Q: We plan on adding an overhang to our house to cover a concrete patio, 17-by-28 feet. The concrete is chipped and has some minor cracks and stains. Should we add a fresh layer of concrete over the old? A: You can pour a new concrete floor over the old, or you may find a commercial resurfacing product easier to use for almost the same results.
Mr. Mac's Concrete Resurfacer, marketed by Macklandburg-Duncan Corp., is designed for use by the do-it-yourselfer. It's a polymer product that is applied in a 1/16- to 1/32-inch layer that bonds to the existing concrete and fills small holes, pits and low spots, and levels out the damaged surface.
If the surface you are covering has holes and pits greater than 1/8 inch, use Mr. Mac's Concrete Fix as a patching material prior to applying the resurfacer. For more information on these products and their availability, contact Mr. Mac at 1-800-348-3571.
You may find that resurfacing with this or a similar product is more expensive than the alternative of pouring a new floor over the old. If you wish to add more inches to the surface, you will need to consider another technique. Here is one recommendation from the Portland Cement Association:
Prepare the old slab by cleaning the surface. A dry-cleaning method that will remove any deteriorating concrete on the old surface is recommended. Use a scarifier or grit blaster, which will clean and roughen the surface in preparation for the new concrete layer. These machines can be rented. Any residual oil must be removed.
For a thin resurfacing that is less than two inches thick, a sand-cement grout is used to develop a strong bond between the old slab and the new layer. Edge forms and screeds for this type of work vary. Some solid support is usually required along the sides of the area resurfaced. This is necessary for ease of screeding the topping to the proper elevation.
After the edge forms have been set, the elevation should be checked with an engineer's level or laser and adjusted as necessary to ensure the desired surface tolerance, slope for drainage, and minimum overlay thickness.
Immediately before a fully bonded overlay is placed, a thin coating of bonding grout should be scrubbed into the cleaned, dry surface or cleaned, damp (but not wet) surface, if it has been prewetted overnight. This grout should consist of a 1/16- to 1/8-inch layer containing one part portland cement, one part concrete sand from which the plus No. 8 material has been scalped and about half part of water mixed to give a creamy paint-like consistency.
Broom the grout into the surface to ensure the entire surface area receives a thorough, even coating. The rate at which grout is applied should be limited to the rate at which topping concrete can be placed. The grout should not be allowed to dry to a whitish appearance before the topping concrete is placed lest the grout act as a debonding layer instead of improving the bond.
Concrete for resurfacing floors must be of high quality and resistant to surface abrasion, with a reasonably low water-cement ratio. Mix design will vary depending on the thickness of concrete to be placed. A superplasticizer can be added to the concrete mix to obtain a more workable concrete.
Hand or machine methods can be used to place and finish the topping, such as strike-off, compact, float, trowel and cure.
Proper and adequate curing is more important in bonded resurfacing than in ordinary concrete work because of the potential for rapid, early drying of the thin concrete overlay. Protect against rapid drying with coverings such as wet burlap, plastic sheets, or waterproof paper as soon as possible to do so without marring the surface.
Keep concrete covered and wet for at least seven days when using normal portland cement in temperatures greater than 50 degrees, and for at least four days in warm temperatures or for high-early-strength portland cement. Send inquiries to Here's How, Copley News Service, P.O. Box 190, San Diego, Calif. 92112-0190. Only questions of general interest can be answered in the column.