Fixing bad concrete with a spray-on overlay.

Q. I have a client who wants me to use a "Kool Deck" style spray-on overlay on a badly deteriorated pool deck. The deck has spalled areas and some cracks that are about 1/8 inch wide. If I go ahead and do this job, will it stay down and bridge the cracks?

A. Unfortunately a spray-on

overlay won't cover up cracks, although it could bond to the spalled concrete if the base concrete matrix is sound, but you will want to patch the spalled areas before applying the overlay. Mixing overlay cement with less water than what you would use for the overlay makes a good patching cement. Patches must be completely dry before the base coat is applied. In terms of problems that cause durability issues with this type of overlay, contractors agree that preparation is the key. "Many companies try too hard to sell overlay finishes on old, spalled concrete, says Aubrey Black, Construction/Increte Systems, Lookout Mountain, Ga. "But bad concrete surfaces often mean poor concrete below, so I avoid these jobs. I haven't had a job failure in the past five years." Randy Martin from Florida Spa and Pool, Leesberg, Fla., agrees, "If a job has problems, it's always because of poor prep." He advises using a light acid wash or carefully pressure washing the base slab. If acid is used, care must be taken to flush the slab so the surface pH ends up in the neutral range. His crews also frequently wash existing concrete with a 12% chlorine solution (using swimming pool chlorine) to get rid of algae that could interfere with a proper bond.

As far as the cracks, trying to repair a moving crack (and most cracks on outdoor surfaces open and close with temperature fluctuations) is impossible from a practical standpoint. Joe Primavera with Sundeck Products recommends saw cutting into a crack and placing an expansion joint product. Others think that using cracks to create a stone pattern by grinding or taping pattern lines is a better approach.

To apply this sort of overlay, use two applications of material. The first, the "base coat," can be applied with a squeegee or trowel. When a pattern is installed, the base coat is integrally colored to provide the joint color. The second coat, which is sprayed on, is the "texture coat." The texture coat is usually "knocked down" with a trowel immediately after spraying. Usually two men are involved--one spraying and one following immediately with a trowel to flatten the tops of the spatter marks, providing a surface that's easy to walk barefoot on when dry.