ASTM F2170 In-Situ Relative Humidity Testing
Scope and method
ASTM F2170 measures relative humidity inside a concrete slab using in-situ probes. It is the direct method for the RH that a floor covering will experience. Holes are drilled to a specified depth, cleaned, and fitted with probes that equilibrate with the concrete pore air.
Core rules
- Depth is 40% of slab thickness for slabs drying from one side; use 20% where drying from both sides.
- Test frequency: three tests for the first 1,000 ft², plus one per additional 1,000 ft² or fraction.
- Allow the probe to equilibrate; 72 hours is the standard minimum after insertion. Follow the probe manufacturer's calibration and the standard's cleaning steps.
- Locations should represent the slab, including areas likely to be wettest, and holes should be spaced and away from edge effects.
Worked example
A 5,000 ft² slab is 150 mm thick and dries from the top. Required depth = 0.40 × 150 = 60 mm. Test count = 3 for the first 1,000 ft² + 4 for the remaining 4,000 ft² = 7 tests. A result read two hours after probe insertion is not valid because the hole has not equilibrated; the reading must wait the specified period.
Traps
Common errors are drilling too shallow, failing to clean dust from the hole, reading too soon, testing in an unconditioned space, and using a surface meter as a substitute. Do not average away a high reading at one location; report the location and value.