d. Thickness of Base and Subbase. Enter Figure with the CBR of the subbase
material and determine the total required thickness of base and surfacing.
e. Repeat Step d for other available subbase materials to determine the
most economical combination of base and subbase material and thickness.
f. Minimum Base and Subbase. Use a minimum base thickness of 6 inches.
Minimum subbase thickness should be 4 inches.
g. Minimum Thickness of Surface. Minimum thickness of asphalt concrete
surfacings should be as follows:
Type of Surface
Minimum Thickness (inches)
))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))
Primary Road
3
Secondary and Tertiary Roads
2
Parking Area
2
Driveway
1.5
Sidewalk
1
Surfacing Used by Tracked Vehicles
4
Where exceptionally stable bases, such as cement-treated aggregate, natural
cementing materials (limerock), or asphalt concrete bases are used, the 2-
inch minimum thickness may be reduced to 1.5 inches.
4. COMPACTION AND SETTLEMENT. Nonuniform settlement or inadequate compaction
of the pavement components can result in premature pavement rutting and
cracking. See requirements for subgrade compaction and settlement analysis
contained in Section 4. Subbase and base course compaction requirements are
contained in Sections 5 and 6.
5. FROST DESIGN. In areas having freezing temperatures, ascertain the need for
designing for frost effects from state, county, or city highway departments.
Determine the depth of frost penetration from the highway department or local
utility companies. Use local design procedures. For a complete discussion of
frost effects and two methods of design, see NAVFAC DM-21.
Section 8. DESIGN OF BITUMINOUS SURFACES
1. FUNCTIONS AND REQUIREMENTS. Bituminous surfaces provide a resilient,
waterproof, load distributing medium that protects the base course against the
detrimental effects of water and the abrasive action of traffic. Bituminous
surfaces permit slight adjustments in the pavement structure, due to
consolidation, without detrimental effect, and are more readily adaptable to
stage construction. The types of bituminous surfacings commonly used for roads
and streets are given in Table 1.
2. TERMINOLOGY. See Figure 1 for a description of flexible pavement components
and see Table 8 for terminology related to bituminous mixes.
3. BITUMINOUS MATERIALS. Bituminous materials used in road construction
include asphalts, tars and tar-rubber blends. The applicable specifications
for bituminous materials are given in Table 9.
a. Asphalt. Asphaltic materials are the usual choice for use in bituminous
pavements due to their widespread availability and economy. Asphaltic
materials are available as asphalt cements, liquid asphalts, or emulsified
asphalts.
(1) Asphalt cements are most widely used in hot-mix asphalt concrete
mixtures as binder and wearing courses. The material is available in varying
degrees of hardness (penetration) and is a solid at room temperature.
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