Journal of Composites for Construction

The use of glass fiber–reinforced polymer (GFRP) bars in reinforced concrete (RC) structures has grown significantly in recent years, most notably in bridge deck applications, as they provide an innovative solution to address stringent durability requirements in severe environmental conditions. However, concerns regarding the degradation of mechanical properties of GFRP bars and their bond to concrete at elevated temperature, as well as a lack of fire design guidance, remain obstacles to their widespread use in buildings. This paper presents a review of the available literature regarding the fire performance of GFRP-RC structural members, as well as a summary of relevant research needs. First, it addresses the effects of elevated temperature on the thermal and mechanical properties of GFRP bars. Second, experimental and numerical studies of GFRP bars’ bond behavior at elevated temperature, and of the fire performance of GFRP-RC beams and slabs, are discussed. Finally, the available fire design guidance is discussed, and recommendations for future research are given.

Due to the ductile properties of large-rupture-strain fiber-reinforced polymer (LRS-FRP) laminates and their confined concrete, the failure patterns and ultimate deformation capacities of LRS-FRP-strengthened reinforced concrete (RC) beams were considerably different from those of their carbon FRP (CFRP)-strengthened counterparts under static and impact conditions. This study investigates the dynamic behaviors of LRS-FRP-strengthened beams under impact using high-accuracy numerical models and simplified analytical approaches. First, high-fidelity finite-element (FE) models were created to evaluate the static and impact responses of LRS-FRP-strengthened RC beams with varied failure patterns recorded in prior experiments. Subsequently, based on the validated FE model, the effects of critical variables, such as impact energy, longitudinal FRP strengthening ratio, wrapping length, and types of FRP, on the dynamic response were investigated. In addition, a simplified design model for a rapid evaluation of the performance and safety conditions of FRP-strengthened RC beams under impact was presented, which considered the energy conservation during the impact process, material strain rates, and influences of failure patterns. Extensive data from the static and impact tests with various failure patterns were used to verify the accuracy of the model. Finally, the simplified model yielded an impact mass/impact velocity (m–v) diagram, demonstrating that LRS-FRP-strengthened RC beams outperform their CFRP counterparts in terms of anti-impact performance by obtaining a larger ultimate deformation capacity and greater safety margin.

The flexural performance of reinforced concrete (RC) beams could be effectively improved by applying a near-surface mounted (NSM) fiber-reinforced polymer (FRP) at the beam soffit. However, such NSM FRP flexurally-strengthened beams frequently failed due to FRP debonding, which limited the full utilization of the FRP strength. In some experimental studies, FRP U-jackets have been used as the anchorage to mitigate or prevent debonding failures in NSM FRP flexurally-strengthened beams. These studies showed excellent anchoring performance of the FRP U-jackets. The authors recently developed a finite-element (FE) approach that could accurately predict the behavior of RC beams that had been flexurally strengthened with NSM FRP (NSM-strengthened beams), which were anchored with FRP U-jackets. Based on a parametric study that was undertaken, which used the simplified version of the FE approach, this paper proposed a strength model for the most common debonding failure mode in NSM-strengthened beams with FRP U-jackets. The proposed strength model consisted of an equation for the maximum NSM FRP strain (ɛ_f) at debonding failure. Once the maximum FRP strain was known, the load-carrying capacity of the strengthened beam could be obtained through a section analysis. Comparing the predictions made by the proposed strength model with the test results showed that the proposed strength model could provide close predictions.

A bending-active arch derives its curved shape from elastic bending of an initially straight strut. The recent revival of interest in this special structural form has been largely driven by the use of pultruded hollow-section fiber-reinforced polymer (FRP) profiles. Stability is a crucial concern for bending-active arches, because their unique forming method demands a sufficiently high flexibility (slenderness) to facilitate curvature development. However, previous theoretical results, especially those associated with postbuckling behavior, have not been verified with test results due to the lack of loading tests. To fill this gap, this study conducted central point loading tests on FRP bending-active arches, which successfully captured the full-range postbuckling response. The test data fulfilled the intended goal of verifying a nonlinear load–deflection (equilibrium path) analysis. In this paper, the results of the loading tests are first presented. The differential equations governing the equilibrium path are next formulated and then solved as a boundary value problem (BVP) using a numerical BVP solver. Finally, the results of parametric studies performed using the verified BVP solver are presented. The results show that the effect of gravity on the mechanical behavior of FRP bending-active arches is negligible, whereas the forming-induced prestresses have a negative effect on the buckling load. In the range of practical arch shapes, the loss in buckling load due to this effect is approximately 10%.

This paper reports on the compressive properties of a new steel wire– and carbon/glass hybrid fiber–reinforced polymer (S-CG HFRP) bar. The influence of the carbon and glass fiber volume proportion (V_c/V_g), steel wire replacement ratio (V_s), fiber volume ratio (V_f), fiber arrangement, and ratio of the unbraced length to the bar diameter (L_u/d) on the compressive behavior of the S-CG HFRP bars was experimentally investigated. The failure mechanisms were analyzed, revealing that the failure mode changed from fiber microbuckling to fiber macrobuckling with an increase in L_u/d. Adding steel wires resulted in a decrease in the compressive modulus of the S-CG HFRP bars. The compressive strength of the S-CG HFRP bars was not effectively improved by adding steel wires or carbon fibers and was governed by the L_u/d ratio. The results showed that the compressive modulus and strength of the S-CG HFRP bars were approximately 90% and 28% of their tensile modulus and strength, respectively. Finally, the relationship between the compressive and tensile properties of the S-CG HFRP bars was established. Simplified empirical equations were proposed to estimate the compressive strengths of S-CG HFRP bars with different L_u/d ratios.

The effectiveness of textile-reinforced mortars (TRM) utilized as jacketing overlays that are applied to short, shear-critical reinforced concrete (RC) columns was investigated experimentally and verified analytically in this study. Moreover, the possibility of replacing the standard ordinary portland cement (OPC) matrix with an alternative alkali-activated material (AAM) in the TRM retrofitting technique was explored. Seven rectangular-shaped RC columns were constructed with a shear span-to-height ratio of 1.5. Four of the seven specimens were constructed to be more ductile with closely spaced shear reinforcement, and the rest had their stirrups positioned at larger spacings. Both series of columns were retrofitted with two and four layers of TRM jacket overlays, which were made from uncoated carbon textiles and AAM or OPC-based mortar. They were subjected to cyclic loading combined with a constant vertical load. The control (unretrofitted) specimens exhibited brittle failure, and the retrofitted specimens performed significantly better for deformation capacity, with a clear shift in the failure mode toward shear–flexure. The load capacity of the jacketed columns was governed by yielding in the longitudinal reinforcement. An important result is that AAM-based TRM jacketing is not inferior to its OPC-based counterpart.

A strengthening technique incorporating prefabricated prestressed carbon fiber–reinforced polymer (CFRP) was proposed and evaluated. The flexural performance of bare stone beams strengthened with prefabricated prestressed CFRP-reinforced stone plates was investigated by testing 11 stone beams. The test variables considered involved the diameter and prestress level of the CFRP bars, the reinforcement ratio, and the surface condition of the stone plate. The stress transfer efficiency between different components during the prestressing and strengthening processes was monitored with the aid of strain gauges that were attached to the CFRP bars and stone beams. The monitoring results showed that the CFRP bars prestress could be effectively maintained by the temporary device and then transferred to the stone beam to be strengthened. Further four-point bending tests indicated that the strengthened stone beams exhibited a failure mode with obvious deflection and multiple flexural cracks. The strengthened beams presented three stages of load versus deformation responses and showed significantly higher load-carrying and deformation capacities than the bare stone beams. The flexural behavior of the strengthened stone beams for different parameters was analyzed. Finally, models were proposed to calculate the cracking and ultimate moments, and the initial stiffness of the strengthened stone beams.

This study investigated the anchoring effect of wedge anchorages on prestressed basalt fiber–reinforced polymer (BFRP) laminates. Compared with traditional carbon fiber–reinforced polymer (CFRP) laminates, BFRP laminates have a larger ultimate fracture strain, lower elastic modulus, and smaller prestress loss, making them cost-effective prestressed materials. Fifteen fiber-reinforced polymer (FRP) laminate–anchorage assemblies were prepared to investigate the effects of FRP type, anchorage type, mat type, and presetting load level on the static tensile behavior of FRP laminate–anchorage assemblies. The differences in the failure modes, load versus displacement (L–D) curves, and strain development of the FRP laminates were analyzed. The results demonstrated that, first, the anchorage efficiency of the CFRP laminate–anchorage assembly was higher than that of BFRP. Since the elastic modulus and transverse shear strength of the BFRP laminates were much lower than those of the CFRP laminates, the application of the traditional CFRP anchorage easily damaged the BFRP laminates. Therefore, a specific design and optimization are required for BFRP laminate anchorages. Second, an anchorage with a retracted stressing end wedge was prone to obtain higher normal stress and thus achieve a higher anchoring capacity. Third, the sandpaper mat improved the friction between the FRP laminate and wedges and alleviated the stress concentration phenomenon of the FRP laminate at the stressing end of the anchorage. Finally, an increased presetting load on the wedges could slightly improve the assemblies’ ultimate bearing capacity and stiffness; however, the enhanced presetting load aggravated the stress concentration phenomenon at the stressing end of the anchorage.

This paper presents a new prestressing reinforcement technique based on prestressed near-end enhanced embedment (NEEE) carbon fiber–reinforced polymer (CFRP) strips for concrete beams. This technique offers significant economic and performance advantages over existing methods by using prestressed externally bonded (EB) or near-surface mounted (NSM) CFRP strips, thus reducing the cost of permanent mechanical anchors required and avoiding the heavy labor of cutting grooves in the concrete cover. Fourteen concrete beams strengthened with prestressed EB, NSM, and NEEE CFRP strips were tested under monotonic and fatigue loading. The test results showed that the prestressed NEEE CFRP-strengthened specimens exhibited better static and fatigue performances than the others. This was because of the unique nature of the NEEE technique, which can effectively anchor the prestressed CFRP reinforcement. Additionally, the bending CFRP strips within the NEEE section can restrain the transitive behavior of the CFRP tensile stress from the midspan to the beam end, inhibit concrete cover separation, and fully utilize the strength of the material. The restraining effect can be further enhanced by increasing the bending angle of the CFRP strips, e.g., by reducing the length of the inclined embedment section. However, if this length becomes too short (e.g., 150 mm in this study), the bonding performance of the surface-bonding sectional CFRP–concrete interface, which is positively correlated with the static and fatigue performances of the reinforced beam, is weakened. The use of a shallow embedment construction in the surface-bonding section can improve the bonding performance significantly, thereby increasing the structural stiffness, reducing the stress amplitudes of the tensile steel bars, and improving the load-carrying capacity and fatigue life of the strengthened beams.

The use of low-emission geopolymer concrete (GPC) and noncorrodible basalt-fiber-reinforced polymer (BFRP) bars is an effective strategy in the bid for net zero emissions and making sustainable and durable structures. To date, however, there have been no studies on the impact response of prefabricated/precast segmental concrete beams (PSCBs) constructed using GPC and BFRP bars. This experimental study, therefore, was intended to partially fill this knowledge gap. The key objectives were to investigate the impact behavior of the segmental versus traditional monolithic beams, the effect of impact location, and the performance of GPC versus ordinary Portland cement (OPC) concrete beams. The test results showed that, with the energy absorption capability derived from the opening and sliding of joints, the PSCB experienced less damage than its monolithic counterpart under similar impact conditions. The joints, however, reduced the global stiffness of the PSCB, resulting in the PSCB having a higher displacement, lower impact and reaction forces, but a longer impact force duration and greater impulse, compared to the corresponding monolithic beam. Under the impact loads, the PSCB had a higher tendon force but smaller reinforcement strain than the monolithic beam. Impacting at the joints mobilized the energy absorption capability more effectively, resulting in a reduction in impact-induced damage. The impact performance of both the monolithic and segmental GPC beams was quite similar to that of their OPC counterparts. Thus, GPC can be adopted as a sustainable alternative to OPC in the construction of concrete structures against impact loads. In this study, a three-dimensional finite-element model was also developed in order to obtain a better understanding of the impact behavior of segmental and monolithic beams.

Fiber-reinforced polymer (FRP) composites are widely used in the strengthening and retrofitting of reinforced concrete (RC) members to enhance their flexural and shear capacities. Strengthening with FRP systems has been proven effective due to their superior properties such as high strength-to-weight ratio and corrosion resistance. However, the debonding of FRP laminates from the concrete substrate is a major concern as it results in a brittle member failure. Anchorage systems could delay early debonding of the FRP laminates and improve the FRP-to-concrete bond. Carbon FRP (CFRP) spike anchors are commonly used as anchorage systems due to their compatibility with CFRP laminates and versatility to suit a varied range of applications. However, research to study the effects of different anchor parameters on strength enhancement has been limited. Hence, this study investigated the effect of CFRP anchor diameter, embedment depth, and dowel angle on the capacity of concrete beams strengthened with externally bonded CFRP laminates. All beams were tested using the four-point bending flexural test. Test results of anchored strengthened beams showed an increase in load-carrying capacity and CFRP strain utilization of up to 50% and 84%, respectively, when compared to the unanchored strengthened beams. In addition, larger embedment depth and insertion angle showed a significant effect on the capacity of the specimens. Finally, anchoring CFRP laminates with different anchor parameters delayed debonding failure with different anchor failure modes. Increasing the anchor diameter up to 12 mm prevented anchor rupture. Furthermore, increasing the embedment depth increased the beam capacity but resulted in anchor rupture. Anchoring CFRP laminates at large angles shifted the anchor rupture failure to partial anchor pullout. This study demonstrates that CFRP spike anchors, when properly designed and installed, can delay the brittle debonding failure mode, thus enhancing the load-carrying capacity of strengthened members.

The maintenance or replacement of critical infrastructure exposed to aggressive environments is a major problem in many countries. To address this issue, alternative noncorrosive materials, such as fiber-reinforced polymers (FRPs), have been proposed to substitute conventional steel bars. However, large-scale tests of beams with small shear-span-to-depth ratios (deep beams) and FRP bars have shown a significant reduction of shear strength compared to similar beams with steel reinforcement. Therefore, the main objective of this paper is to model and explain the mechanisms that govern the reduction in strength. Twelve test specimens from the literature are analyzed based on a crack-based assessment framework for steel-reinforced concrete members, which is extended to account for FRP bars. The crack-based approach shows that large strains in the flexural FRP reinforcement diminish the aggregate interlock resistance and cause premature shear-induced flexural failures in members without shear reinforcement. It is also shown that the model adequately captures local and global deformations, as well as the effects of beam slenderness, longitudinal reinforcement axial stiffness, concrete strength, and crack geometry on the shear response of internally FRP-reinforced concrete deep beams without shear reinforcement.

This study proposes numerical modeling of fiber-reinforced cementitious matrix (FRCM) composite systems in direct shear (DST) or tensile (DTT) tests with any setup. In the interfacial fracture theory framework, the nonlinear differential problem is treated through the finite difference method and the incremental solution to the equilibrium in terms of displacements is found by updating the secant stiffness of the cohesive material law. Relying on independent solutions which consider the system in the different phases since the initial condition, without and with fractures, and reconnecting the related solutions, a new procedure for the construction of the load slip diagram of DTT is implemented, allowing estimation of load drops and stiffness degradation caused by cracks opening. The model is benchmarked on results available in the literature and experimental tests carried out by the authors. A thorough parametric analysis of DTT highlights the influence on the response of matrix tensile strength and its distribution along the reinforcement. In particular, it is shown that the ratio between bond stress and mortar tensile strength can determine bilinear or trilinear diagrams and that the position of cracks widely influences the load and slip capacity of the system.

Although carbon fiber–reinforced polymer (CFRP) composites have been extensively used to rehabilitate many deficient bridges, data warranting their long-term performance is lacking. Current durability testing of CFRP composites involves accelerated conditioning as a part of material specification requirements to ensure that they maintain mechanical and physical properties during their service life. However, relating accelerated conditioning test data to material performance under real-time aging is not possible without field data. Work conducted at the University of Delaware in the early 1990s resulted in the first full-scale application of externally bonded CFRP on a publicly owned bridge in the United States. The original intent of CFRP strengthening was to extend the service life of the Foulk Road bridge in Wilmington, Delaware by 5–10 years. As one of the earliest CFRP-rehabilitated bridges in the United States, it offers a unique opportunity to study CFRP durability over a time span of 26 years. Field evaluation and laboratory testing (calorimetry, spectroscopy, scanning electron microscopy, and tensile testing) of CFRP samples collected from several girders were employed to investigate CFRP degradation and bond quality. The results indicate that after 26-year-long service life, the condition of CFRP repairs considerably deteriorated.