4.1 CFRP Sheets
Numerous studies have been conducted to investigate the effectiveness of CFRP sheets in strengthening RC beams subjected to flexural stresses. In a comprehensive study conducted by Ashour et al. (
2004), an empirical assessment of 16 RC beams strengthened with CFRP laminates was performed. A predictive model was developed as a means to determine the flexural load capacity of the investigated beams. Findings highlighted that the reinforced beams demonstrated increased load capacities, albeit with a trade-off of diminished ductility, when juxtaposed with their control counterparts. Remarkably, the primary mode of failure for most of the reinforced beams was the brittle peeling failure of the adjacent concrete cover to the CFRP sheets, regardless of their proximity to their flexural capacities. The extension of CFRP sheets to envelop negative or positive moment zones did not preclude the occurrence of peeling failure, even in scenarios, where tensile rupture of the CFRP sheets transpired. With regard to continuous beams, the increased bending moment capacity due to external reinforcement surpassed the load capacity of the continuous beam. The study proposed that, based on elastic principles, to avert the brittle peeling failure of the FRP laminates, the calculated shear stresses at the adhesive layer should be kept below 0.80 N/mm
2.
Pham and Al-Mahaidi (
2004) investigated the failure causes and the effects of various factors on debonding modes in 18 rectangular-reinforced concrete beams. The study's findings can be summarized as follows. First, debonding occurred at the mid-span and end of the beams due to significant shear stress in the concrete, which measured about 1 MPa. Second, two critical characteristics impacted FRP performance: the ratio of FRP bond length in the shear span to concrete depth and the ratio of laminate stiffness to tension reinforcement stiffness. The effectiveness of the FRP was shown to rise with bond length and decrease with FRP quantity applied. Furthermore, parameters such as concrete cover and shear reinforcement quantity were discovered to have a negligible effect on the debonding process. Steel clamps were discovered to be an effective solution for preventing end debonding and increasing beam ductility by retaining friction between the delaminated fibers and the concrete. It was also discovered that even after FRP debonding, the RC beams kept their initial strength. The researchers also discovered that beam theory produced reasonably accurate estimates of FRP strain levels.
In a detailed study conducted by Hosny et al. (
2006), the behavior of RC T-beams, enhanced with hybrid FRP laminates, was explored. The comprehensive testing of twelve beams revealed that the use of CFRP or GFRP laminates significantly increased the maximum load-carrying capacity, though at the cost of reduced ductility. However, improved ductility was achieved when a mixture of CFRP and GFRP laminates was utilized. An optimal reinforcement strategy was identified, involving the placement of CFRP laminates on the beam's sidewalls and GFRP laminates at the base. The study also affirmed that using the strain compatibility method can accurately predict the beams' behavior, provided a strain limit of 50% of the maximum values for both CFRP and GFRP is respected.
Toutanji et al. (
2006) conducted a thorough examination of the flexural behavior of eight RC beams subjected to four-point bending. This comprised 8 beams fortified with carbon fiber sheets bound with an inorganic epoxy, including one control beam. The findings demonstrated a positive correlation between the number of carbon fiber layers and the load-bearing capacity of the RC beams. Beam failure for those strengthened with three and four layers of carbon fiber occurred due to carbon fiber sheet rupture, while FRP delamination was the cause of failure in beams with five and six layers. Strengthened beams showed less ductility compared to the control beam. Consistent with previous research, the ultimate strain of the CFRP reinforcement, bonded with an inorganic epoxy, was established to be 0.0060 mm/mm. To further substantiate these findings, a moment–deflection model was developed and validated using data from this study and other investigations, yielding a satisfactory degree of agreement. The research underscores the need for further exploration into the bond-controlled failure of inorganic epoxy systems and the debonding failure of FRP attached to RC beams.
Hamid et al. (
2009) evaluated the usefulness of externally bonded CFRP sheets in enhancing the flexural strength of HSC beams. Six beams with varied arrangements of CFRP sheets were tested experimentally. The beams’ behavior was studied using FE models. The following were the important findings: the FE models agreed well with the experimental data, allowing for exact prediction and design principles for FRP strengthening. The bonding of CFRP sheets improved flexural strength, particularly in beams with lower steel reinforcing ratios. Tensile steel stresses exceeded CFRP strains. The concrete fibers’ compressive strain remained linear until failure, unaffected by concrete cracking or tension steel yielding. CFRP decreased compression fiber strain. Despite a brittle failure mechanism, steel and CFRP-reinforced beams showed enough deformation capacity. As steel reinforcing grew, the extra strength supplied by CFRP decreased. CFRP raised the strength of poorly reinforced beams substantially but had a lower effect on moderately reinforced beams. results for energy ductility were almost twice as high as results for displacement ductility. In the bending moment zone, CFRP strain was uniform, slightly greater at load sites, and dropped linearly along the shear span.
Kim and Shin (
2011) performed an in-depth exploration of the structural properties of RC beams that had been retrofitted with hybrid FRPs. Utilizing four-point bending tests, the study evaluated a range of factors, such as ductility, crack development, failure mechanisms of the samples, and the repercussions of preloading on the retrofitted RC beams. The research outcomes demonstrated that the employment of hybrid FRPs led to improvements in the ultimate strength and stiffness of the beams. It was found that the sequence of application of distinct FRPs had a considerable effect on the strength, rigidity, and ductility of the retrofitted beams. The beams displayed optimal strength and ductility when glass fiber was applied prior to the use of carbon fiber. Furthermore, the research determined that preloading influenced the structural behavior of the beams, lessening the strengthening benefits of hybrid FRPs in comparison with beams without preloading. However, this adverse effect could be counteracted by repairing cracks before the attachment of FRPs. Finally, the study highlighted that failure occurred in the retrofitted RC beams before the hybrid FRP sheets reached their fracture point. This observation underscores the necessity for a reevaluation of retrofitting design strategies to optimally utilize the potential of hybrid FRPs.
Attari et al. (
2012) undertook a comprehensive analysis to determine the effectiveness of external fortification systems on RC beams, utilizing GFRP and CFRP fabric. The research encompassed the examination of various reinforcement setups, including the application of unidirectional glass and carbon fibers with U-anchorages, and the use of a bidirectional glass–carbon fiber hybrid fabric. The study was based on the repeated loading of seven RC beams, and the ensuing assessment of their strength, rigidity, ductility, and failure mechanisms. Concurrently, the researchers developed an analytical model with the aim of accurately predicting the flexural failure of RC components. The outcomes of the study indicated a strong correlation between the model's predictions and the observed behavior of RC beams under load. In addition, the experimental results suggested that the use of a dual-layer glass–carbon FRP fabric as a reinforcement strategy for RC structures presents a cost-effective solution.
Jankowiak (
2012) investigated RC beams reinforced with CFRP strips numerically and experimentally. The beams were tested to see how efficient the reinforcement was in terms of load bearing capability. Different preloading states were investigated, and analysis was performed using FE modeling. The following are the key findings: due to debonding of the CFRP strips, all reinforced beams broke brittlely. When compared to beams without strips, the improvement in load bearing capability varied from 24% to 30%. The reduction in deflection at failure ranged from 33% to 37%. When beams were unloaded to dead weight before adding the strips, the most efficient strengthening was found. The utilization of adequate concrete fracture energy and a proper description of the tensile concrete were critical for accurate results. The numerical model was tested effectively and proven beneficial for assessing RC beams reinforced with composite materials, potentially providing a future alternative to labor-intensive and expensive laboratory experiments.
Dong et al. (
2013) conducted an empirical study on the flexural and flexural–shear fortification of RC beams using CFRP and GFRP sheets. The research scrutinized various enhancement arrangements involving CFRP and GFRP sheets and evaluated their impact on the performance of retrofitted RC beams. The study's findings suggested that the flexural–shear strengthening configuration significantly augmented the stiffness, peak strength, and strain-hardening behavior of RC beams, to a greater degree than merely applying flexural strengthening. The study also encompassed the development of theoretical computations to project the bending and shear capacities of the tested beams, with these predictions subsequently contrasted against the equivalent experimental data.
Ali et al (
2014) investigated the effect of CFRP mechanical anchorages on the flexural performance of CFRP sheets and plates used in externally strengthened RC beams. The study's primary goal was to evaluate the load-bearing capability and ductility of the beam specimens. This study's significant findings included: beams with anchors had a greater delamination load than those without anchors, albeit further statistical analysis with a larger sample size is required to fully assess the significance of this increase. The ductility of the beam specimen without reinforcement was the highest, whereas the ductility of the beams reinforced with CFRP sheets and plates and supported by CFRP anchors was the lowest. The addition of anchors had no discernible effect on the flexural stiffness of the beam specimens.
In their research, Xie et al. (
2014) examined the impact of pre-damage level, shear span-depth ratio, and CFRP thickness on the flexural performance of pre-damaged RC beams fortified with CFRP. The empirical findings led to the following conclusions: the application of CFRP substantially enhances the flexural load-bearing capacity of pre-damaged RC beams while not affecting stiffness, yield loads, or ultimate loads. The shear span–depth ratio plays a significant role in influencing the failure mechanism and load-bearing capacity of reinforced RC beams. As the shear span–depth ratio decreases, there may be a transition in the failure mechanism from IC debonding to cover separation at the CFRP end, with IC debonding beams presenting higher load-bearing capacities. CFRP usage is improved by increasing the shear span ratio. Higher CFRP levels contribute to improved load bearing capacity and flexural stiffness in beams failing due to IC debonding, but this tendency does not apply to beams collapsing due to cover separation near the CFRP end. However, increasing the thickness of the CFRP affects the ductility of the strengthened beams.
Cosgun (
2016) investigated the use of CFRP to strengthen RC beams. The study included experimental and computational assessments of 16 RC beams with different concrete strength classes and reinforcing schemes. The following are the important findings: the ultimate load-bearing capabilities of CFRP strengthened flexural and shear beams were comparable. Shear failure was seen in all CFRP-reinforced flexural beams, which was most likely caused by increased stiffness in the center area. In addition to shear failure, shear beams exhibited plate end debonding, demonstrating the impact of reinforcing layout. The deflection values and ductility of the CFRP strengthened beams were comparable. The effect of CFRP on load–displacement behavior was comparable for beams of various concrete strength classes. Although the failure processes vary, the ultimate load capacity and ductile behavior of CFRP-reinforced flexural and shear beams with various reinforcing schemes were similar. The numerical analysis results matched the experimental data well, with a difference of 2–7%, showing consistency and adequate accuracy.
Salama et al., (
2019) evaluated the flexural capacity of RC beams that had been externally reinforced with side-bonded CFRP sheets using epoxy adhesives. Nine RC beams were evaluated, with the performance of side-bonded and standard bottom-bonded strengthening methods compared. The results revealed that the side-bonded approach was marginally less efficient at improving flexural characteristics. The flexural strength of bottom-bonded specimens improved by 62–92%, whereas side-bonded specimens increased by 39.7–93.4%. However, ductility at failure was reduced by 42.3–62.5% when compared to the control beams. Steel yielding occurred prior to CFRP debonding, with larger reinforcement ratios resulting in quicker debonding. Fiber depth and stress distribution were blamed for the efficiency difference. Increasing the breadth of side-bonded CFRP sheets resulted in very minor gains. Flexural strength was appropriately anticipated by the ACI440.2R-08 design requirements. Overall, side-bonded CFRP strengthening was less effective but still generated considerable strength gains.
Choobbor et al., (
2019) examined the efficacy of flexural strengthening in RC beams utilizing hybrid BFRP and CFRP sheets. Nine RC beams' experimental and numerical findings were examined. The research indicates that the flexural strength of reinforced beams increased by 28–75% in comparison with beams that were not strengthened. In addition, increasing the number of basalt sheets in the hybrid laminate improved ductility by as much as 31.1% for beams having the same layer count. In addition, altering the arrangement of the FRP sheets did not impact the functionality of beams featuring two or three layers of FRP laminates. The accuracy of finite-element (FE) models in estimating ultimate load bearing capacity and deflection of RC beams reinforced with hybrid FRP laminates was validated, with percentage deviations ranging from 1% to 11% and 2% to 12%, respectively.
Hadi et al., (
2022) studied numerous FRP systems to find the best efficient way for retrofitting and strengthening RC beams. The study's findings led to the following conclusions. For starters, there was a remarkable agreement between the experimental results and the American Concrete Institute (ACI) connections. Second, among the many FRP mechanisms examined, the hybrid strengthening approach demonstrated more ductility than the control beam and other FRP mechanisms. Furthermore, the hardening slope and absorbed energy were found to be larger in all FRP mechanisms than in the control beam, with the hybrid type displaying the greatest values.
Nawaz et al., (
2022) investigated the flexural behavior of all lightweight concrete (ALWC) beams reinforced with CFRP sheets. Experiments were carried out on fifteen specimens, with variables, such as reinforcement ratio, number of CFRP layers, and pre-loading taken into account. The results revealed that CFRP boosted the ultimate load bearing capacity of the beams substantially, with a variety of improvements. Delamination was the most common failure mode, which was slowed by CFRP sheets. With more CFRP layers, stiffness, yield load, and post-cracking stiffness improved, but ductility dropped. The strength of pre-loaded specimens was greater. Flexural strength predictions based on various design standards overstated testing results. More study on other lightweight aggregates and large-scale beams with varied concrete strengths and CFRP reinforcement is proposed.
In the experimental study conducted by Hashemi et al. (
2022), low-strength concrete (LSC) elements, which are susceptible to seismic and static loads, were reinforced using FRP strengthening. Two primary aspects were investigated: first, the impact of rebar planting to increase the initial compressive strength for FRP reinforcement, and second, the effectiveness of CFRP confinement in enhancing the strength of rebar-embedded specimens. A total of 38 standard concrete cylinders were tested, with variables, including rebar dimensions, concrete strength, and CFRP sheet count. Statistical analysis revealed the significant role played by CFRP confinement, combined with rebar embedment, in increasing the load-bearing capacity of LSC concrete, as supported by experimental results. Rebar planting demonstrated a strength enhancement of up to 53%, rendering certain LSC specimens eligible for CFRP confinement.
4.2 Near Surface Mounted (NSM) Bars
Extensive study has been carried out to determine the efficiency of NSM bars in strengthening the structural reinforcement of RC beams subjected to flexural stresses. Dias and Barros (
2011) scrutinized the effectiveness of the NSM technique in augmenting the shear strength of T-section RC beams with low concrete durability. The experimental approach of this study involved assessing the effects of different CFRP shear fortification configurations on various parameters. These included load-bearing capacity, stiffness, peak stress levels in CFRP laminates, and the mechanisms leading to failure. The NSM approach was found to be successful in RC beams with a low concrete strength of 18.6 MPa. Following shear crack development, the CFRP shear strengthening designs enhanced both maximum load and load-carrying capacity. Concrete strength was important, with increased strength resulting in better efficacy. Inclined laminates outperformed vertical laminates in terms of shear capacity, while increasing the proportion of laminates enhanced shear capacity. However, an increase in the percentage of existing steel stirrups had a negative impact on the NSM technique's effectiveness. The proposed formulation for NSM shear strengthening provided reliable estimates, with predicted CFRP contribution at 75% of experimental results.
Singh et al., (
2014) evaluated the performance of flexure- and shear-strengthened beams to control beams. A parametric research was carried out using numerical modeling, and the results were confirmed using experimental data. Based on numerous characteristics, the research gave guidance for the appropriate positioning and use of FRP bars. The diameter and strength of the FRP bars were discovered to be key considerations. FRP bars set at a 45° angle to the beam axis were most effective for shear strengthening, whereas vertical bars were least effective. The study also determined the optimum shear strength by determining the minimum groove distance, groove width, and spacing between NSM FRP bars. The geometry of the NSM FRP bars has no effect on strength increase.
Haddad and Almomani (
2017) studied the possibility of restoring the flexural performance of thermally damaged concrete beams using NSM CFRP strips. Two sets of beams were exposed to varied circumstances, including 2 h of heating at 600 °C. The NSM CFRP strips were used to repair/strengthen half of the beams in each set, while the other half functioned as controls. Four-point loading tests were used to assess mechanical performance, which included load capacity, stiffness, ductility, and bond strength. Heating considerably lowered load capacity while improving ductility, according to the findings. Strengthening intact beams with NSM CFRP strips increased load capacity and stiffness, but mending heat-damaged beams resulted in mechanical performance loss. With longer NSM CFRP strip embedment lengths, the overall performance factor suggested that mending heat-damaged beams was possible. The minimal contribution of the restoration techniques to restoring the original mechanical performance was ascribed to considerable bond strength loss. With an average prediction error of 9%, analytical projections for ultimate load capacity closely matched experimental data.
Chellapandian et al., (
2019) performed flexure experiments on RC beams with and without FRP strengthening utilizing various approaches, such as NSM, external bonding (EB), and combinations of both. The study team performed a complete three-dimensional, nonlinear finite-element analysis, which was supplemented with analytical predictions generated from the beams’ sectional responses. This confirmed finite-element (FE) model laid the groundwork for additional experiments, such as calculating the ideal NSM edge distance and CFRP ratios. The findings of this study provided some crucial insights. For starters, the use of Hybrid FRP strengthening significantly increased load-bearing capacity, with hybrid-strengthened beams retaining a significant amount of residual capacity following the rupture of NSM laminates. Second, hybrid FRP strengthening outperformed standalone NSM or EB strengthening in terms of energy absorption capacity. Finally, for both unconfined and FRP-confined concrete, theoretical predictions based solely on sectional calculations tended to underestimate experimental data.
Dong et al., (
2019) conducted an in-depth study on the performance of concrete beams reinforced with FRP bars, anchored using high-strength cement grout and corrugated sleeves. This methodology-controlled crack expansion, with ten beams tested under four-point bending conditions. Results demonstrated that FRP-reinforced beams had superior flexural capacity compared to steel-reinforced beams. The use of grouted FRP bars within sleeves did not affect failure modes, with these beams showing more deflection before concrete crushing, suggesting an early warning sign of impending failure. Axial stiffness of FRP bars impacted the behavior of FRP-reinforced beams, with an increase in stiffness improving flexural capacity, structural rigidity, and reducing crack width. The use of high-strength grout and corrugated sleeves led to a notable reduction in crack widths, thus improving serviceability performance. In addition, the study highlighted some discrepancies in current standards (e.g., ACI 440.1R, CSA S806-12, GB 50608-2010) for predicting crack spacing and strain coefficients, indicating the need for further research to refine crack-width equations.
Panahi et al., (
2021) studied the efficiency of FRP composite-based flexural strengthening solutions for reinforced concrete beams. NSM, EB, and a mixture of the two were among the approaches investigated. The impacts of material characteristics, geometry, and configurations on the flexural behavior of the strengthened beams were studied using ABAQUS software. The results showed that beams with NSM FRP rods had much higher flexural capacity and stiffness than control beams, while mid-span deflections were lower. With increasing material strength and embedding length, the ultimate bending moment and stiffness of the beams rose. The study also discovered that increasing the diameter of FRP rods increased the ultimate bending moment while decreasing mid-span deflection and ductility. The strengthened beams’ load–deflection behavior followed a tri-linear pattern, beginning with linear elastic behavior, then reinforcement usage and enhanced load-carrying capacity, and finally an increase in mid-span deflection. Furthermore, broader sheet widths boosted the load-carrying capability of beams reinforced with EB FRP sheets, albeit at the expense of lower ductility compared to narrower sheets.
4.3 FRP Bars and Grids
Some researches concentrated on the usage of FRP bars and grids in concrete components. Liu et al. (
2020) investigated the crack development in GFRP- and CFRP-reinforced beams using lightweight concrete (LWC) and self-compacting lightweight concrete (SFLWC). They found that the use of SFLWC and increased clear span length decreased maximum crack width at lower loads, and higher reinforcement ratios also reduced crack width throughout the loading process. All CFRP-reinforced beams met the 0.7 mm fracture width requirement at service load, whereas some GFRP-reinforced specimens showed non-conservative crack widths. The research found that the inclusion of steel fibers effectively reduces the fracture width at service load for lower reinforcement ratios, although it slightly augments it for higher ratios. The experimental results corroborated the reliability of ISIS-M03 and ACI 440.1R formulas for predicting crack width, while flagging inaccurate estimates presented by GB 50608. A novel model for predicting crack width in NWC beams reinforced with FRP, which accommodates both lightweight aggregates and steel fibers, was proposed. This model demonstrated a commendable level of accuracy in its predictions.
In their 2021 research, Guo et al. conducted four-point bending tests to examine the shear behavior and enhancement effects of RC beams fortified with polypropylene-engineered cementitious composite (PP-ECC) and FRP grids. The study explored varied reinforcement levels of FRP grids and diverse beam shear span ratios. Findings indicate that the application of ECC layer alone resulted in a minor increase in peak loads, but it exhibited a smoother failure process and successfully conveyed shear forces through the bridging effect of PP-ECC. The shear resistance was contributed by both vertical and horizontal FRP grids, though the latter accounted for only 30% of the resistance when compared to the former. The interaction between stirrups and FRP grids occurred ahead of shear cracks, followed by partial debonding at the interface, particularly in specimens with multiple layers. Furthermore, an analytical method for predicting shear capacities in RC beams reinforced with ECC/ECC–FRP grids was proposed, and its accuracy was validated through a comparison with the test results.
Hassanpour et al. (
2022) studied the effectiveness of using GFRP bars as compressive reinforcement in RC beams, considering singly and doubly reinforced configurations. The study evaluated their impact on load-bearing capacity, ductility, stiffness, and failure modes. The findings revealed a limited enhancement of flexural strength, with a maximum 5% increase in flexural capacity for doubly reinforced beams and an 8% strength gain in GFRP–RC cylinders compared to controls. Concrete’s compression strain limited this improvement. For singly reinforced specimens, the presence of compressive GFRP bars reduced stiffness (up to 16%) but enhanced curvature (up to 10%) and ductility (up to 35%). In addition, a novel ductility assessment method based on deformations near peak and peak load capacity was introduced, predicting ductility gains from compressive bars and identifying distinct failure modes.
Kadhim et al. (
2022) examined the effectiveness of carbon–fiber-reinforced UHPC overlays in strengthening RC beams. The research employed a robust FE model validated against experimental data. A parametric study of 68 models investigated key factors, including reinforcement ratios, concrete strength, overlay thickness, and interface conditions. The CFRP-reinforced UHPC overlays significantly enhanced ultimate load capacity and ductility, eliminating cracking failures. Varying the CFRP reinforcement ratio resulted in a substantial ultimate load increase (112–463%) compared to control beams. The study also developed an analytical model for design purposes based on parametric study outcomes and regression analysis.