Moisture Tests Exceed Limits but the Schedule Pushes Ahead
A 4,200 sq ft slab-on-grade warehouse is being converted to a clinic. The 1972 slab has no vapor retarder; VCT is specified. Before VCT, the contractor performed two moisture tests: calcium chloride gave 6.5 lb/1,000 sq ft/24 h, and in-situ RH probes gave 82 percent. The adhesive data sheet limits the slab to 5 lb and 80 percent RH. pH is 10.5. After a rain, white efflorescence appears along exterior door thresholds, but the center of the slab looks dry. The general contractor wants to seal the slab with a clear curing compound and start layout Monday because the clinic opens in 10 days. The VCT is on site, and the pressure-sensitive adhesive is in date. The flooring crew has a 10-ft straightedge but no hygrometer or dehumidifier. The owner's rep asks whether the two tests conflict and whether the sealer is an acceptable fix.
1. Can the VCT installation proceed Monday? Interpret the two moisture results against the adhesive limits and the visual evidence.
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No. Both results exceed the adhesive limits: 6.5 is greater than 5 lb and 82 percent is greater than 80 percent RH. They are not conflicting; calcium chloride measures vapor emission rate and in-situ RH measures internal RH, and a slab can fail either. The efflorescence at the thresholds suggests localized moisture movement, not a uniformly dry slab. pH 10.5 is also high enough to affect some pressure-sensitive adhesives. A clear curing compound is not a vapor retarder and could weaken bond; do not accept it as a fix. Layout Monday should be stopped until a moisture mitigation design is approved by the adhesive and flooring manufacturers. The 10-day schedule is not a technical justification.
2. What assessment and remediation steps should be completed, and what documentation should be issued before work resumes?
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Document the existing tests and locations; probe the slab on a grid, especially near doors and exterior walls. Identify the moisture source: site grading, downspouts, irrigation, plumbing, or missing vapor retarder. Have a qualified contractor propose a slab moisture mitigation system with a manufacturer's warranty and adhesion test. Re-test after cure using the same methods, then run a mockup. Issue an RFI and change order because the slab condition differs from the bid assumption; update the schedule and notify the owner in writing. Do not install VCT over a sealer unless the sealer is specifically rated as a moisture barrier and approved by both manufacturers.