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12/15/2004

WHY TEST CYLINDER INITIAL CURING IS SO IMPORTANT

"How Producers Can Correct Improper Test Cylinder Curing," from the November 1997 issue of The Concrete Producer focuses on why poor curing (temperatures lower than 60F or higher than 80F, and drying) during the critical 48 hours before a cylinder is taken to the laboratory can significantly reduce the strength at both 7 and 28 days. If the cylinders stay on the jobsite longer than 48 hours, which isn't allowed by ASTM C 31, "Standard Practice for Making and Curing Concrete Test Specimens in the Field," strength loss can be much greater under poor initial curing conditions. But with a curing box, the effect of exceeding the 48-hour limit might still not be as bad as leaving the cylinders exposed to the weather.

Here's why a curing box is a good investment:

* ACI 318-02, Building Code Requirements for Structural Concrete," says further action is needed if a single test result (average of two cylinders, usually tested at 28 days) falls more than 500 psi below the specified compressive strength. Further action is also needed if any arithmetic average of three consecutive tests falls below the specified compressive strength. So either a single test result that's quite low, or a series of three test results that are just slightly low, may trigger steps needed to increase the average strength (and cost) of the concrete. It may also delay the project while the design professional figures out what to do about the low test. If coring is required to verify strength adequacy, another bad thing may happen. The core may be strong enough, but if it's from a floor, it may not be long enough. Pulling a 5-1/2-in. core out of what's supposed to be a 6-in.-thick floor isn't going to make the owner happy, and may cost the contractor a lot more money than buying a curing box and making sure the cylinders are well cured while on the jobsite.

• The curing box can also prevent some headaches for the design professional by reducing the chances of a low test result being caused by faulty initial curing. Having to decide whether to use as is or take some corrective action takes time, perhaps requiring analysis of how the low strength will affect the strength of the structure. So both the contractor and engineer benefit from having a curing box on site.
  
• And of course the concrete producers benefit because they're less likely to be accused of supplying inferior concrete.

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