ASTM F2170 is the standard test method for determining relative humidity (RH) in concrete floor slabs using in-situ probes. It is the most commonly specified internal moisture test for commercial flooring installations and one of the most important documents a project team can request before making a go/no-go decision on flooring.
What ASTM F2170 measures
ASTM F2170 measures the relative humidity inside a concrete slab using probes placed in drilled holes at a defined depth — typically 40% of the slab depth for slabs drying from one side, and 20% for slabs drying from both sides. The reading captures conditions at that depth, where moisture concentration determines the long-term moisture emission potential of the slab after flooring is installed.
Why RH testing predicts flooring performance
RH testing reflects internal moisture conditions that drive long-term moisture movement into flooring. The surface of a slab can appear dry — and can produce low readings on surface-based instruments — while the interior of the slab still holds substantial moisture that will continue to release upward after installation. Adhesives, coatings, and moisture-sensitive flooring systems are vulnerable to this post-installation moisture release, which causes adhesive re-emulsification, bubbling, delamination, and flooring failure. F2170’s in-situ approach captures the actual internal state rather than the surface condition.
Field steps (specific)
The test procedure involves drilling holes in the slab at the specified depth, installing sleeves, and allowing the holes to equilibrate with the slab’s internal humidity for the period specified by the standard (typically 72 hours for in-situ sleeves). After equilibration, calibrated RH probes are inserted, and readings are taken after a defined acclimation period. Temperature must also be recorded, as RH readings are temperature-dependent. Test locations, depths, probe calibration records, RH readings, and temperature readings should all be documented and reported.
How many tests and where to place them
Place probes in representative areas of the slab and in known risk zones — near exterior walls, below-grade areas, locations with prior water events, areas near penetrations, and any zone with visible staining or suspected moisture history. ASTM F2170 provides minimum test frequency guidance as a function of total area; project teams and their consultants should consider placing additional probes at risk zones regardless of the formula count. The goal is results that reflect the slab’s true highest-risk moisture conditions, not just its average conditions.
Interpreting results for a go/no-go decision
RH results must be compared to the flooring manufacturer’s specified maximum allowable RH before installation — not a generic number, but the manufacturer’s written installation requirement for the specific product being installed. Common thresholds range from 75% to 90% RH depending on the system. Results above threshold require a documented decision: delay installation and allow further drying, improve drying conditions (heat, ventilation), or install a manufacturer-approved mitigation system. For more on method selection, see IFTI’s ASTM F2170 vs F1869 comparison and our full concrete moisture testing services.
How F2170 compares to F1869 (calcium chloride)
F2170 is an internal RH test; F1869 is a surface emission test that measures the moisture vapor emission rate (MVER) from the slab surface over 24 hours. F2170 is generally considered to provide better predictive value for long-term moisture performance because it captures internal slab conditions rather than surface conditions, which can be influenced by ambient temperature and humidity. Some project specifications require both; others specify one method. The right choice is determined by the spec and the flooring manufacturer’s written requirements. Also see IFTI’s guide to calcium chloride moisture testing.
Frequently Asked Questions
What does ASTM F2170 test?
The relative humidity inside the concrete slab at a defined depth using in-situ probes.
Is F2170 better than calcium chloride?
Often yes for predicting internal moisture and long-term flooring performance, but the project spec and flooring manufacturer’s requirements determine which method to use.
Where should probes be placed?
In representative areas of the slab plus risk zones: exterior edges, below-grade areas, near penetrations, and locations with prior moisture history.
Can RH change after you test?
Yes. Changes in site conditions, ambient temperature, ventilation, and continued curing can all affect RH readings over time, which is why timing and documentation of conditions at test time matter.
What if RH is too high?
Delay installation, improve drying conditions (heat and ventilation), or install a manufacturer-approved mitigation system. Never install over a slab that exceeds the product’s specified RH limit without a documented mitigation plan.
