Dry on Top Doesn't Mean Dry

The Wet Edge, No. 008. Short field notes for coating crews.

A slab can look dry, feel dry and still be pushing moisture up from underneath. Coating it is like putting a lid on a simmering pot: the moisture has nowhere to go but into the bond line.

That's why the plastic sheet test (ASTM D4263) isn't enough by itself. It shows whether moisture reaches the surface in a day, but it doesn't put a number on it. The calcium chloride test (ASTM F1869) gives you an emission rate. The in-situ RH probe (ASTM F2170), set in a drilled hole at 40% of slab depth, reads what's going on inside the concrete, and it's the most reliable of the three.

The numbers: many standard floor coatings want no more than 3 lbs per 1,000 sq ft in 24 hours, or 75 to 80% RH. Moisture-tolerant systems go much higher. Your product's data sheet sets the limit.

Why it matters: moisture failures show up as blisters and disbonding weeks after the crew's gone, and fixing a floor means grinding it off.

The practical move: test before you lock in the schedule, not the morning you mix. New concrete also needs its full cure, usually 28 days. If the numbers come back high, call it early. A moisture-mitigating primer is a change order, not a surprise.

Bottom line

A dry-looking slab can still be wet inside. Test with RH probes or calcium chloride before you schedule, and let the data sheet set the limit.

Further reading: 3 Ways to Test Concrete for Moisture (Tnemec). Standards: ASTM D4263, ASTM F1869, ASTM F2170.

Talk to a consultant. Questions on a job? Atlantic Coating Consultants supports contractors in PA, NJ, DE, MD, DC, WV and OH with product recommendations, field support and crew training. Call 484-644-5303 or email atlantic@tnemec.com.