3.3 Compressive Strength Cylinders: Fabrication, Curing & Acceptance (ASTM C31, ASTM C39)

3.3 Strength Cylinders, Curing & Acceptance

The project uses strength tests to evaluate delivered concrete and may use separate field-cured specimens to evaluate protection or construction-stage readiness. Track sample identity, curing regime and test age through the field and laboratory reports.

Fabricate ASTM C31/C31M-17 specimens

For strength acceptance, use the size specified for the testing program, 4×8 inches or 6×12 inches. ASTM C31/C31M-17 Section 6.1 requires the diameter to be at least three times the nominal maximum coarse aggregate size. A 1-1/2-inch aggregate needs a diameter of at least 4-1/2 inches, so a 6-inch cylinder fits that case. When the nominal maximum aggregate exceeds 2 inches, use the C31 wet-sieving provision over a 2-inch sieve. Apply the mold-size criterion as well.

Cast upright on a level, rigid surface free from disturbance. Unless another method is specified, use rodding or internal vibration for slump of at least 1 inch, and internal vibration below 1 inch. Self-consolidating concrete follows the separate ASTM C1758 procedure.

Rod the bottom layer through its depth without damaging the mold. Penetrate about 1 inch into the previous layer on upper layers. Tap the outside 10 to 15 times after each layer to close rodding holes. Use an open hand where a mallet could dent a susceptible mold. Finish the top as required and label the specimen without disturbing it.

Match the curing regime to the purpose

Standard curing. Protect against moisture loss and maintain the required temperature for the initial period up to 48 hours after molding. For specified strengths below 6000 psi, use 60°F to 80°F. At 6000 psi or greater, use 68°F to 78°F. Record minimum and maximum temperatures. A cover can protect moisture, while the initial curing environment must also maintain the specified temperature. After initial curing and within 30 minutes of mold removal, place cylinders in a conforming moist room or water storage tank at 73.5 ± 3.5°F with free moisture on the surfaces, subject to the test method's preparation exceptions. ASTM C31 transport provisions require waiting at least 8 hours after final set, protecting from jarring, freezing and moisture loss, and limiting transportation to 4 hours.

Field curing. Store cylinders in or on the structure as near the represented concrete as practical and provide the same temperature and moisture environment and similar protection. Use these specimens when specified to evaluate curing and protection, form or shore removal, or other construction-stage strength. ACI 318-19 standard-cured acceptance results come from the separate standard curing process. Under Section 26.5.3.2(d), use companion specimens from the same sample when this evaluation is required. At the age designated for specified strength, Section 26.5.3.2(e) considers protection and curing adequate if the field-cured average is at least 85% of the companion standard-cured average or exceeds the specified strength by more than 500 psi. This check is separate from the core strength criterion.

Test cylinder strength under ASTM C39/C39M

For example, under C39/C39M-18, check cylinder condition, axis and bearing ends. The permitted end departures are at most 0.5 degrees from perpendicular and 0.002 inch from planeness unless prepared by the permitted grinding, sawing or capping procedure. Apply the appropriate practice for bonded caps under ASTM C617 or unbonded caps under ASTM C1231, including restrictions for the actual strength and cap system.

Test moist-cured specimens in moist condition and within the age tolerance. At 28 days, the time tolerance is ±20 hours. Center the specimen. Apply load continuously without shock. During at least the latter half of anticipated loading, use the rate of movement corresponding to a stress increase of 35 ± 7 psi/s. Record the maximum load and fracture type. Calculate compressive strength by dividing maximum load by measured cross-sectional area, with the applicable length-to-diameter correction when the ratio is 1.75 or less. Report the individual strength to the nearest 10 psi in inch-pound units. If reporting the average of companion specimens, use their unrounded strengths to calculate it.

Evaluate the concrete strength tests

ACI 318-19 Section 26.12.2.1 sets sampling frequency for each mixture placed each day. Apply all three minima: once per day, once per 150 yd³ and once per 5000 ft² of slab or wall surface. If that would give fewer than five tests for a mixture on the project, test at least five randomly selected batches or every batch when there are fewer than five. When total quantity of a mixture is less than 50 yd³, the building official may approve satisfactory evidence in place of tests.

Under Section 26.12.1.1(a), a single test is the average of at least two standard-cured 6×12-inch cylinders or three standard-cured 4×8-inch cylinders from the same C172 sample at point of delivery, tested at 28 days or the other age designated for $f'_c$. The test result is a companion average, so use that average in both criteria below.

A foundation with $f'_c=4000$ psi has five consecutive acceptance tests of 4350, 4100, 3850, 4200 and 3450 psi. The successive three-test averages are 4100, 4050 and approximately 3833 psi. The last window fails. The last individual result, 3450 psi, also falls below the 3500 psi floor. Section 26.12.3.1(b) requires steps to increase subsequent strength tests when either criterion fails. The below-floor individual result also triggers the Section 26.12.6 investigation provisions.

Investigate low results

Verify specimen identity and the sampling, molding, curing, transport and test records. Under Section 26.12.6.1, steps are required to protect structural adequacy when a standard-cured test is below its permitted individual limit or field-cured tests indicate deficient protection or curing. If the likelihood of low strength is confirmed and calculations indicate significantly reduced adequacy, the provision permits ASTM C42 testing of three cores from the affected area for each below-limit test. Condition cores in watertight containers and test at least 5 days after last wetting and no later than 7 days after coring unless otherwise approved under the provision. The represented area meets the core strength criterion when the average reaches at least 85% of specified strength and each reaches at least 75%. When core results fall short and adequacy remains in doubt, the building official may order a Chapter 27 strength evaluation. Record the disposition and verify the approved corrective work.