Behavior Of Reinforced Concrete Beams Research Articles

Effects of pre-cracked reinforced concrete in compression zone on prefabricated RC beam behavior

Cracks can occur in the compression zone of a beam cross section, in the middle part of the span due to different reasons. For instance, due to accidents happened during transportation of the prefabricated Reinforced Concrete (RC) beams on manufacturing site. In this study, effects of such cracks on the behavior of RC beams are investigated experimentally. Compression strength of concrete and intensity degree of pre-created cracks were adopted as test parameters. A total of eight RC beam specimens were tested under the effect of bending, applying three-point loading. According to the results it was observed that the pre-cracked RC beams had almost the same strength with the reference beams; but bending stiffness of the RC beams decreased when the intensity of the cracks increased relatively.

Suggestions for repairing various defects in the NSM strengthening mechanism with hybrid rebars for RC beams: Experimental and numerical study

NSM bars enhance flexural strength of RC components but degrade ductility and transform the failure mechanism from ductile flexure to brittle shear. Failure due to debonding and other causes still has an impact on the strengthening system's efficiency. The focus of this research will be on how successful hybrid steel-CFRP/GFRP rebar strengthening is in enhancing the overall behaviour of reinforced concrete (RC) beams under flexure. Six rectangular RC beams were cast and strengthened using various semi-NSM procedures (120 mm width by 300 mm depth by 3000 mm length), then tested to failure in four-point bending tests. The beams were classified into four categories: (I) control beam, (II) strengthening with mild steel bar, (III) strengthening with mild steel-CFRP/GFRP hybrid rebars, and (IV) strengthening with high tensile steel-CFRP/GFRP hybrid rebars. To enhance the overall behaviour of RC beams, particularly beam ductility, and to avoid corrosion, FRP confinement was utilized in association with steel bars. End anchoring with strengthening rebars aims to limit unexpected de-bonding of NSM bars at higher loads, resulting in superior flexural performance of the RC members. In compared to the others, strengthening with mild steel-GFRP rebars enhances ductility and beam stiffness. This study also suggests and validates a 3D numerical model to simulate the performance and failure of NSM Strengthening mechanism with hybrid rebars for RC beams using the finite element (FE) software “ANSYS”. Regarding the behaviour of load-deflection, moment-curvature, load-crack width, and mode of failure, the FE results were accurate representations of the experimental results. An additional FEM specimen was examined to corroborate the effect of the experimental parameters.

Flexural Behaviour of Reinforced Concrete Beams using Alkali Activated Binders

This article discusses several different percentages of alkali activators that can be used in place of conventional cement with ground granulated blast furnace slag (GGBFS) resulting in alkali activated binder (AAB). They are labelled as G10 percent, G7 percent, G6 percent, and G5 percent, respectively, since they include 90 percent, 93 percent, 94 percent, and 95 percent of GGBFS, as well as 10 percent, 7 percent, 6 percent, and 5 percent alkali as an activator, respectively. A comparison of specimens formed of reinforced beams utilising alkaline binders and conventional cement is shown in this study. One of the six varieties of reinforced concrete beams is composed of Portland slag cement, four of the beams are cast with varied chemical concentrations of AAB concrete, and the last beam is produced with Ordinary Portland cement. They are examined using four-point bend test. The behaviour of reinforced concrete beams composed of AAB and conventional cement is analysed in this study, and the crack types and ultimate loads of failure are also demonstrated. It was established that the combination of alkali activated binders known as G10 percent was the most successful one since it led to good performance when compared to the OPC concrete.

Studies on Strength and Flexural Behaviour of Reinforced Concrete Beams with the Corroded Steel Reinforcements as a result of Sodium Chloride (NaCl)

Corrosion on embedded reinforcement is a serious structural failure which its deterioration are rarely concerned. Reinforcing steel are frequently found left unattended on corrosive environment where potential corroding substances might expose surface of rebar resulting in corrosion initiation. This paper reports result of a study conducted to investigate the impact of a rebar corrosion on flexural strength of reinforced concrete beam induced by 10.5% NaCl. Tensile test was conducted in this study to determine the degradation of tensile strength on rebar and was later utilized to analyse the reduction of maximum load bearing capacity on reinforced concrete beam. Both flexural rebar and tensile rebar were corroded using manual basting method which is carried out every day from 08.00 A.M-10 P.M (every 2 hours step). The corrosion area on flexural steel samples is at the maximum flexural strength area (400 mm in the middle of the span) and is later used on flexural testing purposes after casting with concrete, the tensile steel samples which is 600 mm span of rebar is later utilized for tensile testing purposes and is corroded in the middle of the span (185 mm in the middle of the span). Based on test results, it was observed that steel bar suffered significant loss of weights due to corrosion induced by 10.5% NaCl with respect to its corroding duration three and four months are 1.85% and 3.34% reduction of weight from its normal form and its tensile strength degraded, respectively, 6.78 % and 9.86%. Meanwhile, the reinforced concrete beam with corroded steel bars happened to experience degradation of maximum load capacity which are, respectively, 10% and 15% reduction for three and 4 months of corroding duration.

Effect of industrial waste metal chips on flexural behavior of re-inforced concrete beams

Recycling waste and by-products are necessary for environmentally friendly production and reducing carbon footprint. It is possible to use waste materials as additives and admixtures to improve the strength, ductility, and toughness of the reinforced concrete (RC) elements. Industrial waste metal chips have a high potential to be used as an admixture which improves the general behavior of RC elements, as proven by preliminary studies. In this study, various industrial waste metal chips were added into concrete as an admixture in different sizes and proportions to experimentally investigate the effect on the flexural strength of RC beams. Five RC beam specimens with identical dimensions and different metal chip waste admixtures were cast for the experiments. The load-displacement behavior of beams was examined by conducting a three-point bending test. As a result, it has been observed that the use of waste metal chips can significantly increase the flexural strength and ductility of the beams. The maximum load of one specimen with steel chip admixture is found to be 1.75 times higher than the waste-free reference beam. It is concluded that the usage of waste metal chips as an admixture is a viable solution to enhance the flexural behavior of the RC beams.

Effect of loss of bond on the behaviour of RC beams under service load conditions

Deviation of the flexural reinforcement spacing from code specifications appears to underly the cause of the loss of bond (LoB) between concrete and steel that preceded the unexpected collapse of the top-floor balcony of a building under service load conditions on 15 October 2021 in Athens. Although rare, LoB has also been identified as the cause of collapse of buildings under similar conditions in the UK and Canada. However, the codes of practice for reinforced concrete (RC) design neither make reference to the effect of LoB on structural behaviour, nor include LoB in the parameters that the formulae currently used for assessing load-carrying capacity are dependent on. In view of this, a description of the effect of LoB on the function of RC beams is provided in this paper and this description is used as the basis for the derivation of a formula linking LoB with load-carrying capacity. The validity of the proposed formula is verified using published experimental information on the subject. The proposed formula is used for the structural assessment of the collapsed balcony, taking into account the design details, physical state and loading conditions of structural members similar to the one that collapsed.

Flexural and Shear Behaviour of Reinforced Concrete Beams Modified with Polyethylene Terephthalate Fibre and Blue Gum Eucalyptus Wood Ash

Flexural and Shear Behaviour of Reinforced Concrete Beams Modified with Polyethylene Terephthalate Fibre and Blue Gum Eucalyptus Wood Ash

Does Metakaoline Replacement Adversely Affect the Cyclic Behavior of Non-Strengthened and Strengthened RC Beams: An Experimental Investigation

The need to reduce carbon emissions has recently become prevalent in light of concerns related to climate change. Since the cement industry causes approximately 8% of global CO2 emissions, it might be an urgent necessity to include cement replacement materials within the concrete industry. An important question arises about if such replacement negatively affects the cyclic or seismic behavior of reinforced concrete (RC) elements. This research presents an experimental investigation of the effect of using different percentages of metakaolin replacement on the monotonic and cyclic behavior of RC beams. The investigated parameters include the flexural strength, ductility and energy dissipation capacity of the tested beams. The current paper also aims to study the effect of using the CFRP-strengthening technique with 15% metakaolin replacement on the behavior of RC beams under the same loading protocols. The experimental results reveal that metakaolin can be used as a partial substitute for cement up to 20% without negative effect on the concrete behavior under both loading protocols. For cyclic loading, the percentage of replacement did not negatively affect the ductility; rather, it provided some improvement.

A comparative study on hybrid fatigue stress-life model of RC beams strengthened with NSM CFRP

Near Surface Mounted Carbon Fiber-reinforced Polymers (NSM FRP) are becoming widely used in the rehabilitation field of concrete structures. Several studies were conducted to evaluate the fatigue performance of Reinforced Concrete (RC) bridges strengthened with NSM FRP composite materials. Most stress-life (S-N) fatigue models of RC beams are based on fatigue testing that has been conducted in the region of load cycles ranging between 103 and 107. This might cause an inaccuracy in the stress-life predictions when the common S-N formulations are used. This paper presents a review of the recent research on the fatigue behavior of RC beams and proposes a hybrid fatigue stress-life model of RC beams strengthened with NSM CFRP. An analytical fatigue S-N model was developed based on experimental fatigue data of constant amplitude of tension-tension cyclic loading obtained from the literature. A hybrid formulation was used, combining both exponential and power regression analyses to enhance the accuracy of prediction in low and high cycle fatigue regions. Moreover, Muratov’s method was used to determine the endurance limit of strengthened RC beams. A three-dimensional finite element RC beam strengthened with NSM CFRP was modeled to investigate the accuracy of the proposed hybrid model. The RC beam was modeled in ANSYS with similar dimensions and material properties to experimental specimens in the literature. The hybrid model was compared to experimental and numerical data, and satisfactory precision was observed with a correlation coefficient of 0.86. The proposed hybrid model helps to provide comprehensive fatigue stress-life estimation for design purposes.

Creep Behaviour of Reinforced Concrete Beam under Long-Term Bending

Concrete structure after casting will be subjected to external load, peripheral components and other constraints, the overall performance of the structure under a long-term loading state will change continuously. In practical engineering, reinforcement is usually calculated and configured based on external load, but the role of reinforcement in the continuous loading process of concrete structure is not fully considered. Based on the elastic creep theory, this paper deduces the concrete creep calculation formula considering the influence of reinforcement, and analyses the law of the influence of reinforcement ratio and loading age on the strain and stress of concrete. The results show that the reinforcement can effectively restrain the deformation of concrete and reduce the load level of concrete.

Behavior of RC Beams Strengthened with NSM-CFRP Strips Subjected to Fire Exposure: A Numerical Study

The use of Near-Surface Mounted (NSM) Carbon-Fiber-Reinforced Polymer (CFRP) strips is an efficient technology for increasing flexural and shear strength or for repairing damaged Reinforced Concrete (RC) members. This strengthening method is a promising technology. However, the thin layer of concrete covering the NSM-CFRP strips is not adequate to resist heat effect when directly exposed to a fire or at a high temperature. There is clear evidence that the strength and stiffness of CFRPs severely deteriorate at high temperatures. Therefore, in terms of fire resistance, the NSM technique has a significant defect. Thus, it is very important to develop a set of efficient fire protection systems to overcome these disadvantages. This paper presents a numerical study that investigates the fire behavior of thermally insulated RC beams flexurally strengthened with NSM-CFRP strips and subjected to fire exposure according to the ISO 834 standard. The numerical study considered three-dimensional finite element models in the ABAQUS software that have been developed to simulate and predict the performance (thermal and structural response) of fire endurance tests on strengthened, uninsulated strengthened, and thermally insulated beams strengthened with NSM-CFRP strips, which were exposed to fire and had different fire insulation schemes. The insulation used was plaster from local material with a thickness range of 25 to 50mm. The variation of the thermal and mechanical properties with the temperature of the constituent materials was considered. All beams' mechanical and thermal responses were adequately simulated using numerical models. The results of the numerical simulations were in good agreement with the experimental data. The fire behavior of the NSM-CFRP strengthened RC beams was examined and particularly the efficiency of the NSM strengthening system during the fire. The behavior in the fire of the NSM-CFRP strengthening system on the RC beams thermally protected with different fire insulation schemes was assessed. Finally, the effectiveness of fire insulation was studied.

Study on Flexural Behaviour of Reinforced Concrete Beam With Effective Openings

Reinforced Concrete (RC) beams are essential structural elements that transfer loads from the slabs to the columns through flexure and shear. Openings in Reinforced Concrete (RC) beams are required for a variety of architectural and mechanical reasons. The purpose of this article is to investigate the flexural behaviour of an RC beam with circular openings running the length of it. The three-dimensional nonlinear finite element method was used to investigate a supported RC beam with circular web openings of varying diameters using ANSYS, a finite element software package. The study’s primary characteristic is the variation in diameter and location of reinforcement around the openings. Five RC beams with simple supports were constructed, and tested were conducted under two-point loads. Beams were constructed with one conventional specimen without any openings in beams and two beams with unstrengthen circular openings located in the flexure zone with varying diameters such as 80mm and 100mm considered as a control beam. In contrast, the remaining two beams provided effective diagonal reinforcement around the openings. The load-bearing capacity and load-deflection behaviour of such openings in beams were studied and analysed in detail. To increase the ultimate strength of the beam, effective diagonal reinforcement and short stirrups at the top and bottom of openings are used to prevent the beam from collapsing prematurely.

Experimental and analytical study on the behavior of RC beams with externally bonded carbon-FRCM composites

This paper presents both experimental and analytical study results for predicting the flexural capacity of reinforced concrete (RC) beams with externally bonded carbon fabric reinforced cementitious matrix (carbon-FRCM). Firstly, the available prediction approaches for FRCM-strengthened RC in flexure were summarized. The prediction formulas were deficient in the fabric slippage failure. Nine carbon-FRCM specimens with different layers of carbon fabric (CF) were tested under uniaxial tensile loading. Four RC beams were strengthened in flexure with carbon-FRCM having different layers of CF, which were tested under four-point bending. From the tensile test results, the apparent strain of CF was derived from the global strain of carbon-FRCM, and the two-layer carbon-FRCM was an optimized selection in the design of multilayer carbon-FRCM. From the bending test results, the utilization efficiency of CF was slightly higher in the fabric slippage failure than that in the peeling-off of concrete cover, and the failure mode was differentiated as the layers of CF increased. Additionally, a new procedure of prediction formulas in flexure for carbon-FRCM-strengthened beams was proposed, using the apparent strain of CF to assess the strengthening performance of beams at different load-carrying scenarios. The prediction of the proposed formulas agreed well with the experimental results.

Flexural behaviour of RC beams strengthened with prestressed steel wire ropes polymer mortar composite

ABSTRACT This paper presents an experimental study on the flexural behavior of reinforced concrete (RC) beams strengthened with prestressed steel wire ropes-polymer mortar composite (PSWR-PM). The effects of reinforcement ratio of steel wire ropes, strengthening procedures, and unbonded length of the steel wire rope-to-polymer mortar interface were studied with large-scale bending tests on RC beams strengthened by PSWR-PM. The test results showed that the proposed strengthening technique could substantially increase the load carrying capacity of the RC beams. Both strengthening procedures were effective in enhancing the flexural behaviour of the RC beams, and strengthening procedure II could even restrain the debonding failure. The unbonded length of the steel wire rope-to-polymer mortar interface had a minor impact on the ultimate capacity when the anchoring length of the steel wire rope in the polymer mortar was sufficient. Moreover, a theoretical model was proposed to predict the flexural strength of RC beams strengthened with the PSWR-PM technique and the predictions showed good agreement with the experimental results.

Impact Resistance Behavior of Reinforced Concrete Beams Deteriorated due to Repeated Freezing and Thawing

Many existing reinforced concrete (RC) structures constructed more than 50 years ago now require maintenance. This is especially true in cold, snowy regions where significant frost damage deterioration of RC structures becomes a severe problem. In this study, falling-weight impact tests were performed to investigate the impact resistance behavior of RC beams degraded by frost damage. An RC beam was subjected to approximately 900 freeze-thaw cycles to emulate the frost damage before the execution of the impact test. The surface of the beam was remarkably scaled, and its coarse aggregate was exposed. The degree of deterioration was evaluated by the distribution of ultrasonic propagation velocity. The following conclusions were drawn. (1) The ultrasonic propagation velocity of RC beams was significantly reduced following 872 freeze-thaw cycles. At the upper edge of the RC beam, the ultrasonic wave propagation velocity decreased from 4,000 m/s to 1,500 m/s in some parts. This corresponds to a relative dynamic elastic modulus of approximately 14%. (2) The residual deflection of RC beams with frost damage increased at most by 20% compared with beams without frost damage. The increase in residual deflection was primarily related to the peeling of concrete at the collision site and the opening of multiple bending cracks. (3) According to the existing residual deflection calculation formula, an increase of 20% in the residual deflection corresponds to a decrease of about 17% in the bending capacity of the RC beam. When the relationship between the degree of frost damage deterioration and the impact resistance of RC structures is defined, existing structures subjected to accidental impact force from rockfalls are safer and can be maintained more efficiently.

Behaviour of RC beams strengthened with FRP strips under combined action of torsion and bending

Many researchers worldwide have extensively used fibre-reinforced polymer (FRP) strengthening materials to enhance the shear and flexural strengths of reinforced concrete (RC) beams. However, Studies on strengthening of RC beam subjected to combined torsion and bending moment using both spiral and vertical strip configuration of CFRP that explored in this study is rare. This study aims to demonstrate the behaviour of RC beams strengthened with FRP sheets (strips) with different configurations and subjected to combined actions of torsion and bending moment. Eight beams with a dimension of 15 × 25 × 200 cm were cast. One of the beams was not strengthened, but the others were strengthened with carbon FRP. The angle of twist at torque intervals, first cracking torque, ultimate torque and ultimate twist angle of the conventional and strengthened beams during the testing process were compared. Results showed a significant improvement in the torsional performance of RC beams using carbon FRP. The fully wrapped beams performed better than the beams with strip wrapping due to the influence of various wrapping configurations. Amongst the wrapping configurations of FRP fabrics, the 45° spiral strip wrapping configuration was the most effective for RC beam strengthening in terms of torsion resistance. Highlights Reinforced concrete (RC) beams strengthened with fiber reinforced polymer composite were tested under combined bending and torsional moment; The effect of composite orientation, spacing and number of plies on the torsional response; Ultimate torsional moments of RC beams; Twist angle of rotation of control and strengthened beams; Analytical prediction for CFRP material contributions to the ultimate torsional moment of strengthened RC beam.

Behavior of RC Beams Strengthened Using Steel-Wire–Carbon-Fiber-Reinforced Plates

The steel-wire–carbon-fiber-reinforced plate (SCFR plate) is a relatively new strengthening technology for concrete structures. In this paper, a series of lateral impact tests on SCFR plates and conventional carbon-fiber-reinforced plates (CFR plates) were first performed, followed by tensile tests of both the SCFR plates and the CFR plates. It is found that the SCFR plates can provide the same level of tensile strength as CFR plates, whilst having evident advantages in terms of better ductility and lateral resistance. It is also found that increasing the amount of the steel wire can improve the lateral resistance of the SCFR plate. In addition, the SCFR plate shows the advantage of a reduction in lateral damage, which is commonly experienced by CFR plates during transportation, construction, and maintenance. In the second stage of the research, flexural tests of both SCFR and CFR plate-strengthened reinforced concrete (RC) beams were performed. The failure modes and crack patterns of the RC beams were investigated. Results show that the SCFR plate-strengthened beam exhibits enhanced ductility compared to that strengthened by traditional CFR plates, thereby enhancing the flexural capacity of the RC beams. On the basis of the test results, a formula is designed to predict the flexural capacity of SCFR plates; good agreement is achieved.

RC beam models damaged and strengthened with GFRP strips under bending loading and free vibration

Four reinforced concrete (RC) beam models undamaged, damaged by notches and strengthened by external bonded (EB) glass fiber reinforced polymer (GFRP) strips have been experimentally studied. The paper aims to assess the availability of strengthening with EB GFRP strips in RC beams having a deep localized damage. The behavior of RC beams has been analysed under bending loading until failure. Furthermore, the assessment of RC beam models foresaw nondestructive control of damaged and strengthened models by free vibration tests to obtain frequency values at different damage degree. Damage, artificially obtained by notches with different width, on the midspan section and on the lateral location of beams, has been analysed. The envelope of frequency response functions (FRFs) obtained by dynamic tests was elaborated and changes of natural frequency values are then correlated to damage both to non-strengthened beam with notches and to strengthened beam models. Results of static tests on RC beams strengthened with filled mortar in the notched sections and EB GFRP strips have allowed to validate the strengthening of RC elements with composite material characterized by relatively low elastic modulus; further, it has been assessed maintenance of bond between concrete surface and GFRP strips until failure under bending loading.

Behaviour of RC beams with a fibre-reinforced polymer (FRP)-strengthened web opening

The beam opening (BO) technique, which involves (1) the creation of a web opening in the beam to weaken its flexural capacity and (2) the fibre-reinforced polymer (FRP) shear strengthening around the opening to avoid shear failure of the beam, was proposed to realize the strong column-weak beam hierarchy in existing reinforced concrete (RC) frames, which were designed using previous codes and thus violated such hierarchy. In the present study, six large-scale T-section beams with a web opening of different sizes and made by different methods (pre-formed and post-cut) were tested to assess the effectiveness of BO technique and the test results are presented. The experimental study shows that the post-cutting method is feasible for making web openings in existing RC beams, the BO technique can effectively reduce the flexural capacity of the beam to the desired value if the size of web opening is appropriate, and the FRP strengthening can well compensate the loss of shear capacity and ductility of the beam caused by the creation of web opening. The present study provides a basis for the safe use of BO technique to achieve the strong column-weak beam hierarchy in existing RC frames.

Behaviour of Reinforced Concrete Beams With Circular Transverse Openings Under Static Loads

Transverse opening in a reinforced concrete beam allows the crossing of mechanical and electrical services through the beam. However, it affects the strength of a beam. Understanding its structural behaviour is crucial to ensure a safe design of the beam. For that, an experimental study was carried out on reinforced concrete beams with circular transverse openings. The four-point load test was conducted to study the effects of the size and the position of the opening on the beam performance under the shear and flexural loads. In addition, three reinforcing methods for the opening were tested. The beams were evaluated in terms of the load-displacement responses, mechanical properties, deflections, and failure modes. The opening with the diameter not exceeding 0.25 times beam height affected about 20% of beam strength (without reinforcements at the opening). The diagonal bar reinforcing method effectively restored the beam strength for the opening size not exceeding 1/3 of beam height. The equation model proposed conservatively predicted the ultimate capacity of the beam with a transverse opening.