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Short answer
ASTM C173/C173M (written “ASTM C 173” in some catalogues) is the standard test method for the air content of freshly mixed concrete by the volumetric method. It reads air from a calibrated, transparent meter scale after the concrete has been agitated in water, so it measures the air in the mortar and works for concrete made with any type of aggregate, including lightweight and porous aggregate.
Definition and scope
The full title is Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method. It is issued by ASTM Committee C09 on Concrete and Concrete Aggregates (subcommittee C09.60). The edition shown as active on the ASTM store is C173/C173M-26; C173/C173M-24a is listed as historical, and the revision list runs back through 24, 23, 16, 14, 12, 10B, 10A, 10, 09, 08A, 08, 07, 01E01 and 01.
The scope covers freshly mixed concrete “containing any type of aggregate, whether it be dense, cellular, or lightweight.” Two housekeeping rules sit in the scope:
- SI and inch-pound values are each regarded as standard, but each system is used independently of the other and the values are never combined.
- Fresh hydraulic cementitious mixtures are caustic and can cause chemical burns on prolonged exposure; the standard states that it does not address all safety concerns.
How the volumetric method is used in practice
The test is run on a sample of fresh concrete in a “rollermeter”: a bowl with a transparent graduated neck. The following is a scoped summary of the performance checklist that NICET publishes for the 2016 edition (C173/C173M-16, equivalent to AASHTO T 196), not a reproduction of the standard. The current edition governs, so confirm any value against it.
Equipment named in the NICET checklist
- A bowl of at least 0.075 ft³ (2.0 L), with a top section at least 20% larger than the bowl.
- A transparent scale graduated in increments of no more than 0.5% from 0 to 9% or more, with graduations accurate to ±0.1% of the bowl volume.
- A calibration cup of 1.00 ±0.04% of the bowl volume, used only to add water when the air content exceeds the range of the meter.
- Isopropyl alcohol checked at 70% by volume, a 16 ±2 mm tamping rod and a mallet of about 600 ±200 g.
The sequence at a glance
- Fill the bowl in two layers, rodding each layer 25 times and tapping the sides 10 to 15 times; the rod penetrates about 25 mm into the lower layer, and an excess of 3 mm or less above the rim is acceptable.
- Add water and isopropyl alcohol, then invert and roll the meter for at least 45 seconds without holding it inverted for more than 5 seconds at a time.
- Wait for the liquid level to stabilise, meaning a change of no more than 0.25% in 2 minutes. If it has not stabilised within 6 minutes, the test is discarded.
- Read to the nearest 0.25%. A second reading within 0.25% of the first is the final reading; otherwise a third reading is taken, and if readings still disagree the test is discarded.
- Calculate: final meter reading, minus the correction factor, plus the number of calibration cups of water added, reported to the nearest 0.25%.
Why the method suits lightweight and porous aggregate
ASTM C173 states that the method measures the air in the mortar fraction of the concrete and is not affected by air inside porous aggregate particles. For that reason it is described as appropriate for concretes made with lightweight aggregate, air-cooled slag or highly porous natural aggregate. Other routes to air content, including the gravimetric method of ASTM C138, are compared with it in the CASRAI guide on ASTM C138 and the other air-content test routes.
Validity checks: when a C173 result cannot be used
This checklist pulls together the conditions in the standard and the NICET checklist under which a result is invalid and the test is repeated.
| Check | What invalidates the test | Source |
|---|---|---|
| Foam in the neck | Foam above the water level equivalent to more than 2% air; repeat with more alcohol at the initial fill | ASTM C173/C173M-26, section 4.2 |
| Alcohol added late | Adding alcohol after the initial meter fill is not permitted | ASTM C173/C173M-26, section 4.2 |
| Packed concrete in the bowl | Undisturbed, tightly packed concrete found when the bowl is emptied | NICET checklist (2016 edition) |
| Unstable liquid level | Level still changing by more than 0.25% in 2 minutes after 6 minutes | NICET checklist (2016 edition) |
| Disagreeing readings | No two successive readings within 0.25% of each other | NICET checklist (2016 edition) |
Where it matters for labs and testing programmes
Specimen making for concrete testing is covered in the guide to ASTM C31 making and curing concrete test specimens. In a testing laboratory the practical points are the ones the checklist makes explicit: the meter scale and the calibration cup are measuring equipment with stated tolerances, so they belong on the equipment schedule. For how verification fits into a quality system, see what calibration is, how to set a calibration interval, and what ISO/IEC 17025 accreditation covers. Written steps for the bench belong in a standard operating procedure that cites the edition in force.
Common misunderstandings
“The meter reading is the air content”
Not directly. The checklist’s calculation subtracts a correction factor and adds any calibrated cups of water used, then reports to the nearest 0.25%.
“A fresh result predicts the hardened air content exactly”
The standard itself notes that the air content of the hardened concrete may be higher or lower than the fresh result, and lists consolidation, bubble stability, timing, environmental exposure and pumping as reasons.
“A little foam is fine”
Foam up to the equivalent of 2% air is the limit stated in the standard; more than that means repeating the test with more alcohol at the initial fill.
“SI and inch-pound figures can be mixed”
The standard says each system is used independently and the values are not combined.
“ASTM C173 and ASTM C 173 are different standards”
Both spellings point to the same test method. The full designation carries the edition year, for example C173/C173M-26.
Frequently asked questions
What does ASTM C173 measure?
The air content of freshly mixed concrete, by the volumetric method. It measures the air contained in the mortar fraction of the concrete.
Can ASTM C173 be used with lightweight aggregate?
Yes. The scope covers concrete containing any type of aggregate, whether dense, cellular or lightweight, and the method is not affected by air inside porous aggregate particles.
Can alcohol be added later to clear foam?
No. Isopropyl alcohol is added when the meter is first filled with water, and adding it afterwards to dispel foam is not permitted. Foam above the equivalent of 2% air makes the test invalid.
Will the fresh air content match the hardened concrete?
Not necessarily. The standard notes that the air content of hardened concrete may be higher or lower than the fresh result.
Sources
- ASTM International, C173/C173M-26, Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method (store abstract, active edition), checked 2026-10-29.
- ASTM International, C173/C173M-24a (historical edition and revision list), checked 2026-10-29.
- NICET, Performance Examination Checklist, ASTM C173/C173M-16 (AASHTO T 196M / T 196-11), dated 09/2020, checked 2026-10-29.
