Air Content of Fresh Concrete

Concrete that will freeze while wet needs a system of tiny, closely spaced air bubbles in its paste. When water in the capillary pores freezes and expands, these bubbles give it somewhere to go, relieving the pressure that would otherwise crack the paste. The bubbles are created on purpose with an air-entraining admixture and are called entrained air. Larger, irregular voids trapped during placing are entrapped air and do little for durability (see PCC Durability).

Paving concrete in freeze–thaw climates typically targets a total air content of about 5 to 8 percent, depending on aggregate size and exposure. Too little air risks freeze–thaw damage and scaling; too much lowers strength, roughly 5 percent of compressive strength for each percent of extra air (Taylor et al., 2019¹). Because air content varies with mixing, temperature, admixture dose and handling, it is measured often in the field.

All three field methods measure total air, entrained and entrapped together. None of them shows the size or spacing of the bubbles, which is what actually governs freeze–thaw resistance.

Pressure Method

The pressure meter (AASHTO T 152, ASTM C231) is the most common field method. It relies on Boyle's law: when pressure is applied to a sealed container of concrete, the only thing that compresses is the air, so the drop in pressure tells you how much air there is.

The common Type B meter has a bowl of known volume and a lid with an air chamber and a gauge. The technician (Western Alliance for Quality Transportation Construction, 2025²):

1. fills the bowl in three equal layers, rodding each layer 25 times and tapping the sides with a mallet to close the rod holes and release large entrapped bubbles;
2. strikes off the top flush with a strike-off plate and cleans the rim;
3. clamps on the lid, fills the space between lid and concrete with water through the petcocks, and pumps the air chamber to its initial pressure;
4. opens the valve between the chamber and the bowl, taps the bowl, and reads the air content directly from the gauge.

Two corrections or limits apply:

• Aggregate correction factor. Aggregate particles contain some pores that the test also compresses. The correction factor is found by testing the aggregates alone in the meter, in the proportions used in the mix, and is subtracted from the gauge reading.
• Porous aggregate. The pressure method is not suitable for lightweight aggregate, air-cooled slag or other highly porous aggregate, where the aggregate correction is large and uncertain. Use the volumetric method instead.

Meters are calibrated regularly against a known volume of air, and the calibration is checked whenever the meter is dropped or repaired.

Volumetric Method

The volumetric meter, or roll-a-meter (AASHTO T 196, ASTM C173), measures air by displacing it with water. A known volume of concrete in the base is covered by a top section, which is filled with water to a mark. The meter is then inverted and rolled until all the air has risen out of the concrete into the neck. The drop in the water level, read on the graduated neck, is the air content. Isopropyl alcohol is added to break up foam so the level can be read.

The volumetric method works with any aggregate, including lightweight, because it doesn't depend on compressibility. It is slower and more physically demanding than the pressure method, and incomplete rolling gives low results (Taylor et al., 2019¹).

Gravimetric Method

The unit weight test (AASHTO T 121, ASTM C138) also gives an estimate of air. If the mix proportions and the specific gravities of all the materials are known, the theoretical air-free density can be calculated; the difference between that and the measured density is air. The method is only as good as the specific gravities and batch weights, so it is mostly used as a cross-check: a sudden change in unit weight on a job usually means the air content has changed. See Unit Weight and Yield of Fresh Concrete.

Air-Void Quality

Total air content is a proxy for what matters, the spacing of the bubbles. Spacing is measured on hardened concrete with a microscope (ASTM C457), which takes days. A newer field device, the Super Air Meter, adds a second, higher-pressure step to the pressure test and reports a "SAM number" that correlates with spacing (Taylor et al., 2019¹). Some agencies use it for information or in pilot specifications.

Practical Points

• Test air at the point of placement when the specification says so. Pumping, long conveyors and slipform vibration can all change air content.
• Air is lost over time and with retempering. If water or admixture is added at the site, mix it in fully and retest.
• Hot concrete generally needs more air-entraining admixture to reach the same air content. Fly ash with high carbon content adsorbs admixture and can cause air to drop from load to load.

Standard Test Methods

• AASHTO T 152, ASTM C231: Air content of freshly mixed concrete by the pressure method
• AASHTO T 196, ASTM C173: Air content of freshly mixed concrete by the volumetric method
• AASHTO T 121, ASTM C138: Density (unit weight), yield and air content (gravimetric) of concrete

@misc{pavint-air-content-of-fresh-concrete,
  author = {{Pavement Interactive}},
  title = {Air Content of Fresh Concrete},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/cement-tests/air-content-of-fresh-concrete/}},
  note = {Accessed [date]}
}