Experimental Investigation and Fracture Analysis of Debonding between Concrete and FRP Sheets

The last few years have witnessed a wide use of externally bonded fiber reinforced polymer (FRP) sheets for strengthening existing reinforced and prestressed concrete structures. The success of this strengthening method relies on the effectiveness of the load-transfer between the concrete and the FRP. Understanding the stress transfer and the failure of the concrete–FRP interface is essential for assessing the structural performance of strengthened beams and for evaluating the strength gain. This paper describes an experimental investigation of the interfacial bond behavior between concrete and FRP. The strain distributions in concrete and FRP are determined using an optical technique known as digital image correlation. The results confirm that the debonding process can be described in terms of crack propagation through the interface between concrete and FRP. The data obtained from the analysis of digital images was used to determine the interfacial material behavior for the concrete–FRP interface (stress versus relative displacement response) and the fracture parameter GF (fracture energy). The instability in the test response at failure is shown to be the result of snapback, which corresponds with the elastic unloading of the FRP as the load carrying ability of the interface decreases with increasing slip.     

Numerical Modeling of Concrete-FRP Debonding Using a Crack Band Approach

In this study, numerical procedures are proposed to predict the structural behavior of concrete members strengthened with fiber-reinforced polymeric (FRP) sheets or plates. The concept of damage band or crack band is introduced and used for predicting the debonding failure of the concrete-epoxy interface formed when FRP sheets or plates are externally bonded to a concrete substrate. In the crack band approach, all the processes taking place during the failure of a concrete-epoxy interface are smeared in a band of fixed width. This makes the approach attractive from a modeling point of view since continuum theories, along with softening relations, can be used to model the damage which causes debonding of the interface. In order to validate this approach, numerical predictions, using the concept of crack band, are compared against experimental results obtained from tests of concrete blocks and reinforced concrete beams strengthened with FRP. In particular, the capability of the proposed numerical approach to predict the load-displacement response, strain distributions, failure sequences, damage distribution, and failure mechanisms experimentally observed is verified. Results presented in this study indicate that the concept of crack band is appropriate when modeling concrete-epoxy interfaces under general states of stresses.     

Analytical Solution for Fracture Analysis of CFRP Sheet–Strengthened Cracked Concrete Beams

Fiber-reinforced polymer (FRP) composite materials have been widely used in the field of retrofitting. Theoretical analysis of FRP plate- or sheet-strengthened cracked concrete beams is necessary for estimating service reliability of the structural members. In previous studies, the effect of a perfectly bonded FRP plate or sheet was equivalent to a cohesive force acting at the bottom of crack to delay the crack propagation in concrete and reduce the crack width. However, delamination between FRP and cracked beam is inevitable due to interfacial shear stress concentration at the bottom of crack. The intention of this paper is to present an analytical solution for fracture analysis of carbon FRP (CFRP) sheet–strengthened cracked concrete beams by considering both vertical crack propagation in concrete and interfacial debonding at CFRP-concrete interface. The interfacial debonding is modeled as the interfacial shear crack propagation in this paper. Four different stages are discussed after initial cracking state of the concrete. At the first stage, only fictitious crack propagation occurs in the concrete. At the second stage, macrocrack propagates in the concrete without interfacial debonding. At the third stage, both vertical macrocrack propagation in the concrete and horizontal shear crack propagation at the CFRP-concrete interface occur in the strengthened beam. The tensile stress in the CFRP sheet and interfacial shear stress along the span are formulated based on the deformation compatibility condition at the CFRP-concrete interface at this stage. Finally, macroshear crack propagates at the interface until the CFRP sheet is completely peeled out from the beam, and then the member is fractured. The applied load is determined as a function of the referred two crack lengths at different stages. At the beginning, the applied load increases to one peak value with the full propagation of fictitious crack at the first stage. At the third stage, the applied load is improved to another peak value due to the relatively high cohesive effect of the CFRP sheet. Then the two peak values are determined by the Lagrange multiplier method. The validity of the proposed analytical solution is verified with the experimental results and numerical simulations. It can be concluded that the proposed analytical solution can predict the load-bearing capacity of CFRP sheet-strengthened cracked concrete beams with reasonable accuracy.     

Numerical Modeling of FRP Shear-Strengthened Reinforced Concrete Beams

An attractive technique for the shear strengthening of reinforced concrete beams is to provide additional web reinforcement in the form of externally bonded fiber-reinforced polymer (FRP) sheets. So far, theoretical studies concerning the FRP shear strengthening of reinforced concrete members have been rather limited. Moreover, the numerical analyses presented to date have not effectively simulated the interfacial behavior between the bonded FRP and concrete. The analysis presented here aims to capture the three-dimensional and nonlinear behavior of the concrete, as well as accurately model the bond–slip interfacial behavior. The finite-element model is applied to various strengthening strategies; namely, beams with vertical and inclined side-bonded FRP sheets, U-wrap FRP strengthening configurations, as well as anchored FRP sheets. The proposed numerical analysis is validated against published experimental results. Comparisons between the numerical predictions and test results show excellent agreement. The finite-element model is also shown to be a valuable tool for gaining insight into phenomena (e.g., slip profiles, debonding trends, strain distributions) that are difficult to investigate in laboratory tests.     

Flexural Strengthening of RC Beams with Externally Bonded CFRP Systems: Test Results and 3D Nonlinear FE Analysis

This paper presents experimental results and a numerical analysis of the reinforced concrete (RC) beams strengthened in flexure with various externally bonded carbon fiber-reinforced polymer (CFRP) configurations. The aim of the experimental work was to investigate the parameters that may delay the intermediate crack debonding of the bottom CFRP laminate, and increase the load carrying capacity and CFRP strength utilization ratio. Ten rectangular RC specimens with a clear span of 4.2?m, categorized in two series, were tested to evaluate the effect of using the additional U-shaped CFRP systems on the intermediate crack debonding of the bottom laminate. Two different configurations of the additional systems were proposed, namely, continuous U-shaped wet layup sheets and spaced side-bonded CFRP L-shaped laminates. The fiber orientation effect of the side-bonded sheets was also investigated. A numerical analysis using an incremental nonlinear displacement-controlled 3D finite-element (FE) model was developed to investigate the flexural and CFRP/concrete interfacial responses of the tested beams. The finite-element model accounts for the orthotropic behavior of the CFRP laminates. An appropriate bond-slip model was adopted to characterize the behavior of the CFRP/concrete interface. Comparisons between the FE predictions and experimental results show very good agreement in terms of the load-deflection and load-strain relationships, ultimate capacities, and failure modes of the beams.     

Interfacial Behavior and Debonding Failures in FRP-Strengthened Concrete Slabs

It has been demonstrated, through laboratory investigations and various field projects, that the external bonding of fiber- reinforced polymer (FRP) laminates is an effective technique for the structural enhancement of reinforced concrete slabs. In such applications, failure is generally governed by debonding of the FRP laminate. Nevertheless, numerical simulations to date of FRP-strengthened slabs have usually been based on the assumption of full bond between the concrete and FRP. In this study, the interfacial behavior between the FRP laminates and the concrete substrate is accounted for by introducing appropriate bond-slip models for the interface in a nonlinear finite-element analysis of FRP-strengthened two-way slabs. The numerical model is capable of simulating slabs strengthened in shear or in flexure; it can be applied to arbitrary FRP configurations, and can also accommodate both passive as well as prestressed FRP strengthening schemes. Results are presented in terms of load-deflection relationships, ultimate load capacities, failure modes, and interfacial slip and stress distributions. When compared to test results reported in the literature, the analysis is shown to lead to excellent predictions in that, for the entire set of FRP-strengthened specimens considered, the average of the numerical-to-experimental load capacity ratios is 0.966, with a standard deviation of 0.066. Furthermore, in all cases when FRP debonding was observed experimentally, the analysis correctly predicted the mode of failure.     

Intermediate Crack Debonding in FRP-Strengthened RC Beams: FE Analysis and Strength Model

Reinforced concrete (RC) beams strengthened in flexure with a bonded fiber-reinforced polymer (FRP) plate may fail by intermediate crack (IC) debonding, in which debonding initiates at a critical section in the high moment region and propagates to a plate end. This paper first presents a finite-element (FE) model based on the smeared crack approach for concrete for the numerical simulation of the IC debonding process. This finite-element model includes two novel features: (1) the interfacial behavior within the major flexural crack zone is differentiated from that outside this zone and (2) the effect of local slip concentrations near a flexural crack is captured using a dual local debonding criterion. The FE model is shown to be accurate through comparisons with the results of 42 beam tests. The paper also presents an accurate and simple strength model based on interfacial shear stress distributions from finite-element analyses. The new strength model is shown to be accurate through comparisons with the test results of 77 beams, including the 42 beams used in verifying the FE model, and is suitable for direct use in design.     

Generic Debonding Resistance of EB and NSM Plate-to-Concrete Joints

It is well established that adhesively bonding plates to the surfaces of reinforced concrete members is an efficient retrofitting approach. Specifically, two techniques have emerged: Using thin externally bonded (EB) sheets/plates and near-surface mounted (NSM) strips/bars. A good amount of research has been undertaken worldwide to understand the fundamental behavior describing such adhesively bonded plate-to-concrete joints. Unfortunately, until now, no generic model exists to determine the debonding resistance of both retrofitting techniques. In this paper, a generic analytical model is derived to determine the debonding resistance of any adhesively bonded plate-to-concrete joint using an idealized linear-softening local interface bond-slip relationship. The model is derived using a unique definition of the debonding failure plane and confinement ratio such that it is suitable for both the externally bonded and near-surface mounted techniques. The model is validated by comparison with existing push-pull data as well as 14 new push-pull tests with varying plate cross-section aspect ratios. Comparison with an existing well-known model demonstrates the suitability of the proposed generic model. The model can be used to predict the intermediate crack debonding resistance of strengthened reinforced concrete members.     

Evaluation of Bond Strength in Concrete Elements Externally Reinforced with CFRP Sheets and Anchoring Devices

Use of anchoring devices can be useful to avoid or delay end debonding failure in reinforced concrete elements externally bonded with fiber-reinforced plastic materials. Many theoretical formulations are now available to predict bond strength, but no design provisions have been suggested to take into account the beneficial effect of anchorage devices. This paper presents the results of experimental bond tests performed on concrete blocks externally strengthened with carbon fiber sheets. The prime focus is the evaluation of effect given by three different types of anchorage systems upon increasing debonding load. A simple model is introduced to predict the influence of the examined anchorage systems on the debonding load. Its accuracy is confirmed by comparisons with the experimental results.     

Bond Efficiency of EBR and NSM FRP Systems for Strengthening Concrete Members

This paper reports the results of an experimental program to investigate the bonding behavior of two different types of fiber-reinforced polymer (FRP) systems for strengthening RC members: externally bonded carbon (EBR) plates and bars or strips externally applied with the near-surface-mounted (NSM) technique. The overall experimental program consisted of 18 bond tests on concrete specimens strengthened with EBR carbon plates and 24 bond tests on concrete specimens strengthened with NSM systems (carbon, basalt, and glass bars, and carbon strips). Single shear tests (SST) were carried out on concrete prisms with low compressive strengths to investigate the bonding behavior of existing RC structures strengthened with different types of FRP systems. The performance of each reinforcement system is presented, discussed, and compared in terms of failure mode, debonding load, load-slip relationship, and strain distribution. The findings indicate that the NSM technique could represent a sound alternative to EBR systems because it allows debonding to be delayed, and hence FRP tensile strength to be better exploited.     

Fatigue Behavior of Externally Strengthened Concrete Beams with Fiber-Reinforced Polymers: State of the Art

This paper presents the recent progress and achievement in the application of fiber-reinforced polymers (FRP) on strengthening reinforced/prestressed concrete beams subjected to fatigue loading. Although the performance of FRP-strengthened structures under monotonic loading has been intensively investigated, fatigue behavior is relatively less known to date. This paper summarizes most of the currently available literature, including the codes and design manuals, on reinforced/prestressed concrete beams externally strengthened with FRP. The review focuses specifically on the fatigue life as a function of the applied load range, bond behavior of externally bonded FRP, damage accumulation, crack propagation, size effects, residual strength, and failure modes. Research needs including considerations for design guidelines are presented.     

Energy-Based Modeling Approach for Debonding of FRP Plate from Concrete Substrate

For concrete beams and slabs strengthened with bonded fiber reinforced plastic (FRP) plates, plate debonding from the concrete substrate is a common failure mode. In this paper, the debonding process is modeled as the propagation of a crack along the concrete/adhesive interface, with frictional shear stress acting behind the crack tip. Crack propagation is taken to occur when the net energy release of the system equals the interfacial fracture energy. The analysis is first performed for the special case with constant shear stress along the debonded interface, and then for the general case with slip softening in the debonded zone. From the results, a direct correspondence between energy-based and strength-based analyses can be established for arbitrary softening behavior along the interface. Specifically, through the proper definition of an effective interfacial shear strength, the conventional strength-based approach can be employed to give the same results as the much more complicated energy-based analysis. Also, based on the relation between the effective shear strength and other material parameters, it is possible to explain the very high interfacial shear stresses observed in experimental measurements. As an application example, distribution of plate stress and interfacial shear stress for the linear softening case is derived. The model results are found to be in good agreement with experimental measurements, showing that the simple linear softening model can describe the debonding process in real material systems.     

Stress Transfer and Fracture Propagation in Different Kinds of Adhesive Joints

To effectively and efficiently utilize fiber-reinforced plastic (FRP) laminates (plates or sheets) in strengthening civil infrastructures, a design strategy integrating the properties of FRP reinforcement and composite structural behavior needs to be adopted. The interfacial stress transfer behavior including debonding should be considered to be one of the most important effects on the composite structural behavior. In this paper, two kinds of nonlinear interfacial constitutive laws describing the pre- and postinterfacial microdebonding behavior are introduced to solve the nonlinear interfacial stress transfer and fracture propagation problems for different kinds of adhesive joints in FRP/steel-strengthened concrete or steel structures. Expressions for the maximum transferable load, interfacial shear stress distribution, and initiation and propagation of interfacial cracks (debonding) are derived analytically. In addition, numerical simulations are performed to discuss the factors influencing the interfacial behavior and the theoretical derivations are validated by finite-element analysis.     

Damage Approach for the Prediction of Debonding Failure on Concrete Elements Strengthened with FRP

In this study, experimental and numerical procedures are proposed to predict the debonding failure of concrete elements strengthened with fiber-reinforced polymers (FRPs). Such debonding is modeled as a damage process, which takes place in a band along the bond line (crack band). Three-point bending tests were designed to obtain the softening curve of the crack band. The numerical simulations are conducted using a plastic-damage model. In this approach, the damage resulting in debonding is defined using the softening curve of the crack band. Numerical results are validated against experimental results obtained from single-lap shear tests. The numerical models were capable of predicting the experimentally observed load versus strain behavior, failure load, and failure mechanism of the single-lap shear specimens. The predictive capabilities of the numerical approach presented here were further investigated by means of a parametric study of the single-lap shear test. Results from this study indicate the applicability of the crack band approach to predict the behavior of concrete–FRP joints; they also indicate that the failure load determined from a single-lap shear test is geometry dependent.     

Front and Side View Image Correlation Measurements on FRP to Concrete Pull-Off Bond Tests

Understanding the transfer of force by bond between externally bonded fiber-reinforced polymer (FRP) reinforcement and concrete is an important step in formulating good models for predicting debonding failures observed in externally bonded reinforcement strengthened systems. In this paper, a 3D optical displacement measurement system was used to capture the full-field displacements from the front and side view in pull-off bond specimens. The experiments were carried using six specimens with carbon FRP (CFRP) strips having different axial stiffnesses but a constant bond length to the concrete substrate. Using the optical measurements, it was possible to obtain the in-plane displacement or slip and the out-of-plane displacement or separation between the CFRP strip and the concrete. It was demonstrated, that the usual assumption of pure shear stresses in such pull-off tests is not true and that the bond behavior is a two-dimensional problem involving shear and peeling stresses. The bond behavior in CFRP strip to concrete pull-off tests was characterized by three stages: (1) the initiation of the first crack; (2) the initiation of debonding; and (3) failure by complete debonding. Based on the test results it was found that there was a dependency between the maximum bond shear stress, the maximum fracture energy of the FRP-concrete interface, and the stiffness of the FRP. However, the slip values after initiation of debonding (Stage 2) were independent of the FRP stiffness. The measured anchorage force and anchorage length were in good agreement with predictions from existing code equations.     

Interface Behavior in Fiber-Reinforced Polymer-Strengthened Beams Subjected to Transverse Loads: Maximum Transferred Force

Experimental results of externally bonded strengthened beams, available to date in the literature, have shown the existence of some types of brittle failures that involve the premature laminate debonding before reaching the design load. Since the design procedure to obtain the laminate area to strengthen a reinforced concrete element should avoid these premature failures, there is a need to understand the mechanism of the debonding process in order to prevent it. This paper studies the evolution of the debonding process through the propagation of an interfacial crack by using nonlinear fracture mechanics assuming a bilinear constitutive law for the interface. The transference of stresses between the laminate and the concrete through the interface is first analyzed in a simple case of a pure shear specimen. Later on, this formulation is extended to a general case of a beam subjected to transverse loads. As shown in the analysis performed, the transferred force between both adherents should be limited to a maximum value to prevent laminate debonding. This theoretical maximum transferred force has been obtained for each case studied and will be the basis for the development of a future design proposal.     

Debonding in RC Beams Shear Strengthened with Complete FRP Wraps

Substantial research has been conducted on the shear strengthening of reinforced concrete (RC) beams with bonded fiber reinforced polymer (FRP) strips. The beams may be strengthened in various ways: complete FRP wraps covering the whole cross section (i.e., complete wrapping), FRP U jackets covering the two sides and the tension face (i.e., U jacketing), and FRP strips bonded to the sides only (i.e., side bonding). Shear failure of such strengthened beams is generally in one of two modes: FRP rupture and debonding. The former mode governs in almost all beams with complete FRP wraps and some beams with U jackets, while the latter mode governs in all beams with side strips and U jackets. In RC beams strengthened with complete wraps, referred to as FRP wrapped beams, the shear failure process usually starts with the debonding of FRP from the sides of the beam near the critical shear crack, but ultimate failure is by rupture of the FRP. Most previous research has been concerned with the ultimate failure of FRP wrapped beams when FRP ruptures. However, debonding of FRP from the sides is at least a serviceability limit state and may also be taken as the ultimate limit state. This paper presents an experimental study on this debonding failure state in which a total of 18 beams were tested. The paper focuses on the distribution of strains in the FRP strips intersected by the critical shear crack, and the shear capacity at debonding. A simple model is proposed to predict the contribution of FRP to the shear capacity of the beam at the complete debonding of the critical FRP strip.     

Fatigue Performance of RC Beams Strengthened in Shear with CFRP Fabrics

In recent years, a tremendous effort has been directed toward understanding and promoting the use of externally bonded fiber-reinforced polymer (FRP) composites to strengthen concrete structures. Despite this research effort, studies on the behavior of beams strengthened with FRP under fatigue loading are relatively few, especially with regard to its shear-strengthening aspect. This study aims to examine the fatigue performance of RC beams strengthened in shear using carbon FRP (CFRP) sheets. It involves six laboratory tests performed on full-size T-beams, where the following parameters are investigated: (1) the FRP ratio and (2) the internal transverse-steel reinforcement ratio. The major finding of this study is that specimens strengthened with one layer of CFRP survived 5 million cycles, some of them with no apparent signs of damage, demonstrating thereby the effectiveness of FRP strengthening systems on extending the fatigue life of structures. Specimens strengthened with two layers of CFRP failed in fatigue well below 5 million cycles. The failure mode observed for these specimens was a combination of crushing of the concrete struts, local debonding of CFRP, and yielding of steel stirrups. This failure may be attributed to the higher load amplitude and also to the greater stiffness of the FRP which may have changed the stress distribution among the different components coming into play. Finally, comparison between the performance of specimens with transverse steel and without seems to indicate that the addition of transverse steel extends the fatigue life of RC beams.     

Analysis of FRP-Strengthened RC Beam-Column Joints

Analytical models are presented in this study for the analysis of reinforced concrete joints strengthened with composite materials in the form of externally bonded reinforcement comprising unidirectional strips or flexible fabrics. The models provide equations for stresses and strains at various stages of the response (before or after yielding of the beam or column reinforcement) until the ultimate capacity is reached, defined by concrete crushing or fiber-reinforced polymer (FRP) failure due to fracture or debonding. Solutions to these equations are obtained numerically. The models provide useful information on the shear capacity of FRP-strengthened joints in terms of the quantity and configuration of the externally bonded reinforcement and may be used to design FRP patching for inadequately detailed beam-column joints. A number of case studies are examined in this article, indicating that even low quantities of FRP materials may provide significant enhancement of the shear capacity. The effectiveness of external reinforcement increases considerably if debonding is suppressed and depends heavily on the distribution of layers in the beam and column. The latter depends on the relative quantities of steel reinforcement crossing the joint panel and the level of axial load in the column. Analytical shear strength predictions were in good agreement with test results found in the literature, thus adding confidence to the validity of the proposed models.     

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 numerical results obtained from the FE model.