· by Michael Blair, Nashville's Garage Floor
Before every predicted freeze, TDOT and Metro spray the roads with salt brine, and after every one, every car in the region carries it home. It drips off the undercarriage onto the garage floor, dries into a white crust, and gets wet again the next time the car comes in. Over a few winters that cycle scales the surface of bare concrete, and it does the same to a coating that was not built to take it. The damage is slow and it starts where the tires sit.
How salt damages concrete
Salt does not dissolve concrete. It works by holding water in the surface pores and by increasing the number of times that water freezes and thaws. Each freeze expands the water in the pores; each thaw lets more brine in. On an unheated garage floor in a Nashville cold snap, a wet, salty patch under the car can go through several cycles in a week, and the surface begins to scale, flaking off in thin layers and exposing the aggregate.
The chlorides also migrate down into the slab over years and reach any reinforcing steel or wire mesh, which then rusts and expands, cracking the concrete from inside. That is a long process on a residential slab but it is why a floor that has been salted for decades cracks in ways a dry floor does not.
What it does to coatings
A properly applied epoxy or polyaspartic coating is a barrier: the brine sits on top and never reaches the concrete, and it wipes up. The coating itself does not react with salt. Where coatings fail with brine is at the edges and at any pinhole or chip, because brine gets under the film, the concrete beneath scales, and the coating lifts with it. A coating applied over a floor that already had salt in it, or over concrete that was not properly profiled, also loses its bond as the salt draws moisture up from below.
Cheap one-part floor paints and water-based coatings from the home center are not barriers in this sense; they are thin, they wear through at the tire tracks in a season or two, and the brine gets to the concrete under them.
- Bare concrete: scaling at the tire tracks, then spreading
- Thin floor paint: worn through at the tracks, salt under the film
- Epoxy or polyaspartic over properly prepared concrete: brine wipes off
- Any coating with chips or open edges: failure starts there
Managing it through the winter
The simplest measure is to get the brine off the floor before it dries and before it freezes. A squeegee to the drain or the door after a wet drive, or a rinse on a mild day, removes most of it. A containment mat under each car catches the drips and can be emptied outside; they are the best option on a bare floor that is not going to be coated. Do not let the crust build up over the winter and wash it in spring; by then it has done its work.
Rinse the undercarriage of the car when the temperature allows, for the car’s sake as much as the floor’s. And keep the garage door closed in a cold snap; a slab that stays a few degrees warmer goes through fewer freeze cycles.
Protecting the floor for good
A penetrating siliconate or silane sealer on bare concrete reduces the water and brine the surface absorbs and is the least expensive protection; it needs reapplying every few years and does nothing for a floor that is already scaling. A full coating system, with the concrete ground or etched, any cracks and scaling repaired, and a topcoat rated for chemicals and hot tires, is the durable answer and the one that makes the winter drips a matter of a mop.
If the floor is already scaling, the loose material comes off, the surface is repaired with a polymer-modified overlay, and the coating goes over that. Coating over scaling concrete without repairing it is the most common reason garage floor coatings fail at the tire tracks in their first winter.