Pressure to Certify an Elevated Slab
A technician is on site at a 15,000 ft² elevated composite deck pour: 4 in. of lightweight concrete over galvanized deck, no vapor retarder, 11 weeks old, building enclosed, HVAC running at 72°F and 50% RH. The flooring contractor is behind schedule and asks for ASTM F2170 holes at 1.6 in. and only three holes near the elevator lobby, where the slab is driest, because a full layout will delay installation. The technician complies and reads 74, 76, and 78% RH. The contractor then asks that the report state that the slab passes 75% RH for the entire 15,000 ft². The technician's firm has been told future work depends on a favorable report.
1. Evaluate the test plan: depth, number, location, and the fact that this is an elevated lightweight slab.
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Depth: 1.6 in. is 40% of a nominal 4 in. slab and does not reach the deck, so the arithmetic is right. But lightweight concrete over deck varies in thickness and density, and the deck profile creates a different drying path than a slab-on-grade. The technician should verify actual thickness at the holes rather than rely on nominal.
Number and location: three holes clustered at the elevator lobby cannot represent 15,000 ft², multiple pours, perimeter zones, or areas near penetrations and drains. The lobby is likely the driest, warmest, and most ventilated area, so the selection is biased low. F2170 requires a representative layout and minimum test density; this plan fails the intent regardless of the arithmetic.
Elevated slab: with no vapor retarder and a lightweight mix, drying is slow and uneven. Eleven weeks may be insufficient, and the deck can trap moisture. A 74–78% result from three favorable holes is not evidence that the rest of the slab is below 75%.
The plan is inadequate and the area-wide conclusion is unsupported.
2. How should the technician respond to the request, and what professional-practice principles apply?
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The technician should decline clearly and in writing.
A report may state only what the data support: three readings at three named locations. It cannot state that the entire 15,000 ft² passes 75%. Doing so would misrepresent the scope of the investigation, which is a professional and ethical failure, not a customer-service decision.
The offer of future work in exchange for a favorable report creates a conflict of interest. The technician should disclose it to a supervisor and document the conversation. If pressure continues, escalate through the firm's quality manager and, if necessary, the owner's representative. Applicable principles include honesty and objectivity, avoiding conflicts of interest, reporting limitations, and not altering or selectively reporting data.
The constructive response is to offer a compliant sampling plan with a realistic timeline, explain why the lobby holes are not representative, and let the contractor make schedule decisions based on real data rather than a false certification. If the contractor refuses, test only what is authorized and label the report as limited.
3. What should the report contain, and how should the limitations be stated?
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The report should be a factual record, not a marketing document.
Include: project and client; date and time of drilling and reading; ASTM F2170 as the method; slab thickness verified at each hole and depth; calibration and verification of probes; hole cleaning and sealing method; equilibration period; ambient and slab temperatures and RH; a plan or description of each location; individual readings; range and average; and any deviations.
State the conclusion as: these three locations read 74, 76, and 78% RH under the stated conditions; results apply only to those locations and that time. State explicitly that the remainder of the 15,000 ft² was not tested and that no area-wide pass is claimed.
Add limitations: lightweight concrete over deck without a vapor retarder may continue to dry or redistribute moisture; seasonal HVAC changes can shift RH; readings near a limit require denser testing.
Do not write 'slab passes 75%' for the entire area. If the client wants an area-wide conclusion, require the representative layout first.