Lesson 1/Learning Event 1
Learning Event 1
FLEXIBLE PAVEMENTS AND ROADS
Pavement (including the surface and underlying courses) is divided into two broad classifications or
types rigid and flexible. The term "rigid pavement" is applied when the wearing surface is constructed
of portland cement concrete. Pavement constructed of concrete will possess considerable flexural
strength which will permit it to act as a beam and allow it to bridge over minor irregularities which may
occur in the base or subgrade upon which it rests, hence it is "rigid." All other pavement or bases are
In flexible pavements, the distortion or displacement occurring in the subgrade is reflected in the base
course and on upward to the surface course. Thus the term "flexible" is used to denote the tendency of
all courses in this type of structure to conform to the same shape under traffic. Flexible pavements are
used almost exclusively in the theater of operations for road and airfield construction since they are
adaptable to almost any situation and capabilities of a normal engineer troop unit.
This Learning Event will be concerned primarily with the design considerations.
Flexible Pavement Structure
A typical flexible pavement structure (Figure 1) illustrates the terms used in this subcourse when
referring to various layers. All flexible pavement will not have every layer shown in Figure 1. For
example a two layer structure consisting of only a compacted subgrade and a base course is a complete
flexible pavement. The word "pavement" when used by itself refers to only the leveling, binder and
surface courses, while flexible pavement refers to the entire pavement structure from the subgrade up.
The design of flexible pavement must be based on complete and thorough investigations of subgrade
conditions, borrow area and sources of select materials, subbase and base materials. The Battalion Soils
Section of the Combat Heavy Battalion will determine the California Bearing Ratio (CBR) and other
soil properties of available materials and the subgrade. (Refer to Engineer Subcourse 5453, Soils
Engineering, for more information on testing of soil.)
Distribution of Loads
A pavement composition, Figure 2, shows two typical sections of flexible pavement, one with a thick
and one with a thin base course. In either case, the subgrade is the foundation which eventually carries
any load applied at the surface. Airfields or roads usually must be leveled and shaped; consequently, the
subgrade is defined as the natural soil which is compacted or treated to receive the base and wearing
surface. The subbase and base are composed of higher quality material than the subgrade. This material
may be imported or selected from the site.