ABSTRACT
Pavement performance is strongly influenced by the underlying soil layers, particularly with respect to stability, bearing strength, consolidation over time, and moisture susceptibility. Frequently one or more layers are placed between the soil subgrade and the pavement. In flexible or asphalt pavements, the layer directly under the pavement
is the base and the layer between the base and the subgrade is termed the subbase. Base course materials have to meet tighter quality specifications than subbase materials. Since subbase quality materials may be used directly under a concrete pavement, this layer is typically termed the subbase. This is the terminology used in the 1993 AASHTO Design Guide (AASHTO 1993: I-4). Concrete pavements distribute loads much more widely than asphalt
pavements, and the pressures on the subbase and subgrade are low. As a result, the bearing capacity of the underlying layers is less critical, and there is no need to use stiff base materials except for pavements carrying the heaviest loads. In the literature, however, both terms, base and subbase, have been used
to describe the layer under a concrete pavement. It is rare for concrete highway pavements to have both a base and a subbase layer. In contrast, airfield pavements that handle heavy wide-body aircraft often have both a base and a subbase (Kohn and Tayabji 2003: 41). These layers may be unstabilized, or stabilized with portland or asphalt
cement. They may also be densely graded and relatively impermeable, or deliberately gap graded to provide drainage. The PCA/ACPA publication Subgrades and Subbases for Concrete Pavements addresses selection and construction of these layers (PCA/ACPA 1991). Another publication that discusses subbases and bases, specifically for airfields, is Design and Construction Guide for Stabilized and Drainable Bases (Hall et al. 2005).
