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November 15 - November 21, 2015

How can I determine when concrete has cured sufficiently to be coated, besides waiting the 28 days typically specified?

Selected Answers

From Rodney White of Independent Consultant on November 24, 2015:
The point of my short previous answer is that every coating manufacturer will state a range of acceptance (tolerance) of pH and/ or moisture content to ensure successful application. Those directives should always be followed, without exception. Those conditions may occur as early as 7 days from placement, to 28 days or longer.The point is to quantify the conditions periodically, until the conditions fall within the manufacturer's stated range. This range will probably differ from manufacturer to manufacturer, and from product to product.

From William Slama of International Paint/Ceilcote Products on November 23, 2015:
Many coating and lining suppliers require the ASTM Plastic sheet test (D4454). It assures that the coating (or primer)is able to contact the dry concrete surface and start its curing (or drying) process before any water comes up under it. As most know, the 28-day requirement is an old standard that evolved only because concrete specs usually require a 28-day strength test. That is not really necessary per any of the manufacturers specs that I am aware of. In most cases, 7 to 14 days of cure are adequate for application of systems over concrete. Suppliers usually specify a minimum surface tensile strength of the concrete prior to coating. That ranges from 150 psi for thin film coatings to 300 psi for thicker linings. The intent of that is to assure that future curing stress, thermal stress, and physical stresses to the surface do not cause the coating or lining to fracture the concrete surface.

From David Zuskin of Indepedent on November 20, 2015:
Tape a 12" x 12" sheet of polyethylene to the concrete and check 24 hours later for moisture under the polyethylene. This is an ASTM test I believe (I do not normally work with concrete coatings).

From Rodney White of Independent Consultant on November 18, 2015:
Measur the pH and moisture content.

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