Pavement design must balance the need for the desired performance against the cost of building or rehabilitating a pavement.
Standard sections are often used to simplify the design of pavement features such as parking lots and subdivision streets, however many standard designs were developed decades ago, and their utility reflects the needs of their time. Conditions and considerations have changed. Today's pavements serve a variety of purposes and must be able to withstand the specific traffic associated with those purposes. For example, during the development of low volume roads like city streets, heavy vehicles often load the pavements while delivering construction equipment, concrete, and other materials like steel and bricks. If the pavements are being constructed first, the design should consider an increased thickness of the asphalt layers along the delivery routes to prevent the initiation of failure before the facility opens. The design of subdivision streets should also consider heavy traffic such as school buses and waste trucks. Parking lot design should incorporate a marked, heavier section designated for deliveries, a feature often used by large chain stores with great success.
It is also important to remember just because the standard section says that the thickness of the asphalt surface is two inches, there is no guarantee it will be constructed to that thickness. In the end, the thickness is often dictated by the precision of the grading of the base, the need to match the planned elevation, and the need to match curb lines and drainage inlets. But be aware, small deviations from the planned thickness can have a profound effect on the cracking performance. For instance, if the pavement section is designed for two inches but the as-built thickness is only 1.5 inches, then the asphalt pavement is missing 25 percent of the section. There is a rule-of-thumb that each additional inch of asphalt will double the fatigue life of the pavement. Thus, to ensure cracking resistance, it may be advisable to increase the standard section to 3 inches. For Access to an easy-to-use structural pavement design method based upon the 1993 AASHTO Guide for Pavement design, go to pavexpressdesign.com.
Dr. David Newcomb, P.E., “Quality Asphalt Pavements: A Primer for Cities and Counties,” Texas Asphalt Magazine, Fall 2018