Moisture in concrete is like a slow leak behind a wall. Everything can look fine, right up until the finish fails.
For contractors, flooring installers, and facility managers, concrete moisture testing is less about passing a spec and more about avoiding call-backs. A slab that “seems dry” can still hold enough moisture to blister a concrete epoxy coating, haze a sealer, or soften an adhesive.
Two tests show up on plans again and again: calcium chloride (MVER) and in-situ relative humidity (RH). They don’t measure the same thing, so they don’t predict risk the same way.
What each test measures, and why the difference matters
Calcium chloride testing (ASTM F1869) is a surface-emission test. You prep a small area, place a dish of calcium chloride under a sealed dome, and return later to see how much moisture the salt absorbed. The result is a Moisture Vapor Emission Rate (MVER), usually shown as pounds per 1,000 sq ft per 24 hours.
That sounds useful, but it’s reading what the slab is releasing from the top, under the room conditions during the test window. If HVAC is off, air is dry, or the surface has been heated by sun, the number can shift.
RH testing with in-situ probes (ASTM F2170) measures the internal moisture condition of the slab. Probes go into drilled holes, commonly at 40 percent depth for slabs drying from one side. That’s the key point: RH testing samples the moisture deeper in the concrete, where the “real” reservoir lives.
Here’s the practical analogy: calcium chloride is like checking sweat on your skin. RH is like taking your core temperature. Once you install flooring, you trap moisture, and the slab’s internal moisture moves toward the surface. That post-install “re-balance” is why RH testing often predicts flooring risk better.
A surface test tells you what the slab is doing today. RH tells you what it’s likely to do after you cover it.
A quick comparison helps frame the choice:
| Item | Calcium Chloride (ASTM F1869) | In-situ RH (ASTM F2170) |
|---|---|---|
| Measures | Surface vapor emission | Internal slab moisture condition |
| Reported as | MVER (lbs/1,000 sq ft/24 hrs) | %RH at depth |
| Sensitive to room air | High | Lower (still needs conditioning) |
| Best at predicting trapped moisture | Limited | Strong |
Where each test fits in real-world slabs and schedules
Most failures don’t happen because someone skipped testing. They happen because the team used the wrong test for the risk.
Slab-on-grade with a vapor retarder below: RH testing usually tells the more important story. Moisture can stay elevated for a long time, especially with mixes that hold water longer. A calcium chloride test may look acceptable under dry air, even though the slab still has high internal RH. If you’re installing epoxy coating for concrete, resilient flooring, or a basement concrete coating, internal moisture is the enemy.
Elevated decks and suspended slabs: These often dry differently because air can move around the structure, and schedule pressure is common. RH testing is still a strong fit because it shows whether the slab has stabilized internally, not just dried at the surface.
Fast-track construction: This is where surface testing can mislead the most. Early HVAC, temporary heat, or dehumidifiers can “dry” the top while the middle stays wet. If you’re trying to coat quickly (for example, an epoxy coating for garage floor in a new-build home or a commercial concrete epoxy coating in a tenant space), RH data gives you a clearer go or no-go call.
When moisture mitigation coatings are on the table: If RH comes back high, you may pivot to a mitigation system, then a build like moisture barrier epoxy plus broadcast, then polyaspartic coating as the wear layer. Some newer product categories reference updated standards and may allow installation at higher RH, but only if prep, thickness, and verification match the manufacturer’s requirements. Testing still matters because it drives the system choice and the warranty trail.
If you’re hiring a garage floor epoxy coating company, it’s fair to ask which method they use and how they document results. Good installers don’t guess, they measure.
Interpreting results against adhesive, coating, and finish limits
This is where jobs are won or lost: the slab can “pass” one number and still fail the floor.
Manufacturers set limits in different ways. Some publish a maximum %RH (from ASTM F2170). Others list an MVER limit (from ASTM F1869). Many also require a pH range. For decorative work like concrete polishing and concrete staining, moisture can still cause blotchy color, sealer whitening, or adhesion loss.
Two practical rules help:
- Don’t convert MVER to RH (or the other way around). They aren’t interchangeable because they measure different conditions.
- Follow the most restrictive requirement across the whole assembly, primer, adhesive, patch, coating, and topcoat.
“Meets spec” only counts if it meets your product’s spec, under service conditions, with clean documentation.
Also look at where the numbers come from. A calcium chloride test on a dense, steel-troweled surface can under-report emission if that surface acts like a temporary lid. On the other hand, RH probes placed too shallow, or in a dusty hole that never equilibrates, can give junk readings.
If results come back high, you still have options. You can wait, improve conditioning, or specify a mitigation approach (with clear prep steps, shot-blast profile, and re-test requirements). This is where tight concrete dealing (prep coordination, patching decisions, and sealing details) keeps trades aligned.
A concise jobsite checklist (with QA and chain-of-custody)
Good concrete moisture testing is boring when it’s done right. It’s repeatable, documented, and hard to argue with later.
Use this field checklist to keep data clean:
- Condition the space: Bring temperature and ambient RH close to service conditions, then hold steady before and during testing (standards often call for a conditioning period).
- Plan locations: Follow the probe count and distribution in the applicable standard and project spec, then avoid edges, cracks, sump areas, and obvious wet spots unless you’re investigating a problem.
- Control dust and silica: Drill with proper dust collection, wear respirators as required, and clean holes so sensors can equilibrate.
- Verify equipment: Check probe calibration or verification against the manufacturer’s reference before use, then log serial numbers.
- Seal correctly: Cap and seal test holes per the probe system instructions so ambient air doesn’t skew readings.
- Document everything: Record slab age, thickness, mix notes if known, ambient conditions, exact locations, timestamps, and readings. Add photos that show the location context.
- Protect the chain-of-custody: Note who took each reading, who handled the device, and where reports are stored. Keep raw data, not just a summary page.
That paperwork feels like overhead until there’s a dispute. Then it becomes your best insurance.
Conclusion
Choosing between calcium chloride and RH isn’t a preference call, it’s a risk call. Calcium chloride shows surface emission during the test window, while RH shows internal moisture that can rise after you install a coating or floor. For most modern finishes, especially concrete epoxy coating systems, RH testing gives a clearer view of failure risk. If you want fewer surprises, treat concrete moisture testing like a scope item, not a checkbox.
Comments are closed