Crack-induced Debonding Research Articles

Study on the Stress Threshold of Preventing Interfacial Fatigue Debonding in Concrete Beams Strengthened with Externally-Bonded FRP Laminates

Externally-bonded FRP laminate is widely used in structural strengthening due to the many advantages of FRP materials. Further enhancement of the strengthening effect can be achieved by inducing prestress into the FRP laminate. However, FRP debonding is still a main issue of this strengthening method, especially the Intermediate Crack-induced debonding (IC debonding). To better understand the impact of FRP debonding on the strengthening effect, a series of parameter analyses were conducted in this study based on the fatigue life prediction model proposed by the authors. The proposed model involves the fatigue damage accumulation of components of the beam, the mutual interaction between each component, and the impact of FRP fatigue debonding. As a result, a stress threshold for preventing FRP fatigue debonding in strengthening the concrete beam was proposed, which aimed to avoid safety hazards caused by IC debonding in practical engineering.

Bending performance of RC slabs strengthened by CFRP sheets using sprayed UHTCC as bonding layer: Experimental study and theoretical analysis

Debonding failure is one of the main drawbacks of reinforced concrete (RC) structures strengthened with externally bonded (EB) fiber reinforced polymer (FRP) systems. Due to the huge application potential of sprayed ultra-high toughness cementitious composite (UHTCC) in strengthening and retrofit, this study, with the aim of delaying or preventing intermediate crack-induced debonding of EB FRP sheets, experimentally and analytically investigated the flexural behavior of RC slabs strengthened by EB CFRP sheets with sprayed UHTCC as bonding layers, which has been rarely reported in previous research works. For this purpose, five RC slabs with low strength concrete and low reinforcement ratio, including one control specimen and four strengthened specimens repaired by EB CFRP sheets with sprayed UHTCC as bonding layers, were manufactured and tested under four-point bending load. Variable parameters in the four strengthened specimens included the thickness (10 mm, 20 mm, and 30 mm) and length (600 mm and 900 mm) of the sprayed UHTCC bonding layer. The experimental results indicated that the specimens strengthened by EB FRP sheets with sprayed UHTCC as bonding layers experienced higher load-bearing capacity (24.43 %–120.92 %) and toughness (43.15 %–166.14 %), compared to the control specimen. The thickness of the UHTCC bonding layer had a significant effect on the structural performance. By increasing the thickness from 10 mm to 20 mm and from 20 mm to 30 mm, the ultimate load was enhanced by 32.69 % and 33.81 %, while the ultimate deflection was decreased by 14.99 % and 41 %, respectively. The test findings also showed that sprayed UHTCC had a strong cracking-control ability and that no debonding failure occurred between the UHTCC layer and the CFRP sheet, while interface slip was observed between the concrete substrate and the UHTCC layer. The sectional analysis method was then used to propose an analytical model for analyzing the composite action between the concrete substrate and the strengthening material, as well as a model for predicting the load-bearing capacity of strengthened specimens. Comparing the analytical results to the experimental ones revealed that the proposed models can properly predict the flexural behavior of RC slabs strengthened by EB CFRP sheets with sprayed UHTCC as bonding layers under four-point bending load.

Study on the intermediate crack-induced debonding strain of FRP-strengthened concrete members using the updated BP neural network

Fiber reinforced plastics (FRP) are often used to enhance the capacity of the reinforced concrete beam (RC beam). However, the debonding failure is often observed due to the effect of the complex environment and the random loads. The debonding failure includes three types: plate end interfacial debonding (PE debonding), critical diagonal crack-induced debonding (CDC debonding), and intermediate flexural crack-induced interfacial debonding (IC debonding). In order to investigate the IC debonding strain of RC beam strength by FRP, this paper proposed some data-driven models to explore the IC debonding strain, based on the machine learning approaches The concrete strength, shear span proportion, the proportion of anchorage length to shear width, tensile reinforcement proportion, steel yield strength, stirrup reinforcement ratio, FRP stiffness, and the proportion of sheet span to beamwidth were regarded as the input parameters. The IC debonding strain was regarded as the output. It was found that the BP model can predict the IC debonding strain well. However, the BP data-driven model is easy to fall into a local minimum, and it is very difficult to converge, which has a negative effect on the accuracy of the model. The Sparrow Search Algorithm (SSA) was proposed to update it. The results indicated that the neural network optimized by SSA with lowest relative error, which can predict the IC debonding strain well. In addition, a study on the importance of each input found that the concrete strength, shear span proportion, and reinforcement yield strength will have a big impact on the IC debonding strain.

Intermediate crack-induced debonding in CFRP-retrofitted notched steel beams at different service temperatures: Experimental test and finite element modeling

Carbon fiber-reinforced polymer (CFRP)-retrofitted steel beams are likely exposed to significant service temperature variations due to seasonal and daily temperature changes. However, there is lacking research on the structural performance of CFRP-retrofitted steel beams under service temperature variations. This paper presents an experimental study of fourteen beams including one intact beam, one notched beam and twelve CFRP-retrofitted notched beams tested at various service temperatures from -20°C to 80°C. The structural properties including load-deflection curves, load capacities at initial and ultimate debonding, crack mouth opening displacements at the notch and the CFRP strain measurements were examined and compared at different service temperatures. The debonding loads of CFRP-retrofitted beams were increased with the temperatures from -20°C to 60°C and decreased at 80°C. A formula was proposed to derive the local bond-slip behavior of the CFRP-to-steel interface at different temperatures from the CFRP strain measurements. The shear stiffness of the interface generally decreased with the service temperature growth, while the interfacial fracture energy first increased and then decreased. A finite element model was proposed to gain insight into the effects of interfacial thermal stress and temperature-dependent bond properties on the bond behavior and structural performance of CFRP-retrofitted beams at different service temperatures.

A new method to investigate the intermediate crack-induced debonding strain

In order to investigate the interfacial debonding (IC) strain of the reinforced concrete structures strengthened by fiber-reinforced polymer (FRP), this paper proposed some data-driven models to explore it, considering the effect of random factors both in material and environment. The concrete strength, shear span proportion, the proportion of anchorage length to shear width, tensile reinforcement proportion, steel yield strength, stirrup reinforcement ratio, FRP stiffness, the proportion of sheet span to beamwidth, ratios of environmental temperature to the maintenance standard temperature, and ratios of environmental humidity to the relative humidity were regarded as the inputs, the IC debonding strain was regarded as the output. The results indicate that the neural network optimized by the sparrow search algorithm not only has a relatively small error but also has stronger robustness. In addition, a study on the importance of the inputs on the IC debonding strain indicates that the concrete strength, FRP stiffness, and ratios of environmental temperature to the maintenance standard temperature are the most significant factor to influence the IC debonding strain.

Edge and Crack-Induced Debonding Analysis of FRP-Strengthened Concrete Beams Using Innovative Beam Finite Elements

In order to analyze the edge debonding and intermediate flexural crack–induced debonding of fiber-reinforced polymer (FRP)-strengthened concrete beams, several innovative beam finite elements were developed in this paper. The proposed elements were capable of modeling the concrete, adhesive layer, and FRP sheet in one macroscopic element. The explicit formulations of the proposed element stiffness matrices were given to simulate the adhesive layer in different loading stages. Comparisons against several numerical examples available in the literature were given to validate the proposed elements. The effects of the FRP tapering ends, load, and crack location on the interfacial shear stress within adhesive layers were effectively investigated using the proposed elements. It was found that the proposed elements could conveniently predict the edge and flexural crack–induced debonding behavior of FRP-strengthened beams with accuracy and computational efficiency.

Experimental and Numerical Investigations on the Behavior and Failure Mechanism of RC Beams Strengthened with Near-Surface Mounted High-Strength Aluminum Alloy Bars

This paper presents experimental and numerical investigations on the flexural behavior of reinforced concrete beams strengthened with near-surface mounted high-strength aluminum alloy (AA) bars subjected to four-point bending load. A total of five RC beams, including one control beam and four beams strengthened using near-surface mounted (NSM) AA bars with or without CFRP U-warp end anchorage, were constructed and tested monotonically up to failure. To get a better understanding of the flexural behavior and failure mechanism of the tested specimens, 3D finite element models were developed. A comparison between the finite element (FE) and experimental results was carried out and therefore confirmed the ability of the developed FE models to accurately predict the flexural performance of the RC beams strengthened with NSM AA bars. In addition, FE simulations provided an in-depth investigation of the intermediate crack-induced debonding (ICD) and concrete cover separation (CCS) failures. It is observed from the experimental and FE results that compared to the control specimen, more than 34% increase in the load-carrying capacity of the NSM strengthened specimens was achieved. In addition, the CFRP U-wrap end anchorage contributed to the prevention or delay of the CCS and ICD failures and hence increased the load-carrying capacity and enhanced the displacement ductility of the strengthened beam specimens. Furthermore, a FE parametric study was carried out to investigate the influence of different parameters on the performance of the RC beams strengthened with NSM AA bars, and recommendations based on the parametric study for efficient application of the NSM strengthening technique were given.

Effects of temperature variation on intermediate crack-induced debonding and stress intensity factor in FRP-retrofitted cracked steel beams: An analytical study

Fiber-reinforced polymer (FRP)-retrofitted steel structures in service are likely to experience significant temperature variations due to seasonal and diurnal service temperature changes. Even without the material system degradation, such temperature variations may influence the intermediate crack-induced (IC) debonding mechanism in FRP-retrofitted steel structures, mainly due to the interfacial thermal stress induced by the different thermal expansion coefficients of the FRP plate and the substrate steel. This paper presents a closed-form analytical solution for the full-range debonding process of an FRP-retrofitted notched steel beam under combined thermal and mechanical loading. A bilinear bond-slip relationship is used for describing the non-linear property of the bonding layer. The effects of thermal stress on the interfacial stress distribution, the IC debonding load and the stress intensity factor (SIF) at the notch are examined for the FRP-retrofitted steel beam at various service temperatures. The analytical solution is verified through the comparison between analytical and finite element (FE) results. Parametric investigations have indicated that a temperature decrease may lead to a reduced IC debonding load while an increased SIF, and vice versa. Such effect is more significant when a thicker and stiffener FRP plate is applied to retrofit the steel beam.

Effect of Stirrups on Plate End Debonding in Reinforced Concrete Beams Strengthened with Fiber Reinforced Polymers.

Plate end (PE) debonding is one of the critical debonding failure modes that may occur in reinforced concrete (RC) beams strengthened with externally bonded fiber reinforced polymers (FRPs). This study investigated the effect of internal steel stirrups on the PE debonding failure load of FRP-strengthened RC beams. The dimensions of the beams were 3400 × 400 × 200 mm. The beams were strengthened with carbon FRP (CFRP) sheets bonded to the soffit of the beams. The beams were divided into two series based on the distance of the cutoff point of the CFRP sheets from the nearest support. This distance was 50 mm or 300 mm, and the amount of steel stirrups was varied in terms of varying the stirrup diameter and spacing. The beams were simply supported and tested under four-point bending. The test results indicate that the effect of stirrups on the load carrying capacity of the beams was more pronounced for the beams with CFRP sheets extended close to the supports. It was also indicated that beams with larger amounts of stirrups failed in PE debonding by concrete cover separation while beams with lower amounts of stirrups failed in PE by either PE interfacial debonding or critical diagonal crack-induced debonding. The beams were analyzed using several analytical models from design guidelines and the literature. The result of analysis indicates that most of the available models failed to reflect the effect of stirrups in predicting PE debonding failure load of the beams. On the other hand, the models of El-Sayed et al. and Teng and Yao were able to capture such an effect with the best predictions provided by El-Sayed et al. model.

Investigation of crack growth in CFRP-concrete interface under fatigue loading

In this paper, the fatigue crack growth behavior along the interface between carbon fiber reinforced polymer (CFRP) sheets and concrete induced by intermediate flexural crack in CFRP-sheet strengthened concrete beams was studied, both experimentally and theoretically. The fatigue crack growth behavior of the intermediate crack-induced debonding was investigated by modified beam tests. The major variables considered in the test were loading amplitude, concrete strength, and the width ratio of CFRP-sheet to concrete. Test results indicated that three stages were observed in the evolution of CFRP-concrete interfacial crack propagation under cyclic loading, including debonding initiation, steady propagation, and instability propagation. Furthermore, the fatigue crack propagation rate grew with the increase of the load level, concrete strength, or the width ratio of CFRP-sheet to concrete, and was reduced as load cycles increased in the steady propagation stage due to the interfacial friction existed in the debonding zone. Based on the test results and fracture mechanic method, a crack growth prediction model, which considered the effects of concrete strength and width ratio of CFRP-sheet to concrete, was proposed for predicting the whole crack growth process and fatigue life of intermediate crack-induced debonding of CFRP-concrete interface under cyclic loading. The predicting results using this proposed model agreed well with test results.

Experimental investigation of corrosion-damaged RC beams strengthened in flexure with FRP grid-reinforced ECC matrix composites

In this study, composites fabricated using a fiber-reinforced polymer (FRP) grid-reinforced engineered cementitious composite (ECC) matrix were used for the flexural strengthening of corrosion-damaged reinforced concrete (RC) beams in combination with a patch repair technique. After the longitudinal reinforcements were corroded to the expected ratio by the electrochemically accelerated method, the corrosion-damaged concrete cover was removed, and then repaired with FRP grid-reinforced ECC matrix (FGREM) composites. The investigated test variables were mainly FRP grid types (carbon and basalt), strengthening amounts, corrosion level, and longitudinal reinforcement ratio. The experimental results indicated that the bearing capacities of the corroded beams were sufficiently improved using this strengthening technique. The intermediate debonding at the concrete substrate-to-ECC interface caused by flexural cracks could be completely suppressed. The use of a carbon fiber reinforced polymer (CFRP) grid with higher stiffness could produce a weaker ductile behavior and prematurely trigger critical diagonal crack-induced debonding. Furthermore, the flexural bearing capacities of the basalt-FGREM strengthened beams could be reasonably predicted according to the ACI-549.4R-13 provisions. However, the predicted results were relatively dangerous for carbon-FGREM strengthened beams, which failed under the control of the critical diagonal crack-induced debonding mode.

Debonding development in cracked steel plates strengthened by CFRP laminates under fatigue loading: Experimental and boundary element method analysis

Carbon fiber reinforced polymer (CFRP) laminates can effectively enhance the fatigue life of steel structures. However, few studies have investigated the influence of crack-induced debonding in the CFRP-steel interface on the CFRP strengthening efficiency and the relationship of crack propagation and debonding development. This study experimentally and numerically – with the boundary element method (BEM) – investigated crack propagation and debonding development in CFRP-strengthened cracked steel (Q345, Q460, and Q690) plates. The fatigue test specimens were subjected to a maximum stress of 50% steel yield stress and a stress ratio of 0.1, during which real-time changes in CFRP strain distribution were recorded by digital image correlation (DIC). The CFRP strain gradient calculations showed that crack-induced debonding was crack length-dependent. The numerical fatigue life results were in good agreement with the experimental data when considering debonding and overestimated the experimental data without considering debonding, demonstrating the necessity of considering crack-induced debonding in calculation and design. Furthermore, a relationship was obtained between crack propagation and debonding development.

Shear behavior of flexural CFRP-strengthened RC beams with crack-induced delamination: Experimental investigation and strength model

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.

Fatigue behavior of RC beams strengthened with FRP considering the influence of FRP-concrete interface

As an indispensable component of structure strengthened with FRP, the bond behavior of FRP-concrete interface is very important. In this paper, a theoretical model is established to analyze fatigue behavior of FRP-strengthened RC beam, and the bond-slip relationships are unified into one model. Two failure criteria are proposed according to steel reinforcement rupture and the intermediate crack-induced (IC) debonding. Through the theoretical model, the theoretical results show that the failure mode changes from the steel reinforcement rupture to the IC debonding as the peak loading increases. Therefore, the fatigue life prediction is more accurate by considering the corresponding failure modes.

Mitigation of IC debonding in FRP-plated concrete slabs using patch anchors

Reinforced concrete (RC) slabs can be strengthened using fiber reinforced polymers (FRPs) to enhance their flexural and shear resistance. However, premature debonding of the FRP material limits the level of strengthening achievable and represents an under-utilization of the FRP material. As a result, a variety of anchorage systems have been explored by researchers in an attempt to prevent or delay this type of failure. This paper presents a new FRP patch anchorage system which can be used to mitigate the commonly-observed failure mode of intermediate crack-induced (IC) debonding. This is the first experimental application where patch anchors are investigated in flexurally-strengthened RC slabs. Seven slabs 2700 mm × 400 mm × 150 mm in size were tested. Of these, two were unstrengthened, three were strengthened with 100 mm wide × 1.4 mm thick FRP sheet and two were strengthened in the same manner and anchored with four patch anchors. The use of patch anchors showed significant enhancement of both the strength and ductility of RC slabs compared with unanchored specimens.

Combined effects of wetting–drying cycles and sustained load on the behaviour of FRP-strengthened RC beams

This paper presents the results of an experimental study designed to investigate the performance of reinforced concrete (RC) beams strengthened with fibre-reinforced polymer (FRP) sheets subjected to wetting–drying cycles and a sustained load. Twenty-one RC beams were manufactured and externally bonded with carbon-FRP (CFRP) or glass-FRP (GFRP) sheets. Two U-shaped FRP anchorages were also applied on the ends of the beams to prevent end peeling failure. The FRP-strengthened beams were subjected to combined wetting–drying cycles in 5% NaCl solution and sustained loads for 0, 90, 180, and 360 days at load levels of 0%, 30%, and 60% of the ultimate loading capacity of the unstrengthened beam. During this period, the mid-span deflections and cracks of the beams were monitored, following which the beams were monotonically loaded to failure under a four-point bending. The failure patterns, mid-span deflections, and debonding loads were also examined. The test results showed that during sustained loading, the time-dependent and residual deflections of the FRP-strengthened RC beams increased with the increase in their periods of sustained loading and load levels. Intermediate crack-induced (IC) debonding of the CFRP-strengthened RC beams was induced by concrete separation and CFRP–concrete interface debonding, whereas that of the GFRP-strengthened RC beams was mainly caused by GFRP–concrete interface debonding. Combined wetting–drying cycles and a sustained load had an adverse effect on the yielding and debonding load of the FRP-strengthened RC beams, and the ductility of the GFRP-strengthened RC beams was higher than that of the CFRP-strengthened beams. In addition, a model considering the effects of wetting–drying cycles and sustained loads was proposed to predict the debonding load of FRP-strengthened RC beams, and it showed a reliable prediction.

Experimental Investigation of Fatigue Debonding Growth in FRP-Concrete Interface.

An externally bonded fiber reinforced polymer (FRP) plate (or sheet) is now widely used in strengthening bending members due to its outstanding properties, such as a high strength to weight ratio, easy operating, corrosion and fatigue resistance. However, the concrete member strengthened by this technology may have a problem with the adhesion between FRP and concrete. This kind of debonding failure can be broadly classified into two modes: (a) plate end debonding at or near the plate end, and (b) intermediate crack-induced debonding (intermediate crack-induced (IC) debonding) near the loading point. The IC debonding, unlike the plate end debonding, still needs a large amount of investigation work, especially for the interface under fatigue load. In this paper, ten single shear pull-out tests were carried out under a static or fatigue load. Different load ranges and load levels were considered, and the debonding growth process was carefully recorded. The experimental results indicate that the load range is one of the main parameters, which determines the debonding growth rate. Moreover, the load level can also play an important role when loaded with the same load range. Finally, a new prediction model of the fatigue debonding growth rate was proposed, and has an excellent agreement with the experimental results.

Bond enhancement for NSM FRP bars in concrete using different anchorage systems

Studies have shown that the near-surface mounted (NSM) FRP technique provides better bond performance than the externally bonded (EB) FRP method in strengthening applications. However, undesirable intermediate crack-induced debonding failures and end debonding failures are still frequently observed in NSM FRP reinforcements. In this study, the additional ribs (ARs) anchorage system and the wire mesh mortar protection layer (WML) anchorage system were proposed in an attempt to eliminate such premature debonding failures in NSM FRP strengthening system. The testing results of 20 direct pull-out specimens were used to verify the feasibility of the two proposed anchorage systems in improving the bond behaviour of NSM FRP bars. The individual use of the ARs anchorage system and WML anchorage system enhanced the bond strength of the NSM FRP bars up to 40.7% and 69.7%, respectively. The combined use of the two developed anchorage systems was found to be the most effective in increasing the bond performance of the NSM FRP bars, providing a maximum bond enhancement of 114.3%.

Debonding analysis of curved RC beams externally bonded with FRP plates using CZM

External bonding of fiber-reinforced polymer (FRP) plates has been widely used to strengthen reinforced concrete (RC) structures. The effectiveness and efficiency of this technique largely depend on the bonding behavior between the strengthening FRP plate and the RC substrate. This work initially extends the state space method combined with its numerically stable solution to present an elastic analysis of the interfacial shear stress in curved RC beams externally strengthened by FRP plates. The processes of the plate end debonding and intermediate crack-induced (IC) debonding in the FRP-strengthened curved beam is analyzed for the first time by introducing the cohesive zone model (CZM) for simulation of the adhesive layers. Compared with other analytical methods, the present method is advantageous in its application to much more complex situations, such as the general bond-slip laws, arbitrary boundary conditions and load cases, and variable cross-sectional properties of the curved beams and FRP plates. Finally, some numerical examples are given, and the results demonstrate that the tapering at the ends of FRP plates can significantly reduce the interfacial shear stress, and the debonding area will extend to the whole interface immediately once the load reaches the peak value for the plate end debonding.

Reliability-based design of FRP flexural strengthened reinforced concrete beams: Guidelines assessment and calibration

In the past few decades, many guidelines and standards for FRP strengthened reinforced concrete (RC) structures have been presented based on the theory of reliability-based limit state with the load and resistance factor design (LRFD) approach. The LRFD method consists of two formats: the resistance reduction factor format and the material partial safety factor format. In this paper, several well-recognized design guidelines with these two formats for FRP flexural strengthened RC beams have been assessed based on model error and reliability index. First, the model error of each design guideline was evaluated based on a collected test database. Then, the reliability index was obtained by the importance sampling (IS) method, in which the optimum density function is determined by the first-order reliability method (FORM). Finally, the calibration of resistance reduction factor or FRP partial safety factor was performed with different target reliability indexes. The results show that: (1) The model error of each failure mode is different for each design guideline. For flexural failure, China’s GB 50608 2010 has the lowest model error. For intermediate crack-induced (IC) debonding, UK’s TR 55 2012 shows the highest model error. For end debonding, Italy’s CNR-DT 200 R1/2012 is less conservative. (2) It is difficult to meet the pre-given reliability level for some failure modes in several design guidelines using the originally suggested design parameters. For example, GB 50608 2010, fib T5.1 2019 and JSCE 2001. (3) The calibrated resistance reduction factors or the FRP partial safety factors are provided to satisfy the target reliability level. However, it is meaningless and uneconomical to adopt partial safety factors that are too large or reduction factors that are too small to fit into a design. Other options can be chosen such as applying additional anchorages to avoid end debonding, increasing the FRP bonding area, and reducing the FRP thickness to avoid the probability of IC debonding failure.