IC 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.

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.

Reliability assessment of IC debonding in FRP strengthened RC beams using neural network

IC debonding is a common failure mode in fiber-reinforced polymer (FRP) strengthened concrete beams. Guidelines proposed several models to predict the ultimate strain in the FRP when IC debonding occurs. The reliability of these models has great importance in the design context. Most studies used assumptions that decrease the accuracy of the reliability analysis. In this study, by using the finite element method and neural network, the use of simplifying assumptions is reduced, and the accuracy of analysis is increased. To this end, the bond behavior between the FRP sheet and concrete is accurately modeled by finite element modeling. The neural network is used to find the relationship between the inputs affecting the debonding behavior and the outputs obtained from the finite element analysis, providing an implicit relationship for the reliability analysis. The results indicate the high accuracy of the neural network (NNET) method. The NNET method's error in predicting the beam's ultimate load for nearly 50% of the samples was in the range of 0–5%. The reliability index obtained from the NNET method for all models was smaller than the normal method. The NNET method suggests that the effect of large live loads on reducing the reliability index is much more than the value predicted by the normal method. To achieve the reliability index in the range of 3.3–3.5, the resistance reduction factor of 0.8 can be suggested for the ACI IC debonding model.

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.

Fatigue Life Prediction of CFRP-Strengthened RC Beams with Flexural Crack under Hygrothermal Environments

The durability of reinforced concrete (RC) beams strengthened with carbon fiber-reinforced polymer (CFRP) is a worldwide concern in structural engineering. As an important part of the strengthened beam, the performance of the CFRP–concrete interface under hygrothermal environments is a delicate problem. In this paper, the fatigue behavior of CFRP-strengthened RC beams is analyzed by a theoretical model. In the model, CFRP–concrete interface degradation under hygrothermal environments is involved. Since interface debonding and rebar fracture induced by intermediate cracking are two typical failure modes, the damage models of rebar and the CFRP–concrete interface are established. Based on the theoretical model, the failure mode of CFRP-strengthened RC beams can be predicted, and fatigue life can be determined. The results showed that IC debonding is more likely to occur under hygrothermal environments. The accurate prediction of failure modes is essential for fatigue life prediction.

Inclined FRP U-jackets for enhancing structural performance of FRP-plated RC beams suffering from IC debonding

Intermediate crack debonding (i.e., IC debonding) commonly controls the failure of FRP-plated RC beams. Such IC debonding often occurs with the maximum strain of the FRP soffit plate being far lower than its rupture strain, thus leading to a low utilization of the high strength of expensive FRP material. It is of increasing interest of the research and engineering community to explore an effective anchorage for enhancing the structural performance of FRP-plated RC beams suffering from IC debonding. Inclined FRP U-jacketing is among the easiest and most effective options for this purpose. This paper presents an experimental study to systematically investigate the effect of inclined FRP U-jacketing on the structural performance of FRP-plated RC beams suffering from IC debonding. Eight full-scale RC beams were tested with the width, height and inclination of the U-jacket as the experimental variables. Findings from the present tests show that inclined FRP U-jacketing could successfully lead to concrete crushing failure from IC debonding in the control specimen without anchorage, and significant increase in the load and mid-span deflection of up to 55.8% and 229% over these of the control specimen.

Mitigation of intermediate crack debonding in FRP-plated RC beams using FRP U-jackets

Intermediate crack debonding (i.e., IC debonding) is a common failure mode of reinforced concrete (RC) beams flexurally strengthened with a fiber-reinforced polymer (FRP) plate. Due to IC debonding failure, the tensile strength of the FRP plate cannot be fully utilized. While a number of options to suppress this failure mode have been explored, the easiest option is still to install U-jackets of an appropriate layout outside the FRP plate. This paper presents the results of an experimental study into the effectiveness of such U-jacketing on delaying or suppressing IC debonding failure, an issue which has not yet been systematically investigated so far. Eight large-scale RC beams were tested in the present study to investigate the effects of different forms of FRP U-jacketing on IC debonding failure. The test results indicated that vertical FRP U-jackets have a rather limited effect on IC debonding failure. Moreover, strong vertical FRP U-jackets may cause significant out-plan bending in the soffit plate during the deformation process of the beam, leading to the premature rupture of the FRP soffit plate. The tests also showed that inclined U-jacketing is much more effective than vertical U-jacketing, particularly when inclined U-jacketing is provided in a low moment region.

Identification of intermediate debonding damage in FRP-plated RC beams based on multi-objective particle swarm optimization without updated baseline model

Fiber reinforced polymer composites (FRP) have found widespread usage in the repair and strengthening of concrete structures. FRP composites exhibit high strength-to-weight ratio, corrosion resistance, and are convenient to use in repair applications. Externally bonded FRP flexural strengthening of concrete beams is the most extended application of this technique. A common cause of failure in such members is associated with intermediate crack-induced debonding (IC debonding) of the FRP substrate from the concrete in an abrupt manner. Continuous monitoring of the concrete–FRP interface is essential to prevent IC debonding. Objective condition assessment and performance evaluation are challenging activities since they require some type of monitoring to track the response over a period of time. In this paper, a multi-objective model updating method integrated in the context of structural health monitoring is demonstrated as promising technology for the safety and reliability of this kind of strengthening technique. The proposed method, solved by a multi-objective extension of the particle swarm optimization method, is based on strain measurements under controlled loading. The use of permanently installed fiber Bragg grating (FBG) sensors embedded into the FRP–concrete interface or bonded onto the FRP strip together with the proposed methodology results in an automated method able to operate in an unsupervised mode.

Experimental Investigation on Effective Bond Length of Flexurally CFRP-Strengthened RC Beams Failed by Intermediate Crack Debonding

The models to calculate the effective bond length determined by single-lap or double-lap shear tests are more suitable for the end-debonding failure; and the improved models based on the simply shear test models to calculate the effective bond length of flexural CFRP-strengthened beams failed by intermediate crack debonding (IC debonding) are also lack of the support of experimental data. To investigate the effective bond length of flexural CFRP-strengthened beams, a total of 9 flexurally CFRP-strengthened beams were tested. The experimental results show that the existing models are very conservative for calculating the effective bond length in flexural FRP-strengthened RC beams failed by IC debonding. Based on the experimental results, this paper presents a model to calculate the effective bond length of flexural FRP-strengthened RC beams failed by IC debonding, and a model to calculate the plastic hinge length at the shear-flexure span.

Finite-Element Modeling of Intermediate Crack Debonding in FRP-Plated RC Beams

Intermediate crack-induced debonding (IC debonding) is a common failure mode of RC beams strengthened with externally bonded fiber-reinforced polymer (FRP) reinforcement. Although extensive research has been carried out on IC debonding, much work is still needed to develop a better understanding of the failure mode and a more reliable strength model. This paper presents an advanced finite-element (FE) model on the basis of the smeared-crack approach for predicting IC debonding failure. Existing FE models of the same type are generally deficient in capturing localized cracks (both their pattern and widths). This deficiency is overcome in the proposed FE model through the accurate modeling of interfaces between the concrete and both the internal steel and the external FRP reinforcements. The capability and accuracy of the proposed model are demonstrated through comparisons of its predictions with selected test results. The importance of accurate modeling of localized cracking is also explained using numeric...

Experimental investigations and verification of debonding strain of RHSC continuous beams strengthened in flexure with externally bonded FRPs

Although many in-situ RC beams are of continuous constructions, there has been very little research on the behavior of such beams with external reinforcement. This article presents an experimental program conducted to study the flexural behavior and redistribution of moment of reinforced high strength concrete (RHSC) continuous beams strengthened with carbon and glass-fiber-reinforced polymer (CFRP and GFRP) sheets. The program consists of six RHSC continuous (two-span) beams with overall dimensions equal to 250 × 150 × 6000 mm. One beam was not strengthened and was tested as a control beam. Five beams were strengthened with CFRP and GFRP in flexure along their sagging and hogging regions. The main parameters including type of FRP (GFRP or CFRP), the different ratios of CFRP sheet and effectiveness of end anchorage. The test results showed that the use of GFRP sheet in strengthening of continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase the ultimate strength of them. The use of end anchorage in strengthened continuous beams increased the ultimate strength and moment redistribution. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. Also existing international codes and model such as ACI, fib, JSCE, Teng and Toutanji for prediction of IC debonding strain or stress of strengthened continuous beams are verified. Verifications were carried out based on the test results in this research and the published literature on RC continuous beams strengthened with FRP.

Debonding analysis of FRP–concrete interface between two balanced adjacent flexural cracks in plated beams

A cohesive interface modeling approach to debonding analysis of adhesively bonded interface between two balanced adjacent flexural cracks in conventional material (e.g., concrete or wood) beams strengthened with externally bonded FRP plates is presented. Both the strengthened beam and strengthening FRP are modeled as two linearly elastic Euler–Bernoulli beams bonded together through a thin adhesive layer. A bi-linear cohesive model, which is commonly used in the literature, is adopted to characterize the stress-deformation relationship of the FRP–concrete interface. Completely different from the single-lap or double-shear pull models in which only the axial pull force is considered, the present model takes the couple moment and transverse shear forces in both the substrates into account to study the second type of intermediate crack-induced debonding (IC debonding) along the interface. The whole debonding process of the FRP–concrete interface is discussed in detail, and closed-form solutions of bond slip, interface shear stress, and axial force of FRP in different stages are obtained. A rotational spring model is introduced at locations of the two adjacent flexural cracks to model the local flexibility of the cracked concrete beam, with which the relationship between the local bond slip and externally applied load is established and the real bond failure process of the FRP-plated concrete beam with the increasing of the externally applied load is revealed. Parametric studies are further conducted to investigate the effect of the thickness of adhesive layer on the bond behavior of FRP–concrete interface. The present closed-form solution and analysis on the local bond slip versus applied load relationship for the second type of IC debonding along the interface shed light on the bond failure process of structures externally strengthened with FRP composite plates and can be used effectively and efficiently to predict ductility and ultimate load of FRP-strengthened structures.

A critical steel yielding length model for predicting intermediate crack-induced debonding in FRP -strengthened RC members

Yielding of the internal steel reinforcement is an important mechanism that influences the Intermediate Crack-induced debonding (IC debonding) behavior in FRP-strengthened RC members since the FRP is required to carry additional forces beyond the condition of steel yielding. However, rational design practice dictates an appropriate limit state is defined when steel yielding is assured prior to FRP debonding. This paper proposes a criterion which correlates the occurrence of IC debonding to the formulation of a critical steel yielding length. Once this length is exceeded the average bond stress in the FRP/concrete interface exceeds its threshold value, which proves to correlate with the average bond resistance in an FRP/concrete joint under simple shear loading. This proposed IC debonding concept is based on traditional sections analysis which is conventionally applied in design practice. Hence complex bond stress-slip analyses are avoided. Furthermore, the proposed model incorporates not only the bond properties of FRP/concrete interface but also the beam geometry, and properties of steel and FRP reinforcement in the analysis of IC debonding strength. Based upon a solid database, the validity of the proposed simple IC debonding criterion is demonstrated.

Strengthening of one-way spanning RC slabs with cutouts using FRP composites

This paper reports the results of tests on fibre reinforced polymer (FRP) strengthened one-way spanning reinforced concrete (RC) slabs with central cutouts. Four wide slabs with cutouts were tested in addition to two narrow slabs without cutouts. Different positions of applied line loads for the slabs with cutouts resulted in different slab bending action and hence different FRP behaviour for the strengthened slabs. All FRP-strengthened slabs achieved a higher load-carrying capacity than their unstrengthened control counterparts. In addition, all strengthened slabs failed by debonding initiating at intermediate cracks (IC debonding) and in the case of the slabs with cutouts, the critical cracks were diagonal and originated from the corners of the cutout. The ability of the FRP to redistribute stresses around the cutout, the failure mechanisms, as well as pre- and post-debonding behaviour of the strengthened slabs was therefore assessed for different load application positions. Strains on the FRP, concrete and internal steel reinforcement, as well as deflections at different positions on the slab surfaces are also reported. An analytical model, which is based on the ultimate moment of resistance about critical crack lines, is also reported and it its predictions are found to correlate well with the experimental results. The analytical model is able to capture the different slab bending actions in addition to the debonding failure of the strengthened slabs.

Analysis of Interfacial Debonding of RC Beams Strengthened with CFL

The interfacial bonding property between carbon fiber laminate (CFL) and concrete is a key issue in the application of RC beams strengthened with CFL. In this paper, fracture mechanics method is used to develop a calculation formula of stress intensity factor for the CFL-concrete interfacial crack. In combination with a series of tests, the interfacial fracture toughness is discussed, and the influencing factors and process of the intermediate crack-induced debonding (IC debonding) are also analyzed. The tests results show that IC debonding is generally induced by the flexural or flexural-shear cracks in the bottom of RC beams around the mid-span, and develop along the interface towards the end of the beam. A method to determine IC debonding derived from theoretic analysis and the tests results demonstrates that the interfacial failure can be effectively avoided by limiting CFL strain in the range of the debonding strain.

Moment redistribution parametric study of CFRP, GFRP and steel surface plated RC beams and slabs

It is common practice these days to retrofit reinforced concrete (RC) beams and slabs by adhesive bonding FRP or steel plates to their surfaces. However, research has shown that these external plates can debond prematurely at relatively low strains such that the ability of the plated section to redistribute moment to other sections is severely limited and to such an extent that guidelines often preclude moment redistribution. This restriction may severely limit the use of plating in buildings where ductility is a requirement. Tests on steel and FRP plated continuous beams have shown that moment redistribution can occur and a design procedure has been developed to determine the amount of moment redistribution for any type of adhesively bonded plated section, and for any type of plate material, such as steel or FRP plates. In this paper, an analysis approach to quantify the amount of moment redistribution is described and the concepts of positive and negative moment redistribution introduced. A parametric study is then used to illustrate how the plate material and geometric properties affect moment redistribution; in particular, the study looks at the effect of using carbon FRP plates and glass FRP plates, as well as steel plates that have been designed to either debond prior to yielding or yield prior to IC debonding. In summary, the paper will show that plated sections can redistribute moment and, hence, the present restriction can be removed which should extend the use of retrofitting by plating. Furthermore, the moment redistribution analysis procedure allows the engineer the freedom to choose the properties of the plate to design for moment redistribution.