Failure Of Reinforced Concrete Beams Research Articles

Effect of stirrups on shear performance of geometrically-similar reinforced concrete deep beams: An experimental study

In practical engineering, reinforced concrete (RC) beams with stirrups are the predominant form of RC beams. The geometric dimensions of RC beams are large, and the brittle shear failure of them maybe show the characteristics of size effect. However, current research primarily focuses on the size effect of shear failure in RC beams without stirrups, with less emphasis placed on those with stirrups. Furthermore, when the minimum stirrup ratio is exceeded, there is no consensus regarding on whether the size effect still exists. Therefore, the objective of the present work is to discover the influence of the stirrup ratio on the size effect in RC deep beams. In this work, the shear failure experiment of RC deep beams considering the structural size (maximum beam height of 900 mm) and stirrup ratio (0%∼0.942%) is designed and completed. The experiment results indicate that: 1) for the beams without stirrups, clearly, the corresponding size effect on the shear strength is the most obvious, with beam depth increases from 300 mm to 900 mm, the nominal shear strength decreases approximately by 41%; 2) as the stirrup ratio increases from 0 to 0.942%, the size effect is weakened with the presence of stirrups; 3) however, size effect cannot be completely eliminated even when the stirrup ratio reaches as high as 0.942%; (4) generally, for the stirrup ratio of 0.3%, there is a decrease in nominal shear strength by 20%, indicating that consideration of size effects should be taken seriously in practical concrete structure design; (5) moreover, Jin et al.’s size effect law is also compared and validated by the present test data.

Fatigue life prediction for the reinforced concrete (RC) beams under the actions of chloride attack and fatigue

Pitting corrosion of tensile steel bars, usually causes the failure of RC beams under the actions of chloride attack and fatigue. Based on the Fick’s 2nd law and Paris law, a fatigue life prediction model for RC beams under these combined actions is proposed. Pitting corrosion is considered as a surface crack. Fatigue life is predicted by parameters of the service environment, service time and fatigue loading, which are available data for in-service RC structures. The model can produce reliable fatigue life predictions according to the validation results, and it is proved that the crack initiation significantly affects the fatigue life due to its large proportion. In addition, parametric studies indicate that the fatigue life obviously decreases as the stress ratio decreases from 0.4 to 0.1, and when the service time increases, the fatigue life rapidly decreases at first and then gradually decreases after 10 years of service. The fatigue life markedly reduces as the environmental temperature (greater than0 degree) increases, but the relative humidity rarely affects it.

Effect of FRP U-jackets on the behaviour of RC beams strengthened in flexure with NSM CFRP strips

The near-surface mounted (NSM) fibre-reinforced polymer (FRP) strengthening technique has become a viable strengthening method for reinforced concrete (RC) beams. Premature debonding failure of the NSM FRP strips is a common failure mode though that severely limits the effectiveness of the FRP intervention that results in reduced beam strength and deformability. The application of FRP U-jackets near the ends of the NSM strips offers a potential solution to the debonding problem. This paper presents the first systematic experimental investigation on the use of FRP U-jackets for delaying or preventing debonding failure in RC beams strengthened in flexure with tension face NSM FRP strips. A total of 11 large-scale RC beams were tested, with the investigated parameters being (i) arrangement of NSM carbon FRP (CFRP) strips, (ii) U-jacket material type, (iii) layout, width and inclination angle of FRP U-jackets. Failure modes, load-deflection and strain distribution responses of all tested beams were recorded. The test results bring clarity and understanding to the tested parameters and they also demonstrate the effectiveness of the U-jacket intervention.

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

Effect of Pit Distance on Failure Probability of a Corroded RC Beam

The present paper studies effect of the variation of pit distance on structural reliability of a reinforced concrete (RC) beam, with particular emphasis on the interference of localized corrosion on adjacent tensile rebars. The research question leading the inquiry of this article is how does average distance between corrosion pits in rebars affect the probability of failure in RC beams. In this paper, by using Monte Carlo Simulation (MCS), probabilities of failure in a corroded RC beam with different pit distances are quantified. Uncertainties in material properties, geometry, loads, corrosion modelling, pit distances and pit interference are taken into account. Statistical data reported in literature regarding the extent and location of corrosion is utilized to undertake a parametric study of corresponding probability distribution functions. According to results, variation of pit distance has significant influence on probabilities of failure. This influence increases if the effect of interference of localized corrosion is taking into account.

IC debonding failure in RC beams strengthened with FRP: Strain-based versus stress increment-based models

Since the advent of the use of fibre-reinforced polymers (FRP) to strengthen reinforced concrete (RC) and its employment in civil engineering in the early nineteen nineties, much of the research has concerned the enhancement of bending capacity. Failure modes are well understood and can be predicted with some accuracy. The models adopted by the most highly reputed international standards for the specific case of inter-crack (IC) debonding are based on limiting the strain on the laminate. These methods, an offshoot of fracture mechanics analysis, are calibrated with the results of point load beam tests. This paper proposes a new interfacial fracture energy model by characterising the FRP-concrete interface with beam-type tests. The existing fracture energy models, as well as the proposed model, were applied to an existing stress increment-based formulation, in which no calibration was conducted with experimental results and yet proved to predict inter-crack debonding failure moderately better than the models in use. The performance of each IC debonding model is assessed through an exhaustive statistical analysis with experimental data of point load RC beams tests gathered from literature. The behaviour of the models in uniformly loaded RC beams is discussed and the implications of each approach are drawn.

Finite element analysis of end cover separation in RC beams strengthened in flexure with FRP

The use of externally-bonded (EB) or near-surface mounted (NSM) FRP reinforcement in the strengthening of reinforced concrete (RC) beams in flexure has become increasingly popular in recent years. Such beams are likely to fail by end cover separation in which a major crack in the concrete initiates at a cut-off point of the FRP reinforcement and propagates along the level of steel tension bars, leading to the detachment of the FRP reinforcement together with the cover concrete. Due to the complexity of this failure mode, no reliable finite element (FE) approach for its accurate prediction has been published despite many previous experimental and theoretical studies on the problem. This paper presents a novel FE approach for predicting end cover separation failures in RC beams strengthened in flexure with either externally bonded or near-surface mounted FRP reinforcement. In the proposed FE approach, careful consideration is given to the constitutive modelling of concrete and interfaces. Furthermore, the critical debonding plane at the level of steel tension bars is given special attention: the radial stresses exerted by the steel tension bars onto the surrounding concrete are identified to be an important factor for the first time ever and are properly included in the FE approach. The proposed FE approach is shown to provide accurate predictions of test results, including load–deflection curves, failure loads and crack patterns.

Mechanisms of Diagonal-Shear Failure in Reinforced Concrete Beams analyzed by AE-SiGMA

Serious shear failures in reinforced concrete (RC) structures were reported in the Hanshin-Awaji Earthquake. In particular, it was demonstrated that a diagonal-shear failure could lead to disastrous damage. However, mechanisms of the diagonal-shear failure in RC beams have not been completely clarified yet. In this study, the diagonal-shear failure in RC beams is investigated, applying acoustic emission (AE) method. To identify source mechanisms of AE signals, SiGMA (Simplified Green's functions for Moment tensor Analysis) procedure was applied. Prior to four-point bending tests of RC beams, theoretical waveforms were calculated to determine the optimal arrangement of AE sensors. Then, cracking mechanisms in experiments were investigated by applying the SiGMA procedure to AE waveforms. From results of the SiGMA analysis, dominant motions of micro-cracks are found to be of shear crack in all the loading stages. As the load increased, the number of tensile cracks increased and eventually the diagonal-shear failure occurred in the shear span. Prior to final failure, AE cluster of micro-cracks was intensely observed in the shear span. To classify AE sources into tensile and shear cracks, AE parameter analysis was also applied. As a result, most of AE hits are classified into tensile cracks. The difference between results obtained by the AE parameter analysis and by the SiGMA analysis is investigated and discussed.

OS3-5-5 Fracture Mechanisms of Diagonal-Shear Failure in Reinforced Concrete Beams analyzed by AE-SiGMA

Diagonal shear failure could lead to the serious damage in concrete structures due to earthquakes. However, mechanisms of diagonal shear failure in reinforced concrete (RC) beams have not been completely clarified yet. Generally, the diagonal shear failure occurs, depending on the ratio of the shear span to the beam depth. In the case that the ratio is smaller than or equal to 2.7, the shear failure occurs as the diagonal tensile failure. In this study, diagonal shear failure in RC beams is investigated, applying acoustic emission (AE) method. AE method is one of nondestructive testings for concrete structures with a high capacity for diagnostics and health monitoring. Presently, it is known that fracture mechanisms are identified by AE waveform analysis. The moment tensor analysis of recorded AE waveforms can identify cracking kinematics of location, crack-type and crack orientation. The analysis is implemented as SiGMA (simplified Green's functions for moment tensor analysis) procedure. Prior to testing RC beams, theoretical waveforms were calculated in order to determine location of AE sensors. Theoretical waveforms were synthesized by introducing the dislocation model and Green's functions in a half space. Then, cracking mechanisms in the experiments are investigated by applying the SiGMA procedure to detected AE waveforms. Since classification of crack mechanisms can be analyzed by AE parameter, the results of AE parameter analysis and those of SiGMA analysis are compared and discussed.

Using radial basis function neural networks to model torsional strength of reinforced concrete beams

The application of radial basis function neural networks (RBFN) to predict the ultimate torsional strength of reinforced concrete (RC) beams is explored in this study. A database on torsional failure of RC beams with rectangular section subjected to pure torsion was retrieved from past experiments in the literature; several RBFN models are sequentially built, trained and tested. Then the ultimate torsional strength of each beam is determined from the developed RBFN models. In addition, the predictions of the RBFN models are also compared with those obtained using the ACI 318 Code equations. The study shows that the RBFN models give reasonable predictions of the ultimate torsional strength of RC beams. Moreover, the results also show that the RBFN models provide better accuracy than the existing ACI 318 equations for torsion, both in terms of root-mean-square error and coefficients of determination.

Debonding Failures of RC Beams Strengthened with Near Surface Mounted CFRP Strips

This paper presents the results of a series of tests conducted on reinforced concrete (RC) beams strengthened in flexure with near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. As the main focus of the research is on debonding failure mechanisms, the only test variable investigated was the embedment length of the NSM strip and the NSM strip was extensively strain-gauged to monitor its bond behavior. Load-deflection curves, failure modes, strain distributions in the CFRP strip, and local bond stresses at the CFRP–epoxy interface from the tests are all examined in detail and compared with the predictions of a simple analytical model where appropriate. Of the four embedment lengths investigated, all but the shortest one led to a notable increase in the load-carrying capacity and, to a lesser extent, in the postcracking stiffness of the beam. Debonding was found to be the primary failure mode in all cases except for the beam with the longest embedment length. Also reported in this paper are results from preliminary bond tests used to characterize the local bond-slip behavior of the NSM system. Apart from gaining a better understanding of debonding failures in RC beams with NSM FRP strips, the test results reported in the paper should be useful for future verification of numerical and analytical models.