Prepping a Slab for Coating: Grinding, Moisture and Cracks

There was a stain in the middle of a Cedar Park garage floor shaped like a kidney, maybe four feet across, and it came back every February no matter what got rolled over it. Two coatings had already let go in that spot. When the third contractor peeled a chunk of the failed film, the underside carried a chalky white bloom and the concrete below was darker than the slab a foot away. Nobody had a coating problem. They had a drainage and base problem left over from somebody’s dirt work in 2011.

I do earthwork and compaction, so I get pulled into coating jobs only after a floor has failed twice and somebody wants to know where the water is coming from. It is not the concrete.

Grinding fixes the top eighth of an inch

Prep means opening the surface so a coating can key into it. Diamond grinding or shot blasting takes off the laitance, old curing compound, and the slick skin the power trowel left, leaving a profile like 60 to 120 grit sandpaper. What it cannot do is change what sits under the slab. If vapor is moving up through the concrete, a perfect profile just gives it a better-bonded film to push off.

Where the water comes from under a Central Texas garage

Around Cedar Park, most houses sit on an engineered pad: cut into caliche and limestone on the high side, fill on the low side, much of it placed in boom years when crews moved fast.

• Fill placed in thick lifts holds water. Fill belongs in 6 to 8 inch loose lifts at 95 percent of standard Proctor, moisture within 2 percent of optimum. Dumped in at 12 to 18 inches, the bottom of every lift stays loose, and loose ground stays damp.
• There is often no capillary break. A slab poured onto fine-grained fill or caliche screenings wicks water up like a paper towel. Four inches of clean, open-graded stone breaks that path, and a vapor retarder is the other half. The residential code has carried an under-slab vapor retarder requirement for interior slabs for many cycles, but plenty of older garages never got one, or got one shredded by boots before the pour.
• The site keeps feeding it. Irrigation heads along the garage wall, a downspout at the apron corner, and a driveway sloping back toward the door keep a subgrade wet year round. Out on the Blackland clay east of the tollway, that same water is swelling the clay.

Test the slab before you buy a coating

Two standard tests measure different things. The calcium chloride test reads vapor leaving the top surface over three days, in pounds per 1,000 square feet per 24 hours, and most coatings want that at 3 or below. The in-situ relative humidity probe reads humidity in a hole drilled to 40 percent of slab depth, capped by most manufacturers between 75 and 85 percent. The RH probe is the better test on a slab drying from one side, which is every slab on grade. Testing runs $50 to $150 per location, and a two-car garage wants three.

The cheap screen is an 18 by 18 inch square of plastic sheet taped down for 24 hours. Condensation under it means keep going with real tests; a dry one does not clear you. More in moisture vapor from below.

Cracks tell you which problem you have

Run a hand across it. If both sides are level and the crack wanders, that is shrinkage: it happened in the slab’s first weeks and it is a filling job. Chase it with a crack chaser wheel, fill with rigid polyurea, broadcast sand.

If you can feel a lip, or the crack runs in a straight line from the wall toward the driveway, the dirt moved. That line is usually the plumbing trench, backfilled loose when the house went up. Filling it only hides it until the next wet cycle. Those slabs get stabilized first, commonly with polyurethane foam injection at $1,200 to $4,000 for a garage bay, and coated afterward.

What prep costs and who should do it

Mechanical grinding runs $1.50 to $3.50 per square foot, more where old glue or a failed coating comes off. A moisture mitigation primer, a dense epoxy layer meant to hold back vapor pressure, adds $2 to $5 per square foot. The coating lands around $4 to $9 per square foot for a full-broadcast flake system. A 400 square foot garage done properly is realistically $3,000 to $5,000, a third of it prep nobody sees again. When picking who does it, a garage floor epoxy contractor in Austin who insists on moisture testing before quoting is telling you something about how the rest of the job will go.

Frequently asked questions

How long should a new slab cure before it gets coated? Twenty-eight days is a strength number, not a dryness number. A 4 inch slab in Central Texas often needs 60 to 90 days to reach a level a coating tolerates, and a wet subgrade can hold it above that indefinitely.

Can a floor be coated over a crack that is still moving? Not with a rigid filler. A moving crack telegraphs through the coating within a season. Either stabilize the ground under it first, or treat it as a joint with a flexible sealant and accept a visible line. That judgement is most of the job on a coating quote — an epoxy flooring installer in Las Vegas prices the crack treatment separately for exactly this reason.

Does a vapor retarder help an existing garage? Not retroactively, since it belongs under the slab. What helps is cutting off the water supply: move irrigation heads back from the wall, extend downspouts past the apron, and regrade so the driveway sheds away from the door.