Concrete is porous, and moisture moves up through a slab from the ground below. Polished concrete handles that better than most floors because nothing is sealing the surface shut, but it still has to be measured before work starts.
We test every slab with ASTM F2170 relative humidity probes before anything is ground. It’s a slower start than most contractors offer, and it’s the reason we rarely get called back to fix something. If you want your own slab tested rather than a general answer, request an estimate. Someone comes out.
What follows is what the test is for, what a high reading means, and what happens next when it reads higher than it should.
Why moisture matters here specifically
Central Texas gives slabs two things to deal with at once.
Clay soil holds water. The Blackland prairie east of Austin swells when it’s wet and shrinks in drought, and a slab sitting on it has moisture underneath it for months at a time. West of the city the limestone drains faster, so the problem is smaller but not absent.
That difference is large enough to change what we expect before we arrive. A slab in Hutto or Pflugerville is more likely to read high than one in West Lake Hills, and the readings across a single floor vary more on clay than they do on rock.
Humidity does the rest. A slab releasing vapor into a humid room releases it more slowly, which extends how long a new pour takes to reach a workable reading. A floor that would have been ready in three months somewhere dry can take longer here, and the difference is seasonal rather than fixed.
Neither factor is a reason to avoid polished concrete. Both are reasons to find out where a specific slab actually sits before committing to a date.
Neither of these makes polishing a bad choice. They make measuring it first a necessary one.
What moisture does to a floor
The failures depend entirely on what’s on top of the slab.
Under a coating, vapor has nowhere to go. Pressure builds under the film until it lifts, usually starting as a blister nobody notices and spreading over a season. That’s the most common reason we’re called to look at a coated floor, and by the time it’s obvious the repair means taking the whole thing off rather than patching the affected part.
Under a polished surface, there’s no film to lift, because nothing was added. What you get instead is more subtle: dark patches where vapor is concentrated, or efflorescence, which is salts carried to the surface by rising moisture and left behind as a white powder when the water evaporates.
Neither is dramatic, and neither means the floor is coming apart. They’re indications that moisture is moving, which is useful information rather than a failure.
Efflorescence is a symptom rather than damage. It cleans off, and it comes back if the underlying moisture isn’t understood. Treating the powder without addressing what’s carrying it to the surface is a cycle rather than a repair.
A slab that reads high can also affect how a penetrating sealer performs, which is why the reading changes what we specify rather than just whether we proceed.
The practical version: on a slab with a moderate reading we’d choose a different sealer and set different expectations, and on a slab with a high reading we’d talk about timing before talking about finish.
How we test
ASTM F2170 is the in-slab relative humidity method. It’s the one that measures what’s happening inside the concrete rather than at the surface.
Probes go into holes drilled to a specified depth, the holes are sealed, and readings are taken once the probes have equilibrated. That takes about three days, which is the part clients find inconvenient and the part that makes the result meaningful.
Surface tests measure the surface. The surface isn’t where the problem lives, and a slab can read acceptably at the top while carrying far more moisture at depth. That difference is the entire reason the in-slab method exists.
We test before grinding rather than after, because the reading changes decisions that come earlier in the sequence: which sealer suits the slab, how aggressively to open the surface, and whether the job should start at all. Testing afterwards tells you why something went wrong. Testing first stops it.
That last one comes up more than people expect. A new pour that isn’t ready is not a floor to work around. It’s a floor to come back to, and there’s a page covering the other situations where we’d say no.
What happens when a slab reads high
There are three honest answers and we’ve given all of them.
Wait. On a recent build, a slab that isn’t ready usually becomes ready. Concrete keeps releasing moisture for months after pouring, and the answer is a date rather than a product. That conversation is easier to have before a floor is scheduled than after, which is another argument for testing early.
Adjust the specification. A slab with a persistent but moderate reading can often be polished with a sealer chosen for those conditions. Polished concrete is more forgiving here than coated systems because the surface stays breathable, so a reading that would rule out a coating doesn’t necessarily rule out a polish.
Decline. Some slabs have a moisture source that isn’t going away, usually a missing or damaged vapor retarder under the slab, or water finding its way in from outside. Polishing that floor produces a result nobody is happy with, and we’ll say so rather than take the work.
That third answer is the one worth asking any contractor about. A floor with an unresolved moisture source will fail whatever goes on it, and the question is only how long it takes and how visible it is.
Vapor retarders
Slabs poured on ground in Texas are normally laid over a vapor retarder, a sheet membrane between the ground and the concrete. Its job is stopping ground moisture entering the slab in the first place.
Older buildings sometimes don’t have one, and slabs poured in eras when the practice was less consistent are the usual case. A converted space is another, because the addition may have been built to a different standard than the original, and that shows up in the readings as well as in the grind.
We can’t install one under an existing slab. Nobody can, short of removing the floor. What we can do is find out whether there is one, and factor that into what’s realistic for your floor.
That’s worth saying plainly because it occasionally gets sold otherwise. There are surface products marketed as moisture barriers, and on a slab with a genuine vapor problem they relocate the failure rather than fix it.
Garages without a vapor retarder are the most common version of this we see, and it’s part of why coated garage floors in this climate fail as often as they do. A garage slab is also the one most likely to have been poured to a lower standard than the house it’s attached to, which compounds the problem.
Does polished concrete trap moisture
No, and it’s the most common misconception about the finish.
A densified and penetrating-sealed slab stays breathable. The densifier reacts with free lime inside the concrete and hardens it from within rather than forming a layer, and a penetrating sealer soaks in rather than sitting on top. Vapor still passes through, which is exactly what you want on a slab sitting on damp ground.
That’s the opposite of a coating, which is designed to be a barrier and behaves like one in both directions.
It’s also why polished concrete suits slabs with moderate moisture readings that would rule out a coated system entirely. On a floor where a coating would need remediation first, polishing sometimes just works.
What to ask a contractor
Whoever you use, three questions are worth asking before work starts.
Do you test slab moisture, and with what method? Surface tests and in-slab tests measure different things, and only one of them tells you what’s happening below the surface. A contractor who tests at all is ahead of most, and one who names the method is telling you they know why it matters.
What happens if the reading is high? An answer that involves proceeding regardless is the wrong answer.
Have you seen the slab? Nobody can assess moisture from a photograph, and any quote that arrives without a site visit was written without the information that matters most.
Those three questions cost nothing to ask and they separate contractors quickly. The answers also tell you something about how the rest of the job will go, because a company that skips the measurement usually skips other things too.
One last thing worth knowing: a high reading is not a verdict on your floor. It’s a reading on a particular day, and slabs change. The most common outcome of a high result is a different start date rather than a different floor.
Established 2012, working across Austin and Central Texas. Fully insured, no subcontractors, estimates free.
Call (512) 900-5938 or request an estimate and we’ll test the slab before anyone commits to anything.