High-Moisture Slab With Adhesive Residue and Porcelain Tile
A general contractor is preparing a 600 ft2 ground-floor break room in a medical clinic for 24 x 24 rectified porcelain tile. The slab is 6 in. concrete on grade, 20 years old. The floor has carpet adhesive residue, several hairline cracks, and gypsum patch compound. Calcium chloride testing gives 5.2 lb/1000 ft2/24 h; pH is 10.5. The specification calls for an unmodified thin-set because the owner wants a low-VOC assembly and the tile is porcelain. The installer plans to grind the adhesive, vacuum, prime with a generic concrete primer, and set the tile with a 1/2 x 1/2 in. notched trowel, back-buttering only the cut tiles. The clinic will occupy the space in 10 days.
1. Interpret the moisture and pH results. Can the installer proceed with the specified unmodified thin-set and generic primer? Explain.
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The calcium chloride result of 5.2 lb/1000 ft2/24 h exceeds the common 3.0 lb limit in ASTM F1869 for many thin-set installations. That does not automatically mean tile cannot be installed, but it means the adhesive system must be approved for that moisture level or a moisture mitigation system must be used. A generic primer is not a substitute for a vapor-retarding membrane. The pH of 10.5 is also above the typical 10.0 maximum many thin-set manufacturers publish; the slab should be cleaned and the manufacturer consulted before assuming a primer will solve it. The specified unmodified thin-set is a poor choice for 24 x 24 porcelain even apart from moisture: porcelain has low absorption, and large-format tile needs a modified thin-set or an approved additive to achieve bond and coverage. The installer should not proceed with the proposed generic primer and unmodified thin-set without written approval.
2. What surface preparation and crack treatment are required before any tile or membrane is installed?
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The adhesive residue and gypsum patch must be removed, not merely vacuumed. Grind or scarify the slab to expose clean concrete, then HEPA-vacuum. Gypsum patch is not suitable under a moisture-sensitive assembly and may need to be removed entirely. The hairline cracks should be evaluated: if they are shrinkage cracks and not structural, treat them with a crack-isolation membrane or follow the moisture-mitigation manufacturer's detail. Do not bridge active cracks with thin-set alone. The slab should be sound, clean, and within the manufacturer's flatness tolerance; grind high spots and fill low spots with a compatible patch. pH should be retested after cleaning if the manufacturer requires it. Prime only with the primer specified for the selected membrane or thin-set, not a generic product.
3. Recommend a technically defensible assembly and schedule. Address the 10-day occupancy deadline.
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Use a moisture mitigation system rated for at least 5.2 lb and approved for the slab pH, or select an adhesive system with a written manufacturer allowance for that moisture level. Then use a modified thin-set or an unmodified thin-set with the manufacturer's additive, mixed exactly to the data sheet. For 24 x 24 porcelain, use a notch size that, with back-buttering, gives full coverage; 1/2 x 1/2 in. may be marginal, so check with a lift test. Back-butter every tile, not only cuts. Honor the cracks with the specified membrane detail and leave perimeter and movement joints. The 10-day schedule is tight: moisture mitigation often requires cure time, and tile, grout, and sealant need time before occupancy. If the membrane cannot cure and be tested in time, the opening should be delayed or a different flooring system selected. Do not let the schedule drive an unapproved assembly.