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.     

Effects of steel fiber reinforcement on surface wear resistance of self-compacting repair mortars

Self-compacting repair mortars (SCRM), as new technology products, are especially preferred for the rehabilitation and repair of reinforced concrete structures. The self-compactability of repair mortars may bring considerable advantages at narrow mould systems. However, due to the high powder content and absence of coarse aggregate, plain SCRMs are susceptible to surface abrasion, especially in case of repair of surfaces under high rates of abrasion (floors, slabs). Steel fiber reinforcement can be an excellent solution for the abrasion resistance problem of SCRMs. However, the optimum amount of fiber reinforcement to sustain self-compactability should be pre-determined. In this study, the optimum superplasticizer dosage and the maximum possible amount of fiber addition, which maintain the self-compactability and stability, was determined for mortars incorporating steel fibers. In addition, the mechanical performance and abrasion resistance of SCRMs prepared by using these fibers were determined. It was concluded that steel fibers can have rheological and mechanical synergistic effects, and that optimised fiber – superplasticizer dosage combinations can better improve the wear resistance while maintaining adequate flow properties for FR-SCRM.     

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.     

Compatibility of repair mortars with concrete in a hot-dry environment

Strengthening, maintenance and repair of concrete structures are becoming more recognised in the field of civil engineering. There is a wide range of repair mortars with varying properties, available in the market and promoted by the suppliers, which makes the selection of the most suitable one often difficult. A research programme was conducted at Leeds University to investigate the properties of cementitious, polymer and polymer modified (PMC) repair mortars. Following an earlier publication on the intrinsic properties of the materials, this paper presents results on the compatibility of these materials with concrete. The dimensional stability is used in this study to investigate the compatibility of the repair mortars and the parent concrete. Composite cylindrical specimens (half repair mortar/half concrete) were prepared and used for the measurements of modulus of elasticity and shrinkage. The results of the different combined systems were obtained and compared to those calculated using a composite model. The variations between the measured and calculated values were less than 10%. The paper attempts to quantify the effect of indirect differential shrinkage on the permeability and diffusion characteristics of the different combined systems.     

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.

     

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.     

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 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.

     

Bond of ribbed galvanized reinforcing steel in concrete

The ASTM beam end test (ASTM A944) has been used to compare the bond and slip behaviour of deformed (i.e. ribbed) galvanized, epoxy-coated and black steel bars in concrete. The objective was to determine whether galvanizing adversely affects bond strength. From a series of thirty specimens, the average bond strength of black steel and galvanized steel reinforcement used in these tests has been determined and bond stress has been shown to act uniformly over the embedded bar area. A slip value of approximately 0.4 mm has been confirmed to be associated with bond failure by concrete splitting. The results indicated that while epoxy coating resulted in a significant loss in bond strength of the order of 20% compared to black steel, there is no adverse effect on bond with the use of galvanized steel. Chromate treatment of galvanized bars is deemed unnecessary since there was no evidence of long term reduction in bond due to the possible effects of hydrogen gas evolution resulting from the reaction between zinc and wet concrete.     

Effect of polypropylene fibre reinforced mortars on steel reinforcement corrosion induced by carbonation

Low amounts of polypropylene fibres are added to concrete as a secondary reinforcement to control cracking. Whether this addition might improve the corrosion resistance of the concrete reinforcement by increasing the resistance to carbonation, via reducing penetrating paths, is the subject addressed in the present paper. Crack control by the fibres in plastic state mortars and crack evolution with time have been studied. Furthermore, the influence of crack width on the steel bar corrosion induced by carbonation has also been monitored. Circular specimens made of mortar have been employed in the experimental phase of the study, using a water/cement ratio of 0.50 and cement/sand ratio of 2/1. The polypropylene fibre content was 0% and 0.5% by volume. Low modulus polypropylene fibres may control the crack width in specimens subjected to inadequate curing conditions. No relationship between crack width and time for corrosion initiation has been observed. However, a beneficial effect of fibre addition on the corrosion rate was found.     

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 between deformed reinforcement and normal and high-strength concrete with and without fibers

This paper describes an experimental study that consisted of pullout tests of deformed reinforcing bars in NSC and HSC specimens, with and without hooked-end steel fibers. Two types of test setups were applied, direct and flexural tests, and three bar diameters were tested (8, 12 and 20 mm). The experimental setups were based on standard RILEM pullout (direct) and beam tests, with several modifications. The experimental program included study of the effects of concrete strength and inclusion of steel fibers on the bond strength, as well as the influence of bar geometry and concrete cover. Discussion of the results shows coupling of these effects and proposes an empirical expression that represents this coupling. The results from the current study are also compared with the design bond strengths specified in American and European standards as well as a known model.     

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.     

Bond behaviour of flat stainless steel rebars in concrete

In recent years there has been an increasing interest in applying stainless steel (SS) reinforcement in concrete structures to avoid corrosion derived durability problems. Concrete-steel bond behaviour and applicability of existing standards have not yet been extensively studied with respect to SS. Moreover, an interest in applying flat rebars as reinforcement elements has emerged. The idea is based on the optimization of the rebar surface in contact with the concrete as well as on the reinforcement optimization for shallow slabs. To advance in the evaluation of the bond between concrete and flat SS strips and to compare with the behaviour of standard carbon steel (CS) round reinforcement 72 bond tests have been conducted. The test program consists of pull-out tests to centrally reinforced concrete specimens. Traditional concrete and self compacting concrete are used for embedding. For a better analysis and understanding of the failure aspect of the specimens, fluorescent epoxy injection has been conducted. Results are compared to the CEB-FIP 1990 bond model. The authors conclude that, the use of SS instead of CS is not considerably influencing the bond capacity of the reinforcement when ribbed samples are used while parameters as reinforcement shape or rib pattern are more deterministic. An adaptation of the CEB-FIP 1990 bond model is proposed for flat reinforcements tested in this work.     

钢纤维增韧微膨胀钢管混凝土界面粘结性能研究

进行了30根钢纤维微膨胀钢管混凝土试件推出试验,系统研究了核心混凝土与管壁的界面粘结性能,阐述了推出试验过程中试件界面粘结破坏特征,并对界面粘结性能的影响因素进行了分析。结果表明,钢纤维微膨胀钢管混凝土界面粘结强度较普通钢管混凝土明显提高。径厚比和含钢率是钢纤维微膨胀钢管混凝土界面粘结强度的主要影响因素。钢纤维的掺加导致微膨胀钢管混凝土界面粘结性能降低,其体积掺量建议不宜超过0.75%。含钢率达到22%时,钢纤维掺量与界面长度对界面粘结性能影响不明显。     

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.