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Detection of Thermal Cracks in Early-Age Concrete by AE

T. Watanabe, T. Ohno, C. Hashimoto

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.

Keywords


thermal cracks; AE method; blast-furnace slag; fly ash, early-age cracks

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References


Hashimoto, S., Hashimoto, C., Watanabe, T. and Mizuguchi, H. Study on Evaluation on Settlement Cracks occurring on the Concrete Surface above Steel Bars of Reinforced Concrete Slab by Nondestructive Testing, Proceedings of the 10th International Conference on Structural Faults and Repair, 2003, CD-ROM.

Ohtsu, M., Acoustic Emission Theory for Moment Tensor Analysis, Research in Non- Destructive Evaluation, ASNT, 1995, 6: pp.169-184.

Son, H. N., Hosoda, A. and Watanabe, T. Characterization of Microcracking in Very Early Age Concrete subjected to Elevated Temperature by AE, Proceedings of The Japan Concrete Institute, 2001, 31: pp. 323-328


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