UFC 3-260-02
30 June 2001
Table 19-9
Time Intervals for the 356-millimeter (14-inch) Overlay
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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