Stéphane Laurens

Numerical study of the linear polarisation resistance technique applied to reinforced concrete for corrosion assessment

The linear polarisation resistance (LPR) technique is increasingly being implemented to assess the corrosion rate of steel reinforcements in concrete. However, a lack of reliability is often observed experimentally. In order to improve the physical comprehension of the LPR technique, FEM simulations were performed. Several measurement devices using annular and rectangular probe geometries were simulated and the numerical experimentation allowed the relevance of RILEM recommendations for on-site measurements to be tested. It was shown that the LPR measurement using annular counter-electrodes with confining devices was a real three-dimensional physical problem and the non-uniform distribution of the polarising current on the steel surface is actually a major source of error since it is not taken into account by the usual protocols. Moreover, it was found that some complexity was induced by the use of guard-rings. La technique de résistance de polarisation linéaire (Rp) est utilisée de plus en plus fréquemment pour évaluer la cinétique de corrosion des aciers dans le béton armé. Cependant, la technique souffre d’un manque de fiabilité. Afin de mieux appréhender la physique de la mesure, des simulations numériques ont été réalisées. Plusieurs dispositifs ont été simulés. Ces expériences numériques ont permis d’éprouver la robustesse du protocole RILEM dédié à la mesure de Rp in situ. Les résultats montrent que les mesures réalisées au moyen de sondes annulaires constituent un problème physique tridimensionnel qui doit être mieux pris en compte par le protocole RILEM. En particulier, la distribution non uniforme du courant polarisant constitue une source d’erreur importante dans la mesure où elle n’est pas considérée dans la recommandation RILEM. Enfin, les simulations ont révélé une forme de complexité supplémentaire induite par l’utilisation d’anneaux de garde pour le confinement du courant polarisant.

An alternative to the Lyapunov exponent as a damage sensitive feature

When using Lyapunov exponents as a damage sensitive feature (DSF) in vibration based structural health monitoring, the computation time represents an important issue for real time applications. To overcome this problem, a new feature, the Jacobian feature vector (JFV), is proposed, based on the same algorithm but allowing faster computation. Whereas for Lyapunov exponents calculation, the trajectories in the state space are followed for more than a thousand time steps, the new feature is formed by the Jacobian matrix of the dynamics after a few time steps. The new feature and the Lyapunov exponents are tested on two case studies: a mass/spring/damper model and a laboratory three story structure. Both the DSF show a good performance in detecting damage for the numerical study, even if the Lyapunov exponents present a slightly better ability. Regarding the experimental data, Lyapunov exponents failed to detect damage while the JFV is clearly able to reveal structural changes. Therefore, the JFV offers a robust alternative to Lyapunov exponents and allows one to save computation time.

Non destructive evaluation of concrete contamination by chloride: Comparison of methods

ABSTRACT This paper aims to analyse the sensitivity of some NDT methods (radar, resistivity and capacitive technique) to chloride contamination. In laboratory, four different concretes with different water to cement ratio and two kinds of aggregates were conditioned with different saturation degrees of two solutions (30 g/l or 120 g/l of NaCl). NDT measurements show that the attenuation of radar waves is sensitive to the presence of chlorides but depending on the saturation degree of the solution while velocity is not affected. Resistivity is also very sensitive. The real permittivity measured at low frequency with capacitive technique is modified by the presence of chlorides. All the techniques were carried out on a real site. From the analysis of laboratory results, an attempt of evaluation of both moisture content and level of chloride contamination is proposed and compared to values obtained from cores extracted on a very near area.

An alternative to Lyapunov Exponent as Damage Sensitive Feature

When using Lyapunov exponents as damage sensitive feature (DSF) in vibration based structural health monitoring, the computation time represents an important issue for real time applications. To overcome this problem, a new feature, the Jacobian Feature Vector (JFV), is proposed based on the same algorithm but allowing faster computation. Whereas for Lyapunov exponents calculation, the trajectories in the state space are followed for more than a thousand time steps, the new feature is formed by the Jacobian matrix of the dynamics after few time steps. The new feature and the Lyapunov exponents are tested on two case studies : a mass/spring/damper model and a laboratory three story structure. Both the DSF are showing a good performance in detecting damage for the numerical study, even if the Lyapunov exponents present a slightly better ability. Regarding the experimental data, Lyapunov exponents failed to detect damage while the JFV is clearly able to reveal structural changes. Therefore, the JFV offers a robust alternative to Lyapunov exponents allowing to save computation time.

Effects of Reinforcement Corrosion on the Mechanical Behavior of Reinforced Concrete

The purpose of this chapter is to present the effects of reinforcement corrosion on the mechanical behavior of reinforced concrete. The corrosion of reinforcements leads to a reduction in the resistant cross-section of the bars (which modifies the constitutive relation of steel in tension) and to a modification of the steel-concrete bond phenomenon owing to the de-confinement of the bars, linked to the creation of the corrosion cracks: these two phenomena alter the mechanical behavior of reinforced concrete in terms of both bearing capacity and deflections in service and deflections at failure.

Which Parameter to Quantify Corrosion Intensity

The corrosion process is always described as an increase in the degree of corrosion as a function of time. For example, the universally-quoted Tuutti model uses the depth of corrosion parameter as the ordinate.

Predicting the Service Life of Structures

The aim of this chapter is to present the possible predictions concerning the different stages of aging in structures likely to corrode: corrosion initiation, corrosion propagation, prediction of the occurrence of corrosion cracks, service limit state (maximum openings of corrosion-induced cracks, spalling), ultimate limit state (loss of bearing capacity, loss of ductility). For this, the four-phase phenomenological model of the corrosion process is used which applies to all structures.

In situ Corrosion Diagnosis

The aim of this chapter is to present the different possibilities for establishing a state-of-corrosion diagnosis. We will not be referring to the determining of causes of corrosion linked to carbonation depth or to a certain rate of chlorides present in the vicinity of the steel reinforcements, but to the measurement of parameters linked to corrosion development.

Prediction of Bearing Capacity and Behavior in Service of Corroded Structures

The aim of this chapter is to present an assessment method using the calculation of corroded structures. Using the diagnosis of corrosion of a structure based on the analysis of the cracks created by corrosion, we propose to analyze the behavior of the structure and to determine its residual bearing capacity as well as its residual ductility, and therefore its capacity to redistribute force through the formation of a plastic hinge.

Steel Corrosion in Reinforced Concrete

Steel corrosion constitutes the most important cause of premature aging and deterioration on reinforced-concrete structures on an international scale. Its technical, financial and societal consequences are considerable. Its economic impact in particular is hard to assess as it would require incorporation of both direct costs (surveillance, maintenance, repairs and rehabilitation) and indirect costs (operating losses relating to immobilization of the structure, insurance, etc.) to define a relevant measurement of the cost relative to steel corrosion in concrete. Across all sectors as a whole, certain sources assess the total cost of corrosion at around 4% of GDP on average in industrialized countries. The specific case of steel corrosion in concrete certainly contributes significantly to this percentage and the sums allocated annually to the rehabilitation of corroded reinforced-concrete structures stands at billions of Euro.