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 get the appropriate curing.
Concrete specifications are less explicit than they should be regarding how to ensure adequate curing and who is responsible for it.
Although ASTM C31 specifies the curing conditions for concrete cylinders for acceptance testing, only about half of projects meet them.
The costs of poor curing
What happens when cylinders don’t get proper curing in the first 48 hours? Compressive strength measurements can be up to 20% lower. That forces the concrete producer to increase the strength, adding to the cost.
These changes to obtain higher strength may also make the concrete more susceptible to shrinkage and cracking. In addition, test results may be more variable, making it difficult to meet strength specifications.
Every time there’s a low test result, the engineer must investigate to ensure that the strength of the in-place concrete is acceptable. Normally construction comes to a halt for coring and testing of the relevant member. It usually takes a week or longer to condition and test the cores, so the delay is not trivial.
Often delays incur penalties, further adding to the cost. If the strength test result was artificially low because of improperly cured cylinders, the time and money have been wasted.
Taking responsibility for curing concrete cylinders
A recent paper in Concrete International recommends that the testing agency be explicitly responsible for onsite curing. The paper also recommends specifying preplacement meetings to allow all 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 necessary in summer. In winter, an insulated container and warm water from the concrete truck should be sufficient to keep the temperature above 60 °F.
Download the entire paper from Concrete International, August 2018, pp. 28-35.
The authors are Karthik H. Obla, Orville R. (Bud) Werner, John L. Hausfeld, Kevin A. MacDonald, Gregory D. Moody, and Nicholas J. Carino