Partenaires et organisations internationales
(Anglais)
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A, B, HR, DK, FIN, F, D, IS, I, L, NL, N, P, RO, SI, E, S, CH, GB
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Résumé des résultats (Abstract)
(Anglais)
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Reinforcement corrosion is the main cause of damage to concrete structures. The location of corroding zones is usually done by potential mapping. However, the method provides no information on the rate at which corrosion occurs. Corrosion rate measurements on site are of great practical interest in controlling the effects of corrosion protection measures, in predicting the residual life of a structure and in deciding when remedial measures on a structure have to be taken. Several electrochemical techniques can be used to measure the corrosion rate of steel in concrete. Whatever the technique used, the main problem in measuring the polarisation resistance and, subsequently, the corrosion rate of rebars on-site is the non-uniform current distribution between the small counter electrode (CE) on the concrete surface and the large rebar network in reinforced concrete construction. The non-uniformity in homogeneous conditions (homogeneously passive reinforcement or homogeneously active corrosion) consists of the vanishing of electrical signals with increasing distance from the CE along the rebar as well as in non-homogeneous distribution of the signal around the rebar circumferences and between the rows of the reinforcement. In the case of local corrosion with small actively corroding areas coupled to large passive areas, the non-uniformity of the signal distribution is further influenced by the differing polarisability of the active and the passive areas. Methods based on the polarisation resistance method have been implemented in portable corrosion rate measuring instruments and used to evaluate rebar corrosion rate. Some corrosion measuring devices use the double counter electrode method: a central counter electrode (CE) together with an additional surrounding guard electrode (GE). Others use unconfined signals. GE electrodes are used in an attempt to confine the excitation of the CE current within a defined and constant area, so that errors caused by the lateral non-uniform current distribution along the length of a rebar can be eliminated. In order to investigate the effect of current distribution on the measurements of the corrosion rate when using a device with or without confinement, measurements of the polarisation resistance performed with two- and three-dimensional electrical networks have been numerically simulated for reinforced concrete beam and slab models. The models allow the simulation of on-site measurements. The calculations show that neither a modulated GE nor an unconfined CE are able to deliver accurate values of the polarisation resistance within the whole range of corrosion activity when the constant diameter of CE signal spread is used. Using the constant diameter of confined area for differing test condition leads in both cases to serious errors. At high corrosion activity, an 'overconfinement' takes place through the GE and may be a source of for serious underestimation of the corrosion rate. In such conditions, the non-confined measurements deliver values closer to the true corrosion rate. For the passive state of reinforcement, more accurate values of corrosion rate were achieved with the GE confinement. When the corrosion is not uniform, the corrosion rate of a corroding spot may be seriously underestimated by a factor as high as 18. However, errors are by factor 1.6 - 4 higher when an arrangement with GE confinement is used. For a more accurate assessment of the corrosion rate, actual testing conditions have to be considered. Assuming uniform rebar corrosion, it is possible to evaluate the actual corrosion rate from either confined or non-confined measurements on site, when parameters like concrete resistivity, concrete cover depth and rebar layout are known. The effects of these parameters on the errors of corrosion rate measurement have been evaluated with the use of 3-D modelling of on-site conditions with non-confined test arrangements and an appropriate correction coefficient has been introduced. The results of these calculations have been further used for the estimation of the corrosion rate of reinforcement in laboratory and on site measurements with a portable non-confined device based on the galvanostatic transient technique. Parallel to the performed galvanostatic pulse measurements, other corrosion risk parameters such as carbonation depth of concrete, concentration of Cl- ions and visually judged corrosion grade of rebars have been measured. There has been good correlation between the rebar corrosion risk parameters, the visually judged corrosion grade of rebars and corrected values of corrosion rate.
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