Ductility Of Reinforced Concrete Beams Research Articles

Study on the effect of pre-stressed level on the force performance of wedge steel plate jacket and steel wire mesh-PCM strengthened RC beams

In this study, the shear behavior of reinforced concrete (RC) beams strengthened with wedge steel plate jacket (WSPJ) and high-strength steel wire mesh-polymer cement mortar (PCM) was investigated. The total contained 6 strengthened beams and 3 control beams, the test variables were the level of prestressing applied to the steel wire and the shear-span ratio of the RC beam. Our findings indicate that shear failure and bending failure were two types of strengthened beams failure, and the WSPJ and steel wire mesh-PCM strengthening technology can effectively improve the shear bearing capacity and ductility of RC beams. Furthermore, it delayed and inhibited the emergence and development of oblique cracks. Under the same shear-to-span ratio, the characteristic load as well as the shear bearing capacity of strengthened RC beams were significantly increased with the increase of prestressing level. The ductility is increased, and the damage mode of strengthened RC beams was changed. Finally, a modified model based on Truss-Arch model was proposed to forecast the shear capacity of strengthened beams. This model considers the impacts of the prestressing level and the shear-to-span ratio and then validated by the experimental data.

A New Approach for Determining the Curvature Ductility of Reinforced Concrete Beams

Abstract This paper presents a numerical parametric study of the moment-curvature and curvature ductility of doubly-reinforced beams with different parameters. The effects of the strength of the concrete and the amount of the reinforcement, including the tensile and compression reinforcement on the complete moment-curvature behavior and the curvature ductility factor of the beam sections, have been studied. A new predictive formula for the ductility factor of beam sections that considers the different parameters has been developed. In a continuation of the study, the flexural ductility of beams designed with different parameters according to the ductility factor proposed by different researchers was investigated. Based on the results of the numerical analysis, the proposed predictions for the curvature ductility factor were verified by comparisons with other predictive formulas. The proposed formula offers fairly accurate and consistent predictions for the curvature ductility factor of beam sections. It is shown that the concrete’s compression strength and the amount of reinforcing steel, including the compression reinforcement ratios, have an effect on the curvature ductility factor of beam sections.

Effect of U-wrap anchors on the strength and ductility of externally bonded RC beams with mortar bonded GSM sheets

Strengthening of deficient reinforced concrete (RC) structures had been one of the major concerns of researchers over the past few decades. Thus, new materials are being discovered to ease the process of externally strengthening RC structures. One of the various strengthening materials is Galvanized Steel Mesh (GSM) that is available in different cord densities. High and low cord-density GSM sheets can be externally bonded to RC structures using epoxy adhesives, while only low cord-density GSM sheets can be externally bonded with cement Mortar. In this experimental study, low cord-density GSM sheets (3.15 cords/cm) were bonded with cement mortar to the soffit of RC beams surfaces. A total of four RC beams were cast, three of which were externally strengthened in flexure with GSM sheets and bonded with mortar and the remaining beam was kept as a control un-strengthened specimen. In order to avoid the brittle delamination failure mode, one of the strengthened specimens was anchored with end U-Wrap carbon-fiber reinforced polymer (CFRP) sheets, while the other strengthened specimen was anchored with intermediate U-Wrap CFRP sheets, and the remaining third strengthened specimen was not anchored, to serve as a control strengthened specimen. All specimens were subjected to four-point loading test and the load-deflection response curve of each specimen was recorded. It was observed that flexural strengthening using GSM sheets increased the ultimate-load capacity of strengthened specimens by a range of 41.8 to 51.4 %, when compared to control un-strengthened specimen. While, no strength increase was observed when comparing anchored strengthened specimens to non-anchored strengthened specimens. However, the ductility of end anchored specimens increased by 142.1 and 270% when compared to control and non-anchored specimens, respectively. In addition, the ductility of intermediate anchored specimens increased by 134.1 and 257.3 % when compared to control and non-anchored specimens, respectively. Thus, it can be concluded that GSM laminates if properly anchored can enhance the strength and ductility of RC beams in flexure.

Numerical Study on The Structural Behavior of RC Beams after Exposure to Elevated Temperature

This paper presents a numerical simulation of the structural response of reinforced concrete (RC) beams under elevated temperature using the commercial finite element package ABAQUS. A numerical model is firstly suggested by selecting the appropriate geometrical and material properties of the RC beam model at elevated temperature. Thereafter, the suggested numerical model was validated against the experimental tests conducted in this study. The validation results in terms of temperatures- time histories; load-mid span deflection of the RC beams have confirmed the accuracy of the suggested numerical model. The validated numerical model was implemented in conducting a parametric study to investigate the effects of two important parameters on the behavior and failure of RC beams under elevated temperature. These parameters are the effect of the high ranges of elevated temperatures; and the effect of heating rate. The parametric study results have revealed that the failure load and the ductility of RC beams under elevated temperature are not considerably influenced by changing the heating rate. It has also been concluded that the ultimate load capacities of RC beams have considerably decreased by 55.49%, 74.72%, and 81.31% comparing with the control RC beam when they exposed to temperature values of 600 ºC, 700 ºC, and 800ºC respectively. These conclusions may be used in the design of RC beams subjected to fire induced temperature.
 Numerical model

Study on Anti-Explosion Behavior of High-Strength Reinforced Concrete Beam Under Blast Loading

To investigate the anti-explosion behavior of high-strength reinforced concrete (RC) beam subjected to blast load, the ANSYS/LS-DYNA finite element analysis software was applied. Based on anti-explosion test results, the effects of reinforcement strength grade, reinforcement ratio and stirrup ratio on dynamic response, failure mode, resistance curve and ductility of RC beams under uniform blast load were studied. The anti-explosion performance of RC beam can be effectively improved by increasing the strength grade of the high-strength reinforcement. When the shear capacity is high enough, the ultimate capacity of high-strength RC beam can be significantly enhanced by increasing its reinforcement ratio. Anti-explosion performance may deteriorate due to the change of failure modes when the reinforcement ratio is increased to a certain extent. Increasing stirrup ratio can improve the shear capacity of high-strength RC beam to guarantee the full utilization of the advantage of high flexural capacity. For high-strength RC beam with sufficiently shear capacity, the further increase of stirrup ratio has a slight effect on the anti-explosion ability.

Cyclic Response of Steel Fiber Reinforced Concrete Slender Beams; an Experimental Study.

Reinforced concrete (RC) beams under cyclic loading usually suffer from reduced aggregate interlock and eventually weakened concrete compression zone due to severe cracking and the brittle nature of compressive failure. On the other hand, the addition of steel fibers can reduce and delay cracking and increase the flexural/shear capacity and the ductility of RC beams. The influence of steel fibers on the response of RC beams with conventional steel reinforcements subjected to reversal loading by a four-point bending scheme was experimentally investigated. Three slender beams, each 2.5 m long with a rectangular cross-section, were constructed and tested for the purposes of this investigation; two beams using steel fibrous reinforced concrete and one with plain reinforced concrete as the reference specimen. Hook-ended steel fibers, each with a length-to-diameter ratio equal to 44 and two different volumetric proportions (1% and 3%), were added to the steel fiber reinforced concrete (SFRC) beams. Accompanying, compression, and splitting tests were also carried out to evaluate the compressive and tensile splitting strength of the used fibrous concrete mixtures. Test results concerning the hysteretic response based on the energy dissipation capabilities (also in terms of equivalent viscous damping), the damage indices, the cracking performance, and the failure of the examined beams were presented and discussed. Test results indicated that the SFRC beam demonstrated improved overall hysteretic response, increased absorbed energy capacities, enhanced cracking patterns, and altered failure character from concrete crushing to a ductile flexural one compared to the RC beam. The non-fibrous reference specimen demonstrated shear diagonal cracking failing in a brittle manner, whereas the SFRC beam with 1% steel fibers failed after concrete spalling with satisfactory ductility. The SFRC beam with 3% steel fibers exhibited an improved cyclic response, achieving a pronounced flexural behavior with significant ductility due to the ability of the fibers to transfer the developed tensile stresses across crack surfaces, preventing inclined shear cracks or concrete spalling. A report of an experimental database consisting of 39 beam specimens tested under cyclic loading was also presented in order to establish the effectiveness of steel fibers, examine the fiber content efficiency and clarify their role on the hysteretic response and the failure mode of RC structural members.

Shear Strengthening of Reinforced Concrete Beams Using CFRP Wraps

The need to retrofit existing reinforced concrete (RC) structures have increased over the decades due to corrosion of steel reinforcement inside the concrete, neglect and overuse, and increased loading. Experimental and numerical studies in this research field showed that using fiber-reinforced Polymer (FRP) materials to strengthen RC members in shear, flexure, and column confinement applications is an effective method to retrofit RC structures. This strengthening technology has numerous advantages over conventional steel plating, such as providing high strength-to-weight ratio, versatility, durability, and ease of use to strengthen RC members. The purpose of this paper is to study the effect of strengthening shear deficient RC beams with externally bonded (EB) carbon fiber-reinforced polymer (CFRP) sheets with different wrapping configurations. Two shear strengthening wrapping schemes of CFRP sheets will be investigated; U-Wrapped and completely Wrapped. Although the completely wrapped scheme is more effective, however, due to its limitations, the U-Wrapped scheme became the most commonly used method in shear strengthening of RC structures. The drawback of this strengthening scheme is the premature failure caused by debonding of the CFRP laminates, without utilizing its full strength. The main aim of this study is to compare the performance of a U-Wrapped T-beam with two completely wrapped rectangular beams. The first strengthened rectangular specimen (WBR1) has a depth equivalent to the T-beam’s web height, while the second strengthened rectangular specimen (WBR2) has a depth equivalent to the T-beam’s total depth. In addition, a control T-beam and two rectangular beams were cast and tested as benchmark specimens. The experimental test results showed that the beam strengthened by U-Wrapped CFRP sheets increased the control beam’s shear strength by 114.82%, while the increase in shear capacity of the completely wrapped equivalent WBR1 and WBR2 beams over their control unstrengthened specimens was 69.28% and 201.63%, respectively. In addition, the completely wrapped scheme provided more ductility compared to that of the U-Wrapped T-beam specimen, that failed by CFRP sheets debonding in a brittle manner. Thus, it could be concluded that an ideal way to increase the shear capacity and ductility of RC beams is to completely wrap the beams using CFRP sheets. However, if the completely wrapping scheme is not possible due to geometrical obstructions, U-wrapping scheme could be effective in increasing the shear capacity of RC beams but will fail in a brittle mode by sheet debonding without utilizing its full strength. Anchoring the CFRP U-Wraps could be a viable solution to enhance the performance of strengthened RC beams that should be investigated in future research studies.

Ductility of RC beams under torsion

In this article, the torsional ductility of reinforced concrete (RC) beams with rectangular cross section is studied. For this, the experimental results of several RC beams found in the literature were compiled and analyzed. A torsional ductility index was used to characterize the torsional ductility of the studied beams. The following variables study were considered: compressive concrete strength, torsional reinforcement ratio and cross section type (plain or hollow). The influence of each variable study on the torsional ductility is studied and important findings are pointed out which could help for the design of RC beams under torsion. An additional comparative analysis with the rules from some codes of practice in use is also performed. It is shown that, in general, the codes are too much restrictive as far as the maximum torsional reinforcement is concerned. As a consequence, this can leads to the unacceptance of several beams with ductile behavior.

Detailing of concrete-to-concrete interfaces for improved ductility

Recent research has shown that reinforced concrete (RC) beams with concrete-to-concrete casting interfaces where plastic hinges are likely to develop, may experience reduced ductility in comparison to similar structural elements casted at once, due to a potential shear slippage along the casting interfaces. Although of relevant importance for both precast and cast in situ RC structures, this problem is still not addressed in current codes and standards, which limit the safety check of casting interfaces to the verification of their strength based on improved expressions of the “shear friction theory”, the latter proposed in the 60’s. However, recent research has shown that friction strength of casting interfaces depends on interface width opening, and it is significantly reduced after the yield of the bending reinforcement. During the formation of plastic hinges, shear stresses run preferentially across the compressed zones of the interfaces, reducing their strength, and ultimately the specimens’ ductility.In this paper, different and alternative details for interfaces are proposed to improve global behaviour, and in particular, ductility of RC beams with casting interfaces located on plastic hinges regions. An experimental campaign was carried out to study the effect of: (i) epoxy and latex based adhesion promoters’ usage between castings; (ii) web reinforcement; (iii) geometry of interfaces; (iv) and shear level.Results show that both epoxy and latex based adhesion promoters, currently used in construction, hardly improve the tensile strength of casting interfaces, to a point that the interface presence has negligible impact on the cracking pattern. A much better result was observed from the use of a web reinforcement crossing the interface perpendicularly. Although this solution revealed itself also incapable to avoid preferential cracking along the interfaces, it proved to be efficient in limiting shear slippages. The adoption of inclined interfaces either perpendicular or parallel to the expected direction of shear cracks proved also to be an efficient solution. Finally, the likelihood of experiencing a shear slippage along the interface is strongly dependent on the existing shear level after the formation of a plastic hinge.

Flexural behavior of reinforced concrete beams strengthened with externally bonded Aluminum Alloy plates

The objective of this experimental investigation is to study the viability and effectiveness of using Aluminum Alloy (AA) plates as externally bonded flexural reinforcement for reinforced concrete (RC) beams. Ten RC beams were prepared and nine of them were strengthened with externally bonded 2 mm and 3 mm thick AA plates with different mechanical properties. Four strengthened beams had no end wraps or anchorages. Single-layer and double-layer U-wrap CFRP sheets were used in the transverse direction as end anchorages for four strengthened beams and one beam had three double anchorages (two at the ends and one at mid-span). The beams were tested under monotonic load until failure. The goal is to study the effect of using AA plates as externally bonded flexural strengthening material and to explore the effect of end anchorages on the flexural strength and ductility of these beams. The increase in strength over the control unstrengthened specimen ranged from 13% to 40% while the ductility significantly surpassed that of beams strengthened with CFRP sheets. It is observed that the use of end anchorages enhanced the ductility but not the strength of the tested beams. It is also observed that beams without end anchorage failed predominantly in flexure with full de-bonding while beams with end anchorage failed by localized de-bonding and flexure. Furthermore, the performance of the tested beams was compared with numerical predictions by a computer program developed in this study. The results of the numerical models were in close agreement with the measured experimental data. It was concluded that AA plates could be used as an external strengthening material to enhance both the strength and ductility of RC beams in flexure.

Predicting shear strength of SFRC slender beams without stirrups using an ANN model

Shear failure of reinforced concrete (RC) beams is a major concern for structural engineers. It has been shown through various studies that the shear strength and ductility of RC beams can be improved by adding steel fibers to the concrete. An accurate model predicting the shear strength of steel fiber reinforced concrete (SFRC) beams will help SFRC to become widely used. An artificial neural network (ANN) model consisting of an input layer, a hidden layer of six neurons and an output layer was developed to predict the shear strength of SFRC slender beams without stirrups, where the input parameters are concrete compressive strength, tensile reinforcement ratio, shear span-to-depth ratio, effective depth, volume fraction of fibers, aspect ratio of fibers and fiber bond factor, and the output is an estimate of shear strength. It is shown that the model is superior to fourteen equations proposed by various researchers in predicting the shear strength of SFRC beams considered in this study and it is verified through a parametric study that the model has a good generalization capability.

Concurrent flexural strength and ductility design of RC beams via strain-gradient-dependent concrete stress-strain curve

The authors have previously conducted an experimental study that showed that strain gradient would improve the maximum concrete stress and strength of reinforced concrete (RC) members under flexure. As a continued study, the authors herein will extend the investigation of strain gradient effect on flexural strength and ductility of RC beams to higher strength concrete up to 100 MPa by theoretical analysis. In this study, the flexural strength of RC beams is evaluated using nonlinear strain-gradient-dependent stress–strain curves of concrete applicable to both normal-strength and high-strength concrete. On the basis of this, a parametric study is conducted to investigate the combined effects of strain gradient and concrete strength on the flexural strength and ductility of RC beams. It was evident from the results that both the flexural strength and ductility of RC beams would be improved with strain gradient considered. From the results, two formulas are proposed for the strain-gradient-dependent concrete stress block parameters α and β. A constant value of 0.0032 is proposed for the ultimate concrete strain in flexural strength design with strain gradient effect considered. Lastly, for practical engineering design purpose, design formulas and charts have been presented for flexural strength and ductility of RC beams incorporating strain gradient effect.

Experimental Study on Flexural Behavior of Damaged Concrete Beams Strengthened with NSM-CFRP Strips

In order to investigate the strengthen effect of different embedment lengths of the NSM strip on different damage levels. A series of tests were conducted on damaged reinforced concrete (RC) beams in flexure strengthened with near surface mounted (NSM) carbon- fiber-reinforced polymer (CFRP) strips, and initial cracking load, ultimate capacity, loading-deflection curves, and failure modes are examined and analyzed in the paper. The results showed that not only the initial cracking loads and ultimate capacities of the beams are significantly increased，but also the flexural stiffness of the beams in the yield and ultimate behavior stages are improved by using NSM-CFRP strips. The strengthen effect on lower damage level RC beams has no obvious difference with that on non-damaged RC beams. Anchoring of the strip end can increase the ultimate load capacities and decrease the ductility of RC beams. Debonding was found to be the primary failure mode in all cases.

Strengthening RC Beams in Flexure Using New Hybrid FRP Sheet/Ductile Anchor System

This paper explores a new hybrid fiber-reinforced polymer (FRP) sheet/ductile anchor system for rehabilitation of reinforced concrete (RC) beams. The advantages of the proposed strengthening method is that it overcomes the problem of low ductility that is associated with brittle failure mode in conventional methods of strengthening beams using epoxy-bonded FRP sheets. The proposed system leads to a ductile failure mode by triggering yielding to occur in a steel anchor system (steel links) rather than by rupture or debonding of FRP sheets, which is sudden in nature. Four half-scale RC T-beams were tested under four-point bending. Three retrofitted beams were strengthened using one layer of carbon FRP sheet. The results of the two beams that were strengthened with the new hybrid FRP sheet/ductile anchor system were compared with the results from the beam strengthened with conventional FRP bonding method and the control beam. The results show the effectiveness of the proposed strengthening system in increasing flexural capacity and ductility of RC beams.

Effects of Using High-strength Concrete on Flexural Ductility of Reinforced Concrete Beams

The complete moment-curvature behavior and flexural ductility of reinforced concrete beams made of normal- or high-strength concrete have been analyzed by a newly developed theoretical method that uses the actual stress-strain curves of the materials and takes into account the strain reversal of the tension reinforcement in the analysis. It was found that as expected the flexural ductility decreases with the tension steel ratio and increases with the compression steel ratio. However, the variation of the flexural ductility with the concrete grade is quite complicated; at fixed tension and compression steel ratios, the flexural ductility increases as the concrete grade increases but at a given degree of being under- or over-reinforced, the flexural ductility decreases as the concrete grade increases. In order to better reveal the combined effects of the steel ratios and the concrete grade, the flexural ductility has been plotted against the flexural strength for different steel ratios and concrete grades. ...

Shear Ductility of Reinforced Concrete Beams of Normal and High-Strength Concrete

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