7.1 Scope, Equipment, and When the Volumetric Method Fits

What ASTM C173 Measures

ASTM C173/C173M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method, determines how much air — entrained plus entrapped — is present in a freshly mixed sample. The principle is mechanical, not pneumatic: a measured volume of concrete is mixed with water inside a sealed rollameter (volumetric air meter), and the apparatus is rolled and agitated until the air is physically washed out of the concrete and rises into a graduated neck. The drop in liquid level equals the volume of air, which is read directly as a percent of the concrete volume on the calibrated neck.

Because the air is separated physically rather than compressed under pressure, the result is independent of the aggregate's own air voids. This is the single most important fact about C173 on the exam: **no aggregate correction factor is required. Air content is reported to the nearest 0.25 %.

When You Must Use C173 Instead of C231

The pressure method, ASTM C231, relies on Boyle's Law — applying a known pressure and reading how much the air volume compresses. That logic only holds when the aggregate itself contains essentially no compressible air voids. With lightweight aggregate, highly porous aggregate, cellular concrete, or slag, the aggregate pores compress under pressure and corrupt the reading, so C231 is invalid for those mixes.

C173 is slower and more physical, but it works on every aggregate, which is why it is the universal fallback.

The reason air content is worth all this effort is durability. Entrained air provides the microscopic pressure-relief chambers that let saturated concrete survive freeze-thaw cycling and deicer scaling. Too little air and the concrete fails in service; too much air and compressive strength drops roughly 5 % for every 1 % of added air. 5 %**), and the C173 field reading is the gate that accepts or rejects the load before it is placed. A technician who cannot run C173 correctly cannot test a lightweight bridge-deck mix at all, because no pressure meter is allowed on that concrete.

The Rollameter Apparatus

The volumetric air meter comes in two mating parts that clamp together into a watertight assembly:

• Measuring bowl (base): A rigid metal bowl of known, calibrated volume that holds the concrete sample. Bowl capacity is at least 0.075 ft³ (2 L).
• Top section: A funnel-shaped upper unit with a clear, graduated neck marked in percent air. It seats on the bowl with a gasket and is secured by clamps to form a leak-proof seal. The neck is the scale you read.

Supporting equipment the candidate must be able to identify and check:

Equipment Condition Checks Before Testing

Exam scenarios often hinge on spotting a defective meter before the test starts. Confirm the gasket is intact and the clamps seat squarely — a leak invalidates the test. Verify the neck scale is legible and undamaged, the bowl rim is clean and not dented (a dent changes the calibrated volume), and that you have isopropyl alcohol on hand. The meter must have been calibrated for its specific bowl-and-top combination; the two halves are a matched set and are not interchangeable between meters. Recognizing these conditions — not memorizing a serial number — is what the Grade I exam tests.

C173 in the Field-Test Family

Grade I candidates run seven core ASTM field tests, and C173 has to be placed correctly among them. Sampling follows C172; consistency is C143 (slump); density and yield are C138 (unit weight); specimens are molded under C31; temperature is C1064. Air content is measured by either C231 (pressure) or C173 (volumetric) — they are alternatives, not partners, and you choose based on the aggregate. A useful mental model: C231 is the everyday meter for normal-weight concrete, and C173 is the rollameter you reach for when the aggregate would lie to a pressure gauge.

Two facts anchor every C173 exam question. First, the result is a direct volumetric reading — air is read off the neck as a percent, not back-calculated from a pressure dial. Second, no aggregate correction factor exists for this method, so any answer choice that adds one is wrong. Keep those two ideas front of mind and most scope-level questions answer themselves.