Why Test at All?
Concrete moisture content varies dramatically by slab age, season, and drainage conditions. A Houston-area garage poured in summer during a drought reads very differently in January after 6 inches of rain. Testing at the time of installation — not at the time of sale — is the only reliable data point. ASTM International provides two accepted testing methods for moisture content and vapor emission.
ASTM F1869 — Calcium Chloride Test
The calcium chloride test measures moisture vapor emission rate (MVER) by placing a sealed container of calcium chloride on the concrete surface for 60–72 hours. The chloride absorbs moisture vapor, and the weight gain determines MVER in lbs/1,000 sq ft/24 hours. This is an inexpensive, widely used test. Limitation: it measures only the top 1/2" of the slab, missing moisture that migrates from deeper layers.
ASTM F2170 — In-Situ Relative Humidity
The RH test drills a hole 40% of the slab depth, inserts a calibrated humidity probe, and measures relative humidity within the slab after 24-hour equilibration. This test captures moisture moving through the full slab thickness — more accurate for thick slabs and high-water-table conditions like much of the Katy area. The action threshold is 75–80% RH for standard coatings; moisture-tolerant systems handle up to 95% RH.
What We Do with the Results
Readings below 3 lbs (F1869) or 75% RH (F2170): standard epoxy primer, any decorative system. 3–10 lbs / 75–85% RH: moisture-tolerant epoxy primer required, limited to mid-range decorative options. Above 10 lbs / 85% RH: heavy-duty moisture-barrier system, source investigation, possible crack sealing required before coating. We document all test results and include them in the project file.
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