ASTM C192

ASTM C192, the Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory, defines how concrete cylinders and beams are batched, molded, and cured under tightly controlled laboratory conditions. The practice is maintained by ASTM Committee C09 on Concrete and Concrete Aggregates, was first approved in 1944, and is published as a dual designation: C192 in inch-pound units and C192M in SI units. The current edition is ASTM C192/C192M-26.

The purpose of the practice is to remove preparation and curing variability from the test result, so that a strength test reflects the intrinsic properties of the concrete mixture itself rather than inconsistencies in how the samples were made or stored. C192 is the laboratory counterpart of ASTM C31, which governs specimens made in the field from delivered concrete.

The practice applies to concrete that can be consolidated by rodding or vibration. The standard lists four uses for the information that laboratory specimens generate: proportioning mixtures for project concrete, evaluating different mixtures and materials, correlating strength with the results of nondestructive tests, and research.

ASTM C192 does not cover:

• Specimens made in the field from delivered concrete (that is ASTM C31)
• Mixtures that cannot be consolidated by the rodding or vibration procedures the practice describes, such as roller-compacted concrete
• Acceptance testing of concrete as delivered to a project

The two practices answer different questions. C192 specimens, made under ideal conditions, measure what a mix design is capable of. C31 specimens, made at the job site from delivered concrete, measure what actually arrived.

The distinction affects acceptance directly. Under ACI 318, acceptance of delivered-concrete strength rests on field-sampled, standard-cured C31 cylinders tested per ASTM C39, not on laboratory trial-batch results. If field cylinders fail while the original laboratory qualification passed, the investigation usually starts with delivery, handling, and jobsite curing rather than the mix design. ACI 318 also treats field-cured cylinders that test below 85 percent of their standard-cured companions as a signal to improve jobsite curing and protection, unless the field-cured cylinders still exceed the specified strength by more than 500 psi.

Cylinder dimensions follow the governing test method. When none is specified, the length is twice the diameter. In every case, the cylinder diameter (or the minimum cross-section of a beam) must be at least 3 times the nominal maximum size of the coarse aggregate (see the ASTM D448 stone size chart for standard gradations). In practice, 6 x 12 inch and 4 x 8 inch cylinders dominate US strength testing: one strength test averages two 6 x 12 cylinders or three 4 x 8 cylinders. Beams for flexural testing are molded with their long axis horizontal.

The slump of the concrete determines the consolidation method:

For rodded cylinders, the number of layers and the rod size scale with the cylinder diameter:

Internal vibrators are not permitted in cylinders smaller than 4 inches in diameter or in beams narrower or shallower than 4 inches. For machine-mixed laboratory batches, the practice prescribes 3 minutes of mixing, a 3 minute rest, and a 2 minute final mix, with all materials brought to room temperature (68 to 86°F) before batching.

The curing requirements are specific:

• Cover specimens immediately after finishing to prevent moisture loss, and record ambient temperatures during initial curing
• Remove specimens from molds 24 ± 8 hours after casting; for mixtures with prolonged setting times, wait until 20 ± 4 hours after final set
• Moist cure at 73.5 ± 3.5°F (23.0 ± 2.0°C) from the time of molding until the moment of testing
• Maintain free water on the entire specimen surface at all times, using a moist room meeting ASTM C511 or a saturated limewater storage tank
• Keep specimens in a vibration-free environment for the first 48 hours of curing
• Flexural beams get an extra step: immersion in saturated limewater for at least 20 hours immediately before testing, with surfaces kept wet until the test ends, because even slight surface drying measurably lowers flexural strength

C192 produces the specimens; companion ASTM standards run the tests and define the equipment:

• ASTM C39: compressive strength of cylindrical specimens
• ASTM C78: flexural strength of beams using third-point loading
• ASTM C469: static modulus of elasticity and Poisson’s ratio
• ASTM C496: splitting tensile strength of cylinders
• ASTM C617 and C1231: bonded capping and unbonded neoprene caps for cylinder ends before compression testing
• ASTM C143 (slump) and C231 or C173 (air content): fresh-property tests run while specimens are molded; the slump result decides rodding versus vibration
• ASTM C511: the moist rooms, cabinets, and storage tanks that curing depends on

Laboratory trial batches made and cured per ASTM C192 are how producers qualify a mixture for a performance-based specification. Rather than dictating exact quantities of cement, sand, and stone, the engineer specifies the required outcome, and the producer demonstrates through trial-batch testing that the proposed mixture can deliver it. ACI 301 and ACI 318 accept laboratory trial mixtures (with data developed within 24 months of submittal) or documented field test records as the basis for mixture qualification.

Qualification and acceptance are separate steps. Once concrete ships to a project, ACI 318 judges compliance with the specified strength using field-sampled C31 cylinders tested per ASTM C39: every average of three consecutive strength tests must meet or exceed the specified strength, and no single test may fall more than 500 psi below it (for specified strengths up to 5,000 psi).