Alteration in concrete properties depending on conditions and setting period after hydrothermal treatment

Abstract

The ultimate compressive strengths of specimens exposed to hydrothermal treatment increase steadily during hardening under moist conditions. Hardening in extremely moist conditions (in wet chips, for example) is particularly effective. After 14 days hardening in an atmosphere with a humidity of 95% and a temperature of 18–22°C, the strength of the concrete rises by 16–28% and can reach standard values. Where the hardening temperature is about 8–15°C, conditions of maximum moisture should be obtained so that the rate of increase of the strength will be of the same order. Determinations of water absorption give a picture of the structural changes in concrete for various types of conditions during hardening. When the concrete dries out, its open porosity increases by 10–30%. Hardening in moist conditions after hydrothermal treatment raises the frost resistance of concretes without additives. Hardening in the air has no beneficial effect. Concretes containing a complex plasticizer (sulfite liquor) or an air-entraining agent (air-entraining resin) also exhibit a frost resistance several times greater than additive-free concretes after hydrothermal treatment. If concretes are to attain their design properties after hydrothermal treatment, the most important factor is the provision of a very humid atmosphere for hardening in the initial stage. The duration of such a stage is about 14 days.