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.     

Shear Strengthening of RC Beams with EB FRP: Influencing Factors and Conceptual Debonding Model

This paper deals with the shear strengthening of RC beams using externally bonded (EB) fiber-reinforced polymers (FRP). Current code provisions and design guidelines related to shear strengthening of RC beams with FRP are discussed in this paper. The findings of research studies, including recent work, have been collected and analyzed. The parameters that have the greatest influence on the shear behavior of RC members strengthened with EB FRP and the role of these parameters in current design codes are reviewed. This study reveals that the effect of transverse steel on the shear contribution of FRP is important and yet is not considered by any existing codes or guidelines. Therefore, a new design method is proposed to consider the effect of transverse steel in addition to other influencing factors on the shear contribution of FRP (Vf). Separate design equations are proposed for U-wrap and side-bonded FRP configurations. The accuracy of the proposed equations has been verified by predicting the shear strength of experimentally tested RC beams using data collected from the literature. Finally, comparison with current design guidelines has shown that the proposed model achieves a better correlation with experimental results than current design guidelines.     

Moment Redistribution in FRP and Steel-Plated Reinforced Concrete Beams

Research on retrofitting reinforced concrete (RC) beams and slabs using externally bonded (EB) fiber reinforced polymer (FRP) or steel plates has reached the stage where the flexural strength can be determined with confidence. Research has also shown that EB plated structures tend to debond at relatively low strains and to such an extent that guidelines often preclude moment redistribution which can severely restrict the use of plating. However, recent research on retrofitting using FRP and steel near surface mounted plates (NSM) has shown that NSM plates tend to debond at high strains which can allow substantial amounts of moment redistribution. A moment redistribution approach has been developed for both NSM and EB plated beams that allows for the wide range of debonding strains that can occur. This allows RC beams to be retrofitted for both strength and ductility which should help expand the use of this convenient and inexpensive form of retrofitting.     

Cracking Characteristics of RC Beams Strengthened with FRP System

The cracking characteristics of fiber-reinforced polymer (FRP) strengthened reinforced concrete (RC) beams in both the short- and long-term is addressed in this paper. First, an empirical equation based on regression analysis of test results obtained from 36 beams was derived for the evaluation of crack widths in FRP-strengthened RC beams under short-term loading. The equation accounts for the effective concrete area in tension, steel stress, proximity of tensile longitudinal reinforcement, and primary crack height. Next, the long-term crack widths of glass FRP-strengthened RC beams under sustained loads were studied. Beams strengthened with glass FRP laminates showed improved cracking characteristics with smaller crack widths compared to conventional RC beams. Based on the investigation, two empirical equations are presented to compute the long-term crack widths in FRP-strengthened beams.     

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.     

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.     

Full Torsional Behavior of RC Beams Wrapped with FRP: Analytical Model

Torsion failure is an undesirable brittle form of failure. Although previous experimental studies have shown that using fiber-reinforced polymer (FRP) sheets for torsion strengthening of reinforced concrete (RC) beams is an effective solution in many situations, very few analytical models are available for predicting the section capacity. None of these models predicted the full behavior of RC beams wrapped with FRP, account for the fact that the FRP is not bonded to all beam faces, or predicted the ultimate FRP strain using equations developed based on testing FRP strengthened beams in torsion. In this paper, an analytical model was developed for the case of the RC beams strengthened in torsion. The model is based on the basics of the modified compression field theory, the hollow tube analogy, and the compatibility at the corner of the cross section. Several modifications were implemented to be able to take into account the effect of various parameters including various strengthening schemes where the FRP is not bonded to all beam faces, FRP contribution, and different failure modes. The model showed good agreement with the experimental results. The model predicted the strength more accurately than a previous model, which will be discussed later. The model predicted the FRP strain and the failure mode.     

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.     

Structural Performance of RC Beams Poststrengthened with Carbon, Aramid, and Glass FRP Systems

The use of fiber-reinforced polymers (FRPs) to poststrengthen concrete structures started to be investigated in the mid-1970s and today is widely recognized as an attractive technique to be used in civil structures, especially when aggressive environments prevent the use of materials that are susceptible to corrosion, such as steel. Different FRP poststrengthening techniques have been developed and applied in existing structures, aiming to increase their load capacity. Most FRP systems used nowadays consist of carbon fibers embedded in epoxy matrix. Regardless of the advantages and the good results achieved using carbon fiber-reinforced polymers, some new possibilities, such as the use of prestressing and lower cost fiber materials, are being analyzed in an attempt to provide viable alternatives for a more efficient, safe, and rational use of FRP systems. The main purpose of the present work was to make a comparative analysis of the behavior of reinforced concrete beams poststrengthened with carbon, aramid, and glass FRP subjected to static loading tests. Experimental results were evaluated against theoretical ones obtained through an analytical model that considers a trilinear behavior for the load versus displacement curves. The experimental results indicate that all FRP systems applied have appropriate structural performance for use in poststrengthening applications of RC. The choice of the more suitable system would, therefore, be strongly influenced by circumstances regarding cost limitations and level of reinforcement required.     

New Approach for Estimating the Deflection of Beams Reinforced with FRP Reinforcement

Due to concerns with corrosion, the use of fiber-reinforced polymer (FRP) as a replacement to conventional steel reinforcement has greatly increased over the last decade. Researchers have identified the distinctive mechanical and bond properties of FRP reinforcement that prevent the use of existing relationships to establish serviceability of concrete structures reinforced with such products. Although studies have modified these empirical relationships to describe the behavior of structures reinforced with FRP reinforcement, this paper will provide a new approach to estimate deflection of concrete beams by considering material properties of the reinforcement and incorporating the effects of tension stiffening. Accuracy and precision of the approach was established by performing a statistical analysis on a database containing 171 FRP-reinforced concrete beams. Results were compared to those from existing proposed relationships and indicate the potential of the method to estimate deflection at various service conditions.     

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.     

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.     

Flexural Behavior of Continuous GFRP Reinforced Concrete Beams

The results of testing two simply and three continuously supported concrete beams reinforced with glass fiber-reinforced polymer (GFRP) bars are presented. The amount of GFRP reinforcement was the main parameter investigated. Over and under GFRP reinforcements were applied for the simply supported concrete beams. Three different GFRP reinforcement combinations of over and under reinforcement ratios were used for the top and bottom layers of the continuous concrete beams tested. A concrete continuous beam reinforced with steel bars was also tested for comparison purposes. The experimental results revealed that over-reinforcing the bottom layer of either the simply or continuously supported GFRP beams is a key factor in controlling the width and propagation of cracks, enhancing the load capacity, and reducing the deflection of such beams. Comparisons between experimental results and those obtained from simplified methods proposed by the ACI 440 Committee show that ACI 440.1R-06 equations can reasonably predict the load capacity and deflection of the simply and continuously supported GFRP reinforced concrete beams tested.     

Evaluation of Bending Strength of RC Beams Strengthened with FRP Sheets

This paper deals with reinforced concrete beams strengthened by means of externally bonded fiber-reinforced polymer (FRP) sheets. The scope of the work is to discuss and compare an exact and an approximate approach to the computation of the flexural load-carrying capacity of the strengthened beam. The two approaches differ from one another in the way they take into account the extent of the load already acting throughout strengthening operations. To achieve this goal a numerical model is presented and validated by comparing its output with that of 46 experimental tests taken from the literature. The numerical model is then adopted to perform a numerical parametric analysis of a wide range of practical applications, excluding all cases of FRP delamination, and useful conclusions are drawn.     

Failure Analysis of FRP-Strengthened RC Beams

Fiber-reinforced polymer (FRP) application is a very effective way to repair and strengthen structures that have become structurally inefficient over their life span. This paper investigates the applicability of existing models for the prediction of debonding failure in RC beams externally strengthened with FRP. It is very important to predict the limit at which FRP debonds from the beam in order to arrest premature failures. The existing models lack the thoroughness of bond predictability. This is mainly due to the development models on the basis of small amount of tested data. Hence, there is a need to compare the existing work to an extensive database of strengthened beams. Existing experimental work was collected from literature to create a database of 163 beams tested in three point and four point bending tests. Various models are applied to this database and behavior of each model is analyzed using statistical parameters and degree of uncertainty in prediction.     

Strengthening RC Beams in Flexure Using New Hybrid FRP Sheet/Ductile Anchor System

This paper explores a new hybrid fiber-reinforced polymer (FRP) sheet/ductile anchor system for rehabilitation of reinforced concrete (RC) beams. The advantages of the proposed strengthening method is that it overcomes the problem of low ductility that is associated with brittle failure mode in conventional methods of strengthening beams using epoxy-bonded FRP sheets. The proposed system leads to a ductile failure mode by triggering yielding to occur in a steel anchor system (steel links) rather than by rupture or debonding of FRP sheets, which is sudden in nature. Four half-scale RC T-beams were tested under four-point bending. Three retrofitted beams were strengthened using one layer of carbon FRP sheet. The results of the two beams that were strengthened with the new hybrid FRP sheet/ductile anchor system were compared with the results from the beam strengthened with conventional FRP bonding method and the control beam. The results show the effectiveness of the proposed strengthening system in increasing flexural capacity and ductility of RC beams.     

Analysis of the Flexural Behavior of Partially Bonded FRP Strengthened Concrete Beams

An investigation was conducted on the flexural behavior of partially bonded fiber-reinforced polymer (FRP) strengthened concrete beams focusing on the improvement of ductility. An analytical model was developed based on the curvature approach to predict the behavior of beams strengthened with partially bonded FRP systems. The result of the analysis showed that ductility of the partially bonded system was improved while sustaining high load carrying capacity in comparison to the fully bonded system. To verify the analytical model, an experimental program was carried out with reinforced concrete beams strengthened with the externally bonded FRP system. A comparison of the analytical prediction and experimental results showed good agreement.     

Tests on the Ductility of Reinforced Concrete Beams Retrofitted with FRP and Steel Near-Surface Mounted Plates

Reinforced concrete beams are now commonly retrofitted using externally bonded (EB) fiber reinforced polymer (FRP) plates as the technique is both inexpensive and unobtrusive. However, tests have shown that EB carbon FRP plates tend to debond at low strains, which can severely limit the ductility or moment redistribution to such an extent that guidelines often preclude moment redistribution. This paper reports the moment redistribution achieved in tests on nine near full-scale two-span continuous reinforced concrete beams that were retrofitted with near-surface mounted (NSM) plates. The plates were either carbon FRP or high yield steel strips which were adhesively bonded within saw grooves cut into the concrete cover on the tension face or sides of the beam. It was found that the debonding strains of these NSM plates were considerably larger than those associated with EB plates and that substantial amounts of moment redistribution occurred. These tests suggest that NSM plates can be used to increase the strength of reinforced concrete structures with little, if any, loss of ductility.     

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.     

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.