THE EFFECTS OF IONIC MOBILITIES ON ALKALI-SILICA REACTION PROGRESSION IN CONCRETES

摘要

The external and internal factors affecting the migration of water and alkali ions through Portland cement concretes are reviewed. Laboratory cells have been designed to simulate some of the mechanisms identified and have been used to measure water, alkali solution and alkali ion diffusion rates over an extended period. Direct measurements of the apparent diffusion rates for samples of alkali reactive and non-reactive structural and laboratory concretes were determined. These results are related to the age and reactivity of the concrete for both water and 1M alkali solutions. All the concretes were found to be less permeable to alkali solution than to pure water. Diffusion rates for Na~+, K~+ and mixed alkali solutions through the same concretes were measured by direct chemical analysis of samples of the solutions extracted from special sealed cells each side of the 100mm by 30mm thick concrete test discs. Apparent diffusion coefficients were calculated using Fick's First Law. The coefficients increase with time and K~+ ions were found to diffuse through concrete more readily than Na~+ ions. The data obtained is compared with previously published data. The measured diffusion coefficients were found to vary depending on the concrete. The Gaussian Error Function solution to Fick's Second Law was used with alkali diffusion data and generally accepted threshold values of alkali concentration for reaction, to provide examples of model calculations showing how aft alkali-silica reaction may be expected to develop and progress in a concrete. The examples also allow a prediction to be made of the length of time in years that alkali-silica reaction may be expected to remain active in an affected concrete under specified environmental conditions.