Concrete
Does road salt damage concrete?
Salt does not attack concrete chemically, but it makes the surface freeze and thaw far more often than it otherwise would, and that is what flakes the top off.
The numbers
- The flaking is called scaling, and it takes the top layer off in sheets and chips.
- Salt causes it by melting ice that then refreezes, over and over, inside the surface.
- New concrete is at its most vulnerable through its first winter.
- Air entrainment, proper finishing and proper curing are what a slab's resistance to it comes down to.
- Whatever goes down on a slab, follow the manufacturer's label for it.
What scaling is
Scaling is the top few millimeters of a slab coming off in flakes and patches, leaving the sand and stone exposed. It usually shows up in spring, it shows up worst where cars drip and where somebody salted heavily, and once it starts it keeps going.
It is a surface failure rather than a structural one at first. The slab is still carrying its load. What has gone is the dense finished layer on top that was keeping water out.
Why salt makes it happen
Concrete has water in its pores. When that water freezes it expands and pushes on the pore walls, and good concrete has microscopic air bubbles deliberately mixed into it to give that expansion somewhere to go.
Salt does two things. It melts ice, so meltwater soaks into the surface that would otherwise have stayed frozen on top of it, and it drops the freezing point, so the surface crosses freezing many more times in a winter than the air does. More cycles, more pressure, and the top layer eventually gives up.
The first winter
New concrete has not finished gaining strength and its surface is still vulnerable, so the industry guidance is to keep deicing chemicals off it through its first winter. It is the piece of advice worth passing on to whoever clears your walks in northwestern Wisconsin.
After the first winter, keeping the joints sealed and rinsing the slab in spring does most of the rest. A slab already scaling has to be looked at to say whether it is surface only or has gone deeper, and a photograph of it in spring is where that conversation starts.
Where these numbers come from
- Deicer guidance for new concrete follows the American Concrete Institute and Portland Cement Association recommendation to keep deicing chemicals off a slab through its first winter.