5.5 Gravimetric Air and Theoretical Density

Air Content By Mass-Based Density Comparison

ASTM C138 includes a gravimetric air content calculation. Unlike the pressure method or volumetric method, it does not directly measure air with a meter. It compares the measured density of the fresh concrete with the theoretical density of the same mixture if no air were present. The difference is attributed to air volume.

The key term is theoretical density. It represents the air-free density calculated from the masses and absolute volumes of the mixture ingredients. Cement, supplementary cementitious materials, water, aggregates, admixtures, and their specific gravities or volumes all affect this value. The field technician may be given the theoretical density or mixture data in a written problem.

Measured C138 density is usually lower than theoretical air-free density because air occupies volume but adds little mass. The gravimetric air concept is commonly expressed as air percent = (T - D) / T x 100, where T is theoretical density and D is measured density. The units of T and D must match.

Gravimetric air concept table:

This method is powerful because it can use the same C138 test that also supports yield. It is also sensitive. If the batch weights are wrong, aggregate moisture is not accounted for, or the theoretical density belongs to a different mix, the air content can be wrong even when the C138 density was measured perfectly.

A negative air result is a warning sign, not a realistic concrete property. If measured density is greater than theoretical air-free density, check the numbers. Possible causes include using the wrong theoretical density, mixing metric and inch-pound units, including measure tare in the net mass, or testing a sample with too much coarse aggregate. The answer is not to report negative air as ordinary field behavior.

Gravimetric air is different from C231 pressure air and C173 volumetric air. C231 uses pressure behavior in a sealed meter and subtracts aggregate correction. C173 rolls and reads air volume in a neck. C138 uses mass and density comparison. The final percentages may be compared, but the mechanisms and error sources are different.

The exam may also ask which information is needed. Measured density alone is not enough for gravimetric air. You need theoretical density or the mixture information needed to calculate it. You also need accurate C138 density, because the air formula uses that measured value directly.

Use this process in calculation questions:

1. Confirm that measured density and theoretical density use the same units.
2. Subtract measured density from theoretical density.
3. Divide the difference by theoretical density.
4. Multiply by 100 to express percent air.
5. Check whether the answer is reasonable and nonnegative.
6. If it is strange, look first for bad units, bad tare, or wrong theoretical density.

Gravimetric air rewards careful thinking. It is not a new field procedure so much as a controlled calculation built on C138 density and mixture proportion data.