# Making and Curing Concrete Test Specimens

Making cylinders and beams in the field, initial curing on site, transport and final curing, and the handling errors that most often cause low strength results.

- Canonical URL: https://pavementinteractive.org/reference-desk/testing/cement-tests/making-and-curing-concrete-test-specimens/
- Last updated: 2026-11-01
- Standards: AASHTO T 22, ASTM C39, AASHTO R 100, AASHTO T 231, ASTM C1231, ASTM C31

Concrete strength for acceptance is almost always measured on specimens made in the field: cylinders for compressive strength, and on many paving jobs beams for flexural strength. The specimens are tested weeks later in a laboratory, and the result is taken to represent the concrete in the pavement or structure. That only holds if the specimens were made, cured and handled the same way every time. Most low strength results that turn out not to reflect the concrete trace back to the first 48 hours of a specimen's life (National Ready Mixed Concrete Association, 2014[^1]).

The practice for making and curing specimens in the field is AASHTO R 100 (ASTM C31). How they are broken is covered in [Compressive Strength](https://pavementinteractive.org/reference-desk/testing/cement-tests/compressive-strength/) and [Portland Cement Flexural Strength](https://pavementinteractive.org/reference-desk/testing/cement-tests/portland-cement-flexural-strength/).

## Two Purposes, Two Curing Regimes

Specimens are made for one of two purposes, and the purpose decides how they are cured:

- **Standard-cured specimens** check that the concrete as delivered meets the specified strength. They are cured under controlled laboratory conditions so that results from different days and jobs can be compared. Acceptance is based on these.
- **Field-cured specimens** estimate the strength of the concrete in place, for decisions such as when to remove forms, open a pavement to traffic, or stress post-tensioning. They are cured next to the concrete they represent, under the same conditions.

Results from the two regimes are not interchangeable. Field-cured specimens should never be used for acceptance, or standard-cured ones for opening decisions (Taylor et al., 2019[^2]).

## Molds and Sizes

The standard cylinder is 150 by 300 mm (6 by 12 inches). Smaller 100 by 200 mm (4 by 8 inch) cylinders are allowed by most specifications when the aggregate is small enough, because they are lighter and need less concrete; the cylinder diameter must be at least three times the nominal maximum aggregate size. Standard beams are 150 by 150 mm (6 by 6 inches) in cross section. Molds must be watertight, hold their shape, and sit level on a firm surface (Western Alliance for Quality Transportation Construction, 2025[^3]).

## Making the Specimens

Molding starts within 15 minutes of getting the sample (see *Sampling Freshly Mixed Concrete*). The specimens are made where they will spend their first day, so they don't have to be moved while soft.

- **Consolidation.** Concrete with a slump of 25 mm (1 inch) or more is usually rodded; stiffer concrete is consolidated with an internal vibrator. For rodding, a 150 by 300 mm cylinder is filled in three equal layers and a 100 by 200 mm cylinder in two, each layer rodded 25 times with a rod of the specified diameter, and the outside of the mold tapped 10 to 15 times after each layer to close the rod holes.
- **Finishing.** The top is struck off and finished flat with a trowel or float, then covered to prevent evaporation.
- **Labeling.** Each specimen is marked on the mold, not the lid, with an identification that links it to the field record.

## Initial Curing on Site

Standard-cured specimens spend up to 48 hours on site before they go to the laboratory. During this time they must be kept:

- in a temperature range of about 16 to 27°C (60 to 80°F), with a tighter range for high-strength concrete;
- protected from moisture loss, with lids or wet coverings; and
- still: away from vibration, traffic and direct sun.

A curing box with a thermostat and a max–min thermometer is the usual way to meet these conditions. Cylinders left in the sun on a hot day, or allowed to freeze overnight, will break low no matter how good the concrete is (National Ready Mixed Concrete Association, 2014[^1]).

## Transport and Final Curing

Specimens are moved to the laboratory only after they have set, and are cushioned and kept from drying, freezing or jolting on the way. At the laboratory they are removed from the molds and placed in final curing, moist storage at 23 ± 2°C (73.5 ± 3.5°F) with free water on their surfaces at all times, until the test age (Western Alliance for Quality Transportation Construction, 2025[^3]).

## Capping

The ends of a cylinder must be flat and square to the axis before it is broken, or the load concentrates on a high spot and the cylinder fails early. Ends are either ground, capped with a sulfur mortar (AASHTO T 231), or tested with unbonded neoprene pads in steel retainers (ASTM C1231). Pads must be of the hardness the method calls for, are replaced after a set number of uses, and are checked for cracks and splits.

Calculator: Concrete cylinder strength — https://pavementinteractive.org/tools/cylinder-strength/

## When Strength Comes in Low

Before concluding that the concrete is deficient, check the specimens (National Ready Mixed Concrete Association, 2014[^1]):

- Were they sampled from the middle of the load, from a remixed composite?
- Were they consolidated properly, with no rock pockets or voids visible on the broken pieces?
- Were they kept within the initial curing temperature, and away from sun and freezing?
- Were they moved while still soft, or bounced in a truck bed?
- Were the ends flat, and the pads in good condition?
- Did the break pattern look like a normal cone or column fracture?

If the specimens are suspect, in-place strength can be checked with cores (see *Pavement Cores*).

## Standard Test Methods

- AASHTO R 100, ASTM C31: Making and curing concrete test specimens in the field
- AASHTO T 22, ASTM C39: Compressive strength of cylindrical concrete specimens
- AASHTO T 231: Capping cylindrical concrete specimens
- ASTM C1231: Unbonded caps for hardened concrete cylinders

## Sources

[^1]: National Ready Mixed Concrete Association (NRMCA) (2014). CIP 9: Low Concrete Cylinder Strength. Concrete in Practice series. National Ready Mixed Concrete Association. Alexandria, VA. https://www.nrmca.org/wp-content/uploads/2021/01/09pr.pdf
[^2]: Taylor, P.; Van Dam, T.; Sutter, L. and Fick, G. (2019). Integrated Materials and Construction Practices for Concrete Pavement: A State-of-the-Practice Manual, Second Edition. National Concrete Pavement Technology Center, Iowa State University, for the Federal Highway Administration. Ames, IA. https://intrans.iastate.edu/app/uploads/2019/05/IMCP_manual.pdf
[^3]: Western Alliance for Quality Transportation Construction (WAQTC) (2025). Transportation Technician Qualification Program: Concrete Workbook. WAQTC/NAQTC. Hosted by the Alaska Department of Transportation and Public Facilities. Published October 2025. https://dot.alaska.gov/stwddes/desmaterials/mat_waqtc/assets/pdf/waqtcmanuals/workbook_concrete.pdf