Performance of high-alumina cement concrete strored in water and dry heat at 25, 35 and 50oC

Abstract

in Canada, is nevertheless frequently used in this country for special applications. Canadian Standards Association Committee A 23.1, Concrete Materials and Methods of Concrete Construction, in Appendix A of its 1977 edition has recommended against the use of this cement for structural purposes because of the recent structural failures associated with its use in England. This investigation is a continuation of the work undertaken in 1975 at CANMET and involves a study of the performance of high-alumina cement stored in water and dry heat at 25°, 35° and 50°C. A series of 0.056-m 3 (2-ft 3 ) concrete mixes was made in the laboratory using crushed gravel and natural sand as coarse and fine aggregates respectively. Thirty 102 x 203-mm (4 x 8-in,) cylinders were cast from each mix. Following the initial moist-curing period of 24 hours at 18°C, one third of the cylinders were subjected to standard moist curing at 21°C, one third were cured in water at 25°C and one third were cured in dry heat at 25°C. A similar procedure was repeated for 35° and 50°C exposure conditions. The curing period varied from 1 day to 365 days for each exposure condition. At selected ages, the weights and pulse velocity of the cylinders were determined before the specimens were tested in compression. Small samples of mortar from the tested specimens were examined by X-ray diffraction (XRD) and differential thermal analysis (DTA) to determine the phase composition of thé samples and hence to estimate the degree of conversion. High-alumina cement concretes exposed to moist curing at 21°C show continuous gain in strength at least up to one year regardless of the water/cement ratio of the concrete. However at 25° and 35°C under both dry and humid conditions there is a loss in strength with age; the loss in strength increases with an increase in the water/cement ratio of the concrete. For continuous exposure at 50°C under both humid and dry conditions, the change in strength with age depends upon the water/cement ratio of the concrete. At a water/cement ratio 0.31 there is only an insignificant drop in strength at early ages, in spite of the high degree of conversion, following which the strength increases with age, reaching a value of 125 per cent of one-day strength at age 365 days. This is significant as it suggests a means by which very high strengths can be achieved and maintained at least up to one year. The degree of conversion should not be used as a measure of compressive strength of high-alumina cement concrete because of lack of correlation between the two parameters.