Q-My husband and I are going to have a new blacktop driveway installed. I love blacktop, but I dislike the cracks that develop in it. What, if anything, can be done to minimize or eliminate cracking in blacktop? Should I simply install a thicker layer of blacktop?
A-If I knew how to eliminate cracks in blacktop, I wouldn’t be writing this column-I’d be retired and on a beach somewhere in the Caribbean. However, I can tell you what you can do to help to minimize cracking.
The best place to start is to explain just what blacktop is. Blacktop is a generic term for hot mix asphalt or asphalt concrete. The makeup of asphalt concrete is similar to that of regular concrete in many respects. Blacktop consists of aggregate and asphalt cement, while concrete comprises aggregate and cement. Both of them derive a great deal of strength from the size and amount of aggregate.
Asphalt cement is a product of the refining process of crude oil. There are different grades of asphalt cement that are based on chemical makeup and viscosity. At or about 80 degrees Fahrenheit, paving-grade asphalt is a semisolid. When heated to 300 degrees, it becomes a thin liquid.
Asphalt concrete is made by mixing this liquid with a mixture of dried aggregate. The entire batch will now stick to itself and just about anything else it touches.
This manufacturing process gives blacktop its unique flexibility. But the material poses both an advantage and a disadvantage.
The advantage is its ability to stretch somewhat without cracking. The disadvantage is that in and of itself, blacktop has very little tensile strength. Rather, it derives the greatest portion of its strength from its base or foundation. Herein lies the reason for most blacktop cracks.
The base for many blacktop driveways is crushed gravel, which often is applied directly onto the soil. But for the base to attain its maximum strength, it must be placed on compacted soil.
Furthermore, the base and the soil beneath it must not be allowed to become saturated with water. Water softens the soil and allows the base to move, which then allows the blacktop to crack. Poor subsoil drainage can turn the strongest soils into mush. If this mush contaminates the gravel base, the base rapidly loses strength.
Years of research and development have produced some methods to help solve these problems. Instead of using crushed gravel for a base, a different type of asphalt concrete can be used. This initial layer has very large aggregate, which gives it fantastic strength. Also, manufacturers have developed fabrics that can be laid on the soil before either of the bases is applied. These fabrics prevent soil mush from traveling up into the base, should the soil become oversaturated with water.
Crack prevention starts at the beginning of the job. Thickening of the final blacktop layer is not the answer.
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Have questions about the remodeling process? Write to Tim Carter, c/o The Chicago Tribune, P.O. Box 36352, Cincinnati, Ohio 45236-0352. Questions will be answered only in the column.
For a listing of associations and publications that can provide helpful blacktop information and other tips on residential blacktop, please send $1.50 and a business-size stamped, self-addressed envelope to Tim Carter at the above address. Ask for Builder Bulletin No. 50.
You can obtain a free order form for a wide variety of individual job bid sheets by sending a business-size SASE to the same address.
