Concrete Slab Evaluation: Moisture Testing, pH & Mitigation Decisions

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Concrete Slab Evaluation: Moisture Testing, pH & Mitigation Decisions. Don't let the plain title fool you. Here's the part that actually matters on the job: A slab is accepted by numbers, not by touch — relative humidity, pH, and surface condition are measured the way the standard says, written down, and compared with the product's published limits before anything is glued to it. Learn it well and you will stop owning floor failures that were built into the concrete months before you arrived.

The intro course introduced ASTM F2170 and F1869 and said why they matter. This lesson is about running them correctly, reading the slab's history before you drill, and making the mitigation call when the numbers come back high.

Reading the Slab Before You Drill

Every slab has a history, and most of it is on the drawings or visible on the surface:

• Vapor retarder. The structural sheets show whether a sheet meeting ASTM E1745 was placed directly under the slab, as ACI 302.1R recommends. A slab on grade with no retarder, or with the retarder buried under a layer of granular fill, is fed water from below indefinitely and may never pass a test; the fix is a mitigation system or an adhesive warranted for the condition, not more drying time.
• Mix and thickness. Lightweight concrete and elevated slabs on metal deck dry from the top only and dry slowly. Thicker slabs take longer. A common rule of thumb is a month of drying per inch of thickness under good conditions, and the conditions are rarely good.
• Surface treatments. Curing compounds, sealers, densifiers, and burnished finishes block adhesive bond and slow drying. Old cutback adhesive, paint, and gypsum patch are contaminants until removed. Black mastic in a pre-1980 building is presumed asbestos until tested, which is a wet-scrape job, never a grind.
• Post-tensioned slabs. Never drill RH probe holes or anchor anything into a post-tensioned slab without locating the tendons; the structural drawings and a scan come first.
• Cracks and joints. Note every control joint, construction joint, and crack with its width. They matter for prep and for tile movement joints later.

Running ASTM F2170 by the Standard

In-situ RH is the test most manufacturers now name, and it has rules that are easy to get wrong:

• Service conditions first. The building must be at its normal operating temperature and humidity for at least 48 hours before testing, and the HVAC must be the permanent system or an equivalent. Readings taken in an unconditioned shell are not valid.
• Count and location. Three probes for the first 1,000 square feet and one for each additional 1,000. Spread them across the floor, including one within three feet of an exterior wall, and map and number each location.
• Depth. Probes go to 40 percent of the slab thickness when the slab dries from the top only, and 20 percent when it dries from both sides, such as a slab on metal deck with vented deck. Measure the actual thickness; do not assume the drawings.
• Hole prep. Drill dry with the diameter the probe sleeve calls for, vacuum the dust completely, seal the sleeve to the slab, and cap it. Dust left in the hole throws off the reading.
• Equilibrium. Under the current edition the probe reads after 24 hours in the sealed hole. Record RH, temperature, date, time, and probe serial number, and photograph the location.

The report is a document, not a text message. Sign it, keep it with the job file, and send a copy to the GC the day the readings are taken.

Calcium Chloride, pH and Meters

ASTM F1869 still appears in many specifications. Grind off a small area to remove surface treatments, wait 24 hours, seal the dish and dome for 60 to 72 hours, and weigh the dish to calculate the moisture vapor emission rate in pounds per 1,000 square feet per 24 hours. It reads only the top inch or so of the slab, so it can pass a slab that fails F2170, and it is not appropriate on lightweight concrete. When the spec allows either, run RH.

ASTM F710 also requires a surface pH test: a few drops of distilled water on the clean slab, read with pH paper or a meter after a short wait. The common acceptable range is 7 to 10, with the flooring manufacturer's number governing; high alkalinity attacks adhesives even on a slab that is dry enough.

ASTM F2659 covers non-destructive electronic meters. They are screening tools for finding wet spots, comparing areas, and deciding where to drill. They are not acceptance tests, and no manufacturer accepts them as one.

Reading Results Against the Data Sheet

The manufacturer's installation instructions set the limit for each product and adhesive, and the number varies: many standard adhesives are warranted to 75 to 85 percent RH or 3 pounds MVER, while high-moisture adhesives for some resilient products carry limits of 90 percent RH or higher. Read the limit for the adhesive you will actually use, not the floor covering alone, and read the pH limit next to it.

When readings exceed the limit, the options are the same every time: more drying under real service conditions and a retest; a different adhesive or product warranted for the reading; or a mitigation system. A single high probe near an exterior wall usually means a local problem, such as a missing retarder lap or a wet planter outside, and it is investigated, not averaged away. No installer should ever accept the words feels dry in place of a report.

Mitigation Systems and Their Limits

ASTM F3010 covers two-component resinous membrane-forming mitigation systems with a very low permeance. The slab is shot-blasted to the profile the manufacturer requires, cracks and joints are treated, the membrane is applied at the specified thickness, and in most systems a primer and a cementitious skim coat or self-leveling underlayment go over it before any adhesive, because flooring adhesives are not warranted directly to epoxy. Moisture-tolerant adhesives and some sheet membranes serve as lighter-duty alternatives where the product allows. None of them solves standing water, hydrostatic pressure, or an active leak, and none bridges a moving crack or joint.

Going Deeper (Intermediate)

Gypsum underlayments in multifamily buildings are tested by the gypsum manufacturer's method, often an F2170 variation, and must be primed or sealed before adhesives. Lightweight concrete can read high for a year or more. Radiant slabs are tested with the system running at operating temperature. Existing adhesive residue is handled per F710: remove what can be removed, and use only the adhesives and patches whose manufacturers approve installation over what remains.

Advanced / Pro-Level

Pros settle responsibility before the slab is poured. The pre-installation meeting records who tests, whose meter and probes are used, when the building will be conditioned, and who pays for mitigation if the slab fails: the concrete contractor who skipped the retarder, the GC who could not condition the building, or the owner who shortened the schedule. The flooring proposal carries a mitigation alternate priced per square foot so the change order is a number, not a negotiation.

Practice Challenge

A 14,000 square foot slab on grade will receive glue-down LVT whose adhesive is limited to 85 percent RH. Your F2170 probes read between 78 and 83 percent, except two near the loading dock wall at 91 and 92. The GC wants to start Monday. How many probes were required, and what do you do? (Answer: 3 for the first 1,000 square feet plus 13 more, 16 probes total; the field passes, but the two dock readings fail, so you report all readings in writing, investigate the dock wall for a missing retarder or outside water source, and either mitigate that zone or switch to an adhesive warranted at the reading, rather than installing over a failed test.)

In Practice

An installer who drills to 40 percent depth, waits the 24 hours, and files a signed report can hand the GC a sheet of numbers when a floor is questioned two years later. One who touched the slab, called it dry, and glued the floor down owns the replacement.

Common Mistakes to Avoid

• Testing before the building has been at service conditions for 48 hours
• Drilling probes to a guessed depth instead of 40 percent of the measured thickness
• Using an electronic meter reading as an acceptance test
• Comparing readings with the flooring's limit instead of the adhesive's limit
• Drilling a post-tensioned slab without locating the tendons

Takeaway: A slab is accepted by numbers, not by touch — relative humidity, pH, and surface condition are measured the way the standard says, written down, and compared with the product's published limits before anything is glued to it.

Educational content only — not design, engineering, or safety advice, and not a substitute for hands-on training under a qualified installer or the judgment of a licensed professional. Standards editions and code adoption vary by jurisdiction. Follow the TCNA Handbook, ANSI A108/A118/A137, the ASTM F710 family of standards, NWFA and CRI guidelines, the building and accessibility codes as adopted where you work, the project specifications, and the manufacturer's written instructions for every product you install.