Concrete Construction editor-in-chief, Bill Palmer recently wrote a blog on the most common misconception about the tested strength of a concrete cylinder. In a follow up to his thoughts, I’d like to advise concrete producers that the correct testing of the cylinders is how we get paid.
Speaking from experience
When I first owned Gallup Sand and Gravel Co., cylinders were usually left under the trailer for 28 days and then taken to the closest testing facility 140 miles away and tested. Generally they did not make strength, and in some cases, they did. As a result of this improper curing of test cylinders, we started our own testing laboratory, and were able to get it CCRL approved.
As we began making cylinders as a quality control check for the customer, we invested in water-cured boxes like the intelliCure made by FLIR Systems. We took it to the job and asked the technicians to make the cylinders and properly do initial curing of the cylinders. As a result we had very few low breaks at 28 days because of the water curing.
The water curing box quickly paid for itself. The ready-mix producer could prove the concrete being shipped to the job was correct, and in return, promote that high quality concrete as a marketing tool.
Currently, six different curing methods are allowed under ASTM C 31, Standard Practice for Making and Curing Concrete Test Specimens in the Field.
I believe the producer should get credit for all of the potential strength of the concrete and propose a change to ASTM C 31: that all cylinders be required to be cured immersed in water for the first 24 hours.
Ideally all testing labs should have a temperature-controlled curing box on the jobsite in which to store cylinders for the first 24 to 48 hours. And, I believe ASTM needs to remove the following inadequate curing techniques from C 31:
(2) store in properly constructed wooden boxes or structures,
- (3) place in damp sand pits,
- (4) cover with removable plastic lids,
- (5) place inside plastic bags,
- (6) cover with plastic sheets or nonabsorbent plates if provisions are made to avoid drying and damp burlap is used inside the enclosure.
As we push to have performance-based specifications, I believe the immediately immersed molded specimens with plastic lids in water, saturated with calcium hydroxide, is the ideal way to initially cure concrete. However, in my work we just used tap water. The computers which batch the concrete are quite precise. The only variable is the testing and initial curing. With this initial curing in water between 60˚ F and 80˚ F, we can eliminate a lot of problems and meetings over cylinders that don’t meet the requirements. I am very interested to see what other producers think of this idea.