Takashi Idemitsu

The Design Method for the Demolition of Concrete with Expansive Demolition Agents

Recently, some chemical expansive agents have a tendency to be used for demolition. With these agents, demolition work can be done in safety almost without any pollution, since high expansive pressure is obtained gradually by only mixing the agents with water and pouring the slurry into boreholes. To estimate the time for demolition works, it is necessary to clarify the amount of the expansive pressure acting on the inner surface of the boreholes and the mechanism of fracture of materials to be demolished. The authors carried out experimental and theoretical studies to clarify the problems mentioned above, and proposed a demolition design method.

INFLUENCE OF THE WETTABILITY AND ANGLE OF FORM ON THE CHARAC-TERISTICS OF SURFACE VOIDS IN SELF-COMPACTING CONCRETE

高流動コンクリートは材料分離抵抗性を高くした締固め不要なコンクリートであるために表面気泡を発生しやすい. 本研究では, 表面気泡発生に係る影響要因のうち, はく離剤の種類, 型枠の材質, 型枠の傾斜角度などに注目し, 表面気泡の発生メカニズムを検討した. 実験の結果, 深さと直径の比が大きい表面気泡ほどコンクリート内部に発生することを確認した. 次に, 水平型枠面上の液滴の最大高さから接触角を測定し, 型枠表面の濡れを数値化した. この手法で、型枠表面ごとの濡れを測定し, それらがモルタルの表面気泡に及ぼす影響を調べた. 更に, 人工的に型枠表面に気泡を発生させて, 型枠に付着する気泡の最大容積と型枠角度との関係を調べ, 表面気泡の低減方法について検討した.

A METHOD FOR THE DESIGN OF PC BOX GIRDER BRIDGES WITHOUT CROSS GIRDERS

現場工事にとって施工の省力化とコスト低減は, 最も重要な課題である. 近年, 施工の容易さから大型の1室箱断面を有するプレストレストコンクリート橋が採用される例が多い. 本研究は, 現場施工の省力化を目ざすために箱桁内の中間横桁を省略することを提案するもので, これにより生ずる応力や変形を求めるための簡易計算法を示し, 実際の設計に適用した場合の影響について考察した. これにより中間横桁の省略の可能性を示した.

Corrosion of steel in concrete under the influence of chloride.

The Japan Society of Civil Engineers has recommended the allowable chloride content of fine aggregate in RC and pretensioned PC member to be 0.1% and 0.03% by weight, respectively. In Kyushu about 90% of the fine aggregates used for concrete works are sea sand. However, the relationship between the salt content of concrete and corrosion of reinforcing bar and PC wire has not been clearly known and the basic concept for the recommended salt content is not firmly established yet. Under these circumstances, the authors have made experimental studies on rusting of reinforcing bar and PC wire in concretes.Although the experiments are still being conducted, the results up to date are as follows.(1) The measured values of salt contant in the specimens placed on land were found to be reduced to about one half of the designed amount, but in other cases in sea and on coast reached up to 1-2% near the surface. It is thought that the phenomenon for the latter cases was caused by infiltration of chloride ions.(2) Though the PC wires were covered with rich mix concrete, their rusting was remarkable as compared with that of reinforcing bars. In the case when the salt content of concrete was 0.2-2.0%, the pitting due to rust began influencing upon the ultimate tensile strength of PC wires.(3) When the salt content of concrete was 0.5-1.0%, the percentage of rusting weight of steel became remarkable. Every effort should be made to keep the salt content as low as possible when placing concrete.