Question: I have an issue with a customer about her sidewalk surface spalling. She called me in February saying it was popping off in pieces. The sidewalk was placed about seven months earlier in June of the previous year. I am sure she is salting the sidewalk and causing this. I have a bag of rock salt in my garage that says, “Do not place on concrete less than two years old.” Is that good advice? And can I use that statement as my defense?
Answer: I'm afraid the short answers are that yes, it's good advice, but no, it's not a good defense. The bible in the concrete industry these days is “Design and Control of Concrete Mixtures,” by Steven Kosmatka, Beatrix Kerkhoff, and William Panarese. Published by the Portland Cement Association, it's a book that every contractor should own. It covers a wide range of concrete topics, including this one.
The authors say that mixes with the proper strength and air entrainment, low water-cement ratios, and appropriate curing should have no trouble resisting scaling due to the application of salt—even heavy applications. What are the “proper” values for those parameters?
In general, 3000-psi concrete should be strong enough to resist spalling as long as it has proper air entrainment. That strength comes from having enough cement in the mix and then using a water-cement ratio of 0.45 or less. Because concrete's strength develops over time, it has to be cured, which is largely maintaining the “proper” temperature and moisture levels while allowing time for its strength-building chemical reactions—hydration of the cement—to occur. For standard concrete, that means not letting it dry out, freeze, or get too hot—but that usually only happens on massive concrete placements. If the curing goes well, it will reach its design strength.
Air entrainment creates voids with tiny bubbles in the concrete paste where water in the concrete can expand as it freezes. Air entrainment of about 6% is recommended for concrete that will be exposed to freeze/thaw cycles. This raises a sticky issue: Entrained air can be forced out of the concrete in local areas by overfinishing the surface, among other things. The result is that local areas of the surface can be left with a much lower entrained air level and therefore be more susceptible to spalling.
So how does salt cause a problem? Simply using salt to melt ice and snow on concrete surfaces can exaggerate the effects of freezing and thawing. Applying salt to an ice-covered sidewalk leaves a liquid with a reduced freezing point. That gets down into the concrete and if the temperature drops far enough, that means the concrete goes through an additional freeze/thaw cycle. Of course, it's these cycles that cause a problem.