DESIGN CONSIDERATIONS - 3.4

CONSTRUCTION PRACTICES

Preparation of Subgrade

Remove all large rock, debris, and topsoil from the area to be paved. All vegetation, including root systems, should be removed. To prevent future growth, the subgrade should be treated with an approved herbicide. Install all drainage and utility facilities and then properly backfill and compact.  

The subgrade must be properly shaped to meet true lines and elevations and compacted to not less than 95 percent of maximum laboratory density. The surface of the compacted subgrade should not vary more than ¾ inch from the established grade.

Areas showing pronounced deflection under construction traffic indicate instability in the subgrade. If the situation is not corrected by reworking and additional rolling, the areas must be removed and replaced with suitable material and compacted or stabilized using a geotextile. The use of Asphalt Concrete base or course granular material is recommended.

Untreated Aggregate Base

The crushed aggregate base course may consist of one or more layers placed directly on the prepared subgrade. The material must be spread and compacted to the required thickness, grades, and dimensions indicated in the plans or as specified. The minimum compacted thickness of each lift should be no less than two times the size of the largest aggregate particle, or 4 inches, whichever is greater. The maximum compacted lift thickness should be 6 inches.

Binder and Surface Courses

The upper lifts of the pavement may consist of one or more courses of Asphalt Concrete placed on the previously constructed Asphalt Concrete base. In general, the top or wearing course must not be constructed to a depth greater than 3 inches. Where a thickness greater than 3 inches is indicated, it should be placed in two courses consisting of a binder and a surface or wearing course.

The minimum lift thickness must be 1 inch, but this thickness should never be less than two times the maximum particle size.

Tack Coat

A tack or bond coat of CSS-1, SS-1, MC-70 or an approved alternate should be applied between each course at an undiluted rate of 0.02 to 0.05 gallons per square yard. The surface must be cleaned of all dust, dirt, or other loose material before the bond coat is applied. If emulsion is used, it should be diluted with equal parts of water or as specified in the proposal.