High In-Situ RH Readings on a Recreation Centre Slab Without a Vapour Retarder
You are assessing a 650 m² ground-floor slab for tile in the changerooms and corridor of a new recreation centre. The slab was placed nine months ago, and the vapour retarder under it was value-engineered out. The concrete finisher applied a curing compound. The general contractor wants tile installed in three weeks. You commission three ASTM F2170 in-situ relative-humidity probes, drilled to 40% of the slab depth and allowed to equilibrate for 72 hours. Readings at 20 °C are 86%, 88%, and 87%. A surface pH test reads 11. The specified thin-set is a standard cementitious product whose manufacturer limits substrate relative humidity to 75%. The slab also has a 3 mm undulation over 3 m and two shrinkage cracks. The building's HVAC will not run until after occupancy.
1. Interpret the F2170 results. Are three probes sufficient, and what else should be verified before choosing a repair?
Read the worked answer
The readings are 11 to 13 percentage points above the adhesive limit of 75%, so the slab is not acceptable for the specified thin-set as-is. The F2170 method is appropriate only if the probes are at 40% of slab depth and have equilibrated; 72 hours is the standard minimum. Three probes are not enough for 650 m² (about 7,000 ft²). The test method calls for at least three tests for the first 1,000 ft² and one additional test for each additional 1,000 ft² or fraction, so this area needs roughly nine probes. Also verify the curing compound type and coverage, check pH 11 against the adhesive manufacturer's limit, and inspect the two cracks for activity. A surface-only moisture test is not a substitute for in-situ RH. Confirm the slab is not receiving direct rain or standing water and that the building will be enclosed and conditioned before installation.
2. What remediation options are available, and which would you recommend? Explain why simply switching to a 'high-RH' adhesive may not be enough.
Read the worked answer
Options include waiting for drying, using a moisture mitigation system, or using an uncoupling or membrane system rated for the measured conditions. Waiting is unreliable because the slab has no vapour retarder and has been covered with curing compound; nine months of age does not guarantee it will reach 75%. The practical recommendation is to mechanically remove the curing compound by diamond grinding or shot blasting to CSP 3–5, then apply a 100% solids epoxy moisture mitigation membrane at the manufacturer's specified thickness, with aggregate broadcast and pinholes repaired. Confirm in writing that the membrane system tolerates the measured RH and pH. A 'high-RH' adhesive alone does not address slab vapour drive, curing compound, or the risk of adhesive re-emulsification; it only raises one limit. Address the cracks with a crack-isolation membrane or repair detail compatible with the chosen system.
3. What verification and site conditions must be met before tile installation proceeds?
Read the worked answer
After mitigation, verify membrane thickness, continuity, pinholes, and adhesion with pull tests or the manufacturer's quality-control method. Do not rely on a new F2170 reading through the membrane to prove the slab has dried; the membrane is the control layer, and the manufacturer's limits govern. Grind high spots to meet the specified flatness, fill or isolate the shrinkage cracks, and confirm the substrate is clean and sound. The HVAC must be operating and the space maintained within the adhesive and membrane manufacturer's temperature range, typically about 18–29 °C, with no direct sun or drafts. Record batch numbers, application dates, and cure times. Install a mock-up area, let it cure, and check bond before releasing the full 650 m².