Bond strength tests on deformed reinforcement in normal weight concrete

The primary aim of the research which is described in this paper was to investigate the effect of lateral pressure on the bond strength of deformed reinforcement set in normal weight concrete. A large number of tests was undertaken in which the main variables affecting bond strength were investigated. These included concrete strength, reinforcing bar diameter, concrete cover, lateral pressure, position of casting and bar spacing. A brief review of the small number of earlier investigations involving lateral pressures is provided. The paper provides details of the test specimens, the test apparatus and procedure, as well as typical results selected from approximately three hundred results. It draws conclusions on the appropriateness of the test method and the effects of the variables on the bond stress ratio and on bond strength.

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Résumé Le but principal de ces recherches était d'étudier l'effet de l'effort tranchant sur l'adhérence des armatures crantées placées dans du béton ordinaire. Un grand nombre d'essais ont été effectués pour étudier les principales variables qui influencent l'adhérence, y compris la résistance du béton, le diamètre des barres de renforcement, l'épaisseur d'enrobage, l'effort tranchant, la mise en place et le positionnement des barres. On résume le petit nombre de recherches antérieures sur ce sujet. L'article décrit les éprouvettes, l'appareillage et les procédures d'essais, ainsi que des résultats types choisis parmi environ 300 résultats d'essais. Il présente des conclusions sur l'applicabilité de la méthode d'essais et les effets des variables sur le rapport contrainte d'adhérence et sur l'adhérence.

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Editorial note Prof. Dr. P. E. Regan is a RILEM Senior Member and a member of the Editorial Group of TC 090-FMA, Fracture Mechanics of concrete-Applications.     

Bond over direct support of deformed rebars in normal and high strength concrete with and without fibers

This paper describes results of the second stage of a research on the bonding of deformed bars in normal and high strength concrete with and without fibers. The current study focused on the effect of confinement conditions, which are present over a direct support of a reinforced concrete beam, and compares these results with data obtained in the first stage of the research. The experimental work consisted of pullout tests, which were based on the standard RILEM flexural bond test (??beam test??) of deformed steel bars with diameters of 12 and 20?mm. Results revealed an increase in the normalized bond strength due to support conditions. An unstable post-peak slip of the tested rebar was observed when located over the support, whereas bars bonded near mid-span exhibited controlled bond?Cslip throughout the test. Up to peak load, however, bond?Cslip relations exhibited higher energy absorption over the support than near mid-span. Inclusion of fibers had only a minor effect on bond strength but had a significant influence on pre-peak energy absorption. A previously developed model of bond strength shows good agreement with current results when applying an increase factor due to the support effect.     

Bond behavior between steel reinforcement and recycled concrete

In this paper the bond behavior of recycled aggregate concrete was characterized by replacing different percentages of natural coarse aggregate with recycled coarse aggregate (20, 50 and 100 %). The results made it possible to establish the differences between the conventional concrete bond strength and the recycled concrete bond strength depending on the replacement percentage. It was thus found that bond stress decreases with the increase of the percentage of recycled coarse aggregate used. In order to define the influence of recycled aggregate content on bond behavior, normalized bond strength was calculated taking into account the reduced compressive strength of the recycled concretes. Finally, using the experimental results, a modified expression for maximum bond stress (bond strength) prediction was developed, taking into account replacement percentage and compressive strength. The obtained results show that the equation proposed provides an experimental value to theoretical prediction ratio similar to that of conventional concrete.     

Bond behaviour between recycled aggregate concrete and deformed steel bars

The results of thirty pullout tests carried out on 8 and 10 mm diameter deformed steel bars concentrically embedded in recycled aggregate concrete designed using equivalent mix proportions with coarse recycled concrete aggregate (RCA) replacement percentages of 0, 25, 50, 75 and 100 % are reported towards investigation of bond behaviour of RCA concrete. Bond strengths of the natural aggregate concrete and the RCA concrete was found to be comparable, particularly for the 10 mm rebars, and the RCA replacement percentage had an insignificant effect on peak bond stress values. However, for both the bar sizes, when the measured bond strengths were normalized with the respective compressive strengths, then the normalized bond strengths so obtained across all the RCA replacement percentages were higher for the RCA concrete compared to the natural coarse aggregate concrete. Further, higher normalized bond strength values were obtained for the 8 mm rebars compared to the 10 mm bars. An empirical bond stress versus slip relationship between RCA concrete and deformed steel bars has been proposed on the basis of regression analysis of the experimental data and it is conservatively suggested that anchorage lengths of 8 and 10 mm diameter deformed bars in RCA concrete may be taken the same as in natural aggregate concrete.     

Mechanical reinforcement of concrete with bi-component fibers

For some applications the reinforcement of concrete with fibers is an economical alternative to conventional steel bar reinforcement. Steel fibers have been the first choice for many years because of their high tensile strength and high elastic modulus. Low modulus fibers, such as polyolefin based fibers generally are thought to be less suitable for this purpose.However, it is shown that polyolefin fibers with sufficient tensile strength can, applying a novel bi-component approach, successfully enhance the mechanical properties of concrete. The effect of the introduction of nanoparticles into the fiber polymers and of a fiber surface structuring on the fiber pull-out characteristics and the fiber–matrix bond strength is presented.The performance of bi-component fiber reinforced concrete is studied in 4-point bending and square slab tests. Ductile post-peak behavior of such fiber reinforced concrete is achieved, making this new fiber technology interesting for applications in pre-cast elements, industrial floors and earth quake protecting systems.     

Bond between steel reinforcement bars and Electric Arc Furnace slag concrete

This paper deals with the bond between steel reinforcement and recycled aggregate concrete, including Electric arc furnace (EAF) slag as full replacement of natural coarse aggregates. Pull-out tests were carried out according to RILEM standard on specimens made with six concrete mixtures, characterized by different w/c ratios and types of aggregates. Plain and ribbed steel reinforcement bars were used to observe the influence of steel roughness. Experimental bond-slip relationships were analyzed, and results show similar bond mechanisms between the reference and EAF concrete specimens. Significant bond strength enhancement is observed in concretes with low w/c ratio, when EAF slag is used as recycled coarse aggregate. Experimental results in terms of bond strength were also compared to analytical predictions, obtained with empirical formulations.     

Bond of steel reinforcement with microwave cured concrete repair mortars

This paper investigates the effect of microwave curing on the bond strength of steel reinforcement in concrete repair. Pull-out tests on plain mild steel reinforcement bars embedded in four repair materials in 100 mm cube specimens were performed to determine the interfacial bond strength. The porosity and pore structure of the matrix at the steel interface, which influence the bond strength, were also determined. Test results show that microwave curing significantly reduces the bond strength of plain steel reinforcement. The reduction relative to normally cured (20 °C, 60% RH) specimens is between 21 and 40% with low density repair materials and about 10% for normal density cementitious mortars. The corresponding compressive strength of the matrix also recorded similar reduction and microwave curing resulted in increased porosity at the interface transition zone of the steel reinforcement. A unique relationship exists between bond strength and both compressive strength and porosity of all matrix materials. Microwave curing reduced shrinkage but despite the wide variation in the shrinkage of the repair mortars, its effect on the bond strength was small. The paper provides clear correlations between the three parameters (compressive strength, bond strength and porosity), which are common to both the microwave and conventionally cured mortars. Therefore, bond-compressive strength relationships used in the design of reinforced concrete structures will be also valid for microwave cured elements.     

Failure mechanism of normal and high-strength concrete with rice-husk ash

Mineral admixtures generally lead to a densification of the concrete internal structure. In this sense, the failure mechanism could be modified so that the concrete exhibits a more brittle behaviour. This paper discusses the effects of rice-husk ash (RHA) additions to concrete, based on analyses of the mechanical behaviour of normal and high-strength concrete. The stress–strain response in compression and load vs. CMOD (or deflection) in bending were analysed. It appears that the incorporation of RHA in concrete increases the strength, particularly for lower water/binder ratio concretes. The analysis of the failure mechanism indicates a tendency for more brittle failure behaviour in RHA concretes. For the same strength level, however, the energy of fracture was reduced no more than 10%, which is much smaller than the variations that may be produced by a change in the type or size of coarse aggregate.     

Bond and interfacial properties of reinforcement in self-compacting concrete

This paper reports a study carried out to assess the impact of the use of self-compacting concrete (SCC) on bond and interfacial properties around steel reinforcement in practical concrete element. The pull-out tests were carried out to determine bond strength between reinforcing steel bar and concrete, and the depth-sensing nano-indentation technique was used to evaluate the elastic modulus and micro-strength of the interfacial transition zone (ITZ) around steel reinforcement. The bond and interfacial properties around deformed steel bars in different SCC mixes with strength grades of 35 MPa and 60 MPa (C35, C60) were examined together with those in conventional vibrated reference concrete with the same strength grades.The results showed that the maximum bond strength decreased when the diameter of the steel bar increased from 12 to 20 mm. The normalised bond strengths of the SCC mixes were found to be about 10–40% higher than those of the reference mixes for both bar diameters (12 and 20 mm). The study of the interfacial properties revealed that the elastic modulus and the micro-strength of the ITZ were lower on the bottom side of a horizontal steel bar than on the top side, particularly for the vibrated reference concrete. The difference of ITZ properties between top and bottom side of the horizontal steel bar appeared to be less pronounced for the SCC mixes than for the corresponding reference mixes.     

Bond characteristics of corrding reinforcement in concrete beams

The results of an experimental study of the bond characteristics of reinforced concrete beams subjected to reinforcement corrosion are presented. Beam specimens recommended by the joint RILEM/CEB/FIP Committee were used, which comprised two halves of a reinforced concrete beam rotating about a hinge mechanism. Corrosion was induced at different levels of rebar diameter loss: 0, 0.3, 0.4, 0.5, 1, 2 and 5 percent by impressing direct current of intensity 0.8 and 2.4 mA/cm². The specimens were tested under four point bending to induce bond failure and load-free end slip curves were plotted. The tests showed that at up to 0.4% degree of corrosion, no free-end slip occurred in the reinforcement bars until complete breakdown of bond at failure. At higher degrees of reinforcement corrosion, free-end slip commenced immediately upon application of load and increased linearly with increasing load. The free-end slip at maximum load was a function of the degree of reinforcement corrosion. At small degrees of corrosion, the bond strength increased with increasing degree of corrosion, showing a maximum increase of over 25% at 0.4% corrosion. Higher degrees of corrosion led to a sharp decrease in bond strength.

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Résumé Cet article présente les résultats d’une étude expérimentale sur les caractéristiques d’adhésion pour des poutres en béton armé où l’armature est exposée à la corrosion. Des échantillons de poutres recommandés par le comité joint RILEM/CEB/FIP étaient employés sous forme de deux moitiés d’une poutre en béton armé tournant autour d’un pivot. La corrosion était induite à plusieurs niveaux de réduction de la section d’armature à savoir 0; 0,3; 0,4; 0,5; 1, 2 et 5% par le biais d’un courant direct d’intensité 0,8 et 2,4 mA/cm². Les échantillons ont été testés sous des conditions de quatre points de flexion pour induire l’affaiblissement d’adhésion des courbes de glissement ont été tracées. Les essais ont démontré que jusqu’à 0,4% de corrosion, il n’y avait pas de glissement par l’armature jusqu’à rupture totale d’adhésion. Pour des niveaux de corrosion élevés, le glissement a commencé dès l’application de la charge augmentant linéairement avec l’augmentation de celle-ci. Le glissement sous charge maximale est une fonction du degré des armatures. Pour un faible niveau de corrosion, l’adhésion augmente avec le degré de corrosion, pour un accroissement de l’adhésion de plus de 25% pour un degré de corrosion de 0,4%. Des niveaux de corrosion élevés induisent une réduction significative de l’adhésion.

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Editorial Note Prof. P. Mangat is a Senior Member and the Chairman of RILEM TC LPC: Long term performance characteristics of fibre cement composites.     

Experience with high-strength concrete in combination with high-strength steel in precast reinforced and prestressed concrete

This paper deals with the experience gained before the war with centrifugally moulded (spun) concrete masts, tubular and half-tubular beams, reinforced with high strength steel and after the war with prestressed, vibrated high strength precast concrete, including lightweight concrete, with the advantage of gap-grading, particularly when employing phased frequencies at vibration.     

Bond of reinforcement in eccentric pullout silica fume concrete specimens

The objectives of the study were to investigate the role of specimen type and specimen size on the effect of silica fume on bond strength of reinforcing bars, and to evaluate the effect of bar size and concrete confinement on the anchorage characteristics of the reinforcing bars in silica fume concrete. Forty-eight eccentric pullout specimens were tested. The variables were the percentage replacement by weight of cement by silica, the concrete cover over the reinforcing bar, and the bar size. Failure of the specimens was governed by splitting of the concrete cover over the anchored bar. Test results indicated that as the percentage of silica fume increased, the maximum load capacity and the stiffness of the load-slip curve of the anchored bar decreased, regardless of bar size or the concrete cover used. The results showed that concrete confinement slightly improved the bond resistance in concrete containing more than 10 percent silica fume.

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Résumé Les objectifs de cette étude concement l'influence de la nature et de la taille des échantillons sur l'action de la fumée de silice dans les forces de liaison entre le béton et les armatures d'acier. L'évaluation de l'effet du diamètre des armatures et du confinement du béton sur les caractéristiques d'ancrage des armatures utilisées avec du béton avec comme adjuvant la fumée de silice a été également effectuée. Quarante-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 montrent que plus le pourcentage de la fumée de silice monte, plus la capacité maximale de charge ainsi que la courbe glissement—charge appliqué des armatures ancrées diminuent, quel que soit le diamètre des armatures ou l'enrobage du béton. Un léger confinement, dans le cas d'un béton à plus de 10 pour cent de fumée de silice, sert à augmenter légèrement ces forces de liaison.

     

变形钢筋/超高性能混凝土搭接黏结性能

超高性能混凝土(UHPC)是一种高强度、高韧性和高耐久性的水泥基复合材料。为了研究钢筋/UHPC的搭接黏结性能,进行了21组考虑搭接长度、纤维掺量和配箍率影响的钢筋搭接对拉拔出试验,3组考虑锚固长度影响的钢筋直接拔出锚固试验;试验出现了劈裂拔出破坏和钢筋拉断破坏2种破坏模式;钢筋/UHPC平均黏结强度随钢筋埋置长度的增大而减小,随配箍率的增大而增大;钢纤维掺量的增大,有利于增大对UHPC的约束作用,增加配箍率和适当增大纤维掺量均能减小钢筋/UHPC的临界搭接长度;结合前人的试验结果,拟合得到平均锚固和搭接黏结强度计算公式及临界锚固和搭接长度计算公式,根据混凝土结构设计规范,建立了钢筋/UHPC锚固和搭接长度简化算法,计算结果较为准确。      

钢筋混凝土柱纯扭破坏尺寸效应数值分析

纤维增强聚合物(FRP)筋作为非腐蚀材料替代钢筋已成为解决锈蚀问题的创新方案,被应用于预应力结构、海洋平台、沿海码头和长期处于浸蚀性化学环境的构件中。关于FRP筋混凝土构件的纯扭承载力设计方法只有加拿大规范CSA S806-12提出,且以借鉴现有的钢筋混凝土构件纯扭承载力计算公式的形式给出,考虑了FRP筋与钢筋间材料力学参数的区别。该文通过已验证了的三维细观数值模拟方法,研究了最大横截面宽度为1000 mm的FRP筋混凝土柱纯扭破坏行为,揭示了其名义抗扭强度尺寸效应规律。基于模拟数据,对比了现有的FRP筋混凝土构件纯扭承载力计算公式,并在中国规范GB 50010−2010关于钢筋混凝土构件纯扭承载力计算方法的基础上,提出了考虑尺寸效应影响的FRP筋混凝土构件纯扭承载力的计算方法。通过对比现有的试验数据,验证了所提公式的正确性和合理性。

     

Behavior of normal and high-strength concrete cylinders confined with E-glass/epoxy composite laminates

This paper is aimed at studying the behavior of concrete cylinders with varying compressive strength wrapped with E-glass/epoxy fiber reinforced polymer (GFRP) jackets and subjected to uniaxial compressive loads. A comprehensive experimental program which involves 54 plain concrete cylinders was conducted in this study. The cylinders evaluated in this study, were divided into six groups, and each group contain a control cylinder without confinement to quantify the amount of gain obtained using the GFRP laminates. Experimental results indicated that the use of GFRP jackets substantially increases both the compressive strength and ductility of unreinforced concrete cylinders. In this paper, the influences of two parameters influencing the behavior of the GFRP confined cylinder is investigated. These parameters are: the number of composite plies (i.e. composite thickness) and concrete compressive strength. The results of this study showed that: (i) compressive strength and ductility of the concrete cylinders increases with number of composite layers; and (ii) effect of confinement is substantial for normal strength concrete and marginal for high-strength concrete. A semi-empirical theoretical model is also presented in order to predict stress–strain relationship of GFRP confined concrete cylinders. The model results showed an excellent agreement with experimental values.     

Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete

This paper summarizes the results of an experimental investigation into the bond behavior between recycled aggregate concrete (RAC) and deformed steel rebars, with the main variables being the recycled coarse aggregate replacement ratio (RCAr) and water-to-cement ratio of the concrete mixture. The investigation into splitting cracking strength indicates that the degradation of the bond splitting tensile stress of the cover concrete was affected by not only the roundness of the coarse aggregate particles but also the weak interfacial transition zone (ITZ) between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete. In this study, a linear relationship between the bond strength and the density of the RCA was found, but the high compressive strength reduced the effects of the parameters. To predict the bond strength of RAC using the main parameters, a multivariable model was developed using nonlinear regression analysis. It can be inferred from this study that the degradation characteristic of the bond strength of RAC can be predicted well, whereas other empirical equations and code provisions are very conservative.     

Bond strength between concrete substrate and metakaolin geopolymer repair mortar: Effect of curing regime and PVA fiber reinforcement

The suitability of repairing Portland cement concrete with geopolymer mortars is explored as a viable way to replace Portland cement in concrete repairs and reduce their carbon footprint. Bond tests are performed through non-standard slant shear tests with variable bond plane inclination to assess concrete-geopolymer shear bond strength under different combinations of normal and shear stresses at the concrete-geopolymer interface. Interfacial cohesion and friction coefficients, two inherent mechanical properties of the substrate-repair interface, are extrapolated from experimental data and compared among different types of geopolymer repairs. The adoption of different curing temperatures for the geopolymer repair mortar (20°C and 45°C) and its reinforcement with various contents of Polyvinyl Alcohol fibers (volume fractions V_f = 0%, 0.5%, and 1%) are investigated to optimize the substrate-repair bond. Mechanical tests are supported by statistical analysis and microscope observation.     

Cracking behaviour of concrete beams reinforced with a combination of ordinary reinforcement and steel fibers

The paper presents an experimental and theoretical study on the cracking behaviour of concrete beams having longitudinal tension reinforcement and various combinations of volume and aspect ratio of steel fibers. Five full-scale beams with a concrete compressive strength of 42 MPa were tested. The mechanical properties of the steel fiber concrete under tension were determined by means of the four-point bending test specified in the Belgian standard NBN B15-238. The experimental results show that the addition of steel fibers decreases both the crack spacing and the crack width. A modification of the model of Nemegeeret al. to predict crack widths is suggested.