Experimental and numerical study on the behavior of concrete subjected to biaxial tension and shear

In this article an experimental and numerical study on the behavior of concrete subjected to biaxial loading is outlined. For this purpose the unique biaxial machine available at the Stevin Laboratory was used. Two load-paths were pursued, namely axial tension at constant shear and proportional tension/shear. The recently developed lattice model was used to simulate the two load-paths. The remarkable feature of the lattice model is its ability to simulate curved overlapping cracks which resemble the experimental findings.     

Effect of aging on bond of GFRP bars embedded in concrete

This paper presents an experimental investigation of the durability of the bond between GFRP bars and concrete, specifically as it relates to degradation of the GFRP-bar surface and behavior of the bar–concrete interface. The GFRP bars were embedded in concrete and exposed to tap water at 23 °C, 40 °C, and 50 °C to accelerate potential degradation. The bond strengths before and after exposure were considered as a measure of the durability of the bond between the GFRP bars and concrete. In addition, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) were used to characterize how bar aging affected the bond between the GFRP bars and the concrete. The results showed that aging did not significantly affect the durability of the bar–concrete interface under the conditions used in this study.     

GFRP筋与自密实混凝土黏结性能的试验研究

为了研究玻璃纤维增强树脂复合材料(GFRP)筋与自密实混凝土(SCC)的黏结性能,制作了66个GFRP/SCC试件进行中心拉拔试验,研究SCC混凝土保护层厚度、GFRP筋直径和黏结长度以及SCC中添加纤维种类等因素对两者黏结性能的影响,并对试件的破坏形式进行分析。结果表明:试件主要出现了三种破坏形式,即劈裂破坏、拔出破坏、拔出带缝破坏;通过电镜扫描发现SCC浇筑方向对GFRP筋与SCC黏结界面的结构有一定影响,GFRP筋上部界面与SCC黏结更紧密。当SCC保护层厚度由4D增大至7D时,黏结强度提高了约44.05%;当GFRP筋黏结长度由5D增大至15D时,黏结强度降低了约65.43%;当GFRP筋直径由12 mm增大至16 mm时,黏结强度降低了约22.57%;SCC中添加聚丙烯纤维、钢纤维、聚丙烯纤维+钢纤维的试件黏结强度比不添加纤维的试件黏结强度分别提高12.80%、15.16%、15.09%。可以通过适当增加SCC保护层厚度、在SCC中添加纤维等措施来提高GFRP/SCC试件的黏结强度。      

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。      

Local bond stress-slip relationships of glass fibre reinforced plastic bars embedded in concrete

The bond relationships between glass fibre reinforced plastic bars with polyester matrix and concrete is investigated in order to obtain information concerning the possible use of these bars for the reinforcement of concrete constructions, where the use of ordinary steel bars could have certain disadvantages due to the possibility of corrosion. The results obtained are used to develop a model of the stress-slip relationship and to estimate the anchorage lengths needed to embed these bars in concrete casts.

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Resume On a effectué une recherche sur la relation d'adhérence entre des barres de résine polyester renforcée de fibres de verre (GRP) et le béton, dans le but d'obtenir des informations sur l'emploi éventuel de telles barres comme armatures pour les ouvrages en béton armé, dans lesquels l'emploi de barres ordinaires en acier présente plusieurs inconvénients, dont ceux liés au phénomène de la corrosion. Les résultats obtenus ont permis d'établir un modèle de la relation contrainte d'adhérence-glissement, et de formuler une estimation des longueurs nécéssaires pour l'ancrage de telles barres dans des coulée de béton.

     

Experimental study of bond behaviour between concrete and FRP bars using a pull-out test

This paper presents the results of an experimental programme concerning 88 concrete pull-out specimens prepared according to ACI 440.3R-04 and CSA S806-02 standards. Rebars (reinforcing bars) made of carbon-fibre and glass-fibre reinforced polymer (CFRP and GFRP), as well as steel rebars, with a constant embedment length of five times the rebar diameter were used. The influence of the rebar surface, rebar diameter and concrete strength on the bond–slip curves obtained is analysed. In addition, analytical models suggested in the literature are used to describe the ascending branch of the bond–slip curves. To calibrate the analytical models, new equations that account for the dependence on rebar diameter are presented.     

Numerical calibration of bond law for GFRP bars embedded in steel fibre-reinforced self-compacting concrete

An experimental program was carried out at the Laboratory of Structural Division of the Civil Engineering Department of the University of Minho (LEST-UM) to investigate the bond behaviour of glass fibre reinforced polymer (GFRP) bars embedded in steel fibre reinforced self-compacting concrete (SFRSCC) for the development of an innovative structural system. Thirty-six pull-out-bending tests were executed to assess the influence of the bond length, concrete cover, bar diameter and surface treatment on the bond of GFRP bars embedded in SFRSCC. This paper reports the results of a numerical study aiming to identify an accurate GFRP–SFRSCC bond–slip law. Thus, the above mentioned pullout bending tests were simulated by using a nonlinear finite element (FE) constitutive model available in FEMIX, a FEM based computer program. The bond–slip relationship adopted for modelling the FE interface that simulates the interaction between bar and concrete is the key nonlinear aspect considered in the FE analyses, but the nonlinear behaviour of SFRSCC due to crack initiation and propagation was also simulated. The evaluation of the values of the relevant parameters defining such a bond–slip relationship was executed by fitting the force versus loaded end slip responses recorded in the experimental tests. Finally, correlations are proposed between the parameters identifying the bond–slip relationship and the relevant geometric and mechanical properties of the tested specimens.     

Experimental investigation on the bond of reinforcing bars in high performance concrete under cyclic loading

The bond performance between reinforcing bars and concrete under cyclic loads were experimentally investigated in this paper. In particular, the effects of steel fibers and hybrid fibers (including steel fibers and carbon fibers) on␣the bond performance were analyzed. Based on the experimental results, the deterioration laws of the peak bond stress, the stiffness of the unloading branch, frictional bond resistance and remnant ultimate bond stress under cyclic loads were summarized. Meanwhile, the increasing law of the remnant slip and the effect of the loading order on the deterioration of the bond performance were studied. Then, two damage factors d _u and D _u were introduced, where d _u is a factor that reflects the damage of the peak bond stress under constant controlling displacement and D _u is also a factor that reflects the damage of the remnant ultimate bond stress under changeable controlling displacement. Making use of the two damage factors and other key variables, the bond constitutive relationships between reinforcing bars and concrete under cyclic loads could be expressed by formulas. The computed τ–s relationships agreed well with those obtained by experiments.     

Evaluation of the bond behavior of steel reinforcing bars in recycled fine aggregate concrete

This paper describes pullout test results on deformed reinforcing bars in natural and recycled fine aggregate (RFA) concrete. The effects of bar location and RFA grade on bond strength between reinforcing bar and recycled aggregate concrete (RAC) were evaluated through the experimental program. A total of 150 pullout specimens were fabricated for the experiment. Two reinforcing bar orientations were considered with respect to the casting direction; vertical bars and horizontal bars, the latter of which was prepared to evaluate top-bar effect. Considered variables included four RFA replacement ratios (RFArs), two water-absorption grades (RFA-A: 5.83%, RFA-B: 7.95%) of RFA and three reinforcing bar locations (75, 225 and 375 mm height from the bottom of the casting mold). In addition, to evaluate the thermal and aging effect on bond behavior between the reinforcing bar and RFA concrete, some parts of pullout specimens had exposed to rapid freeze–thaw environment or been cured at air during 28 or 730 days. Test results demonstrated that bond strength does not seem to be affected by the RFAr for higher RFA grades (RFA-A), at least up to 60% RFAr. In contrast, the RAC including lower RFA grade (RFA-B) showed clear decreases in bond strength with increasing RFAr, similar to the trend observed for compressive strength. For horizontal pullout specimens, RFA concrete specimens showed higher bond strength gap between top and bottom bars than natural aggregate concrete (NAC) specimens. Bond strengths of the horizontally cast pullout specimens were affected by the flowability of concrete rather than the RFAr or RFA grade. No noticeable degradation occurred during freeze–thaw cycling of the RAC specimens, indicating that the RFA used in this study is appropriate for use in freeze–thaw environments.     

CFRP筋的疲劳性能

进行了不同循环应力下CFRP筋的常温疲劳试验,选取CFRP筋70％的极限拉伸应力作为最大循环应力,在R=0．5（最小循环应力与最大循环应力的比）和R=0的应力率下,测量了CFRP筋的疲劳寿命曲线,研究了CFRP筋的疲劳性能．结果表明：CFRP筋的最大循环应力应控制在70％极限拉伸应力以下;在R=0．5应力率下,最大循环应力下降5％的极限拉伸应力时,CFRP筋的疲劳寿命增长10倍左右;最大循环应力分别为60％和50％极限拉伸应力时,R=0应力率下CFRP筋的疲劳寿命分别为R=0．5应力率下疲劳寿命的百分之一和十分之一．这说明CFRP筋具有很大的脆性,需要有足够的强度安全系数,才能发挥作用;根据试验拟合的疲劳寿命曲线,CFRP筋的疲劳性能远高于Q235光圆钢筋．     

Experimental investigations on the fatigue behaviour of cold-worked deformed bars

The importance of fatigue of reinforcing steel in reinforced concrete structural members is discussed. The mechanism of fatigue failure of reinforcing bars and the factors affecting it are explained. The details of an experimental investigation for the determination of the fatigue characteristics of cold-worked deformed bars embedded in concrete beams are described. The results of the fatigue tests are discussed and recommendations for design are given.     

A study on the bond strength of tension lap splices in self compacting concrete

The primary objective of the research reported in this paper was to evaluate the effect of using self compacting concrete on the bond strength and mode of bond failure of tension lap splices anchored in normal strength concrete (NSC). Studies on the effect of transverse reinforcement on anchored reinforcement in self compacting concrete to prevent brittle mode of failure is limited. To meet this objective, full-scale NSC beam specimens were tested. Each beam was designed with bars spliced in a constant moment region at mid-span with various levels of stirrup confinement. The slip of the reinforcement with respect to concrete is also measured by providing notches at the end of the splices. Test results indicated that there is an increase in the bond strength when self compacting concrete is used in place of vibrated concrete. Ductility and splice strength increased as the confinement increased. When the stirrup spacing is less than 150 mm, the failure in the splice region was by yielding of steel. The influence of confinement on the crack formation of the beams is also reported.     

Influence of damage on the fractal properties of concrete subjected to pure tension

In a previous work [1] the authors showed how to acquire the meso-structural characteristics of undamaged concrete-like materials by a peculiar laser equipment [2]. In order to extend the analysis to damaged disordered materials, a new direct tension test equipment has been developed, that minimizes flexural effects by freely rotating boundary conditions. Increasing levels of damage are obtained, after reaching the peak load, by proceeding along the descending strain-softening curve. After the desired damage level is reached, the load is removed and the specimen is cut to permit the laser acquisition of the most damaged zone. The progressive rarefaction of the effective stress-carrying cross section is described by means of fractal concepts. It is worth noting that both the fractal dimension and the measure of the stress carrying cross section decrease after the peak load, and vanish when the specimen is broken apart.

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Résumé Lors d'une précédente étude [1], les auteurs ont démontré la possibilité d'apprécier les caractéristiques méso-structurelles des mattériaux désordonnés, comme le béton, au moyen d'un dispositif laser [2]. Afin de pouvoir étendre l'analyse à des matériaux désordonnés endommagés, il a fallu mettre au point un équipement d'essa à traction directe, pour minimiser les effects de flexion tout en permenttant la rotation des sections de extrémité de l'échantillon. Des niveaux d'endommagement croissants sont obtenus par l'application d'une charge supérieure à la valeur de pic, suivant la courbe décroissante de radoucissement (strain-softening). Une fois le niveau d'endommagement souhaité atteint, la charge est enlevée et l'échantillon est coupé. Le dispositif laser est ainsi capable d'analyser la section la plus endommagée. La raréfaction progressive de la section effective est décrite en utilisant des concepts appartenant au domaine de la géométrie fractale. Il est à la fois possible d'observer la réduction de la mesure de la section effective et la diminution de la dimension fractale après que la charge appliquée a franchi le pic, jusqu'à l'annulation lorsque les deux moitiés de l'échantillon sont complètement séparées.

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Editorial Note Prof. Alberto Carpinteri is a RILEM Senior Member.     

Effect of loading rate on the strength of concrete subjected to uniaxial tension

In the context of an international co-operation project between the University of Delft (The Netherlands) and the LCPC, an experimental study was made of rate effects in the behaviour of concrete under tensile stress. Very high speed tests ( between 1 and 80 GPa s⁻¹) were carried out in Delft on a Hopkinson bar, and quasi-static tests ( between 5×10⁻⁵ and 5×10⁻³ GPa s⁻¹) were carried out by the LCPC on a hydraulic press. This investigation had two objectives. 1. To verify on a mini-concrete (diameter of the largest particles 10 mm) a result obtained with a micro-concrete (diameter of the largest particles 2 mm) in the course of a previous study. Rate effects are produced by the presence of pore water in the material. 2. To investigate the influence of the water/cement ratio (i.e., the compressive strength of the concrete) on these rate effects. The three main conclusions that can be drawn from this study are (i) it is indeed the presence of pore water in the concrete which is at the origin of rate effects where this materials is concerned, (ii) the effect of speed on the tensile strengthf ₁ increases with the water/cement ratio, and (iii) in absolute value, the increase in strength (f _tdyn-f _tstat ) seems to be independent of the water/cement ratio.

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Resume Dans le cadre d'une coopération internationale entre l'Université de Delft (Pays-Bas), et le Laboratoire Central des Ponts et Chaussées (LCPC), une étude expérimentale a été menée sur les effects de vitesses dans le comportement en traction du béton. Les essais très rapides ( compris entre 1 et 80 GPa s⁻¹) ont été réalisés à Delft sur une barre d'Hopkinson, les essais quasi-statiques ( compris entre 5×10⁻³ et 5×10⁻⁵ GPa s⁻¹) étant réalisés au LCPC sur une presse hydraulique. Deux objectifs sont visés dans cette étude: (i) vérifier sur un mini-béton (diamètre du plus gros grain égal à 10 mm) un résultant obtenu sur micro-béton (plus gros grain égal à 2 mm) lors d'une étude précédente: c'est la présence d'eau libre au sein du matériau qui induit les effects de vitesse; et (ii) étudier l'influence du rapport eau/ciment (c'est-à-dire de la résistance en compression du béton) sur ces effets de vitesse. Les conclusions principales que l'on peut tirer de cette étude sont les trois suivantes: c'est bien la présence d'eau libre dans les bétons qui est l'origine des effects de vitesse relatifs à ce matériau; l'effet de vitesse sur la résistance en traction f₁ augmente avec le rapport eau/ciment; et en valeur absolue, l'augmentation de la résistance (f _tdyn-f _tstat ) semble indépendante de ce rapport eau/ciment.

     

Modeling the effect of high temperature on the bond of FRP reinforcing bars to concrete

The effect of high temperature on the bond between fiber reinforced polymers (FRP) reinforcing bars (rebars) and concrete was studied. The bond strength exhibited a severe reduction of 80–90% at relatively low temperature (up to 200°C), accompanied by changes in the pullout load-slip behavior. A semi-empirical model was developed in order to describe the extent of reduction in the bond strength as the temperature rises. The model is based on the following parameters: glass transition temperature of the polymer layer at the surface of the rod; polymer's degree of crosslinking; the residual bond strength at high temperature after the polymer of the external layer of the rebar ceased to contribute to the bond. The parameters of the rods that were tested for pullout at various temperatures were introduced into the model. The output curves of bond–temperature relationships showed good agreement with the test results.