations were recommended for future study.

This paper presents the results of an analytical study on reinforced concrete (RC) T-beams strengthened in shear with L-shaped fiber-reinforced polymer (FRP) plates. The main objective of this study is to develop design equations for RC beams retrofitted in shear using L-shaped FRP plates, considering all possible modes of failure in ultimate limit states. Unlike RC beams strengthened with externally bonded (EB) FRP plates and fabrics, prefabricated L-shaped plates feature unconventional failure modes because of their special shape and anchorage. The possible failure modes for RC beams strengthened with L-shaped FRP plates are (1) concrete breakout at the embedded part of the FRP plate in the flange; (2) FRP pull-off at the epoxy/FRP interface; (3) FRP pull-off at the concrete/epoxy interface; and (4) FRP overlap failure at the beam soffit. These failure modes do not occur in RC beams shear-strengthened with EB FRP fabrics and plates. Therefore, the existing design models for EB FRP fabrics and plates cannot predict with sufficient accuracy the contribution of FRP to the shear resistance of RC beams shear-strengthened with L-shaped plates. In this article, new design equations are proposed in light of recent developments and data. These equations distinguish the failure mode and calculate the FRP contribution to the shear resistance of RC beams strengthened with L-shaped FRP plates. In some cases, full embedment of the L-shaped carbon FRP (CFRP) plates in the RC beam flange is not feasible because of the presence of an obstacle (e.g., longitudinal reinforcing steel bars in the flange). The new design equations are applicable to RC beams strengthened with L-shaped FRP plates with different embedment lengths (including no embedment) of the CFRP plates in the RC beam flange. The proposed design equations are validated against experimental data collected from the literature.

Fiber-reinforced polymer (FRP) composites with a large rupture strain (LRS) (i.e., having an ultimate tensile strain larger than 5%) are promising jacketing materials for the seismic retrofit of reinforced concrete (RC) columns. These LRS FRPs are environmentally friendly as their reinforcing fibers can be made from recycled plastics [e.g., polyethylene terephthalate (PET) bottles]; as a result, they are also cheaper than conventional FRPs [i.e., carbon FRP (CFRP), glass GFRP (GFRP), and aramid FRP (AFRP)]. This paper presents the first-ever study on the behavior and modeling of LRS FRP-confined concrete under cyclic axial compression. Experimental results are first presented to examine both the envelope compressive stress-strain curve and the cumulative effect of loading cycles. A cyclic stress-strain model is then proposed and shown to provide close predictions of the test results. The proposed cyclic stress-strain model is formed by combining an existing monotonic stress-strain model for predicting the envelope curve with an existing cyclic stress-strain model for predicting the unloading and reloading paths. This cyclic stress-strain model can be employed in modeling the behavior of LRS FRP-jacketed RC columns subjected to seismic loading.

Results of nonlinear finite-element analyses of commercial pultruded fiber-reinforced plastic (PFRP) I-section profiles subjected to pure compression are presented. One narrow flange and four wide flange shapes are analyzed for 10 values of the column slenderness, involving 50 finite-element (FE) models. The imperfection sensitivity is investigated with reference to different imperfection shapes. For wide flange columns, if the imperfection amplitudes are identified with the limiting amplitudes reported by the pultruders, extremely low ultimate axial loads are obtained, associated with unrealistic failure mechanisms and strong interaction between local and global buckling modes. In contrast, reduced imperfection amplitudes reported in the literature lead to well documented failure mechanisms and to a small influence of the buckling mode interaction. As is known, the local buckling of wide flange shapes is triggered by the flange instability. Conversely, the local buckling of narrow flange shapes generally is triggered by the web instability. Because of this property, the paper shows that at equal cross section area, the narrow flange profiles may exhibit higher ultimate loads with respect to the wide flange profiles in a broad range of column slenderness.

This paper presents the results of an experimental study that was undertaken to investigate the effects of key parameters on the compressive behavior of fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs). A total of 24 normal-strength and high-strength concrete-filled DSTCs were manufactured and tested under axial compression. The key parameters examined included the concrete strength; thickness of FRP tube; diameter, strength, and thickness of inner steel tube; and presence (absence) of concrete filling inside it. The results indicate that both normal- and high-strength concretes in a DSTC system is confined effectively by FRP and steel tubes, resulting in a highly ductile compressive behavior. The results also indicate that increasing the inner steel tube diameter leads to an increase in the ultimate axial stress and strain of concrete in DSTCs. It is observed that the concrete filling of the inner steel tubes results in a slight decrease in the ultimate axial strain and a slight increase in ultimate stress of DSTCs. No clear influence of the strength of inner steel tube is observed on the ultimate condition of concrete in DSTCs. It is found that, for a given nominal confinement ratio, an increase in the concrete strength results in a decrease in the ultimate axial strain of DSTCs.

Seismic retrofit of reinforced concrete (RC) columns using fiber-reinforced plastic (FRP) jacketing has been widely investigated. However, the residual performance of FRP retrofitted columns after surviving earthquake attacks remains as an issue with little research. As a follow up of a previous experimental study, this paper reports the analysis of the axial loading creep behavior of FRP retrofitted RC columns with different degrees of simulated earthquake damage. The damage degree and residual strains in FRP after lateral loading are assessed based on previous maximum lateral displacement excursion. A creep model for damaged columns with FRP jacket is presented based on the age-adjusted effective modulus method for creep of concrete and the Findley’s power law function for creep of FRP jacket. The creep model considers the effects of previous damage, sealed concrete, multiaxial stress state of stresses and stress redistribution. The proposed model is validated against previous creep tests of fiber-wrapped concrete columns by other researchers and the long-term tests conducted by the authors. Finally, a detailed study is carried out to identify the practical design parameters affecting creep of hybrid columns. The analytical results show that the creep of the FRP confined column increases as the damage degree becomes larger and exhibits notable nonlinear feature. The creep life declines and may cause creep rupture of the FRP when retrofitted columns suffer serious earthquake damage or under high axial load.

The deformation of a prestressed concrete (PC) member is controlled by the variation in flexural rigidity (EI) both along the length of the member and with applied loads. Typically, the moment-curvature $\mathrm{M}/\chi$ approach is used to quantify EI with the analysis being mechanically correct prior to cracking as the only empirically derived components are the material stress-strain relationships. Postcracking, however, the $M/\chi$ approach has to be semi-empirical as it cannot directly simulate the effects of tension stiffening. This paper presents a displacement-based $M/\mathrm{\Theta }$ approach for determining the behavior of PC members, with and without supplementary tension reinforcement. Applying the mechanics of partial-interaction (PI) theory, the approach directly simulates the formation and widening of cracks as the reinforcement pulls from the crack face, thus directly allowing for tension stiffening. The PI $M/\mathrm{\Theta }$ approach can quantify the equivalent flexural rigidities (${\mathrm{EI}}_{\text{equ}}$) associated with tension-stiffening which can be used in standard analysis techniques to predict member behavior. The proposed PI $M/\mathrm{\Theta }$ is finally used to predict the behavior of members with both steel and fiber-reinforced polymer (FRP) reinforcement and a good correlation with test results from serviceability to ultimate loading is obtained.

Three-point and four-point bending tests are traditionally used to estimate the shear and flexural properties of pultruded fiber-reinforced plastic (PFRP) structural shapes. In the framework of Timoshenko’s beam model, the rigidities of beams can simply be obtained from linear combinations of the experimental deflections. However, several measurement uncertainties may affect the parameter identification. To analyze this drawback, a new test configuration was compared with the common three-point and four-point bending test configurations through an extensive experimental program concerning wide-flange commercial profiles. A parametric study was conducted by varying the span length, and, in the case of the four-point bending tests, the relative position of the applied loads. The influence of the load and deflection measurement errors was analyzed and proper uncertainty bands were provided for the computed rigidities.

This research aims at creating finite-element models for fiber-reinforced polymer (FRP) shear strengthened concrete beams. It is inspired by the fact that the determination of the structural behavior of shear strengthened beams requires advanced numerical methods of which results are substantiated by credible experimental findings. The models are developed here to assess the shear and interfacial types of behavior of beams strengthened using one of three different schemes, namely, externally bonded (EB), mechanically fastened (MF), and hybrid EB/MF FRP schemes. The interfacial behavior between the EB, MF, and hybrid EB/MF FRP and the concrete is accounted for using interface elements for both vertical and inclined FRP strips. A user-defined subroutine for the microplane constitutive law for the concrete material is incorporated in the model. Results are presented in terms of the ultimate load-carrying capacities, load-deflection relationships, and interfacial stress/slip distributions. Numerical results are validated against available experimental data and show reasonable agreement. Models for hypothetical cases of MF FRP strengthened beams are created to enrich the discussion on the interfacial bearing stress distributions.

Shear-deficient reinforced concrete (RC) structures can be effectively strengthened using external prestressed carbon fiber–reinforced polymer (CFRP) straps. Due to the presence of the external elastic straps, a strengthened beam can continue to carry significant load beyond the stages of crack plane slipping and internal-shear stirrup yielding, and the concrete is subjected to high tensile strain levels. As a consequence, the concrete material models play a significant role in the context of modeling such behavior. The modified compression field theory (MCFT), which is a widely accepted shear theory for unstrengthened RC structures, incorporates the details of the stress-strain behavior of concrete. The MCFT also considers compatibility as a governing factor, which facilitates the inclusion of the strap system into the MCFT formulation. In the current study, modifications were investigated to model CFRP strap retrofitted RC beams associated with either uniform or nonuniform strap spacings. An experimental investigation on strengthened and unstrengthened rectangular RC beams was carried out to validate the MCFT predictions for various strap layouts. The validation process revealed that, in general, the MCFT was able to model the shear response of the retrofitted RC beams, but the representation of the softening of the concrete compressive strain, and stress, was found to be influential in the determination of the ultimate load capacity.

Reinforced-concrete (RC) tanks in water and wastewater treatment plants (WWTPs) experience severe corrosion problems resulting from the application of specific treatment methods or chemicals. Municipalities around the world spend hundreds of millions of dollars annually to replace and repair corroded RC tanks. Designing these tanks requires attention not only to strength requirements, but also to durability and crack control. This paper presents the design procedures, construction details, leakage testing, and monitoring results for the world’s first RC water chlorination tank totally reinforced with glass-fiber-reinforced polymer (GFRP) bars. The project is located in Thetford Mines, Quebec, Canada. The tank is considered one of the most important components in the city’s new water treatment plant. The tank has a volume of over $\mathrm{2,500}{\mathrm{m}}^3$ and its walls are 4,650 mm high. The foundation, vertical walls, and cover slab were totally reinforced with GFRP bars. The tank was designed to satisfy the serviceability and strength criteria in CAN/CSA S806-12 (CSA 2012), ACI 440.1R-06 (ACI 2006), and ACI 350/350R-06. The tank is fully instrumented at critical locations with fiber-optic sensors to collect strain data. Site inspection showed that the tank performed very well and was able to withstand applied loads without problems or leaking during the leakage test and after eight months under the service condition.