Strengthened Reinforced Concrete Beams Research Articles

Effective CFRP retrofit strategy for flexural deficient RC beams

Structural deterioration arises due to aging, environmental effects, deficiencies during design and construction phase, and overloading. Experimental and numerical investigations are carried out in this study to evaluate the performance of control and flexural deficient reinforced concrete (RC) beams under monotonic loading. Three levels of flexural deficiency are considered in this study. After confirming load carrying capacities of control and flexural deficient beams, the flexural deficient RC beams are strengthened with carbon fibre reinforced polymer (CFRP) fabric. CFRP strengthened RC beams are tested under monotonic loading and compared with the performance of control specimen. Further, non-linear finite element analyses are also carried out to evaluate the flexural performance of control, deficient and CFRP strengthened flexural deficient RC beams. There is good correlation between results of experimental and numerical investigations. Numerical approach presented in this study can be adopted for assessing the adequacy of CFRP retrofit measure.

The Use of Bolted Side Plates for Shear Strengthening of RC Beams: A Review

Reinforced concrete (RC) beams may need to be strengthened because of material deterioration, structure aging, change of building function, defective design, and the decrease of structural reliability caused by accidental disasters such as earthquake and fire. Thus, the retrofitting of RC beams has become a crucial problem, especially for the old buildings constructed before 1980 in mainland China. A variety of studies have proven that the bolted side-plating (BSP) method is feasible and effective for rehabilitating RC beams in existing buildings and infrastructures. The aim of this paper is mainly to review the previous studies conducted by the authors on the shear performance of reinforced concrete (RC) beams retrofitted using the BSP technique, including experimental, theoretical, and numerical studies.

Environmental efficiency of strengthening schemes for concrete beams with externally bonded fibre reinforced plastic composites

Externally bonded fibre reinforced plastic (FRP) is one of the most widely used methods in engineering practice to strengthen in-service reinforced concrete (RC) structures. Although some specifications, such as GB50367-2013 (China) and ACI440-2008 (U.S.A), have been issued to guide the design of strengthening schemes, few researches were conducted to evaluate environmental impacts of the strengthening schemes. This study aims to explore the environmental impact of FRP strengthened RC beam considering the structural safety and uncertainty, and to establish sustainable design criteria. Environmental impact and improvement of structural safety of two strengthening schemes designed according to the specifications of China (GB50367-2013) and U.S.A. (ACI440-2008) were evaluated by Life Cycle Assessment (LCA) method and reliability method, respectively. An environmental-efficiency index was introduced by taking an increment of safety as a functional unit in the LCA method. Furthermore, the allowable environmental impact was assumed and uncertainties involved in the LCA method were considered. The exceedance probability, denoting the possibility that environmental impact of a design scheme exceeds a presumed allowable value, is presented and calculated. It can be found that the environmental-efficiency indices of two schemes are close to each other. Moreover, the exceedance probability depends on the allowable environmental impact, thus it should be determined cautiously.

The Influence of CFRP Orientation Angle on the Shear Strength of RC Beams

Background:Carbon Fiber Reinforced Polymer (CFRP) sheets are widely used for strengthening and repairing reinforced concrete structures. Previous experimental studies have shown that strengthening Reinforced Concrete (RC) beams with CFRP sheet can be greatly influenced by the configuration, orientation and properties of the CFRP sheets.Objective:The behaviour of RC beams, strengthened with 60° and 45° inclined CFRP sheets, has not clearly explained.Method:Thus, an experimental program, proposed in this paper, investigates the shear behaviour of RC beams strengthened with CFRP sheets under different orientation angles including 0˚, 45˚, 60˚ and 90˚.Result:The study shows that strengthening RC beams with CFRP is highly influenced by the orientation angle of the sheets. The influence of CFRP sheets is remarkable on increasing the ultimate deflection, ductility and shear strength of RC beams.Conclusion:It is beneficial to strengthen RC beams, which are weak in shear, obliquely using 45˚ or 60˚ inclined CFRP sheets.

Fatigue Performance of RC Beams Strengthened with CFRP under Overloads with a Ladder Spectrum.

Vehicle overload is detrimental to bridges and traffic safety. This paper presents a study on the fatigue performance of typical reinforced concrete (RC) beams of highway bridges under vehicle overload. A definition method of vehicle overload and a construction method of overload ladder spectrum were first proposed based on traffic data acquisition, statistical analysis and structural calculation of the highway bridges in Guangzhou. A fatigue experimental method was also proposed with the three-ladder vehicle overload spectrum, and the fatigue tests of 15 RC beams strengthened with carbon fiber reinforced polymer (CFRP) under three loading levels were then carried out. The fatigue performance and the failure mechanism of the strengthened beams were presented and discussed, and two fatigue life prediction methods were proposed with the established modified Palmgren-Miner rule and the loading level equivalent method respectively. The results showed that the fatigue performance of the strengthened RC beams was severely degraded under overload ladder spectrum compared with that under constant amplitude cyclic load, and the life prediction methods were proved effective.

Behaviour of RC beams strengthened in shear using near surface embedded FRCM

This paper presents an experimental study on the efficacy of a lately introduced “near surface embedded” technique (NSE) for fabric reinforced cementitious matrix (FRCM) for the strengthening of reinforced concrete (RC) beams in shear; considering two test variables: (a) fabric type (glass and carbon) and (b) strengthening configuration (intermittent versus full). For this purpose, five (5) simply supported medium-scale RC rectangular beams have been tested under displacement controlled monotonic three-point loading. The experimental results showed that the FRCM system is effective in strengthening of shear deficient RC beams. An average of 62% gain in the shear capacity has been achieved due to the FRCM strengthening system. Carbon FRCM owning the higher axial rigidity showed better performance than that of glass FRCM. The average gain in the shear capacity for carbon FRCM strengthened beams was 22.5% higher than that for glass FRCM strengthened beams. Moreover, full strengthening configuration showed better performance than that for the intermittent configuration counterparts confirming the significance of the FRCM continuity and its quantity in enhancing the shear capacity of the strengthened beams.

A new approach for predicting the shear capacity of FRCM strengthened RC beams in shear

In spite of the availability of different models proposed to predict the shear capacity of reinforced concrete (RC) beams strengthened in shear using fabric reinforced cementitious matrix (FRCM) system, accurately predicting this value remains a challenge. The simplified compression field theory has shown to accurately predict the shear capacity of RC beams. This paper presents an analytical model based on the simplified compression field theory for predicting the shear capacity of RC beams strengthened in shear using FRCM strengthening system. The model has been validated against a data series of over sixty RC beams strengthened in shear using different FRCM types with different strengthening configuration and orientation. The results showed that the model can reasonably predict the load carrying capacity of the beams. The ratio of theoretically predicted and experimental results for the load carrying capacity ranged between 0.67 and 1.33 while average of this ratio was 1.01 with a coefficient of variation of 0.16.

Reliability assessment for flexural FRP-Strengthened reinforced concrete beams based on Importance Sampling

Due to the uncertainties raised by model error, materials (e.g., concrete, fiber reinforced polymer (FRP) and steel) and environment (e.g., loads), research aimed at reliability analysis and risk assessment of FRP-strengthened structures is necessary. In this paper, reliability analysis of flexural FRP-strengthened reinforced concrete (RC) beams with the ultimate limit state is studied. Four failure modes are considered: concrete crushing, FRP rupture, intermediate crack-induced (IC) debonding and end debonding. First, the computational models for each failure type are assessed in terms of model error based on a comprehensive database that contains 624 FRP strengthened RC beams. Then, the stochastic information for mechanical properties of the FRP and concrete materials is further identified through a series of mechanical tests on 600 concrete compression blocks and 50 FRP tensile specimens. Subsequently, the reliability index is estimated by Importance Sampling (IS) for its satisfied accuracy and efficiency. The results show that: (1) The probability of failure is different for each failure mode. It is necessary to consider all the possible failure modes to provide a comprehensive guide in the design process. (2) Based on the partial safety factor design format, the effect of the FRP partial safety factor γf on the reliability index for concrete crushing, IC debonding and end debonding is negligible. The calibration of γf is only valid for FRP rupture in order to meet the target reliability index. (3) Based on the resistance reduction factor design format, it is effective to calibrate resistance reduction factor ϕ for concrete crushing, FRP rupture, IC debonding and end debonding. Each failure mode should be designed with corresponding ϕ. (4) Additional anchors can improve the reliability index for IC debonding and helps to transfer to other failure modes for end debonding.

A Parametric Study on the Flexural Strengthening of Reinforced Concrete Beams with Near Surface Mounted FRP Bars

FRP rods as lightweight materials with extraordinary properties of high strength to weight ratio, corrosion resistance, potentially high overall durability, tailor ability and high specific attributes are one of the most favorable materials to strengthen existing reinforced structures. The present study aimed to identify the behavior of reinforced concrete flexural beams strengthened with fiber reinforced polymer (FRP) rods through near surface mounted method (NSM). The results of the current study were based on nonlinear finite element software ABAQUS which can accurately simulate the experimental investigations on flexural behavior of reinforced concrete beams strengthened with NSM FRP rods. Validation of the proposed model was confirmed first by making a comparison with the experimental study presented in the literature. A parametric analysis was conducted on validated specimens to investigate the effect of FRP rod diameters, rod arrangements, FRP materials, as well as rods groove intervals on flexural behavior of strengthened reinforced beams. The numerical results of mid-span bending moment deflection, ultimate bending moment, failure deflection and ductility index were reported. For the sake of simplicity to be used by engineers, the results of the current study were drawn in the form of design charts and tables.

STRUCTURAL BEHAVIOR OF THE HYBRID COMBINATION OF CFRP-BFRP SHEETS IN RETROFITTING OF RC BEAMS BASED ON NONLINEAR FINITE ELEMENT ANALYSIS

The use of composite materials such as fiber-reinforced polymers in strengthening and repairing of reinforced concrete elements is widely spreading. However, for successful and costeffective applications, engineers must improve their knowledge with respect to the actual behavior of strengthened structures. The CFRP sheets used in strengthening applications have high strength; however, they are brittle materials with low ductility. Basalt fiber reinforced polymer (BFRP) sheets on the other hand have relatively lower strength compared to CFRP, however they have higher ductility and cheaper than carbon fibers. As a result, there is growing interest among researchers and practitioners in combining different types of FRP sheets to produce an enhanced strengthening system in terms of strength and ductility. This hybrid system is designed to enhance the properties of composites, where it combines the high strength of CFRP and high ductility of BFRP sheets, respectively. This paper presents the nonlinear finite element analysis (FEA) that has been carried out to simulate the behavior of failure modes of reinforced concrete (RC) beams strengthened in flexure using CFRP sheets, BFRP sheets, and their hybrid combination (CFRP-BFRP). Besides, new technical approach is presented numerically to show that concrete cover replacement with high strength concrete one provided better load capacity and failure mode, indicating utmost utilize of the hybrid combination of CFRP-BFRP in strengthening of RC beams. The commercial and general finite element analysis; ABAQUS software is used for modeling and nonlinear analysis. Load deflection relationships, failure mode, ductility, ultimate load and ultimate deflection were obtained and compared with the recent experimental results available in literature. From the analysis, it is found that FEA can predict accurately the load-displacement relation and good agreements were obtained when compared to the experimental data. In addition, the proposed FE analysis can be reliably used as a cost-effective tool to predict the inelastic behavior of strengthened beams.

Parametric study for concrete cover separation failure of retrofitted SNSM strengthened RC beams

ABSTRACTThe side near-surface mounted (SNSM) method is a new flexural strengthening method for reinforced concrete (RC) beams which was proposed to allow near-surface mounted (NSM) strengthening to be applied on beams with small width. As a relatively new strengthening method, further studies are needed to determine the effects of strengthening parameters on the flexural performance of RC beams. In response to that, this paper presents a parametric study on the concrete cover separation failure of SNSM strengthened beams using a simulation method based on the moment-rotation (M/θ) approach.

Effect of CFRP Plate Length on Flexural Behavior of GFRP Reinforced Concrete Beam Strengthening

This research investigates the effect of CFRP plate length on flexural behaviour of GFRP strengthened reinforced concrete beam. CFRP plate was placed on the bottom of the beam to act as tension reinforcement. Ten beams were tested, largely divided into over designed and under designed reinforced beams. Each group had one steel reinforced concrete beam, one GFRP reinforced concrete beam and three GFRP reinforced concrete beam with different length of CFRP plate at the bottom of beam. Tested parameters included flexural strength, stress–strain relationship and deflection. Results indicated that all strengthened beams had improved structural performance where the maximum flexural strength increased by 102% and maximum deflection reduced by 35%. The CFRP plate can significantly improve the load-bearing capacity of strengthened GFRP concrete beams.

Shear strengthening of RC beams with thin panels of mortar reinforced with recycled steel fibres

Abstract The use of thin cement based panels reinforced with relatively high content of Recycled Steel Fibres (RSF) for the shear strengthening of Reinforced Concrete (RC) beams is investigated in the present work. The mechanical properties of this Recycled Steel Fibre Reinforced Mortar (RSFRM) are characterised. The panels are produced by using a mixing technique similar to the one used in the slurry infiltrated fibre concrete technology. Then, their potentialities as a shear strengthening solution for RC beams deficiently reinforced in shear are investigated by performing three-point bending tests with RC beams of “I” shape cross section. The RSFRM panels are applied in the lateral faces of the beams by using the two following strategies: (1) bonded to the concrete substrate by applying exclusively epoxy adhesive; and (2) besides epoxy adhesive, mechanical fasteners are applied. The applicability of an analytical approach for estimating the contribution of RSFRM panels for the shear resistance of RC beams is assessed, and a design example is presented.

Shear Repairing and Strengthening of Reinforced Concrete Beams Using SIFCON

The paper deals with the application of Slurry Infiltrated Fiber Concrete (SIFCON) as shear strengthening and repairing materials for normal strength reinforced concrete (RC) beams. The main objective of this study is to develop new and effective construction materials for strengthening of deficient RC beams. The experimental part of the study is divided into three stages. The first stage involved testing of thirteen shear deficient beams under four-point loading as control beams taking into account the effect of shear span-to-effective depth ratio (a/d = 1.2, 2.2, and 3.0) and amount of longitudinal reinforcement (ρ = 1.29%, 1.76%, and 2.28%). The second stage involved four shear deficient beams strengthened with SIFCON. For purpose of comparison, two of the tested control beams with a/d of 2.2 and ρ of 1.76% and 2.28% were repaired with SIFCON jacket. The test results indicate that all the beams strengthened with SIFCON jackets displayed an excellent shear capacity. The use of SIFCON jackets as external shear reinforcement eliminated the brittle shear failure and increased the ultimate shear strength of strengthened beams from 37 to 53%. Therefore, SIFCON jacket can be considered as a promising material for strengthening and repairing of RC structures. Finally, an analytical model was proposed for predicting the shear behavior of the strengthened RC beams with SIFCON Jackets with an acceptable error of less than 30%.

Flexural behaviour of RC members strengthened with FRCM: State-of-the-art and predictive formulas

Abstract Fabric-reinforced cementitious matrix (FRCM) composites are a promising and cost-effective technology for strengthening applications of reinforced concrete (RC) structures. The experimental and theoretical research on FRCM composites is still quite limited if compared with the amount of work carried out on fiber-reinforced polymer (FRP) systems. In addition, the knowledge gained on FRCMs so far has never been collected and reviewed in a systematic framework. This paper contributes to filling this gap by collecting and reviewing the main experimental research available in the literature on flexural strengthening of RC members with FRCM composites. A large database focused on the flexural behavior of strengthened RC beams is compiled. The experimental evidence is critically examined. Comparison within groups of tests of the database is made in terms of flexural capacity and failure modes. Key aspects of the design of the FRCM reinforcement are also analyzed and open issues for possible future research areas are pointed out. For Steel-FRCM systems, a predictive formula for the strain corresponding to the debonding of the external reinforcement is proposed.

Effect of defects in externally bonded FRP reinforced concrete

In practical engineering works, external bonding (EB) of fiber reinforced polymer (FRP) composite materials to concrete cannot be as perfect as in laboratory. Defects in bonding, therefore, cannot be avoided. An experimental program including single shear pull-out tests on EB-FRP concrete joints and three-point bending tests on reinforced concrete (RC) beams with/without EB-FRP was conducted. The effects of existing cracks in concrete with different spacing and widths, as well as FRP location relative to the centerline of specimen, were studied. Most of the specimens in the experimental program were prepared by unskilled personnel to simulate the “imperfect” workmanship. Test results show that EB-FRP joints with more than 10% concrete pulled off on the bond surface could have stable strength and ductility. Transverse offset of FRP from longitudinal center line does not have a significant effect on bond behavior. Existing cracks have a two-sided effect on bond-slip relationship. Cracking of concrete causes degradation of the bond slip relationship when crack width is large. However, cracking with smaller crack width could enhance the bond behavior to a certain degree. Existing cracks have limited influence on the load-deflection curves of FRP strengthened RC beams.

Strengthening of RC beams using externally bonded laminate (EBL) technique with end anchorages under flexure

This paper presents a detailed study on flexural strengthening of reinforced concrete (RC) beams with externally bonded (EB) technique using carbon fiber-reinforced polymer (CFRP) laminate. The different strengthening methods used were externally bonded laminate (EBL) and externally bonded laminate with end anchorage (EBLA). All the beams were designed for under-reinforced condition. Totally, six beams were cast; out of this, two beams were control beams and remaining were strengthened beams. The study was mainly concentrated on the improvement of ultimate load carrying capacity. The experimentation was done using two-point loading condition. The result indicates that the ultimate load for all the strengthened beams was improved when compared to control beams. The beams strengthened with EBLA technique show maximum increase in ultimate load, i.e., 48.54% with reference to control beams and also delay the debonding to a certain extent.

Strengthening of RC Beams Using Externally Bonded Fibre Reinforced Polymer Composites

An experimental study is presented on the behaviour of reinforced concrete (RC) beams retrofitted externally with glass fibre reinforced polymer (GFRP) fabrics. Out of ten beams, one beam without GFRP and nine beams wrapped in various lay-up patterns with one, two, three and four layers of GFRP fabrics have been tested for flexure under two-point loading. Loads corresponding to the first crack/delamination and ultimate failure of the beams have been recorded and types of failure have also been observed. Load versus deflection graphs have been plotted at salient locations of beams. Thereafter, a critical discussion has been made with respect to increase in the flexural strength of retrofitted beams as compared to the control beam in order to explore the optimal use of GFRP fabrics for strengthening the RC beams. In addition, a design proposal has been developed in extension to IS: 456-2000 to predict the ultimate design strength of RC beams strengthened with fibre reinforced polymer (FRP) fabric sheets. Thereafter, the values of the experimental flexural strength of the strengthened RC beam have been compared with the corresponding values of design flexural strength computed from present formulation and ACI: 440-2R-08 for its validation. The results obtained from the present experimental study show that flexural strength of the strengthened RC beams increases with increase in number of layers for all lay-up patterns. However, the beams wrapped with two layers GFRP fabrics in the tension face and half of the both sides below neutral axis show superior performance with respect to flexural strength, ductility and economy.

Torsional behaviour of reinforced concrete beam wrapped with aramid fiber

Present study gives the torsional behaviour of Reinforced Concrete (RC) beam of M30 grade of concrete strengthened with aramid fiber. The aramid fiber as an externally bonded reinforcement used for increasing flexural strength, an attempt is made to use it for improving torsional behaviour of RC beam. Torsional failure mostly occurs in seismic affected areas subjected to sudden failure. Torsional failure is brittle type failure occurs due to undesirable loading.RC beam strengthened with aramid fiber tested for torsional moment using lever arms subjected to equal static loading which transfer equal torsional moment to the beam. The cross section of beam is 150 mm × 300 mm and of 1 m in length. In that three beams were designed for torsional reinforcement and nine as conventional beams. Beam is designed for torsion as per IS456-2000. Building designed earlier using the codes were insufficient to cater for torsional reinforcement. This research investigates behaviour of such conventional beams for torsion when wrapped with aramid fiber. Torsional moments of such strengthened beams are compared with torsionally designed beams which are designed as per new design codes. The study is restricted to aramid fiber fully wrapped and wrapped in strips at width 100 mm of U shape on three faces of beam by using epoxy resin.Experimental result includes ultimate loads & first cracking loads, angle of twist and twisted shape of the beam. Result shows that fully wrapped RC beam gives more torsional strength as compared to controlled beam and there is significant improvement in torsional strength of beams wrapped in strips. After first crack, beam shows tendency to carry load with increase in angle of twist. Thus it is easy method for strengthening of RC beams.

An Artificial Neural Network Model for Estimating the Shear Contribution of RC Beams Strengthened by Externally Bonded FRP

This paper provides an artificial neural network model for predicting the shear contribution of FRP in reinforced concrete (RC) beams strengthened in shear with externally bonded FRP. Although there are some models and equations for estimating the contribution of FRP, these models, in some cases, have a significant error in the calculation of FRP contribution. One of the reasons for these errors is neglecting the effect of shear span (a) to the effective depth of beam (d) ratio in FRP performance. In this model, mechanical and dimensional properties of RC beams strengthened and strengthening materials, and also the shear span to the effective depth of beam ratio (a/d) are taken as input parameters, and the shear contribution of FRP is the target of the network. After a comprehensive review in existing literature, 96 strengthened RC beams which all of them have FRP rupture failure mode were selected which 92 of them were used for training, validation and testing the network and four of them were used for controlling the generalization of the network. Finally, the outputs of the model have compared with ACI 440.2R, fib 14 and CIDAR guidelines, and the result indicated that the ANN model is more accurate than the existing guideline equations based on experimental result.