Curing Surface Problems

Often, the concrete below the surface is in good shape. The surface peels away because the cement which holds the sand and gravel together was very likely diluted by the installer. This happens when the finisher sprinkles water onto the concrete surface or floats the bleed water back into the concrete while working it. This extra water simply dilutes the cement in the upper surface. With less cement present, the concrete is very susceptible to damage by ice formation or pressures exerted by salt crystallization. With enough cement present in all parts of the concrete, it can withstand years and years of snow, ice, salt, etc. That is a proven fact.

The Fix

Do you drive the Interstate highway system? If so, you probably have seen bridge resurfacing/restoration projects happening. Here in Cincinnati, workers have been applying a one to two 2 inch concrete overlay on top of the old concrete surface. No doubt they use specialized bonding agents and a special mix due to the abnormally high traffic loads and abuse the bridges receive.

You, however, can redo your driveway in a simple fashion that will last for many, many years.

Adhesion - The Key

The concrete overlay you are about to install will succeed if it adheres or sticks to the old surface. If it does not, it will simply peel like your existing surface. Regular Portland cement is an excellent glue or adhesive. Just think - had the original concrete mason/finisher done his/her job, the Portland cement in the original mix would be working just fine.

Adhesion of the new overlay to the old concrete has both a physical and chemical aspect. The Portland cement crystals grow into and chemically bond to the clean, old cement matrix. Mechanically, the greater the surface area of the bond the more crystals will grow and create greater adhesion. This simply means that the rougher the surface, the better the bond!

Thus, it is to your advantage to remove all loose concrete, dust, etc. from the surface. A pressure washing to blast away concrete that appears sound but actually isn't is not a bad idea!

Grease and oil stains will interfere with adhesion. You must remove them using soap, solvents or a combination of the two.

Once all of the loose material and contaminants have been removed, wash the surface to be repaired with soap, water and a stiff broom. We want it squeaky clean. Rinse to remove all soap, dirt, dust, etc.

The Overlay - How Thick?

The thickness of the overlay is critical. It is often dictated by conditions other than the slab you are going to repair. In other words, the slab may abut a garage slab or city sidewalk. The current levels of these two surfaces may be level! What do you do then? I'll tell you in a moment.

I have successfully installed overlays that are only 1/2 inch thick. These can easily be accomplished by a novice homeowner. The thin overlay allows a large area to be covered by a minimal amount of material. For example, if you install a 1/2 inch overlay on your driveway, and your driveway measures 648 square feet, you only need one cubic yard of overlay material! That is not much. Your original driveway probably required 10 times as much material.

Concrete Overlay Literature Sources

The following two organizations have the absolute best information on all aspects of concrete and concrete installation. Each has an online Bookstore listing all of their publications. Their literature is authoritative, easy to understand and user friendly. Check it out!

• American Concrete Institute (www.concrete.org)
• Portland Cement Association (www.cement.org)