Q. I have a client who wants to place a thin polymer-modified cementitious overlay over a 20-year-old exposed-aggregate pool deck. The concrete contractor originally finished the deck by seeding select aggregate into the fresh concrete and then removing the top mortar layer using a broom with a water-spray attachment. Can this kind of surface be successfully overlaid?
A. Jeff Potvin, director of engineering for Increte Systems Inc., says if the cementitious matrix of the exposed-aggregate substrate is in good shape, you can overlay it. To check the condition of the matrix, pressure wash a small portion of the deck at about 3500 psi. If the pressure washing damages the concrete matrix, you should tear out the deck and build a new one. If a few aggregate particles come off but the matrix is sound, proceed with the overlay. Start by applying a degreaser with a pump sprayer to remove oils, such as those from spilled foods or suntan lotions. Brush in the degreaser with a nylon-bristle broom, wait 10 to 15 minutes, and then wash off the surface with a garden hose or pressure washer. If water still beads on the surface, repeat the process.
Next, acid-etch the surface and rinse it thoroughly immediately afterward. If acid is allowed to dry on the surface, byproducts of the reaction between acid and concrete cause a yellow film to form. This film will decrease bond between the overlay and the concrete substrate.
After any water puddles on the rinsed surface have dried, mix the polymer-modified overlay to a wetter-than-normal consistency, and apply it with a squeegee to fill the spaces between the exposed-aggregate particles. After this has cured for a day, apply a leveling coat (also called a slurry or base coat) with a squeegee. The surface will then be ready for a spray-applied or a dash-coat overlay.
Potvin warns that this procedure isn't recommended for surfaces on which the exposed aggregate is bonded with epoxy.