The most commonly employed tests on fresh air entrained concrete measure the total air content. The pressure method, described in ASTM C231, is widely used and is based on Boyle’s Law. It is assumed that solid constituents in fresh concrete and water are incompressible so that volume change under pressure is due to the contraction of air voids. Volume change and difference in pressure are directly related to air content in two air meter designs, described as `Type A’ and `Type B’ respectively. Another approach, the volumetric method, such as that described in ASTM C173, uses an apparatus that allows water to replace air voids in agitated concrete while monitoring the displaced air volume. A third technique, the gravimetric method, described in ASTM C138, compares the difference between actual and theoretical unit weights to estimate the volume represented by air.
Specifications typically define a target mean volume of air to be entrained as a percentage of the concrete and these test methods are therefore adequate. However, the requirement in service is for a certain volume of air as a percentage of the cement paste, properly distributed through a myriad of bubbles conforming to limitations on diameter and spacing. Thus the air content test is useful as a quality control check but it does not provide any indication of the distribution of the air content.
The need for a test method better related to the performance requirements of entrained air led to the development of an alternative method that can measure air content, spacing factor and specific surface in a short period of time. The principle of the test is that the rate of rise of an air bubble in water is related to its size. The test method, reported by Price (1996), exploits the ability to monitor the change in buoyancy of a plate supported by a liquid and air bubbles. The test involves injecting a sample of fresh concrete into a viscous liquid at the base of a column of water. Entrained air bubbles are released and rise through the column where they strike a plate. The change in buoyancy of the plate with time is monitored. The buoyancy change can be related to air content, specific surface and spacing factor.
