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ASTM C78 is the standard test method for the flexural strength of concrete, determined on a simple beam loaded at its third points. The result is reported as the modulus of rupture. It is used mainly for concrete in slabs and pavements, and the number depends heavily on how the beam was made, cured and conditioned.
What ASTM C78 covers
The full title is “Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading).” It is also written ASTM C 78 or, in its combined SI and inch-pound form, C78/C78M. Section 1.1 of the scope says the method “covers the determination of the flexural strength of concrete” using a simple beam with third-point loading. The standard is maintained by ASTM Committee C09 on Concrete and Concrete Aggregates (subcommittee C09.61) and sits in Volume 04.02 of the ASTM book of standards.
Flexural strength is a bending measure. The Shimadzu application page describes the modulus of rupture as the calculated tensile-face stress in the beam at the maximum bending moment.
Which edition is current
As of 2026-10-29, the ASTM store page for C78/C78M-16 is marked Historical and lists C78/C78M-22 as the active version. Because the active edition changes over time, a lab report should name the exact edition used, for example “ASTM C78/C78M-22”, rather than citing “ASTM C78” alone.
Units
Section 1.2 states that values in either SI units or inch-pound units are to be regarded separately as standard. Each system is to be used independently of the other, and mixing values from the two may result in non-conformance with the standard.
How the test is used in practice
Beam specimens are prepared and cured under one of three other ASTM practices: C31/C31M, C192/C192M or C42/C42M. The test then loads the beam to failure, and the result is reported as modulus of rupture. The Wiss, Janney, Elstner Associates (WJE) laboratory page describes the main uses as checking compliance with specifications, mixture proportioning, mixing uniformity, and checking placement using sawed beams, and notes that the test is used primarily for concrete in slabs and pavements.
A testing-machine vendor application page (Shimadzu) describes a typical beam as 6 × 6 × 20 in (150 × 150 × 500 mm), with 100 × 100 mm and 150 × 150 mm cross sections also common. It also lists reporting and handling points: surfaces should be smooth and sides square, the orientation of the tension face matters, the span length must be reported, and load is applied at a uniform rate without shock. Treat these as practical notes from a vendor, and take exact dimensions, span and loading rate from the current edition of the standard itself.
Where the beam comes from: C31, C192 and C42
The three specimen standards are the least obvious part of C78. They decide whether the beam represents field-made concrete, laboratory-made concrete or concrete already in place.
| Specimen standard | How it relates to a C78 beam |
|---|---|
| C31/C31M | Named in C78 as a route for preparing and curing beams (field-made specimens) |
| C192/C192M | Named in C78 as a route for preparing and curing beams (laboratory-made specimens) |
| C42/C42M | Named in C78 as a route for beams; sawed beams are discussed in the CASRAI guide to ASTM C42 cores and sawed beams |
Which of these a lab follows should be stated in the report, because it changes what the number can be compared with.
What changes the number: a reporting checklist
The ASTM significance section states that, for the same specimen size, measured strength varies with specimen preparation, curing, moisture condition and whether the beam was molded or sawed. WJE adds that the measured modulus of rupture generally increases as specimen size decreases, and that for identical specimens C78 results are on average lower than those from the companion method C293/C293M. A report that lets a reader compare two results should therefore record:
- The edition of the standard (for example C78/C78M-22) and the unit system used.
- The specimen standard followed (C31, C192 or C42) and whether the beam was molded or sawed.
- Specimen size and the span length.
- Curing history and the moisture condition at test.
- Whether the result came from C78 (third-point loading) or C293, since the two are not interchangeable.
Where it matters for labs and quality programs
Because the result depends on the edition, the specimen route and the conditioning, the lab’s written standard operating procedure for the method should name the exact edition. For related background, see what calibration means in a lab and A2LA, which describes one accreditation body.
Common misunderstandings
“Flexural strength is the same as compressive strength”
It is not. C78 measures resistance to bending and reports modulus of rupture. It is a separate test with separate specimens.
“ASTM C78 and ASTM C 78 are different standards”
They are the same designation written two ways. What matters is the edition suffix.
“Any beam works”
The standard points to C31, C192 or C42 for preparation and curing. Specimen size, moisture and molded versus sawed origin all affect the result.
FAQ
What is ASTM C78 used for?
It determines the flexural strength of concrete using a simple beam with third-point loading, mainly for concrete in slabs and pavements, for compliance checks, mix proportioning and placement checks.
What does ASTM C78 report?
The modulus of rupture, which is the tensile-face stress at the maximum bending moment.
Is the SI or inch-pound system required?
Either may be regarded as standard, but each must be used independently of the other.
How does C78 relate to C293?
Both are flexural-strength methods. For identical specimens, C78 results are on average lower than C293/C293M results (WJE).
Sources
- ASTM International, C78/C78M-16 standard page (scope, significance and use, status), checked 2026-10-29.
- Wiss, Janney, Elstner Associates, ASTM C78 laboratory testing page, checked 2026-10-29.
- Shimadzu, ASTM C78 application page (vendor), checked 2026-10-29.
