Search

An experimental program was conducted to examine the seismic performance of flexural beam members strengthened with carbon fiber-reinforced polymer (CFRP) sheet and anchored using carbon fiber splay anchors. This research study extends the use of carbon fiber anchors to secure the top and bottom CFRP flexural sheets under cyclic loading. In this study, low strength concrete is examined, with lightly reinforced steel ratio (0.0034) to test full-scale rectangular beams. Five reinforced concrete beams were cast, strengthened, and tested under reversed cycles of four-point bending to failure. The beams were categorized into a control specimen, strengthened specimens with thin sheets with and without fiber anchors, and strengthened specimens with thick sheets with and without fiber anchors. They were tested cyclically under displacement control according to the AC 125 loading protocol. The failure modes ranged from classical ultimate failure for the control beam to rupture and intermediate crack (IC) debonding for the beam with unanchored thin CFRP sheets. On the other hand, debonding took place for the thicker sheets with no anchors, while the thicker sheets with fiber anchors underwent cover delamination failure mode despite the fact that the sheets were extended very close to the simple supports. The hysteresis response loops were compared for the different specimens and the analytical envelop curves were predicted resulting in a close agreement to the experimental finding. • Seismic performance of flexural beams CFRP strengthened with and without carbon fiber anchors is successfully examined. • Thicker CFRP sheets yield lower energy dissipation, ductility, but higher strength and seismic pinching. • Failure modes varied from sheet debonding to CFRP rupture and cover delamination. • The use of anchors at sheet ends was not very effective in extending the hysteretic cycles beyond those without anchors. • The need to apply fiber anchors along the entire shear span to harness improved behavior is clearly evident. [ABSTRACT FROM AUTHOR]

Plate end debonding is a common failure mode for fiber reinforced polymer (FRP) strengthened beams. FRP sheets and shape memory alloy (SMA) composites can effectively slow down plate end debonding and improve beam ductility owing to the anchorage effect of SMA wires on the FRP ends. This work aims to comprehensively study the anchorage performance of FRP/SMA composite strengthened beams through both model analysis and experimental investigation. The plate end debonding strength models of existing FRP strengthened beams were initially summarized and analyzed, and an improved model was proposed to predict the debonding strength of FRP/SMA composite strengthened beams at the plate end. Moreover, the anchorage performance and flexural behavior of beams strengthened with carbon fiber reinforced polymer (CFRP) and SMA composites were studied through experiments. The failure mode of the FRP sheet strengthened beams was plate end debonding failure, while the plate end debonding failure of the CFRP/SMA composite strengthened beams occurred at the strengthening zone end, and the peeling failure was delayed owing to the anchorage effect of SMA wires on the ends of the strengthening CFRP sheets. The recovery effect of SMA wires could successfully introduce prestress into CFRP sheets. Moreover, the CFRP strain corresponding to plate end debonding increased because of the anchorage of SMA wires on the CFRP sheet in the strengthening zone. The strength and ductility of the CFRP/SMA composite strengthened beam significantly improved. The proposed model for predicting the plate end debonding strength of FRP/SMA composite strengthened beams was validated through experiments. • An improved model is proposed to predict the debonding strength of beams strengthened with FRP/SMA composites at the plate ends. • The debonding failure was delayed due to the anchorage effect of SMA wires on CFRP sheets in the strengthening zone. • The strength and ductility of the beam strengthened with CFRP/SMA composites have been significantly improved. • The proposed model for predicting the plate end debonding strength of FRP/SMA composite strengthened beams was validated through experiments. [ABSTRACT FROM AUTHOR]

• Efficiency of WWS technique in anchoring longitudinal CFRP sheets was investigated. • Influence of different parameters on the performance of WWS anchors were investigated. • Efficiency of WWS method was much better than that of conventional U-shaped anchors. • Simultaneous use of WWS technique with corner strip led to rupture of longitudinal CFRP. U-shaped straps are an effective means to postpone premature debonding failure when fiber-reinforced polymer (FRP) composites are applied in structures. However, the U-wrap anchors are not cost-effective and in many cases cannot completely prevent the debonding phenomenon. In the current study, an innovative technique named "warp and woof strap" (WWS) is introduced for anchoring longitudinal FRP sheets, in which some parts of FRP fibers are manually made bi-directionally. Unlike the conventional U-shaped anchors, the anchorage system in this method is in direct contact with the tension side of the beam. Totally, 20 reinforced concrete (RC) beams, 120 × 160 × 1400 mm in size, were subjected to the four-point loading to evaluate the efficiency of the introduced anchorage system in comparison to the U-shaped anchorage. The effect of different parameters including the method employed for anchoring the longitudinal carbon fiber reinforced polymer (CFRP) sheet, anchorage height and width, location of FRP anchorage application, use of the corner strip and rounding corner method, and configuration of WWS anchorage were investigated. The experimental results indicated that the efficiency of the WWS method in preventing undesired debonding mode of failure is much better than that of conventional U-shaped anchor. It was also observed that using WWS method can improve the load carrying capacity and ductility up to 32% and 80%, respectively, compared to U-shaped anchors. [ABSTRACT FROM AUTHOR]

• The impact and post-impact performance of RC beam strengthened with CFRP was investigated experimentally and numerically. • The strengthening effect by various CFRP anchorage methods was discussed. • CFRP slipping played an important role in the strengthening effect of strengthened beam. • The post-impact performance was quantified by residual resistance, residual stiffness and energy absorption ratio. • CFRP strengthened beams show better residual performance and resilience. Carbon fiber reinforced polymer (CFRP) external bonding is a common approach to strengthen the bearing capacity of the reinforced concrete (RC) members. In this study, two types of RC beams, i.e., CFRP strengthened beam and conventional RC beam, were tested under drop hammer impact and subsequent quasi-static loading. The strengthening effect of the CFRP on the dynamic responses, failure modes and residual load-carrying performance of the RC beams were fully investigated. Then, a high-resolution explicit numerical simulation was conducted to further investigate the residual load-carrying mechanism of the CFRP strengthened beam. The experimental results demonstrate that even though the CFRP sheet peels off from the soffit of the investigated beam instantaneously upon impact, which provides a little contribution to the impact damage elimination in the mid-span of the beam, the oblique cracks within the flexure-shear zone were reduced moderately. Further, the bonded CFRP sheet can effectively reduce the maximum and residual displacement of the beam and improve the recovery performance of deformation. In addition, the residual resistance, residual stiffness and post-impact energy absorption capacity of the beams with CFRP are shown to be higher than those of the unstrengthened ones. The experimental and numerical results both show that the effective anchoring approach has an important effect on the residual loading performance enhancement. Larger impact energy and more significant impact dynamic effect tend to degrade the bearing performance of the beam more. [ABSTRACT FROM AUTHOR]

• CFRP grid and ECC were employed to repair load-damaged RC beams. • Failure modes, load-deformation response, and flexural capacities were evaluated. • Prediction model for the flexural bearing capacity were proposed. In this study, the flexural behavior of pre-damaged reinforced concrete (RC) beams repaired with carbon fiber reinforced polymer (CFRP) grids and engineered cementitious composite (ECC) was investigated. Fifteen beams were designed and subjected to four-point bending tests. Three types of specimens were tested: undamaged, pre-damaged with removed load (i.e., loaded to the target level and then unloaded), and pre-damaged under sustained load. The main testing parameters were strengthening material (ECC alone, or CFRP grid plus ECC), CFRP grid thickness, main reinforcement ratio, preloaded level, and repairing process. The experimental results showed that most of the repaired beams exhibited flexural failure caused by concrete crushing, and a few others failed by debonding. The proposed repairing technique was effective in enhancing the flexural stiffness and bearing capacity of pre-damaged RC beams. For the repaired beams, the flexural bearing capacities did not significantly decrease as the preloaded level increased from 0 to 0.45 and slightly decreased when the preloaded level exceeded 0.65. A mathematical model based on the plane section assumption was proposed to predict the flexural capacities of the repaired beams, considering whether the load causing the damage was removed or sustained. The predicted capacities were in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

[Display omitted] • RC beams failed in flexure while RC beams + CFRP failed in shear. • RC beams + CFRP failure loads are lower than theoretical flexural and shear strengths. • Higher CFRP ratio delaminates more and failure load approaches a steady value. • Analytical load–deflection agrees with the experiment up to a certain % of CFRP. • FE modeling captures the behavior and an adjusted capacity model is provided. This study investigates the shear behavior and debonding characteristics of initially ductile RC beams (failing in flexure when unstrengthened) at the verge of transitioning to a more brittle shear failure upon strengthening using external flexural carbon fiber reinforced polymer (CFRP). Despite the absence of shear CFRP strengthening, various degrees of improvement in the shear capacity were achieved depending on the flexural CFRP amount. However, steeper shear cracks that bypass the stirrups in addition to the nonyielding stirrups results in shear capacities that cannot be reliably predicted using the ACI and other similar codes' provisions. Instead of adopting capacity models that focus on the shear failure alone or the crack-induced debonding as oneway cause-effect (shear crack-debonding) relationship, a more holistic capacity prediction approach (calibrated with finite element anlaysis) that considers the synergistic effect between the shear failure and the CFRP debonding is provided. Since any enhancement in flexure must be checked against shear, the presented capacity prediction approach could be a more handy means to assess the shear strengthening level needed in such cases where the flexural strengthening is insufficient to achieve the overall structural upgrade sought. The designed beams – i.e. at the verge of changing failure mode with flexural FRP strengthening – and the test program could serve as a bond assessment philosophy in the sense that this upper bound brittle failure mechanism provides more realistic behavior of the crack-induced debonding performance of FRP-strengthened beams. Analytical load–deflection assessment of the tested specimens is also provided. [ABSTRACT FROM AUTHOR]

The premature debonding of Fiber Reinforced Polymer (FRP) sheets has been well studied and understood for normal weight concrete. This type of failure mode can be averted by keeping the maximum FRP strain below the debonding strain of FRP as specified by ACI 440.2R-17. However, the debonding strain in lightweight concrete has not been addressed or evaluated yet. Accordingly, this paper investigates the debonding strain in lightweight concrete using full-scale reinforced concrete T beams strengthened with different layers of carbon fiber reinforced polymer (CFRP). These beams were prepared and strengthened in two series. The first series included three T beams that were cast with normal weight concrete. The second series consisted of another group of three T beams using lightweight concrete. Both series had identical geometry, reinforcement, and span lengths. Furthermore, they had almost identical concrete properties and the same FRP material to effectively examine the deboning strain difference in both types of concrete. The debonding strains of the lightweight and normal weight concrete are compared and the reduction factor is addressed in light of the current design equation in ACI 440.2R-17. [ABSTRACT FROM AUTHOR]

This paper presents an experimental program that was carried out for the purpose of studying the global flexural performance of reinforced concrete beams strengthened internally with Carbon Fiber Reinforced Polymer (CFRP) rods using the Side Near Surface Mounted (SNSM) technique. The CFRP rods were placed laterally, adjacent to the longitudinal steel bars inside precut grooves. The strengthening length and position of CFRP rods as well as the type of filling material were the main variables investigated in this study. Moreover, a detailed comparison between the Side Near Surface Mounted (SNSM) and the Near Surface Mounted (NSM) techniques for the strengthening of RC beams using CFRP rods was conducted as well; in order to validate and assess the effectiveness of the SNSM technique. The test results showed that using the SNSM-CFRP rods technique allowed improving in a significant way the load carrying capacity of RC beams but decreased their ductility along the deflection at maximum load. The results obtained indicated that the failure mode was influenced by the length of CFRP rods and the filling material characteristics, while the strengthening position did not have significant impact. The SNSM strengthening technique can be used as an alternative method to the NSM method, and in some cases, it may be used to prevent the non-conventional failure modes, due to degradation of the NSM strengthening system, such as the pull out of CFRP-rods or premature debonding failure. The conventional analytical model accurately predicted the strength capacity and mid-span deflection of the SNSM strengthened beams. [ABSTRACT FROM AUTHOR]

The strengthening reinforcement is glued to the soffit of the reinforced concrete (RC) beams in the mainstream externally bonded reinforcement (EBR) technique for flexural strengthening. However, the soffit of RC beams could be inaccessible for strengthening due to limited or no space at the soffit because of walls or doors, or the spalling of concrete. To overcome such difficulties, this paper investigates the feasibility of the flexural strengthening of lightweight non-pre-cracked and pre-cracked reinforced concrete (RC) beams using the side externally bonded reinforcement (S-EBR) technique with carbon fiber reinforced polymer (CFRP) fabric. A total of eight full-sized RC specimens were fabricated, strengthened, and tested under four-point loading until failure. The test variables were the internal reinforcement ratio, percentage of pre-cracking loads, and the effect of cracking loads on the flexural performance. The loads, deflection, and ultimate failure of the specimens were recorded and the types of modes of failure were also observed. The S-EBR strengthening technique with CFRP fabric significantly enhanced the flexural capacity of non-pre-cracked and pre-cracked lightweight RC beams compared to the control specimen. An analytical model in accordance with ACI 440.2R-08 was developed to verify the experimental results. The flexural capacity predicted by the model was in reasonable agreement with the experimental results. [ABSTRACT FROM AUTHOR]

• Parametric analysis of pre-cracked RC beams shear-strengthened with CFRP strips is made. • Different failure modes related to CFRP strips are observed. • Shear behaviors and the failure mechanisms of specimens are revealed. • A revised model to evaluate the shear capacity of CFRP shear-strengthened beam is proposed. Experimental studies on shear capacity of eighteen pre-cracked reinforced concrete (RC) beams shear-strengthened with carbon fiber reinforced polymer (CFRP) strips are carried out in this study. The effects of CFRP strips spacing, shear span ratio and pre-cracked degree on the failure mode, shear capacity, load-deflection curve and moment-curvature relationship of strengthened specimens are analyzed. Three failure modes related to the failure of CFRP strips, that is the bonding failure, the debond failure and the fracture failure of CFRP strips, are observed for specimens with different structural parameters. Moreover, with decrease of CFRP strips spacing, the shear capacity, the ultimate deflection and the bending stiffness of specimens increase. With increase of pre-cracked degree, the increment of shear capacity of RC beams shear-strengthened with CFRP strips is lower than that of non-cracked beams. The pre-cracked degree presents slight effect on the stiffness of CFRP strips-strengthened beams. Further, a revised model to determine the shear capacity of CFRP strips is proposed by accounting for the strain distribution of CFRP strips and the pre-cracked degree. Comparison between the experimental results and the calculated values indicates that the proposed formula can versatilely evaluate the shear capacity of CFRP strips strengthened RC beams. [ABSTRACT FROM AUTHOR]