Q: The broom-finished driveway of my house in North Carolina is 25 years old. I bought the house several years ago and have applied rock salt (sodium chloride) to the driveway during the winter to remove snow and ice. This past winter, I tried a commercial deicer that I hoped wouldn't be as damaging to the grass. This deicer contains calcium chloride. When I used it, however, I got deep scaling in the car wheel paths. The entire mortar surface is gone, exposing the coarse aggregate. I'm a concrete contractor and have never seen an older driveway scale after many years of trouble-free service. Does concrete ever lose surface strength with age?

A: We don't think strength loss is the problem. Try to get a Materials Safety Data Sheet (MSDS) for the product you used to find out what other chemicals, if any, the deicer contained. If you see ammonium nitrate or ammonium sulfate listed, that could be the cause of the problem. Both of these materials will damage high-quality concrete.

In a study done by the Portland Cement Association, air-entrained and non-air-entrained concretes were subjected to freezing and thawing with deicer solutions of varying concentrations ponded on the concrete surfaces. As seen in the graph for sodium chloride, damage to air-entrained concrete caused by 200 cycles of freezing and thawing with a ponded sodium-chloride solution peaked at about a 4% concentration of the deicer. The graph for calcium chloride shows little damage at about a 4% concentration but significant damage after 200 freeze-thaw cycles with a 16% concentration of this deicer solution. It's possible that the deicer concentration on your driveway reached 16%, but North Carolina doesn't typically get that many cycles of freezing and thawing.

A third possibility is that you experienced a particularly wet winter, raising the concrete moisture content high enough for freeze-thaw cycles and causing the deicer to damage even good-quality, air-entrained concrete. ACI 201.2R, Guide to Durable Concrete, states that "...under extremely severe conditions, even quality concrete may suffer damage from cyclic freezing, e.g., if it is kept in a state of nearly complete saturation."

References

1. George J. Verbeck and Paul Klieger, "Studies of 'Salt' Scaling of Concrete," Research Department Bulletin RX083T, Portland Cement Association, Skokie, Ill., 1957.

2. ACI 201.2R-92, Guide to Durable Concrete, American Concrete Institute, Farmington Hills, Mich., 1992.