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.     

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.     

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 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.     

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.

     

Experimental investigations on polymer-modified concrete subjected to elevated temperatures

This paper presents the effect of elevated temperature and duration of exposures on polymer-modified concrete (PMC). Styrene Butadiene Rubber latex polymer solids were added in terms of 0, 5, 10 and 20 % by mass of cement. Curing of PMC specimen was done by the combination of wet and dry conditions. At appropriate ages, specimens were exposed to 200–800 °C for 1–3 h. The residual compressive strength was tested at 7 and 28 days. Micro structural properties were studied by XRD and SEM analysis. It was found that PMC and conventional concrete can be exposed to 400 °C for 3 h exposure without any adverse effect on strength properties. Addition of 20 % polymer was detrimental to concrete subjected to elevated temperature. Duration of exposure does not have much influence on the residual compressive strength properties of conventional concrete and PMC specimens.     

Design of reinforced concrete panels subjected to combined tension and shear

Abstract

This paper presents an analytical solution applying the principle of principal stress to the reinforced concrete panel subjected to an in‐plane loading with a combined shear and tension. The results are compared with the design equations specified in the current ACI Code (318–83). It is found that the amount of shear reinforcement in vertical and horizontal directions is controlled by the angle of the principal plane θ ₁. Based on the results of this study, coefficients C_k and C_v, are developed for the calculation of shear reinforcement in horizontal and vertical directions.     

Mechanical behavior of reinforced concrete subjected to impact loading

Understanding the behavior of concrete and reinforced concrete at high strain rates is of critical importance in a range of application. The behavior of concrete and reinforced concrete at strain rates of the order of 10⁴/s and pressure up to 1.5 GPa are studied experimentally. The concrete analyzed has the same composition and processing conditions as the matrix phase in the reinforced concrete. The dynamic compression experiments of reinforced concrete are carried out by one-stage light gas gun apparatus which subjects the reinforced concrete to deformation at strain rates of the order of 10⁴/s with confining pressures of 1–1.5 GPa. The voltage–time signals are recorded by the manganin pressure gauges embedded in the target. The stress–strain curves of reinforced concrete with different impact velocities are obtained using Lagrangian analysis, from which the distribution regulations of other mechanical parameters such as specific internal energy and specific volume in the flow field are acquired. Experimental results indicate that the load-carrying capacities of concrete and reinforced concrete increase significantly with strain rate. The concrete and reinforced concrete are non-linear, rate-sensitive and pressure-dependent.     

Fatigue behavior of concrete subjected to biaxial loading in the compression region

Concrete hollow cylinders subjected to combined compression and torsion were used to simulate concrete airport pavements subjected to biaxial fatigue loading in the compression region. It was found that the increase in the compliance in the post-peak period is due to the damage evolution of the specimen. The static failure mechanisms was explained by fracture mechanics. Similar failure was observed in fatigue loading. It was found that with the crack growth as a parameter, the static response acts as an envelope for the fatigue failure response. The rate of the crack growth under fatigue loading follows a two-stage process: a deceleration stage followed by an acceleration stage up to failure. In the deceleration stage, the growth is governed by the R-curve of the specimen. In the acceleration stage, it is governed by the Paris Law. The previously proposed model in the biaxial tension region was extended to the biaxial compression region. In the biaxial compression region, static and fatigue behaviors under pure compressive loading were modelled in terms of inelastic displacement, rather than crack length.     

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.     

混凝土材料动态力学性能的实验研究

本文利用改装的直锥变截面式φ７４ｍｍ大尺寸Ｈｏｐｋｉｎｓｏｎ压杆对混凝土材料试件（φ７２×３６）进行了冲击压缩实验。为了避免这种装置加载波波头部分对混凝土材料实验数据精度的影响,在实验过程中还采用了予留间隙法。实验结果表明,混凝土材料不仅具有敏感的应变率效应,还具有十分明显的损伤软化效应。