Raoul François

Analyzing crack width to predict corrosion in reinforced concrete

Our aim in this paper is to introduce a set of relationships linking the distribution of reinforcement corrosion and the width of cover crack that results from such corrosion. This work is based on experimental results obtained on the longitudinal reinforcements of two beams naturally corroded over periods of 14 and 17 years. We first compared these experimental results with existing models linking crack width and attack penetration. Noting that such models only partially predict actual experimental data, we put forward a new model using the parameter of reinforcement cross-section loss.

Effect of the leaching of calcium hydroxide from cement paste on mechanical and physical properties

This paper deals with the effect of the leaching process of cement based materials on their mechanical and physical properties. In order to characterize this effect, we have performed experiments on cement paste samples. The leaching process was achieved by the use of a 50% concentrate solution of ammonium nitrate. Both compression tests and water porosity tests were conducted on micro-cylinder samples (10, 12, 14 and 20 mm of diameter) because of the slow kinetics of degradation due to the leaching. The deterioration of the cement paste and the mortar exposed to the action of the ammonium nitrate was manifested by a peripheral zone of less resistance. This process induces mainly a total leaching of Ca(OH)2 and a progressive decalcification of CS H which leads to a gradient of CS ratio in the leaching zone. Both mechanical tests and water porosity tests show that there is a linear variation of the loss of strength and the increase in porosity in relation to the ratio of degraded area over total area of the sample AdAt. It means that both compressive resistance and water porosity of the leaching zone are constant whatever the size of the degraded zone and then whatever the time of exposure to the chemical attack. So we could venture the hypothesis that the dissolution of calcium hydroxide is the essential parameter governing both decrease in strength and increase in porosity.

Mechanical behaviour of corroded reinforced concrete beams—Part 1: Experimental study of corroded beams

Steel corrosion in reinforced concrete leads to crack occurrence along the reinforcement (secondary cracks), to a reduction in bond strength and a reduction in steel cross section. The purpose of this study is to determine the effect of these deteriorations on the global behaviour of reinforced concrete structural elements in their service and ultimate states. Mechanical experimentation was carried out on fourteen-year-old reinforced concrete beams, on two control elements and two corroded beams. A comparative analysis of the results obtained on the beams showed that concrete cracking in the compressive area had no significant influence on the behaviour in service of the corroded elements. However, significant modifications of service behaviour were observed, due to the degradations in the tensile zone, namely: loss of bending stiffness, dissymmetrical behaviour. Finally, the measure of the residual steel cross-section of the corroded re-bars showed that the loss of bending stiffnes due to steel corrosion cannot be merely explained in terms of steel cross-section reduction. Concerning the ultimate behaviour, the loss of steel cross-section is the main parameter which leads to a reduction of bearing capacity and ductility. Another part will explain the separate and coupling effects of bond strength and steel cross-section loss on the mechanical behaviour of corroded beams.RésuméLa corrosion des armatures dans le béton armé entraîne une fissuration du béton d’enrobage qui traduit à la fois une réduction de section des aciers et une perte d’adhérence acier-béton. L’objectif de ce travail est d’étudier l’influence de ces dégradations sur le comportement global, en service puis à rupture, des éléments de structure. Pour ce faire, des expérimentations mécaniques ont été réalisées sur des poutres corrodées, ceci en comparaison avec le comportement d’éléments témoins. L’analyse comparative des résultats obtenus a montré que la fissuration du béton comprimé n’a pas d’influence sur le comportement en service. En effet, ce sont les dégradations en partie tendue qui entraînent des modifications importantes, notamment une réduction de la raideur en flexion et un comportement dissymétrique. De plus, l’analyse des taux de corrosion le long des armatures tendues semble montrer que la perte d’adhérence acierbéton a une influence très significative. En ce qui concerne le comportement à rupture, la réduction de section d’acier est le seul paramètre déterminant. Elle entraîne une diminution de la capacité portante et de la ductilité. Une seconde partie sera dédiée à l’étude plus détaillée de l’effet couplé ou découplé de la réduction de section d’acier et de la perte d’adhérence acier-béton en partie tendue sur le comportement mécanique local et global des poutres corrodées.

MODELLING THE LOSS OF STRENGTH AND POROSITY INCREASE DUE TO THE LEACHING OF CEMENT PASTES

Abstract Pure water leads to a progressive decalcification of hardened cement pastes. The physical and chemical processes of this degradation are relatively well defined today, but the consequences of the leaching on the porosity increase and the loss of strength have been only partially studied. Our work consists of correlating the alteration of these properties with the rate of degradation of the material. The leaching is achieved by an accelerated process. Compressive strength and water porosity are measured after different processing times in an aggressive solution. A physical phenomenon analysis leads us to propose a stress distribution model at rupture, and a calculation model of the porosity increase, taking into account the nature of the degraded hydrates (CSH).

Mechanical behaviour of corroded reinforced concrete beams—Part 2: Bond and notch effects

This paper deals with the effect of steel cross-section and bond strength reduction on the mechanical behaviour of corroded RC-beams.In the case of corroded reinforced concrete members, those effects are always coupled and a previous study (part one) has shown that it is not realistic to forecast the behaviour of corroded beams merely in terms of steel cross-section reduction. The object of the study is thus to understand the separated and coupled effects of the reduction in bond strength and steel cross-section. These investigations are carried out in order to be able to model the behaviour of corroded structural members and to predict how and when repairing is necessary. Different experimental simulations of corrosion were made. The results show the significant impact of coupling between reduction of bond strength and steel cross-section.RésuméCet article traite de l’influence de la réduction de section d’acier et de l’adhérence acier béton en partie tendue sur le comportement mécanique des poutres corrodées.L’étude précédente (partie 1) semble montrer qu’il n’est pas réaliste de vouloir prédire le comportement mécanique des éléments de structures corrodées en ne tenant compte que de la réduction de section des aciers tendus. Par conséquent, l’objectif de ce travail est de mieux comprendre et quantifier les effets couplés et découplés de ces deux paramètres. Pour cela, plusieurs simulations expérimentales des effets de la corrosion ont été réalisées sur poutres ou sur échantillons d’armature non corrodés. Les expérimentations mécaniques sont réalisées en service et à rupture. Les résultats obtenus confirment largement qu’une prédiction réaliste du comportement mécanique résiduel en service des poutres corrodées, ne sera obtenue qu’en prenant en compte de l’effet couplé de la réduction de section d’acier et de l’adhérence acier béton en partie tendue.

Quality of steel–concrete interface and corrosion of reinforcing steel

Abstract This article deals with the influence of steel–concrete interface defects on reinforcing steel corrosion. The defects that are analyzed in this paper relate to the gaps caused by bleeding, settlement and segregation of fresh concrete under horizontal reinforcing bars. These defects are increasing with the concrete depth below the horizontal reinforcement and depend on the bleeding capacity of concrete mixture. Various concrete mixtures including self-compacting concrete (SCC) were tested. The defects at the interface were characterized by the ultimate bond strength recorded in a pullout test and by the defect length under the reinforcement measured with a videomicroscope. The results indicate a good correlation between these two characterization methods. The corrosion was measured by the resistance of polarization and corroded surface area. The results allow us to conclude that the quality of concrete and steel–concrete interface, decreasing with height of concrete section, affects directly the corrosion rate.

Penetration of chlorides in relation to the microcracking state into reinforced ordinary and high strength concrete

Three types of concrete with compressive strengths of about 45, 80 and 100 MPa are exposed in a loaded state in a closed container and subjected to a sequence of wetting by salt fog and drying. For each concrete, the measurements of chloride content on three prismatic specimens allow us: 1/ to study the profiles of chloride ingress into concrete, 2/ to compare the different results obtained in relation to the loading state of each specimen, and 3/ to calculate the apparent diffusion coefficient Da.The results show the influence of concrete strength and the effect of the load applied on the penetration of chlorides into concrete. A linear relationship is found between the chloride’s apparent diffusion coefficient Da and the concrete strength. There is also a relationship between Da and the loading level. These relationships are established independently of the exposure period.RésuméTrois types de bétons de résistance à la compression de 45, 80 et 100 MPa sont conservés sous sollicitation mécanique dans une enceinte et soumis à des cycles alternés de pulvérisation par un brouillard salin et de séchage. Pour chaque béton, la mesure de la concentration des ions chlorures effectuée sur trois échantillons prismatiques, nous permet: 1/ de tracer les profils de pénétration des ions chlorures dans le béton, 2/ de comparer les résultats obtenus en fonction du niveau de chargement de chaque échantillon, 3/ de calculer le coefficient apparent de diffusion Da.Les résultats font apparaître l’influence de la résistance en compression du béton et l’influence du chargement mécanique sur la pénétration des ions chlorures dans le béton. On trouve ainsi une relation linéaire entre le coefficient apparent de diffusion Da et la résistance en compression du béton. On établit également une corrélation entre Da et le niveau de sollicitation mécanique. Ces relations sont établies indépendamment du temps d’exposition.

Effect of Reinforcing Bar Orientation and Location on Bond with Self-Consolidating Concrete

Self-consolidating concrete (SCC) is more workable than vibrated concrete (VC) when it is fresh. This workability supports the use of SCC in congested and restricted areas without adding vibration. However, there are also some differences in the concretes once they have hardened. This article reports on a study of the effect of reinforcing bar orientation and location on steel-concrete bond behavior. The study used SCCs and two VCs and the concrete casting direction is always vertical. For small-size concrete elements, SCC25 shows a better resistance against bleeding than VC25. The difference, however, is not significant for SCC40 and VC40. For samples reinforced with ribbed bars, the orientation of the bars (horizontal or vertical) has a significant and equivalent influence on both 25 MPa (3625 psi) concretes. The VC40 and SCC40 bond strength values are almost equivalent and not affected by the orientation of the bars. For tall concrete elements, voids formation under the horizontal bars was clearly observed for every type of concrete. The authors note that the size of the voids was almost equivalent for SCC25, SCC40, and VC40, but significantly larger in the case of VC25, especially near the top casting surface. Finally, the ultimate bond strengths (not affected by the casting conditions) obtained were approximately 20% higher for SCC than for VC, regardless of the concrete strength, for samples reinforced with ribbed bars.

Effect of loading on carbonation penetration in reinforced concrete elements

Abstract Concrete tensile microcracking due to a mechanical loading enhances the diffusion of aggressive agents from concrete cover to the reinforcements, which leads to the beginning of the propagation period of steel corrosion. The purpose of this work is to quantify the effect of concrete microcracking on CO 2 penetration. Concrete carbonation was studied on two 13-year-old reinforced concrete beams subjected to atmospheric carbonation. Indeed, these beams were exposed to the climate of the south-west of France but in a covered place and were always stored in a loaded state. Because the microcracking network is quite impossible to characterise, the tensile stress in the reinforcements was chosen as the main parameter of the microcracking state. This investigation results on a proposed model whose accuracy predicts the increase of the carbonation depth of the concrete in relation to the tensile stress in rebar.

Influence of creep and shrinkage on cracking in high strength concrete

Abstract Efficient use of high strength concrete (HCS) depends on a thorough understanding of its long term behaviour. However, its good mechanical performance must not blind its durability properties which are affected by cracking, as recent studies have shown. This paper deals with surface microcracking developed both by creep and shrinkage on young age loaded HSC. The HSC cylinders were stored for 400 days in an air conditioned room (50% RH and 20°C). Creep load up to 32 MPa was applied 28 hours after batching. Moreover, the effect of the desiccation was studied in unloaded samples. Microcracking was observed by SEM with replica technique and analyzed with the stereological tools of total projections. The main conclusion is that the intensity of the compressive creep load disturbs the orientation and density of surface microcracking due to desiccation and that recovery after unloading induces surface microcracking, which is strongly oriented in a given direction.