United States Department of Transportation
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Abstract:Hollow censopheres of fly ash may have walls so thin that they will appear to be air voids when they appear on a polished slab prepared for air void determination by ASTM C457. Therefore the following precautions are recommended. 1. The operator of the linear traverse equipment should be aware of the appearance of fly ash and alert to avoid counting as air voids those transected fly ash cenospheres that are thick walled, dark colored, or frothy in appearance. 2. In cases where the air void parameters are barely within the specification and fly ash cenospheres are present, small allowances should be made for the bias caused by the fly ash. 3. If it is suggested that the particular concrete specimen is exceedingly rich in cenospheres which appear to be air voids, the lapped specimen of concrete can, after the air voids determination has been completed, be etched in a 0.3 N solution of HC1 for about 10 seconds to etch away the paste and reveal the glass shells of the cenospheres in relief. A second determination by linear traverse can then be made to count and measure only those fly ash cenospheres that could have been mistaken for air voids. The data is adjusted accordingly. This procedure is necessary only when the air void data are marginally within specification and it is suspected that clear, confusing cenospheres occur on the traverse line more frequently than one for every 50 mm (2 in.) of traverse.
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