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It has been qualitatively suggested that the pressure method for the determination of air content in freshly mixed concrete can be in error when a portion of the entrained air bubbles becomes relatively incompressible due to the increase in internal bubble pressure which accompanies a reduction in bubble diameter. This has been used to explain the observed discrepancy between air meter readings and the air volume reported by microscopic analysis. This paper reports the results of an analytical investigation of the bubble-size effect, in which it is seen that it is unlikely that a sufficient volume of bubbles with the required diameter are present so as to cause the observed discrepancy. In order to demonstrate a significant influence of bubble size on the accuracy of the air meter, one must resort to values for the surface tension of the bubble film which are unjustified. The characteristics of various meters are discussed.
Associate professor, Cornell University, Ithaca, NY
Stock #: CCA10028J