Fast-Track F2170 on an Unconditioned Slab
An 18,000 ft² slab-on-grade in a new distribution center is 21 days old, 6 in. thick, with a 15-mil vapor retarder directly beneath it. The building is not enclosed: exterior doors stand open, the slab is at 48°F, and ambient swings from 45°F at night to 70°F by afternoon. The general contractor needs a moisture report by Friday to release the slab to the resilient flooring installer, whose specification limit is 80% RH by ASTM F2170. A technician drills six holes in a line down the center aisle, each to 2.4 in., vacuums them, inserts sleeves and plugs, and reads each hole after 3 hours because the probe appears to stabilize. Results are 68, 71, 69, 73, 70, and 72% RH. The GC forwards the report as a pass. Two months later, with HVAC running, the same six locations read 83–87% RH and the installed flooring shows dome-shaped blisters along seams.
1. Was the first set a valid ASTM F2170 test? Identify each deviation and explain how it biases the reported RH.
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The hole depth is correct: 40% of 6 in. is 2.4 in. Almost everything else is not.
- Equilibration. The default minimum in F2170 is 72 hours, not 3. A freshly drilled hole is ventilated and cooler than the surrounding pore system; short waits read the disturbed zone and typically read low in a drying slab. The probe showing a stable number for a few minutes is not hole equilibrium.
- Service conditions. F2170 results are tied to representative in-service temperature and humidity. Doors open, a 48°F slab, and daily swings mean the slab is not at service conditions, and the reading is not comparable to an 80% limit derived under occupied conditions.
- Representativeness. Six holes along one aisle do not represent 18,000 ft², multiple pours, or edges and penetrations. A center-aisle line is the easiest place to read low.
- Documentation and technique. There is no record of probe verification, hole cleaning adequacy, sleeve seal, slab temperature, or ambient conditions. Any one of these can shift a result by several points.
The later 83–87% values are consistent with a slab that was still drying and equilibrating internally. The first report is not a valid basis for release.
2. What should have been done before testing, and what retest plan would you specify now?
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Before any test: close the building, run HVAC to the conditions the floor will actually experience, and hold them long enough for the slab and air to stabilize, with temperature and RH logged. Confirm the slab is at least 28 days old or per specification, and that no curing compound, sealer, or adhesive residue remains.
Retest plan:
- Mark a grid that covers every pour, perimeter zones, columns, and any known wet areas; do not reuse the original holes.
- Drill to 40% of nominal thickness, clean thoroughly, insert the sleeve so the hole volume is sealed from room air, and plug.
- Wait the standard default 72 hours; any shorter interval requires demonstrated stability, not schedule pressure.
- Verify the probe against a known salt standard before and after, record slab and ambient temperature at each location, and log the time of drilling and reading.
- Report each location individually and the range, not only an average.
If any result is within a few points of 80%, increase density rather than argue about the average.
3. The owner asks whether the later flooring failure is the tester's fault. How do you frame causation and what documents matter?
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Framing should separate data quality from causation. The first test was not a valid basis for declaring the slab dry, and the report should have stated its conditions and limitations. That is a professional and documentation failure. It does not by itself prove the flooring failed because of moisture.
Other plausible contributors: adhesive or flooring installed outside its own temperature and RH limits, HVAC startup timing, slab thickness or vapor retarder continuity, and prolonged wetting from construction water. Blistering along seams is consistent with vapor drive, but it is not a diagnosis on its own.
Documents to collect: original field sheets and probe verification, photographs of holes and sleeves, thermostat and ambient logs, the specification and product data, RFIs about schedule pressure, the flooring installer's own records, and any post-installation moisture readings. The defensible conclusion is that the first report could not support a pass and the area-wide claim was unsupported, while causation of the floor failure requires a separate investigation.