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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锈蚀钢筋混凝土柱的修正压-剪-弯分析模型研究

为了研究立面差异化锈蚀桥墩在全寿命周期内的震后可恢复性能,该文基于结构震中响应和震后状态提出了一种“先判定损伤等级后评估可修复程度”的桥墩抗震韧性概率评价方法。以地震易损性分析为基础,基于最大位移角-残余位移角组合指标构建桥墩震后可恢复性曲线,给出桥墩各级破坏状态下多级可修复程度的概率计算方法。根据海洋环境的腐蚀特点,建立了立面差异化锈蚀桥墩的有限元模型,通过Pushover分析研究了全服役期内桥墩抗震性能的退化规律。采用增量动力分析(IDA)方法对不同服役年限的桥墩进行地震易损性分析,根据墩顶最大位移角定义桥墩破坏状态、以峰值加速度(PGA)表征地震动强度,从而建立起桥墩时变地震易损性曲线。以残余位移角为参考指标,对桥墩不同服役时间的各级破坏状态进行震后可修复程度的分类与统计,通过构建可恢复性曲线实现桥墩不同破坏状态下各级修复难度的概率评估。研究结果表明,随着服役时间的增长,钢筋锈蚀会导致桥墩抗震性能的不断退化。相比未锈蚀情况,桥墩服役100年后,承载力下降约20.22%,而侧向变形能力降幅可达75.30%。同时,随着服役时间的增长,桥墩各级破坏状态的地震失效概率逐渐增大,且震后修复难度也随之提高。当PGA为0.6 g时,桥墩从未锈蚀到服役100年,发生严重破坏的超越概率由16.92%上升到了35.66%,而震后需要简单修复的概率由14.17%上升至26.49%,并出现了较难修复和难以修复的情况。

     

Probabilistic bond strength model for reinforcement bar in concrete

In order to overcome the disadvantages of traditional deterministic models, a probabilistic bond strength model of reinforcement bar in concrete was presented. According to the partly cracked thick-walled cylinder model, a deterministic bond strength model of reinforcement bar in concrete was developed first by taking into account the influences of various important factors. Then the analytical expression of probabilistic bond strength model of reinforcement bar in concrete was derived by taking into consideration both aleatory and epistemic uncertainties. Subsequently, a probabilistic bond strength model of reinforcement bar in concrete was proposed by determining the statistical characteristics of probabilistic model parameters based on the Markov Chain Monte Carlo method and the Bayesian theory. Finally, applicability of the proposed probabilistic model were validated by comparing with 400 sets of experimental data and four typical deterministic bond strength models. Analysis shows that the probabilistic model provides efficient approaches to describe the probabilistic characteristics of bond strength and to calibrate traditional deterministic bond strength models.     

Effects of thaumasite on bond strength of reinforcement in concrete

The conditions necessary for the formation of thaumasite are well known and much work is in progress to identify concrete mixes resistant to thaumasite form of sulfate attack (TSA). However, there have been no data to indicate how TSA affects the nature and strength of the bond between reinforcement steel and concrete and hence the load capacity of reinforced concrete elements.

During works to repair and strengthen the thaumasite-affected Tredington–Ashchurch Overbridge in Gloucestershire, sections of column were removed and placed in storage. These column sections presented an opportunity to perform pullout tests on full size TSA-affected structural elements and unaffected control specimens from the same structure. In total 63 pullout tests were performed on plain round reinforcement bars embedded in two unaffected and four TSA-affected reinforced concrete elements. The sections were also characterised in terms of estimated in situ cube strength and depth of softened zone.

A statistical analysis of the experimental results indicates that the bond of the plain round reinforcement bars in the unaffected concrete exceeded that of the plain round reinforcement bars in the TSA-affected concrete. TSA reduced the mean experimental bond coefficient by 24% for corner bars and 10% for other bars, representing an average reduction in mean experimental bond coefficient of 15% for all bars.     

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.     

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.     

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.     

Potential use of zinc in the repair of corroded reinforced concrete

A detailed research program was undertaken to explore the possibility of using zinc to enhance the repair of corroded concrete. The variables studied were the water–cement ratio, chloride content, and amount of zinc coverage in the repair area. The results confirmed that using dense repair mortar alone can move the corrosion damage to adjacent unrepaired zones and that the presence of zinc can mitigate this problem. However, the engineering solution is complex since the effectiveness of zinc in the prevention of further corrosion may be influenced by various factors, such as leaking joint, zinc–steel contact area.     

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.

     

纤维增强聚合物基复合材料加固锈蚀钢筋混凝土圆柱轴心受压性能

为了给纤维增强聚合物基复合材料（FRP）加固腐蚀环境下钢筋混凝土圆柱的设计和施工提供参考,促进FRP加固钢筋混凝土圆柱的应用,本文通过加速腐蚀得到类似实际环境中已锈损钢筋混凝土圆柱,采用碳纤维增强聚合物基复合材料（CFRP）条带和玻璃纤维增强聚合物基复合材料（GFRP）条带分别对锈蚀钢筋混凝土圆柱进行加固,最后对加固后圆柱进行轴心受压试验,重点研究钢筋锈蚀率、FRP层数和种类对钢筋混凝土圆柱受压承载力的影响;基于对FRP条带间隔约束效应、钢筋锈蚀对混凝土截面及钢筋力学性能影响的研究与分析,提出FRP条带间隔约束锈蚀钢筋混凝土圆柱轴心受压承载力计算模型。试验实测值与模型计算值之比的平均值为1.020,变异系数为0.063,二者符合较好。     

Bamboo as reinforcement in structural concrete elements

It is a fact that the construction industry is the main consumer of energy and materials in most countries. The pursuit of sustainable development, defined in the Brundtland Report 1987 as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”, has become a major issue when trying to meet the challenges in providing proper housing for the ever-increasing world population. To increase the amount of information concerning bamboo several successful research programs have been carried out since 1979 at PUC-Rio and in Brazil. Vegetable fibres can be used either alone or as reinforcement in different types of matrices such as soil and cement composites. This paper presents the results of some of the recent studies of the microstructure of bamboo as a functionally gradient material. These studies led to the establishment of bamboo’s composite behaviour through the rule of mix. A concise summary regarding bamboo reinforced concrete beams, permanent shutter concrete slabs and columns is discussed. Finally, some recommendations for future studies are proposed with the hope that the newly developed material could contribute, on a large scale, to sustainable development without harming our globe.