ASTM C31: making and curing concrete cylinders in the field

What is ASTM C31 for?

Cylinders made under C31 are the specimens that feed C39 breaks and, through them, the project acceptance check. Standard-cured cylinders are meant to show the potential strength of the concrete as delivered. Field-cured cylinders, when a project calls for them, are meant to show how concrete in the structure is doing under site conditions. The two serve different purposes, so do not mix them in one set and label them clearly.

The practice exists because a cylinder is easy to damage invisibly. Jarring, freezing, drying out or overheating in the first hours can cost strength that no lab procedure can recover.

What equipment do you need to make concrete cylinders?

Single-use or reusable molds of the right size (4 × 8 in and 6 × 12 in are the common sizes), a tamping rod or vibrator suited to the mold and the slump, a mallet, a strike-off tool, a small scoop, and a sample taken under ASTM C172. For initial curing you need a place that holds the cylinders level, protected and inside the required temperature range, plus a maximum-minimum thermometer so you can prove it.

Write the mold size on the field sheet. The acceptance rules count cylinders differently for 4 × 8 and 6 × 12 sets.

How do you make concrete test cylinders in the field?

Obtain a composite sample from the truck as C172 describes, and start molding promptly. Place the molds on a level, rigid surface close to where they will spend their first night, so you are not carrying freshly molded cylinders across a site. Fill each mold in the number of layers the standard sets for your mold size and consolidation method, consolidate each layer, tap the sides to close voids, strike off the top and cover the molds to stop evaporation.

Mark each cylinder with an identifier that ties it to the sample, the truck ticket and the placement. Record the time of molding and the maximum-minimum temperatures during initial curing. Then leave them alone until it is time to move them to the lab.

What temperature should cylinders be kept at during initial curing?

During initial curing, for up to 48 hours, cylinders are kept at 60 to 80 °F. When the specified strength is 6,000 psi or more, the range tightens to 68 to 78 °F. These are sensible default warning ranges for a lab's records, and they should be editable per project because some specifications add their own requirements.

The maximum-minimum thermometer reading is the evidence. Write down both values when the cylinders are picked up. A record that says only "cured on site" cannot show that the range was met, and a reviewer who sees a low break with no curing record has nothing to rule out.

How do handling errors affect strength results?

A set that sat in a hot truck bed or froze overnight does not fail with a warning label. It just breaks low. On its own, one low test may still be above the single-test limit, yet it drags down the running average of three, and that can be the difference between acceptance and an investigation with cores. The worked example below shows how a single soft test sits inside an otherwise healthy series.

Because the strength math cannot tell a curing problem from a concrete problem, the field record has to. Curing temperatures, transport notes and the fracture type from the break are what let a reviewer separate the two.

How do you tell a curing problem from a concrete problem?

Start with the record, not the number. If the maximum-minimum readings fall inside the required range, the cylinders were picked up on time and the fracture types look normal, a low result points more toward the concrete. If the record shows a temperature outside the range, a late pickup or rough transport, the result may describe the handling instead.

Compare companion cylinders too. Cylinders from one sample that break far apart often share a handling story. A whole set that breaks low together, with a clean curing record, is a stronger signal about the concrete itself and is the case where an engineer may ask for further investigation.

What are the most common C31 mistakes?

Molding on the tailgate and then carrying the cylinders. Mold where they will cure.

No thermometer in the curing box, or a thermometer nobody reads. If the range is not recorded, it was not met as far as the record is concerned.

Moving cylinders too early, or bouncing them unprotected across a site in the back of a pickup.

Labels that only make sense to the person who wrote them. Tie every cylinder to the sample and ticket.

Mixing field-cured and standard-cured cylinders in one acceptance set.

Which standards relate to ASTM C31?

ASTM C172 covers sampling the concrete before molding. ASTM C39 covers breaking the cylinders. ACI 318 contains the acceptance criteria applied to the strength tests. The standards index lists every standard explained on this site.