Journal of Composites for Construction

This paper presents a new discrete crack model that can simulate the complex behavior of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) slabs. The model approximates the kinematics of crack openings by a rigid body movement that can be easily embedded in regular finite elements. As such, concrete cracking and its interaction with the FRP can be automatically accounted for in finite element simulations. The proposed technique includes all relevant material nonlinearities related with concrete, steel, and FRP, as well the debonding at interfaces. The model is validated against experimental results on one-way simply supported slabs before assessing in detail the relevance of the discrete simulation of cracks for the analysis of the behavior of the strengthened structure. The numerical model provides important insights on the bond mechanism that cannot be easily determined otherwise. For example, the debond failure is shown to be composed of a critical local stable debonding length that is then followed by global debonding which triggers a rapid loss of strength provided by the FRP. The model also provides the stable bond length from parametric analysis of the optimal strengthening layout. Overall, the model correctly predicts the composite behavior and strength of the FRP-strengthened structure, confirming experimental observations, and expanding the current capabilities of existing analytical and numerical models.

Upgrading existing buildings to new functional requirements may require new openings that can weaken the structure, prompting the need for strengthening. In such cases traditional strengthening solutions, such as creating a reinforced concrete (RC) or steel frame around the opening, imply long-term restrictions in the use of the structure compared to solutions that use externally bonded composites. Two fabric-reinforced cementitious matrix (FRCM) composites were used in this study to restore the capacity of panels with newly created door type openings to that of a solid panel. Five half-scale RC panels acting as two-way action compression members were tested to failure. Two full-field optical deformation measurement systems were used to monitor and analyze the global structural response of each tested panel (i.e., crack pattern, failure mechanism, and displacement/strain fields). The performance of existing design methods for RC panels has been assessed in comparison with the experimental results. The capacity of strengthened panels with small openings ($450\times \mathrm{1,050}\mathrm{mm}$) was entirely restored to that of the solid panel. However, for panels with large openings ($900\times \mathrm{1,050}\mathrm{mm}$), only 75% of the solid panel’s capacity was restored. The capacity of the strengthened panels was about 175 and 150% higher compared to that of reference panels with small and large openings, respectively.

Shear-deficient RC beam-column joints (BCJs) represent one of the main factors behind the seismic damage suffered by existing concrete infrastructure, as well as the associated loss of life. This study presents a novel technique for strengthening shear-deficient RC BCJs. The technique involves embedding carbon fiber–reinforced polymer (CFRP) or steel bars into epoxy-filled holes drilled within the joint core. Six exterior RC BCJs were constructed and tested under displacement-controlled cyclic loading. Five specimens, of which four were strengthened with embedded bars, were designed with shear-deficient joints according to the pre-1980s building codes. The remaining specimen was adequately designed according to the current standard. The test parameters are the type (steel or CFRP) and number (four or eight bars) of embedded bars. The unstrengthened control specimen experienced joint shear failure in the form of cross-diagonal cracks. The strengthened specimens, namely those strengthened with embedded steel bars, exhibited less brittle failure when damage occurred in the beam region at the early stages of loading, suggesting the outset of a beam hinge mechanism. Additionally, the strengthened specimens exhibited enhancements in joint shear strength, ductility, dissipated energy, and stiffness of 6%–21%, 6%–93%, 10%–54%, and 2%–35%, respectively, compared with the control specimen. This paper also presents a mechanics-based design model for RC BCJs strengthened with embedded bars. The proposed model covers all possible failure modes including yielding of the existing steel reinforcement, concrete crushing, and debonding of the embedded bars. The accuracy of the proposed model was checked against the test results. The model gave good predictions with an average predicted-to-experimental ratio of 1.05 and a standard deviation of 0.04.

Fabric-reinforced cementitious matrix (FRCM) composites are a new class of materials used to repair and strengthen existing structures. Mechanical performance varies widely based on the type and volume of fibers, fabric architecture, mortar properties, and bond characteristics. Moreover, test setup and boundary conditions are of the utmost importance for their mechanical characterization because of the complex interaction between fabric and brittle matrix. Therefore, the challenge is to define characterization and acceptance criteria that are (1) suitable for a wide spectrum of existing FRCM systems, and (2) produce representative values for the design of strengthened members. In this paper, three FRCM systems comprising carbon, glass, and steel fabrics were tested in accordance with two different acceptance methods. One method, which has been developed and adopted in the United States, is based on tensile tests on FRCM coupons with a clevis-grip mechanism that allows slippage at the fabric–matrix interface and provides mechanical properties that are directly used for design. The other method, developed within RILEM TC 250-CSM, combines the results of clamping-grip tensile tests on bare textile specimens with those of FRCM-to-substrate shear bond tests to obtain mechanical properties accounting for a broad range of possible failure modes. The results provided by the two methods are discussed and compared in relation to FRCM field applications.

Research studies on reinforced concrete T-beams shear strengthened with externally bonded carbon fiber reinforced polymer (EB-CFRP) have shown that debonding at the CFRP–concrete interface is still a major concern. It is well established that shear strengthening using a full-wrap configuration offers a better solution than side bonding and U-wrap configurations without anchorage systems in preventing premature FRP debonding. However, such a strengthening technique can only be used for rectangular beams for which the four faces are accessible. This is clearly not practical, since most RC beams are cast monolithically with slabs and therefore have a T-section. The objective of the present research was to experimentally assess the effectiveness of a new and innovative EB-CFRP closed-stirrup technology for fatigue upgrade by reproducing the advantages of a full-wrap configuration. The CFRP shear-strengthening system consisted of L-shaped laminates and anchoring ropes to form EB-CFRP closed stirrups. Laboratory tests were performed on full-size RC T-beams with different ratios of internal shear reinforcement under fatigue cyclic loading. The specifications of North American design codes and standards for fatigue limit state design of RC structures were also assessed in this investigation. The EB-CFRP closed stirrups resulted in a decrease in the levels of transverse steel strains and thereby enhanced the fatigue performance of shear-strengthened specimens. The results also demonstrated the existence of an interaction between internal transverse steel and EB-CFRP under fatigue loading. Finally, strengthening with CFRP L-shaped laminates led to a significant increase in the shear resistance of RC T-beams.

This study aims to explore the cyclic bond–slip behavior of basalt fiber-reinforced polymers (BFRP) bars. Experimental pullout tests under monotonic and cyclic loading patterns were conducted in order to examine the effect of the cyclic loading on the bond behavior of BFRP bars. The effects of cyclic loading characteristics, concrete compressive strength, and bar diameter on the bond–slip relationship were examined. In addition, the bond performance of bars with four different surface profiles (smooth, sand-coated, wrapped, and wounded) under different loading protocols was studied and compared to that of deformed steel bars. The results revealed that cyclic loading had a significant effect on the bond performance of BFRP bars and the bond–slip relationship was greatly dependent on the bar surface treatment, as well as the characteristics of the applied cyclic loading protocol. Also, cyclic loading magnified the effect of concrete compressive strength on the bond behavior. A numerical model was proposed to predict the bond–slip relationship of wounded BFRP bars under cyclic loading. The model predictions showed good agreement with experimental results.

This paper presents the potential of cathodic protection for steel beams strengthened with carbon fiber reinforced polymer (CFRP) sheets. A conceptual experimental program is conducted with 19 strengthened beams. Sacrificial anodes are employed to mitigate the initiation and progression of corrosion, which lead to three test categories: unprotected, protected with zinc coins, and protected with zinc spray. A comparative assessment is reported on the behavior of the beams subjected to an accelerated corrosion protocol with an NaCl-based electrolyte solution for up to 144 h, including an unconditioned control beam. Material testing indicates that the strength of the epoxy adhesive bonding the CFRP to steel substrates is preserved, whereas the fiber-resin interface of the CFRP is deteriorated by the electrolyte. The cathodic protection improves the flexural behavior of the conditioned beams in terms of load-carrying capacity, yield characteristics, and ductility. Protection limits beyond which the efficacy of the zinc anodes diminishes are noticed. According to reliability investigations, the safety index of the strengthened beams is reduced due to corrosion damage (the accelerated corrosion degrades the CFRP-steel interface); however, the presence of cathodic protection retards the degree of reduction. A fuzzy-logic model is developed to address the intrinsic uncertainty of corrosion and is used to propose design recommendations for CFRP-strengthened steel beams with cathodic protection.

This paper presents the experimental results of a pilot study performed on the seismic performance of three types of damage-resistant, slender reinforced concrete (RC) shear walls. The study explores three innovative schemes for the mitigation of postearthquake damage, including permanent lateral deformation and concrete damage, in RC shear walls. Each innovative shear wall had an aspect ratio of 2.0 and was reinforced with a hybrid reinforcing system consisting of mild steel and a type of self-centering reinforcement such as shape memory alloy bars, glass fiber reinforced polymer bars, or high-strength steel strands. To mitigate concrete damage, the walls were detailed with fiber reinforced cementitious composites, either engineered cementitious composite or steel fiber reinforced concrete. The specimens were supported as cantilevers, and then were tested up to failure under pseudo-static, cyclic loads. As test results showed, the innovative shear walls had smaller residual drift ratios and mitigated damage with respect to a conventional RC shear wall. The innovative walls also showed significant levels of energy dissipation and ductility throughout testing.

This paper reports on the effect of exposure to cycles of wetting and drying in fresh and salt water on the performance of carbon fiber-reinforced polymer (CFRP)-bonded concrete beams. The test program included 3, 8, and 12 months of exposure of normal and high-strength concrete specimens strengthened with one and two layers of CFRP reinforcement. The results indicated that the exposure caused a reduction in the tensile strength and modulus of elasticity of the epoxy adhesive. The ultimate loads of the small-beam specimens were not significantly affected by the exposure owing to the redistribution of shear stress along the CFRP plates. The degradation of the CFRP-concrete interface, however, decreased the bond strength and fracture energy of CFRP strengthening systems in the small-beam test, especially in the one-layer high-strength concrete configuration. Insights on bond characteristics and failure mechanisms of CFRP-strengthened concrete under the tested exposure conditions are presented.

This paper assesses the seismic response of square concrete columns internally confined with glass fiber–reinforced polymers (GFRP). Tests were conducted under simulated earthquake loads on eight full-scale $305\times 305\mathrm{mm}$ columns reinforced with GFRP rectilinear ties and steel longitudinal bars. Shear versus deflection and moment versus curvature responses along with various ductility parameters were used to evaluate the performance of the columns. Results showed that the GFRP ties provide effective confinement to the concrete core until a rupture strain much larger than the steel yield strain. The influence of tie configuration on specimen behavior was found to be significant. An approximately 4% ultimate drift ratio was observed for properly confined columns tested under axial load of 42% of the column axial load capacity. The current design code provisions and guidelines for GFRP ties were found to be ambiguous and lacking in several aspects; suggestions are made for improvement.

The polymer matrices currently used for the fabrication of fiber reinforced polymer (FRP) reinforcing bars soften near their glass transition temperature $T_g$, resulting in a reduction of mechanical strength. This paper discusses the effect of high temperatures on the bond behavior of three types of glass FRP (GFRP) reinforcing bars using standard pullout tests. The bars were 16 mm in nominal diameter with different surface treatments, including sand-coated and ribbed surfaces. The tests were conducted under steady-state and transient temperature protocols for a temperature range of 25°C–360°C at the concrete-to-bar interface. In addition, the effect of embedment length was investigated for a limited number of specimens. The bond failure modes were assessed in detail for different bars. The results for temperatures below 80°C are relevant for elevated service temperatures, and higher temperatures have important implications for performance in fire. At temperatures such as those that would be experienced in a fire (i.e., above 200°C), bond strength dramatically deteriorated, and the retained strength was below 17%. The glass transition temperature of the matrix material was found to be critical for the bond performance of GFRP bars in fire scenarios. The results in this paper are needed to develop rational design guidelines for embedment and anchorage of GFRP bars to obtain the required fire endurance (e.g., 2–3 h) that has been demonstrated through separate full-scale fire tests.

This study presents the results of an experimental program on the comparative performance of fiber-reinforced polymer (FRP) confined concrete specimens with different wrapping schemes. A total of 32 specimens in four groups were cast and tested under concentric and eccentric axial loads. All specimens were wrapped with the same amount of FRP but with different wrapping schemes, including full wrapping, partial wrapping, and nonuniform wrapping. Specimens in the first group were fully wrapped (Group F). Specimens in the second group were partially wrapped with 30 mm FRP strip spacing (Group P30). Specimens in the third group were partially wrapped with 60 mm FRP strip spacing (Group P60). Specimens in the fourth group were nonuniformly wrapped with a combination of full and partial wrapping (Group FP). Two similar specimens in each group were tested under concentric, 15 mm eccentric, 25 mm eccentric, and 40 mm eccentric axial loads. The test results indicate that fully wrapped specimens outperformed other groups of specimens under both concentric and eccentric axial loads, which were followed by nonuniformly and partially wrapped specimens. With the increase in axial load eccentricity, the performance in all groups significantly decreased. Moreover, with the increase in axial load eccentricity, the failure mode changed from FRP rupture at the compression side to extensive concrete cracking at the tension side. Equations were developed to predict the compressive strength of FRP confined concrete with different wrapping schemes. Experimental and analytical interaction ($P\text{-}M$) diagrams were also constructed to investigate the axial and flexural behavior of different groups of specimens.

Carbon fiber–reinforced polymer (CFRP) grid–reinforced engineered cementitious composites (ECC) have been developed as an alternative to epoxy resin–based FRP plates for the strengthening of concrete structures. To broaden the application of this new composite material for seismic retrofit and strengthening of reinforced concrete elements, an experimental study, including three nonreinforced ECC plates and 30 CFRP grid–reinforced ECC (CFRP-ECC) hybrid plates, was conducted to investigate their mechanical performance under both monotonic and cyclic uniaxial tensile loads. The stiffness of the internal CFRP grid and loading scheme were the two primary test variables. Test results showed that all CFRP-ECC plates failed with a dominate crack in the ECC matrix and rupture of the internal CFRP grid. The envelopes of the cyclic tensile stress–strain curves followed the static stress–stain curves regardless of the cyclic loading scheme adopted. A nonlinear stress–strain model is proposed to model the unloading/reloading paths of CFRP-ECC plates under cyclic uniaxial tensile loading.

A novel shear reinforcing system, wound fiber–reinforced polymer (W-FRP), that capitalizes on the flexibility of carbon fiber to create durable reinforcement cages for geometrically optimized concrete structures, is proposed thereby unlocking new potential to minimize carbon emissions associated with new concrete structures. Fiber-reinforced polymer (FRP) shear design methods have been extensively validated against prismatic beam tests, but variations in geometry have not yet been considered. This paper proposes revised design methods, validated using tests of eight W-FRP–reinforced variable-depth concrete beams, to examine the contributing factors to shear capacity. The corner strength, orientation, and compression concrete confinement provided by W-FRP links, along with the contribution to shear of longitudinal bars, are shown to be key design parameters. Optimizing the W-FRP pattern is found to enhance shear capacity by as much as 50%. The variable-depth geometry tested in this paper uses 19% less concrete than an equivalent-strength prismatic beam. Both reinforcement and geometry optimizations are the key steps toward achieving minimal material use for concrete structures.

An experimental study was conducted to evaluate the effect of the gross-to-cracked section moment of inertia ratio $I_g:I_{cr}$ on the long-term deflection multiplier for glass fiber–reinforced polymer (GFRP) reinforced concrete beams subjected to sustained loading. Ten beams (eight GFRP-reinforced beams and two steel-reinforced beams) were maintained under a constant sustained service load for nearly three years. Long-term deflections were monitored on each beam over the duration of the test. Based on the results, the long-term deflection multiplier was found to be related to the gross-to-cracked section moment of inertia of the beam cross section. These trends were further confirmed by an examination of results published by other researchers and from an analytical study based on simple deflection models.

Concrete slabs, beams, and columns reinforced with glass fiber-reinforced polymer (GFRP) reinforcement have shown considerable deformability under monotonic loading; however, they have not been fully investigated under seismic loading. In efforts to understand the seismic behavior of GFRP-reinforced concrete (RC) moment-resisting frames subjected to seismic loading, this paper presents an investigation of the anchorage of GFRP bars in exterior beam-column joints. Four full-scale exterior beam-column joints were tested under cyclic reversed load simulating seismic loading conditions. Parameters tested in this study included end anchorage details of beam bars in the joint (90-degree hooks or anchor heads) and their surface texture (sand coated or mechanically deformed). Experimental results showed that all specimens successfully sustained 4.0% loading drifts and that surface texture showed an insignificant influence on the overall behavior.

Ten high-strength shear-controlled square columns are constructed and seven of them are strengthened with carbon fiber–reinforced polymer (CFRP) straps. These columns are then tested under simulated seismic load at the standard axial compression ratio range from 0.119 to 0.714. The main variables in this study include the value of axial compression ratio, CFRP layout, number of CFRP sheets, and predamaged condition. The failure modes, lateral load-displacement curves, and several important seismic parameters are analyzed. The test results show that the seismic behavior of the columns is obviously increased by wrapping CFRP sheets around the columns even at an ultrahigh axial compression ratio, which confirms the validity of loosening the allowable axial compression ratio for strengthened columns. Based on the study of ductility and ultimate lateral capacity, it is suggested that the CFRP straps are more suitable to strengthen the columns than is the fully wrapped CFRP layout at the same volumetric ratio of CFRP and confined concrete. Finally, an effective stiffness degradation model and an ultimate shear capacity model are proposed. It is evident that satisfactory accuracy and good agreement between the theoretical predictions and the test data from this research and other relevant literature are achieved for the two proposed models.

This paper presents the interfacial behavior and deterioration mechanisms of carbon fiber–reinforced polymer (CFRP) sheets bonded to a concrete substrate subjected to sulfuric acid based on a test protocol suggested by published guidelines. To represent possible scenarios on-site, two groups were tested: Category 1 is for CFRP-strengthened concrete members experiencing acid exposure owing to a functional change, and Category 2 is concerned with acid-damaged concrete that requires CFRP strengthening. All specimens were conditioned in a 5%-concentration sulfuric acid solution for up to 9 weeks at a typical interval of 3 weeks. Chemical interactions between the test specimens and sulfuric acid are affected by the presence of CFRP bonding, thereby lowering the dissolution of the cement paste. The capacity of the interface decreases due to the acidic environment, which is rapid at an early exposure period between 0 and 3 weeks. CFRP debonding in conjunction with notch-induced concrete cracking is responsible for the specimens’ failure, regardless of exposure period. The specimens in Category 1 reveal an apparent transition from full bond to debonding when failure is imminent, which is different from those in Category 2 accompanied by gradual interface degradation. The extent of composite action between the CFRP and concrete is a function of exposure period and load level. An analytical model is developed to propose performance-based design recommendations that are dependent upon variable safety indices and interfacial deterioration levels.

Available flexural strength formulas mainly focus on rectangular concrete beams exclusively prestressed with fiber-reinforced polymer (FRP) tendons. Moreover, an iterative procedure is required when FRP rupture governs the design. This paper presents a simplified and yet rational analytical approach for flexural capacity of prestressed concrete T-beams with bonded FRP tendons and non-prestressed steel bars. First, a sectional analysis assuming balanced failure is conducted to distinguish tension and compression failure. In conjunction with a statistical analysis of an experimental database of 87 beams, a new transition region between tension- and compression-controlled sections is proposed in terms of ratio of provided-to-balanced reinforcement ($1.0<{\rho }_{ep}/{\rho }_{ep,b}\le 1.5$) instead of the traditional net tensile strain limits in current American design guidelines. Then, a numerical sectional analysis of tension-controlled sections is was used to perform a detailed parametric study of more than 120,000 sections. Based on a multiple regression analysis of the numerical results, simplified equations are developed for the flexural capacity of the sections. Then equations are presented for flexural capacity of compression-controlled sections. Finally, the accuracy of the proposed approach is verified based on the experimental database from 87 beam tests.

This paper presents a confined concrete material constitutive model for use in finite-element analysis, which is able to model accurately the combined confinement effect of fiber-reinforced polymers (FRP) and internal steel reinforcement on the structural monotonic, cyclic, and/or dynamic response of reinforced concrete (RC) columns confined with externally wrapped FRP. The proposed material constitutive model for FRP-and-steel confined concrete explicitly models the simultaneous confinement produced by FRP and steel on the core concrete to predict the combined effect on the structural response of circular RC columns. This modified material model is combined with a force-based frame element to predict numerically the load-carrying capacity of FRP-confined RC columns subjected to different loading conditions. Numerical simulations are compared to experimental test data available in the literature and published by different authors. The numerically simulated responses agree very well with the corresponding experimental results. The proposed model is found to predict the ultimate load for FRP-confined RC circular columns with better accuracy than models that do not consider the simultaneous confinement effects on FRP and steel, particularly for columns subjected to concentric axial loads.

This study aimed at developing a simple procedure to predict the fatigue life of reinforced concrete beams strengthened with externally bonded fiber reinforced polymers (EBFRP). The study tested classical beam theory against 68 data points collected from the literature to validate its use. The calculated stresses showed good agreement with the data reported in the literature, despite the fact that stiffness degradation was neglected in the calculated stresses. This work introduced a new stress versus number of cycles to failure ($S\text{-}N$) model, which was developed by regression analysis between analytically calculated steel stresses and the reported fatigue life of the beams, in order to ensure the compatibility of the model. A modified $S\text{-}N$ model with a mean safety factor of 3.9 was introduced to be utilized for design purposes.