ASTM C31: Making and Curing Concrete Test Specimens in the Field

What the ASTM C31 test does

ASTM C31 is the procedure for making and curing the concrete cylinders that will later be broken to measure strength. The break itself happens in the laboratory under ASTM C39, but the cylinders are born in the field, on your pour, out of a sample from a specific truck. This standard governs every step of that birth: how the molds are filled, how the fresh cylinders are protected, and how they travel to the lab.

It is easy to underestimate this test because nothing about it looks dramatic, a technician filling plastic molds beside a truck. But those cylinders are the legal memory of your pour. Twenty-eight days from now, and twenty-eight years from now, they are the evidence of what strength went into the structure. Cylinders made or cured wrong write false memory, almost always in the direction of reading lower than the truth, which is how good concrete ends up under investigation.

This page explains how specimens are made and cured, why the first night matters more than most people think, and what a complete strength record looks like from the chute to the certificate.

Why careful specimen work protects everyone

A low break triggers real consequences: engineering review, possible coring of the structure under ASTM C42, schedule holds, and hard conversations about who pays. Industry experience says a large share of low breaks trace not to bad concrete but to bad specimen handling, cylinders that froze or baked their first night, rode loose in a truck bed, or sat on a vibrating slab edge while they were still soft.

Every one of those failures is preventable, and this standard is the prevention. Following it means a low break can be trusted as a genuine signal about the concrete, which is exactly when you want to know. A good break can be trusted as proof the structure has what the design assumed.

How ASTM C31 specimens are made

Cylinders are cast from a composite sample taken per ASTM C172, with molding started within 15 minutes of sampling. The standard sizes are 6 by 12 inch and 4 by 8 inch molds. The smaller size is common today and is filled in two equal layers rodded 25 times each, while the larger takes three layers. Each layer is rodded evenly across the section, the mold sides are tapped to close the rod holes, and the top is struck off smooth.

The set size matches the acceptance rule in your specification, commonly two 6 by 12 or three 4 by 8 cylinders per test age, plus any extras for early breaks that release formwork or post-tensioning. Every cylinder is labeled to the truck, the location placed, and the time, because a strength result you cannot trace to a spot in the structure answers nothing.

The part everyone underestimates: initial curing

For their first hours and up to 48, the cylinders must live in a controlled world: temperatures held between 60 and 80 degrees Fahrenheit for typical mixes, moisture retained. No bumps, vibration, or rough handling while the concrete is still soft. On a Texas jobsite that means insulated cure boxes, ice or heat as the season demands, water or wet covers. A location out of the sun and away from traffic.

Transport has rules too: cylinders travel cushioned and upright, protected from freezing, drying, and jolts, and arrive at the laboratory within the time window the standard allows. There they move into final curing, a moist room or lime-water tank held at 73 degrees plus or minus 3, until their test date. That controlled biography, field to box to truck to tank, is what makes the eventual number mean what everyone assumes it means.

It helps to picture the failure mode this prevents. A cylinder that spends its first July night on a tailgate can cure at well over 100 degrees. A winter cylinder left beside the forms can flirt with freezing; both will break low through no fault of the concrete. The cure box is a fifty dollar answer to a fifty thousand dollar question.

What the record looks like

Each set carries its birth certificate: project, date, truck and ticket, placement location, the companion fresh properties, slump per C143, air per C231 or C173, temperature per C1064, plus the initial cure temperature history. When the breaks come back under C39, the strength attaches to all of it. That is the record a building official accepts, a lender's consultant audits, and a litigation expert cannot poke holes in.

Who needs ASTM C31 testing

Contractors need cylinders for acceptance and often for schedule, early-age breaks are what release stripping, stressing, and loading decisions. Owners need the strength record as the permanent proof of the structure they bought. Engineers design around specified strength and rely on properly made specimens to verify it, and building officials require the results on essentially all structural concrete.

Common questions about ASTM C31 cylinders

How we help with ASTM C31 testing

Cylinder fabrication rides with every fresh-concrete stop we make. The unglamorous parts, cure boxes placed before the pour, temperature logs inside them, gentle transport on schedule, are handled as standard practice rather than as favors. Our laboratory's moist room and break machines complete the chain under one quality system, so from the chute to the certificate, your strength record has one custodian and no gaps.

Cylinders and the schedule

The acceptance set answers the design question, but the extra cylinders answer the schedule question, and on fast-cycle structures they are the more valuable specimens on site. Stripping suspended forms, stressing tendons, backfilling walls, and loading a deck each wait on a strength threshold. Each threshold is proven by a field-cured or lab-cured early break ordered for that purpose. We plan the extra cylinder count with your cycle before the first pour. That is because the cheapest schedule insurance in concrete construction is a cylinder that exists when you need to break it.