Rectangular Reinforced Concrete Beams Research Articles

Carbon Fiber Reinforced Polymer (CFRP) for Structural Capacity Enhancement of RC Beams Incorporating Innovative Side Hybrid (SH) Technique

Reinforced concrete (RC) infrastructure is an essential part of modern civilization. However, the serviceability of RC infrastructure in extreme weather has become challenging due to the susceptibility of the initiation of cracks. Hence, the demand for strengthening and retrofitting RC infrastructure is rapidly increasing. The RC specimens strengthened with existing externally bonded reinforcement (EBR) and near-surface mounted (NSM) techniques; however, they suffered a prematurely brittle or debonding failure. Hence, the merging of side near surface mounting (SNSM) and side externally bonded reinforcement (S-EBR) methods ended up resulting in the development of an innovative side hybrid (SH) strengthening approach that is designed to overcome these drawbacks. In this investigation, six rectangular RC beam specimens were flexurally strengthened utilizing carbon fiber-reinforced polymer (CFRP) with the SH technique, and then four-point bending experiments were performed to failure. The beam specimens were categorized into two types: (I) control specimens and (II) specimens strengthened with the SH technique applying CFRP varying bonded length from 1600 mm to 1900 mm. The initial cracking, yield, and ultimate load-bearing capabilities, deflection, failure modes, cracking characteristics, stiffness, energy absorption capacity, and strain on the utmost fiber of concrete, the tensile strain of major steel rebars, SNSM bars, and S-EB plates were assessed from the experimental investigation. The SH technique substantially improved the flexural performance of the beam specimens. The initial cracking load, yield, and ultimate load-bearing capabilities were enhanced remarkably by 387%, 108%, and 163%, respectively, over the reference specimen. The flexural stiffness and energy absorption capacity substantially improved by 120% and 103%, respectively, compared with the reference specimen.

FE Analysis on Size Effect in Torsional Behavior of Rectangular RC Beams with and Without FRP Strengthening

Although torsion is usually considered a secondary consideration in structural design, it can lead to failures, especially in structures with irregular geometry exposed to seismic forces or in unique situations. While some studies have explored the size effect on the torsional behavior of RC beams, none have addressed the impact of reinforcement. This paper investigates the size effect on the torsional behavior of RC beams with varying reinforcement ratios, including both FRP sheets and steel rebars. In this study, the torsional behavior of reinforced concrete (RC) beams with and without FRP strengthening was investigated numerically. Modeling was carried out by Finite element (FE) software ABAQUS and validated using experimental data from literature. The torsional behavior of 24 models with differences in cross-section size and reinforcement ratio were investigated. Concrete damage plasticity, Hashin damage criteria, and von Mises stress were used to investigate the damage in concrete, FRP, and steel, respectively. The cracking torsional moment, ultimate torsional moment, and torque-twist curve of the beams were evaluated and presented. The results indicated the presence of a significant size effect in the torsional strength of beams. After reinforcing the beams with FRP fabrics, the average cracking moment increased by a range of 1.72% to 36% across different groups of beams. An 8.2% increase in ultimate strength is observed with just a 0.4% rise in the FRP strengthening ratio. Post-cracking behavior was seen in 8 models with adequate steel and FRP reinforcement ratio.

The behavior of strengthened RC beams under pure torsion using NSM-CFRP rope

PurposeThe paper aims to introduces an experimental work to investigate the torsional behavior of reinforced concrete (RC) beams strengthened by near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) ropes.Design/methodology/approachIn this research, nine rectangular RC beams of 250 mm × 300 mm cross-section and 1,600 mm in length were constructed and tested considering the studied parameters. These parameters include the length of the CFRP rope, the orientation of the CFRP rope, the arrangement of longitudinal and the scheme of NSM-CFRP ropes.FindingsIn comparison to control specimens, the results demonstrate a considerable improvement in the torsional response of RC beams strengthened with the CFRP rope. Additionally, specimens strengthened with 90° vertical ropes increase torsional moment capacity more efficiently than specimens strengthened with 45° inclined ropes since the stress concentration leads to premature debonding of the CFRP rope. Whereas RC beams' ability to withstand torsional moments is reduced as the distance between reinforcing CFRP ropes is increased. According to test results, adding CFRP ropes to RC beams' bottoms had a slightly positive impact on torsional response.Originality/valueThis paper fulfills an identified need to study how the using of the CFRP rope is effective in strengthening RC beam subjected to torsion moment.

A Key Contribution for Concrete Durability: Harnessing Force‐Magnetic Coupling for Stress State Detection in Reinforced Concrete Beams

Over the past 20 years, more than 200 major bridge‐collapsed accidents have occurred during their service life. The durability of reinforced concrete (RC) beams is a serious threat to the safety performance of the structures. To accurately grasp the service performance of RC beams, a four‐point bending loading experiment was conducted on RC rectangular beams, and magnetic field data were detected. The results show that during four‐point bending loading, the damage modes of RC beams can be categorized into the elastic stress stage, stage of work with cracks, and yield stage. The change rule of the rebar tangential magnetic induction intensity (Bx) curves varies from overlapping each other to rotating counterclockwise, finally generating abrupt changes. The force‐magnetic coupling model is optimized based on the magnetization angle. The “force‐magnetic area parameter” Kσx is proposed to quantitatively analyze the rebar stress. Finally, the stress state assessment model of RC beam rebars is established. The relative error of the assessment results is near 6.61%. The nondestructive testing and assessment of the rebar stress state inside the RC beams are realized through the comparison and verification of the experimental phenomenon analysis and the force‐magnetic coupling model. It lays a theoretical foundation for ensuring the safe operation of bridge structures and building structures during the service life.

Shear behavior of web huge rectangular opening RC beams

Abstract Opening in the web of reinforced concrete (RC) beams will destroy the integrity of the beam and weaken the mechanical properties of the RC beam, such as the bearing capacity and stiffness. The three-point loading test and finite element analysis were carried out on 10 RC beams with a huge rectangular opening in the web. The effects of stirrups around the holes on the shear properties of RC opening beams, such as bearing capacity, deformation capacity, and stirrup strain, were studied, and the shear transfer mechanism of RC beams at the openings was explained by using the strut-and-tie model. The finite element test results are consistent with the actual test results, as shown by the results. When the hole width is small, the reinforcement of stirrups at the hole side can significantly improve the shear capacity of the specimen, but it can also cause chord shear failure. When the hole height is small, with the strengthening of the chord stirrup, the ultimate bearing capacity of the specimen increases, but it can still result in hole-side shear failure. The curve of stiffness degradation is approximately logarithmic. Moreover, according to the finite element stress vector diagram, the shear transfer mechanism of the perforated beam based on the strut-and-tie model has been proved.

Flexural repairing of heat damaged reinforced concrete beams using NSM-BFRP bars and NSM-CFRP ropes

The introduction of Basalt Fiber Reinforced Polymer (BFRP) bars marks a significant milestone in the construction industry's pursuit of advanced and sustainable materials. The application of near-surface mounted (NSM) BFRP bars presents a promising, emerging approach for restoring heat-affected reinforced concrete (RC) elements. This study investigated experimentally and theoretically the flexural performance of rectangular RC beams subjected to a temperature of (650 °C) for 3 h, followed by repairing using NSM-BFRP bars and NSM carbon fiber-reinforced polymer (CFRP) ropes. Among the twelve designed RC beams, seven were repaired with NSM-BFRP bars and three with NSM-CFRP ropes to address flexural deficiencies. The primary focus areas included failure modes, ultimate capacity, and the recovery percentage of original flexural capacity. Results showed that recovering percentages ranging from 88.2 % to 127 %, beams repaired using NSM-BFRP bars gave almost the same behavior as those repaired using NSM-CFRP ropes. Theoretical outcomes agreed well with experimental findings, in alignment with the ACI 440.2R-08 guidelines.

Flexural Strength of Damaged RC Beams Repaired with Carbon Fiber-Reinforced Polymer (CFRP) Using Different Techniques

In this study, an experimental program was developed to investigate the flexural behavior of pre-damaged reinforced concrete (RC) beams that had been repaired and strengthened using carbon fiber-reinforced polymer (CFRP) laminates under a monotonic load. Two techniques were used: externally bonded reinforcement (EBR) and near-surface-mounted (NSM) reinforcement, to repair and strengthen the tested beams. The experimental program involved casting and testing nine simply supported RC rectangular beams; one beam was considered as the reference beam and did not undergo additional strengthening, and the remaining beams were strengthened using CFRP laminates. These eight beams were divided into two main groups for the purposes of strengthening: beams for which the EBR technique was used, and beams for which the NSM technique was used. The primary variables observed in the EBR and NSM groups included four damage percentages obtained according to the preload (20, 40, 60, and 80%) from the ultimate load carried by the reference beam. The experimental results show that decreasing the damage percentage leads to an increase in ultimate strength from about 3.6% to 17.2% for the beams repaired using the EBR technique and from 27.6% to 57% for the beams repaired using the NSM technique; additionally, the NSM method was more effective than the EBR method in terms of the flexural strength and mode of failure. However, using CFRP laminates enhances the flexure capacity of strengthened RC beams.

Prediction of flexural resistance of reinforced concrete beam cross-section by practical formula

This paper presents the process of developing a practical formula for predicting the ultimate bending moment of rectangular reinforced concrete (RC) beams through regression analysis. The data used for regression analysis was generated by using the fiber method to analyze a non-linear batch of commonly encountered RC beam cross-sections. The practical formula was obtained by fitting a linear regression model to the training set and then making predictions on the test set. The coefficient of determination, R2, between the bending moment values calculated from the formula and the results of the non-linear analysis was 0.9948, indicating a good predictive capability of the formula.

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.

Investigação da contribuição do engrenamento dos agregados em vigas de concreto armado e concreto reforçado com fibras de aço

Abstract Shear strength in reinforced concrete (RC) beams, especially in steel fiber reinforced concrete (SFRC) beams is a subject of great interest in structural engineering. In the case of beams without transverse reinforcement, the failure is explained based on a predefined crack pattern and kinematics, and the transfer of shear force accomplished through different mechanisms. Among these mechanisms, the aggregate interlock is present in most of the existing shear strength mechanical models in the literature, with divergences regarding its performance and preponderance. Thus, this paper focuses on evaluating the contribution of aggregate interlock throughout the critical crack formation process up to the ultimate load by performing bending tests on small-scale rectangular RC and SFRC beams without considering the effect of transverse reinforcement. The Digital Image Correlation (DIC) technique is used to track the patterns of shear cracks and their associated kinematics by measuring the relative displacements of opening (w) and sliding (δ). A detailed description of the shear behavior of these beams is provided by quantifying the aggregate interlock using the simplified Walraven model. The results help to understand the level of contribution of the aggregate interlock, and the main differences between structural elements of concrete with and without steel fibers in the scope of the shear strength.

Experimental Flexural Behavior of Externally Prestressed Rectangular Reinforced Concrete Beams and Effective Parameters

Experimental Flexural Behavior of Externally Prestressed Rectangular Reinforced Concrete Beams and Effective Parameters

Flexural Strengthening of Reinforced Concrete Beams with Variable Compressive Strength Using Near-Surface Mounted Carbon-Fiber-Reinforced Polymer Strips [NSM-CFRP

An experimental and analytical investigation was conducted on reinforced concrete (RC) beams strengthened in flexure using the near-surface mounted carbon-fiber-reinforced polymers (NSM-CFRPs) technique. A total of 11 full-scale RC rectangular beams were cast and tested under a monotonic three-point bending test, up to failure. The main test variables adopted in this study were the concrete compressive strength (high, medium, and low), the number of CFRP strips, and the strip length. The results indicated that the use of NSM-CFRPs strips in different configurations efficiently increased the load-carrying capacity of the strengthened RC beams, in which all these beams exhibited a higher moment resistance than the corresponding un-strengthened beam. Results also showed that all strengthening schemes were successful in increasing the flexural capacity of the specimens tested. Such increases ranged between 10.36% and 52.28%. Notably, a significant improvement in the ultimate load ratio was observed with beams having a low compressive strength of 17-MPa, then followed by the beams with medium strength (32-MPa), and finally beams with high compressive strength (47-MPa). The NSM technique reduced the occurrence possibility of the CFRP de-bonding failure mode. Furthermore, the test results were compared with theoretical predictions using the ACI 440.2R17 guidelines and showed a good agreement between these results.

Torsional strengthening using carbon fiber reinforced polymer of reinforced concrete beams subject to combined bending, shear and torsion

The results of an experimental investigation of six large scale rectangular reinforced concrete (RC) beams subject to different ratios of combined bending, shear, and torsion strengthened with externally-bonded carbon fiber reinforced polymer (CFRP) U-wraps are presented. All beams exhibited torsion-dominated behaviour. The presence of discrete CFRP U-wrap strips controlled the cracks well and the resulting torsional cracks were observed to be oriented at about 50°. Regardless of loading ratio, strengthened beams exhibited a 70% increase in torsional capacity. Before cracking, torsion to flexure ratio had a small effect on the flexural and torsional stiffness indicating that twist and deflection have a small counteracting effect on each other. Existing beam capacities were assessed using ACI 318-19 and GB 50010-2010 while CFRP strengthening was assessed based on fib Bulletin 14. Main findings are as follows: the ACI 318-19-predicted torsional capacity was conservative while GB 50010-2010 appeared to result in better capacity-prediction of unstrengthened RC beams. Even though load capacities derived from GB 50010-2010 are consistent with torsion-dominated behaviour, neither standard-based approach addresses the complete interaction of internal forces which is necessary to determine existing in situ strength for FRP strengthening applications. The fib Bulletin 14 approach to FRP strengthening for torsion was extrapolated to U-wraps and shown to overestimate the strengthening effect of the CFRP system applied. Critically, the assumed angle of inclination of torsional cracking was increased by the presence of the U-wraps, reducing the efficacy of the CFRP application.

Shear retrofitting with manually made NSM FRP sheets (MMFRP) of RC beams

This paper presents an experimental study to evaluate the effectiveness of the proposed carbon manually made fibre-reinforced polymer (MMFRP) folded sheet for shear strengthening of nine RC beams using near-surface mounted (NSM) method, in addition to an unstrengthened control beam. In the laboratory, MMFRP folded sheets were manually manufactured. MMFRP is made by folding an FRP sheet into appropriate groove widths. Carbon MMFRP folded sheets with varying strip amounts, FRP areas and development lengths increase the shear strength of rectangular-reinforced concrete (RC) beams. Double, triple and quadruple strips of FRP were intended to test the enhancement of strength capacity when the strip quantity was just increased without modifying the FRP area and development length. In addition, different FRP areas and development lengths were used to analyse the FRP area and development length of NSM-FRP strips. The advantages of beam strengthening employing different MMFRP folded sheets are explored based on experimental findings. In comparison with the control beam, beams with double, triple and quadruple strips demonstrate 68%, 59% and 74% improvements in shear load-carrying capacity, respectively. However, the study discovered that if the number of strips, FRP area or development length is raised, the member’s shear strength capability is estimated to improve.

Shear Strengthening of Reinforced Concrete Beam Using Wire Mesh–Epoxy Composite

This experimental research aims to study the use of wire mesh–epoxy composite (WMEC) as a shear-strengthening technique for reinforced concrete (RC) beams by focusing on the following parameters: (1) presence of shear reinforcement in the shear span; (2) type of strengthening technique (U-jacketing, vertical U strip, or inclined strip); and (3) number of wire mesh layers (three or six layers). Nine simply supported rectangular RC beams were tested under two monotonic point loads. The testing specimens were divided into two groups: (1) five beams without shear reinforcement and (2) four beams with shear reinforcement. Load–deflection relationship, shear ductility index, beams’ stiffness, energy absorption, crack propagation, mode of failure, and strain were studied for all testing specimens and compared with those of the control beams to measure the improvement from WMEC addition. Results showed that all WMEC types enhanced the shear capacity. Among the three shear-strengthening types, the continuous U-jacket scheme had a higher effect, increasing the shear capacity between 33.4 and 95.9% and the shear ductility index by 23% relative to those of the reference specimens. The shear capacity improvement by WMEC for the beams without shear steel reinforcement was greater than that for the beams with shear reinforcement under the same shear-strengthening configuration. When the number of wire mesh layers was doubled, the ultimate load was further increased from 33.4 to 57.8%. This research showed that WMEC is a practical and excellent shear-strengthening technique for RC beams. Doi: 10.28991/CEJ-2022-08-06-09 Full Text: PDF

Improved Equations for the Torsional Strength of Reinforced Concrete Beams for Codes of Practice Based on the Space Truss Analogy.

Design codes provide the necessary tools to check the torsional strength of reinforced concrete (RC) members. However, some researchers have pointed out that code equations still need improvement. This study presents a review and a comparative analysis of the calculation procedures to predict the torsional strength of RC beams from some reference design codes, namely the Russian, American, European, and Canadian codes for RC structures. The reliability and accuracy of the normative torsional strengths are checked against experimental results from a broad database incorporating 202 RC rectangular beams tested under pure torsion and collected from the literature. The results show that both the readability and accuracy of the codes’ equations should be improved. Based on a correlation study between the experimental torsional strengths, and geometrical and mechanical properties of the beams, refined yet simple equations are proposed to predict torsional strength. It is demonstrated that the proposed formulation is characterized by a significant improvement over the reference design codes. The efficiency of the proposed formulae is also assessed against another equation earlier proposed in the literature, and an improvement is noted as well. From the results, it can be concluded that the proposed equations in this study can contribute to a more accurate and economical design for practice.

Experimental and numerical investigation on low-strength RC beams strengthened with side or bottom near surface mounted FRP rods

This article examines experimentally and numerically the flexural performance of low-strength reinforced concrete (RC) beams strengthened using near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) rods. Five RC rectangular beams were tested in four-point bending until failure. Four of these beams were strengthened using Bottom NSM (BNSM) or Side NSM (SNSM) rods. The results are discussed in terms of observed damage, load capacity, and midspan deflection. It is shown that the SNSM strengthening solution enhanced the cracking load of the beams by up to 19%. Similarly, the yield and ultimate load-carrying capacities of the strengthened beams increased by up to 31% and 64%, respectively. It is also shown that moment-curvature and FE approaches predict well the deflections of the strengthened RC beams within 20% and 10% of accuracy at failure. The results from nonlinear Finite Element (FE) analyses showed a better agreement with the experimental results up to failure (within a 10% of accuracy). For the beams presented in this study, the crack widths calculated by Eurocode 2 match reasonably well the measured values.

Machine learning-based adaptive degradation model for RC beams

Reliable seismic damage assessment of structural systems requires analytical models that are able to capture the cyclic deterioration of components. In this paper, an adaptive-updated degradation model (AUDM) for reinforced concrete (RC) beams is proposed based on the machine learning approach. The proposed model is capable of updating its constitutive parameters from the embedded artificial neural networks, based on the analysis results derived from the previous step. It offers the possibility for generating deterioration without constructing empirical equations, updating the hysteretic shape as the accumulation of damage, and accounting for the effect of variable shear-flexural interaction in nonlinear analysis. The details of the proposed model, including the backbone curve, the hysteretic behavior and the cyclic deterioration rule, were first described. An experimental database consisting of 100 cantilever rectangular RC beams under cyclic loading was collected from the existing literature to identify the constitutive parameters in the proposed model. Artificial neural networks for predicting the constitutive parameters in the proposed model were developed and trained based on the identified data, which was then embedded into the open-source computational platform OpenSees to implement AUDM. The accuracy of the proposed model over the existing commonly-used degradation model is revealed in OpenSees based on the test results of RC beams under cyclic loading. It was found that the proposed AUDM was shown to be superior to the existing degradation models in terms of the lateral bearing capacity, the dissipated energy, the cyclic deterioration and the hysteretic shapes under different failure modes, with the relative error of energy dissipation capacity in the range between −0.19 and 0.19 and the maximum root square error for strength deterioration less than 0.259.

Performance of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer Strips.

Carbon fiber reinforced polymers (CFRP) have shown considerable potential in the repair and rehabilitation of deficient reinforced concrete (RC) structures. To date, several CFRP strengthening schemes have been studied and employed practically. In particular, strengthening of shear damaged RC members with CFRP materials has received much attention as an effective repair and strengthening approach. Most existing studies on strengthening shear-deficient RC members have used unidirectional CFRP strips. Recent studies on strengthened T-beams demonstrated that a bidirectional CFRP layout was more effective than a unidirectional layout. As such studies are limited, in this study, the feasibility of bidirectional CFRP layouts for the shear strengthening of rectangular RC beams was experimentally evaluated. Bidirectional layout details with CFRP anchors as well as rehabilitation timing were considered and investigated. The test results showed that the members with a bidirectional CFRP layout carried less shear strength capacity than those with unidirectional layouts for the same quantity of CFRP material. Nevertheless, the bidirectional CFRP layout allowed for a uniformly distributed stirrup strain compared to the unidirectional CFRP layout at the same load level, which increased the efficiency of the transverse reinforcement. Additionally, the shear contribution of CFRP material according to the CFRP strengthening timing was verified.

Experimental Research on the Flexural Performance of RC Rectangular Beams Strengthened by Reverse-Arch Method

Carbon fiber-reinforced polymer (CFRP) reinforcement technology has been widely used in the reinforcement of reinforced concrete (RC) beams. At this stage, high prestressed CFRP board reinforcement is often used in actual reinforcement. However, most reinforced bridges are designed for a long time, and the design value of the protective layer is low, and it is impossible to achieve a large prestressed tension. Therefore, this paper proposes the reverse-arch method to paste the CFRP board and apply low prestress to strengthen the symmetrical RC beam. Through the three-point forward loading test, the cracking load, ultimate load, crack width, mid-span deflection, strain and failure mode of a reverse-arch method-pasted CFRP board-reinforced beam, a directly pasted CFRP board-reinforced beam and an unreinforced beam are compared. The results show that the load-bearing capacity and stiffness of the test beam can be improved by pasting CFRP plates with anti-arch method, but the ductility of the test beam is reduced. Compared with the unreinforced beam, the maximum cracking load and ultimate load are increased by 56% and 63% respectively. The reverse-arch method can produce low prestress, improve the stiffness and bearing capacity of members, and has a good prospect of engineering application.