UFC 3-260-03
15 Apr 01
Subgrade Strength Category
Characteristic
Upper Limit
Lower Limit
Value
K1
K1
K1
Code
Rating
CBR
CBR
CBR
A
High strength
--
--
108 (400)
13
135 (500)
15
B
Medium strength
108 (400)
13
54 (200)
8
80 (300)
10
C
Low strength
54 (200)
8
27 (100)
4
40 (150)
6
D
Ultra-low strength
27 (100)
4
--
--
20 (75)
3
1
K in MN/cubic meter (pounds per cubic inch).
c. For Army, Navy, and Marine Corps airfield pavements, the PCN is based upon the maximum
loading of the most critical aircraft which can be allowed to use the airfield for the following 20-year period.
This loading is based upon the average day-to-day traffic that has been converted to equivalent passes of
the critical aircraft and extrapolated for the next 20 years. It should be noted that for airfield pavements
limited to aircraft weighing less than 5,670 kilograms (12,500 pounds), the PCN is not applied and weight-
bearing limits are reported directly in terms of maximum allowable gross weight of aircraft.
d. The PCN value assigned for Air Force airfield pavements will be based upon the allowable load-
ing for 50,000 passes of the Group 10 (C-17) aircraft. This requires each pavement feature to be evalu-
ated for the Group 10 (C-17) at 50,000 passes and a PCN assigned for that allowable loading from
figure 8-11.
e. When the critical aircraft and allowable gross weight have been established, the pertinent relation
between gross weight and PCN must be attained (for the proper pavement type and subgrade class). As
mentioned earlier, the gross weight versus PCN relations can be computed using the standard methods or
can be obtained from figures 8-1 through 8-11. By entering the proper figure with the allowable gross
weight of the critical aircraft, the limiting PCN can be determined.
f. By the same process, a separate PCN for the frost-melting period can be determined and
reported. Some guidance needs to be included, when reporting for a frost-melting period, to indicate
during which period the more limiting value must be applied.
3. TIRE PRESSURE LIMITATION. An aspect of ACN/PCN reporting is limitation of tire pressure
through application of categories for reporting in accordance with the following tabulation.
Pressure Limited to:
Code
Rating
MPa
PSI
W
High
No limit
No limit
X
Medium
1.50
217
Y
Low
1.00
145
Z
Very low
0.50
73
Rigid or rigid-overlay pavement can sustain the High (W) category except where the rigid layer is very thin
(less than 102 millimeters (4 inches)) or is thoroughly shattered (pieces less than about 0.6 meters (2 feet
wide)).
8-2
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