UFC 3-260-02

30 June 2001

Time Interval

Time at the Beginning

Time Between

Number

of Each Interval, years

Intervals, years

1

0.000

2

6.33

6.33

3

34.00

27.67

4

73.77

39.77

5

130.92

57.15

6

213.05

82.13

c. Compute the Cumulative Damage in the Overlay. Once the base slab performance curve is

established, the damage in the trial overlay can be assessed. The procedure basically consists of

computing the tensile stresses at the bottom of the overlay, calculating the number of coverages to initial

cracking and complete failure, and calculating and cumulating the damage in the overlay for each time

period. This process is demonstrated in Table 19-10. Columns 1 and 2 contain the interval number and

the magnitude of the interval in years, respectively. Column 3 contains the average SCI's within each

interval that is used to compute the reduced modulus of elasticity (Equation 19-4). Columns 4, 5, and 6

contain the design aircraft, traffic rate, and pass-to-coverage ratio. Column 7 contains the computed

tensile stresses at the bottom of the overlay. These stresses are computed with the elastic layer

computer program JULEA assuming the interface between the overlay and the slab is unbonded since a

bond-breaker layer is used. Column 8 contains the number of coverages to initial cracking (Co) of the

overlay for each aircraft. The damage (Do) is computed in column 9 and accumulated in column 10

(DAMo). In a similar fashion the damage to complete failure (DAMf) is calculated and accumulated in

columns 11, 12, and 13. This process is repeated for each time interval as is shown in the table.

d. Determine Required Overlay Thickness. The cumulative damage for initial cracking (DAMo) and

complete failure (DAMf) of the trial overlay for each time interval can now be plotted. Figure 19-12

shows the plot for the 356-millimeter (14-inch) overlay. From this plot, the years to initial cracking and

complete failure of the overlay can be obtained by reading the years at which the DAMo and DAMf

curves cross a cumulative damage of 1.0. For the 356-millimeter (14-inch) overlay shown in

Figure 19-12, these values correspond approximately to 26 years to initial overlay cracking and 50 years

to complete failure of the overlay. Similar curves can be generated for the 406-millimeter (16-inch) and

457-millimeter (18-inch) overlay trials. Figure 19-13 summarizes the analysis performed on the 305-,

356-, and 406-millimeter (12-, 14-, and 16-inch) overlay trials. The values obtained from Figure 19-12

are used to generate the composite overlay performance curve. From Figure 19.13, for the case of the

356-millimeter (14-inch) overlay, the overlay performs at an SCI of 100 for 4.0 years before it starts to

deteriorate. It then deteriorates linearly with the logarithm of time until it reaches a complete failure

condition (SCI=0.0) after 50 years. Finally from Figure 19-13, the life of each overlay trial can be

obtained for the design overlay SCI of 80. These values are 4.2 years, 29.6 years, and 81.7 years for

the 305-, 356-, 406-millimeter (12-, 14-, and 16-inch) overlays, respectively. To obtain the required

thickness for the design life of 20 years, a plot of the overlay thicknesses versus the life of each overlay

is generated as illustrated in Figure 19-14. From this figure, a 426-millimeter (16.8-inch) overlay would

be required for a design life of 20 years and a SCI of 80.

19-19

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