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Short answer
ASTM C138/C138M is the standard test method for density (unit weight), yield and air content (gravimetric) of freshly mixed concrete. It determines the density of fresh concrete and gives the formulas used to derive yield, cement content and air content from that density. The ASTM store listed edition 24a as active on 2026-10-29.
Definition and scope
The full title is Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete. It is issued by ASTM Committee C09 on Concrete and Concrete Aggregates, under subcommittee C09.60, and is published in the Annual Book of ASTM Standards, Volume 04.02. The “M” in C138/C138M signals that the method carries both SI and inch-pound units; each system is regarded as standard and is used independently of the other.
Section 1.1 of the scope covers the determination of the density of freshly mixed concrete. It also supplies formulas for three derived properties:
- Yield, defined in the standard as the volume of concrete produced from a mixture of known quantities of the component materials (earlier editions also describe it as “yield (or relative yield)”).
- Cement content, for which the scope also gives a formula.
- Air content, the gravimetric determination that gives the method its name.
“Unit weight” and “density” are the same property
Note 1 of the standard states that unit weight was the previous terminology used to describe the property. Specifications, mixture submittals and older lab records may still say “unit weight” while a current edition of C138 says “density”. They refer to the same measurement, so a lab that searches its records for only one of the two words can miss data.
How it is used in practice
The method works on a sample of freshly mixed concrete, before it hardens, and the results describe that plastic material rather than the cured product. According to the abstract on the ASTM store, the apparatus consists of a balance or scale, a tamping rod, an internal vibrator, a measure (a container of known volume), a strike-off plate, a mallet and a scoop. The standard itself sets out the procedure; this page does not reproduce it, and anyone running the test should work from the licensed edition their specification calls for.
Because density, yield and air content all come from the same measurement, a single C138 result can feed several records. In practice it sits alongside the specimen-making work described in the CASRAI guide to ASTM C31 for making and curing concrete test specimens in the field: C31 covers the cylinders and beams that are later tested for strength, while C138 characterises the fresh batch they came from.
The standard’s own safety statement (section 1.4) notes that fresh hydraulic cementitious mixtures are caustic and may cause chemical burns. Staff should follow the lab’s own written procedures; see CASRAI’s definition of a standard operating procedure.
C138 compared with C231 and C173: three routes to air content
C138 is one of three ASTM methods for the air content of freshly mixed concrete. The scope descriptions on the ASTM store pages for C231 and C173 position them against each other as follows.
| Method | Approach | Limits stated on the ASTM page |
|---|---|---|
| C138/C138M | Gravimetric: air content is derived from the measured density of the concrete | Also yields density, yield and cement content from the same measurement |
| C231/C231M | Pressure method | Needs an aggregate correction factor; not for lightweight aggregate, air-cooled slag, highly porous aggregate or non-plastic concrete |
| C173/C173M | Volumetric method | Usable with any aggregate (dense, cellular or lightweight); measures air in the mortar fraction only |
The C231 page adds that for concrete made with dense aggregate, the pressure method should give about the same results as the gravimetric and volumetric methods. The C173 page cautions that hardened concrete may contain more or less air than the fresh-state result, for reasons such as consolidation, bubble stability and pumping. A fresh-state air value is therefore a statement about the plastic concrete at the point of sampling.
Edition status on the ASTM store, as of 2026-10-29: C138/C138M-24a is active (updated 6 August 2024); C138/C138M-16 is listed as historical. For the companion methods, C231-22 and C173-16 are shown as historical and C231-26 and C173-26 as active. Always cite the edition named in the project specification, not simply the newest one.
Where it matters for labs
Testing laboratories that perform C138 typically operate under third-party accreditation. One accredited testing laboratory’s public page for the method states that it tests to C138/C138M-17a and that its work is accredited by AASHTO (R18) and ANAB under ISO/IEC 17025. For background on how these bodies relate, see CASRAI’s comparison of laboratory accreditation bodies and the entry on ANAB.
A lab record that cites C138 is clearer when it states:
- the full designation including the edition suffix (for example C138/C138M-24a), because the editions differ and old ones become historical;
- which property is being reported: density, yield or air content, since all three come from one test;
- which air-content method produced a value, when C231 or C173 results appear in the same report as C138;
- the unit system used, SI or inch-pound.
Those four items follow directly from the scope, the edition history and the unit statement quoted above. If your instruments are part of the record, see how a calibration certificate is read, since the balance and measure in the equipment list are the instruments the result depends on.
Common misunderstandings
“C138 only measures density.”
Its title names three properties. Density is the measured quantity; yield, cement content and air content are calculated from it.
“Unit weight and density are different tests.”
Unit weight is the older term for the same property, according to Note 1 of the standard.
“Any air-content method can be swapped for any other.”
The three methods are built differently, and C231 and C173 each carry stated limits on aggregate type. Which one applies is decided by the specification and the materials, not by convenience.
“The fresh air value predicts the hardened air content.”
The C173 page states that hardened concrete may contain more or less air than the fresh result.
“The current edition is always the one to cite.”
The right edition is the one the contract documents reference. ASTM keeps superseded editions on its store as historical.
Frequently asked questions
What does ASTM C138 measure?
The density (unit weight) of freshly mixed concrete, with formulas for yield, cement content and gravimetric air content derived from it.
What does the M in C138/C138M mean?
The designation covers both SI and inch-pound units; each system is regarded as standard and used independently of the other.
Is unit weight the same as density in C138?
Yes. Note 1 of the standard says unit weight was the previous terminology for the property the method determines.
How does C138 differ from C231 and C173?
C138 is gravimetric, C231 is the pressure method and C173 is the volumetric method. For concrete made with dense aggregate, C231 states that the pressure method should give substantially the same air contents as the other two.
Which edition of C138 should be cited?
The one named in the project specification. On 2026-10-29 the ASTM store listed C138/C138M-24a as active and C138/C138M-16 as historical.
Sources
- ASTM International, C138/C138M-24a, Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete, store page checked 2026-10-29.
- ASTM International, C138/C138M-16 (historical), checked 2026-10-29.
- ASTM International, C231/C231M-22, Air Content of Freshly Mixed Concrete by the Pressure Method, checked 2026-10-29.
- ASTM International, C173/C173M-16, Air Content of Freshly Mixed Concrete by the Volumetric Method, checked 2026-10-29.
- Wiss, Janney, Elstner Associates, ASTM C138 laboratory testing page, checked 2026-10-29.
