TM 5-822-13/AFJMAN 32-1018
CHAPTER 6
FLEXIBLE PAVEMENT DESIGN
6-1. Design Requirements.
(c) An initial pavement section is determined
using the minimum thickness requirements from
Flexible pavement designs will provide the following:
TM 5-822-5/AFM 88-7, Chap. 3 or by estimation. The
a. Adequate thickness above the subgrade and
resilient modulus of the base and of the subbase is
above each layer together with adequate quality of
determined from figure B-l and the initial thickness.
the select material, subbase, and base courses to pre-
(d) The vertical strain at the top of the
vent detrimental shear deformation under traffic and,
subgrade is computed using JULEA for each axle load
when frost conditions are a factor, to control or re-
being considered in the design.
duce to acceptable limits effects of frost heave or per-
(e) The number of allowable coverages for each
mafrost degradation.
computed strain is determined from the subgrade
b. Sufficient compaction of the subgrade and of each
strain criteria using equation 5-5.
layer to prevent objectionable settlement under traffic.
(f) The value of n/N is computed for each axle
load and summed to obtain the cumulative damage.
drainage of base course during spring thaw.
(g) The initial thicknesses are adjusted to make
d. A stable, weather-resistant, wear-resistant,
the value of the cumulative damage approach 1. This
waterproof, nonslippery pavement.
may be accomplished by first making the computations
6-2. Determination of Pavement Thickness.
for three or four thicknesses and developing a plot of
a. Bituminous concrete. When the computed thick-
thickness versus damage. From this plot the thick-
ness of the bituminous concrete is a fractional value,
ness that gives a damage of 1 may be selected.
it will be rounded to the nearest full or half inch thick-
(2) Frost conditions. Where frost conditions ex-
ness. Values falling midway between the full and half
ist and the design thickness is less than the thickness
inch values will be rounded upward.
required for complete frost protection, the design must
b. Conventional flexible pavements.
be based on weakened subgrade condition. In some
(1) General. Conventional flexible pavements for
cases, it may be possible to replace part of the
roads, streets, and open storage areas consist of rela-
subgrade with material not affected by cycles of
tively thick aggregate layers with a thin (3- to 5-inch)
freeze-thaw but which will not meet the specifications
wearing course of bituminous concrete. In this type
for a base or subbase. In this case, the material must
of pavement, the bituminous concrete layer is a mi-
be treated as a subgrade and characterized by the
nor structural element of the pavement, and thus, the
procedures given for subgrade characterization. For
temperature effects on the stiffness properties of the
information on designing for frost conditions, see
bituminous concrete may be neglected. Also, it must
TM 5-822-5/AFM 88-7, Chap. 3.
be assumed that if the minimum thickness of bitumi-
c. All-bituminous concrete pavements. The all-
nous concrete is used as specified in TM 5-822-5/AFM
bituminous concrete pavement differs from the con-
88-7, Chap. 3, then fatigue cracking will not be con-
ventional flexible pavement in that the bituminous con-
sidered. Thus, for a conventional pavement, the de-
crete is sufficiently thick (greater than 5 inches) to
sign problem is one of determining the thickness of
contribute significantly to the strength of the pave-
pavement required to protect the subgrade from shear
ment. In this case, the variation in the stiffness of the
deformation. The steps for determining the required
bituminous concrete caused by yearly climatic varia-
thickness for nonfrost areas are:
tions must be taken into account by dividing the traf-
(a) Since summer temperature condition is con-
fic into increments during which variation of the
sidered most severe for subgrade shear failure, i.e.,
resilient modulus of the bituminous concrete is at a
largest subgrade vertical strain under load, a modu-
minimum. One procedure is to determine the resilient
lus value of 200,000 psi (considered to be small for bi-
modulus of the bituminous concrete for each month,
tuminous concrete) is used for the bituminous
then group the months when the bituminous concrete
concrete.
has a similar resilient moduli. Since the bituminous
(b) The traffic data determine the design load-
concrete is a major structural element, the failure of
ings and coverages.
this element due to fatigue cracking must be checked.
6-1
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