Why trapped vapour destroys a coating
A slab on grade without a working vapour barrier wicks soil moisture upward continuously and evaporates it off the surface. You never notice, because bare concrete lets it go. Put an impermeable coating on top and that vapour has nowhere to escape, so pressure builds at the bond line until it lifts the film in domed bubbles, typically in year two.
The two methods
Calcium chloride seals a pre-weighed dish of salt under a dome for 60-72 hours and re-weighs it. In-situ RH drills to 40% of slab depth, inserts a sleeve, and reads humidity inside the concrete after equilibration. In-situ RH is the more reliable test because it measures the whole slab rather than just the surface.
What a high reading means
Not that the floor cannot be coated. It means the system starts with a 100% solids moisture-vapour-barrier primer rated to resist the pressure, typically to 95%+ RH, with the decorative build going on top as normal. That adds $2-$4 per square foot and it is not optional where the readings call for it.
The slabs most at risk
Basements, which contact soil on multiple faces. Slabs poured before vapour barriers were routine, which locally means most residential construction before the 1990s. New concrete still releasing water. And anything at the bottom of a slope where groundwater collects. A taped plastic sheet left overnight showing condensation underneath is a free early indicator.
Want this answered for your slab?
Free site visit with a moisture reading, then a written fixed price. Across the BC Interior.