Which test does my adhesive manufacturer want?

Read the technical data sheet, because it will name one and give a limit. Most modern flooring adhesives specify in-slab relative humidity to ASTM F2170 with a cap somewhere between 75 and 85 percent. Some still specify calcium chloride to ASTM F1869 with a limit in pounds. A few name both. Testing to the wrong standard means you have a number the warranty does not recognise.

Can I just use a hand-held moisture meter?

For a quick indication, yes. For a decision, no. A surface meter reads the top few millimetres, and the top of a slab is the driest part of it — the moisture that matters is deeper and it will migrate upward once you seal the surface with a floor. A surface reading that looks reassuring is exactly the trap the standardised tests exist to avoid.

Why does the test take three days?

Because the probe has to equilibrate with the concrete around it before the reading means anything, and ASTM F2170 specifies 72 hours for that. A probe read at two hours reads low, which is the dangerous direction — it says the slab is drier than it is, and a floor gets bonded down on the strength of it.

The slab is ten years old. Do I still need to test?

For a glue-down floor, yes. A slab reaches equilibrium with the ground beneath it, not with the calendar. On the clay east of the escarpment that ground carries a genuine vapour load through a wet season regardless of when the concrete was poured. Age tells you the slab has finished curing; it does not tell you what is coming up through it.

Climate & Local

Calcium Chloride Test vs RH Probes for Concrete

August 25, 2026 8 min read By Leander Flooring

Two tests, two different things

If you are getting a floor glued down to a concrete slab, somebody should be testing it. There are two standardised ways to do that and they measure different things, which is why the numbers are not interchangeable.

Calcium chloride — ASTM F1869

A measured dish of anhydrous calcium chloride is sealed under a dome on a prepared patch of slab for 60 to 72 hours. The salt absorbs moisture, and the weight it gains converts to a moisture vapour emission rate — pounds of water emitted per 1,000 square feet per 24 hours.

It measures what is coming out of the surface during the test period.

Typical limits sit around 3 to 5 lbs, depending on the adhesive.

In-slab relative humidity — ASTM F2170

Holes are drilled to a specified depth — 40 percent of the slab thickness for a slab drying from one side — sleeved, capped and left to equilibrate. A probe then reads the relative humidity inside the slab.

It measures what the slab actually holds, which is what will migrate upward once you seal the top with a floor.

Typical limits sit somewhere between 75 and 85 percent RH.

Why RH probes largely won

Calcium chloride has a real weakness: it only samples the surface, and the surface is the driest part of a slab.

Put a floor over it and the slab can no longer dry upward. Moisture that was sitting deeper redistributes toward the top and reaches the adhesive. A slab that passed a surface test can therefore still fail an adhesive, and that is not a rare theoretical case — it is the classic mechanism behind a floor that failed eighteen months after a test said it was fine.

In-slab RH measures the reservoir rather than the emission, which is the number that predicts what happens after the floor goes down.

Most modern flooring adhesive data sheets now specify F2170 for that reason. Some still cite F1869, a few cite both, and the one that matters is whichever your adhesive names — because that is the number a warranty claim will be judged against.

Where this bites in Central Texas

Two situations here make the test decisive rather than a formality.

New slabs, and there are a lot of them. Liberty Hill, Jarrell, Hutto and Manor have added enormous numbers of houses in the last decade. Concrete keeps releasing moisture for months after it is poured, and where a homeowner wants to replace the builder’s carpet with glued-down plank in year two, the reading genuinely decides whether the job goes ahead now or later.

Clay ground. East of the Balcones Escarpment, on Taylor Marl and Blackland, the soil beneath the slab holds water. That slab does not dry downward the way a slab poured on free-draining limestone does, and it can carry a real vapour load through a wet season no matter how old it is.

Up on the Edwards limestone through Cedar Park and Bee Cave, readings are frequently comfortable. Out east they frequently are not. Same builder, same year, different answer — and only a test distinguishes them.

What happens if it fails

Nothing dramatic. It changes the specification rather than cancelling the job.

A moisture-tolerant adhesive, where the reading is only slightly over.

A moisture mitigation coating — a two-part epoxy barrier rolled onto the prepared slab — where it is meaningfully over. This is a real line item, it is not cheap, and it is the most commonly omitted item on a competing quote, precisely because it only becomes visible after somebody tests.

A floating floor instead, which is not bonded to the slab and is far more tolerant of vapour. On the eastern clay this is often the better answer anyway, because floating construction also rides the seasonal slab movement.

Waiting, on a genuinely new slab. Occasionally that is the honest recommendation.

What to ask for

Ask for the number, in writing, with the standard it was measured to and the date. Ask what the adhesive’s published limit is. And ask what the plan is if the reading comes back over.

A contractor who has those three answers has done the work. One who says the slab “looks fine” has looked at the driest part of it.