Search Results (14)

Externally bonded carbon-fiber-reinforced polymer (CFRP) can increase the flexural capacity of steel beams, but the benefit is often limited by the performance of the adhesive interface. This study develops and validates a three-dimensional finite-element model (FEM) with an explicit cohesive-zone representation of the adhesive layer. It reproduced benchmark four-point bending tests in terms of peak load, corresponding mid-span deflection, and the transition from end/intermediate debonding to laminate rupture. A one-factor-at-a-time parametric analysis is carried out to examine the influence of (i) member geometry (beam depth; flange and web thickness), (ii) CFRP configuration (bonded length; laminate thickness), and (iii) bond quality (cohesive normal strength). Within the ranges studied, cohesive strength and bonded length are the primary variables controlling both capacity and failure mode: strengths below about 25 MPa and short plates lead to debonding-governed response. Increasing strength to around 27 MPa and bonded length to 650–700 mm delays debonding, promotes CFRP rupture, and produces the largest incremental gains in peak load, while further increases in length give smaller additional gains. Increasing laminate thickness and steel depth or flange/web thickness always raises peak load, but under baseline bond conditions failure remains debonding and the added material is only partially mobilized. When cohesive strength is increased above the threshold, additional CFRP thickness becomes more effective. A linear regression model is fitted to the FEM dataset to express peak load as a function of bonded length, cohesive strength, laminate thickness, and steel dimensions, and is complemented by a failure-mode map and a cost–capacity chart based on material quantities. Together, these results provide quantitative trends and simple relations that can support preliminary design of CFRP-strengthened steel beams for similar configurations. Full article

Steel-reinforced grout (SRG) composites are a newly developed retrofitting technique, which is considered an alternative to other fiber-reinforced composites to increase the load-carrying capacity of existing structures. This work presents an experimental campaign aimed at investigating the response of reinforced concrete (RC) beams strengthened with SRG externally applied to the tension side of the member to improve flexural capacity. The number of fiber sheet layers and fiber sheet density have been varied to evaluate the effectiveness of the retrofitting system. For some beams, different solutions of anchors at the ends of the beams have been considered to delay the premature debonding of the SRG. Moreover, single-lap direct shear tests have been carried out on concrete prisms strengthened with the same SRG composite to evaluate the bond behavior of the system. Failure modes, load responses, and corresponding flexural capacity (beam tests) and debonding loads (shear tests) are reported. The moment–curvature curves derived from cross-sectional analysis are compared with the corresponding experimental curves. The strain when the loss of composite action occurs is obtained from the curvature measured experimentally and compared with the values from formulas for the strain available in the literature and the strain at debonding in single-lap shear tests. Full article

With 3D printing technology, fiber-reinforced polymer composites can be printed with radical shapes and properties, resulting in varied mechanical performances. Their high strength, light weight, and corrosion resistance are already advantages that make them viable for physical civil infrastructure. It is important to understand these composites’ behavior when used in concrete, as their association can impact debonding failures and overall structural performance. In this study, the flexural behavior of two designs for 3D-printed glass fiber composites is investigated in both Portland cement concrete and polymer concrete and compared to conventional fiber-reinforced polymer composites manufactured using a wet layup method. Thermogravimetric analysis, volume fraction calculations, and tensile tests were performed to characterize the properties of the fiber-reinforced polymer composites. Flexural testing was conducted by a three-point bending setup, and post-failure analysis was performed using microscopic images. Compared to concretes with no FRP reinforcement, the incorporation of 3D-printed glass-fiber-reinforced polymer composites in cementitious concrete showed a 16.8% increase in load-carrying capacity, and incorporation in polymer concrete showed a 90% increase in flexural capacity. In addition, this study also provides key insights into the capabilities of polymer concrete to penetrate layers of at least 90 microns in 3D-printed composites, providing fiber bridging capabilities and better engagement resulting in improved bond strength that is reflected in mechanical performance. The polymer material has a much lower viscosity of 8 cps compared to the 40 cps viscosity of the cement slurry. This lower viscosity results in improved penetration, increasing contact surface area, with the reinforcement consequently improving bond strength. Overall, this work demonstrates that 3D-printed fiber-reinforced polymer composites are suitable for construction and may lead to the development of advanced concrete-based reinforced composites that can be 3D-printed with tailored mechanical properties and performance. Full article

Deterioration of infrastructure is a major challenge in the civil engineering industry. One of the methods that has been deemed effective in upgrading reinforced concrete (RC) structures is using externally bonded fiber-reinforced polymer (FRP) composites. However, the efficacy of this retrofitting technique is limited by the premature debonding failure of the FRP at the concrete-FRP interface; thus, the full capacity of the FRP is rarely utilized. Anchorage systems were proposed as a feasible solution to suppress or delay debonding failure. This paper presents an experimental investigation on the use of end U-wraps and carbon FRP (CFRP) spike anchors to anchor CFRP plates bonded to flexure-deficient RC beams. The experimental program consisted of seven RC beams with the length of the CFRP plate, type of anchorage, and the number of anchors as experimental variables. Test results indicated a remarkable enhancement in the ultimate load-carrying capacity when longer CFRP plates were used to strengthen the beams. In addition, anchoring the plates enhanced the strength of the CFRP-plated beams by 16–35% compared to the unanchored specimen, depending on the anchorage type and scheme. Finally, fib Bulletin 90 (2019) provisions provided the most accurate predictions of the moment capacity of the strengthened specimens. Full article

The strengthening of steel beams using hybrid fiber-reinforced polymers (HFRPs) has gained enormous attention over the last decades. Few researchers have investigated the effectiveness of the fastening techniques without a bonding agent to overcome the undesirable debonding failure of the bonded FRP–steel system. This paper reports the outcomes of experimental and numerical investigations conducted on steel beams strengthened by HFRP using steel bolts. Twenty-two steel beams were tested in four-point loading to investigate the effect of the HFRP length and the bolt arrangement on the flexural behavior of the strengthened systems. The observed failure modes, load-deflection relations, deflection profiles, and strain measurements were also studied. The tested beams showed a ductile behavior, with 15.1 and 22.2% enhancements in the yield and ultimate flexural capacities, respectively. Simplified empirical equations were developed to predict the ultimate load of the bolted HFRP–steel beams. ANSYS software was used to model the beams’ behavior and investigate the effects of the HFRP thickness, bolt spacing, steel grade, loading scheme, and beam length on the effectiveness of the adopted fastening technique. Increasing the HFRP length enhanced the utilization of HFRPs as well as the beam’s ductility, with a reduction of up to 51.2% in the mid-span deflection. Moreover, the strain compatibility of the HFRP–steel beams was improved with an 87.2% reduction in the interfacial slippage. The bolt arrangement showed an insignificant effect on the overall performance of the beams. The numerical results verified the effectiveness of the fastening technique in enhancing the flexural performance of the steel beams, with gains of up to 16.7% and 34.5% in the yield and ultimate load-carrying capacities, respectively. Full article

With the assessment of intermediate crack debonding (ICD) being a subject of main importance in the design of reinforced concrete (RC) beams strengthened in flexure with externally bonded fibre-reinforced polymer (FRP), several approaches to predict the debonding loads have been developed in recent decades considering different models and strategies. This study presents an analysis of formulations with different levels of approximation collected in the fib Bulletin 90 regarding this failure mode, comparing the theoretical predictions with experimental results. The carried-out experiments consisted of three RC beams strengthened with carbon FRP (CFRP) tested under a four-point bending configuration with different concrete strengths and internal steel reinforcement ratios. With failure after steel yielding, higher concrete strength, as well as a higher reinforcement ratio, lead to a higher bending capacity. In addition, the performance of the models is assessed through the experimental-to-predicted failure load ratios from an experimental database of 65 RC beams strengthened with CFRP gathered from the literature. The results of the comparative study show that the intermediate crack debonding failure mode is well predicted by all models with a mean experimental-to-predicted failure load ratio between 0.96 and 1.10 in beams tested under three- or four-point bending configurations. Full article

The hybrid retrofit system using FRP and concrete overlay applied on the top of slabs has proven effective in strengthening and overcoming logistical constraints, compared with conventional strengthening techniques using externally bonded composite materials to the underside of the slabs. Nevertheless, the performance of retrofitted slabs is governed by debonding failure due to the low bond strength between CFRP and concrete overlay. Thus, this study investigates the behavior of flexural strengthened slabs with FRP retrofit systems and the effect of bond–slip laws on debonding failure. Firstly, two full-scale RC slabs with and without a retrofit system were tested in a four-point bending setup as the control specimens. Then, the same retrofitted slab was simulated by utilizing the commercial program ABAQUS. A sensitivity analysis was conducted to consider the influence of bond–slip laws to predict the failure mechanism of the retrofitted slabs based on load–deflection relationships. The results showed that the strengthened slab enhanced the load-carrying capacity by 59%, stiffness by 111%, and toughness by 29%. The initial stiffness of 0.1K₀ and maximum shear stress of 0.13τ_max, compared with the corresponding values of Neubauer’s and Rostasy’s bond–slip law, can be used to simulate the global response of the retrofitted slab validated by experiment results. Full article

An experimental and analytical investigation was conducted on reinforced concrete (RC) beams strengthened in flexure using the near-surface mounted carbon-fiber-reinforced polymers (NSM-CFRPs) technique. A total of 11 full-scale RC rectangular beams were cast and tested under a monotonic three-point bending test, up to failure. The main test variables adopted in this study were the concrete compressive strength (high, medium, and low), the number of CFRP strips, and the strip length. The results indicated that the use of NSM-CFRPs strips in different configurations efficiently increased the load-carrying capacity of the strengthened RC beams, in which all these beams exhibited a higher moment resistance than the corresponding un-strengthened beam. Results also showed that all strengthening schemes were successful in increasing the flexural capacity of the specimens tested. Such increases ranged between 10.36% and 52.28%. Notably, a significant improvement in the ultimate load ratio was observed with beams having a low compressive strength of 17-MPa, then followed by the beams with medium strength (32-MPa), and finally beams with high compressive strength (47-MPa). The NSM technique reduced the occurrence possibility of the CFRP de-bonding failure mode. Furthermore, the test results were compared with theoretical predictions using the ACI 440.2R17 guidelines and showed a good agreement between these results. Full article

Sandwich structures fabricated from an aluminium skinned foam enclosed within a carbon fibre reinforced composite structure have the potential application for high-performance on- and off-road automotive vehicles. The deformations and failure of these types of structures are presented, and results indicate that the application of aluminium face sheets with aluminium foam (AF) aids to prevent the delamination of the outer layers of carbon fibre reinforced polymers (CFRP). The load carrying capacity has been increased by utilising a manufacturing method to maintain the adhesion between the core and the skins until the failure stage is reached. The core shear and de-bonded issue associated with this type of sandwich structure can be addressed by this manufacture method. The peak average flexure load capacity of an aluminium foam sandwich structure (AFSS) with a completely wrapped around CFRP skin was 2800 N with a mass of 191 g. This compares favourably with previously used AFSS without the skins, which had a peak average load of 600 N and a mass of 125 g. An initial finite element model for comparison purposes has been developed to represent the structure’s behaviour and predict the associated failure loads. It is proposed that CFRP wrapped around AFSS enhances the structural performance without significant weight gain. Full article

The advantages of using prestressed carbon fiber reinforced polymer (CFRP) for strengthening and retrofitting structures have been reported in recent years. In this regard, most of the studies on prestressed CFRP technique have been carried out in the laboratory test with small-scale and no damage (reinforced concrete) RC beam. However, the real structures that need to be retrofitted in service are often degraded or damaged due to early cracking. This paper aims at studying the effect of prestressed CFRP method on full-scale and damaged RC beams. The damaged levels of four full-scale damaged hollow RC beams taken from an old bridge were evaluated. One damaged beam was tested to check the residual capacity, and the other three were strengthened with prestressed composite strengthened CFRP and steel-carbon fiber reinforced polymer (SCFRP). The flexural behavior of non-strengthened and prestressed strengthened beams was investigated. During the experiments, the failure modes, deflection, yield and ultimate load, strains of concrete, steel reinforcements, and SCFRP were measured and analyzed. The results showed that the stiffness at the elastic stage was increased by 64.9%, 66.9%, and 67.1% after strengthened by SCFRP with 30%, 40%, and 60% prestressing level. Moreover, the ultimate load of damaged hollow RC beams were improved by 19.53%, 21.82%, and 31.9%, respectively. The flexural behavior of the severely damaged RC beam with strength reduction coefficient of 0.65 can be recovered after being strengthened by SCFRP with 40% prestressing levels. Meanwhile, SCFRP-concrete interface debonding failure occurred when the prestressing level exceed 60%, and the characteristics of brittle failure became more evident with increased prestressing level of the SCFRP. Full article

The combined seismic and energy retrofit of existing aged buildings represents a topic of importance for the building stock. The current study investigates the out-of-plane performance of a specific type of thermo-insulation scheme with panels attached on the external facades of multistory buildings. The investigation was carried out through flexure tests of prototype masonry specimens. From the comparison of their flexural performance, with or without thermo-insulating attachments, the influence of thermal insulation on the out-of-plane behavior of clay brick masonry is demonstrated. It was found that when the thermo-insulating attachment is in tension from such out-of-plane flexure of the masonry facade it performs in a satisfactory way and gives an increased flexural capacity for the assembly. The thermal insulating panels, although partially debonded from the masonry substrate at a limit-state, do not collapse, even when the masonry panel develops large flexural cracks. This is due to the presence of the used plastic anchors. When the thermo-insulating panel is subjected to compression during such an out-of-plane flexure the resulting increase in the out-of-plane load bearing capacity is relatively small. Based on these observations it can be concluded that such thermo-insulating panels may also lead to a less vulnerable seismic performance than that of the same masonry panel without this type of thermo-insulating attachment. This was also confirmed when the in-plane behavior was considered from a separate investigation already published. The employed numerical modeling was successful in simulating the most important aspects of the out-of-plane response of the tested masonry wallets with or without thermo-insulating attachments. The good agreement with observed performance as well as the general nature of this numerical simulation confirms its validity for further use. Full article

In this study, the structural behavior of small-scale wood beams externally strengthened with various fiber strengthened polymer (FRP) composites (i.e., flax FRP (FFRP), basalt FRP (BFRP), E-glass FRP (“E” stands for electrical resistance, GFRP) and their hybrid FRP composites (HFRP) with different fiber configurations) were investigated. FRP strengthened wood specimens were tested under bending and the effects of different fiber materials, thicknesses and the layer arrangements of the FRP on the flexural behavior of strengthened wood beams were discussed. The beams strengthened with flax FRP showed a higher flexural loading capacity in comparison to the beams with basalt FRP. Flax FRP provided a comparable enhancement in the maximum load with beams strengthened with glass FRP at the same number of FRP layers. In addition, all the hybrid FRPs (i.e., a combination of flax, basalt and E-glass FRP) in this study exhibited no significant enhancement in load carrying capacity but larger maximum deflection than the single type of FRP composite. It was also found that the failure modes of FRP strengthened beams changed from tensile failure to FRP debonding as their maximum bending load increased. Full article

Externally Bonded Reinforcement (EBR) technique has been widely used for flexural strengthening of concrete structures by using carbon fiber-reinforced polymers (CFRP). EBR technique offers several structural advantages when the CFRP material is prestressed. This paper presents an experimental and numerical study on reinforced (RC) slabs strengthened in flexure with prestressed CFRP strips as a structural strengthening system. The strips are applied as an externally bonded reinforcement (EBR) and anchored with either a mechanical or a gradient anchorage. The former foresees metallic anchorage plates fixed to the concrete substrate, while the latter is based on an accelerated epoxy resin curing followed by a segment-wise prestress force decrease at the strip ends. Both anchorage systems, in combination with different CFRP strip geometries, were subjected to static loading tests. It could be demonstrated that the composite strip’s performance is better exploited when prestressing is used, with slightly higher overall load carrying capacities for mechanical anchorages than for the gradient anchorage. The performed investigations by means of a cross-section analysis supported the experimental observation that in case a mechanical anchorage is used, progressive strip debonding changes the fully bonded configuration to an unbonded end-anchored system. The inclusion of defined debonding criteria for both the anchorage zones and free length between the anchorage regions allowed to precisely capture the ultimate loading forces. Full article

Debonding failure is the main failure mode in flexurally strengthened reinforced concrete beams by externally bonded or near surface mounted fibre reinforced polymer (FRP) composites. It is believed that FRP debonding will be initiated if the shear stress on the concrete-FRP interface reaches the tensile strength of concrete. However, it was found through experimental and analytical studies that the debonding mechanism of FRP composites has the potential of shear failure in combination with debonding failure. Moreover, the shear failure probably influences the debonding failure. Presently, there are very little experimental and analytical studies to investigate the influence of shear resistance of reinforced concrete (RC) beam on FRP debonding failure. The current study investigates and analyzes the effect of shear resistance on FRP debonding failure based on test results. The analytical results show that the shear resistance of RC beam has a great effect on flexural debonding load-carrying capacity of FRP-strengthened RC beam. The influence of shear resistance on flexural debonding load-carrying capacity must be fully considered in flexural strengthening design of RC beams. Full article