Concrete Moisture Testing Before Coating: ASTM F2170 and F1869

Why a dry-looking slab is not a dry slab

A concrete slab can be bone dry to the touch, hold a chalk line cleanly, and still be carrying enough moisture inside it to lift a coating within months. The surface is simply the one part of the slab that is in contact with moving air, so it is the first part to dry and the least representative of the whole. Everything below it is still equilibrating, and the moment a coating seals the surface, that equilibration stops working in your favour: the slab's internal moisture redistributes upward until the whole depth approaches a single relative humidity, and whatever pressure that generates arrives at the underside of the coating film.

This is why the two dominant mechanisms behind coating failure — inadequate surface preparation and substrate moisture — are both decided before a single coat is applied, and why sources disagree about which of the two causes more failures. Preparation problems announce themselves at the bond line. Moisture problems announce themselves later, as disbondment, blistering or a coating that simply lets go across an area with no obvious cause. ACI 302.2R-06, Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials, is the reference guide for how slabs hold, move and release moisture, and for the mitigation options when they hold too much.

Singapore makes all of this harder rather than easier. Ambient relative humidity is high year-round, so a slab has less drying potential than the same slab in a temperate climate; slab temperatures stay warm, which raises the vapour pressure inside the concrete; and slab-on-grade construction without a functioning damp-proof membrane gives the slab a permanent moisture source from below that no amount of waiting will exhaust. Add the surface-drying effect of a warm, air-moved space and you get the characteristic local trap: a slab that feels dry, reads dry on a surface meter, and is far above the limit two-thirds of the way down. Construction moisture in a new slab leaves slowly, and it leaves from the surface inward — which means the interior is always the last part to reach equilibrium and the only part worth measuring.

ASTM F2170 and ASTM F1869 compared

Two ASTM test methods dominate quantitative slab moisture testing. ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes, measures the relative humidity inside the slab. Holes are drilled and sleeved, probes are inserted to a specified depth, and the slab is allowed to equilibrate for 24 hours before a reading is taken. The depth is not arbitrary: probes are set at 40% of slab depth where the slab can dry from one side only, which is the normal case for a slab on grade, and at 20% of depth where the slab can dry from two sides, as with a suspended slab open above and below.

ASTM F1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, measures something quite different: the moisture vapour emission rate, or MVER, expressed in pounds per 1,000 square feet per 24 hours. A dish of anhydrous calcium chloride is weighed, sealed under a dome on the slab for 60 to 72 hours, and weighed again; the mass gained is the moisture that left the slab in that period. The test conditions are prescriptive — 23.9 ± 5.5 °C and 50 ± 10% relative humidity, held for 48 hours before the test as well as during it. The method is not applicable to lightweight aggregate concrete.

The widely held professional view, and the reason F2170 has largely displaced F1869 in modern specifications, comes down to what each method can physically see. The calcium chloride test only captures what is leaving roughly the top 20 mm of the slab during the hours the dome is in place, which makes it acutely sensitive to surface conditions and to the ambient conditions in the room — a slab whose surface has been dried by air movement can produce a comfortable MVER while remaining saturated below. In-situ RH, by contrast, reads the moisture condition inside the slab, which is precisely the condition the slab will equilibrate toward once a coating seals the surface. That is the number that predicts what happens after the coating goes down. This is a prevailing view rather than a universal one, and F1869 remains a legitimate method where a specification calls for it.

What moisture level is too high — and who actually decides

This is where most write-ups on the subject quietly mislead, so it is worth being exact. The figures everybody quotes — 75% internal relative humidity for ASTM F2170, and 3 lb per 1,000 ft² per 24 hours for ASTM F1869 — are conventionally specified maxima. They are not requirements of the standards. ASTM F2170 and ASTM F1869 are test methods. They tell you how to obtain a number that is reproducible and comparable; they do not tell you what number is acceptable. The acceptance limit is set by the coating manufacturer, in the technical data sheet, or by the project specification. Describe 75% as commonly specified rather than as something the standard mandates — the distinction is real, it is the difference between a test method and an acceptance criterion, and a specifier will notice.

The practical consequence is that the correct threshold varies by product. Some epoxy manufacturers accept MVER results in the 3–5 lb range; others hold to 3 lb; others do not use MVER at all and specify an internal RH limit. A slab at 78% internal RH is a failure against one specification and acceptable against another. Reading the number off a chart on the internet, rather than off the technical data sheet of the product you are about to apply, is how a compliant test result ends up authorising a non-compliant application.

For a Sparco system, the limit that governs is Sparco's own. Sparco's TDS for Sparco Epoxy Bonding Primer #100 and for Sparcofloor WBE 400 caps the maximum permissible concrete substrate moisture content at 5%. It is important to be precise about what that figure is, because it is easy to conflate three different things. A moisture-content figure is a measure of the mass of water in the concrete relative to the concrete itself. Internal relative humidity (F2170) is a measure of the humidity of the air within the pore structure. MVER (F1869) is a rate of moisture leaving the surface. These are three distinct measurements on three distinct scales, and there is no reliable universal conversion between them. If a specification calls for a moisture-content percentage and the site has run an in-situ RH test, the specification has not been demonstrated. Establish at the outset which measurement the specification and the product TDS actually call for, and test for that.

• 75% internal RH (F2170) — commonly specified maximum, not a requirement of the standard
• 3 lb/1,000 ft²/24 h (F1869) — commonly specified maximum; some epoxy manufacturers accept 3–5 lb
• Sparco's TDS (Epoxy Bonding Primer #100, Sparcofloor WBE 400) — maximum permissible concrete substrate moisture content 5%
• Moisture content, internal RH and MVER are three different measurements — confirm which one the specification calls for before testing

Running the test properly, and what the field indicators are worth

The sequence below is where results are won or lost, and the first step is the one most often skipped. The space must be conditioned to its intended service conditions before testing, and held there — ASTM F1869 requires its conditions to be maintained for 48 hours before the test as well as throughout it. A test run in an unconditioned, sealed-up shell in Singapore, with the doors shut and no air handling, measures a slab in a state it will never be in again once the building is occupied. It will read high. A test run with fans blowing across the surface will read low. Neither is the slab's real condition.

Depth discipline matters just as much. An F2170 probe hole drilled to the wrong depth returns a number that is precise, repeatable, and about the wrong part of the slab. For a slab on grade, drying from one side only, the probe sits at 40% of slab depth; for a slab drying from two sides, at 20%. Reading a probe before the full 24-hour equilibration has elapsed will report a value biased by the heat and disturbance of drilling — almost always in the optimistic direction. Test multiple locations across the slab, weighted toward the areas you expect to be worst: near external walls, at construction joints, over any known damp-proof membrane defect, and in the middle of the largest pour.

Field indicators have a place, and it is not the acceptance test. The plastic-sheet indicator — taping a sheet of clear polythene to the slab, leaving it, and looking for condensation or a darkened slab beneath — is a qualitative go/no-go screen. It can tell you that a slab is obviously wet. It cannot tell you that a slab is dry enough, because it detects nothing below the surface zone and will pass a slab sitting at 90% internal RH. Similarly, a calcium-carbide meter or an electrical-impedance meter gives a fast, useful reading for screening a large area and deciding where to place the real tests, but neither is an acceptance test and neither should appear on a sign-off sheet as one. Use the screens to find the problem areas. Use ASTM F2170 or ASTM F1869 to decide.

Where a slab is highly absorbent, water-based epoxies such as Sparcofloor WBE 400 and Sparcofloor WBE 410 are formulated for that condition, as is Sparco Epoxy Bonding Primer #100. This is a statement about absorbency, not about moisture tolerance: none of these products is a moisture-tolerant primer, a damp-proof membrane or a moisture mitigation system, and the TDS limit of 5% maximum substrate moisture content applies to them exactly as it does to anything else. A slab above the limit needs to dry, or needs mitigation designed for the purpose. Selection should be confirmed through technical review.

Common mistakes in concrete moisture testing

Almost every bad moisture test is bad in one of six ways, and none of them look like errors at the time. The test gets run, a number comes out, the number is written down, and the coating goes on. The defect only becomes visible when the coating disbonds, at which point the test record is produced as evidence that the slab was dry — which it was, in the sense that somebody measured the wrong thing carefully.

The most consequential of these is the last one. A slab that looks dry, in a warm and well-ventilated building, in a country where the surface of a slab is almost always drier than its interior, is exactly the slab that most needs testing. The appearance of dryness is produced by the same mechanism that hides the problem.

• Testing before the space is conditioned to service conditions — an unconditioned shell reads high, a fan-dried slab reads low, and neither is the slab's real state
• Drilling the probe hole to the wrong depth — 40% of slab depth when the slab dries from one side, 20% when it dries from two; a precise reading of the wrong depth is still wrong
• Reading an in-situ RH probe before the 24-hour equilibration has elapsed — the result is biased by drilling heat and disturbance, almost always optimistically
• Treating 75% RH as a law rather than a specified limit — ASTM F2170 is a test method, and the acceptance limit comes from the coating manufacturer or the specification
• Testing one location in a large slab — moisture is not uniform; test near external walls, at construction joints, over suspected membrane defects, and mid-pour
• Skipping the test on a slab that "looks dry" — surface appearance reflects the top few millimetres and reveals nothing about the interior
• Substituting a plastic-sheet indicator or a hand-held meter for a quantitative test — these are screens for locating problems, not acceptance criteria
• Comparing a moisture-content percentage against an internal RH limit — three different measurements, no reliable conversion between them

Checklist: before you accept a slab as dry enough to coat

Work through this before the primer is ordered to site, not on the morning it arrives. If the slab fails, the remedy is time or mitigation, and both need to be in the programme rather than discovered on the critical path. Note that in a tropical climate the drying assumptions imported from temperate-climate guidance are optimistic, and a new slab-on-grade without a functioning damp-proof membrane may never reach the limit unaided.

Keep the record. Test location plan, depths, equilibration times, readings, slab and ambient temperature and RH, the date and the instrument calibration. Along with the surface preparation record, this is the documentation that settles a dispute over a failed floor.

• Confirm which measurement the specification and the product TDS call for — internal RH, MVER, or moisture content
• Confirm the acceptance limit from the coating manufacturer's technical data sheet, not from a generic figure (Sparco's TDS: maximum 5% substrate moisture content)
• Condition the space to its intended service conditions and hold it — ASTM F1869 requires 48 hours before and during the test
• For ASTM F2170, set probes at 40% of slab depth where the slab dries from one side, 20% where it dries from two sides
• Allow the full 24-hour equilibration for in-situ RH probes before taking any reading
• For ASTM F1869, run the test for 60–72 hours at 23.9 ± 5.5 °C and 50 ± 10% RH, and confirm the slab is not lightweight aggregate concrete
• Test enough locations to represent the slab — external walls, construction joints, suspected membrane defects, and the centre of the largest pour
• Use plastic-sheet indicators and hand-held meters to screen and locate, never to accept
• Record readings alongside slab temperature, ambient temperature and ambient RH, with date, time and location
• Confirm instrument calibration is current and documented
• If the slab fails, plan for drying time or purpose-designed mitigation — no Sparco primer is a moisture mitigation system
• Re-test after any water ingress, wet trade, or extended programme delay before coating proceeds

Frequently asked questions