Michelle L. Wilson, Portland Cement Association
(ASTM C231) (ASTM C173)
(ASTM C138)
Several techniques are available for measuring the air content of fresh concrete. This article describes five techniques for use with fresh concrete and one technique for use with hardened concrete. The reader is referred to the appropriate AASHTO or ASTM standard for full details of each procedure. Failure to maintain and calibrate equipment and to properly follow test procedures are primary causes of problems in the measurement of air content. Samples should always be obtained in accordance with AASHTO T 141 (ASTM C172) Standard Method of Test for Sampling Freshly Mixed Concrete.
Pressure Method—AASHTO T 152 (ASTM C231)
AASHTO T 152, Standard Method of Test for Air Content of Freshly Mixed Concrete by the Pressure Method is based on Boyle’s law, which states that the volume occupied by air is proportional to the applied pressure. Two types of meters designated A and B are covered by the standard. The Type A meter is rarely used. With the Type B meter shown in the photograph, a separate air chamber is connected through a valve to the test bowl that is filled with concrete. With the valve closed, the separate chamber is pressurized to a predetermined operating pressure. When the valve is opened, the air expands into the test chamber, and the pressure drops in proportion to the air contained within the concrete sample. The pressure gauge is read in units of air content.
Sources of error in the pressure method include incomplete sample consolidation; over vibration; error in the pressure gauge which may result in incorrect application of pressure or in gauge malfunction; calibration tests; sampling methods; aggregate correction factor; and leaks in the needle valve, petcocks, or a poor fit when the mating surfaces are not clean.
The pressure meter should not be used for concrete made with lightweight aggregates. In these instances, the volumetric method should be used.
Volumetric Method—AASHTO T 196 (ASTM C173)
AASHTO T 196, Standard Method of Test for Air Content of Freshly Mixed Concrete by the Volumetric Method relies on displacement of air with water in a vessel of pre-calibrated volume. To perform the test, the concrete is consolidated into the bowl, the apparatus filled with water, and a measured quantity of 70% isopropyl alcohol is added to dispel the foam generated during agitation. Next, the meter is inverted and agitated to free the concrete from the base and to displace air from the concrete into the water. The meter is then “rolled and rocked” until all the air has been dispelled from the concrete and the water level is stable. The air content of the concrete is read directly from the sight tube.
Major sources of error in the volumetric air test are failure to dispel all the air from the concrete during the agitation process, and difficulty in reading the liquid level in the sight tube. Other sources of error include possible variations in percentage of alcohol, use of alcohols other than isopropyl, and failure to allow sufficient time for stability of the reading.
Gravimetric Method—AASHTO T 121 (ASTM C138)
AASHTO T 121, Standard Method of Test for Density (Unit Weight), Yield, and Air Content [Gravimetric] of Concrete determines air content of fresh concrete by comparing measured density or batch volume to calculated density or volume. The density (unit weight) is determined by weighing a known volume of fresh concrete. The air content is computed using two independent equations given in AASHTO T 121. A significant discrepancy in the results from the two equations is an alert to check test equipment, procedures, sampling, mix ingredients, and proportions.
The test is sensitive to consolidation and strike-off of the concrete in the container; accurate weighing; and the need for precise batch weights, moisture contents, and densities of all constituent materials.
Chace Air Indicator—AASHTO T 199
AASHTO T 199, Standard Method of Test for Air Content of Freshly Mixed Concrete by the Chace Indicator is identical in concept to the volumetric air meter, but the air collected in this hand-held device has been liberated from a small fraction of mortar. The sample size is so small that this is a semi-quantitative test at best, and should not be a substitute for the more accurate pressure, volumetric, and gravimetric methods. It should not be used for determining the compliance of air content with the specifications.
Air Void Analyzer
The air void analyzer (AVA) determines the volume and size distributions
of air voids; thus an estimation of the spacing factor, specific surface,
and total amount of entrained air can be made. Air bubbles from a
sample of fresh concrete rise through a viscous liquid, enter a column of
water above it, then rise through the water and collect under a
submerged buoyancy recorder. The viscous liquid retains the original
bubble sizes. Large bubbles rise faster than small ones. The change in
buoyancy is recorded as a function of time and can be related to the
number of bubbles of different sizes. For more details on this test, see
HPC Bridge View Issue No. 34, July/August 2004.
Air-Void System—ASTM C457
ASTM C457, Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete describes procedures for microscopical determination of the air content of hardened concrete and of the specific surface, void frequency, spacing factor, and paste-air ratio of the air-void system in the hardened concrete. Differences between the air content measured on fresh and hardened concrete from the same batch are generally not more than ± 2 percentage points.
Further Information
Further information about air content is available in the following publications:
Whiting, D. A. and Nagi, M. A., Manual on Control of Air Content in Concrete, EB116, National Ready Mixed Concrete Association and Portland Cement Association, 1998, 42 pp.
Kosmatka, S. H., Kerkhoff, B., and Panarese, W. C., Design and Control of Concrete Mixtures, EB001, 14th edition, Portland Cement Association, Skokie, Illinois, 2002, 358 pp.