The spec says the epoxy floor coating must go on a concrete slab that passes ASTM F2170 at under 75% RH, and the GC wants to know why a plastic sheet test isn't good enough.

Tell the GC the plastic sheet test (ASTM D4263) is a qualitative "go/no-go" indicator only, not a quantitative measurement. It detects surface condensation, not the internal moisture vapor emission rate (MVER) or relative humidity (RH) deep in the slab.

Why it fails for epoxy specs:

1. Depth: Plastic sheet tests the top ¼"–½". Epoxy fails from moisture migrating from the bottom up. ASTM F2170 probes at 40% slab depth (20% if drying one side), capturing the equilibrium RH the coating will actually see after installation.
2. Quantification: Epoxy manufacturers warrant products based on specific %RH limits (e.g., <75%, <80%, <85%). A plastic sheet gives "wet/dry," not "72% RH." You cannot verify compliance or select the correct primer/moisture mitigation system without a number.
3. False Negatives: Low ambient humidity, HVAC, or surface sealers can dry the surface crust while the core remains at 95% RH. The plastic sheet reads "dry"; the epoxy blisters in 6 months.
4. Liability: If you install per a failed plastic sheet test and the floor fails, the manufacturer will deny the warranty claim citing non-compliance with ASTM F2170 (the industry standard per ACI 302.2R).

Code/Standard Reference: ASTM F2170 is the mandated method in ACI 302.2R-06 Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials and virtually all epoxy manufacturer data sheets.

Bottom line: The plastic sheet is a free preliminary check. The F2170 test ($50–$100/hole) is the insurance policy. Don't risk a $15–$30/sq ft epoxy job on a $20 test.

Tell the GC: The plastic sheet test (ASTM D4263) only detects visible moisture at the surface at the time of testing — it’s a qualitative, pass/fail check for liquid water or high surface dampness. It does not measure internal relative humidity (RH) within the concrete slab, which is what determines long-term moisture vapor emission that can cause epoxy coating failure (blistering, delamination, adhesion loss).

ASTM F2170 measures the equilibrium relative humidity inside the slab at 40% depth (via in-situ probes), giving a true indicator of the slab’s moisture condition over time. Concrete can appear dry on the surface but still emit significant vapor drive from within — especially if curing compounds, poor drainage, or high groundwater are present. Epoxy coatings are vapor barriers; if applied over a slab with >75% RH (per F2170), trapped moisture builds pressure underneath, leading to failure.

Plastic sheet tests miss this because they don’t quantify vapor transmission rate or internal RH — they only catch obvious, gross moisture. Relying on them risks costly rework. F2170 is the industry-standard, quantitative method required by coating manufacturers (e.g., Sherwin-Williams, Sika, BASF) and referenced in ACI 302.2R and ASTM F3010 for flooring success.

Tell the GC: “If we skip F2170 and rely on the plastic sheet test, we’re gambling with the coating’s lifespan — and the liability falls on us if it fails. Let’s do it right the first time.” (Word count: 198)