TM 5-822-13/AFJMAN 32-1018
d. Pavements with a chemically stabilized base
subbase layers are determined from the chart in fig-
ure B-l of appendix B. The strains calculated using
course. For a pavement having a chemically stabilized
JULEA are tabulated in table 6-1. Based on the strain
base course and an aggregate subbase course, dam-
age must be accumulated for subgrade strain and for
values, the allowable coverages (Ni) are determined
horizontal tensile strain at the bottom of the asphalt
from figure 5-1. The corresponding damage for each
surface layer. Normally in this type of pavement, the
thickness under each axle load is computed as ni/Ni
and is tabulated in the last column of table 6-1. The
base course resilient modulus is sufficiently high
( 100,000 psi) to prevent fatigue cracking of the bitu-
total damage for each thickness is the sum of the dam-
age of each axle load. A plot of the damage against
minous concrete surface course (where the bituminous
the thickness indicates that the required thickness is
concrete surface course has a thickness equal to or
greater than the minimum required for the base course
22 inches for a damage of one, which is the same thick-
given in TM 5-825-2), and thus, this mode of failure is
ness derived using the design index method in TM 5-
only a minor consideration. For most cases, a very
822-5/AFM 88-7, Chap. 3. The subbase is therefore 12.0
conservative approach can be taken in checking for
inches. Table 6-1 shows that the damage caused by
this mode of failure; i.e., all the traffic can be grouped
the passenger cars is so small that their inclusion in
into the most critical time period and the computed
the damage computation could actually be neglected.
bituminous concrete strain compared with the allow-
6-4. Design Example for an All-Bituminous Concrete
able strain. If the conservative approach indicates that
(ABC) Pavement.
the surface course is unsatisfactory, then the damage
should be accumulated for different traffic periods. For
Design an ABC pavement for the same condition
the pavement having a stabilized base or subbase, the
shown in previous example. For computation of the
stabilized layer is considered cracked for the purpose
fatigue damage and subgrade damage, monthly tem-
of design. The cracked stabilized base course is rep-
perature variations are considered; the corresponding
resented by a reduced resilient modulus value, which
variations of bituminous concrete modulus are shown
is determined from the relationship between resilient
in tables 6-2 and 6-3, respectively. Three pavement
modulus and unconfined compressive strength shown
thicknesses of 8, 10, and 12 inches are used for dam-
in figure 4-3. When the cracked base concept is used,
age computation. Normally for ABC design the
the subgrade criteria generally control the design. The
subgrade damage will be the controlling criteria and
section obtained should not differ greatly from the
thus the thickness for satisfying the subgrade criteria
section obtained by use of the equivalency factors pre-
is first determined. The design is carried out in the
following steps:
sented in TM 5-822-5/AFM 88-7, Chap. 3.
a. Subgrade failure.
6-3. Design Example for a Conventional Flexible
(1) The subgrade strains are computed for each
Pavement.
thickness under each axle load for each month using
JULEA computer program. The bituminous concrete
Design a conventional flexible pavement to support
moduli for each month are shown in the last column
the following traffic:
of table 6-3. Because the effect of passenger cars on
Passenger car
2,000 operations per day
damage computation was proven to be negligible in
3-axle trucks
200 operations per day
the previous example, damage computation for pas-
As stated in TM 5-822-8/AFM 88-7, Chap. 3, this traf-
senger cars was not done.
fic results in a design index of 6 and the required pave-
(2) The allowable coverages Ni for each pavement
ment thickness is determined to be 22 inches for a
are computed from the failure criteria shown in fig-
subgrade CBR of 4.
ure 5-1.
a. Assume each axle of the passenger car carries
(3) The damage increments for each month are
1,500 pounds and the front axle of the truck (single-
computed. The strains, allowable coverages, and the
axle, single wheel) carries 9,000 pounds and the rear
cumulative damage for the 32-kip tandem-axle, dual-
axle of the truck (dual-axle, dual wheels) carries 32,000
wheels loads are tabulated in table 6-4. Cumulative
pounds. The total number of operations and their cor-
damage for the 9-kip load is negligible. It is seen that
responding coverages (n1) for each axle load are tabu-
nearly all the subgrade damage in the flexible pave-
lated in table 6-1. The design using the layered elastic
ment is done during the warmer months, i.e., May,
method is discussed in the following paragraph.
June, July, August, and September.
b. Three pavement thicknesses (16, 20, and 24
(4) Similar computations are made for 9-kip single-
inches) are assumed for the design. The bituminous
axle, single-wheel loads. A plot of the cumulative dam-
concrete surface and base layers are 4 and 6 inches,
age for both 32-kip and 9-kip loads and pavement
respectively. The subgrade strains are computed for
thickness indicates that for a damage of one, the
each thickness under each axle load using the layered
required ABC thickness is 10.05 inches. For design
elastic method. The modulus values of the base and
purpose lo-inches are used.
6-2
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