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Twenty laboratories from ten countries took part in an international comparative test to determine the air-void system of hardened concrete. The method of measurement did not seem to have a significant influence on results, but good grinding techniques and the magnifications were important. Further tests were performed at the Viennese Cement Research Institute with the aim of improving the precision of the results. It is recommended that the entrapped and the entrained air voids be counted separately and that the spacing factor be calculated from the entrained air voids only.

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