Journal of Composites for Construction

The residual flexural behavior of reinforced concrete (RC) beams strengthened with a fabric-reinforced cementitious matrix (FRCM) composite system after exposure to elevated temperatures was analyzed and discussed in the paper. Ten RC beams, two unstrengthened and eight strengthened with a polypara-phenylene-benzo-bisthiazole (PBO) FRCM system, were tested at ambient temperature (20°C) under three-point bending after being exposed to temperatures of 100°C, 200°C, and 300°C. Test results were analyzed in terms of failure modes, failure loads, load–deflection curves, strain, stress distributions, and ductility. The obtained results evidenced that the load-bearing capacity of the strengthened beams remained roughly constant for temperatures ranging from 20°C to 200°C (from 13% to 23% higher than that of the unstrengthened beams). On the other hand, the collapse of PBO FRCM-strengthened beams exposed to a temperature of 300°C occurred for load values that were, on average, 12% lower than those of strengthened beams at room temperature. The effects of exposure to elevated temperatures on the PBO FRCM-to-concrete debonding were also discussed. A semiempirical model based on the experimental results was defined to estimate the residual debonding strain of the composite system as a function of temperature. The accuracy of the proposed model as well as those of some different analytical procedures available in the literature was assessed through a comparison with experimental results.

Concrete box girders are significant structural elements in various engineering applications due to higher torsional stiffness and low self-weight. The use of glass fiber–reinforced polymers (GFRPs) as internal bars has been adopted for many structural applications. So far, the torsional behavior of reinforced concrete (RC) box girders reinforced with GFRP bars has not been addressed. Therefore, this paper presents experimental data on the torsional behavior of RC box girders reinforced with GFRP bars and stirrups. Six box girders measuring 4,000 mm in length, 380 mm in height, and 380 mm in width, with a wall thickness of 100 mm, were tested under pure torsional moment over a clear span of 2,000 mm. The test parameters included the type and amount of torsional reinforcement. The test specimens comprised four box girders entirely reinforced with GFRP reinforcement, one box girder with only longitudinal GFRP bars, and one box girder reinforced with steel reinforcement as a reference specimen. The test results indicate that the torsional strength of the steel and GFRP box girders was significantly affected by stirrup capacity; stirrup stiffness did not affect the torsional strength. Increasing the web’s GFRP reinforcement ratio increased the torsional strength and stiffness. An analytical iterative softened membrane model for torsion (SMMT) was modified to predict the entire torsional behavior of the GFRP-reinforced concrete box girders. The model was validated by comparing the analytical results to the experimental results of the four GFRP-reinforced concrete box girders. The comparison indicates that the model was able to predict the cracking and ultimate torsional strength and the corresponding twist with reasonable agreement. In addition, a noniterative Rahal model was modified to predict the ultimate torsional strength of FRP-reinforced concrete members. At last, the experimental ultimate torsional strength was assessed with the corresponding value calculated according to available fiber-reinforced polymer design codes and guides.

Even though several stress–strain models have been proposed for fiber-reinforced polymer (FRP)-confined concrete columns subjected to axial compressive loading, very few models can predict an axial response featuring postpeak strain-softening behavior. Furthermore, the reliability of most of these models is limited to only a certain concrete strength class (either normal-, high-, or ultrahigh-strength concrete). This study aimed to develop an analytical model for determining the axial response of FRP-confined concrete applicable to cases with different levels of confinement stiffness and concrete strength. For this purpose, this research proposed a new confinement stiffness threshold dependent on the coupled concrete strength and column dimension size effects to classify quantitatively FRP-confined concrete’s behavior in two distinguished subcategories: strain-hardening behavior and postpeak strain-softening behavior. For FRP-confined concrete with strain-hardening response, a parabolic–linear stress–strain relation was developed, where a new formulation was derived for the slope of the linear second portion, calibrated by 583 test data. To simulate FRP-confined concrete with postpeak strain-softening behavior, a new methodology was proposed whose key components were calibrated by using 121 test data. With these features, the proposed model can objectively account for the integrated influence of concrete strength and confinement stiffness on stress–strain response. The predictive performance of the developed stress–strain model was evaluated by comparing the predictions of a wide range of relevant experimental test data, which confirms the model’s reliability and accuracy. Compared to the other existing stress–strain models, the proposed model performed better.

Single-kenaf fiber-reinforced polymer composites are typically characterized by relatively low strength and stiffness properties that make them unsuitable for structural applications. However, they are lightweight, economical, and ecofriendly. This paper presents a study on the manufacturing and mechanical characterization of bidirectional kenaf (K) fiber-reinforced epoxy composites hybridized with carbon (C) fibers in various stacking sequences and the effects of hybridization on salient physical and mechanical properties. Single and hybrid fiber composites were fabricated utilizing the vacuum infusion molding technique. The density, tensile, flexural, and interlaminar shear properties in hybrid composites increased significantly when carbon fiber volume increased from 9% to 16%. Stacking sequences in a hybrid affected the mechanical properties of the composites. The highest tensile strength and modulus were shown by the seven-layer hybrid composite with an alternate K/C stacking sequence and C layers as skin layers, i.e., C/K/C/K/C/K/C, among all tested hybrid composites. Sandwich design in the hybrid (C₂/K₃/C₂) had higher flexural strength (+300%), flexural modulus (+414%), interlaminar shear strength (+278%), lower water absorption (−46%), and thickness swelling (−30%) compared to single-fiber kenaf/epoxy composites. Density increased by 5% in hybrid composites. The highest fracture toughness (+134%) was achieved using the dual sandwich design structure hybrid (C/K₂/C₂/K₂/C). The developed composite has applications in stairways, walkways, and bridges.

Reinforced concrete (RC) corbels are commonly utilized in bridges and industrial buildings to support primary beams and girders. Using glass fiber–reinforced polymer (GFRP) reinforcement in corbels can be advantageous due to its corrosion-resistance properties. However, GFRP reinforcement, with a lower modulus of elasticity and shear strength than steel, could affect the capacity of direct shear. This paper presents the experimental results of nine full-scale, double-sided corbels reinforced with either GFRP or steel bent bars. Large-scale double-sided corbels were constructed and tested for failure under monotonic concentric loads. The test parameters included the reinforcement type (GFRP and steel), the main reinforcement ratio (0.5% and 0.7%), the shear span-to-depth ratio (a/d = 0.33 and 0.66), and the amount of crack-control horizontal reinforcement (0.7% and 1.3%). The predictions of corbel capacity using the Canadian standards for FRP-RC structures were conservative, especially for the corbels with crack-control reinforcement. In contrast, the predictions of the American and European codes overestimated the corbel strength, particularly for the higher a/d ratio of 0.66.

The open mesh textile reinforcements used today in textile-reinforced mortar (TRM) strengthening applications are composed of identical fiber rovings in both orthogonal directions, regardless of the type of application. However, for reinforced concrete (RC) members, such as beams and columns, where fibers are utilized in one main direction, such textile geometry can lead to ineffective and excessive use of raw materials and, therefore, uneconomic design. This paper experimentally investigates the use of hybrid textile reinforcement for flexural and shear strengthening in 22 RC beams. Four types of hybrid textiles were designed and manufactured to allow for the environmentally friendly and cost-efficient use of the fibers. Expensive high-strength carbon and weaker glass fibers were used in the main and secondary directions, respectively. Hybrid carbon–glass textiles, which are composed of weaker glass fiber rovings with larger spacings in the secondary direction, showed similar effectiveness with the standard carbon textiles with 50%–50% weight distribution per direction. A cost estimation revealed that massive savings on raw materials can be achieved when hybrid layouts are used instead of conventional textiles. Therefore, the results of this paper could provide a new scope for the optimization of current TRM systems for various retrofitting applications and might assist in the development of a more economical, environmentally friendly, and balanced design of textile reinforcements.

The external bonding of fiber-reinforced polymer (FRP) sheets is a widely used method for strengthening and rehabilitating existing concrete structures. The premature failure of these structures is frequently attributed to debonding at the FRP–concrete interface, highlighting the need for interface studies. In this study, the interface properties of carbon fiber–reinforced polymer (CFRP) and steel fiber–reinforced concrete (SFRC) were investigated. A series of double-shear tests was designed with variations in the initial angle between the CFRP sheet and SFRC to reflect different mixed-mode scenarios. A streamlined cohesive zone model (CZM) was proposed for both the CFRP–SFRC interface slip and steel fiber slip. The complete process of CFRP sheet delamination from SFRC under mixed-mode conditions was simulated using a finite-element software. The simulated results were in good agreement with the experimental results.

The flexural and durability performance of reinforced ultrahigh-performance fiber-reinforced concrete (UHPFRC) beam structures is expected to improve if the steel bars are replaced with carbon fiber–reinforced polymer (CFRP) bars with high tensile strength and corrosion resistance. Nevertheless, the interfacial bond characteristics between CFRP bars and UHPFRC may deteriorate when subjected to elevated ambient temperatures during service. To investigate the bond–slip behavior of CFRP bars embedded in UHPFRC at elevated ambient temperature, a series of direct pullout tests were conducted under elevated ambient temperatures ranging from 20°C to 80°C. The testing used CFRP bars with two diameters of 10 and 12 mm. Based on the test results, two temperature-dependent bond–slip models (T-MBPE and T-MCMR-SC models) and a temperature-dependent bond strength model were developed. The results indicated that the bond strength decreases dramatically as the ambient temperature increases. The bond strengths of the specimens with 12-mm-diameter CFRP bars are higher than those with 10-mm-diameter CFRP bars, which is mainly due to the greater rib height of the 12-mm-diameter CFRP bars. Compared with the proposed T-MBPE model, the constructed bond–slip relationships of the proposed T-MCMR-SC model were more consistent with the experimental results. The proposed temperature-dependent bond strength model performed well in characterizing the bond degradation under elevated ambient temperatures, with a prediction error of less than 10% compared to the experimental results. The research outcomes can provide an experimental and constitutive basis for using CFRP bars in UHPFRC structures at elevated temperatures.

Using geopolymeric recycled aggregate concrete (GRAC) rather than conventional concrete to fill glass fiber–reinforced polymer (GFRP) tubes to produce high-performance structural members is a promising green construction technique. In this article, an experimental study investigating the impact behavior of GFRP tube-confined GRAC under axial impact loadings is presented. Moreover, split Hopkinson pressure bar (SHPB) impact tests were conducted, with several variable parameters: (1) confinement ratio (0, 6-ply, and 8-ply GFRP tubes); (2) recycled aggregate (RA) replacement ratio (0%, 50%, and 100%); and (3) strain rate (ranging from 29.4 to 263.3 s⁻¹). In addition, the experimental results of the impact behavior of GFRP-confined GRAC were compared with those of confined ordinary portland cement (OPC) concrete. The test results showed that the confinement of the GFRP tube can remarkably improve the impact resistance of GRAC. The dynamic increase factor (DIF) of confined GRAC was higher than that of confined OPC-based concrete. Similar to OPC-based concrete, the incorporation of RA negatively influenced the impact properties of GRAC, but these influences could be alleviated by GFRP tube confinement. The compressive properties of GFRP-confined GRAC exhibited a strong strain-rate dependency, while the DIF slightly decreased with an increasing confinement ratio. Finally, a model was developed to predict the dynamic strength of GFRP-confined GRAC by reasonably considering the coupling effects of the RA replacement ratio and confinement ratio.

The recently published ACI 440.11-22 code permits the use of glass fiber–reinforced polymer (GFRP) bars as compression reinforcement in concrete columns subjected to combined axial and flexural loads. However, owing to a lack of experimental investigations, there are no current codes specifically for basalt fiber–reinforced polymer (BFRP) reinforced concrete structures. In this study, the behavior of short and slender concrete columns under axial and flexural loads was investigated. Twelve concrete columns were constructed and tested. Test variables were column slenderness ratio (short versus slender columns), eccentricity-to-depth ratio, and reinforcement type (BFRP versus steel). The test results showed the effectiveness of BFRP bars as internal reinforcement in short and slender concrete columns subjected to concentric and eccentric loadings. To generate axial load–moment interaction diagrams, the columns were tested under four different levels of load eccentricity-to-depth ratios of 0, 22.2%, 44.4%, and 66.7%. Regardless of the reinforcement type or slenderness ratio, columns tested under concentric or small eccentricity loadings exhibited a brittle compression-controlled material failure mode, while that tested under high eccentricity loads showed a tension-controlled failure mode. The developed second-order moment is higher in BFRP RC columns than in steel RC columns, this can be attributed to the lower modulus of elasticity of BFRP bars. Subsequently, the research program was expanded to develop an analytical model considering the second-order effects, and then the predicted axial load–moment interaction diagrams by the developed model were verified against experimental test results in the current study and available tested columns in the literature, the verification proved the accuracy of the new model. The findings of this study are an important step toward establishing code guidelines for employing BFRP bars and ties as internal reinforcement in concrete columns.

Different rehabilitation techniques have been utilized to extend the service life of wooden utility poles, which are mainly affected by degradation and decay. This paper presents an evaluation of the performance of wooden utility poles rehabilitated using sprayed glass fiber–reinforced polymer (GFRP) composites and near-surface-mounted (NSM)-GFRP bars. Seven full-size (305-mm diameter) new wooden poles and five full-size old wooden poles, taken out of service, were tested under monotonically increasing lateral load. The test parameters included the thickness (4, 6, and 8 mm) and length (1.0 and 2.0 m) of the sprayed-GFRP coating, and rehabilitation methods (sprayed-GFRP composites, NSM-GFRP bars). The results showed that the sprayed-GFRP coating can restore the load-carrying capacity and enhance the stiffness of both old and damaged poles. In addition, the load-carrying capacity of the wooden poles was not affected by the increase in GFRP thickness after the thickness reached 6 mm. It was also concluded that using the NSM-GFRP bars is not cost-effective compared to the sprayed-GFRP composites. A simple analytical procedure was introduced to estimate the load-carrying capacity of retrofitted poles and to calculate the required thickness of the sprayed-FRP layer, which yielded reasonably conservative results.

Prestressing tendons made of fiber-reinforced polymers (FRPs) are a promising alternative to conventional steel tendons in prestressed concrete structures owing to their corrosion resistance. However, the shear strength of FRP prestressed concrete beams is still not well understood. This paper presents a theoretical model for predicting the shear strength of prestressed concrete beams with FRP tendons or strands, with and without FRP shear reinforcement. The model is an extension of the compression chord capacity model (CCCM), originally proposed for steel-reinforced concrete structures, which has been adapted to account for the particularities of FRP as active and passive reinforcement. The model is applicable to rectangular, T, and I sections and accounts for reductions in shear strength caused by bond loss in FRP tendons. Experimental validation of the model was performed by comparing the theoretical predictions for 55 shear tests found in the literature. Good accuracy was obtained in predicting the ultimate shear capacity of beams and identifying shear bond failures observed in some tests.

Reinforced concrete (RC) columns are prone to shear failure under seismic loads, particularly in the case of short columns. However, this brittle failure may have more adverse effects on large-sized columns. To address this issue, a common solution is to retrofit RC columns using fiber-reinforced polymer (FRP) laminates to improve seismic performance. The primary objective of this study is to assess the shear performance and size effect of rectangular RC columns confined using carbon fiber–reinforced polymer (CFRP). To account for the heterogeneity of concrete, a mesoscale numerical approach is developed using a random aggregate model. The investigation focuses on impacts of the axial compression ratio and CFRP volumetric ratio. Results show that the cross-sectional size has little effect on the final failure mode of the columns. However, the width of the main diagonal crack is reduced as the cross-sectional height increases. It can be observed that the size effect on the CFRP rupture hoop strain distribution is primarily manifested in the strain value rather than the shape of the distribution. The size effect is evident in the total shear strength of columns and in the CFRP shear contribution, with the effect becoming more pronounced with increasing axial compression ratio. Taking into account the influences of size on the CFRP shear contribution and shear strength of RC columns as well as the CFRP confinement effect on concrete, the calculation model of shear capacity is established based on a new proposed FRP effective strain model available for rectangular columns. The proposed model can provide a more accurate prediction on shear capacity of rectangular CFRP-confined RC columns compared with existing design codes.

This paper presents experimental and numerical studies on sandwich panels with flax fiber–reinforced polymer (FFRP) faces and polyisocyanurate (PIR) foam cores. The panels are subjected to two-way bending under impact loads at the center, simulating applications like building cladding systems exposed to wind-borne debris. Nine large-scale panels were fabricated and subjected to impact loads; 412 tests were performed. Each panel was 1,220 mm × 1,220 mm with a nominal thickness of 80 mm. The main test parameters were core-to-face thickness ratio based on one, two, or three FFRP layers (core-to-face thickness ratios of 65.1, 32.6, and 21.7) and impact energy (50%, 70%, and 95% of failure energy). For each face thickness, three identical panels were fabricated and tested. The impact tests were performed using a 140-mm-diameter drop weight ranging from 10.5 to 20 kg, with a varying height of up to 3,250 mm. The results showed that the panels are susceptible to internal damage accumulating after impacts, such as core shear failure. Analyses of the test data showed that the impulse duration of a panel increased with an increase of damage. A finite-element model was also developed to predict the behavior of these panels under low-energy impacts. The model accounted for the nonlinear behavior of both the FFRP faces and foam cores. The model was used to perform a parametric study to examine the effect of core thickness, face thickness, and core density. It showed that impulse duration and maximum deflection increased with a decrease in face thickness, core density, and core thickness.