Bonded Carbon Fiber-reinforced Polymer Strips Research Articles

Fatigue performance simulation of reinforced concrete beams externally strengthened with side bonded CFRP sheets

Damaged Reinforced Concrete (RC) beams are commonly strengthened by bonding Carbon Fiber Reinforced Polymer (CFRP) strips to their soffits. However, inaccessible or narrow soffits limit the use of bottom-bonded CFRP strips. The Side Bonded (SB) CFRP technique overcomes this, yet studies on SB-CFRP-reinforced beams' fatigue behavior are limited. This paper presents a Finite Element (FE) model for simulating the fatigue behavior of RC beams externally strengthened with SB-CFRP sheets. The model incorporates cyclic-dependent CFRP-concrete interface degradation. Existing experimental results are utilized to validate its accuracy. Computational analyses are undertaken to explore the effects of CFRP dimensions, load and prestress levels, and end-U-shaped wrapping on fatigue performance. A simple model is proposed to predict fatigue life considering load and prestress levels. The FE model effectively predicts fatigue performance. Parametric studies indicate that narrow CFRP strips are unable to prevent concrete failure under high loads. Fatigue failure modes include rebar ruptures and CFRP delamination. Besides, the end-U-shaped wrapping reduces interface damage, extending fatigue life. The study emphasizes the sensitivity of vibration excitation method to CFRP debonding. The proposed equation efficiently predicts the fatigue life of RC beams with externally bonded CFRP strips on their sides.

Fatigue performance improvement of U-rib butt-welded connections of steel bridge decks using externally bonded CFRP strips

U-rib butt-welded connections in steel bridge decks are known to be highly susceptible to fatigue cracking, which can pose a direct threat to the safety and durability of bridge structures. Carbon fiber-reinforced polymer (CFRP) materials have shown promising potential for fatigue strengthening. This study investigated the effectiveness of CFRP application on the fatigue performance of U-rib butt-welded connections by employing full-scale fatigue tests and finite element analysis. Two single U-rib specimens with welded splice joints were initially tested to initiate real fatigue cracks under cyclic loading. Different techniques of CFRP installation were then proposed to promote the fatigue performance of cracked butt-welded connections. The failure modes and fatigue strength of the tested specimens were evaluated before and after strengthening. Research results showed that fatigue cracks were observed at the arc transition of butt welds, where remarkable incomplete penetration was also discovered. The fatigue strength grade of the tested U-rib butt-welded details was less than 71 MPa stipulated by Eurocode 3. However, after being repaired by CFRP sheet bonding, the equivalent fatigue life of U-rib butt welds with different damage degrees can be extended by 0.5–16.6 times that of the original ones. These findings suggest that CFRP strengthening should be applied instantly on U-rib butt-welded connections once fatigue cracks are found.

Importance of Concrete Mesoscale Structure on Bond-Slip Behavior of Externally Bonded CFRP Strips

Currently, the effect of the concrete substrate on the load-bearing capacity of externally bonded carbon fiber-reinforced polymer (CFRP) strips is determined in the guidelines solely on the basis of the concrete strength. In addition to strength, surface preparations can improve the bond resistance by exposing the aggregates of the substrate. However, the interlocking also depends on the parameters that derive from the inner concrete structure such as the aggregate grain dimension and shape, as well as the air void distribution. Therefore, the influence of the concrete inner structure on the bond is investigated with double shear tests by comparing specimens with equivalent concrete strength and varying grain shape and dimension. Bond parameters are calculated using a novel procedure based on optical measurements. Furthermore, crack paths are analyzed by means of computed tomography (CT) scans, which prove to be an essential inspecting technique for characterizing the inner concrete structure. The fracture energy is approximately 30% higher for concrete with a maximum grain size of 8 mm compared to an equivalent mortar with 4 mm maximum grain size.

Reinforced concrete slabs strengthened with externally bonded carbon fibre-reinforced polymer strips under long-term environmental exposure and sustained loading. Part 1: Outdoor experiments

Carbon fibre-reinforced polymers (CFRPs) can be used to increase the load-bearing capacity of concrete bridge deck slabs and their lateral cantilevers. In this study, experiments were performed to explore the long-term performances of loaded RC slabs strengthened on the top surface using non-prestressed and prestressed externally bonded (EB) CFRP strips. A real bridge application was simulated by applying mastic asphalt on the top surface. The effects of four years of outdoor exposure to elevated temperatures and solar radiation were considered. The results show that specimens strengthened with non-prestressed EB CFRP strips exhibit satisfactory behaviours, but a substantial increase in strain is shown in the CFRP strips over time. A slab strengthened with prestressed EB CFRP strips anchored with a so-called gradient anchorage experiences premature failure, owing to elevated temperatures and sustained loads.

Experimental study on flexural behaviour of reinforced concrete beams strengthened with passive and active CFRP strips using a novel anchorage system

The paper presents the research on reinforced concrete (RC) beams strengthened with carbon fibre reinforced polymer (CFRP) strips with various configurations in terms of anchoring and tensioning. The five full-scale RC beams with the total length of 6.0 m were strengthened with passive strips, without and with mechanical anchorages at their ends, as well as with strips tensioned by the novel prestressing system with three various prestressing levels ranging from 30 to 50% of the CFRP tensile strength. All RC beams were tested under static flexural load up to failure and they were investigated in a full range of flexural behaviour, including the post-debonding phase. The main parameters considered in this study include the use of mechanical anchorages, the effect of tensioning the strips and the influence of the various prestressing levels. Several performance indicators have been established to evaluate the beams’ behaviour. The study revealed that the RC beams strengthened using tensioned CFRP strips exhibited a higher cracking, yielding and ultimate moments as compared to the beams with passively bonded CFRP strips. Moreover, increasing the beams’ prestressing level has a significant positive influence on the performance of strengthened beams. However, it did not affect the ultimate load-bearing capacity of the beams. The optimal prestressing level for the novel system has been determined as 60% of CFRP tensile strength.

Static and Dynamic Stiffness of Reinforced Concrete Beams Strengthened with Externally Bonded CFRP Strips.

This paper presents experimental investigations of reinforced concrete (RC) beams flexurally strengthened with carbon fiber reinforced polymer (CFRP) strips. Seven 3300 mm × 250 mm × 150 mm beams of the same design, with the tension reinforcement ratio of 1.01%, were tested. The beams differed in the way they were strengthened: one of the beams was the reference, two beams were passively strengthened as precracked (series B-I), two beams were passively strengthened as unprecracked (series B-II) and two beams were actively strengthened as unprecracked (series B-III). Moreover, the strengthening parameters differed between the particular series. The parameters were: CFRP strip cross-sectional areas (series B-I, B-II) or prestressing forces (series B-III). The beams were statically loaded, up to the assumed force value, in the three-point bending test and deflections at midspan were registered. After unloading the beams were suspended on flexible ropes (the free-free beam system) and their eigenfrequencies were measured using operational modal analysis (OMA). The static measurements (deflections) and the dynamic measurements (eigenfrequencies) were conducted for the adopted loading steps until failure. Static stiffnesses and dynamic stiffnesses were calculated on the basis of respectively the deflections and the eigenfrequencies. The qualitative and quantitative differences between the parameters are described.

Nonlinear FEA of mechanical anchorages on CFRP-to-concrete bonded joint

The debonding of carbon fibre-reinforced polymer (CFRP) strips from the surface of concrete is one of the main premature failure modes for concrete beams that are externally strengthened with CFRP strips. Many anchorage systems are developed to prevent or delay the debonding process in these beams in order to improve the ultimate load capacities. In this study, nonlinear finite-element analyses (FEAs) are performed by employing ABAQUS software to describe the load-deflection behaviour and ultimate loading capacities of concrete beams whose flexure has been strengthened using externally bonded CFRP strips with mechanical anchorages. First, the nonlinear finite-element model results are validated using the results of an experimental study previously carried out by the authors. In the experimental study, the variables investigated are the CFRP strip width and the number and the arrangement of the mechanical anchorages. A good agreement is demonstrated between the numerical and the experimental results. Then, a parametric study is conducted to investigate the effects of the mechanical anchorages on the ultimate load capacities of concrete beams. Consequently, the variable, the CFRP bonding length, is included into the finite-element models that are corrected with the experimental results, and equations involving many variables concerned with the ultimate loading capacities are suggested. Finally, an ultimate load capacity multiplier is proposed enabling the calculation of the ultimate load capacities of beams mechanically anchored with CFRP strips, with regard to the CFRP strip width, the bonding length, the number of mechanical anchorages and the arrangement of the variables.

Shear transfer of concrete strengthened with externally bonded CFRP strips inclined to shear plane

Concrete with steel reinforcement across a shear plane was externally bonded using carbon fiber-reinforced polymer (CFRP) strips with different inclination angles to find optimal strengthening effect of shear resistance. Thirty-five Z-type specimens were tested and classified into two groups, i.e., the shear transfer group and shear component group, to investigate shear resistance and shear transfer behavior based on a component model. In this model, the shear resistance was contributed by six shear components, i.e., friction, aggregate interlock, adhesion, shear dilation, dowel action, and resistance parallel to the shear plane provided by CFRP strips. Results found that inclination angles of about 70 and 45° were optimal and associated with the maximum shear resistance for specimens with CFRP reinforcement ratios of 0.298% and 0.149%, respectively. Compared to the cases of CFRP strips perpendicular to the shear plane, additional percentage increases of the maximum shear resistance in a range of 8–23% were obtained in the cases using CFRP strips with optimal inclination angles. These extra benefits were mainly attributed to the increases in the friction and aggregate interlock, shear dilation, and resistance parallel to the shear plane provided by CFRP strips. A best-fitting expression was also proposed to predict the ultimate shear transfer capacities of strengthened reinforced concrete using CFRP strips with optimal inclination angles.

Influence of the asphalt pavement on the short-term static strength and long-term behaviour of RC slabs strengthened with externally bonded CFRP strips

Lateral cantilevers of Reinforced Concrete (RC) box-girder bridges might have a lack of negative bending resistance in cross-direction due to the increase of traffic loads or the degradation along the time of the material properties. Externally Bonded (EB) Carbon Fiber Reinforced Polymer (CFRP) strips on the top side of these bridge slabs are a possible strengthening technique for such purpose. The paper presents a study regarding the short- and long-term behaviour of such reinforcements where the specific case of a lateral cantilever element is investigated. Strengthening of negative bending moments on the upper deck side implicates the exposure of the composite reinforcement to both short- and long-term elevated temperature scenarios. During the construction phase, the application of warm mastic asphalt induces elevated peak temperatures in the epoxy resin of about 80°C with a subsequent slow cooling phase over several hours. In this investigation, the temperature evolution on several locations was measured. Then, the short-term residual bond strength of the strengthened system after such an exposure is checked with both lap-shear and large-scale tests. The long-term behaviour with the seasonal temperature fluctuation is another concern. For this purpose, a cantilever slab with a real asphalt layer under sustained load was installed and continuously monitored in time over one year.

Nonlinear 3D finite element simulation of reinforced concrete flat plates retrofitted with externally bonded CFRP strips

In this study, a comprehensive 3D non-linear finite element modelling procedure is presented for the simulation of reinforced concrete flat plates, retrofitted with externally bonded carbon fibre-reinforced polymer (CFRP) strips and subjected to punching shear loads. The presented modelling procedure may be used to conduct parametric numerical analyses on the response of reinforced concrete elements similar to considered in this study. The modelling procedure is applied on: (i) a solid, un-retrofitted flat plate and (ii) eight flat plates with openings and retrofitted with externally bonded CFRP strips. In the procedure, debonding of CFRP strips from concrete surfaces is accounted for by defining appropriate bilinear shear stress–slip and normal stress–gap relationships. In the model, the corner lift-offs observed at the corners of the flat plates are also considered by defining appropriate restrains and frictional support conditions. Results obtained from the proposed modelling procedure are verified using experimental data available in the literature. From the verification, accuracy of the proposed modelling procedure is confirmed in terms of structural responses such as load displacement relationships and crack propagations of the test specimens.

Bond slip behavior of anchored CFRP strips on concrete surfaces

In this study, strain distributions and the bond slip behaviors at the carbon fiber reinforced polymer (CFRP) strips and reinforced concrete interface were investigated. 14 concrete specimens with externally bonded CFRP strips were tested. Main variables considered in the experimental study were width of the CFRP strips, bond length of CFRP strips and the number of anchorages. The test results obtained from the experimental study are comparatively presented in terms of strain distributions on CFRP strips with anchorages, stress – displacement behaviors, initial stiffnesses, energy dissipation capacities of test specimens. From the test results it is observed that using anchorages is very effective in terms of ultimate load capacity, strain distribution and energy dissipation capacity. The results of the experimental study are compared with analytical results obtained from several bond strength models available in the literature. A bond slip model was proposed based on the results of the conducted tests. It was observed that bond stress of the CFRP strips increased with increasing number of anchorages.

Experimental and Theoretical Investigation on Shear Strengthening of RC Precraced Continuous T-beams Using CFRP Strips

Carbon fiber reinforced polymer (CFRP) sheets are externally bonded to reinforced concrete (RC) members to provide additional strength such as flexural, shear, etc. It has been widely accepted that carbon fiber reinforced polymers (CFRPs) can be used effectively to strengthen reinforced concrete (RC) members. This paper is intended to study and use externally bonded CFRP strips to repair and strengthen of RC continuous T-beams, as well as influence of material (CFRP) on repair of shear defect on RC continuous T-beams. And this defect that will repair by using different CFRP strips under sustained loading. Total of three RC continuous T-beams with identical size of 150x320x3650mm, flange width= 400mm and flange thickness = 120mm, and the orientation will involves 0/90 degree and 45/135 degree in 3 sides wrap schemes All beams will be tested under sustained loading. Tests result shows that the effectiveness and shear capacity of the CFRP strengthened specimens. The shear enhancement of the CFRP strengthened beams varied between 26.57% and 38.56% over the control beam. This study confirms that the CFRP strip technique significantly enhances the shear capacity of reinforced concrete shear beams.

Strengthening of the Net Section of Steel Elements under Tensile Loads with Bonded CFRP Strips

AbstractThe use of carbon-fiber-reinforced polymer (CFRP) is an increasingly common solution for the strengthening of structures. However, the majority of research and applications have focused on the retrofitting of concrete structures. The applications of adhesively bonded CFRP to enhance the load-carrying capacity of metallic elements has been widely studied in the aeronautical industry, but it is also a promising technique for the area of civil engineering. This paper presents an experimental study that was designed to verify the effectiveness of the use of CFRP for the strengthening of the net section of steel elements under tensile loading. A series of tensile tests were conducted with different bond lengths, number of layers, and surface preparations of the steel. The specimens consisted of double lap joints and steel plates with various hole configurations. The ultimate load, the failure mode, and the effective bond length for CFRP-strengthened specimens were determined. The results showed that us...

Behaviour of Reinforced Concrete Beams Strengthened by CFRP Wraps with and without End Anchorages

Use of Carbon Fibre Reinforced Polymer (CFRP) strips as externally bonded reinforcement, is a technically sound and practically efficient method of strengthening and upgrading reinforced concrete (RC) members. Externally bonded CFRP strips help in improving the structure performance by reducing deflections and/or cracking and increasing ultimate strength. The ultimate capacity of the strengthened beam is controlled by either compression crushing of concrete, rupture of CFRP and flexural shear cracking induced de-bonding atconcrete-CFRP interface. Present study deals with the use of externally bonded CFRP wraps instead of strips to strengthen RC beams in flexure with and without end anchorages. Six beams strengthened with CFRP wraps at bottom with and without anchorages, were tested with different a/d ratios. It was observed that use of CFRP wraps resulted in significantly increasing the stiffness and ductility of the RC beams along with increasing load carrying capacities of the strengthened RC beams with end anchorages.

Strengthening of defected beam–column joints using CFRP

This paper presents an experimental study for the structural performance of reinforced concrete (RC) exterior beam–column joints rehabilitated using carbon-fiber-reinforced polymer (CFRP). The present experimental program consists of testing 10 half-scale specimens divided into three groups covering three possible defects in addition to an adequately detailed control specimen. The considered defects include the absence of the transverse reinforcement within the joint core, insufficient bond length for the beam main reinforcement and inadequate spliced implanted column on the joint. Three different strengthening schemes were used to rehabilitate the defected beam–column joints including externally bonded CFRP strips and sheets in addition to near surface mounted (NSM) CFRP strips. The failure criteria including ultimate capacity, mode of failure, initial stiffness, ductility and the developed ultimate strain in the reinforcing steel and CFRP were considered and compared for each group for the control and the CFRP-strengthened specimens. The test results showed that the proposed CFRP strengthening configurations represented the best choice for strengthening the first two defects from the viewpoint of the studied failure criteria. On the other hand, the results of the third group showed that strengthening the joint using NSM strip technique enabled the specimen to outperform the structural performance of the control specimen while strengthening the joints using externally bonded CFRP strips and sheets failed to restore the strengthened joints capacity.

Numerical Analysis of Underground Concrete Arch Structure Strengthened with Near-Surface Bonded CFRP Strips

To strengthen underground arch structure, the method with near-surface bonded carbon fiber reinforced polymer (CFRP) strips is proposed. A nonlinear finite element program is built to analyze concrete arch structure strengthened with CFRP. Factors that influence the strengthening effect are discussed. After arch strengthened with near-surface bonded CFRP strips, the cracking load, the yield load and the ultimate load has been greatly improved, the stiffness has also been greatly improved. The bearing capacity and the stiffness of arch structure increase with the layer quantity of CFRP increasing, increase with the strip quantity of CFRP in units increases. In the condition of the layer quantity and the strip quantity of CFRP being identical, the increasing ratio of load decreases with the reinforcement ratio increasing.

Temporary bond strength of partly cured epoxy adhesive for anchoring prestressed CFRP strips on concrete

Externally bonded Carbon Fiber Reinforced Polymer (CFRP) strips have been used for strengthening reinforced concrete structures. This paper presents an experimental study on the debonding of externally bonded CFRP strips anchored to a concrete substrate by a commercial epoxy adhesive. The study represents the basis for the characterization of an innovative ‘gradient method’, giving the possibility to anchor prestressed CFRP strips to concrete without the use any mechanical anchorage systems such as plates and bolts. Bond between the two components is achieved by an epoxy adhesive able to carry loads after an accelerated curing process under elevated temperatures. The effect of heating configuration/duration, strip thickness and bond length on the temporary bond resistance have been investigated using prestressed and non-prestressed CFRP-strips. Besides the optimization of the heating elements necessary for the curing process, curing parameters for an optimal temporary bond strength could be determined. Twenty-five minutes of heating and curing at 90°C was found to be an optimum heating configuration, resulting in better short-term mechanical performances than after conventional curing at room temperature for several days. The main reason is a temporarily softer adhesive which allows the use of the full bond length by reducing shear force peaks.

Structural Performance of CFRP-Strengthened RC Slabs in a Corrosive Environment

This study was undertaken to address the effect of the main steel corrosion on the structural performance of RC slabs strengthened with carbon-fiber-reinforced polymer (CFRP) strips and exposed to a corrosive environment. A total of eight specimens ( 500×100×1,500 mm ) were constructed and tested under monotonic static loading. Three specimens were CFRP-strengthened and corroded, three specimens were CFRP-strengthened and kept at room temperature, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Three different strengthening schemes were applied: (1) externally bonded CFRP strips; (2) externally bonded CFRP strips provided with CFRP anchors; and (3) near surface mounted (NSM) CFRP strips. During the corrosion process, the specimens were placed in a small tank filled with sodium chloride (NaCl) solution concentration (3%) which covered only the slabs’ bottom third, and corrosion was induced by means of an impressed current. The corrosion process lasted f...

Behavior of RC T-section beams strengthened with CFRP strips, subjected to cyclic load

This paper presents results of an experimental investigation on T-section reinforced concrete (RC) beams strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) strips. Specimens, one of which was the control specimen and the remaining six were the shear deficient test specimens, were tested under cyclic load to investigate the effect of CFRP strips on behavior and strength. Five shear deficient specimens were strengthened with side bonded and U-jacketed CFRP strips, and remaining one tested with its virgin condition without strengthening. The type and arrangement of CFRP strips and the anchorage used to fasten the strips to the concrete are the variables of this experimental work. The main objective was to analyze the behavior and failure modes of T-section RC beams strengthened in shear with externally bonded CFRP strips. According to test results premature debonding was the dominant failure mode of externally strengthened RC beams so the effect of anchorage usage on behavior and strength was also investigated. To verify the reliability of shear design equations and guidelines, experimental results were compared with all common guidelines and published design equations. This comparison and validation of guidelines is one of the main objectives of this work. The test results confirmed that all CFRP arrangements differ from CFRP strip width and arrangement, improved the strength and behavior of the specimens in different level significantly.

板曲げを受けるＣＦＲＰ板接着鋼板のはく離強度

For rapid repair and strengthening of steel members, carbon fiber reinforced polymer (CFRP) strips can be conveniently used. However, their deboding mechanism and design parameters are still to be clarified to prevent any debonding problem. Bending tests of steel plates with bonded CFRP strips are carried out to see the debonding stresses and failure mechanism. Then, differential equations are derived to express axial and shear forces in CFRP strip and steel plate, as well as shear and normal stresses in adhesive. It is shown that the debonding strength of CFRP bonded to steel plate in bending can be evaluated by the maximum principal stress developed in adhesive at the end of the CFRP strips.