Bituminous mixtures are frequently used as base courses beneath high-type bituminous pavements, particularly for rear-area Wields which carry heavy traffic. Such base courses may be used to advantage when locally available aggregates are relatively soft and otherwise of relatively poor quality, when mixing plant and bituminous materials are readily available, and when a relatively thick surface course is required for the traffic. In general, a bituminous base course may be considered equal on an inch-for-inch basis to other types of high-quality base courses. When a bituminous base course is used, it will be placed in lifts not exceeding 3 1/2 inches in thickness. If a bituminous base is used the binder course may be omitted, and the surface course may be laid directly on the base course.

Figure 3-18.-Typical section.