Prevent Blacktop Cracks From The Start
Prevent Blacktop Cracks
DEAR TIM: My husband and I are going to have a new blacktop driveway installed. I love blacktop, but I dislike the cracks that develop in blacktop. What, if anything, can be done to minimize or eliminate cracking in blacktop? Should I simply install a thicker layer of blacktop? K. C.
DEAR K. C.: Your questions are excellent. If I knew how to eliminate cracks in blacktop, I wouldn't be writing this column. I would be retired and on a beach somewhere in the Caribbean! However, I can tell you what you can do to help to minimize cracking.
I feel the best place to start is to explain just what blacktop is. A grasp of this information will help you better understand blacktop's limitations. Blacktop is a generic term for hot mix asphalt or asphalt concrete. The makeup of asphalt concrete is similar to the makeup of regular concrete in many respects.
Blacktop consists of aggregate and asphalt cement. Concrete consists of 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. These grades are based on chemical makeup and viscosity. At or about 80 degrees F, paving grade asphalt is a semisolid. When heated to 300 degrees F it becomes a thin liquid.
Asphalt concrete is made by mixing this product with a mixture of dried aggregate. The aggregate becomes coated with the asphalt cement in the mixing process. The entire batch will now stick to itself and just about anything else it touches.
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This manufacturing process gives blacktop a unique characteristic. Blacktop is somewhat flexible. This flexibility is both an advantage and a disadvantage. The advantage is that it can stretch somewhat without cracking.
Blacktop's disadvantage is that in and of itself, it has very little tensile strength. Blacktop derives the greatest portion of its strength from its base or foundation. Herein, lies the source of a majority of the reasons for crack development in blacktop.
The base for many blacktop driveways is crushed gravel. This base is more often than not applied directly on the soil. The base and the soil have to be able to support the loads which will be placed on it. For the base to attain its maximum strength, it must be placed on compacted soil.
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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 your blacktop to crack. Poor subsoil drainage can turn the strongest soils into mush. If this mush contaminates the gravel base, the base rapidly looses strength.
Years of research and development have produced some methods which help to 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 characteristics.
Also, manufacturers have developed geo-tech fabrics which can be laid on the soil before either of the bases are applied. These fabrics prevent soil mush from traveling up into the base, should the soil become over saturated 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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