Fine cracks formed in steam curing of reinforced concrete elements

Atmospheric pressure steam curing has been widely used for precast concrete products, as the high curing temperature accelerates early concrete strength development. Though steam curing has an economic advantage over ordinary moist curing, it has one problem that should not be ignored. Sometimes fine cracks are formed on the surface of precast concrete products. It has been proposed that these fine cracks are formed during a certain stage of steam curing. The purpose of this investigation was to reveal the condition and mechanism of fine crack formation on the surface of steam cured concrete. A new X-ray technique, using a contrast medium, was developed and used to detect fine cracks on the surface of concrete. As a result of the study, three different types of fine cracks, namely mortar cracks, vapor cracks and separation cracks between aggregates and pastes were observed. They occurred at different stages of steam curing. The condition of developing cracks was also studied and compared with the standard specification of the Japan Society of Civil Engineers. (author)