Journal of Composites for Construction

This paper aims to evaluate the effectiveness of hybrid composite plates (HCPs) technique for the shear strengthening of the reinforced concrete (RC) T cross-section beams. HCP consists of a thin plate of strain hardening cementitious composite (SHCC) reinforced with carbon fiber reinforced polymer (CFRP) laminates. Two HCPs with different CFRP laminates percentage (${\rho }_{fw}=0.08\%$ and ${\rho }_{fw}=0.14\%$) were adopted for the shear strengthening of the beams. The HCPs were bonded to substrate in two different ways. In the first case, the HCPs were bonded using epoxy adhesive, whereas in the second case they were bonded using epoxy adhesive and fixed by mechanical anchors. The effectiveness of this technique was limited by the tensile strength of the concrete cover of the strengthened beams. Therefore, in the second case, mechanical anchors prevented a premature debonding of the HCPs and a certain concrete confinement was applied in the zone of the beam to be strengthened, resulting in favorable effects in terms of shear strengthening. Advanced finite element method (FEM) based numerical simulation was performed by using a constitutive model, whose predictive performance was demonstrated by simulating the experimental tests carried out. After demonstration of the good predictive performance of the numerical model, a parametric study was carried out to study the influence of the shear reinforcement ratio, and the influence of thickness of the HCPs on the beam’s load carrying capacity.

Flexural strengthening of reinforced concrete using near-surface mounted (NSM) fiber-reinforced polymer (FRP) composites is a widely studied topic, and a promising technology. However, overlooked applications such as prestressed concrete and one-way slabs require further experimental investigation and theoretical refinement in order to develop comprehensive design guidelines. This paper investigates the behavior of prestressed hollow-core slabs strengthened in flexure with NSM carbon FRP strip reinforcement. Seven full-scale simply supported slab specimens were tested to failure under two-point monotonic load configuration. Experimental variables included two different internal prestressing reinforcement ratios, and three different NSM strengthening reinforcement ratios. This paper examines cracking behavior and modes of failure, load-strain relationships, deflections, and strengthened capacities. Also, a comparison between experimental results and theoretical predictions by relevant Canadian and American standards or guidelines is presented.

This paper summarizes the details of a numerical and experimental research program that was conducted to study the debonding of carbon fiber–reinforced polymer (CFRP) patches from cracked steel members under fatigue loading. A preliminary numerical model was developed to investigate the influence of patch debonding on the fatigue life of cracked steel elements. Results indicated that altering the shape and increasing the size of the debonded region could change the calculated crack growth rate by up to 54 times. To validate the model, six steel edge–notched tension coupons were patched with CFRP materials and tested under fatigue loading, while full-field strain distributions were monitored using a digital image correlation (DIC)–based measurement system. Based on the experimental results the numerical model was refined to incorporate the interfacial traction-separation behavior. A parametric study was conducted using the refined numerical model. The results indicate that the size and shape of the debonded region, and therefore the fatigue crack propagation rate, are not only dependent on the fatigue detail and the crack length, but also on the maximum magnitude of the applied fatigue load and the properties of the bonded interface.

This paper presents an experimental study on the effect of fiber-reinforced polymer (FRP) wrapping on the fatigue performance and bond strength of reinforced concrete beams containing a lap splice at a constant moment region under cyclic loading. In addition, the study presents the slip response of the steel bars. Fifteen beams were investigated that were divided into three groups, namely: unwrapped, glass fiber-reinforced polymer (GFRP) wrapped, and carbon fiber-reinforced polymer (CFRP) wrapped. The test variables were the presence or absence of FRP wrapping, the type of the FRP wrapping (GFRP or CFRP), the type of loading (monotonic or fatigue loading), and the fatigue load range. The results of the tests under monotonic load showed a significant increase in load and deflection for the FRP-wrapped beams compared to the unwrapped beams. The increase in the unwrapped beams load deflection results for the CFRP-wrapped beams was greater than that for the GFRP-wrapped beams. Also, the FRP-wrapped beams had higher fatigue strengths than the unwrapped beams. The bond stress slip response under monotonic and fatigue loading is compared.

An experimental and analytical investigation of the fracture performance of glass fiber-reinforced polymer (GFRP) bars embedded in concrete beams with cracks is presented. Beams with cracks were conditioned with sustained flexural loads in outdoor, 60°C-alkaline solution, and tap water environments for up to 9 months, after which they were subjected to eccentric three-point flexure tests to evaluate fracture performance and microscopic tests to analyze the changes in microscopic structures through scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The results showed that the cracks aggravated the influence of the environments on GFRP-RC beams but did not change the mechanisms of mechanical degradation of the GFRP bars. Based on the research conclusions, the same durability tests and eccentric three-point flexure tests were conducted to determine the permissible crack values and treatment recommendations for GFRP-RC beams with different precrack widths. These results will provide a certain theoretical basis for the crack specifications of GFRP RC structures.

This paper presents a rational approach for evaluating the fire resistance of reinforced concrete (RC) beams strengthened with different types of fiber-reinforced polymers (FRP). This approach is based on conventional fire design principles for RC beams, but incorporates the contribution of FRP and fire insulation in fire resistance calculations. Simplified equations are proposed for evaluating cross-sectional temperatures in fire-exposed FRP-strengthened RC beams with and without fire insulation. These temperatures are used to determine the loss of strength in steel, FRP, and concrete using temperature-dependent strength properties. Employing the strength in different materials, the moment capacity of FRP-strengthened beams is calculated, at any given fire exposure time, by applying force equilibrium and strain compatibility principles. At each time step, the limit state is applied to define the failure of an FRP-strengthened RC beam, and the time to failure is taken as the fire resistance of the beam. The validity of the proposed approach is established by comparing predicted fire response parameters with those obtained from fire resistance tests and detailed finite-element analyses. Further, the applicability of the proposed approach in practical situations is illustrated through a design example. Results from the proposed approach clearly indicate that a typical FRP-strengthened beam without supplemental fire insulation experiences a significant loss of moment capacity after approximately 20 min into fire exposure. However, a similar FRP-strengthened beam with supplemental fire insulation retains much of its moment capacity until 2 h into fire exposure.

An experimental study to investigate the stiffness and strength enhancement in a structural steel channel section strengthened by six different carbon fiber–reinforced polymer (CFRP) wrapping configurations is described in this paper. An approach of transforming the singly symmetric open section such as a channel section to a closed section by CFRP wrapping as a means to increase the stiffness and strength is demonstrated. A total of 21 specimens, both CFRP reinforced and bare steel specimens, were tested in four-point bending. Two different CFRPs, unidirectional and bidirectional fabrics, were used in wrapping the specimen. While the unidirectional layers contribute to the stiffness and strength, the bidirectional layer primarily contributes to confining the former in addition to increasing the resistance to lateral torsional buckling (LTB) of the specimens. The results indicate that the CFRP-strengthened closed sections confined by bidirectional fabrics are effective in enhancing the strength and stiffness compared to CFRP skin–strengthened sections (perimeter of bare steel channel sections overlayed with CFRP). The effectiveness of the closed section can be further improved by increasing the unidirectional CFRP layers prior to the final wrapping by bidirectional fibers. The variation in stiffness for all the CFRP configurations from the initial loading of specimens up to the ultimate is also investigated. This paper demonstrates that the strength and stiffness of steel channel sections can be significantly enhanced by means of appropriate CFRP wrapping configuration.

High strength steel is widely used in buildings, bridges, railways, and other structures. However, limited fatigue experiments have been conducted for cracked high strength steel plates with carbon fiber-reinforced polymer (CFRP) sheets. This paper describes an investigation into the fatigue behavior of unstrengthened and CFRP strengthened cracked Q345, Q460, and Q690 steel plates. First, the mechanical properties of these steels, which are dependent on the chemical composition and metallographic structure, were analyzed. Microalloying was found to refine the grain size and thereby enhance the strength. Second, 10 bare steel plates and nine strengthened steel plates were tested under fatigue loading at different stress ranges and stress levels (defined as the ratio of the maximum stress to the yield stress). For the unstrengthened specimens, the results show that the fatigue behavior at a 175.5 MPa stress range is best for Q690, followed by Q460 and then Q345. However, at the same stress level, the fatigue life of Q690 was shortest, followed by Q460 and then Q345. For the strengthened specimens, different failure modes were observed, and serious debonding occurred in Q690 steel under a 402.5 MPa stress range. CFRP strengthening technology can effectively increase the fatigue life (1.3–3.1 times) for all specimens except the Q690 specimen. The microstructure of the fracture surface was then investigated by scanning electron microscopy (SEM). Finally, a fit of material-related constants in fracture mechanics theory was obtained for the three categories of steel, and the fatigue life was predicted accurately using Paris’ law.

Carbon fiber–reinforced polymers (CFRPs) are commonly used for strengthening steel and concrete structures, with different types of CFRP used for strengthening different structural elements. A number of studies have focused on strengthening of steel members using low and normal-modulus CFRP. However, there is a lack of understanding on the use of ultrahigh modulus (UHM) CFRP for strengthening steel structures under impact loading. This paper presents experimental and numerical investigations on the effect of high load rates on the ultimate joint capacity, failure mode, effective bond length, and strain distribution along the bond interface between CFRP and steel plates in double-strap joints. Two methods of capturing strain were used in this program: (1) Image correlation photogrammetry was used for specimens tested under quasi-static tensile load, and (2) foil strain gauges were used for specimens tested under impact tensile loading. UHM CFRP was used to strengthen the joints using epoxy. The results show a significant increase in bond strength, and different failure modes were observed.

The hybrid composite beam (HCB) is a novel idea that combines conventional construction materials (i.e., steel and concrete) with fiber-reinforced polymer (FRP) composites in a new configuration. This hybridization aims to optimize the beam’s structural performance and produce a structural element that is more durable than conventional members. This study examined the durability of a commercial glass FRP (GFRP) laminate that was used to encase the HCB elements in a recently constructed HCB bridge. The E-glass/vinyl ester laminate was subjected to five aging regimes. These conditioning regimes simulated an alkaline attack, a salt attack, a salt attack that was preceded by ultraviolet (UV) irradiation exposure, and sustained stresses that were accompanied by controlled thermal cycles and natural weathering. The durability of the E-glass/vinyl ester laminate was examined in terms of changes that occurred in the ultimate tensile strength. A microstructural analysis was performed on both unconditioned and conditioned specimens via optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and Fourier transform infrared (FTIR) spectroscopy. The microstructural analysis revealed that the hydroxide and chloride ions penetrated the laminate through the existing voids and cracks without causing hydrolysis to the vinyl ester resin. Both the alkaline and the salt solutions caused fiber-matrix debonding and reduced the glass fibers load-bearing through physico-chemical processes (leaching and the dissolution of fibers). The tensile strength was reduced greatly under the alkali attack. The mechanical testing and the microstructural analysis provided fundamental insight into the durability and stress corrosion mechanisms of the examined GFRP shell under different environmental effects. This information is valuable to enhance the GFRP shell’s durability.

In the present study, natural fiber (i.e., flax fiber) reinforced epoxy (FFRP) plates and tubes were prepared with a filament winding process. FFRP tubes with 150-mm diameter were filled with self-compacting concrete and tested in terms of static and cyclic compressive properties. Basalt fiber–reinforced epoxy (BFRP) plates and tubes were prepared and tested for comparison. The FFRP plate possesses a nominal tensile strength and modulus of 333 MPa and 48.5 GPa, respectively, and exhibits a nonlinear tensile strain-stress curve. Compared with the static tensile modulus of the FFRP plate, the strain modulus–controlled low-frequency cyclic test steadily decreases with the number of load-unload cycles as a result of cumulative damage, and the ultimate stress is also reduced by 36%. According to the static compression results, FFRP tubes exhibit a notable confinement effect on concrete cylinders. The confinement effectiveness coefficient determined by the static compression test ranges from 4 to 7 for the FFRP tubes wound with four to 12 layers of flax fibers. Cyclic compression results show that the compression stress of the concrete-filled FFRP tubes is lower than that of the static compression stress under the same compression strain. This finding is attributed to the cumulative damage of the FFRPs during the cyclic process, similar to the FFRP coupons. On the contrary, the cyclic ultimate compression strain and stress of the concrete-filled BFRP tubes are notably close to the static values.

It is often found that a major limitation to retrofitting RC beams by adhesive bonding fiber-reinforced polymer (FRP) plates is premature debonding, which restricts the FRP strains to well below that at fracture. Tests have shown that the FRP peak strains can be increased by anchoring the plate ends of adhesively bonded plates and also by prestressing the anchored plates before adhesive bonding, both of which make the system much more efficient. In this paper, a mechanics-based numerical approach is developed that quantifies the stiffness, strength, and ductility of FRP retrofitted RC beams, in which the FRP plates are anchored, prestressed, and adhesively bonded. The approach can cope with any degree of anchorage and prestress and whether adhesively bonded or not. The approach is generic as it can be applied to any type of RC beam, anchor, and FRP plate and any distribution of load. Furthermore, it allows for discrete flexural cracks and for the formation of softening hinges should these occur. Worked examples are given as a possible design approach. This paper provides a design tool for engineers to develop and quantify their own FRP retrofitting systems for beams with anchored plates.

The use of externally bonded fiber-reinforced polymer (FRP) composites for upgrading the out-of-plane flexural resistance of unreinforced masonry (URM) walls is experimentally and analytically investigated in this study. A total of six hollow concrete block walls were tested to failure using an airbag and a reaction frame to obtain a uniform load on the wall. The masonry walls were placed horizontally and tested as one-way slabs with span direction perpendicular to the bed joints. The first wall was left unstrengthened to be used as control specimen; the other five walls were strengthened using different schemes of externally attached glass-fiber-reinforced polymer (GFRP) sheets. The main parameters studied experimentally were FRP reinforcement ratio and stiffness. In addition to the experimental program, an analytical model was developed to predict the ultimate moment capacity of the URM walls. The procedure outlined in standard guidelines was also utilized to compute the flexural capacity of walls. Besides the URM wall specimens tested in this study, another 42 test results of URM walls available in literature were used for analytical model validation. A design model was also proposed and new design aspects were introduced.

This paper presents the findings of a research study that was conducted to assess the effect of moderately elevated temperatures, up to 50°C, on the bond behavior of steel beams strengthened with externally bonded high-modulus carbon-fiber-reinforced polymer (CFRP) plates. In the first phase of the testing, seven steel-CFRP bonded double-lap shear coupons were tested at different temperatures to characterize the bond behavior. In the second phase of testing, steel beams were strengthened with different lengths of high-modulus CFRP plates and loaded under four-point bending at 25, 40, and 50°C. The test results indicate that the debonding load of bonded joints at 50°C is between 56 and 116% greater than the debonding load of similar joints at 25°C for the specific system tested in this study. This increase was attributed to an increase of the toughness of the tested adhesive at 50°C despite the lower tensile strength and stiffness of the material. The research findings suggest that the proposed strengthening system for steel beams is suitable for application at service temperatures up to 50°C.

The behavior of slender high-strength concrete columns reinforced with glass fiber-reinforced polymer (GFRP) bars and spirals subjected to concentric and eccentric axial loads was evaluated. Large-scale tests were conducted for nine circular concrete columns (three short and six slender) reinforced with internal GFRP-spirals and either steel, GFRP, or a combination of steel and GFRP longitudinal bars. The short and slender columns had slenderness ratios equal to 10 and 49, respectively. Axial load tests were conducted with loads placed concentrically (short columns) and at two eccentricities (slender columns) to observe the general behavior associated with different geometric and loading conditions. The behavior of slender columns with small eccentricity (8.3% of the column size) was governed by material failure, while that of slender columns with large eccentricity (33% of column size) was governed by a buckling failure. The research shows that GFRP spirals and GFRP longitudinal bars are a viable method of reinforcement for slender concrete columns subjected to eccentric axial loads.

This paper presents a probabilistic procedure for deriving design models for the ultimate strength and strain of fiber–reinforced-polymer (FRP)-confined concrete. First, a large database of axial compression tests performed on circular FRP-confined concrete specimens is collected for calibrating an ultimate strength model, based on the Drucker-Prager criterion, and an ultimate strain model, based on the ultimate dilation rate. The database is also employed for deriving a probabilistic model for the FRP strain efficiency factor. The calibrated models, though simple, show superior performance over some of the models in the literature. Then, using the Central Limit Theorem and considering uncertainty in the mechanical properties of the concrete and FRP material as well as their correlation, analytical probabilistic design models for the ultimate strength and strain of FRP-confined concrete are derived. These models can be used in the design and reliability analysis of FRP-confined columns.

This study evaluated the effectiveness of using externally bonded carbon fiber–reinforced polymer (CFRP) sheets to enhance the seismic performance of nonductile reinforced concrete shear wall specimens representative of low aspect ratio walls. The specimens were designed to replicate structural deficiencies typically found in older concrete structures constructed during the 1960s and 1970s. The nonductile reinforcement details included insufficient shear reinforcement, lack of boundary elements, and low concrete compressive strength. The specimens were repaired or strengthened using vertical and horizontal CFRP sheets and then cyclically tested to simulate seismic loading. A tube anchor system was used to transfer the tensile force carried by the CFRP sheet to the foundation of the specimens. Experimental results showed that the CFRP retrofitting system was able to restore the original strength and initial stiffness of severely damaged specimens and significantly increase the flexural strength, ductility, and energy dissipation capacity in strengthening applications.

This paper presents a modified damage-plasticity model for fiber-reinforced polymer (FRP)-confined normal-strength and high-strength concrete (NSC and HSC). The proposed model is based on a concrete damage-plasticity model from the literature, which is improved through accurate incorporation of the effects of the confinement level, concrete strength, and nonlinear dilation behavior of FRP-confined concrete. The proposed model uses a new and accurate failure surface and flow rule that were established using a comprehensive and up-to-date experimental test database and it incorporates an analytical rupture strain model for the FRP jacket. Finite-element (FE) models incorporating the proposed damage-plasticity model are developed and validated for concretes having up to 110-MPa compressive strength confined by different types of FRP under a wide range of confining pressures. Comparisons with experimental results show that the model’s predictions of (1) axial stress-axial strain, (2) lateral strain-axial strain, (3) axial stress-volumetric strain, (4) plastic volumetric strain-axial plastic strain, and (5) plastic dilation angle-axial plastic strain relations are in good agreement with the test results of FRP-confined NSC and HSC. The accurate predictions of the compressive strength and ultimate axial strain of FRP-confined concrete were achieved by establishing the hardening/softening rule and flow rule based on the level of confining pressure and modeling the failure surface of the confined concrete by incorporating the effect of unconfined concrete strength.

The bond between carbon-fiber-reinforced polymer (CFRP) and steel is crucial for strengthening of steel structures using CFRP materials. However, existing studies on this issue are still limited. To better understand the bond behavior of CFRP-to-steel bonded interfaces, this paper reports an experimental study on the behavior of CFRP plate-to-steel bonded joints with a nonlinear adhesive by the single-shear testing method. The three-dimensional digital image correlation (3D-DIC) technique was used to measure the displacements and strains of the specimens. The effects of the bond length and adhesive thickness on the bond behavior are evaluated. The results show that failure occurred within the adhesive layer (i.e., cohesive failure) for all tested specimens. The ultimate load increased with increasing bond length until the effective bond length was reached. The ultimate load also increased as the adhesive thickness was increased from 0.5 to 2.0 mm. The bond-slip relationship exhibited an approximate trapezoidal shape for such bonded joints. The key parameters of the trapezoidal bond-slip relationship were obtained for specimens with different adhesive thicknesses. This study indicates that the 3D-DIC technique is suitable for application in studies of the interfacial behavior between CFRP plate and steel.

In the last decade, the shear strength of concrete members with rectangular cross sections reinforced with fiber-reinforced polymers (FRPs) has received considerable attention. Yet no research seems to have investigated circular concrete members reinforced with FRP reinforcement under shear loads. This paper presents the results of an investigation of the shear strength and behavior of six circular concrete specimens reinforced with glass-FRP (GFRP) bars and spirals. The specimens, which measured 3,000 mm in length by 500 mm in diameter, were tested under four-point bending. The test parameters included the shear span-to-depth ratio ($a/d$) ranging from 1.5 to 3.0 and the GFRP spiral reinforcement ratio with different spiral spacings (100, 150, and 200 mm) and spiral diameters (13 and 15 mm). As designed, the specimens failed in shear due to GFRP spiral rupture or flexural-shear failure for the specimens with $a/d>2.5$ and strut crushing combined with spiral rupture for the specimens with $a/d<2.5$. The experimental results were compared to the current sectional models and the strut-and-tie model in codes and design guidelines as well as to the available analytical approach, which is based on the modified compression field theory. The comparison indicates that the shear capacity of FRP-reinforced concrete members with circular cross sections may be determined with the shear design provisions developed for rectangular sections within a variable degree of conservativeness.

This paper presents results of finite element (FE) modeling and experimental testing of reinforced concrete beams strengthened in shear with fabric-reinforced cementitious matrix (FRCM). The studied parameters included the number of FRCM layers (one and two layers), matrix type (mortar and epoxy), and amount of internal stirrups (no stirrups and stirrups with spacings of $0.6d$ and $0.3d$, where $d$ is the depth of the tensile steel). Test results showed that the shear-strength gain after strengthening was in the range of 51–145%. The shear-strength gain decreased with an increase in the amount of internal stirrups. Doubling the number of FRCM layers resulted in a nonproportional increase in the shear strength. The use of epoxy adhesive rather than a cementitious mortar as a matrix insignificantly increased the shear-strength gain. The effect of increasing the amount of FRCM or varying the matrix type on the shear strength was less pronounced for the specimens with internal stirrups. The FE models developed in this study were capable of predicting the nonlinear shear response of the tested specimens. A comparison between predicted and experimental results confirmed the accuracy and validity of the developed FE models.