Q. We're going to apply an overlay to a deteriorated parking deck. In some areas, we have to remove concrete surrounding unbonded post-tensioning tendons. The tendons are encased in plastic sheathing. Is there any surface preparation method we can use that will remove the concrete but not the sheathing?

A. We spoke to a few manufacturers of sandblasters and shotblasters and they said that using either of these methods will most likely damage, if not remove, the sheathing. However, Stan Smith, associate principal at J.R. Harris & Co., Denver, faced this challenge on a recent parking deck repair project. Mr. Smith said that a shotblasting demonstration performed on the project resulted in no apparent damage to the sheathing. But shotblasting removes the concrete surface slowly and is not an economical method if a significant amount of concrete needs to be removed. Using chipping hammers, it's difficult to remove concrete from around the tendons without damaging the sheathing. Also, waterblasting the surface at pressures high enough to remove concrete will remove the sheathing as well. If you don't have to remove the concrete surface but just need to wash it, you may be able to pressure-wash the surface at pressures low enough to keep the sheathing intact. But Gerard McGuire, executive director of the Post-Tensioning Institute, advises against waterblasting over tendons encased in sheathing. By doing so, he says, you risk forcing water through any existing small holes or tears in the sheathing, increasing the risk of corrosion of the tendons. One possible solution is to replace sheathing that is removed by surface preparation. Mr. Smith said that J. R. Harris & Co. is looking into the feasibility of applying a flexible, synthetic rubber coating to areas of the tendons where sheathing has been removed.

Have any of our readers confronted the challenge of removing concrete near post-tensioning tendons encased in plastic sheathing? We'd like to hear about your experiences. Send answers to Repair Q & A Editor, The Aberdeen Group, 426 S. Westgate St., Addison, IL 60101.