Detection of Thermal Cracks in Early-Age Concrete by AE

Abstract

Early-age cracking after casting is a fundamental problem for the durability of concrete structures.   Recently, thermal cracks and early-age cracks have been observed in concrete containing Portland cement with blast-furnace slag in Japan.  In order to reduce the number of these cracks, fly ash is substituted for cement or sand.  Fly ash is one of the popular admixtures as a waste material from coal-fired power plants.  To evaluate the effect of fly ash on early-age cracks, experimental studieswere performed.  In order to detect micro-cracking in concrete, acoustic emission (AE) measurement was applied.   In experiments, two types of fly ash, typeⅡ and Ⅳ categorized by JIS were employed. The ratio to substitute cement and sand was 10% and 20%.  As a result, micro-cracks in concrete were detected by the AE method.   Resultsshow that substitution of fly ash for cement and sand is effective to reduce micro-cracking due to thermal changes which occurs from 20 to 30 hours elapsed after casting of concrete.