Background

The need and use of bases and subbases for pavements has been well known for thousands of years. The Romans built over 53,000 miles of roads primarily to facilitate the movement of troops and supplies beginning in about 500 BC [Hays 2016]. The Romans recognized the benefits of “protecting” the natural earth subgrade from the repeated loading of their carts and chariots. Roads such as the Appian Way (Figure 1) were constructed of multiple layers of stones (subbase, base, and surface) and were sloped to drain water away from the road. This Roman road and the few others that remain today are tourist attractions. They are not active roadways and would not be able to handle modern traffic.

Early roads had fairly thick bases and subbases (Figure 2). In the early 1900s, with the use of asphalt and cement bound surface layers, base and subbase thicknesses were decreased.

As shown in Figure 4, loads applied to a PCCsurfaced rigid pavement are spread over a large area of subgrade, compared to loads applied to an asphalt concrete-surfaced flexible pavement. This permits the use of thinner bases for rigid pavements than for flexible pavements.