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.     

Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio

The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP (Vf). In all, 14 tests were performed on 4,520-mm-long T-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1)?the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2)?the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3)?the effect of FRP strip location with respect to internal transverse-steel location; (4)?the effect of FRP strip width; and (5)?the effect of internal transverse-steel reinforcement on the CFRP shear contribution.     

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Using Nonmetallic Anchor Systems

This paper presents the flexural behavior of reinforced concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets using nonmetallic anchor systems. The developed nonmetallic anchor systems replace the permanent steel anchorage. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The flexural behavior of the tested beams, including detailed failure modes of each nonmetallic anchor system, is investigated. The study shows that the developed nonmetallic anchors are more effective in resisting peeling-off cracks compared to the permanent steel anchors and the beams strengthened with the nonmetallic anchors provide comparable load-carrying capacity with respect to the steel anchored control beam.     

Performance of Mechanically Fastened FRP Strengthened Concrete Beams in Flexure

The use of adhesively bonded fiber-reinforced polymer (FRP) materials has become widely accepted for use in flexural strengthening applications; however, the method of attachment presents drawbacks in application. These include extensive time and labor investments, as well as a tendency of the system to fail in a brittle manner. This paper presents a study of a series of reinforced concrete beams each strengthened in flexure with an FRP strip attached with large diameter concrete screws. The concrete screws were arranged in a variety of patterns. The effect of fastener number and spacing, as well as the effect of fastener pattern on the behavior of the beam, was investigated through the use of two groups of specimens. The beams in each group were tested to failure to verify the behavior of the strengthening system. Measured behavior was then used to determine an analytical approach for prediction of load response behavior of mechanically fastened systems. It was found that the strengthening method investigated improved the flexural capacity of the specimens 12 to 39% with little or no loss in ductility.     

Shear Strengthening of RC T-Beams Using Mechanically Anchored Unbonded Dry Carbon Fiber Sheets

This research studies the feasibility and effectiveness of a new method of strengthening existing RC T-beams in shear by using mechanically anchored unbonded dry carbon fiber (CF) sheets. This method eliminates the debonding of epoxy-bonded carbon-fiber-reinforced polymer (CFRP) sheets and utilizes the full capacity of dry CF sheets. In this method, dry CF sheets are wrapped around and bonded to two steel rods. Then the rods are anchored to the corners of the web-flange intersection of the T-beam with mechanical bolts. This makes a U-shaped dry CF jacket around the web which increases the shear strength of the T-beam using the privilege of higher tensile strength and modulus of elasticity of dry CF compared to composite CFRP. A total of three RC T-beams with shear span-to-depth ratio of 2.0 were tested under increasing monotonic load till failure. The pilot tests were done as a proof-of-concept of the effectiveness of the proposed method in increasing the shear capacity of the RC T-beams. The first T-beam, which was tested as the control beam, failed in shear. The second beam was strengthened by using a U-shaped CFRP sheet that was externally bonded to the web of the beam in the shear zones. The third beam was strengthened by using anchored U-shaped dry CF sheet. The test results showed that the beam strengthened by the new mechanically anchored dry CF had about 48% increase in shear capacity as compared to the control beam and 16% increase in shear capacity as compared to the beam strengthened by CFRP epoxy-bonding method.     

Fatigue Behavior of RC Beams Strengthened with NSM CFRP Rods

This paper presents experimental results of reinforced concrete beams strengthened using near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) reinforcement. A total of nine beam specimens were tested under fatigue loads. In addition, two specimens were tested for monotonic capacity. The beams were 3,500 mm long with a cross section of 254 mm deep by 152 mm wide. Different load ranges were considered in the fatigue tests to construct the fatigue life curves. The test results showed that under monotonic loading, the beam strengthened with NSM CFRP rod exhibited increases of 26 and 50% in the yield and ultimate load over the control beam, respectively. Under cyclic loading, the fatigue life for the strengthened beams was 24% higher than that of the control unstrengthened beams. An analytical model using sectional analysis and strain-life approach was developed to estimate the fatigue life of the specimens at various cyclic load ranges. A good agreement between the experimental results and analytical prediction of the fatigue life was obtained.     

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.     

Strengthening RC T-Beams Subjected to Combined Torsion and Shear Using FRP Fabrics: Experimental Study

In several cases of loading and geometrical configurations, flexure beams, and girders are subjected to combined shear and torsion. Failure of a structural element under combined shear and torsion is brittle in nature. Externally bonded fiber-reinforced polymer (FRP) fabrics are currently being studied and used for the rehabilitation, repair, and retrofit of concrete structure. The objective of this study is to investigate the strengthening techniques for T-beams subjected to combined shear and torsion. Six half-scale beams—two control specimen and four strengthened beams—were constructed and tested using a specially designed test setup that subjects the beam to combined shear and torsion with different ratios. Four strengthening techniques using carbon FRPs were tested. The experimental results were reported and analyzed to assess the effectiveness of the proposed strengthening techniques. An innovative strengthening technique namely the extended U-jacket showed promising results in terms of strength and ductility while being quite feasible for strengthening. Future areas of research are being outlined.     

Simulation of Plastic Hinges in FRP-Plated RC Beams

Structural engineers inherently rely on the ductility of a member to absorb energy, give prior warning of failure, control the drift of columns, and redistribute moments in beams. However, quantifying the ductility of reinforced concrete members has been a seemingly difficult or even intractable structural engineering problem mainly because of the softening characteristics of concrete. In this paper, a numerical model of the hinge region of a reinforced concrete beam bonded with fiber reinforced fiber-reinforced polymer (FRP) tension face plates is described, which simulates four mechanisms that contribute to the rotation of the member and, significantly, defines the limit to rotation imposed by concrete softening.     

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.     

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.     

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.     

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.     

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Development of Nonmetallic Anchor Systems

This paper presents a novel anchoring technique for strengthening reinforced concrete beams with prestressed carbon fiber- reinforced polymer (CFRP) sheets. Permanent steel anchors are commonly used for the application of prestressed CFRP sheets. The steel anchors are, however, susceptible to corrosion and may not blend into the aesthetics of the original structure. As a result, it may be preferable to remove the steel anchors after transferring the required prestress to the structure with minimal losses of sustained prestress. A technique for replacing the steel anchors with nonmetallic anchors is investigated and reported herein. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The developed nonmetallic anchorages successfully transfer the sustained prestress in the CFRP sheets with insignificant prestress losses. A closed-form solution for the transfer of prestress is developed and compared to the experimental results.     

Fatigue Performance of CFRP Strengthened RC Beams under Environmental Conditioning and Sustained Load

Carbon fiber-reinforced polymers (CFRPs) have become increasingly important in recent years in bridge rehabilitation. Significant research has been done on the static behavior of CFRP-strengthened reinforced concrete (RC) structures; however, the fatigue behavior of such structures with interface defects subjected to harsh environmental conditions still needs to be investigated. Hence, an experimental program has been carried out to investigate the fatigue behavior, under a load range, which generates service load stress levels, of RC beams strengthened with CFRP fabrics. The effect of aggressive environments was studied by subjecting the test members to freeze–thaw, extreme temperature, ultraviolet light exposure, and relative humidity cycles. All beams survived 2 million fatigue cycles without showing significant bond degradation between composite and substrate. However, significant flexural stiffness degradation was observed in the conditioned specimens. The presence of defects also affected specimen stiffness; however, limited growth in defect size was observed due to fatigue cycling.     

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.     

Experimental Investigation on Torsional Behavior of Solid and Box-Section RC Beams Strengthened with CFRP Using Photogrammetry

The construction boom over the last century has resulted in a mature infrastructure network in developed countries. Lately, the issue of maintenance and repair/upgrading of existing structures has become a major issue, particularly in the area of bridges. Fiber- reinforced polymer (FRP) has shown great promise as a state-of-the-art material in flexural and shear strengthening as external reinforcement, but information on its applicability in torsional strengthening is limited. The need for torsional strengthening in bridge box girders is highlighted by the Westgate Bridge in Melbourne, Australia, one of the largest strengthening projects in the world for externally bonded carbon FRP (CFRP) laminates. This paper reports the experimental work in an overall investigation of torsional strengthening of solid and box-section reinforced concrete beams with externally bonded carbon FRP. This was found to be a viable method of torsional strengthening. Photogrammetry was a noncontact measuring technique used in the investigation. The deformation mechanisms were found to be unchanged in the strengthened specimens. Furthermore, it was found that the crack widths were reduced and aggregate interlocking action improved with the strengthened beams.     

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.     

Deflection Calculation of FRP Reinforced Concrete Beams Based on Modifications to the Existing Branson Equation

Fundamental concepts of tension stiffening are used to explain why Branson’s equation for the effective moment of inertia Ie does not predict deflection well for fiber reinforced polymer (FRP) reinforced concrete beams. The tension stiffening component in Branson’s equation is shown to depend on the ratio of gross-to-cracked moment of inertia (Ig/Icr), and gives too much tension stiffening for beams with an Ig/Icr ratio greater than 3. FRP beams typically have an Ig/Icr ratio greater than 5, leading to a much stiffer response and underprediction of computed deflections as observed by others in the past. One common approach to computing deflection of FRP reinforced concrete beams has been to use a modified form of the Branson equation. This paper presents a rational development of appropriate modification factors needed to reduce the tension stiffening component in Branson’s original expression to realistic levels. Computed deflections using this approach give reasonable results with the right modification factor, and compare well with a more general unified approach that incorporates a realistic tension stiffening model. Comparison is made with the existing and past correction factors recommended by ACI 440 for predicting deflection of FRP beams. The method presently used by ACI 440 gives reasonable estimates of deflection for glass and carbon FRP reinforced beams. However, this method underestimates deflection of aramid FRP reinforced beams and is restricted to rectangular sections. A proposal is made for adoption of a simple modification factor that works well for all types of FRP bar and beam cross-sectional shape.     

Effects of Corrosion of Steel Reinforcement on RC Columns Wrapped with FRP Sheets

This study investigated the effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in protecting reinforced concrete (RC) columns from corrosion of steel reinforcement. Thirty small-scale RC columns and four midscale RC columns were used in this study. The small-scale columns were used for a comprehensive parametric study, whereas the midscale columns were used to evaluate design guidelines proposed based on the results of the small-scale column tests. The test columns were conditioned under an accelerated corrosion process and then tested under uniaxial compression up to failure. The test results showed that although CFRP sheet wrapping decreased the corrosion rate, the corrosion of steel reinforcement could continue to occur, eventually showing a decrease in ultimate axial compression capacity. Design guidelines were proposed based on the small-scale RC column tests and evaluated through a comparison with the test results of midscale RC columns. The proposed design guidelines introduced a concept of effective area to account for the corrosion damage, such as internal cracking and cross-sectional loss of steel reinforcement.