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CURRENT TOPICS [J. F. J, Entrained Air Indicator.--A improved entrained air indicator, designed for faster and more accurate determinations of entrained air content in freshly mixed concrete, has been developed by the Central Scientific Co., Chicago.
Made of cast magnesium alloy, the new entrained air indicator is more durable than the former steel models and its lighter weight makes it more convenient for cement contractors to carry in the field.
Air-entrained concrete is one of the most recent developments of concrete research and is destined to save the taxpayers a great deal of money in highway maintenance costs. The mixture contains billions of microscopic air cells which provide expansion chambers for water when it freezes inside the concrete, thereby reducing internal pressures which cause cracking, chipping and peeling.
The amount of entrained air required is usually between 3 and 6 per cent of the volume of the concrete, but may be varied under special conditions. Determining and controlling the percentage of entrained air is done by the Cenco air indicator which tests concrete samples by the pressure method. Air-entrained concrete is produced by introducing small amounts of resinous or fatty materials such as beef tallow or agents of the soap family.
A recent survey of fourteen experimental highway sections has proved that pavements made with air-entrained concrete withstand severe winter exposure, including frequent applications of ice-removing chemicals. Currently, thirty of the forty-eight states require its use in all pavements and eleven additional states permit its use under certain conditions. In addition to highway projects, air-entrained concrete has stood up on heavily traveled streets in Chicago, Milwaukee, Minneapolis, Detroit and other large cities.