Q.: Our garage floor has several popouts, some as large as a hand and 1/2 to 3/4 inch in depth. What causes them, and what is the best way to repair them? We live in Minnesota and thought the salt in the winter time was the culprit. Any information would be appreciated.

A.: Popouts are roughly cone-shaped pits left in the surface of flatwork after a small piece of concrete has broken away by internal pressure. This pressure is generated when a piece of chert, soft fine-grained limestone, shale, hard-burned lime, hard-burned dolomite, pyrite, or coal expands. The first two are natural constituents of some aggregates; the others sometimes find their way into aggregates as impurities. In some materials, the expansion is caused by freezing or absorption of moisture; in others it is caused by a chemical change. For example, popouts can occur from the chemical reaction between alkalies in concrete and reactive siliceous aggregates. Popouts range in size from about 1/4 inch to 2 inches (60 to 50 mm) in diameter. But some can be larger, like those in your garage slab. Because popouts usually do not significantly diminish the integrity of concrete flatwork, they are sometimes tolerated. Nevertheless, they are usually unsightly and interfere with the performance of any slab required to be smooth. On floors with hard-wheeled traffic, popouts can degenerate into larger imperfections. Early repair should minimize further problems in high-traffic areas.

Impurities in the concrete may be beyond the control of the floor constructor since they usually occur inadvertently in the production and handling of ready-mixed concrete or its constituents. The presence of naturally occurring chert of soft fine-grained limestone, however, can be a continuing problem in some locales.

So you see, you were right that if water got into the aggregates and then froze during the winter in Minnesota, the aggregate impurity would expand upon freezing and pop the concrete around it. You have some options for repair:

  1. Grind the slab surface and resurface it with a thin coat of concrete-epoxy repair products
  2. Demolish the existing slab and replace it (expensive and popouts could reoccur)
  3. Prepare the surface of the popouts (clean, rough finish) and apply patching compound specifically for concrete repair. A reputable contractor can do this repair or you can attempt it yourselves after reading up on surface preparation and manufacturer's instructions for patching concrete.

Ask a good contractor to recommend good repair products and advise you on the best way to repair your garage slab. It is best to get the advice of a qualified contractor or engineer who can take a look at the surface imperfections in the slab.

One of the best sources of information on popouts is ACI 302.1R, Guide For Concrete Floor and Slab Construction." You can order this document from ACI at: www.aci-int.org.