Shear Strength Of Reinforced Concrete Beams Research Articles

Experimental Investigation of the Shear Strength of RC Beams Extracted from an Old Structure and Strengthened by Carbon FRP U-Strips

In recent years, old reinforced concrete (RC) buildings have increasingly been protected as historic symbols in Italy. The conversion of these buildings can resolve the increasing need for new structures, reducing much social, economic, and environmental impact. Retrofitting solutions by carbon fiber reinforced polymer (CFRP) reinforcements are very common nowadays. Code predictions present uncertainties due to the brittle behavior and the debonding of CFRP strips resulting from concrete cracking. Therefore, especially in the case of old beams, experimental validation is necessary. This paper deals the experimental evaluation of the shear strength of two beams extracted from an old RC building in Rome built in 1929 with modest-quality concrete. Preliminary tests were carried out to evaluate the beam elastic response and the material characteristics. These beams were tested until failure (three-point load shear/bending tests) after strengthening for shear by CFRP U-jacketing and for bending by new steel rebars. The results obtained, together with the extensive data taken from the literature, were compared with the predictions by design code equations. The influence of construction details on beam shear strength and the experimental maximum CFRP strain were also analyzed. Code predictions can be effective to estimate the shear strength of the retrofitted beams.

Torsional strengthening of RC beams using NSM CFRP rope and innovative adhesives

The strength of reinforced concrete (RC) members can be increased by bonding carbon fibre-reinforced polymers (CFRP) using externally-bonded reinforcement (EBR) and near-surface mounted (NSM) techniques. However, to date few studies in the area of torsional strengthening have been performed using NSM methods. Although the efficacy of increasing the flexural and shear strength of RC beams has been well reported using the NSM technique bonded with epoxy resins, the poor performance of epoxy in high-temperature environments and toxic fumes have been motivating factors for using new cement-based adhesives as a replacement adhesive for epoxy in several studies. This paper investigates the torsional behaviour of RC beams strengthened using NSM FRP laminates and FRP ropes. An epoxy adhesive was used in four beams and a new cement-based adhesive was utilised in the remaining four beams and used to evaluate the torsional capacity. Both CFRP laminates and CFRP ropes were embedded into pre-cut grooves. According to the results, the torsional behaviour of the beams strengthened with CFRP ropes was superior to that of the beams strengthened with CFRP laminate when using epoxy resin and cement-based adhesive. Generally, the epoxy was more effective in increasing the torsional strength than the cement-based adhesive.

Influence of polypropylene fibres on the shear strength of RC beams with web reinforcement

The use of various types of fibres within concrete matrix for enhancing structural performances of concrete or reinforced concrete (RC) members has been gaining interest in recent years. While a considerable amount of research has been conducted for using steel fibres, the studies on the other types of fibres are limited. For the purpose of extending our knowledge on polypropylene fibres in RC beams, an experimental programme was carried out for investigating the shear behaviour of polypropylene fibre-reinforced concrete (PFRC) beams. This paper presents the results of beams with web reinforcement divided into three groups according to the shear span-to-effective depth ratio and the amount of web reinforcement. Each group consists of a reference RC beam and three beams with fibre contents of 1.0, 2.0 and 3.0%. The simply supported beams were loaded at mid-span. Experimental results show that the addition of polypropylene fibres enhanced the shear behaviour of RC beams with web reinforcement. Furthermore, an expression for the contribution of web reinforcement was derived by considering the effectiveness of web reinforcement and it was introduced to the equation proposed previously for predicting the shear strength of PFRC beams without web reinforcement. The predictions agree with the experimental results.

Shear strength of reinforced concrete beams strengthened in shear using externally-bonded FRP composites

A shear strength estimation model for reinforced concrete (RC) beams, the so-called dual potential capacity model (DPCM), has previously been proposed by the authors. In this study, the DPCM is extended to estimate the shear strength of RC beams strengthened in shear by using externally-bonded fiber-reinforced polymer (FRP) composites. The proposed model has been derived so as to take into account the effects of the type of FRP composite, fiber-bonding configuration, and fiber layout, and it can also determine the critical shear failure modes of RC beams strengthened by externally-bonded FRP composites. To verify the accuracy of the proposed model, shear test results of 227 RC beams strengthened by externally-bonded FRP composites were collected from previous studies, and it is shown that the analysis results estimated by the DPCM agree well with those test results.

Shear strength of RC beams subjected to axial load

Due to the complex mechanism of the shear failure, most shear design equations in codes and guidelines are based on empirical approaches and consequently should only be used within the bounds of the test population from which they were developed. To accommodate this issue, a mechanics-based segmental approach, which can simulate a reinforced concrete (RC) beam with any type of concrete and reinforcement and which can explain the physical process of shear failure has been proposed to determine the shear capacity of RC beams with and without stirrups. In this paper, this segmental approach is extended by incorporating axial load in the mechanics-based model; furthermore, a closed form solution for assessment is also derived. By simulating 61 published test specimens, the proposed approaches are validated with good correlation between the predicted and measured strengths. Being mechanics based, this approach has a great potential to be developed for all types of RC structures with all combinations of loading for use in the development of or refinement of simplified design approaches.

Torsional strengthening of reinforced concrete beams using different configurations of NSM FRP with epoxy resins and cement-based adhesives

The strength of reinforced concrete (RC) members has been improved with a good deal of success using carbon fibre-reinforced polymers (CFRP) with two application techniques i.e. externally-bonded reinforcement (EBR) and the near-surface mounted (NSM) technique. However, to date no studies have been reported on torsional strengthening using NSM methods, as the research on torsional strengthening has been conducted using EBR techniques only. Thus far, NSM strengthening has been successfully employed with epoxy in improving the flexure and shear strength of RC beams. As toxic fumes and poor performance in high-temperature environments are major problems associated with using epoxy, some studies have used a new cement-based adhesive as an alternative to epoxy. This paper presents the results of experimental work on torsional strengthening using the NSM technique in a specialized torsional rig system. Eight RC members were tested to evaluate the strength enhancement provided by two different NSM FRP configurations. Six beams were strengthened with CFRP laminates embedded into pre-cut grooves. The laminates were applied to all four faces of two beams (full wrapping) and to only three faces of the other four beams (u-wrapping). An epoxy and a new cement-based adhesive were examined in this study. For each type of adhesive, one beam with full wrapping and two beams with u-wrapping were considered. Two control beams were initially tested and a comparison was made with the results of the strengthened beams in terms of the total performance and the ultimate strength of the beams. The results show that the u-wrapping configuration provides a reasonable improvement to the torsional behaviour of the beams when using epoxy resin as the bonding agent. On the other hand, cement-based adhesive is less efficient in increasing the torsional strength than epoxy. A theoretical design capacity method was adopted in this study to compare its prediction to the experimental data.

Experimental investigation into the use of NSM FRP to increase the torsional resistance of RC beams using epoxy resins and cement-based adhesives

Carbon fibre-reinforced polymers (CFRPs) have been used to increase the strength of reinforced concrete (RC) members using both externally bonded reinforcement (EBR) and near surface mounted (NSM) techniques with a great deal of success. However, currently no studies have been conducted on torsional strengthening using NSM methods as current research on torsional strengthening has been conducted using EBR techniques only. It is well documented that flexure and shear strength of RC beams has been improved using NSM strengthening technology applied with epoxy adhesives. However, due to epoxy adhesive problems associated with toxic fumes and poor performance in high-temperature environments, a new cement-based adhesive has been developed and used as an alternative to epoxy in several studies. This paper reports on experimental results of ten RC beams tested in torsion using a specialized torsional rig system. Two control beams were considered and eight beams strengthened with CFRP laminates applied to all four faces of the beam and embedded into pre-cut grooves using the NSM technique were prepared. An epoxy adhesive was used in four beams and a new cement-based adhesive was used as a substitute for epoxy in the remaining four beams. For each type of adhesive, two different spacings between the grooves were examined. The performances and the ultimate strengths of the beams are presented and the results expressed in terms of the maximum torsional capacity with respect to the peak angle of twist. According to the results, the torsional behaviour of the beams was improved in the strengthened beams to varying degrees when using epoxy resin as the bonding agent. On the other hand, the NSM technique with the cement-based adhesive was also an effective method in increasing the torsional strength.

Dual potential capacity model for reinforced concrete beams subjected to shear

AbstractThe shear resistance mechanisms of a reinforced concrete (RC) member with shear reinforcement can be divided into the contributions of the concrete and the shear reinforcement. The shear resistance mechanisms of concrete can be further divided into the shear resistance of the intact concrete in the compression zone, the aggregate interlock in the cracked tension zone and the dowel action of the longitudinal tension reinforcement. The shear demand curves and potential shear capacity curves for both tension and compression zones have been derived in this study, with the assumption that the shear failures of RC members are dominated by the flexural‐shear strength. The shear capacity model was also proposed on this basis. In the proposed model, the crack width and the local stress increase in reinforcement were calculated based on the bond behaviour between the reinforcement and its surrounding concrete. Further, the crack concentration factor was introduced to consider the formation and propagation of the critical shear crack that developed from the flexural cracks. The results of a total of 1,018 shear tests were collected and compared with the analysis results provided by the proposed model. It was demonstrated that the proposed model provides a good estimate of the shear strengths of RC beams.

Shear strengthening of reinforced concrete beams with Hybrid Composite Plates (HCP) technique: Experimental research and analytical model

This paper presents a study on the efficiency of Hybrid Composite Plates (HCPs) in enhancing the shear strength and stiffness of reinforced concrete (RC) beams. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates applied to the sides of reinforced concrete beams according to the Near Surface Mounted technique (NSM). Due to the excellent bond conditions between SHCC and CFRP laminates, these reinforcements provide the necessary tensile strength capacity to the HCP. To examine the efficiency of HCPs as a shear strengthening technique, a total of 17 RC beams are tested. Seven of these beams have a rectangular cross-section and ten have a T cross-section. The influence of the percentage and inclination of the CFRP laminates on the shear strengthening effectiveness of HCPs is investigated. Two different processes for applying the HCPs to the beams’ concrete substrate are examined: (1) using epoxy adhesive; and (2) using mechanical anchors in addition to the epoxy adhesive. It is demonstrated that when only epoxy adhesive is used, the shear strengthening contribution of the HCPs is limited by the tensile strength of the concrete substrate of the strengthened beams. When mechanical anchors are applied in addition to the epoxy adhesive, the shear strengthening potential of HCPs is fully mobilized. An analytical model is proposed to predict the shear strength of RC beams strengthened with HCPs. It is demonstrated that the analytical model predicts with good accuracy the shear strength of RC beams strengthened with HCPs.

Shear behavior of Reinforced Concrete (RC) beams with circular web openings without additional shear reinforcement

The introduction of an opening into a Reinforced Concrete (RC) beam leads to a reduction to both the beam's stiffness and its overall structural capacity due to stress concentrations and local cracking around the opening. This paper uses both the experimental and finite element (FE) method to analyze the shear behavior of RC beams with opening. The shear span-to-depth ratios (av/d), opening size, and opening location were considered as the main parameters. The beams were divided into three series according to the shear span-to-depth ratios. Three reinforced concrete beams without openings and one beam with openings were tested experimentally under two-point loading. These beams were considered as control beams so as to validate the FE model using ANSYS14.5 software. A total of 31 specimens were analyzed using ANSYS14.5 while considering the above mentioned parameters. Results showed that the openings located in a high shear region (the line connecting the load and support points) led to the early collapse of the beam. In addition, by comparing the experimental results with those of the non-linear FE analysis, it was concluded that both results were in accordance. An analytical equation for predicting the shear strength of RC beams with circular openings was then proposed, and the obtained results later compared with those from FE model. Furthermore, the developed FE models can serve as a numerical platform for performance predictions of RC beams with different web opening geometries.

Assessing reduction in concrete shear strength contribution

Design codes use a strength reduction factor that is less than unity. This factor reflects the degree of uncertainty associated with the probability of under-strength members and inaccuracies in the design equations, the importance of the member in the entire structure and the behaviour of a member at ultimate load. A failure probability based on a second moment probabilistic analysis procedure is used in this study to compute the strength reduction factor in predicting the contribution of concrete to shear strength of reinforced-concrete beams according to the American Concrete Institute code ACI 318. For different coefficients of variation of concrete and failure probabilities, the shear strength reduction factor is investigated using experimental studies available in the literature. It is assumed that the random variables are statistically independent, and the correlation effects are not taken into account. It is found that a strength reduction factor of 0·75 for shear is valid in design according to ACI 318 for a coefficient of variation of concrete compressive strength of 0·18 and a failure probability of 10−5.

Influence of Displacement Ductility on Concrete Contribution to Shear Strength

Shear strength of reinforced concrete (RC) members is composed of the contributions of the nominal shear strength provided by transverse reinforcement and concrete. The shear strength of RC members under cyclic lateral loading degrades much faster than the flexural strength. Based on this state, Seismic Codes tend to be excessively conservative and do not take into account the contribution of concrete in certain cases. The aim of this study is to investigate the influence of displacement ductility on concrete contribution to shear strength using finite element analyses (FEA). Based on the agreement between the FEA and experimental results selected from literature, a simple relation is proposed for the prediction of the concrete contribution to shear strength of RC beams. The relation proposed takes into account a reduction of the normalized concrete contribution for increasing inelastic displacement demands, with a small residual strength at large ductility levels.

Shear Strength of RC Beams with Steel Stirrups

AbstractBecause of the complexity of the shear failure mechanism, most current design codes and guidelines are based on highly empirical approaches and consequently should only be used within the bounds of testing regimes from which they were derived. Hence this limitation restricts their applicability to innovative materials such as high-strength concrete and fiber-reinforced polymer (FRP) reinforcement. To solve this issue, a mechanics-based segmental approach, which can analyze a reinforced concrete (RC) beam with any type of concrete and reinforcement and which can explain the physical process of shear failure, has been developed to quantify the shear capacity of RC beams without stirrups. In this paper, with the partial interaction analysis of transverse reinforcements directly linked with that of longitudinal reinforcements, the segmental approach is extended by incorporating steel stirrups in the mechanics-based model; furthermore, a simplified closed form solution for design is derived. By evaluat...

Crack damage mitigation and shear behavior of shear-dominant reinforced concrete beams repaired with strain-hardening cement-based composite

This research aims to study: (1) the crack damage mitigation and shear behavior of reinforced concrete (RC) beams that have been repaired using strain-hardening cement-based composite (SHCC) via experimental testing and (2) the contribution of the SHCC layers to the shear strength of the repaired RC beams via predictions. Five cantilever RC beams with a shear span-to-depth ratio of 2.8 were subjected to cyclic concentrated loading. The study variables include two types of tensile performance of the SHCC (with low or high strength in tension) and two repair methods (patching and layering). The experimental results show that the use of a SHCC layer leads to a substantial increase in the shear strength and ductility of the RC beams after the peak load. During the tests, all of the SHCC repaired beams showed delamination along the interface between the concrete and SHCC, and the shear resistance started to drop. However, the results also indicate that SHCC layers can be effective repair material for enhancing the control of cracking to help protect the concrete from the migration of aggressive agents in severe environments. In order to predict the shear strength of RC beams that have been repaired with SHCC, two methods were used in this study; one is based on Dinh's proposed model that considers the shear strength in both the compression and tension zones, and the other method considers the shear strength of the reinforcement, such as a stirrup or fiber-reinforced polymer (FRP) laminate that considers only the tensile strength across cracks. These two methods were able to predict the contribution of the SHCC layer to the shear strength of the RC beams, and the predicted shear strength values were very similar between the two methods.

초기 재령 철근콘크리트 보의 구조 실험을 통한 동바리 되세우기 안전성 평가

Significant concerns of reshores lie in their safety when shores are being replaced while the construction load of the upper floor is applied. Reinforced concrete (RC) beams at early ages were tested to investigate the concerns. Tests on cylinder specimens showed that the compressive strengths and elastic moduli of early-age concrete are reliably predicted using the equations in KCI code and, therefore, the nominal strength and stiffness can also be predicted for RC beams with ages other than 28 days. The RC beams of 2 or 3 days old showed a shear-compression failure and the RC beams of 7 or more days old showed a typical flexural behavior. Because the shear strength of RC beams is more affected by the compressive strength of concrete than their flexural strength is, the shear capacity needs to be checked in case of reshoring. Tests also found that beam-column joints have sufficient stiffness even at early ages and the joints can be safely assumed to be fully restrained regardless of the concrete age. From the test results, it was drawn that reshoring at 5 days of concrete pouring is safer than keeping the shores by 55%.

Effect of flexural CFRP sheets on shear resistance of reinforced concrete beams

The current method of strengthening reinforced concrete (RC) beams in shear is by externally bonding carbon fiber reinforced polymer (CFRP) sheets to the vertical sides of the beam with different orientations and spacing using epoxy adhesives to arrest diagonal cracks. However, in certain applications, the sides of the beam might not be accessible for shear strengthening. The aim of this paper is to investigate the effect of externally bonded CFRP sheets on the load-carrying capacity of shear-deficient RC beams when attached to the beam’s soffit. This will examine the contribution of flexural longitudinal reinforcement ratio in enhancing the shear strength of RC beams. Thirteen beams were cast without transverse reinforcement in the shear span and were divided into three groups with different longitudinal steel reinforcement ratios. The tested specimens failed in shear as a result of a diagonal-tension crack. The increase in the concrete shear capacity of strengthened specimens was in the range of 10–70% compared to the control specimens. This concludes that flexural longitudinal reinforcement ratio has a significant effect on the shear strength of RC beams. The shear strength of the tested specimens were also predicted using different codes guidelines and standards.

Strut and tie modeling for RC short beams with corroded stirrups

Corrosion of steel reinforcement is one of the major problems that shorten the service life of reinforced concrete (RC) structures. Steel stirrups, due to their location as an outer reinforcement, are more susceptible to corrosion problems and damage. However, there is limited research work in the literature on the effects of stirrup corrosion on the shear strength of RC beams. This paper attempts to evaluate analytically the residual shear strength of RC short beams with corrosion-damaged stirrups. The shear strength of short or deep beams are generally determined using the strut and tie model. The corrosion effects were implemented in the model to make it capable of predicting the residual shear capacity of RC beams with corroded stirrups. The effect of corrosion is implemented considering the reduction in geometry of the concrete cross section due to spalling and reduction in effective compressive strength of concrete due to corrosion cracks. The proposed strut and tie model which accounts for the corrosion effects was verified using the experimental data available in the literature, and good agreement was found.

Effect of longitudinal CFRP strengthening on the shear resistance of reinforced concrete beams

The flexural strengthening by external bonding of fiber reinforced polymer (FRP) to the tension face of reinforced concrete (RC) beams has proven to play similar role as that of the internal longitudinal reinforcement in increasing flexural strength and stiffness of the beams. Internal longitudinal reinforcement is also known to influence the shear strength of RC beams particularly the concrete shear strength component Vc. However, the effect of longitudinal FRP strengthening on Vc of RC beams has not yet been investigated. It is well established that Vc of flexural members can be reliably quantified from shear tests on beams without shear reinforcement. Therefore, RC beams without stirrups were constructed and strengthened with carbon FRP (CFRP) reinforcement to assess the effect of longitudinal strengthening on the shear strength of the beams. The study included five strengthened beams and two unstrengthened beams; all were tested under three-point bending. The test results indicated that the shear strength of the strengthened beams increased over that of the unstrengthened beams. The strength increase was up to 35%. The beneficial effect of the longitudinal strengthening was accounted for by considering the FRP reinforcement ratio in addition to the steel reinforcement ratio. Both reinforcement ratios were combined together in an equivalent reinforcement ratio accounting for the difference in the modulus of elasticity of the two materials. The equivalent reinforcement ratio was introduced in the conventional design methods for steel-reinforced beams to assess the shear strength of the strengthened beams. The experimental shear strength of the test beams were compared with the predictions of the modified design methods and good agreement was found.

Shear strength predicting of FRP-strengthened RC beams by using artificial neural networks

AbstractIn this study, the efficiency of artificial neural networks (ANN) in predicting the shear strength of reinforced concrete (RC) beams, strengthened by means of externally bonded fiber-reinforced polymers (FRP), is explored. Experimental data of 96 rectangular RC beams from an existing database in the literature were used to develop the ANN model. Eight different input parameters affecting the shear strength were selected for creating the ANN structure. Each parameter was arranged in an input vector and a corresponding output vector that includes the shear strength of the RC beam. For all outputs, the ANN model was trained and tested using a three-layered back-propagation method. The initial performance of back-propagation was evaluated and discussed. In addition, the accuracy of well-known building codes in predicting the shear strength of FRP-strengthened RC beams was also examined, in a comparable way, using same test data. The study shows that the ANN model gives reasonable predictions of the ultimate shear strength of RC beams (R2≈0.93). Moreover, the study concludes that the ANN model predicts the shear strength of FRP-strengthened RC beams better than existing building code approaches.

CONTRIBUTION OF CONCRETE TO SHEAR STRENGTH OF RC BEAMS FAILING IN SHEAR

Reinforced concrete (RC) beams with light transverse reinforcement are vulnerable to shear failure during seismic response. In order to prevent brittle shear failures at beam plastic hinge regions of earthquake-resistant structures, the Turkish Earthquake Code and ACI318 require the use of sufficient transverse reinforcement to resist the total expected shear demand. These codes tend to be excessively conservative and, in some cases, the contribution of the concrete to the shear strength is neglected. The aim of this study is to investigate the contribution of concrete to shear strength of RC beams failing in shear experimentally. The beams were tested under monotonically increasing reversed cyclic loading to determine the concrete contribution to shear strength. It is observed that the concrete contribution to the shear strength at ultimate state ranges from 18% to 69% of the ultimate strength.