Journal of Composites for Construction

The use of curvilinear glass fiber‒reinforced polymer (GFRP) bars as the longitudinal reinforcement in curved reinforced concrete (RC) elements has generated significant interest. Even so, there is a lack of comprehensive data on the long-term durability of nonpultruded curvilinear GFRP bars in an alkaline environment. This study represents a pioneering effort to assess the long-term durability of nonpultruded curvilinear GFRP reinforcement, as well as its physical and mechanical properties, to address this research gap. Over 400 bar samples—including #5 curvilinear and straight GFRP bars—were prepared and tested. The physical properties were evaluated in terms of fiber content, cross-sectional area, glass-transition temperature, cure ratio, and water absorption. Tensile, transverse shear, and interlaminar shear tests were performed to assess the mechanical properties. The durability properties were investigated by submerging the samples in an alkaline solution at four different temperatures (24°C, 40°C, 50°C, and 60°C) for three different exposure durations (1,000, 3,000, and 6,000 h). Morphological studies were conducted with scanning electronic microscopy (SEM); elemental analysis was determined with energy-dispersive X-ray spectroscopy (EDS). Additionally, the bars underwent thermal analysis with differential scanning calorimetry (DSC) and chemical analysis with Fourier transform infrared spectroscopy (FTIR). Then, a long-term prediction model was developed to predict the service-life durability of the curvilinear GFRP bars in an alkaline environment. The experimental results indicate that the physical, mechanical, and durability properties of the nonpultruded curvilinear GFRP bars met all the requirements for use as reinforcing bars. In addition, the SEM, EDS, DSC, and FTIR analyses revealed that the curvilinear GFRP bars were highly durable in the alkaline environment. Furthermore, according to the long-term prediction model developed, the tensile strength retention of the curvilinear GFRP bars after 100 years of service life in an alkaline environment was >77%.

Current tunnel design provisions do not consider the shear resistance of transverse reinforcement in precast concrete tunnel lining (PCTL) segments. Such reinforcement is often designed as a minimum for shrinkage and temperature resistance. This study evaluated the behavior and shear strength of PCTL segments reinforced with glass fiber‒reinforced polymer (GFRP) bars with and without closed shear ties. A total of five full-scale rhomboidal PCTL specimens with a 1,500 × 250 mm rectangular cross section and an arched length of 2,100 mm were constructed and tested under three-point loading up to failure. The testing parameters included the configuration of the transverse reinforcement (bars versus closed ties), the spacing of closed shear ties, the longitudinal reinforcement ratio, and concrete strength. The load–deflection behavior, cracking, failure mechanisms, shear capacities, and strain in the reinforcement and concrete are discussed herein. The results indicate that all specimens experienced shear failure because of the diagonal tension failure or shear compression mode. The presence of shear stirrups enhanced the structural performance of the GFRP-reinforced PCTL segments. In addition, increasing the closed-tie ratio, longitudinal reinforcement ratio, and concrete strength increased the shear strength of the segments. A comparison between the test results and predictions made with the critical shear crack theory (CSCT) was introduced in the theoretical approach to evaluate the applicability of existing FRP design code provisions. The predictions produced according to FRP design codes can be classified between conservative and nonconservative predictions. The modified CSCT yielded predictions closest to the experimental values with an accuracy of 98%.

Externally bonded fiber-reinforced polymer (EB-FRP) laminates have become popular for strengthening existing reinforced concrete (RC) structures. However, the high tensile strength of the FRP laminate is often not fully utilized due to premature debonding failure of the FRP-to-concrete interface, typically occurring in a thin layer beneath the bond interface. Numerical simulations have gained significant attention as a supplement to experimental tests, as they have the ability to provide valuable insights into the debonding process. However, most existing numerical models for EB-FRP joint debonding are unable to explicitly consider cracks within different concrete phases [i.e., mortar and interfacial transition (ITZ)], or precisely capture the corresponding failure mechanisms involving mortar cracking, ITZ debonding, and kinking. This study proposes a novel mesoscale phase field model for concrete, which is capable of accurately modeling complex failure behaviors, including mixed-mode fracture in both the mortar and ITZ, as well as friction on cracked surfaces. The ITZ is regularized using an auxiliary interface phase field and then the overall mixed-mode failure behaviors in both the mortar and ITZ are modeled using a unified damage phase field. To validate the proposed mesoscale model, three pull-off tests of FRP-to-concrete bonded joints, which have been well reported in the existing literature, are simulated. Moreover, the model is used to investigate the effects of adhesion and the FRP laminate on the debonding behavior of the FRP-to-concrete joints.

Abstract

Reinforced concrete (RC) structural members, such as bridge girders and columns, flanged and spandrel beams, and edge frame beams, may be subjected to combined actions in the general case of loading. Despite extensive research on the behavior of RC members strengthened with fiber-reinforced polymer (FRP) composites under pure actions, questions have been raised about the strengthened RC members subjected to combined actions, particularly actions including torsion. As a contribution to this demanding field of research, the first theoretical model capable of predicting the full load–deformation response of RC members strengthened with conventional FRP-strengthening configurations, and subjected to torsion combined with biaxial bending, was developed based on the principles of the well-known combined-actions softened truss model (CA-STM). The proposed model considered the influence of various parameters, such as FRP stiffness, on the constitutive laws of materials. In addition, the computational operations and duration were significantly reduced, and the numerical stability of the solution procedure was enhanced by extending the refined efficient solution algorithm for FRP-strengthened RC members for the first time. Furthermore, the efficiency and contribution of the FRP system and the governing failure mode was accurately predicted. Three categories of equations, including equilibrium equations, compatibility equations, and the constitutive relationships of materials, were incorporated into the proposed model, then a system of 16 nonlinear equations, containing 16 unknown variables, was solved to obtain the full load–deformation response of an FRP-strengthened member subjected to combined actions, including torsion. A detailed comparison conducted between the test results of a database with various strengthening patterns collected from the literature and the predictions of the proposed model showed that the model was able to estimate the ultimate strength, with a mean value and coefficient of variation of 1.11% and 13%, respectively, in addition to predicting the overall load–displacement response well.

Practical Applications

This research highlights the importance of considering the combined effects of bending and torsion on the capacity of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) members, particularly in members where torsion is critical. The addition of flexural moment in certain situations can lead to a reduction in the torsional capacity of the member’s cross-section. While in specific situations, where the strengthening system is asymmetrically applied or the internal steel arrangement of the section is asymmetrical, the torsional capacity can even increase compared to the case when subjected to pure torsion. Therefore, the arrangement of the strengthening system plays a crucial role in determining the failure mode and strength of the member. The model proposed in this study, along with its resulting findings, will provide valuable assistance to engineers in determining the optimal percentage of an FRP system and its pattern in these situations. This will allow them to achieve the highest possible effectiveness of the strengthening system in cases involving combined actions, including torsion and flexure.

With the increasing scarcity of natural gravel resources and increased production of construction and demolition waste, recycled aggregates offer prospects for concrete structures. However, the adhered loose and porous mortar and microcracks in recycled aggregates led to impaired mechanical behavior of recycled aggregate concrete (RAC) and reduced durability in harsh environments, significantly restricting its engineering application. Compression-casting can solve the problems of low strength and poor durability of RAC; however, it induces significant brittleness of concrete in compression. This study proposed an improvement in the ductility of compression-cast RAC through confinement using fiber-reinforced polymer (FRP) laminates. A total of 32 FRP-confined compression-cast cylinder specimens were tested under uniaxial compression, with variables including concrete mixture proportion, FRP thickness, and concrete casting method. The experimental results showed that the ultimate axial stresses of the unconfined RAC and FRP-confined RAC increased by up to 34.8% and 14.5%, respectively, and the ultimate axial strain decreased by up to 49.0% with compression-casting. Furthermore, the compression-casting method resulted in an 8.5% reduction in the measured average rupture strain of the FRP. The confinement efficiency was not significantly affected by compression-casting, as indicated by a slightly greater slope of the strength enhancement ratio with an increase in the confinement ratio for FRP-confined compression-cast RAC. Stress–strain models of FRP-confined compression-cast and normal RAC were proposed based on test data from this study and the literature. It was found that the proposed model improved the prediction accuracy of the stress–strain behavior compared with existing models.

Steel reinforcement is widely used in reinforced concrete (RC) constructions. However, the durability of the steel RC elements is often compromised due to corrosion under adverse environmental conditions. Glass fiber‒reinforced polymer (GFRP) bars have become an attractive alternative for replacing steel bars in several RC applications due to their excellent durability properties. However, the GFRP RC columns exhibit lesser stiffness and fail in a brittle manner compared to steel RC columns. Also, the second-order moments and deformations can increase in the GFRP RC columns due to the low elastic modulus of GFRP bars. The pseudoductility and postcracking behavior of GFRP RC columns can be improved by adding discrete fibers. In this study, 12 steel and GFRP RC slender columns are tested under eccentric compression (e/h ratio of 0.5), and the results are presented. The test matrix consists of (1) steel RC columns without fibers; (2) GFRP RC columns without fibers; and (3) GFRP RC columns with four different fiber dosages of (1) 0.50% and 1.0% of macrosynthetic polyolefin (PO); and (2) 0.5% and 1.0% of hybrid (HB) combination of PO and steel fibers. The test results are compared with analytical predictions considering the geometrical and material nonlinearities. The test results show that adding HB fibers resulted in higher strength and pseudoductile failure than columns reinforced with only PO fibers. HB fibers significantly increased flexural rigidity and the ultimate strength of slender GFRP RC columns.

The use of carbon fiber–reinforced polymer (CFRP) grid-reinforced engineered cementitious composite (CFGRE) systems to strengthen concrete-filled steel tubes (CFSTs) offers potential to improve the structural response as well as the resistance to high temperatures and corrosion. This study consists of an experimental investigation on the behavior of eccentrically loaded square CFSTs strengthened with CFGRE systems. Nineteen specimens were tested using a compression-testing machine with loads, deformations, and recorded test observations. The investigated parameters included the number of CFRP grid layers, width-to-thickness ratio of a steel tube, concrete strength grade, and eccentric ratio. The strengthened columns exhibited ductile failure. Due to stress concentrations, the CFGRE systems ruptured at the corners on the compression side. After strengthening, the yielding and ultimate loads of the CFST columns increased from 21.8% to 53.5% and 21.1% to 34.9%, respectively. Although an increase in the number of CFRP grid layers did not significantly affect the bearing capacities of the square specimens, it significantly enhanced the ductility of the columns. An increase in the eccentric ratio weakened the confining effect. An average decrease of 9.7%, 27.6%, and 41.4% was observed in the ultimate loads of the strengthened columns when the eccentric ratio increased from 0 to 0.1, 0.25, and 0.4, respectively. A five-point N–M interaction model was developed based on the principles of force equilibrium and deformation compatibility to predict the eccentric bearing capacities of strengthened columns. The prediction results exhibited high accuracy.

Strengthening using large-rupture-strain fiber-reinforced polymer (LRS-FRP) laminates can effectively enhance the deformation and energy absorption capacity of reinforced concrete (RC) components under static and seismic loads. This study explores the application of LRS-FRP, particularly polyethylene terephthalate FRP (PET-FRP), to increase the impact resistance of RC columns in instances wherein high deformability and energy absorption capacity are necessary. Six RC column specimens, preloaded axially with heavy blocks and laterally impacted using a pendulum apparatus, were wrapped with PET-FRP. The dynamic responses of the columns, including the impact force, axial force, and lateral and axial displacements, were carefully recorded. Local strengthening schemes applied at the impact points of the columns and ends effectively enhanced their anti-impact performance without transferring damage to unstrengthened areas. As regards middle-impact specimens, PET-FRP wrapping changed the failure mode from shear to flexure failure, thereby increasing the lateral-impact resistance and deformation recovery capacity and preventing collapse owing to the axial bearing capacity loss. In bottom-impact specimens, PET-FRP improved ductility in the shear failure mode and maintained a consistent trend in the damage degree of flexure and shear failure columns across varying axial compressive ratios. Within an axial compression ratio of < 0.32, the axial force reduced flexural and shear damages. However, excessive axial preloading (axial compression ratio = 0.64) increased the flexural deformation because of the P-delta effect and shear damage owing to the increased risk of FRP debonding.

Glass fiber–reinforced polymer (GFRP) composites have been used in civil construction because of their high strength, lightweight, and corrosion resistance. However, their thermal sensitivity due to the polymer resin requires further understanding of the fire performance of these structures, because the heat from the fire could cause glass transition and decomposition of GFRP composites and degrade their mechanical properties. Fire dynamic simulations were conducted in this study to investigate heat propagation, which considered airflow aerodynamics, in typical single-story residential houses with walls and roofs in the form of GFRP web–flange sandwich structures. The temperature progressions and gradients in the GFRP wall and roof members were quantified, which allowed for the identification and further understanding of the locations and development of glass transition and decomposition in the GFRP composites during fire exposure. The effects of the building floor layouts, fire source locations, additional fire resistance gypsum plasterboard, and a fresh air supply were further discussed and clarified.

Glass fiber–reinforced polymer (GFRP) composites are widely used materials in construction. Their use in structural applications, especially in civil infrastructure, requires structural analysis and design of their connections with proper safety and reliability levels. This work implements complex machine learning tools and schemes on a large amount of pin-bearing strength data on pultruded GFRP materials from the literature to extract correlations between various input parameters that affect the resulting bearing strength. This is performed to bridge the gaps in the findings that are associated with these data and to verify the recommended formulas in the corresponding design manuals and codes that are used for these materials in construction. The results of the derived gradient boosting model (GBM) could be used to predict the bearing strength of any pultruded composite without implementing experimental testing, therefore reducing operational costs and time. The derived model was exploited to investigate the effect of the hole diameter, the angle between the applied load and the pultrusion direction, and the effect of service temperature on the bearing strength of pultruded composites. The results imply the need to modify the used formulas in design standards; therefore, the proposed modifications are highlighted to properly predict the bearing strength for composite materials to be used in construction.

An experimental campaign was undertaken to characterize the mechanical properties of newly proposed aluminum alloy ribs (ARs)-anchored glass fiber–reinforced polymer (GFRP) bars via tensile, short beam shear and pullout tests. The investigated variables consisted of bar surface treatments (smooth, shallow-ribbed, and deep-ribbed) and anchorage design parameters (radial extrusion and initial wall thickness). The results derived from the tensile test showed that the tensile strength of the ARs-anchored GFRP bars could be designed and optimized by adjusting the radial extrusion and initial wall thickness of the anchorage. The short beam shear test further validated that the presence of the ARs on the GFRP bar increased its interlaminar shear strength by up to 25%. The pullout test conducted on specimens with favorable anchorage design parameters based on tensile and short beam shear tests demonstrated significantly high bond strengths between 11.45 and 46.63 MPa at the bar–anchorage interface, with enhancements ranging from 60% to several times compared with that of the bar–concrete interface under different parameter combinations. The proposed ARs-anchored GFRP bars present an effective alternative to conventional GFRP bars in applications where a short anchorage length is preferred.

Hybrid double-skin tubular columns (DSTCs), which consist of an outer fiber-reinforced polymer (FRP) tube, an inner steel tube, and a concrete annular layer between the two tubes, possess many advantages, such as high load resistance and large deformation capacity and excellent durability. The use of ultrahigh-performance concrete (UHPC) or engineered cementitious composites (ECCs) instead of normal strength concrete (NSC) in the annular layer, given the advantages such as the ultrahigh strength of UHPC and strain hardening and high toughness feature of ECC, endows the DSTCs with additional high performance, such as an enhanced load-carrying capacity (with UHPC) and an improved cracking resistance (with ECC). The behavior of such DSTC specimens with UHPC or ECC that are subjected to eccentric loading was investigated for the first time, to the best of the authors’ knowledge, and the results are reported in this paper. The DSTC specimens with NSC and ECC under eccentric compression exhibited similar great ductility with a bilinear shape for the axial load–deformation curves. In addition, the initial cracking of the FRP tube at the tension side of DSTC specimens with ECC or NSC occurred later than that of DSTC specimens with UHPC, which might imply the better durability performance of DSTC specimens with ECC or NSC during the serviceability state.