If you have ever been involved with a construction project, the odds are good that you have probably been on the receiving end of a Concrete Test Report. Found on most reports is information that discloses whether a curing box was present and used for the initial curing of the cylinders. But should the knowledge that the cylinders were placed in a curing box provide comfort to those on the receiving end of the report? To answer this question, one must turn to the ASTM C-31 standard.

When cylinders are molded on a project site, they are typically cast and then cured following one of three methods: first as “standard cured”, second as “field cured”. A third method, not spelled out in C-31, would represent curing that does not follow the procedures spelled out and required by the first two methods. To understand how a curing box comes into play, a review of the first two methods is needed.

“Standard curing” is used when one wants to a) determine if the concrete as delivered is acceptable per building code and the contract documents, b) check the adequacy of the mix proportions for strength or c) use the results for quality control purposes. ASTM C-31 defines specific steps to be followed if cylinders are to be considered standard cured, particularly for the curing process. Cylinders are initially cured for up to 48 hours, in a temperature range of 60 deg F – 80 deg F, in an environment that is shielded from direct sunlight and radiant heating sources, and in an environment that prevents moisture loss. Note that among this list of necessary elements, there is no requirement that a curing box be used, nor is there language that allows use of a curing box to take the place of the remaining steps. ASTM C-31 also requires the temperature of the storage to be “controlled” with the temperature recorded using a maximum-minimum thermometer.

“Field curing” is used when one wants to a) determine when a structure can be put into service or b) compare the results to standard cured cylinders to determine the adequacy of the curing of the concrete structure. Cylinders used for field curing are stored on or in the structure and protected as near as possible in the same way as the structure. They should be provided the same temperature and moisture environment as is the structure. Note that for field cured cylinders, there is no attempt to control the temperature or moisture of the cylinders, but rather they are allowed to experience the same conditions as does the structure. A curing box is therefore unnecessary.

So why use a curing box in the first place? ASTM C-31, in Note 8, does mention the use of a box, but only as a means to create a satisfactory moisture environment for the initial curing of cylinders cast under the standard curing method. The same note also discusses procedures that can be used to create a satisfactory temperature environment: ventilation, ice, a thermostatically controlled heating or cooling device. It is important to pay attention to the caution given in the note that heat generated during the early ages may raise the temperature above the required storage temperature.

So, when is a curing box not a curing box? When it is incorrectly used and the required elements listed in ASTM C-31 are not followed. For standard cured cylinders, the intent of which is to determine concrete acceptance per the contract documents and building code, this means:

1. The cylinders must be kept between 60 deg F and 80 deg F during the initial curing period.
2. The cylinders must be kept out of direct sunlight and other conditions that affect temperatures.
3. The temperature range of the initial curing must be recorded to verify compliance.
4. The initial curing period must last no longer than 48 hours at which point the cylinders must be transported to a facility for final curing (in a 73.5 deg F environment) until breaking.
5. The curing conditions (initial curing times and min/max temperatures) must be reported.

A concrete test report stating a curing box was used but failing to record and report the items listed above gives all in receipt of the report a false sense of security. If the requirements are not verified, then the results of the testing  cannot be used for concrete acceptance determination.

George R. Kuhn, Jr. 
Chairman – VRMCA Technical Committee