UFC 3-260-02
30 June 2001
d. As an example of the overlay design procedure, it is assumed that an existing pavement
(type B traffic area) is to be upgraded to an Air Force medium-load pavement. The existing
pavement consists of 75 millimeters (3 inches) of AC, 150 millimeters (6 inches) of base
(100 CBR), and 533 millimeters (21 inches) of subbase (50 CBR). The subgrade is a lean clay with
a CBR of 6, and has a density of 95 percent in the top 150 millimeters (6 inches) and 90 percent
below 150 millimeters (6 inches). From Figure 10-18, the total thickness of new pavement
required is 1,143 millimeters (45 inches) and the thickness of base and surface required over the
50 CBR subbase is 178 millimeters (7 inches). However, from Table 8-5, the minimum surface and
aggregate base course required for medium load and traffic area B is 25 millimeters (10 inches).
From Table 6-2, it is noted that the in-place subgrade density is adequate. Based on above
information, the following analysis is made:
(1) New design criteria requires a 1,143-millimeter (45-inch) pavement section above the
subgrade and, from Table 8-5, 254 millimeters (10 inches) above the subbase.
(2) Existing pavement section is 758 millimeters (30 inches) with 225 millimeters
(9 inches) above the subbase.
(3) In this example, the existing thickness would require an additional inch of pavement to
meet the minimum thickness asphalt and the thickness required above the subbase. A 1-inch
overlay however is not sufficient to protect the subgrade which requires 1,143 millimeters
(45 inches) (or equivalent) of pavement above it. Any thickness of asphalt exceeding the minimum
of 100 millimeters (4 inches) can be converted to an equivalent thickness of subbase. This
excessive thickness of asphalt is equal to the thickness of overlay plus any existing thickness of
asphalt minus the minimum thickness. The required thickness of overlay (to) is then determined as
follows:
(17-6)
%
%
&
%
%
'
where
tm = minimum thickness of asphalt
to
= thickness of overlay
te
= thickness of existing asphalt
Ef = equivalency factor for converting asphalt to an equivalent thickness of subbase
tSb = thickness of existing base course
tb
= thickness of existing subbase
Tp = total thickness of pavement required above the subgrade
For this example, the equation is
%
%
&
%
%
'
17-16
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