Two Moisture Tests, One Slab, and a Monday Start

The in-situ RH probe result governs because it measures the equilibrium moisture condition within the slab pore structure, which is what drives moisture vapour emission through the finished floor. ASTM F2170 at 40 mm depth gives a direct RH reading that can be compared with the adhesive manufacturer's published limit. ASTM F1869 measures moisture emission at the surface over 24 hours and is sensitive to surface conditions: power-troweling, curing compounds, dust, and soluble salts can all distort the result. A slab can emit at a rate that appears acceptable while the deeper concrete remains at 86% RH, especially where there is no vapour retarder and outdoor humidity is high. The 4.2 lb result does not override the 86% probe reading. The adhesive limit is 80% RH; 86% is over the limit, so installing on Monday would risk adhesive re-emulsification, blistering, and debonding. The correct decision is to treat the slab as too wet and either mitigate the moisture or select a system whose manufacturer accepts the measured condition in writing.

I would specify a moisture-suppression system compatible with the slab and the adhesive: typically a two-coat epoxy or urethane vapour-retarder coating applied to a prepared, sound, contaminant-free surface, with the manufacturer's coverage rate and recoat window followed. Because the slab has no vapour retarder and reads 86% RH, a surface coating must be selected on the basis of its published perm rating and its ability to withstand the measured RH, not on the basis of a generic 'sealer'. I would require the slab to be mechanically prepared, cracks and bugholes filled, and the coating applied at the specified thickness with wet-film gauge checks. After cure, I would require a second ASTM F2170 probe test in the same locations, plus adhesion pull tests of the coating, before accepting the floor. The resilient adhesive must also be approved for use over the mitigation system. If the coating cannot reduce the reading below the adhesive limit, the schedule must change or a different moisture-tolerant system must be engineered. The three-week window is tight because each coat and the verification test need cure time, so the work should start immediately rather than waiting for fixtures.

I would issue a written field report and a request for information that state the test methods, locations, dates, calibration information, and results, and that identify the adhesive manufacturer's 80% RH limit. The report should explain that ASTM F1869 and ASTM F2170 measure different things and that the probe result is the governing one for this adhesive. I would not rely on a verbal conversation. The report should recommend a hold on installation until mitigation and re-testing are complete, and it should ask the general contractor and owner to confirm the schedule impact in writing. I would also notify the adhesive and mitigation manufacturers' technical services and keep their written recommendations in the project file. If the contractor directs installation to proceed, I would document that direction and the manufacturer's position before covering the slab. Clear documentation protects the installer, the owner, and the flooring manufacturer, and it gives the project team a factual basis for deciding whether to mitigate, reselect materials, or delay.