Blacktop Driveways Over Concrete
DEAR TIM: The surface of my concrete driveway is eroding after years of wear and tear. I am thinking about resurfacing my existing concrete driveway with blacktop. It is my intention to add two inches of blacktop to the existing concrete after all of the loose debris is removed. Is this a good idea? Do you see any problems? Are there other alternatives? Patrick O., Lexington, KY
DEAR PATRICK: What you plan to do is done on hundreds of thousands of miles of roadways and driveways each year in the USA. Blacktop can be successfully installed over concrete. But there are issues as you might expect. In addition, you do have an alternative paving method that may surprise you.
One positive aspect of installing blacktop over concrete is the concrete provides a firm base for the flexible asphalt paving. Experts who work each day in the blacktop industry, refer to blacktop as a flexible pavement. It does not have near the strength of concrete when you measure them inch for inch. The asphalt cement that bonds the pieces of sand and gravel in the blacktop can and does move over time. It is a unique material in that it possesses qualities of both a solid and a liquid.
Concrete is very similar to blacktop in that it also contains sand and gravel pieces. But the Portland cement ingredient that crystallizes and bonds tenaciously to the sand and gravel is as hard as rock. In fact, and geologist will tell you that regular concrete looks and feels exactly like a sedimentary rock called conglomerate.
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Blacktop derives its strength from the base beneath it. New blacktop driveways are routinely installed over a compacted crushed-gravel base. This base stone is often eight to ten inches thick and when installed on firm compacted soil, it makes a superb foundation for the blacktop. But if your concrete driveway is crack-free and the individual slabs are not tilted, the blacktop covering will perform as well or better on it than the gravel.
But keep in mind that you may have crack problems at a later date with your blacktop. My guess is your current concrete drive has control or expansion joints at 10 - 15 foot intervals. Or perhaps there are a few random cracks and some slight displacement between the cracked pieces of concrete. If this is the case, these cracks will eventually telegraph through the new blacktop covering. Your blacktop contractor can saw cut in crisp lines in the blacktop directly over the concrete control and expansion joints if you like. These thin lines can be filled with special crack sealers.
You may have other problems. The added two inches of blacktop might cause safety issues where the driveway abuts stairs and sidewalks. The added thickness of the blacktop can reduce the riser height at stairs. A tapered two-inch lip can create a stumbling hazard when people walk from adjacent paved surfaces onto the driveway.
Keep in mind that your blacktop driveway will become a maintenance item every 3 - 5 years. To keep blacktop from degrading due to exposure to the elements and liquids such as gasoline, oil and hydraulic fluids, you must keep a coat of premium sealer on it at all times.
If I were you, I would seriously consider installing a thin concrete overlay on your driveway. You can apply a layer of cement stucco as thin as 3/8 inch that will last for another 40 or more years. It is possible to add an even thicker layer of concrete with small stones in it, but then you have the same safety issues I have already discussed.
There are professional companies that can stamp and colorize the thin overlays if you want a truly distinctive driveway. Stamped concrete is not a do-it-yourself (DIY) project unless you have lots of experience and many friends who can assist you.
The concrete overlay project is very DIY friendly as you don't have to do the entire driveway on one day or even a weekend. You can apply the overlay to one or two sections of driveway at a time. The trick to making sure the driveway looks the same once complete lies in getting all of the materials you need at the same time. You must buy all the sand at one time and the bags of cement must be from the same manufacturer and batch.
Beware of blacktop contractors that say all will be well if they install blacktop over broken and crumbling concrete. It will look fine for awhile, but wait until you start to drive on it or heavy trucks pull into your drive. If you live in a cold climate, all sorts of cracks will appear half-way through the coldest part of the winter when the soil freezes and heaves the driveway.
If you decide to install the concrete overlay, you must make sure all loose material is removed and the concrete is slightly damp. Immediately before applying the overlay mix, paint on a thin coat of cement paint. Cement paint is made by mixing Portland cement powder with clear water.