Noboru Sakata

Acoustic emission evaluation of an arch dam during construction cooling and grouting

Abstract Acoustic emission (AE) monitoring was conducted in order to ensure safe construction of an arch dam under severe climate conditions. The AE was detected during the secondary cooling in the winter time while the construction was suspended due to low temperature and heavy snow. In the spring time just before the construction restarted, AE was monitored during grouting into a joint of dam blocks. A multi-channel digital signal processing system was used to perform a quantitative AE waveform analysis based on the moment tensor analysis as well as the conventional AE parameter measurement. Low frequency AE sensors (15 kHz resonant) were employed to detect AE signals since the signals with ordinary frequencies attenuate greatly during the propagation in concrete of the dam. Twelve sensors were buried from the dam surface and four were mounted on the corridor floor inside the dam. Neither critical AE activity nor intense AE cluster were observed in the AE monitoring. Thus, it was confirmed that the dam was safe during both the secondary cooling and the grouting. The AE monitoring was proven to be very useful for the safety inspection of a dam under construction in severe conditions.

Durability of Combination Type Self Compacting Concrete After Twenty-one-year Exposure

Three large size specimens composed of a combination type self compacting concrete in the range of slump flow 45-76cm were manufactured 21 years ago. The concrete was placed without compaction. These specimens have been exposed to nearshore for 11 years, after that, these were exposed to 20km inland for another 10 years. Then the authors measured the properties of the concretes such as strength, porosity, carbonation depth, and chloride ion diffusivity. As a result, the porosity of the concretes was almost constant depending on neither slump flow nor flow distance. Then, it was found that the combination type self compacting concrete was more durable than conventional concrete with the same W/C. Thus, the carbonation depth of the self compacting concrete was 15-80 percent and the chloride ion diffusion coefficient was 1-10 percent of conventional concrete, respectively.21年前に，スランプフロー45～76cmの範囲で流動性の異なる3つの併用系高流動コンクリートを用いて，それぞれ締固めを行わずに，約6m流動させて大型ブロックを作製した。このブロックを11年間海岸近くに暴露した後，さらに10年間内陸部に暴露し，打設後21年経過したコンクリートの強度，空隙率，中性化深さ，塩化物イオン量などを測定した。その結果，材齢21年の試験体表面部の圧縮強度は流動距離によるばらつきが少ないこと，空隙率はスランプフローや流動距離によらず13～16%とほぼ同じであること，中性化深さは同一水セメント比の普通コンクリートと比較して15～80％程度であること，塩化物イオンの見掛けの拡散係数は同一水セメント比の普通コンクリートと比較して1/10～1/100程度であることが分かった。このことから，併用系高流動コンクリートは普通コンクリートに比べて高い耐久性を有することが確認された。