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.     

Design Proposal to Avoid Peeling Failure in FRP-Strengthened Reinforced Concrete Beams

According to the available experimental work, the most common failure in existing structures strengthened by plate bonding is the laminate peeling off. In the last few years, an important effort in the development of mathematical models to avoid premature peeling failures has been made. However, a suitable and reliable design method to predict debonding due to the shear flow between crack discontinuities or at the laminate end is still not available. This paper describes a new design procedure for structures strengthened by plate bonding to avoid peeling failure at any location. After calculating the laminate area required for flexural strengthening, a two-step procedure to prevent peeling failure is proposed. The first step, to avoid peeling failure along the span, is based on a shear-bending interaction diagram associated with the theoretical maximum transferred force between laminate and support along the crack spacing before laminate debonding. This interaction diagram can be obtained through the application of nonlinear fracture mechanics. The second step consists of checking for peeling failure at the laminate end. The bonded length between the laminate end and the nearest crack should be enough to transfer the laminate tensile force acting on this crack. The proposed method has been verified with available experimental results assembled in a database. A good agreement between the experimental and predicted failure load has been obtained. Finally, an application example is presented to show the applicability of the method.     

Prediction of Interfacial Bond Failure of FRP–Concrete Surface

The use of fiber-reinforced polymer (FRP) for strengthening concrete structures has grown remarkably during the past few years. In spite of exhibiting superior properties, the safety of usage is questionable as FRP undergoes brittle debonding failure. The aim of this study is to review and compare the existing research on bond failure between FRP and concrete substrates. Among the different failure modes, there has been little research in terms of intermediate crack-induced interfacial debonding and fewer strength models are developed for predicting such failures. Conducting a simple shear test on the FRP bonded to a concrete substrate can simulate this type of failure mode. Twelve specimens were tested to study the influence of concrete strength and the amount of FRP on the ultimate load capacity of a FRP–concrete bond under direct shear. Existing experimental work was collected from the literature and consists of an extensive database of 351 concrete prisms bonded to FRP and tested in direct shear tests. The analytical models from various sources are applied to this database and the results are presented.     

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.     

FRP-Reinforced Concrete Beams: Unified Approach Based on IC Theory

In general, steel-reinforced concrete involves a ductile steel material and a very strong and ductile bond between the steel reinforcement and concrete, so that debonding rarely governs the design. In contrast, fiber-reinforced polymer (FRP) reinforcement is a brittle material with a weak and brittle bond, making debonding a major issue. Consequently, there has been an extensive amount of research on FRP debonding and in particular intermediate crack (IC) debonding. This paper shows that the very good research by the FRP research community on the mechanics of IC debonding can be applied to a wide range of apparently disparate reinforced concrete behaviors to produce a unified approach. Hence, a single mechanism, or unified approach, based on IC debonding is proposed in this paper for dealing with moment rotation, tension stiffening and deflections, member ductility and moment redistribution, shear capacity, confinement, and fiber concrete for FRP RC beams.     

Study of Intermediate Crack Debonding in Adhesively Plated Beams

The main disadvantage of reinforced concrete beams retrofitted with steel or fiber reinforced polymer (FRP) plates adhesively bonded to the external surfaces is the premature debonding of the plates before reaching the desired strength or ductility. One of the main mechanisms of debonding failure is intermediate crack (IC) debonding, which is initiated by the formation of flexural cracks in the vicinity of the plates causing slip to occur at the plate/concrete interfaces. Much of the existing research focuses on the bond–slip relationship at the plate/concrete interface, with a lack of attention on the IC debonding behavior of flexural members. In this research, a model is described for IC debonding of plated RC beams that is based on partial interaction theory. To allow a better understanding of the IC debonding behavior of plated members, studies are carried out using the proposed model to study the effects of variations in crack spacings and rate of change of moment, and it is shown that both of these factors as well as the number of cracks in the beam can have large effects on the local behavior and the resultant strains in the plated member.     

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.     

Modeling Debonding Failure in FRP Flexurally Strengthened RC Members Using a Local Deformation Model

Reinforced concrete (RC) beams and slabs can be strengthened by bonding fiber-reinforced polymer (FRP) composites to their tension face. The performance of such flexurally strengthened members can be compromised by debonding of the FRP, with debonding initiating near an intermediate crack (IC) in the member away from the end of the FRP. Despite considerable research over the last decade, reliable IC debonding strength models still do not exist. The current paper attempts to correct this situation by presenting a local deformation model that can simulate IC debonding. The progressive formation of flexural cracks, and the associated crack spacings and crack widths are modelled from initial cracking to the onset of debonding. The bond characteristics between the longitudinal steel reinforcement and concrete, and the FRP and concrete, as well as the tension stiffening effect of the reinforcement and FRP to the concrete, are considered. The FRP-to-concrete bond-slip relation is used to determine the onset of debonding. The analytical predictions compare well with experimental results of FRP-strengthened RC cantilever slabs.     

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.     

Debonding Failures of RC Beams Strengthened with Near Surface Mounted CFRP Strips

This paper presents the results of a series of tests conducted on reinforced concrete (RC) beams strengthened in flexure with near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. As the main focus of the research is on debonding failure mechanisms, the only test variable investigated was the embedment length of the NSM strip and the NSM strip was extensively strain-gauged to monitor its bond behavior. Load-deflection curves, failure modes, strain distributions in the CFRP strip, and local bond stresses at the CFRP–epoxy interface from the tests are all examined in detail and compared with the predictions of a simple analytical model where appropriate. Of the four embedment lengths investigated, all but the shortest one led to a notable increase in the load-carrying capacity and, to a lesser extent, in the postcracking stiffness of the beam. Debonding was found to be the primary failure mode in all cases except for the beam with the longest embedment length. Also reported in this paper are results from preliminary bond tests used to characterize the local bond-slip behavior of the NSM system. Apart from gaining a better understanding of debonding failures in RC beams with NSM FRP strips, the test results reported in the paper should be useful for future verification of numerical and analytical models.     

New Approach for Modeling the Contribution of NSM FRP Strips for Shear Strengthening of RC Beams

This paper presents the main features of an analytical model recently developed to predict the near-surface mounted (NSM) fiber-reinforced polymer (FRP) strips shear strength contribution to a reinforced concrete (RC) beam throughout the beam’s loading process. It assumes that the possible failure modes that can affect the ultimate behavior of an NSM FRP strip comprise: loss of bond (debonding); concrete semiconical tensile fracture; mixed shallow-semicone-plus-debonding; and strip tensile fracture. That model was developed by fulfilling equilibrium, kinematic compatibility, and constitutive law of both the adhered materials and the bond between them. The debonding process of an NSM FRP strip to concrete was interpreted and closed-form equations were derived after proposing a new local bond stress-slip relationship. The model proposed also addressed complex phenomena such as the interaction between the force transferred to the surrounding concrete through bond stresses and concrete fracture as well as the interaction among adjacent strips. The main features of the proposed modeling strategy are shown along with the main underlying physical-mechanical concepts and assumptions. Using recent experimental data, the predictive performance of the model is assessed. The model is also applied to single out the influence of relevant parameters on the NSM technique effectiveness for the shear strengthening of RC beams.     

Numerical Cracking and Debonding Analysis of RC Beams Reinforced with FRP Sheet

Bonding a fiber reinforced polymer (FRP) sheet to the tension-side surface of reinforced concrete (RC) structures is often performed to upgrade the flexural capacity and stiffness. Except for upper concrete crushing, FRP sheet reinforcing RC structure may fail in sheet rupture, sheet peeloff failure due to opening of a critical diagonal crack, or concrete cover delamination failure from the sheet end. Accompanying the occurrence of these failure modes, reinforcing effects of the FRP sheet will be lost and load-carrying capacity of the RC structures will be decreased suddenly. This study is devoted to developing a numerical analysis method by using a three-dimensional elasto-plastic finite element method to simulate the load-carrying capacity of RC beams failed in the FRP sheet peeloff mode. Here, the discrete crack approach was employed to consider geometrical discontinuities such as opening of cracks, slipping of rebar, and debonding of the FRP sheet. Comparisons between analytical and experimental results confirm that the proposed numerical analysis method is appropriate for estimating the load-carrying capacity and failure behavior of RC beams flexurally reinforced with a FRP sheet.     

Normalized Test for Prediction of Debonding Failure in Concrete Elements Strengthened with CFRP

This work presents the results of an experimental research program, carried out at the Technical University of Catalonia, to study the debonding behavior of carbon fiber-reinforced polymers (CFRPs) used to strengthen beams in bending. The research is a part of a program that aims to study the strengthening of concrete bridges (both monolithic and segmental) using CFRPs. The overall objective of this paper is to present the results obtained from bond tests performed on material-scale specimens and full-scale tests performed on monolithic and segmental beams. A normalized test is proposed to obtain a more reliable estimate of the debonding strain of CFRPs, which may govern the design of CFRP-strengthened concrete structures. The test is proposed to supplement available design models, as the formula of km included in ACI 440.2R-02 by ACI Committee 440. The results from the tests are checked with the data obtained in large-scale tests, representative of actual bridges. The reported values are significantly lower than those reported in other tests with specimens of a lower size. An explanation is that a size effect can exist, which affects the debonding failure mechanisms. Extrapolation of results—from models calibrated with specimens of reduced dimensions to real structures—may lead to unsafe predictions of the debonding strain. In conclusion, the proposed simplified bond test more accurately estimates the load bearing capacity, which in practical cases is not perfectly well covered by the existing models; for instance when discontinuities (cracks or joints) are present in the concrete where the CFRP is bonded.     

Shear Transfer across Cracks in FRP Strengthened RC Members

The shear capacity of unplated reinforced concrete (RC) beams depends on the transverse shear to form the critical diagonal crack (CDC) as well as the transverse shear capacity across the CDC. The latter depends on the reinforcing bars crossing the CDC as they provide forces normal to the CDC that allow the shear to be transferred by aggregate interlock. For steel reinforcing bars, these normal forces can be assumed to depend on the ductile yield capacity of the reinforcing bar. However, the problem is more complicated when dealing with fiber reinforced polymer (FRP) plated RC beams, as the normal force now depends on the brittle intermediate crack debonding resistance of the plate as well as the brittle nature of the FRP material. In this paper, eight push tests have been used to directly determine the contribution of externally bonded (EB) and near surface mounted (NSM) FRP plates to the shear capacity, and these are compared with further six EB and NSM steel plated members. It is shown that plate reinforcement can substantially increase the shear capacity and, surprisingly, that the brittle FRP plates can provide a more ductile shear mechanism than the ductile steel plates.     

Flexural Behavior of Reinforced Concrete Beams Strengthened with CFRP Sheets and Epoxy Mortar

Experiments were conducted to study the effect of using epoxy mortar patch end anchorages on the flexural behavior of reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) sheets. More specifically, the effect of the end anchorage on strength, deflection, flexural strain, and interfacial shear stress was examined. The test results show that premature debonding failure of reinforced concrete beams strengthened with CFRP sheet can be delayed or prevented by using epoxy mortar patch end anchorages. A modified analytical procedure for evaluating the flexural capacity of reinforced concrete beams strengthened with CFRP sheets and epoxy mortar end anchorage is developed and provides a good prediction of test results.     

Punching Shear Behavior of Fiber Reinforced Polymers Reinforced Concrete Flat Slabs: Experimental Study

This paper presents the results of a two-phase experimental program investigating the punching shear behavior of fiber reinforced polymer reinforced concrete (FRP RC) flat slabs with and without carbon fiber reinforced polymer (CFRP) shear reinforcement. In the first phase, problems of bond slip and crack localization were identified. Decreasing the flexural bar spacing in the second phase successfully eliminated those problems and resulted in punching shear failure of the slabs. However, CFRP shear reinforcement was found to be inefficient in enhancing significantly the slab capacity due to its brittleness. A model, which accurately predicts the punching shear capacity of FRP RC slabs without shear reinforcement, is proposed and verified. For slabs with FRP shear reinforcement, it is proposed that the concrete shear resistance is reduced, but a strain limit of 0.0045 is recommended as maximum strain for the reinforcement. Comparisons of the slab capacities with ACI 318-95, ACI 440-98, and BS 8110 punching shear code equations, modified to incorporate FRP reinforcement, show either overestimated or conservative results.     

Interaction between Steel Stirrups and Shear-Strengthening FRP Strips in RC Beams

RC beams shear strengthened with either fiber-reinforced polymer (FRP) U-jackets/U-strips or side strips commonly fail due to debonding of the bonded FRP shear reinforcement. As such debonding occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups may not have reached yielding. Consequently, the yield strength of internal steel stirrups in such a strengthened RC beam cannot be fully used. In this paper, a computational model for shear interaction between FRP strips and steel stirrups is first presented, in which a general parabolic crack shape function is employed to represent the widening process of a single major shear crack in an RC beam. In addition, appropriate bond-slip relationships are adopted to accurately depict the bond behavior of FRP strips and steel stirrups. Numerical results obtained using this computational model show that a substantial adverse effect of shear interaction generally exists between steel stirrups and FRP strips for RC beams shear strengthened with FRP side strips. For RC beams shear strengthened with FRP U-strips, shear interaction can still have a significant adverse effect when FRP strips with a high axial stiffness are used. Therefore, for accurate evaluation of the shear resistance of RC beams shear strengthened with FRP strips, this adverse effect of shear interaction should be properly considered in design.     

Prediction Models for Debonding Failure Loads of Carbon Fiber Reinforced Polymer Retrofitted Reinforced Concrete Beams

This study focuses on debonding failure in reinforced concrete beams with carbon fiber reinforced polymer composite bonded on the soffit using the wet lay-up method. An experimental study, which involved 26 tests, was carried out. The experiments showed two failure modes: Intermediate span debond and end debond. The first failure is the result of the high bond stress near the tip of a flexure-shear crack, whereas the second type of failure is due to the high shear stress developed in the weakest concrete layer at the tension reinforcement level. The experiments have shown that U-straps can be effective in preventing intermediate span and end debond. Based on experimental observations, two simple and practical theoretical models were developed and verified with the experimental data, together with a large database of other existing tests.     

Interface Shear Stress: A New Design Criterion for Plate Debonding

This paper critically assesses the applicability and reliability of existing analytical techniques to predict and∕or prevent brittle plate debonding failure that occurs in reinforced concrete (RC) beams strengthened with externally bonded steel or fiber-reinforced-polymer composite plates. The experimental results, available to date in literature, have been very carefully reviewed and analyzed for this purpose. A new approach, very different from existing methods, and based on the interface shear stress obtained form elastic analysis of RC beam cross section and the fundamentals of force transfer mechanism in a bonded joint, is presented to predict the premature plate debonding phenomenon. The paper identifies important structural, material, and force parameters that influence this critical interface shear stress value between the bonded plate and concrete. The relations between these parameters and interface shear stress value are also examined and found to be consistent and logical to predict plate debonding at the plate cutoff end. The validity of this new design-oriented approach and scope for further research are also discussed.     

Performance of FRP-Strengthened RC Beams with Different Concrete Surface Profiles

Performance of fiber-reinforced polymer (FRP) composites in repair and retrofit of concrete structures depends to a great extent on the substrate condition to which it is bonded. The present research investigated the effect of concrete surface roughness on the bond behavior and general performance of FRP strengthening systems. The study included flexural testing of 26 specimens with two different carbon FRP systems (wet layup and precured) and three different levels of surface roughness. The influence of six different levels of anchorage by means of U-straps was also evaluated. Additionally, 10 bond specimens were tested in double shear for the wet layup FRP system to compare with the results of beam tests and to further investigate the debonding issue. Bond-dependent coefficient was analyzed with respect to different levels of surface roughness and shear span-to-depth ratio, and was then compared with other test results available in the literature. Surface roughness did not appear to have a significant influence on the overall performance of the FRP strengthening system with or without adequate anchorage, and whether failure was by debonding or rupture of FRP.