Mechanistic Model for the Durability of Concrete Barriers Exposed to Sulphate-Bearing Groundwaters
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MECHANISTIC MODEL FOR THE DURABILITY OF CONCRETE BARRIERS EXPOSED TO SULPHATE-BEARING GROUNDWATERS
ALAN ATKINSON AND JOHN A. HEARNE Materials Development Division, Building 429, Harwell Laboratory, Oxon. OXI 1 ORA, UK.
ABSTRACT' The durability of concretes in radioactive waste repositories is likely to be limited by their reaction with sulphate-bearing groundwaters. In such a situation the concrete slowly degrades by sulphate penetrating into the concrete where it reacts with the cement to cause stress and eventual disruption. Experiments have been carried out to quantify these reactions and the resulting expansions. The data have been incorporated into an overall mechanistic model for sulphate degradation in which the criterion for disruption is the accumulation of a critical amount of stored elastic energy in the reaction zone. The results of this model are in good agreement with those of an earlier empirical model and enable extrapolations to be made to different concretes and groundwaters. INTRODUCTION Concretes are favoured materials for the construction of radioactive waste repositories in many countries. They are likely to be used in considerable quantities in the UK in repositories for the disposal of low and intermediate level wastes. The concretes will probably be used to encapsulate the wastes, to provide the repository structure and to backfill around waste packages. (Here we use the word "concrete" as a generic term to encompass all materials based on Portland cement.) The concretes provide both physical and chemical barriers to the migration of radionuclides from the repository, and therefore it is necessary to be able to predict how the physical and chemical properties of the concretes are likely to change with time. The timescale over which these changes are of interest is much longer than that normally considered in conventional civil engineering applications of concrete and so mechanistic models are required in order to extrapolate data from short term experiments, or observations made in conditions different from those of the actual repository. In an initial appraisal of the likely alteration of structural concrete in a repository situated below the water table in the UK[l] it was concluded that, in many potential repository locations, reaction with sulphate in the local groundwater was the process which would most probably lead to eventual loss of the concrete's mechanical integrity; albeit very slowly. From a survey of results from accelerated tests and long term field experiments the following empirical expression was proposed describing the degradation rate from which the mechanical durability of concrete in sulphate-bearing groundwaters might be estimated; R(mm y-1) = 5.5 CA(%) c0 (M).
(1)
where R is the degradation rate, CA the tricalcium aluminate content of the cement and co the sum of concentrations of sulphate and magnesium ions in the groundwater. This expression was also shown to account for the observed penetration into old concrete exposed in clay for about 40 years[2]. However,
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