Q: I'm looking for information about drilling holes into concrete. A number of tool manufacturers have alluded to a geological map of the United States that shows the varying hardness of concrete across the country. For instance, concrete in the upper northwest is supposedly very hard, requiring a rotary hammer drill to get the job done. In other areas you can get by with a hammer drill or even an ordinary drill.Does this map in fact exist? And if so, can you help me locate a copy?

A.: You're probably referring to maps used by saw-blade manufacturers as an aid to picking the right saw blade. Such maps indicate areas in which aggregates are generally soft, medium or hard. We found one such map on a brochure from Blazer Diamond Products (800-245-5024) and another in a Target catalog (800-288-5040).

Map courtesy Target.