A Mineralogical Consideration on Unusual Cracking of Concrete Structures containing Calcareous Aggregate

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  • MARU Akio
    Inner Mongolia Normal University, China Geological and Mineralogical Engeneering Co. Ltd.

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  • 石灰系骨材を使用して異常ひび割れが発生した構造物コンクリートの鉱物学的考察
  • セッカイケイ コツザイ オ シヨウシテ イジョウ ヒビワレ ガ ハッセイシタ コウゾウブツ コンクリート ノ コウブツガクテキ コウサツ

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Many types of unusual cracking occurs in concrete structures containing calcareous aggregates such as limestone or coral reef, but few studies have been conducted on these phenomena.<BR>It has previously been reported that this unusual occurence of cracking in concrete is due to an alkali-carbonate or an alkali-silica reaction which is derived from limestone containing fine clay or silica. However, the calcareous aggregates are produced in Japan contain a small amounts of silica and there are few dolomitic limestone deposits in Japan.<BR>A mineralogical study was therefore conducted on cracked concrete structure contained calcareous aggregates. The results indicated following common characteristics:(1) A large quantity portlandite cement mineral was formed.(2) Cement minerals such as 3CaO·Al2O3·8-12H20 or 4CaO·Al2O3. 3H2O were formed.(3) In cases where of 4CaO·Al2O3·3H2O appeared, gel filled cracks or air babbles were observed.(4) The components of gel, filled cracks and air babbles in concrete originated from within the cement. The gel was a biproduct of a reaction with in cement, and was not responsible for the cracking of concrete by swelling.<BR>It is thought that cracking in concrete contaning calcareous aggregates is caused by swelling, which is due to the formation of large amounts of portlandite and calcium alminate hydrate. It is also thought that this unusual reaction is related to the powder grade of the cement, the water: cement ratio in the concrete or inadequate curing during concrete placement. This study is on going and the results of which will be reported in the near future.

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