Q: We have a sidewalk that has been through one winter. It was built in August. The top has peeled off to a depth of about 1/8 inch in spots, some about the size of a 25-centpiece and some as big as 2 feet square. Our city uses a thrower to salt the streets. I wonder if the throw from this machine has reached the walk and caused this spalling.
You describe what seems to be deicer scaling caused, as you suspect, by the action of deicing salts on concrete that has an adequate air content. Scaling is commonly experienced not only where the salt is applied directly but where salty slush is splashed up from the street by passing automobiles. Curbs, sidewalks, bridge parapets and bases of concrete lampposts are all subject to such splashing. They absorb some of the salt water, which then does as much damage during subsequent freeze-thaw cycles as it would have had the application been deliberate. Overspray from the passing salt truck could also contribute, as you suggest.The best method of prevention is to provide an adequate air content in all concrete that could conceivably become exposed to deicer salts. The amount of air needed depends on the maximum size of the coarse aggregate:
Maximum aggregate
size, inches
Air content required,
percent by volume
3/8  6 to 10
1/2 5 to 9
3/4 4 to 8
1 3.5 to 6.5
1 1/2 3 to 6
2 2.5 to 5.5
3 1.5 to 4.5

If existing concrete does not contain the necessary amount of entrained air it is possible to give it some protection against deicer scaling by mopping on two coats of linseed oil (ASTM D 260) thinned with an equal amount of mineral spirits. No puddles should be left. Linseed oil emulsions are also available; the manufacturer's instructions should be followed for applying them. Treatment must be renewed about every two years.