An ultimate shear strength theory for reinforced concrete T-beams without web reinforcement

A theory is presented to predict the ultimate shear strength of T-beams of long shear span without web reinforcement. Failure is assumed to occur in the compression zone above the diagonal crack due to the combined shearing and compressive stresses, and is based on Mohr's failure theory. Both equilibrium and compatibility conditions are considered, as well as the shear force due to dowel action of the longitudinal reinforcement. The theory is compared with the test results of the authors and other investigators, and is shown to predict the test results well. The theory is also compared with the ACI-ASCE and the Unified Code methods, and is shown to predict the results better.

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Résumé La rupture en cisaillement des poutres en T en béton armé s'accomplit en deux étapes. La première de ces étapes est caractérisée par une fissure oblique qui survient dans l'ame, et la seconde étape par la ruine définitive; lorsque la ruine se produit, à la force de cisaillement qui s'exerce sur la poutre s'oppose la zone de compression, la structure formée par l'agrégat et l'effet de goujon de l'armature longitudinale. On présente ici une théorie susceptible de prédire la résistance ultime au cisaillement des poutres en T à grand intervalle de cisaillement sans renforcement de l'ame. Il est supposé que la ruine se produit dans la zone de compression au-dessus de la fissure diagonale engendrée par l'action combinée des contraintes de cisaillement et de compression, et elle s'appuie sur la théorie de la rupture de Mohr dans les conditions de sollicitations biaxiales. On examine à la fois les conditions d'équilibre et de compatibilité et l'on suppose que la distribution des contraintes de cisaillement et de compression dans la zone de compression critique au moment de la ruine par cisaillement est uniforme et rectangulaire. On suppose aussi que l'effet de goujon prend 10% de l'effort de cisaillement exercé. Comparée aux résultats d'essai obtenus par les auteurs et par d'autres chercheurs pour un certain nombre de paramètres, la théorie se révèle satisfaisante. La comparaison avec les codes britanniques et américains est à son avantage pour la précision des résultats d'essai.

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Design criteria for early-age bond strength in reinforced concrete

The main objective of the extensive research programme involving 238 test specimens was to provide bond strength criteria between type 2 deformed bars and concrete in models for predicting early-age rracking. The major variables included bar diameter, concrete cover, age of concrete, transverse reinforcement and anchorage length. Both the pull-out load and slip displacement were determined using specially developed tensile bond tests where both steel and concrete were in tension. The bond strength of deformed bars showed a significant age effect and design equations suitable for toth ultimate and serviceability limit states at early age have been developed. BS 8110 and BS 8007 Code values underestimated the ultimate bond strength for the concrete strengths considered, while the same values from BS 8110 and BS 8007 overestimated or compared approximately with, respectively, the average bond stress test results at 0.1 mm slip. AC1-318 Building Code, on the other hand, overestimated the bond strength for concrete cube strengths up to 15 MPa and then became conservative.

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Resume On a réalisé un vaste programme de recherche sur 238 éprouvettes, dont l'objectif principal était d'obtenir des critères d'adhérence entre des barres déformées de type 2 et le béton dans des modèles permettant de prévoir la fissuration aux jeunes ages. Les principales variables étaient: le diamètre de la barre, l'enrobage du béton, son age, le renforcement transversal et la longueur de l'ancrage. On a déterminé à la fois la charge d'arrachement et la longueur de glissement par des essais d'adhérence où l'acier comme le béton étaient soumis à la traction. On a constaté l'importance de l'age des barres, et on a établi des équations variables à la fois pour les états limites ultimes et en service aux jeunes ages. Les valeurs de calcul données par BS 8110 et BS 8007 sous-estiment l'adhérence maximale pour les résistances du béton considérées, tandis que les mêmes valeurs extraites des mêmes documents surestimaient ou rejoignaient à peu près la moyenne des résultats d'essai de la contrainte d'adhérence pour un glissement de 0,1 mm. D'autre part, ACI-318 surestime l'adhérence pour des résistances sur cubes du béton allant jusqu'à 15 MPa, et, au-delà, devient normal.

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Evaluation of size dependent design shear strength of reinforced concrete beams without web reinforcement

Analytical studies on the effect of depth of beam and several parameters on the shear strength of reinforced concrete beams are reported. A large data base available has been segregated and a nonlinear regression analysis (NLRA) has been performed for developing the refined design models for both, the cracking and the ultimate shear strengths of reinforced concrete (RC) beams without web reinforcement. The shear strength of RC beams is size dependent, which needs to be evaluated and incorporated in the appropriate size effect models. The proposed models are functions of compressive strength of concrete, percentage of flexural reinforcement and depth of beam. The structural brittleness of large size beams seems to be severe compared with highly ductile small size beams at a given quantity of flexural reinforcement. The proposed models have been validated with the existing popular models as well as with the design code provisions.     

Shear strength of reinforced aerated concrete beams with shear reinforcement

The paper reports on the analysis of shear strength of reinforced beams made of autoclaved aerated concrete with shear reinforcement. The test data are taken from three different investigations from three countries, in Europe and Japan, and include 61 tests. The analysis of the test data results in regression expressions, suitably modified from a formula used for ordinary concrete members, and shows good agreement with test values. Appropriate expressions are suggested for design.

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Résumé Ce rapport présente l'analyse de la résistance au cisaillement de poutres en béton cellulaire armées (AAC), avec armatures d'effort tranchant. Les résultats d'essais sont pris parmi trois recherches différentes dans trois pays, en Europe et au Japon, et ils regroupent 61 essais. La charge dans la majorité des essais consistait en deux forces symétriques (voir Fig. 1). Dans quelques cas, il y avait une charge en ligne à mi-portée; dans d'autres cas, une charge en ligne était appliquée près d'un appui. Aucun essai n'a été exécuté avec une charge uniformément répartie. Une formule du type de l'équation 1 prenant en compte, les trois variables principales a été utilisée depuis longtemps dans le cas du béton armé sans armature d'effort tranchant. A partir de son développement théorique, cette formule a été adaptée par le passé au béton cellulaire d'ou l'équation 2. Pour les poutres avec armatures d'effort tranchant, l'équation 3 représente l'addition de la résistance au cisaillement due au béton à celle due aux armatures d'effort tranchant. Les résultats d'essais sont présentés dans le Tableau 2, y compris les résistances au cisaillement prédites à partir de l'équation 3 avec les coefficients de régression calculés en utilisant le programme SYSTAT sur un ordinateur personnel. Les résultats des régressions sont donnés dans le Tableau 4 et l'expression finale dans l'équation 4. Dans l'optique du dimensionnement, des valeurs minimales de la résistance ultime au cisaillement avec un seuil de confiance de 90% sont présentées dans l'équation 5. L'analyse rapportée dans cet article fournit des expressions fiables pour la prédiction de la résistance au cisaillement de poutres en béton cellulaire armées avec armatures d'effort tranchant.

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A finite macro-element for corroded reinforced concrete

This paper proposes a model of the mechanical behaviour of corroded reinforced concrete members subjected to bending under service load. The model is based on the formulation of a macro-element to be used in FEM analysis, having a length equal to the distance between two consecutive flexural cracks and a cross-section equal to the member cross-section. The mechanical formulation is directly written in generalized variables (bending moment and curvature) and is based on the concept of the transfer length necessary for the transmission of tensile load from re-bar to tensile concrete thanks to the bond. It is thus possible to take into account the effect of reinforcement corrosion on the bond between re-bar and concrete, by increasing the transfer length versus intensity of corrosion. The variation of the transfer length versus corrosion is expressed using a scalar damage parameter. A first experimental validation is performed on a 17-year-old beam kept in a chloride environment under its service load.     

A new method for modelling reinforcement and bond in finite element analyses of reinforced concrete

A new method of representing the steei in finite element analyses of reinforced concrete structures is described in which the steel and the concrete are analysed separately. The forces between the steel and concrete are used In an interative method which brings the two solutions together. The method converges very rapidly and it is shown that the computational effort is principally dependent on the mesh chosen for the concrete. All the steel in a reinforced concrete structure may be easily included without affecting the layout of the concrete mesh. Results, which are given for three practical problems, include detailed steel and bond stresses for all bars.     

Prediction of the ultimate strength of reinforced concrete beams FRP-strengthened in shear using neural networks

In the last years, a great number of experimental tests have been performed to determine the ultimate strength of reinforced concrete beams retrofitted in shear by means of externally bonded fibre-reinforced polymers (FRP). Most of design proposals for shear strengthening are based on a regression analysis from experimental data corresponding to specific configurations which makes very difficult to capture the real interrelation among the involved parameters. To avoid this, an artificial neural network has been developed to predict the shear strength of concrete beams reinforced with this method from previous tests. Furthermore, a parametric study has been carried out to determine the influence of some beam and external reinforcement parameters on the shear strength with the purpose of reaching more reliable designs. Finally, some modifications of the design expressions are proposed and checked with experimental results.     

Experimental and analytical study of corroded shear-critical reinforced concrete beams

This paper presents the experimental results of two highly corroded shear-critical deep beams subjected to a chloride environment to assess the shear behavior of long-term-corrosion damaged beams. A 26-year-old reinforced concrete beam corroded by exposure to a chloride environment was cut into two small short-shear-span beams, or deep beams, which were tested under three point bending until failure, along with a control beam of the same age, same length and same cross-section (115 × 28 × 15 cm). Cracking and corrosion maps were drawn for the corroded beams in order to assess the corrosion. Force displacement curves were drawn for the corroded and control beams. After testing the beams until failure, the main steel bars and the stirrups were extracted from the beams and loss of mass was measured and plotted for both longitudinal and transverse reinforcement. It was noted that the loss of mass of transverse and longitudinal reinforcements had not had an important influence on the failure mode and it had not affected the load-bearing capacity of the beam. It was found that the stirrups came into action after the failure of the compression strut and thus decreased the ductility of the corroded beams. Analytical investigations were made to compare the experimental shear capacity with the theoretical value. Strut and tie model predictions were more accurate than the conventional sectional methods provided for in ACI 318-08 and Eurocode 2.     

Effect of transverse reinforcement on bond strength of reinforcing bars in silica fume concrete

The objectives of the research program were to investigate the effect of transverse reinforcement on the bond-slip characteristics of tension lap splices in high performance silica fume concrete, to study the validity of the upper limit of 70 MPa imposed by the ACI Building Code 318-95 on the concrete compressive strength for determination of development length, and to evaluate the reliability of the empirical equation of Orangun, Jirsa, and Breen in estimating the bond strength of tension lap splices embedded in high strength concrete and confined with transverse reinforcement. Twelve beam specimens were tested. Each beam specimen included two bars in tension, spliced at the center of the span. The beams were designed in way that bars would fail in bond, splitting the concrete conver in the splice region, before reaching the yield splice in a constant moment region. The variables used were the percentage replacement by weight of cement by silica fume and the amount of confinement over the splice region.     

Role of casting position on bond strength of reinforcement in eccentric pullout silica fume concrete specimens

This paper reports on a study designed to investigate the effect of silica fume, bar size, and concrete confinement on the bond-slip characteristics of bottom cast reinforcing bars anchored in eccentric pullout silica fume specimens. The main objective was to assess the effect of casting position by comparing the test results with those of top cast specimens tested earlier and identical except for the casting position. Forty-eight eccentric pullout specimens were tested. The variables were the percentage replacement by mass of cement by silica, the concrete cover over the reinforcing bar, and bar size. Failure of the specimens was governed by splitting of the concrete cover over the anchored bar. The test results indicated that as the percentage silica fume increased, the maximum load capacity and the stiffness of the load-slip curve of the bottom cast bar decreased regardless of bar size or the concrete cover used. The reduction in the ultimate load due to presence of silica fume was independent of casting position. Companion bottom and top cast anchored bars developed similar ultimate load capacity regardless of the percentage replacement silica fume.

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Résumé Ce rapport se base sur une étude relative à l’effet de la fumée de silice, du diamètre des armatures et du confinement du béton sur les caractéristiques de glissement des armatures moulées d’en bas, dans le cas d’échantillons en béton avec pour adjuvant de la fumée de silice. L’objectif principal était de déterminer l’effet de la position de moulage en comparant les résultats d’essais à ceux d’échantillons testés auparavant et identiques sauf pour la position de moulage. Quanrante-huit échantillons ont été testés par le biais d’une extraction excentrée, les variables étant le pourcentage de substitution du ciment par des fumées de silice (en poids), la couverture de béton et la diminution des armatures. Le mode de rupture des échantillons est dominé par l’éclatement de la couche de béton recouvrant les armatures. Les résultats obtenus ont montré que plus le pourcentage de la fumée de silice augmente, plus la capacité maximale de charge ainsi que la courbe glissement-charge des armatures moulées d’en bas diminuent, quel que soit le diamètre des armatures ou l’enrobage de béton. La réduction de la capacité maximale de charge due à la présence de fumée de silice est indépendante de la position du moulage. En effet, des armatures, semblables moulées d’en haut et d’en bas ont développé des capacités maximales de charge similaires quel que soit le pourcentage de substitution en fumée de silice.

     

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.     

Double shear tests to evaluate the bond strength between GFRP/concrete elements

Externally bonded reinforced systems have been widely used in civil engineering. However, the problems associated with bond between structural elements are not yet fully solved. As a consequence, many researchers have been proposing tests and techniques to standardize procedures and reach better agreement for design purposes. In the present paper, an experimental program is described that was developed to characterize the glass FRP/concrete interface by double shear tests made on 15 cm side cubes with GFRP bonded on two opposite faces. The GFRP wrap had two layers applied by the wet lay-up technique and three classes of concrete were considered. With the support of the experimental program, cohesion and friction angle for GFRP-concrete interfaces were found leading to different envelope failure laws, based on the Mohr-Coulomb failure criterion for each concrete class, capable of predicting GFRP debonding. Results are discussed.     

Restrained shrinkage in repaired reinforced concrete elements

This paper describes an analytical model of restrained shrinkage in repaired reinforced concrete elements. The results of analysis and test show that the effect of restrained shrinkage on the serviceability of the repaired element is quite significant. The computer based analysis proposed can be applied to predict progressive crack development in a repair material subjected to restrained shrinkage during construction. The properties of repair materials, such as ultimate strain in tension, creep coefficient and free shrinkage, are major factors influencing the performance of structural repairs. Restrained shrinkage is related to the stiffness of a repaired section of known structure. The effect of moment redistribution on restrained shrinkage is significant in elements of indeterminate structure. The stress and strain caused by restrained shrinkage in elements of indeterminate structure are higher than in elements of known structure, and an example given in this paper shows an increase in strain of about 30%.     

Shear bond strength in repaired concrete structures

A good bond between old and new concrete is necessary for a successful repair of concrete structures. The bond is usually determined through pure tension tests,e.g., the common pull off test. However, in most applications, the shear bond strength is more interested. A test apparatus for torsion tests has been developed. Laboratory and field tests have been carried out on cast in place concrete and shotcrete repairs. For waterjetted surfaces, the obtained shear bond strength exceeds 3 MPa,i.e., considerably more than the tensile bond strength which usually varies between 1 and 2 MPa. The shear bond strength is also markedly higher than design strength values found in international codes.

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Résumé Une bonne adhérence entre l'ancien béton et le nouveau est nécessaire à la réussite de la réparation des structures en béton. L'adhérence est habituellement déterminée à partir d'essais en traction pure, comme par exemple un essai classique d'adhérence. Cependant, pour la plupart des applications, la résistance au cisaillement de l'adhérence est un problème plus pertinent. Un équipement pour des essais de torsion a été développé. Des tests en laboratoire et sur site, concernant du béton coulé sur place et du béton projeté, ont été menés. Pour les surfaces traitées par hydrodémolition, la résistance au cisaillement de l'adhérence dépasse 3 MPa, ce qui est largement supérieur à la résistance à la traction, qui varie habituellement entre 1 et 2 MPa. Ainsi, la résistance au cisaillement de l'adhérence est sensiblement plus élevée que les valeurs préconsisées par les codes internationaux.

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Editorial note Prof. J. Silfwerbrand is a RILEM Senior Member. He is the Director of the Swedish Cement and Concrete Research Institute, a RILEM Titular Member.     

External reinforcement of high strength concrete columns

With the technology development on the compressive strength of concrete over the years, the use of high strength concrete has proved most popular in terms of economy, superior strength, stiffness and durability due to many advantages it could offer. However, strength and ductility are inversely proportional [J. Mater. Civil Eng. 11 (1999) 21]. High strength concrete is a brittle material causing failure to be quite sudden and ‘explosive' under loads. It is also known that structural concrete columns axially compressed rarely occur in practice. The stress concentrations caused by the eccentric loading further reduce the strength and ductility of high strength concrete. Therefore, studies for high strength concrete columns under eccentric loading are essential for the practical use.

This paper experimentally investigates a number of high strength concrete columns that are externally reinforced with galvanised steel straps and fibre-reinforced polymers subjected to concentric and eccentric loading. The experimental results show that external reinforcement can enhance the properties of high strength concrete columns.     

The influence of corrosion on bond properties between concrete and reinforcement in concrete structures

The rebar corrosion in reinforced concrete is one of the most extensive pathologies affecting the performance of concrete structures. Chloride-induced rebar corrosion damage results mainly from the use of de-icing salts in cold climates and/or exposure to marine environments. Carbonation damage is a further important degradation mechanism. The internal consequences of corrosion are the modification of the steel behavior and degradation of the steel–concrete bond. This work is devoted to the influence of the controlled corrosion on the adherence between steel and concrete. A new geometry of specimen has been designed to: (i) avoid the lateral confining stresses that appear during the classical pullout tests and (ii) permit to impose a known confinement for the study of its influence on the behavior of the interface. Five specimens with different levels of corrosion have been tested in this contribution. Reinforcing bars embedded in concrete were submitted to accelerated corrosion using an external current source. The magnitude of corrosion was measured using both Faraday’s law and the weight loss method. The level of corrosion varied from 0% to 0.76%. The geometry of the specimens allowed us to take series of digital pictures during the tests, which were analyzed using a digital image correlation, the procedure named CORRELI^LMT. The results of pullout tests proposed in this contribution indicate that: (i) levels of corrosion that are less than 0.4% of weight loss improves the bond stress and (ii) levels of corrosion resulting in more than 0.4% of weight loss lead to a reduction of the bond stress value.     

Shear strength of reinforced aerated concrete slabs

The paper reports on the analysis of shear strength of reinforced slabs made of autoclaved aerated concrete without shear reinforcement. The test data are taken from eleven different investigations from six countries, in Europe and Japan, over a period of some twenty years and include 271 tests. The analysis of the test data results in regression expressions, suitably modified from a formula used for ordinary concrete members, and shows good agreement with test values. Appropriate expressions are suggested for design.     

Modeling pull-out resistance of corroded reinforcement in concrete: Coupled three-dimensional finite element model

Aggressive environmental conditions, such as exposure to the sea climate or use of de-icing salts, have considerable influence on durability of reinforced concrete structures due to reinforcement corrosion-induced damage. In the present paper, a recently developed coupled three-dimensional chemo-hygro-thermo-mechanical model for concrete is discussed [1], [2]. The model takes into account the interaction between non-mechanical processes and mechanical properties of concrete (damage). The mechanical part of the model is based on the microplane model. It is validated through a 3D transient finite element analysis of a pull-out of corroded steel reinforcement from a concrete beam-end specimen, which was exposed to aggressive environmental conditions. For the corrosion phase, the influence of the anode and cathode position on the electric potential, current density, corrosion rate and corrosion induced damage is investigated. Moreover, the effect of corrosion on the pull-out capacity of reinforcement and the influence of transport of corrosion products through cracks are studied.