ICRI CSMT cheat sheet

Sources: residual mixing water (w/c ratio), free water from curing, groundwater/hydrostatic wicking, vapor drive from soil, rain ingress, and slab-on-grade vapor transmission through an unvented slab.

Rule of thumb: only water in excess of that needed for cement hydration (~0.23 g water per g cement, plus ~0.19 g for full hydration) remains as free water available to evaporate; lower w/c means lower residual moisture.

Transport mechanisms: capillary flow in the pore network (dominant at high RH), vapor diffusion down a vapor pressure gradient, and liquid flow under hydrostatic or osmotic gradients.

Drying is one-directional in slab-on-grade; the slab reaches a moisture profile with a wetter zone toward the bottom, so surface readings can understate the equilibrium value.

Vapor pressure is temperature-dependent (Clausius-Clapeyron); a warmer slab holds more moisture in the vapor phase, so RH at a given moisture content falls as temperature rises.

Effects: adhesive debonding and re-emulsification, blistering and bubbling of coatings, efflorescence, mold growth above ~70-80% RH, curling/warping, and dimensional movement of resilient and wood floors.

Alkali-silica reaction and delayed ettringite formation require moisture to proceed; keeping RH below a threshold slows but does not guarantee prevention.

Concrete equilibrium RH above ~75-80% is a common trigger for moisture-sensitive adhesives and coatings; verify the specific product's published limit rather than a generic number.

Scope: measures relative humidity in the concrete slab by inserting a probe into a drilled, sleeved hole at a specified depth, typically 40% of slab thickness for slabs on grade.

Required equilibration: the probe must remain in the hole until readings stabilize; ASTM F2170 specifies a minimum equilibration time and a repeat-reading stability criterion before recording the value.

Hole preparation: drill to depth, clean the hole, insert the sleeve so the probe tip sits at the target depth, and seal the top to prevent ambient air exchange.

Number of tests: the standard requires a minimum number of test locations based on slab area and provides rules for increasing locations when results vary; each test location is a distinct hole.

Moisture condition: report whether the slab is in service or has been wetted recently; readings taken too soon after a wet cure or rain event are not representative.

Temperature matters: RH is measured in the pore air; record slab temperature and ambient conditions, because probe readings depend on thermal equilibrium with the concrete.

Acceptance: compare the reported in-situ RH against the flooring/coating manufacturer's limit; F2170 is a quantitative method suitable for specification compliance.

Common error: reading the probe before equilibration, or leaving the hole open so ambient air dilutes the pore humidity, producing falsely low RH.

Scope: measures moisture vapor emission rate (MVER) from a concrete slab using a calcium chloride dish sealed under a dome for a fixed exposure period, reported in lb/1000 ft²/24 h (or metric equivalent).

Setup: clean the slab surface, place the pre-weighed CaCl2 dish under an airtight dome, seal the perimeter, and leave undisturbed for the specified exposure time (typically 60-72 h).

Result: the dish gains mass by absorbing water vapor; the mass gain is converted to an emission rate using the dish area and exposure time.

Surface preparation is critical: the slab surface must be clean and free of curing compounds, sealers, adhesives, and dust, or the test is invalid.

Conditions: record slab and ambient temperature and RH during the test; high ambient humidity or temperature swings bias the result.

Limitations: the test measures vapor emission over a short period and does not directly measure RH at depth; it is sensitive to surface condition and ambient conditions.

Acceptance: compare MVER against the product limit (commonly expressed in lb/1000 ft²/24 h); values above the limit require mitigation before flooring installation.

Common error: testing too soon after slab wetting or without sealing the dome, and reporting a single dish result as representative of the whole slab.

ASTM F2420: measures relative humidity at the surface of a concrete slab using a hood or sealed chamber placed on the surface, with an RH sensor inside; it reports surface RH, not in-situ RH at depth.

ASTM F2659: uses a handheld moisture meter (typically electrical impedance/capacitance) to compare relative readings across a slab surface; it is a qualitative screening tool, not a quantitative acceptance test.

F2659 readings are relative and instrument-specific; they cannot be converted to RH or MVER without calibration to a quantitative method.

Surface methods are fast and useful for mapping moisture variation, locating wet spots, and planning where to place F2170 or F1869 tests.

Surface RH (F2420) can differ from in-situ RH because the surface dries faster and is influenced by ambient conditions; do not substitute it for F2170 when the specification requires in-situ RH.

F2659 is sensitive to surface moisture, salts, and metallic objects (rebar, conduits); readings near embedded metal are unreliable.

Use surface methods as screening and verification, not as the sole basis for accepting a slab for moisture-sensitive flooring.

Record instrument make/model, calibration status, and test locations for any surface method, because results are not transferable between instruments.

Selection rule: use ASTM F2170 when the specification or product requires in-situ RH; use ASTM F1869 when the product limit is stated as MVER; use F2420/F2659 for screening and mapping.

Timing: test after the slab has reached a representative moisture condition, typically after curing and before flooring installation; testing during active wetting or immediately after rain is invalid.

Slab-on-grade with no vapor retarder is at higher risk; a properly installed vapor retarder reduces but does not eliminate vapor drive.

Environmental control: maintain stable temperature and RH in the test area for the equilibration and exposure periods; document conditions.

Number and location: distribute tests across the slab, include areas of concern (near exterior walls, plumbing penetrations, patched areas), and follow the standard's minimum count.

Do not test through sealers, curing compounds, or adhesives; remove them or select a location that represents the exposed slab.

Schedule mitigation (drying, dehumidification, vapor retarder, moisture-tolerant adhesive) before the flooring installation date; results drive the schedule.

When methods disagree, investigate: surface methods may read low on a dry surface while in-situ RH at depth is high; resolve with the quantitative method specified.

Report must identify the standard method (F2170, F1869, F2420, F2659), the test date, and the specific locations (with a plan or coordinates).

Include slab thickness, test depth, and whether the slab is on grade, elevated, or suspended; these affect interpretation.

Record ambient temperature and RH, slab temperature, and any recent wetting or curing history.

For F2170, report equilibration time and the stability criterion met before recording the value.

For F1869, report exposure time, dish area, mass gain, and the calculated MVER with units.

For F2659, report instrument model, calibration date, and that readings are relative and not quantitative.

Interpretation: compare results to the product manufacturer's published limit; state pass/fail against that limit and identify any areas requiring mitigation.

Retain raw data and photographs; results should be traceable and defensible if disputed.

Manufacturer limits are product-specific: some adhesives specify in-situ RH limits (e.g., 75-80%), others specify MVER limits (e.g., 3-5 lb/1000 ft²/24 h); always use the product's published value.

Coatings and epoxy systems are often more moisture-sensitive than resilient flooring; check the coating's required substrate RH and MVER.

Moisture-tolerant adhesives and vapor-reducing primers can raise the allowable limit, but only when the system is specified and installed as a complete system.

Wood flooring requires tighter moisture control; dimensional movement and adhesive performance depend on stable RH.

Do not mix methods: a product limit stated as MVER cannot be verified by an F2170 RH reading without a documented correlation.

Surface preparation (shot blasting, grinding) removes contaminants and opens the surface, but does not lower in-situ RH at depth.

Document the product's published limit and the test result side by side in the report to demonstrate compliance.

When results exceed the limit, mitigation options include extended drying, dehumidification, vapor retarder/ primer systems, or selecting a moisture-tolerant system.

Safety: use PPE (dust mask, eye protection, gloves) when drilling and grinding; control silica dust with wet methods or HEPA vacuum.

Electrical safety: keep cords and equipment away from wet areas; use GFCI protection on site.

Ethics: report results honestly, including unfavorable findings; do not alter data or omit test locations that fail.

Independence: disclose any relationship with the flooring manufacturer, contractor, or product supplier that could affect judgment.

Competence: only perform methods within your training and equipment capability; state limitations clearly.

Confidentiality: protect client data and test results; share only with authorized parties.

Recordkeeping: retain field notes, calibration records, and photographs to support the reported results.

Professional practice: follow the current edition of the referenced ASTM standard and the manufacturer's published instructions; do not substitute personal rules of thumb for the standard.