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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