Question: Home builders here in the Denver area recently have had a rash of scaling driveways. Literally hundreds of driveways constructed over the past two to four years are showing isolated to widespread scaling. Our firm and others have met with the builders to help determine the causes. We have conducted petrographic analysis and chloride ion testing, and have generally concluded that the concrete was of lower quality, poorly placed, finished, and cured, as well as having poor air content or air spacing. Can you comment on this?

Answer: Your conclusions track well with some of the mainstream thinking within the industry. The failure mechanism for scaling is that the surface layer of the mortar portion of the concrete is too weak to resist the expansive forces of water that have gotten into that layer and then frozen.

Various things can lead to that weakness. The concrete may be light on cementitious materials, have a high water content, or both, and simply not develop enough strength. Or premature finishing can seal the surface and trap bleed-water, creating a localized zone of weakness.

Lack of adequate air entrainment is the other major factor contributing to scaling. First, the concrete mixture has to include an air-entraining admixture. But even if the concrete as batched had good air entrainment—a high enough percentage, good void size, and distribution—what counts in terms of preventing scaling is the air entrainment in the surface layer of mortar. Again, excess water, improper finishing, and insufficient curing can decimate the entrained air at that specific location leaving the surface subject to scaling.

Some people continue to insist that deicing salts are a major contributor to scaling. Using deicing salts on new concrete should be avoided the first winter, and they should only be used in moderation thereafter. But the prevailing opinion these days is that properly designed and placed concrete should be able to stand up to the use of typical deicing agents.