5.4 Cylinders (C31), Compressive Strength (C39/T22) & Temperature

Making Cylinders - ASTM C31 / AASHTO T23

Where Sections 5.2 and 5.3 test the fresh concrete on-site, ASTM C31 / AASHTO T23, "Making and Curing Concrete Test Specimens in the Field," governs how the inspector molds specimens that will later be tested for hardened strength. The standard cylinder size is 6 in. diameter by 12 in. tall; a 4 in. by 8 in. alternative is permitted for finer aggregate mixes and is increasingly common because it is lighter to handle and needs less concrete per specimen.

Consolidation method depends on slump:

• For slump greater than 3 in., rod-consolidate: fill the 6x12 cylinder in three layers of approximately equal volume, rodding each layer 25 times with the same 5/8-in. tamping rod used for the slump test (for 4x8 cylinders, two layers, 25 strokes each, with a smaller 3/8-in. rod).
• For slump of 1 in. or less, mechanical vibration is required instead of rodding, because a stiff, low-slump mix will not self-consolidate around a hand-rodded hole.
• Between 1 and 3 in., either method is acceptable.

After each layer is consolidated, the mold is lightly tapped to close surface voids, then the top is struck off flush and covered.

Initial (Field) Curing

The specimen's first hours are its most fragile. ASTM C31 requires initial curing in a location where temperature can be controlled and moisture protected - typically an insulated curing box at the jobsite - holding the specimen at 60-80°F for standard-cure specimens (high-early-strength mixes get a tighter 68-78°F band) for up to the first 48 hours, shielded from direct sun, vibration, and moisture loss. Only after this initial curing window are the cylinders stripped from their molds and transported to the lab for final (standard) curing in a moist room or lime-saturated water bath at 73 +/- 3°F until test day. An inspector who lets field cylinders bake in a truck cab or freeze overnight has compromised the specimen before the lab ever sees it - a bad initial-cure environment can knock a mix's apparent strength down sharply, triggering a false failure investigation into concrete that was never actually deficient.

Compressive Strength - ASTM C39 / AASHTO T22

ASTM C39 / AASHTO T22 governs the actual break test: a capped (or neoprene-pad-confined) cylinder is loaded in a calibrated compression machine at a controlled rate until it fractures, and the maximum load divided by the cross-sectional area gives compressive strength in psi, typically reported to the nearest 10 psi. Cylinder ends must be planar and perpendicular to the axis before loading, achieved by sulfur mortar capping, neoprene pad caps in a retaining ring, or grinding the ends flat - a poorly capped or unground end concentrates load on a high spot and produces an artificially low, unrepresentative break.

Highway specs almost universally test at two ages from the same set of field cylinders:

Because cement hydration and strength gain follow a predictable, front-loaded curve, the 7-day break is commonly used as an early warning: for a typical Type I cement mix, 7-day strength runs roughly 60-75% of the eventual 28-day strength - a rule of thumb only, since the exact ratio shifts with cement type (Type III high-early gains strength faster; fly-ash or Type IV mixes gain it slower) and curing temperature. A low 7-day break does not automatically fail the concrete, but it is the inspector's cue to watch that lot closely and confirm nothing else went wrong (mis-batched water, inadequate curing) before the 28-day result is in.

Worked example: A pavement mix design specifies f'c = 4,000 psi at 28 days. The 7-day cylinder from the same batch breaks at 2,600 psi. Using the 60-75% rule of thumb, the projected 28-day range is 2,600 / 0.75 to 2,600 / 0.60 = 3,467-4,333 psi - bracketing the 4,000 psi target and giving the inspector reasonable confidence the mix is on track, pending the actual 28-day break.

Temperature: The Variable Behind Every Result

Concrete temperature at the time of sampling is recorded alongside every slump and air test because temperature drives the rate of every reaction described above - a hot load hydrates, and loses slump and workability, faster, while a cold load hydrates more slowly and is vulnerable to freeze damage before it gains strength. Most highway specs set both a maximum placement temperature (commonly around 90°F, tightened further for mass placements) and a minimum (commonly 50-55°F, discussed further with cold-weather concreting in Section 5.5), because both the test cylinders and the structure itself are governed by the same thermal reality.