A recent paper in Concrete International recommends changes to the relevant ACI and ASTM specifications as well as the AIA MasterSpec to ensure that concrete cylinders are cured as specified.
Concrete specifications are less explicit than they should be regarding what needs to be done to ensure adequate curing and who is responsible for doing it.
Although ASTM C31 specifies the conditions under which concrete cylinders for acceptance testing are cured, these conditions are met on only about half of project sites.
What happens when cylinders aren’t cured properly in the first 48 hours? The measured compressive strength can be reduced by up to 20%, forcing the concrete producer to use more cement or other ingredients to increase the average strength, adding unnecessarily to the cost.
The changes needed to obtain higher strength may make the concrete more susceptible to shrinkage and cracking. Test results may also be more variable, making it difficult to meet specified strength ranges.
Low strength test results must be investigated to ensure that the strength of the concrete in place is as required. Normally construction must be halted while the relevant member is cored, and the cores conditioned and tested.
Often penalties are assessed for delays, further adding to the cost of the investigation. If the measured strength was artificially low because of improperly cured cylinders, the time and money have been wasted.
A recent paper in Concrete International magazine recommends that the testing agency be explicitly given the responsibility for onsite curing. The paper also recommends that preplacement meetings be specified to allow the various parties to coordinate onsite curing. NRMCA’s checklist for concrete acceptance testing should be used at this meeting.
Adequate onsite curing may be as simple as storing capped specimen molds in the jobsite trailer. If there is no power or no trailer, specimens can be immersed in water in 5 gal buckets with tight-fitting lids and stored in the shade or in a trailer or shed.
Ice may be needed in summer. In winter, an insulated container and warm water from the concrete truck should be sufficient to keep the temperature above the specified 60 °F.
Download the entire paper from Concrete International, August 2018, pp. 28-35
It’s co-authored by Karthik H. Obla, Orville R. (Bud) Werner, John L. Hausfeld, Kevin A. MacDonald, Gregory D. Moody, and Nicholas J. Carino