DEAR TIM: My concrete driveway and sidewalk surfaces are crumbling. The concrete just below the surface appears to be in very good shape. Can a thin concrete overlay be installed over the sound concrete? What needs to be done? Do you think I should hire a professional to do this work? E. R.
DEAR E. R.: This is your day! Thin concrete overlays are possible. Many of the concrete bridges in our interstate highway system are being restored using this proven technology. What's more, if you have some friends, relatives, and neighbors with strong backs and weak minds, it will be a breeze.
Your best results will be achieved with a fully bonded overlay. This method involves placing a 1 to 2 inch thick layer of concrete which physically and chemically bonds to your existing concrete. Thinner layers ranging from 1/2 to 1 inch can be installed. They require extra attention during and after installation. Thin overlays tend to shrink more readily because of rapid moisture loss while drying. This can cause cracking and future delamination.
The amount of cement used in the concrete mixture and the size of the stones (aggregate) is critical to your success. You need to make sure that each cubic yard of concrete you mix has a minimum of 600 pounds of cement. The maximum diameter of any stone in your mixture should not exceed 1/3 the thickness of the overlay. This means that if you intend to pour a 1 inch overlay, no single stone should be larger than 5/16ths of an inch!
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The concrete mixture itself contains a combination of stones, coarse builder's sand, cement, and water. If your overlay is very thin, say 1/2 inch, the stones will most likely be eliminated from your mixture.
Before mixing the first batch of material, you need to do several things. Chip away the existing loose material in several places to insure that the concrete below is, in fact, solid. Remove any grease and oil stains with soap and water and/or solvents. Completely rinse the surfaces to remove grease, dirt, dust, etc. Keeping the area to be repaired wet for 12 hours prior to applying the concrete will vastly improve your chances of success. This is especially true if you intend to do a thin overlay of 1 inch or less.
To fully bond the thin overlay to the old concrete, you need to apply a very thin layer of cement paint. Mix 1 part of cement to 1 part of fine sand. Add just enough water to make a rich, creamy mixture that resembles a high quality latex paint. Have one of your strong backed helpers apply this mixture in a 1/16th inch film just ahead of the concrete mixture. Be sure that there is no standing water in any of the repaired areas. This will dilute the bonding agent! It is vitally important that this paint does not dry before it is covered with the thin overlay.
Once the concrete has been installed, it is finished just like ordinary concrete. You can apply a broom finish or a coarse wood trowel finish for great traction. Apply a liquid curing compound immediately as you finish each section. In addition, cover the overlay with plastic to eliminate any water loss. Avoid doing this work on hot, sunny, or breezy days. Cool, overcast weather is the best for new concrete or concrete repair work!