Calcium Chloride Over a Curing Compound
A 5,200 ft² retail tenant space has a 5 in. slab-on-grade placed 75 days ago. The concrete contractor applied a curing compound at the finisher's recommendation and it was never removed. The slab is now covered with construction dust, and the entrance zone near the storefront has been walked on and is visibly cleaner. The flooring specification permits either ASTM F1869 with a limit of 3 lb/1000 ft²/24 h or ASTM F2170 with a limit of 80%. A testing firm places three F1869 kits, all within 15 ft of the storefront, and reports 2.6, 2.8, and 2.9 lb. The building has no HVAC yet, and the space was 58°F with 70% ambient RH during the test. The flooring contractor calls the result a pass and wants to install VCT next week. The owner's representative asks you to review the report before approval.
1. Identify the problems with this F1869 test and explain how each one affects the result.
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F1869 measures vapor emission from the concrete surface into a sealed dish over 24 hours. That makes it highly sensitive to whatever is on the surface.
- Curing compound. A film-forming curing compound can act as a partial vapor retarder and depress emission. The number may describe the film, not the concrete.
- Dust and contamination. Dust can absorb or release moisture and interfere with the seal. The entrance zone is cleaner than the rest of the slab, so it is not representative.
- Location bias. Three kits clustered within 15 ft of the storefront sample the driest, most walked-on area. The rest of the 5,200 ft² is untested.
- Conditioning. The test was run at 58°F and 70% ambient RH without HVAC. Emission rate is temperature- and humidity-dependent; results are not comparable to a limit established under occupied conditions, and cooler concrete emits less.
- Test count and layout. A clustered set does not meet the intent of random, representative placement across the test area.
A 2.6–2.9 lb result under these conditions is not a credible pass at a 3 lb limit.
2. Compare what F1869 measures with what F2170 measures, and state which method you would use here and why.
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F1869 reports a mass of water vapor emitted per unit area per 24 hours from a covered surface. F2170 reports the equilibrium relative humidity in a hole drilled to 40% of slab thickness. They measure different quantities: emission rate is a surface flux influenced by surface films and air conditions; internal RH is a driving-force measurement less affected by a surface film.
Here the slab has an intact curing compound and unknown surface preparation. That is exactly the condition that makes F1869 unreliable, because the compound can control the flux. F2170 is the better choice for characterizing the slab, provided the holes are properly drilled, cleaned, sealed, equilibrated for 72 hours, distributed across the area, and tested under representative conditions.
The specification allowing either method does not make them interchangeable. Method selection must account for surface condition and the flooring product's stated test method. A sealer can also slow drying, so a high internal RH remains possible and must be measured rather than assumed.
3. Write the review recommendation: what must happen before the floor is approved?
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Recommend disapproval of the current report as a basis for installation.
Required actions:
- Mechanically remove the curing compound and dust from the entire slab, not just the entrance, and verify the surface is sound and clean.
- Enclose and condition the space to the intended service temperature and RH for at least 48 hours before and during testing, with logs.
- Retest with a documented layout covering the full 5,200 ft², including perimeter and interior zones. Prefer F2170 at 40% depth with 72-hour equilibration; if F1869 is used, ensure proper surface preparation, random placement, and correct conditioning.
- Compare results to the flooring and adhesive limits for the method actually used. Do not convert F1869 results into RH or vice versa.
- Require a report showing locations, conditions, calibration, deviations, and a conclusion limited to the tested area.
Until these steps are complete, the 2.6–2.9 lb result should be treated as an invalid pass, not a green light.