Strengthened RC Beams Research Articles

Finite Element Simulation for Nonlinear Finite Element Analysis of FRP Strengthened RC Beams with Bond-Slip Effect

A new simple, efficient and accurate finite element model denoted as FEM-B is developed for the analysis of structural behavior of FRP strengthened RC beams with bond-slip effect. Geometric nonlinearity and material nonlinear properties of concrete and steel rebar are accounted for this model. Concrete, steel, FRP and adhesive are modelled as Solid 65, Link 180, Shell181 and Solid 45 respectively. Concrete is modelled using Nitereka and Neal’s model for compression, isotropic and linear elastic model before cracking for tension and strength gradually reduces to zero after cracking, whereas steel is assumed to be elastic perfectly plastic material. The material of FRP is considered to be linearly elastic until rupture, and adhesive is assumed to be linearly elastic. The bond slip between concrete, adhesive and FRP is based on the bilinear law, which is modelled using spring element Combin 39.The developed new finite element model FEM-B is validated against experimental results, and demonstrates to be effective for the structural analysis of FRP strengthened RC beams.

Moment Capacity Charts for Strengthening of RC Beams With Near Surface Mounted FRP Reinforcement

Near surface- mounted fibre reinforced polymer is a new technique for strengthening
 structural members beside externally applied fibre reinforced polymer (FRP) technology.
 In NSM technique, the FRP material is applied to the tension region of the member inside
 pre-prepared grooves. In externally applied FRP the debonding of the fibres is a major
 issue thus, the full utilisation of FRP material can not be achieved, and the fibres are more
 susceptible to environmental effects. With NSM FRP, these issues can be controlled more
 effectively. ACI Committee 440 [1] has presented guidelines for design and construction
 of externally FRP method for strengthening of concrete structures. Although many
 experiments have been conducted on NSM FRP in flexure strengthening of RC beams,
 no code is introduced yet. Theoretical studies to derive equations for calculating moment
 capacity of strengthened beams have been introduced but mostly require several trial
 procedures to reach the required effective strain for FRP. In this study, a moment
 capacity chart is introduced based on several assumptions in the derivation of flexure
 capacity equation. To avoid the brittle behaviour of FRP, it is suggested that the
 utilization of FRP to be limited to 70% of its ultimate capacity for CFRP, and an
 equivalent area of reinforcement is adopted for simplicity. Experimental results of other
 researchers have been verified with the proposed charts, and a good comparison of results
 have being achieved.

Experimental Studies on Strengthening of Reinforced Concrete Beams Using Ferrocement Wraps for Energy Efficiency

Construction industry is a major consumer of energy and resources, which has an impact on the environment. Ferrocement construction technology is becoming popular throughout the world. Ferrocement is a thin element used for building construction and also for retrofitting, because it can be applied quickly on the surface of the damaged structural element without the requirement of any special bonding material. It requires less skilled labour when compared to other retrofitting solutions existing at present. This paper deals with the study of performance of ferrocement wrapping in strengthening of RC beams. The ferrocement construction has an edge over the conventional reinforced concrete material because of its lighter weight, ease of construction, low self-weight, thinner section and high tensile strength which makes it a favorable material for prefabrication also. Experimental work is carried out for beam having dimensions 250mm x 125mm x 3000mm, to investigate the behaviour of reinforced concrete beams retrofitted with ferrocement to increase the strength of beams in both shear and flexure. Three reinforced concrete beams are casted in order to study different parameters such as strength, stiffness, stability and ductility. M25 grade concrete and Fe 415 steel is used. Beam is designed as per IS : 456-2000 code of practice. The mix design is done as per code IS : 10262-2000. The experimental result concludes that ferrocement technology is the best solution in strengthening of Reinforced Concrete beams to meet the demand in an efficient and economical way for sustainable development.

Strengthening and Repair of Parabolic RC Beam Using Carbon Fiber – Reinforced Polymer Under Repeated Load

The aim of this study is to evaluate the effect of (CFRP) on parabolic beams under repeated load. In this study nine parabolic reinforced concrete beams were prepared having (150 x 200mm) dimensions , two unstrengthened as control beams and others were divided in to two groups (A and B) .
 Group (A) consisted of four beams wrapped with (CFRP) in different ways , group (B) included three damaged beams loaded to about (55%) of the maximum load of control beams and then repaired by (CFRP) . All beams tested under repeated load by (10kN) in one cycle.
 The test results showed the (CFRP) increased the strength of the parabolic RC beams based on the shape and type of the strengthening.

Effect of adhesive replacement with cement mortar on NSM strengthened RC Beam

This paper presents the experimental behavior of reinforcement concrete beams strengthened with near surface mounted (NSM) steel bar. Several researchers have used NSM fiber reinforced polymer (FPR) bar or strip for strengthening reinforced concrete beams. Though FRP has a number advantages like high strength, light weight and corrosion resistance it doesn’t show any ductile behavior, still highly expensive and not easily available in market. On the other hand, steel bar is less expensive, readily available in the market and shows good ductility. In order to get rapid and economic strengthening solution, strengthening with near surface mounted steel bar may become a potential alternative to retrofit reinforced concrete members. Therefore, the main purpose of this paper to investigate the experimental behavior of reinforced concrete beam strengthened with near surface mounted steel bar. Seven full-size beams (one control beams and six strengthened beams) were tested. Steel bars were used for strengthening. The failure load, mode of failure, deflection, and strain behavior are discussed. The study reveals that the near surface mounted technique with steel bars and cement mortar is the most economical strengthening solution to increase the flexural performance of RC beams.

Effectiveness factors for bond strength in FRP shear-strengthened RC beams

A study to describe theoretically the shear interaction effect between internal and external shear reinforcements in FRP-shear strengthening RC beams is presented. Such interaction phenomenon is characterized by the premature and brittleness debonding failure of the FRP and by the under-mobilization of the internal steel stirrups. The average stress (strain) in steel stirrups when FRP debonding occurs may be below the yield value, so not all steel stirrups intercepted by the critical shear crack reach yielding at the shear failure of the beam. In practice, the contributions of both internal shear steel reinforcement and externally bonded FRP are interacting parameters. By means some rationale simplifications to describe the debonding process, the problem has been developed in a way that a closed-form solution can be obtained. The average stresses in the FRP and steel reinforcements intersected by the critical shear crack at the debonding failure have been defined as a function of their strength, using effectiveness factors that have been conveniently expressed for the common FRP shear strengthening configurations, such as side bonding, U-jacketing and complete wrapping. A parametric analysis was also carried out to evaluate the influence of some fundamental parameters on such effectiveness factors.

Hybrid Kenaf Fibre Composite Plates for Potential Application in Shear Strengthening of Reinforced Concrete Structure

This paper presents the finding of an experimental study on kenaf fibre hybrid composite plates for potential application in shear strengthening of RC beam. In the experimental programme, hybrid composite plates had been fabricated using kenaf and carbon fibres with five different mixes i.e., 47% kenaf with 0% carbon; 45% kenaf with 2.5% carbon; 47% kenaf with 5% carbon; 45% kenaf with 7.5% carbon and 45% kenaf with 10% carbon. The physical and mechanical properties of the fabricated hybrid composite plates were then experimentally investigated. Results showed that carbon contents had an influence to enhance the tensile strength of the plates. Hybrid composite plate with 10% carbon content had shown 130% higher tensile strength as compared to that of plate without carbon fibre. The carbon content also had enhanced the modulus of elasticity of plates. Higher percentage of carbon had shown higher tensile strength and modulus of elasticity of hybrid composite plates. All fabricated plates had linear elastic properties under tension. The density of plates increased with the increasing of carbon content in composite plates. The research findings indicated that the developed hybrid composite kenaf could be used for shear strengthening of reinforced concrete beam.

Near surface mounted strengthening of RC beams using basalt fiber reinforced polymer bars

Present paper investigates the efficiency of basalt fiber reinforced polymer bars (BFRP) for near surface mounted (NSM) strengthening of RC beams. An experimental program was carried out to determine the pullout behavior of BFRP bars and based on the results, an BFRP-NSM method has been designed. To assess the bond behavior of BFRP to concrete, pullout tests were carried out. The influences of bonded length, groove size and diameter of bar on the bond behavior were analyzed. Based on the results, a ratio of groove size to diameter has been proposed to utilize the maximum potential of BFRP bars in NSM strengthening. Flexural strengthening of RC beams were carried out by using BFRP-NSM method. The diameter of the bar, number of bar and groove size were considered as the influencing parameter. Based on the studies, it is concluded that it is possible to double the load carrying capability without loosing ductility by using this method.

Investigations on RC Beams Retrofitted with UHSCC Overlay under Fatigue Loading

The present study aims to investigate the use of Ultra High Strength Cementious Composite (UHSCC) overlay strips for strengthening of pre-damaged RC beams. Fatigue loading was conducted in the study, one control beam was tested up to failure. Preloading of 80-90% in terms of number of cycles to failure was applied keeping stress ratio as 0.2 and frequency as 3Hz. Constant amplitude loading spectrum with different maxima minima but keeping stress ratio same was applied to control beam to examine any changes in structural behavior such as deflection and stiffness degradation of beams. The beams used were made of RCC with dimension 1500x100x200mm. The strengthened beams were tested to extract the parameters such as cycles for failure, load, deflection, crack initiation and failure phenomenon. It was observed that the beams retrofitted with UHSCC overlay strips performed efficiently in terms of number cycles to failure in comparison with conventional RCC beams under fatigue loading. Preloaded fatigue beam (FB1) had 17.6% higher load carrying capacity as compared to the control beam. Similarly preloaded fatigue beam (FB2) had 11.7% higher load carrying capacity as compared to the control beam. Acoustic Emission technique is a novel Non-destructive testing method which is used to identify the crack propagation and physical behavior of the beam at selected locations during the onset of loading, which gives a precise idea of the crack initiation and crack propagation under different loading condition.

Bond behaviour between NSM CFRP strips and concrete substrate using single-lap shear testing with cement-based adhesives

The efficiency of strengthening RC beams using the near-surface mounted (NSM) carbon fibre-reinforced polymer (CFRP) method depends on the bond between the NSM CFRP and the concrete substrate. This study presents the results of a study of the bond efficiency of NSM CFRP strips with two different CFRP strip surface types (smooth, rough) and dimensions (1.4 × 10 mm, 1.4 × 20 mm) with cement-based adhesive using single-lap shear tests. The study also contains development of a statistical model to predict the bond strength (pull-out force). It was found that the rough surface CFRP showed superior bond strength (pull-out force) when compared to the smooth surface CFRP. Moreover, there is a very good agreement between the experimental and predicted results.

Experimental study on rupture control of AFRP sheet for flexural strengthening of RC beam

Experimental study on rupture control of AFRP sheet for flexural strengthening of RC beam

Seismic Response of Strengthened RC Beam Column Joints with Haunch Effects

From the study of a large number of buildings, it has been recognized that beam column joint is one of the critical elements of a building system. This element should be designed and detailed properly, especially when the frame is subjected to earthquake loading. Beam column joints are brittle in nature. The breakdown of beam column joint during earthquake is controlled by bond and shear failure mechanism. The main reinforcement of columns just above the joint region, discontinuous bottom beam reinforcement and little or no joint transverse reinforcement are the most critical details of beam column joints in buildings. Therefore, current international code of practices gives more importance to provision of adequate anchorage to longitudinal bars and confinement of core concrete so as to resist shear. Modern codes propose for reduction of seismic forces through provision of special ductility requirements. Details for achieving ductility in reinforced concrete structures are given in IS 13920-1993. In the present study, a two-bay five-storey reinforced cement concrete moment resisting frame for a general building has been analyzed and designed by using STAAD Pro as per IS 1893–2002 code procedures and detailed as per IS 13920-1993 recommendations. An exterior beam column joint has been modelled to 1/5th scale of the prototype and the specimen has been subjected to cyclic loading to study its behaviour during earthquake loading. The experimental programme consists of two interior beam-column joint specimens, one is RC haunch and other one is strengthened RC haunch. The specimens are designated as RC and RCH respectively. In this study key structural issues such as load carrying capacity, load deflection behavior, stiffness degradation, energy dissipation capacity, ductility factor and cracking pattern have also been examined.

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ЖЕЛЕЗОБЕТОННЫХ БАЛОК, УСИЛЕННЫХ УГЛЕПЛАСТИКОМ, С УЧЕТОМ ОШТУКАТУРИВАНИЯ РЕМОНТНЫМ СОСТАВОМ

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ЖЕЛЕЗОБЕТОННЫХ БАЛОК, УСИЛЕННЫХ УГЛЕПЛАСТИКОМ, С УЧЕТОМ ОШТУКАТУРИВАНИЯ РЕМОНТНЫМ СОСТАВОМ

Analytical solution for interaction forces in beams strengthened with near-surface mounted round bars

The use of near-surface mounted (NSM) FRP composites for strengthening RC beams in flexure has become increasingly popular in the last decade. Compared to the externally bonded (EB) FRP strengthening method, the bond between FRP and concrete is much stronger when the NSM FRP strengthening method is used. However, debonding is still a likely failure mode in RC members strengthened with NSM FRP bars. The debonding in an NSM FRP-strengthened beam may initiate from either of the two ends of an NSM bar (i.e. end debonding) in the form of interfacial debonding or concrete cover separation, both of which are closely related to the existence of large localized interaction forces between the NSM bar and concrete near the bar ends in such beams. This paper presents an analytical solution for the interaction forces in RC beams strengthened with NSM FRP round bars, which are one of the most popular types of FRP bars used for NSM strengthening. The key elements of the proposed analytical solution are two interfacial stiffness parameters (i.e. tangential interfacial stiffness and normal interfacial stiffness). The validity of the proposed analytical solution is also verified using a sophisticated 3D FE model of a RC beam strengthened with a NSM round bar.

Strengthening of Reinforced Concrete T- Section Beams Using External Post-Tensioning Technique

This research is carried out to investigate the externally post-tensioning technique for strengthening RC beams. In this research, four T-section RC beams having the same dimensions and material properties were casted and tested up to failure by applying two mid-third concentrated loads. Three of these beams are strengthened by using external tendons, while the remaining beam is kept without strengthening as a control beam. Two external strands of 12 mm diameter were fixed at each side of the web of the strengthened beams and located at depth of 200 mm from top fiber of the section (dps). So that the depth of strands to overall depth of the section ratio (dps /h=0.8). For each strengthened beams, the strands have been tensioned by using a hydraulic jack with constant stress of 600 MPa. The main parameter conducting in this research is the strengthening length ratio (Ls/L) which is equal to the length of strengthening region (Ls) divided to the length of beam (L), these ratios are 0.83, 0.67 and 0.50. The experimental results showed that this technique for strengthening is efficient for reducing cracks width and increasing first cracking, service cracking and ultimate load capacities. The percentage increasing in first crack loads were 100%, 133% and 167%, for service crack loads (0.3 mm) were 63%, 75% and 88% and for ultimate loads were 78%, 89% and 67% for strengthening length ratios 0.83, 0.67 and 0.50 respectiviely as compared with the control beam.

Evaluation of Eco-Efficiency and Performance of Retrofit Materials

In this work three materials namely Fiber Reinforced Polymer (FRP), ferrocement and Textile Reinforced Concrete (TRC) have been evaluated towards their performance efficiency and eco-effectiveness for sustainable retrofitting applications. Investigations have been carried out for flexural strengthening of RC beams with FRP, ferrocement and TRC. It is observed that in the case of FRP, it is not possible to tailor the material according to design requirements and most of the time strengthened structure becomes over stiff. Eco-effectiveness of these retrofitting materials has been evaluated by computing the embodied energy. It is observed that the amount of CO2 emitted by TRC is less compared to other retrofit materials. Further, the performance point of retrofitted RC frames has been evaluated and damage index has been calculated to find out the effective retrofit material. It is concluded that, if RC frame is retrofitted with FRP and TRC, it undergoes less damage compared to ferrocement.

NONLINEAR FINITE ELEMENT ANALYSIS OF FRP STRENGTHENED RC BEAMS

A simple, accurate and efficient finite element model is developed in ANSYS for numerical modelling of the nonlinear structural behavior of FRP strengthened RC beams under static loading in this paper. Geometric nonlinearity and material non-linear properties of concrete and steel rebar are accounted for this model. Concrete and steel reinforcement are modelled using Solid 65 element and Link 180 element, and FRP and adhesive are modelled using Shell 181element and Solid 45 element. Concrete is modelled using Nitereka and Neal’s model for compression, and isotropic and linear elastic model before cracking with strength gradually reducing to zero after cracking for tension. For steel reinforcement, the elastic perfectly plastic material model is used. FRPs are assumed to be linearly elastic until rupture and epoxy is assumed to be linearly elastic. The new FE model is validated by comparing the computed results with those obtained from experimental studies.

Use Of precast SIFCON Laminates For Strengthening of RC Beams

Slurry infiltrated fibrous concrete (SIFCON) is a novel type of high performance fibre reinforced concrete made by infiltrating steel fibre bed with a specially designed cement based slurry. Laboratory experiments have shown that SIFCON is an innovative construction material possessing both high strength and large ductility. In the present study, the use of SIFCON has been investigated as an externally bonded strengthening material on reinforced concrete beams The experimental programme has been carried out to study the behaviour of flexural RC beams with precast SIFCON laminates. A total number of twenty one specimens of size 150mm x 300mm x1800mm corresponding to four test series are cast and tested under static loading to study the load deformation behaviour and ductility associated parameters. The concrete beams has been designed to obtain a concrete grade of M20.For laminates the fibre volume fraction was 7% and 10%. The steel fibres used in the study were hooked end steel fibres having 0.6mm diameter and aspect ratio of 50. Previous results indicate that the strengthening of RC beams with SIFCON laminates has significantly improved the cracking behaviour in terms of significant increase in first crack load and the formation of larger number of finer cracks. The stiffness, ductility and energy absorption are found to be increase to a great extent when the beams are strengthened by three face confinement ( bottom & side faces ).

Effects of Thickness of Ultra High-Performance Fiber Concrete Wrapping on the Torsional Strength of Reinforced Concrete Beam

Abstract: Two groups of rectangular beams, comprising of six specimens, the first group (L) were provided with four longitudinal bars, one at each corner while the second groups of beams (S) were fully reinforced with longitudinal bars and transverse reinforcement. Each group consisted of three beams. Two beams have been strengthened with ultra high performance fiber concrete (UHPFC) on four sides having a thickness of (15mm - 25mm) and one control beam. The variables considered in the experimental study include the transverse reinforcement ratios and the effect of thickness of UHPFC wrap. Experimental results show the effectiveness of the proposed technique at ultimate torque for strengthening beams and behavioral curves. Strengthened RC beams fully wrapped with a thin layer of UHPFC exhibit an enhanced torsional strength when compared to control beam. Results reveal that the transverse reinforcement ratios by 0.66%, increases the UHPFC contribution to torsional strength of strengthened beams with a 15 thick UHPFC; and by up to 7% for strengthened beams with a 25 thick UHPFC, respectively when compared to same strengthened beams without stirrup. It is found that the ultimate torque of beams with a 25 mm thin layer UHPFC is greater than beams with 15 mm by (28% and 28.3%) for the groups L and S, respectively.

Adaptive neuro fuzzy prediction of deflection and cracking behavior of NSM strengthened RC beams

The Near Surface Mounted (NSM) technique is a promising strengthening approach, though unpopular due to reliable serviceability prediction options. In this paper, a soft computing technique is employed to predict the deflection and cracking behavior of NSM strengthened RC beams. The Adaptive Neuro-Fuzzy Inference System (ANFIS) was used to simulate the serviceability behavior of one control and six NSM strengthened RC beams using variable bond lengths (1600, 1800 or 1900mm) with different NSM strengthening materials (steel and CFRP bars). The proposed ANFIS model employed variable load and bond length as inputs while the output parameters were deflection and crack width of the steel and CFRP bars. The ANFIS results were compared with the experimental results using the root-mean-square error (RMSE), coefficient of determination and Pearson coefficient. The results found that the ANFIS approach showed an improvement in predictive accuracy and generalization capability in comparison to the fuzzy approach. The highest level of accuracy with ANFIS was achieved when predicting CFRP bar crack width, where RMSE=8.05E−07. In contrast, the lowest level of accuracy was achieved when predicting CFRP bar deflection, with RMSE=0.045185.