Page 317 - Planning and Design of Airports

Structural Design of Airport Pavements 275

types (ex. P-401/403 asphalt surfaces), or other user preferred types
listed in this section.

Design of Rigid Pavements
Rigid pavements consist of slabs of PCC placed on a subbase that is
supported on a compacted subgrade. Like flexible pavements, a
properly designed rigid pavement provides a nonskid surface which
prevents the infiltration of water into the subgrade, while providing
structural support to aircraft which use the pavement.
The subbase under rigid pavements provides uniform stable sup-
port for the concrete slabs. As a rule, a minimum thickness of 4 in is
required for all subbases under rigid pavements. There are various
types of mixtures which are acceptable for rigid pavement subbases
including:
Item P-154—Subbase Course
Item P-208—Aggregate Base Course
Item P-209—Crushed Aggregate Base Course
Item P-211—Lime Rock Base Course
Item P-301—Soil Cement Base
Item P-304—Cement Treated Base Course
Item P-306—Econocrete Subbase Course
Item P-401—Plant Mix Bituminous Pavements
Item P-403—HMA Base Course
For rigid pavements accommodating aircraft greater than 100,000 lb
maximum gross weight a stabilized subbase is required, which
include items P-304, P-306, P-401, and P-403.

Westergaard’s Analysis
Similar to the CBR method of design for flexible pavements,
prior to 2008, rigid pavement design using nomographs and other
approximation charts based on theories developed by H. M. Wester-
gaard was the FAA standard. Westergaard’s analysis of pavement
design was founded in the mid-1920s and focused on the calcula-
tions of stresses and deflections in concrete pavements due to applied
loading.
Westergaard assumed the pavement slab to be a thin plate resting
on a special subgrade which is considered elastic in the vertical direc-
tion only. That is, the reaction is proportional to the deflection of the
subgrade p = kz, where z is deflection and k is a soil constant, referred
to as the modulus of subgrade reaction. Other assumptions are that the
concrete slab is a homogeneous, isotropic elastic solid and that the
wheel load of an aircraft is distributed over an elliptical area. Although