UFC 3-260-02
30 June 2001
Bo, Bf = remaining life of base pavement to initial cracking and complete failure, respectively. The
remaining life may be estimated from PCI surveys or by computing the damage caused by
applied traffic before overlay. (Bo = 1 - 3 Ci/Co and Bf = 1 - 3 Ci/Cf)
Ci = applied past traffic, coverages
e. Time Periods. The base pavement performance curve (with the overlay in place) is divided into
time periods so that the variation of the base slab support with time can be determined. The first time
period is up to the base slab to. The last time period is the time past the tf. If some traffic has been
applied before overlay, the fatigue life consumed must be subtracted from to and tf because this damage
has already occurred. To calculate the stresses in the overlay, Equation 19-4 is used to determine the
varying base slab support for each of the time periods. If the base slab has begun to deteriorate before
the overlay is placed (SCI is less than 100), the base SCI value at the time of the overlay determines the
initial support condition. If the time to initial cracking computed exceeds to, the time to initial cracking can
be set to to. Doing so is equivalent to assuming that the base pavement will start to deteriorate with the
first coverage of traffic on the overlay. Figure 19-4 illustrates the performance curve for the base slab
subdivided into five time periods.
f. Overlay Performance Curve. Once the base pavement performance curve is established, the
damage is computed and accumulated for each time period. The damage for a time period is computed
as:
' ) (j
' ) (j
(19-7)
'
'
where
(do)j = damage to initial cracking for time period j
(df)j = damage to complete failure for time period j
By plotting the cumulative damage versus time in years, the time to initial cracking and complete failure
for the overlay can be established. These times correspond to the times when the cumulative damage
reaches a value of one. From these time values, a plot of SCI versus logarithm of time (performance
curve) then indicates how long the trial thickness will last for the selected design aircraft, traffic rate, and
design SCI at the end of the composite overlay pavement design life. Figure 19-5 illustrates the
composite overlay performance. If the life of the overlay for the trial overlay thickness is not adequate, a
new overlay thickness is assumed and the process is repeated. If several overlay thicknesses are
assumed, then a plot of thickness versus logarithm of time, like the one shown in Figure 19-6, can be
generated for the selected design SCI, and the design overlay thickness can be chosen.
8. REINFORCED CONCRETE. Limited full-scale accelerated traffic test data are available for the
design of reinforced concrete pavements. The test tracks contained reinforced test sections of varying
thickness and percentages of reinforcement. Comparisons were made between the performance of
plain and reinforced pavements. The improvements in performance were related to the amount of steel
in the concrete slabs. The basis for the comparison was the thickness of unreinforced pavement. The
19-10
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