Bond analysis of corroded reinforced concrete beams under monotonic and fatigue loads

This study investigated the fatigue bond behaviour of corroded steel reinforced concrete beams. Nine beams (152 × 254 × 2000 mm [6 × 10 × 78.74 in.]) were constructed and tested. Bond failure occurred in all the beams. The variables in this test series were: the type of load applied (monotonic or repeated loading), the repeated load range, whether the reinforcement inside the beam was corroded or not, and whether a carbon fibre reinforced polymer (CFRP) repair method was used or not. The fatigue life of the beams varied linearly with the range of applied load with a very shallow slope. Corroding the beams to a low corrosion level decreased the fatigue bond strength by about 30%. Corrosion caused the concrete in between the lugs of the reinforcing bars to be partially crushed due to the formation of the rust products from the corrosion process. This reduced the strength of the concrete keys and increased the rate of slip in the bar under repeated loading.     

Deformation behaviour of mild steel beams under alternating loads

A method has been developed to predict the cyclic behaviour of simple structures subjected to reversed bending under load control conditions. Cyclic moment-curvature relations are developed from pure bending tests under moment control and are applied to predict deflection range and maximum deflection amplitude for a cantilever beam. Disagreement between analytical and experimental results at low life and high load is attributed to a discontinuous yield phenomenon which manifests itself below a certain threshold load range. Cyclic creep is believed to occur depending upon the nature of residual stresses initiated during the first half cycle of reversed loading.     

Experimental study on the plastic rotation capacity of reinforced high strength concrete beams

This paper describes an experimental study on the plastic rotation capacity of reinforced high strength concrete beams. Thirty-six beams with various compressive strengths of concrete, tensile reinforcement ratios, compressive reinforcement ratios, and patterns of loading (1 point loading and 2-point loading) were tested to evaluate the plastic rotation capacity, extreme fiber concrete compressive strain and equivalent plastic hinge length, etc. The same quantities were also obtained from numerical analysis and compared with experimental data. According to the results, the yield curvatures obtained from experiments turned out to be quite close to those obtained from theoretical approach. However, the experimental results for ultimate curvatures were significantly larger than those of theoretical prediction based on the assumption of ε_cu=0.003. Based on these observations, a new formula for ultimate strain is proposed for high strength concrete beams. Also the test results for plastic rotation capacity were found to be closer to those obtained using moment-curvature relationship considering tension stiffening of concrete and shear effect than those obtained using equivalent plastic hinge length. This substantiates that for accurate evaluation of plastic rotation capacity the consideration of tension stiffening of concrete and shear effect is most important.     

Numerical simulation of reinforced concrete beams with different shear reinforcements under dynamic impact loads

The behavior of concrete/reinforced concrete structures is strongly influenced by the loading rate. Reinforced concrete structural members subjected to impact loads behave quite differently as compared to the same subjected to quasi-static loading. This difference is attributed to the strain-rate influence on strength, stiffness, and ductility as well as to the activation of inertia forces. These influences are clearly demonstrated in experiments. Moreover, for concrete structures, which exhibit damage and fracture phenomena, the failure mode and cracking pattern depend significantly on loading rate. In general, there is a tendency that with the increase of loading rate the failure mode changes from mode-I to mixed mode. Furthermore, theoretical and experimental investigations indicate that after the crack reaches critical speed of propagation there is crack branching. The present paper focuses on 3D finite-element study of reinforced concrete beams with different amount of shear reinforcement under impact. The experiments reported in literature are numerically simulated using the rate sensitive microplane model as constitutive law for concrete, while the strain-rate influence is captured by the activation energy theory. Inertia forces are implicitly accounted for through dynamic finite element analysis. However, the impact was modeled not by explicit modeling of two bodies but by incrementing the load point displacement till the maximum value and at the rate reported from the test. The results of the numerical study show that the numerical analysis using the procedure followed in this work can very well simulate the impact behavior of reinforced concrete beams. The static and dynamic reactions, crack patterns and failure modes as predicted in analysis are in close agreement with their experimentally observed counterparts. It was concluded that under impact loads, of the order as simulated in this work (blunt impact with velocity of around 1 m/s), the shear reinforcement does not get activated and therefore the dynamic reactions, unlike static reactions, are almost independent of the amount of shear reinforcement in the beams. However, the presence of shear reinforcement significantly affects the crack pattern and the cracks are well distributed in the presence of shear reinforcement, thus avoiding the formation of shear plugs.     

车载作用下多跨变截面损伤梁的振动特性

将车辆视为7自由度的整车模型,桥梁为多跨变截面的混凝土梁,裂缝分成开口裂缝和呼吸裂缝两种类型,建立了多跨变截面的损伤混凝土梁的车桥耦合方程;编制了相应的计算程序,结果表明了本方法的正确性.数值分析表明:相同条件下.呼吸裂缝所对应DFC的绝对值约为开口裂缝对应绝对值的1.55倍,故忽略呼吸裂缝随载荷张开与闭合的特性是不可取的;裂缝参数、裂缝位置等对耦合系统振动特性影响很大,如当裂缝参数a值从0.2变化到0.4时,对应的DFC绝对值增大1.95倍,且随着裂缝区域出现越多,各阶振动频率减小越快.     

Flexural behavior of reinforced concrete (RC) beams retrofitted with hybrid fiber reinforced polymers (FRPs) under sustaining loads

This paper reports experimental studies of reinforced concrete (RC) beams retrofitted with new hybrid fiber reinforced polymer (FRP) system consisting carbon FRP (CFRP) and glass FRP (GFRP). The objective of this study is to examine effect of hybrid FRPs on structural behavior of retrofitted RC beams and to investigate if different sequences of CFRP and GFRP sheets of the hybrid FRPs have influences on improvement of strengthening RC beams. Toward that goal, 14 RC beams are fabricated and retrofitted with hybrid FRPs having different combinations of CFRP and GFRP sheets. The beams are loaded with different magnitudes prior to retrofitting in order to investigate the effect of initial loading on the flexural behavior of the retrofitted beam. The main test variables are sequences of attaching hybrid FRP layers and magnitudes of preloads. Under loaded condition, beams are retrofitted with two or three layers of hybrid FRPs, then the load increases until the beams reach failure. Test results conclude that strengthening effects of hybrid FRPs on ductility and stiffness of RC beams depend on orders of FRP layers.     

Plastic rotation capacity of high-strength concrete beams

This work describes an experimental study on the plastic rotation capacity of high strength beams. Nine simply supported isotatic beams were tested, by applying comprising two symmetrical concentrated loads at approximately one-third and two-third's span. A method of analysis is defined that leads to the development of a parameter that characterizes the plastic rotation capacity at the failure section by means of a plastic analysis of the tested beams. The influence of concrete strength and the longitudinal tensile reinforcement ratio on the capacity for plastic rotation is examined and discussed. The results are discussed and compared with previous studies.

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Résumé Ce travail décrit une étude expérimentale sur la capacité de rotation plastique de poutres en béton à haute résistance. Neuf poutres isostatiques ont été testées avec une charge constituée par deux forces concentrées et symétriques situées environ au tiers et aux deux tiers de la portée. Une méthode d'analyse est définie, conduisant au développement d'un paramètre qui caractérise la capacité de rotation plastique de la section de rupture, utilisant une analyse plastique des poutres testées. L'influence de la résistance du béton et du taux d'armatures longitudinales de traction sur la capacité de rotation plastique est examinée et discutée. Les résultats sont analysés, discutés et comparés avec des études antérieures.

     

Behaviour of round steel fibre reinforced concrete panels under point loads

The objective of this investigation was to examine methods of testing round Steel Fibre Reinforced Concrete (SFRC) panels to obtain empirical estimates of post-crack load capacity under point loads. The performance of steel fibre reinforced concrete panels under point loads is of interest because such tests are commonly used to determine the suitability of a concrete mix for use in sprayed tunnel lining construction. This investigation has focussed on experimental obstacles to the reliable determination of post-crack load capacity. To examine possible differences in behaviour between continuousin situ tunnel linings and specimen panels representing portions of a lining, four modes of edge support were used to restrain round specimens that varied in both thickness and diameter. Determinate and simple support conditions were examined as possible modes of panel support for routine laboratory testing. Fully clamped and quasi-continuous edge restrained panels were used as approximations of anin situ lining. Performance was measured in terms of peak load capacity and energy absorption.     

Stability of sulphur concrete beams with steel reinforcement

Tests are reported on the structural behaviour of unmodified and modified sulphur concrete beams with steel reinforcement and subjected to air and water curing. The sulphur concrete contained fly ash as a filler and both dicyclopentadiene and dipentene were used as modifiers. The tests were conducted at three months and one year. Unmodified concrete beams showed enhanced strength, stiffness and ductility with ageing when cured in a dry environment. Modified concrete beams showed improved behaviour compared to unmodified beams, but even when dry, they showed strength loss with ageing and their long term stability is open to question. Ageing in a wet environment has a destructive effect on the structural behaviour of all sulphur concrete beams. Loss of strength and softening of sulphur concrete lead to shear failures, and severe corrosion and cracking occur. Stability of sulphur concrete beams can be guaranteed only when they are unmodified and kept dry. In a wet regime sulphur concrete cannot have stability and durability.

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Résumé On rend compte d’essais du comportement structural de poutres de béton de soufre avec armatures d’acier, les unes traitées les autres non, conservées dans l’air et dans l’eau. Des cendres volantes ont été utilisées comme charge et les dicyclopentadiène et dipentène comme agents modificateurs. Les essais ont été conduits sur 3 mois et 1 an. On a constaté une amélioration des propriétés de résistance, de rigidité et de ductilité pour des poutres de béton non traitées dans le vieillissement en atmosphère sèche. Le comportement des poutres de béton traitées a été supérieur à celui des poutres non traitées, mais, même en milieu sec, elles ont accusé une perte de résistance avec le vieillissement et on peut s’interroger sur leur stabilité à long terme. Le vieillissement en milieu humide a un effet destructif sur le comportement structural de toutes les poutres de béton de soufre. La perte de résistance et l’adoucissement du béton soufré conduisent à la rupture par cisaillement; d’autre part une corrosion et une fissuration importantes se produisent. On ne peut garantir la stabilité des poutres de béton de soufre que lorsqu’elles sont non traitées et conservées à sec. En régime humide, le béton de soufre ne peut être ni stable ni durable.

     

Experimental investigation of steel fiber reinforced concrete box beams under bending

Reduction of dead weight of a reinforced-concrete (RC) structure without too much concession in its load carrying capacity has always been an attractive study subject because it engenders (1) a decrease in dimensions of the members, (2) a decrease in the reinforcement steel, and (3) a decrease in lateral inertia forces during severe earthquakes. In this study, nine RC beams of outer dimensions of 300 × 300 × 2000 mm, six of which are box beams, designed and produced using a C20 class steel fiber concrete, (SFRC) with the commonly used steel fiber type of Dramix-RC-80/0.60-BN at a dosage of 30 kg/m³, are tested under bending. The mechanical behaviours of all these nine beams under bending are recorded from the beginning of the test till the ultimate failure of the tensile reinforcement in a two-point beam-loading setup. The proportions of (1) loss in ultimate load versus reduction in dead weight and (2) (ultimate experimental load)/(ultimate theoretical load) of the SFRC box beams are determined for two different box thicknesses. Dimensionless behaviour relationships of all the SFRC beams are determined, and the experimentally obtained relationship between the ratio of (actual ultimate load)/(theoretical ultimate load) and the ratio of (wall thickness)/(beam height) for the SFRC box beams is expressed diagrammatically.     

Non-local behavior of plastic softening beams

Summary This paper deals with the modelling of a plastic beam experiencing softening. This kind of behaviour is observed in steel or reinforced concrete structural members undergoing large rotation amplitudes, which may occur typically for civil engineering structures in the seismic area. The homogeneous cantilever beam loaded by a concentrated force at its extremity is considered. This simple structural problem with gradient bending moment allows an analytical treatment of the evolution problem. It is shown that a local plastic softening model makes the evolution problem ill-posed. Moreover, if we require the plastic curvature to be a continuous variable of the spatial coordinate, Wood’s paradox is encountered. A non-local gradient plastic model is developed in order to overcome this paradox. However classical gradient plastic models may not eliminate the ill-posedness since the beam response may not be continuous with respect to the loading parameter. The new gradient plastic model, presented in this paper, is similar to previous classical gradient models. The main difference is the yield moment considered as a non-local material parameter. This permits to ensure continuity between the elastic and the plastic regions during the loading process. These solutions are controlled by the ratio between the material length and the geometrical length of the beam. The new evolution problem may remain ill-posed as it possesses a finite number of solutions (which can be unique). Closed-form solutions of the unknown deflection are given.     

Experimental and analytical assessment of the behavior of stainless steel reinforced concrete beams

This paper reports the results of a comprehensive experimental test program on high performance and standard AISI 316L stainless, steel reinforced concrete beams. Experimental results were compared with theoretical analysis data using current reinforcement mechanical feature standards and experimental testing-based ones, to identify any resistance, ductility and membrane collapse mode difference. It is also proved the importance of establishing a specific standard on the use of stainless steel reinforcement, in order to enable its more widespread, structural use, and not only for its corrosion resistance.

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Résumé Dans ce travail sont présentés les résultats d'un vaste programme d'essais expérimentaux réalisés, sur des poutres en béton armé normal et à haute résistance; toutes les poutres sont renforcées de barres d'acier inoxydable. Les résultats obtenus à partir de l'analyse expérimentale ont étés comparés avec ceux obtenus par une analyse théorique dans laquelle on a utilisé les valeurs des caractéristiques mécaniques de l'acier, soit standardisées, soit obtenues à partir de l'expérimentation, afin de mettre en évidence pour les poutres, les différences en termes de résistance, de ductilité et de mode de rupture.

     

Effect of steel and synthetic fibers on flexural behavior of high-strength concrete beams reinforced with FRP bars

Six high-strength concrete beam specimens reinforced with fiber-reinforced polymer (FRP) bars were constructed and tested. Three of the beams were reinforced with carbon FRP (CFRP) bars and the other three beams were reinforced with glass FRP (GFRP) bars as flexural reinforcements. Steel fibers and polyolefin synthetic fibers were used as reinforcing discrete fibers. An investigation was performed on the influence of the addition of fibers on load-carrying capacity, cracking response, and ductility. In addition, the test results were compared with the predictions for the ultimate flexural moment. The addition of fibers increased the first-cracking load, ultimate flexural strength, and ductility, and also mitigated the large crack width of the FRP bar-reinforced concrete beams.     

Elasticity analysis of orthotropic beams under concentrated loads

A stress analysis is performed, within the framework of the classical theory of elasticity, on orthotropic beams subjected to concentrated loads. Both three-point and four-point bending are considered. Concentrated loads are represented by a uniform stress distributed over a very small length of the beam. Stresses developed in interlaminar beam tests are examined in detail. Considerable departure from classical beam theory over large portions of such beam specimens is observed.     

Collapse loads of cantilever beams under end shear

Summary Results ofOnat andShield and ofGreen on the collapse loads of symmetrically tapered cantilever beams under a uniformly distributed end shear are extended to cover the entire range of geometric parameters. Close bounds are obtained except for very short beams. In addition, the effect of a parabolic distribution of end shear upon the lower bound is investigated and found to be small. For large beams complete solutions are exhibited. Finally, an optimum design problem is considered: for fixed beam length and end load find the angle of taper which minimizes the weight. The minimum is always achieved for a taper angle (top and bottom) between zero and fifteen degrees.

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Traglast eines einseitig eingespannten, durch Querkraftverteilung, belasteten Balkens

Zusammenfassung Ergebnisse vonOnat undShield, sowie vonGreen, über die Traglast von symmetrisch sich verjüngenden, einseitig eingespannten Balken, die durch eine an ihrem Ende angreifende uniform verteilte Querkraft belastet sind, werden erweitert um den gesamten Bereich der geometrischen Parameter zu erfassen. Mit Ausnahme sehr kurzer Balken werden eng zusammenliegende Schranken erhalten. Zusätzlich wird der Einfluß einer parabolischen Verteilung der Querkraft untersucht und als klein befunden. Für den Fall sehr langer Balken wird eine vollständige Lösung gegeben. Abschließend wird ein optimales Entwurfsproblem betrachtet: Für eine vorgegebene Balkenlänge und Belastung finde man den Verjüngungswinkel, der das Gewicht zu einem Minimum macht. Dieses Minimum wird immer für einen Verjüngungswinkel (zwischen Ober-und Unterseite) zwischen O und 15° angenommen.

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With 3 Figures

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This investigation was supported by the National Research Council of Canada     

Flexural behavior of reinforced concrete beams with cementitious repair materials

There is an urgent need to develop efficient methods for the repairs and rehabilitation of currently existing structures, they are being deteriorated over time, and also the magnitude of loadings keeps rapidly increasing for such structures. Possibly one of the most challenging taks in the rehabilitation processes is to upgrade the overall capacity of deteriorated concrete structures. Recently, considerable efforts are being directed toward developing new construction materials. This paper presents the experimental study for the flexural behavior of reinforced concrete beams repaired by Polymer Cement Mortar (PCM) and Ordinary Portland Cement Mortar (OPCM) in the tension region. Tests were performed for eight reinforced concrete beams with varying reinforcement ratios, repair materials and repair lengths. Emphasis is given to overall bending capacity, deflection, ductility index, failure mode and crack development of repaired beams. The results are compared with those from the control beam.

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Résumé Il est urgent de développer une méthode efficace pour le renforcement et la réparation afin de remettre en état la structure dont le fonctionnement est dégradé en raison de l'accroissement de la charge ou de l'écoulement du temps. Au cours du processus de renforcement, un des paramètres les plus importants est d'améliorer la performance globale des structures en béton endommagées. La tendance d'aujourd'hui est de développer un nouveau matériau. Dans cet article sont menées des études expérimentales sur la flexion de poutres en béton armé réparées ou renforcées avec du mortier de ciment polymère (MCP) et du mortier de ciment portland ordinaire (MCPO) repectivement. Pour des essais, 8 échantillons d'essais ont été fabriqués en variant le rapport d'armature, le matériau de réparation, ainsi que la longueur à réparer, avant de faire les essais. Les résultats confirment: la capacité de la flexion, une flèche, un indice de la ductilité, une rupture apparente, la forme de la fissure des poutres réparées. Et ces résultats ont été comparés avec celui de l'échantillon de référence.

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Shear behaviour of steel fibre reinforced concrete beams

The possibility of substituting traditional transverse reinforcement (stirrups) for steel fibres in precast elements can significantly reduce production costs. In the present paper, the shear behaviour of prestressed elements has been investigated by means of experimental tests on full scale beams. Tests concern beams with conventional as well as steel fibre reinforcement. Experimental results show that the shear behaviour of fibre reinforced concrete beams without conventional reinforcement is similar to, or even better than that of beams with stirrups. When used in beams with stirrups, steel fibres significantly improve their shear strength. A discussion on the contribution of steel fibres on the shear strength is also presented, with reference to the latest RILEM provisions.

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Résumé La possibilité de remplacer des armatures transversales traditionnelles par des fibres d'acier dans des éléments pré-tendus peut apporter des améliorations considérables sur les performances structurales. Dans ce rapport, le comportement en cisaillement des éléments précontraints a été étudié à l'aide de tests expérimentaux sur des poutres à échelle grandeur nature. Les essais portent aussi bien sur des poutres avec armatures traditionnelles que sur des poutres renforcées à l'aide de fibres. Les résultats expérimentaux montrent que les performances des poutres en béton de fibres sans armatures traditionnelles sont similaires, sinon meilleures, à celles des poutres avec armature ordinaire de cisaillement. Lorsqu'elles sont utilisées dans des poutres avec armatures traditionnelles les fibres d'acier améliorent considérablement leur résistance en cisaillement.

     

Minimum steel ratios in reinforced concrete beams made of concrete with different strengths—Theoretical approach

Minimum reinforcement is provided in concrete beams in order to improve their behaviour towards cracking and ductility at failure. Generally, codes of practice equations for the minimum steel ratios, longitudinal and transversal, are mainly empirical and do not include all the influential parameters in them. For this reason and due to the fact that they do lack of a theoretical background, different codes can give values for the minimum steel ratios that greatly differs from one another. Also the validity of these equations may be questioned particularly in the case of high strength concrete beams and prestressed concrete beams for which limited test data are available. In this work, a theoretical approach for the minimum steel ratios that are required for the ductile behaviour at failure in bending, shear and torsion, in concrete beams made of concrete with different strengths is presented. Comparisons are also made between the proposed expressions, the codes expressions and available test results.

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Résumé Un renforcement minimum des poutres en béton armé est prévu pour améliorer leur comportement en cas de fissuration et leur ductilité dans la rupture. En général les formules des normes techniques pour les pourcentages minimum des armatures longitudinales et transversales sont simplement empiriques et ne contiennent pas tous les paramètres influents. Pour cette raison et comme elles ne prennent pas en considération des fondements théoriques, les différents règlements donnent des valeurs très différentes pour les pourcentages minimum d'acier. La validité de ces formules peut être mise en doute particulièrement dans les cas de poutres en béton à haute résistance et des poutres précontraintes pour lesquelles seuls des résultats d'essais très limités sont disponibles. Dans cette communication une approche théorique pour les pourcentages minimum d'acier requis pour le comportement ductile dans la rupture par flexion, effort tranchant et torsion, des poutres en béton armé exécutées avec des bétons de différentes résistances est presentée. Des comparaisons entre les expressions proposées et celles des règlements sont présentées.

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