Q.: I run a pressure-washing business in Georgia. To remove the red clay that stains many driveways, I wet the driveway, then use a low-pressure hand sprayer to apply a dilute solution of muriatic acid (one part commercial-grade muriatic acid to three parts water). After that, I immediately rinse the concrete surface with a pressure washer. I can achieve up to 3000-psi pressures, but seldom have to approach that to get the driveway clean. I've washed hundreds of concrete driveways using this method and never had a problem.
Last June, however, I pressure washed a one-year-old driveway, and five months later the homeowner said it had ruined the concrete surface. I inspected the concrete and did find that the surface is eroding in spots. The erosion is 1/8 to 3/16 inch deep, exposing coarse aggregate. Since I've never had this problem before, I think the concrete must have been of poor quality. Is there any other explanation?
A.: We asked consultant Robert Cain, KRC Associates, Milford, Ohio, about the possibility that residual acid had eroded the concrete surface to the depth you describe. He says that's highly unlikely because pressure washing would have removed any acid that wasn't neutralized by reaction with the concrete. However, Cain says acid opens up the concrete surface and advises that you apply a sealer after the concrete dries. This will help keep the driveway clean for a longer period between washings.
We recommend that you hire a materials consultant to examine the distressed portions of the driveway and, if necessary, take cores that could be used to determine concrete quality.