Reinforced Concrete structures degradation issues impact of Chlorides and of Thermal loadings on Corrosion development
Résumé
Reinforced concrete structures of Fukushima Daiichi Nuclear Power Station have been submitted temporarily to specific environmental conditions such as thermal loading (up to 200DC) and chloride ingress (from seawater) [1]. These conditions are susceptible to lead to corrosion of reinforced concrete components of the NPP. This paper aims at studying environmental conditions encountered by concrete structures, and to propose pertinent parameters for reinforcement corrosion monitoring. The first step of the scenario consists in considering that concrete has been in contact with seawater during several days (we estimate a duration of 15 days in this paper) then pure water replace seawater for cooling fuel. A coupled chemistry-transport software (Hytec) has been used to simulate a CEM V type concrete submitted to such scenario (at ambient T DC). Results show that chloride penetrate into concrete until several millimeters during the 15 days in contact with seawater. As soon as pure water replace seawater, chloride species are leached out from the material. After 2 years, calculations estimate that almost all chloride species disappeared from concrete cover. This result show that reinforcement bars located few centimeters inside concrete have not seen chloride species and should not developed chloride-induced-corrosion. Concrete structures have been submitted to thermal loadings as temperatures were above 200 DC just after the accident, and they stayed above 100 DC for about a half year [1]. Then seawater has been injected inside concrete structures to cool fuel. Previous studies [2] determinate that in such conditions, cold part of the structure is damaged and cracked. Thus we can propose that in a first step, external part of concrete structures have been cracked, then internal parts at high temperature put in contact with seawater at ambient temperature would also cracked. In this context, chlorides from seawater can penetrate through cracks and the degradation issue is linked to corrosion of rebars in cracked concrete. In such conditions, field and laboratory experiments [3, 4] show that quality of steel/concrete interface is the main parameter that has to be considered, more than chloride threshold and crack width as it is commonly specified in codes.
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