Arnaud Castel

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.

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.

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.

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.

Structural performance of a 26-year-old corroded reinforced concrete beam

This article presents experimental results and the modelling of a highly corroded beam subjected to a chloride environment to assess the performance of long-term-corrosion-damaged beams. A reinforced concrete beam corroded by 26 years of exposure to a chloride environment was tested until failure, as was a control beam of the same age. Cracking maps and corrosion maps were drawn for the corroded beam. Force displacement curves for both beams and stress strain curves for the steel bars were also plotted. From the results it appears that corrosion has a significant impact on the load carrying capacity, stiffness and deflection of beams. The modelling approach using Macro Finite Element (MFE) shows good agreement with experimental results in terms of load capacity, stiffness of the beam and ultimate deflection. Cet article présente les résultats expérimentaux et la modélisation du comportement mécanique d’une poutre sévèrement corrodée par 26 années de conservation en atmosphère saline. Après avoir été testée en flexion jusqu’à la rupture, la poutre a été désossée afin de déterminer la carte de corrosion des armatures et de faire des tests de traction sur les armatures corrodées. Les résultats montrent que la corrosion des armatures a une influence importante sur la capacité portante, la raideur en service et la ductilité (flèche à rupture) L’approche de modélisation par macro-élement montre une bonne corrélation en terme de charge ultime, raideur et déplacement maximal.

Concrete cracking due to chloride-induced reinforcement corrosion – influence of steel–concrete interface defects due to the ‘top-bar effect’

This article deals with the influence of the steel–concrete interface condition on reinforcement corrosion and concrete cracking in a chloride environment. Four large concrete members were cast with horizontal reinforcements at different levels in order to create voids under the upper horizontal bars due to the well-known ‘top-bar effect.’ Then, the members were sawn into smaller specimens that included one rebar. Specimens obtained were submitted to wetting–drying cycles in salt water (natural corrosion process) until concrete cracking was observed. Results show that before concrete cracking, the corrosion pattern observed is completely different between the specimens with defect (generalised corrosion) or without defect (local pitting corrosion). The concrete cracking is delayed for specimens affected by the ‘top-bar effect’ in spite of a higher corrosion rate, and occurs at a significantly higher corrosion rate. A new model for uniform steel cross-section reduction calculation from concrete crack width in case of ‘Top-bar effect’ is proposed. L’étude porte sur l’influence de la qualité de l’interface acier-béton sur la corrosion des aciers et la fissuration du béton en milieu salin. Quatre murs en béton de grande hauteur comportant des armatures disposées horizontalement à différent niveaux ont été fabriqués afin de générer des vides par « Top-bar effect ». Ensuite les murs ont été sciés afin d’obtenir des éprouvettes de petite taille avec une seule armature. Les éprouvettes ont ensuite été soumises à des cycles d’humidification/séchage à l’eau salée (processus de corrosion naturel). Les résultats montrent qu’avant fissuration du béton le faciès de corrosion est très différent avec défaut d’interface (corrosion généralisée) ou sans défaut d’interface (corrosion par piqûre). La fissuration du béton est retardée en présence de défaut d’interface mais apparait pour un taux de corrosion bien plus important. Un nouveau modèle pour déduire le taux de corrosion généralisée en fonction de l’ouverture des fissures et en présence de défaut d’interface est proposé.

Study of the impact of localised cracks on the corrosion mechanism

Corrosion is one of the major problems that the reinforced concrete industry is facing nowadays. Much research has been carried out to investigate the influence of cracks on rebar corrosion. In this project ring shaped mortar samples with an 8mm steel bar inside were cracked with different crack widths and then subjected to continuous wetting and drying cycles in salt solution having 35 g/L of NaCl, to accelerate the corrosion process in laboratory conditions. The samples were broken after 1.5 years and 2.5 years to observe the corrosion pattern. Results show that cracks whatever their width allow the corrosion onset at the bottom of cracks and along the steel–concrete interface damaged zone caused by the creation of cracks. This process leads to corrosion cracks formation time dependant of the crack opening, which modify the environmental conditions at the level of steel leading to the generalisation of corrosion all along the re-bars. Cet article étudie les relations entre la fissuration pré-existante d’origine mécanique sur la corrosion des armatures du béton armé. Le programme expérimental utilise un cœur expansif permettant de générer des fissures d’ouvertures contrôlées dans des échantillons armés qui sont ensuite soumis à des cycles d’immersion-séchage en solution saline. Les échantillons ont été dépouillés après 1an et demi et 2 an et demi pour étudier l’évolution de la corrosion au cours du temps. Les résultats montrent que quelque soit l’ouverture des fissures, la corrosion démarre en fond de fissure et se propage le long de l’interface acier-béton endommagée en fond de fissure par la création de la fissure. Ce processus dont la durée est influencée par l’ouverture des fissures, conduit alors à la création de fissures de corrosion qui modifient l’environnement local au contact de l’armature et conduit alors à une généralisation de la corrosion.

Calculation of the Overall Stiffness and Irreversible Deflection of Cracked Reinforced Concrete Beams

The method proposed in this paper aims to assess both the irreversible deflection of cracked reinforced concrete beams when unloaded and their overall stiffness under loading cycles which is relevant for in-situ analysis of existing structures. Only service stage is modeled and time-dependent effects such as creep and shrinkage are not taken into account. The modeling approach combines an existing model based on an empirical moment-curvature analytic relationship with a new Macro-Finite-Element (MFE) both implemented in a linear finite element analysis. MFEs are Beam Finite Elements, characterized by their average moment of inertia. The average moment of inertia is calculated by homogenization of the steel strain, concrete strain and neutral axis modeled between two consecutive bending cracks. The results of the MFE model have been successfully compared to experiments. The irreversible deflection represents a significant proportion of the total deflection in service and the overall stiffness of the cracked beams during the loading cycles is strongly under-estimated by existing models based on empirical moment-curvature relationships.

La microfissuration superficielle a-t-elle une influence sur la profondeur de carbonatation des bétons?

ABSTRACT This work deals with the influence of the superficial microcracking on concrete carbonation. Three types of concrete with strengths ranging between 25 and 60 MPa were subjected to different thermo-hygrometric treatments to create a superficial microcracking, then, to accelerated carbonation tests for 3 months. After their period of preconditioning treatments, the quantification of the superficial microcracking and the measure of the water absorption of the concrete were carried out to observe correlations with carbonation depth. Results show that the influence of the superficial microcracking is strongly dependant of the concrete strength and begins to be significant when concrete strength reaches about 50 MPa.