Q.: In New York State we noted excessive popouts in the flat concrete work which was poured during the latter part of last year. At first we blamed the use of deicing salts. However, closer examination of the damaged portions revealed a piece of either flint or chert aggregate at the bottom of each popout. The popouts occurred only in those portions subjected to deicing salts. Have we made a correct evaluation? Is there a way of counteracting the problem?
A.: Deicing salts do increase the destructive effect of freezing and thawing on concrete that is not air-entrained. It seems reasonable to expect that they also hasten the destruction of porous aggregate particles such as flint or chert. You may not have seen the end of the problem. Popouts normally develop gradually over a period of several years, depending on the exposure, and it is quite possible that eventually you may see additional popouts in areas that have not been subjected to deicing salts.
One method of prevention is to find a source of aggregate that does not contain the offending particles. The easiest way to do this is to check service records of aggregates that are in current use.
Another way, if nonpopping aggregate is scarce, is to place the flatwork in two courses, using the nonpopping aggregate only in the 1 1/2-inch or 2-inch top course. Good practices for 2-course construction should be followed, such as those described in ACI 302. 1R-80, "Guide for Concrete Floor and Slab Construction."
In a few places aggregate producers have set up beneficiation operations to remove the lighter weight porous particles by heavy media separation. This is a good solution if available.
Coatings to prevent moisture buildup in concrete flatwork have been advocated. These are meant to keep the porous particles from becoming saturated because saturated particles are susceptible to damage from freezing. Coatings that significantly reduce the slip resistance should be avoided and no coating should be used that does not allow the concrete to breathe. Nonbreathing coatings can cause moisture rising from the soil to condense near the concrete surface and cause spalling.
Repairs of popouts can be made by breaking out or drilling out the offending particle and replacing with a patch. Coring or drilling is preferable to breaking because it produces a square shoulder. The patching mix should be relatively dry to minimize subsequent drying shrinkage but the surrounding area should be moistened before patching in order to prevent excessive robbing of moisture from the patching mix.
Some owners are satisfied to live with the problem of popouts and do nothing about them. In some circumstances their appearance is not highly objectionable, and they do not usually diminish the structural quality of the flatwork unless associated with some other problem.