Strengthened Reinforced Concrete Beams Research Articles

Efficiency of CFRP NSM strips and EBR plates for flexural strengthening of RC beams and loading pattern influence

The efficiency of Fibre Reinforced Polymer (FRP) materials for strengthening existing Reinforced Concrete (RC) structures according to the Near Surface Mounted (NSM) technique can be greater than the External Bonded Reinforcement (EBR) technique because debonding phenomena are less relevant and thus the tensile strength of the FRP materials is better exploited. Some experimental results and numerical analyses performed in the past showed that the debonding load in RC beams reinforced with EBR-FRP could be influenced by the effect of the loading pattern. Nevertheless, most bending tests available in the literature have been conducted with a 3 or 4 point loading scheme and very few tests have been performed under distributed loads. This issue has not yet been investigated for RC beams reinforced with NSM-FRP.Therefore, the results of flexural tests on RC beams strengthened with both NSM and EBR techniques are discussed in this paper to show that the debonding phenomena for NSM strip strengthened beams are less significant than for EBR plate beams. Moreover, the effect of the loading pattern is analysed to evaluate the sensitivity of failure modes and loads to different distributions of bending moment and shear along the beam. Finally, the experimental results are compared with the predictions of theoretical models and code provisions.

Flexural strengthening of Reinforced Concrete (RC) Beams Retrofitted with Corrugated Glass Fiber Reinforced Polymer (GFRP) Laminates

Abstract Strengthening the structural members of old buildings using advanced materials is a contemporary research in the field of repairs and rehabilitation. Many researchers used plain Glass Fiber Reinforced Polymer (GFRP) sheets for strengthening Reinforced Concrete (RC) beams. In this research work, rectangular corrugated GFRP laminates were used for strengthening RC beams to achieve higher flexural strength and load carrying capacity. Type and dimensions of corrugated profile were selected based on preliminary study using ANSYS software. A total of twenty one beams were tested to study the load carrying capacity of control specimens and beams strengthened with plain sheets and corrugated laminates using epoxy resin. This paper presents the experimental and theoretical study on flexural strengthening of Reinforced Concrete (RC) beams using corrugated GFRP laminates and the results are compared. Mathematical models were developed based on the experimental data and then the models were validated.

Experimental and analytical investigation into the flexural performance of RC beams with partially and fully bonded NSM FRP bars/strips

Thirteen reinforced concrete (RC) beams strengthened with partially and fully bonded near surface mounted (NSM) reinforcement and one unstrengthened beam (CB) were prepared and tested in order to study the effect of bond length, construction details and fiber reinforced polymer (FRP) characteristics on flexural performance. Carbon fiber reinforced polymer (CFRP) bars and strips and glass fiber reinforced polymer (GFRP) bars were used as the NSM reinforcement. Two bonded lengths (384mm and 480mm in each shear span) were studied for beams strengthened with partially bonded NSM reinforcement, while the bond length was 2000mm in beams strengthened with fully bonded NSM reinforcement. The effect of transverse wrapping and end anchorage on the behavior and load capacity of strengthened beams was also studied. The results showed that the RC beam strengthened with fully bonded NSM FRP had greater stiffness and bearing capacity than the beams with partially bonded NSM FRP. The use of NSM FRP with end anchorage or with transverse wrapping for the strengthening of RC beams increased their load carrying capacity and stiffness. The analytical model predicted well the response of the tested beams.

Analytical Model for Torsional Response of RC Beams Strengthened With Ferrocement U-Wraps

Wrapping technology is one of the most effective ways of strengthening concrete elements. Several research articles have confirmed the effectiveness of fibre-reinforced polymers (FRP) as wrapping material for enhancing the strength of reinforced concrete (RC) elements. Wrapping three sides of RC members is a convenient method of strengthening the beams that support the slabs. Its cost effectiveness over FRP makes ferrocement a good substitute as a wrapping material for strengthening RC beams. Limited research is reported regarding the torsional strength of ferrocement U-wrapped RC beams subjected to pure torsional loads. In this paper, an analytical model to predict the torsional response of RC beams with ferrocement U-wraps is presented. This analytical model is based on Hsu’s softened membrane model. The analytical model is validated by conducting experimental tests. In this investigation, 12 RC beams wrapped with ferrocement on three sides are tested under pure torsional loading. The different categories of beams tested in this investigation are under-reinforced beams, partially over-reinforced beams (longitudinal and transverse) and completely over-reinforced beams. The variations of U wraps considered in this investigation are three, four and five layers of mesh reinforcement in the ferrocement. The test results indicate that the cracking torque does not depend on the number of mesh layers present in the ferrocement wrapping. Ferrocement wrapping is more effective in the post-cracking stage than in the pre-cracking stage. The increase in the number of mesh layers in the ferrocement increases the ultimate torsional strength of the wrapped beam irrespective of the categories mentioned. The proposed analytical model is capable of predicting the torsional response with reasonable accuracy.

Finite element analysis of strengthened RC beams in shear with aluminum plates

This paper presents the development of a 3D nonlinear finite element (FE) model to capture and predict the response of shear deficient simply supported reinforced concrete (RC) beams strengthened externally with aluminum alloy plates. Five FE models were developed based on experimental tests conducted by the authors in a previous investigation. The experimental program included four RC beams strengthened in shear with externally bonded structural aluminum alloy plates of grade 5083–0 and tested under four-point loading to failure. The use of this material instead of the conventional fiber reinforced polymer (FRP) materials seemed to be very promising in enhancing both the strength and ductility of the strengthened specimens. The beams were designed to fail in shear and then strengthened with aluminum alloy plates with different strip spacing and orientation. The developed FE models have exact geometry, nonlinear material properties and boundary conditions to that of the experimental specimens. The FE models employed material constitutive laws for the concrete in tension and compression, yielding of the aluminum plates and flexural steel reinforcement. The developed FE models also incorporated the interfacial bond behavior at the aluminum concrete interface. The predicted FE results for the load–midspan deflection are compared to the measured experimental data. Close agreement was found between the predicted and measured results at all stages of loading for the tested specimens. For the maximum load and maximum mid-span deflection, it is observed that the Mean Absolute Percent Error (MAPE) of the prediction for the five specimens is 1.19% and 4.31% and the Normalized Mean Square Error (NMSE) is 0.0005 and 0.004, respectively. It could be concluded that the developed FE model could be used in future investigations to predict the performance of shear deficient RC beams externally strengthened with aluminum alloy plates with different configurations and orientations.

Shear strengthening of reinforced concrete beams strengthened using embedded through section steel bars

A new shear strengthening technique, designated as embedded through section (ETS), has been developed to retrofit existing reinforced concrete (RC) elements. This technique calls for holes to be drilled through the beam section; then bars of steel or FRP materials are introduced into these holes and bonded with adhesives to the surrounding concrete. When concrete cover has not the bond and strength requisites to guarantee a strengthening effectiveness for the Externally Bonded and Near Surface Mounted techniques, ETS strategy can be a competitive alternative since it mobilizes the beam’s concrete core which is, generally, free of damage. To explore the potentialities of the ETS technique for the shear strengthening of RC beams, an experimental program was carried out, composed of RC T-cross section beams shear strengthened by using steel bars. The influence on the shear strengthening efficiency of the inclination and shear strengthening ratio of ETS configurations was evaluated; the study also examined the interaction of ETS bars and existing steel stirrups. An increase of shear capacity up to 109% and 136% in the beams with and without internal stirrups, respectively, was obtained. Inclined ETS bars provided higher increase of shear resistance than vertical ones.

Fire behavior of concrete T-beams strengthened with near-surface mounted FRP reinforcement

This paper presents results from experimental and numerical studies on fire behavior of reinforced concrete (RC) T-beams strengthened with near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. Four RC T-beams, strengthened with NSM FRP reinforcement, were tested by subjecting them to ASTM E119 standard fire exposure and service load conditions. The test variables included load level, restraint conditions, and fire protection applied on the beams. Data from fire resistance experiments is utilized to validate predictions from numerical model to trace the fire response of NSM FRP strengthened concrete T-beams. Results from fire tests and numerical studies indicate that NSM FRP strengthened beams can sustain service loads for three hours under standard fire conditions, even without any external fire protection. In addition, fire induced axial restraint enhances the fire response of NSM FRP strengthened RC beams, while higher load levels lead to much larger deflections and lower fire resistance in these beams.

Fatigue behavior of RC T-beams strengthened in shear with EB CFRP L-shaped laminates

The use of externally bonded carbon fiber-reinforced polymer (EB-CFRP) to strengthen deficient reinforced concrete (RC) beams has gained in popularity and has become a viable and cost-effective method. Fatigue behavior of RC beams strengthened with FRP is a complex issue due to the multiple variables that affect it (applied load range, frequency, number of cycles). Very few research studies have been conducted in shear under cyclic loading. The use of prefabricated CFRP L-shaped laminates (plates) for strengthening RC beams under static loading has proven to be technically feasible and very efficient. This study aimed to examine the fatigue performance of RC T-beams strengthened in shear for increased service load using prefabricated CFRP L-shaped laminates. The investigation involved six laboratory tests performed on full-size 4520mm-long T-beams. The specimens were subjected to fatigue loading up to six million load cycles at a rate of 3Hz. Two categories of specimens (unstrengthened and strengthened) and three different transverse-steel reinforcement ratios (Series S0, S1, and S3) were considered. Test results were compared with the upper fatigue limits specified by codes and standards. The specimens that did not fail in fatigue were then subjected to static loading up to failure. The test results confirmed the feasibility of using CFRP L-shaped laminates to extend the service life of RC T-beams subjected to fatigue loading. The overall response was characterized by an accelerated rate of damage accumulation during the early cycles, followed by a stable phase in which the rate slowed significantly. In addition, the strains in the stirrups decreased after the specimens were strengthened with CFRP, despite the higher applied fatigue loading. Moreover, the addition of L-shaped laminates enhanced the shear capacity of the specimens and changed the failure mode from brittle to ductile under static loading. Finally, the presence of transverse steel in strengthened beams resulted in a substantially reduced gain in shear resistance due to CFRP, confirming the existence of an interaction between the transverse steel and the CFRP.

Automated serviceability prediction of NSM strengthened structure using a fuzzy logic expert system

This paper presents a simplified model using a fuzzy logic approach for predicting the serviceability of reinforced concrete (RC) beams strengthened with near surface mounted (NSM) reinforcement. Existing analytical models lack proper formulations for the prediction of deflection and crack width in NSM strengthened beams. These existing models are based on the externally bonded reinforcement (EBR) technique with fiber reinforced polymer (FRP) laminates, which presents certain limitations for application in predicting the behavior of NSM strengthened beams. In this study seven NSM strengthened RC beams were statically tested under four point bending load. The test variables were strengthening material (steel or CFRP) and bond length (1600, 1800 or 1900mm). For fuzzification, load and bonded length were used as input parameters and the output parameters were deflection and crack width for steel bar and CFRP bar. Experimentally NSM steel strengthened beams showed better performance in terms of crack width and stiffness, although NSM FRP strengthened beams exhibited enhanced strength increment. For all parameters, the relative error of the predicted values was found to be within the acceptable limit (5%) and the goodness of fit of the predicted values was found to be close to 1.0. Hence, the developed prediction system can be said to have performed satisfactorily.

FLEXURAL BEHAVIOURAL STUDY ON RC BEAM WITH EXTERNALLY BONDED ARAMID FIBER REINFORCED POLYMER

This paper presents the flexural behaviour of Aramid fiber reinforced polymer (AFRP) strengthened reinforced concrete (RC) beams of M25 grade of concrete. The experimental program included strengthening and testing of simply supported rectangular cross section beam of size 100mm x 150mm x 1200 mm strengthened with Aramid fiber polymer sheets. Total twenty specimens were tested out of which two beam specimens were tested as control beams and remaining for various damage degrees (i.e. 0%, 70%, 80%, 90%, and 100%). The effects of strengthening on load carrying capacity and effect of damage degree are discussed in detail. The results indicate that the load carrying capacity of beams was significantly increased as the number of layer increased. The validation of the experimental results was done by using ANSYS software. In order to study the flexural behaviour of the beam, the specimens were only subjected to two point loading mechanism only. The beams were wrapped with AFRP sheets in single layer and double layers along the length at the bottom face of the beam. The present work includes Effect of damage degree of the beam and effect of number of layers. Thus it is a feasible method for strengthening and retrofitting of RC beams.

Experimental Study of RC Beams Strengthened with Prestressed Steel-Wire BFRP Composite Plate Using a Hybrid Anchorage System

AbstractSteel wire-basalt fiber reinforced polymer (BFRP) composite plate (SBFCP) is a newly developed plate for strengthening reinforced concrete (RC) structures with both high performance and economy. This paper presents a prestress technique for flexural strengthening RC beams using SBFCPs after investigating mechanical properties of SBFCPs. To achieve higher levels of fiber strength utilization prior to premature debonding failure, a new hybrid anchoring approach that combines adhesive bonding and mechanical fastening is proposed. Six T-beams with 6 m length were tested under four-point bending. The main variable parameters were the composite plate types and prestress levels. The test results demonstrate that the use of SBFCPs for strengthening RC beams is an effective method for improving the flexural performance. Moreover, in comparison with nonprestressed one, prestressed SBFCP technology can further improve the cracking load and overall stiffness with less strain lag, better bonding, and better ut...

Experimental Study on Fatigue Stiffness of RC Beams Strengthened with CFL under Variable Amplitude Overloading

This paper study the effects of stress amplitude on the fatigue performance of reinforced concrete (RC) beams strengthened with carbon fibre laminate (CFL). A total of 15 spcimens were tested under three point bending. Three load levels over 60% bending capacity of the strengthened beams with a stress ratio of 0.1 were applied to three groups of five specimens, respectively. The stress amplitudes of groups 2 and 3 are 10% and 15% larger than group 1, respectively. The loads and deflections during the tests were recorded. The fatigue stiffness of the strengthened RC beams with variable stress amplitude was discussed. The experimental results show that the rates of the bending stiffness deterioration of the retrofitted beams, which increase with the load level, under constant fatigue loading are smaller than those under variable fatigue loading.

Strengthening of shear critical RC beams with various FRP systems

The use of fiber reinforced polymer (FRP) sheets to shear strengthen reinforced concrete (RC) beams has been studied and proven repeatedly. However, numerous strengthening configurations and materials can be combined to maximize the increase in strength and repair. This study investigates the effectiveness of using commercially manufactured carbon FRP (CFRP), glass FRP (GFRP) and fiber reinforced cementitious matrix (FRCM) sheets to increase the shear capacity of RC shear critical beams. Nine RC shear deficient slender beams were tested. The test variables included the use of three different FRPs (CFRP, GFRP, and FRCM), and the presence and type of FRP anchors (CFRP or GFRP). The experimental results revealed that applying FRP sheets increased the overall shear capacity, full depth u-wrapped FRP sheets perform better over companion partial depth u-wrapped FRP sheets, the use of FRP anchors further improved the shear capacity and ductility of failure, FRP strengthening could change the mode of failure from a shear to flexural failure and FRP debonding was delayed with the presence of FRP anchors. A comparison of the experimental load capacities with predicted values indicates that they correlate well with predictions using the current Canadian design codes.

Static and free vibration analysis of RC beams with NSM CFRP rectangular rods

Among the methods available for repairing or strengthening reinforced concrete (RC) beams, the Near Surface Mounted (NSM) technique has been shown to be a suitable method in many cases. Nevertheless, the effect of tensile concrete cracking on the bond mechanisms of Fiber Reinforced Polymer (FRP) rods in rod-resin interfaces and/or in resin concrete interfaces has yet to be investigated.This paper analyses the effects of damages due to bending cracking of concrete and loss of bond of NSM carbon-FRP (CFRP) rectangular rods on the static and dynamic responses of strengthened RC beams. Occurrence of damage in a reinforced concrete element leads to changes in its dynamic response. In many cases, experimental vibration monitoring of strengthening through dynamic tests is a convenient, non-destructive method.An investigation was developed to evaluate the experimental vibration changes of RC beams; three beams were built and examined experimentally by static tests and free vibration. One non-strengthened beam was tested to evaluate frequency value changes due to cracking at increasing bending moment. Two beams were strengthened with NSM CFRP rectangular rods: one was built without defects and one was strengthened with an imposed damage simulating loss of adhesion damage.Comparison between experimental and theoretical frequency values for three beams and the analysis of frequency changes due to damage for bending cracks and loss of adhesion allow discussing the actual behavior of RC beams with NSM CFRP rods.

Influence of prestress level on NSM CFRP laminates for the flexural strengthening of RC beams

This study presents an experimental program to evaluate the influence of the prestressing technique on the flexural behavior of reinforced-concrete (RC) beams strengthened with near-surface-mounted (NSM) carbon-fiber-reinforced-polymer (CFRP) laminates. The experimental program was organized to highlight the benefits of this technique for flexural strengthening of RC beams with low reinforcement ratio, susceptible of not respecting serviceability-limit-state (SLS) conditions, namely the deflection limit. For this purpose, five RC beams were fabricated to be tested under monotonic four-point loading. One beam was kept unstrengthened as a control beam, and another one was strengthened with a non-prestressed NSM CFRP laminate. The remaining beams were reinforced with a NSM CFRP laminate prestressed at 20%, 30% and 40% of its nominal tensile strength. Based on the results, applying the prestress force provided an increase of load carrying capacity corresponding to the concrete cracking and steel yielding initiations compared to the non-prestressed strengthened beam. Moreover, the influence of prestress level on the prevailing failure mode of the tested beams was assessed, and the relevant results are presented and discussed. An advanced numerical strategy was also developed to simulate the tested beams, which was demonstrated capable of being used for the design of this type of structures.

Kenaf Fiber Reinforced Polymer Composites for Strengthening RC Beams

This paper presents the findings of an experimental study to investigate the structural behavior of reinforced concrete (RC) beams externally strengthened with various types of kenaf fiber reinforced polymer composite laminates with 50% fiber volume content. The experimental parameters including flexural strength, stress-strain relationship, deflection, tensile stress and young modulus of beams strengthened by different kenaf fiber reinforced polymer composites are dis- cussed. In order to test the flexural strength of the RC beams, a total of eight beams divided into two series were used. In each series, every two of the beams were strengthened with kenaf/epoxy, kenaf/polyester, and kenaf/vinyl ester com- posite laminates respectively. The remaining two were kept as control specimens. The experimental moment capacity of each beam was calculated using tensile strength and young modulus of the composites. Comparisons were made on the load-deflection, strain behavior and failure mode. Moreover, theoretical moment capacity of each beam was calculated using conventional formulation to verify experimental results. The research findings indicated that all strengthened beams improved structural performance where the maximum flexural strength increase by 40% and maximum deflec- tion reduced by 24%.

Intelligent Predicting System for Modeling of Flexurally – Strengthened Reinforced Concrete Beams with CFRP Laminates

Abstract In the last years, a great number of experimental tests have been performed to determine the ultimate strength of reinforced concrete (RC) beams retrofitted in flexure by means of externally bonded carbon fiber-reinforced polymers (CFRP). Most of design proposals for flexural strengthening are based on a regression analysis from experimental data corresponding to specific configurations which makes it very difficult to capture the real interrelation among the involved parameters. To avoid this, an intelligent predicting system such as artificial neural network (ANN) has been developed to predict the flexural capacity of concrete beams reinforced with this method. An artificial neural network model was developed using past experimental data on flexural failure of RC beams strengthened by CFRP laminates. Fourteen input parameters cover the CFRP properties, beam geometrical properties and reinforcement properties; the corresponding output is the ultimate load capacity. The proposed ANN model considers the effect of these parameters which are not generally account together in the current existing design codes with the purpose of reaching more reliable designs. This paper presents a short review of the well-known American building code provisions (ACI 440.2R-08) for the flexural strengthening of RC beams using FRP laminates. The accuracy of the code in predicting the flexural capacity of strengthened beams was also examined with comparable way by using same test data. The study shows that the ANN model gives reasonable predictions of the ultimate flexural strength of the strengthened RC beams. Moreover, the study concludes that the ANN model predicts the flexural strength of FRPstrengthened beams better than the design formulas provided by ACI 440

Effects of Eccentric Load on Un-Strengthen and CFRP Strengthened RC Beams

CFRP strengthened reinforced concrete (RC) beams are usually tested under either three points or four point concentric loads; the effect of eccentricity is neglected in many studies. In this experimental program eight RC beams tested in two groups, GROUP ‘A’ & ‘B’. In group ‘A’ four RC beam tested, one control beam tested under concentric load and three under the eccentricity of 40, 60 and 80 mm respectively. The remaining four RC beams in group ’B’ strengthened using carbon fibre reinforced polymer (CFRP) strip and tested under the 60mm eccentricity. The experiment has confirmed the behaviour transformation from flexure failure to local failure due to variation in eccentricity. Therefore, the effects of eccentricity cannot be neglected and required special consideration before applying strengthening scheme on RC beam. The RC beams strengthen using CFRP have the ability to enhanced the load carrying capacity of beams tremendously, but the eccentric load effect on ductility, and failure mode of RC beams.

EBROG and EBRIG methods for strengthening of RC beams by FRP sheets

This paper presents an experimental study on reinforced concrete (RC) beam specimens flexurally strengthened by fibre-reinforced polymer (FRP) sheets using conventional externally bonded reinforcement (EBR) method and recently introduced grooving method (GM) in the form of externally bonded reinforcement on grooves (EBROG) and externally bonded reinforcement in grooves (EBRIG) techniques. The concrete beam specimens were strengthened with one, two or three FRP layers, and their load–displacement relationships as well as their failure loads were compared with each other. The performances of EBR, EBROG and EBRIG techniques for flexural strengthening of RC beams were found to be very close when one layer of FRP sheet was used; however, for two FRP layers, beams strengthened with GM in the form of both EBROG and EBRIG techniques experienced up to 25% higher ultimate loads compared to those strengthened with EBR method. Furthermore, for three FRP layers, this increase was 19 and 24%, respectively, for EBROG and EBRIG strengthening techniques compared to EBR method.

RC Beams Strengthened with Mechanically Fastened Composites: Experimental Results and Numerical Modeling

The use of mechanically-fastened fiber-reinforced polymer (MF-FRP) systems has recently emerged as a competitive solution for the flexural strengthening of reinforced concrete (RC) beams and slabs. An overview of the experimental research has proven the effectiveness and the potentiality of the MF-FRP technique which is particularly suitable for emergency repairs or when the speed of installation and immediacy of use are imperative. A finite-element (FE) model has been recently developed by the authors with the aim to simulate the behavior of RC beams strengthened in bending by MF-FRP laminates; such a model has also been validated by using a wide experimental database collected from the literature. By following the previous study, the FE model and the assembled database are considered herein with the aim of better exploring the influence of some specific aspects on the structural response of MF-FRP strengthened members, such as the bearing stress-slip relationship assumed for the FRP-concrete interface, the stress-strain law considered for reinforcing steel rebars and the cracking process in RC members resulting in the well-known tension stiffening effect. The considerations drawn from this study will be useful to researchers for the calibration of criteria and design rules for strengthening RC beams through MF-FRP laminates.