30 June 2001
d. Computer Programs for Computing Cumulative Damage Factor. Two computer codes are used
for computing the subgrade and asphalt damage factors based on Equations 11-3 and 11-4. Both
programs require material strains obtained by the running of the layered elastic computer programs.
The listings of the programs contain an explanation of the input and instructions on the use of the
programs. An example illustrating the use of the programs is given in this chapter in the paragraph
entitled Example Design for Conventional Flexible Pavement.
7. CONVENTIONAL FLEXIBLE PAVEMENT DESIGN. Conventional flexible pavements consist of
relatively thick aggregate layers with a 75- to 125-millimeter (3- to 5-inch) wearing course of bituminous
concrete. In this type of pavement, the bituminous concrete layer is a minor structural element of the
pavement, and thus, the temperature effects on the stiffness properties of the bituminous concrete may
be neglected. Also, it must be assumed that if the minimum thickness of bituminous concrete is used as
specified in Tables 8-2 through 8-5, then fatigue cracking will not be considered. Thus, for a
conventional pavement, the design problem is one of determining the thickness of pavement required to
protect the subgrade. The steps for determining the required thickness for nonfrost areas are:
a. The subgrade resilient modulus is determined based on the soil exploration, climatic conditions,
and laboratory testing. The resilient modulus of the bituminous concrete is assumed to be 1,380 MPa
b. The traffic data determine the design loadings and repetitions of strain.
c. An initial pavement section is determined from the minimum thickness requirements as
determined using Chapter 10 or by estimation. The resilient modulus of the base and of the subbase is
determined based on the chart and the initial thickness.
d. The vertical strain at the top of the subgrade is computed for each aircraft being considered in
e. The number of allowable strain repetitions for each computed strain is determined from the
subgrade strain criteria.
f. The value of n/N is computed for each aircraft and summed to obtain the cumulative damage
g. The assumed thicknesses are adjusted to make the value of the cumulative damage factor
approach 1.0. This may be accomplished by first making the computations for three thicknesses and
developing a plot of thickness versus damage factor. From this plot the thickness that gives a damage
factor of 1.0 may be selected.
8. FROST CONDITIONS. Where frost conditions exist and the design is to be based on a base and
subbase thickness less than the thickness required for complete frost protection, the design must be
based on two traffic periods as described previously. In some cases, it may be possible to replace part
of the subgrade with material not affected by cycles of freeze-thaw but which will not meet the
specifications for a base or subbase. In this case, the material must be treated as a subgrade and
characterized by the procedures given for subgrade characterization.
9. ASPHALT CONCRETE PAVEMENTS. The asphalt concrete pavement differs from the
conventional flexible pavement in that the asphalt concrete is sufficiently thick to contribute significantly
to the strength of the pavement. In this case, the variation in the stiffness of the asphalt concrete caused