UFC 3-260-03
15 Apr 01
adequate performance and service behavior for these materials, judgment may be used for the arbitrary
assignment of a CBR value for evaluation.
5.
PASS/LOAD RELATIONSHIPS.
a. Evaluation Curves. The evaluation of a flexible pavement with respect to thickness above the
subgrade and/or base courses and the selected CBR values can be made for airfields using figures 5-1
to 5-47. These figures may be used to evaluate for specific aircraft and/or Air Force evaluation groups
as indicated in the figures. These curves will be entered with the given thickness, CBR, and number of
passes and determine the allowable aircraft gross weight that can use the pavement. The range of
gross weights shown on the curves bracket the maximum and minimum weights of the aircraft in a
particular group or class. Curves are presented for various traffic areas as defined in TI 825-01/AFM 32-
1124 (I)/NAVFAC DM 21.10, for Army and Air Force and MIL-HDBK-1021/2 for Navy and Marine Corps.
b. Example Use of Evaluation Curves. Assume an evaluation is desired for 3,000 passes of the
C-141 aircraft on a pavement having a subgrade CBR of 4, a 510-millimeter (20-inch) subbase with CBR
of 30, a 152-millimeter (6-inch) base with CBR of 100, and a 102-millimeter (4-inch) surface course for a
total thickness of 762 millimeters (30 inches). The pavement is an Air Force or Army type A traffic area
or a Navy primary traffic area. When making an evaluation, the combinations of CBR and thickness
above a given layer must be evaluated for the subgrade, subbase, and base course to determine the
weakest combination. Enter figure 5-25 with the thickness of cover over a layer, the CBR of that layer,
and the number of passes being evaluated and determine the allowable load for that layer. The layer
having the least allowable load controls the evaluation. In this example, the base with a CBR of 100 and
a cover layer 102 millimeters (4 inches) has an allowable load of 217,700 kilograms (480 kips), the sub-
base with a CBR of 30 and a cover of 255 millimeters (10 inches) has an allowable load of 165,560 kilo-
grams (365 kips), and the subgrade with a CBR of 4 and a cover of 760 millimeters (30 inches) has an
allowable load of 96,160 kilograms (212 kips). Therefore, the pavement is capable of sustaining
3,000 passes of the C-141 at 96,160 kilograms (212 kips). When evaluating for more than one aircraft,
the weakest combination must be determined for all aircraft since the same weak condition may not
govern for all aircraft.
c. Thickness Selection. To evaluate an airfield facility, the pavements must be divided into traffic
areas as described in TI 825-01/AFM 32-1124 (I)/NAVFAC DM 21.10, for Army and Air Force and MIL-
HDBK-1021/2 for Navy and Marine Corps. A uniform thickness may be found for many pavements, and
in these cases the traffic area types should be designated. For pavements designed in accordance with
the traffic area concept, thickness differentials will occur between the various types of traffic areas.
When the pavement has a uniform thickness for the entire width, the selected thickness for evaluation is
no problem. When pavement thicknesses vary for a given feature, each thickness should be evaluated
but only the controlling evaluation for the facility should be reported.
d. Thickness Equivalencies. When a pavement has a thickness of base or surface that exceeds
the minimum thickness required for design, the excess thickness of asphalt is converted to an equivalent
thickness of base course and then added to the existing thickness of base. Any resulting excess thick-
ness of base is then converted to an equivalent thickness of subbase material which is then added to the
subbase thickness for evaluation. The equivalency factor used by the Army and Air Force for asphalt
surfacing is 2.3 and for base course is 2.0. This means that 25 millimeters (1 inch) of asphalt is equal to
58 millimeters (2.3 inches) of subbase, and 25 millimeters (1 inch) of base course is equal to 50 milli-
meters (2.0 inches) of subbase. To illustrate use of these equivalencies, assume that a pavement has a
total thickness of 508 millimeters (20 inches), consisting of 76 millimeters (3 inches) of asphaltic con-
crete, 203 millimeters (8 inches) of base course, and 228 millimeters (9 inches) of subbase. If the mini-
mum thickness of asphaltic concrete is 76 millimeters (3 inches) and base is 152 millimeters (6 inches),
then the existing base course has 50 millimeters (2 inches) of base not needed to meet minimum design
5-3
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