コンクリートの水密性の研究

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  • STUDIES ON THE PERMEABILITY OF CONCRETE
  • コンクリート ノ スイミツセイ ノ ケンキュウ

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This paper describes the results of basic studies conducted on the watertightness of concrete using a method of testing the permeability of concrete devised by the author.<BR>The output method heretofore used as a means of testing water passage through concrete affords ease of analysis of permeability, but in the case of concrete of advanced ages with comparatively high density it not only requires a long period of time to perform the tests, but it is often impossible to measure any outflow of water. The author thus made an investigation of methods of testing permeability which could be applied to concrete of advanced age which had been thoroughly cured. The average depth of penetration of water forced under pressure into concrete was measured and the relation between this and the hydrostatic pressure and the period of time the pressure was applied was theoretically and experimentally studied. The coefficient of diffusion obtained from this relation was used as a measure of watertightness in the depth of penetration method of testing permeability. The reliability of this method was investigated and it was established that even with concrete of advanced age tests could be completed within a short period of time with comparatively little error in test results and that it was a satisfactory method of testing the watertightness of carefully placed and consolidated concrete. Therefore, this method was used in the study of the following : <BR>Experiments were carried out on the effect of the variety of cement, aggregate and admixtures, mix proportions, age of specimens, and methods of curing, on watertightness of concrete. It was found that the use of air-entraining agents and cement dispersing agents of good quality was a convenient means for safely making watertight concrete, especially in cases of lean mixes, when the particle shape of aggregate was unsuitable and when hydrostatic pressure applied was low.<BR>In studying the relation of water-cement ratio and watertightness of concrete in conjunction with the thickness of concrete, it was found that minimum unit water content consistent with achieving required workability was of extreme importance in economically obtaining a watertight concrete member.<BR>As the rate of increase with age in watertightness is greater at early ages, it is of importance to cure thoroughly at the beginning. However, in spite of thorough curing at early ages watertightness decreases greatly on subsequent prolonged drying and it is necessary to continue moist curing until exposure to water conditions in order to obtain a watertight concrete.<BR>It is important for concrete in gravity dams to have great watertightnesss and there has been much valuable research carried out on the watertightness of mass concrete. However, because of limitations of testing apparatuses, many of these studies have been made on mass concrete from which particles of around 60 millimeters or larger have been sieved out and there has been hardly any one that has considered the pros and cons of discussing watertightness mass concrete baseing on permeability results of such test methods. The author used mass concrete of five different mix proportions containing aggregate, the maximum size of which is 100 millimeter aggregate and by comparing the coefficient of diffusion of this mass concrete with concrete obtained by screening out particles larger then 40 millimeters and 25 millimeters from the mass concrete, ascertained that there was a nearly constant relation regardless of mix proportions. Using this relation, even though it may not be possible to conduct permeability tests of the mass concrete itself, an estimate of the watertightness of a carefully placed and consolidated mass concrete can be made to some extent from the test values of small specimens made concrete obtained by wet screening of the mass concrete.

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