UFC 3-260-02
30 June 2001
13. DESIGN EXAMPLE 4, ALTERNATE OVERLAY DESIGN PROCEDURE. This example is based on
predicting structural deterioration from load-induced stresses. A concrete overlay must be designed to
support the traffic mix from example 2. The pavement consisted of a 381-millimeter (15-inch) PCC
pavement originally designed for the C-130 aircraft. It was considered that the concrete flexural strength
of the overlay will be approximately equal to that of the base slab which is 4.48 MPa (650 psi). Only
minor cracking of the base slab was observed; therefore, a SCI of 100 was assumed. It was also
estimated from pavement condition surveys that 90 percent of the pavement life to reach initial cracking
has been consumed. A bond breaker layer will be placed between the base slab and the overlay
(unbonded case). The overlay will be designed for a 20-year operating life and an SCI = 80 at the end of
its life.
a. Establish Base Slab Performance Curve. The first step in designing concrete overlays using the
alternate methodology is to determine the performance of the base slab under the new traffic when the
overlay is placed. For this example, overlay trial thicknesses of 305, 356, and 406 mm (12, 14, and
16 in.) are used. For each trial overlay, the tensile stress caused by each aircraft is computed at the
bottom of the base slab. From these computed stresses the coverages to initial cracking and complete
failure are computed. Table 19-8 presents in detail the necessary data and equations to perform the
calculations. Column 1 of Table 19-8 contains the overlay trial thicknesses, column 2 contains the
design aircraft, and column 3 contains design traffic data for each aircraft. Since the design is for a
20-year life, the traffic rate for the B-52 would be 100 passes (from example 2) divided by 20 years, or
5 passes per year. Column 4 contains the factor to convert passes to coverages. Column 5 contains
the calculated tensile stresses at the bottom of the base slab using the elastic layer computer program
JULEA. The coverages for initial cracking and complete failure of the base slab were computed using
Equations 19-1 and 19-2. The damage rate (DRo) is then computed and accumulated in column 7. The
sum of the damage rates is used to calculate the time for initial cracking (To) in column 8. Similar
calculations are performed in columns 9, 10, and 11 to calculate the damage rate for complete failure
(DRf) and the time for complete failure (Tf) of the base slab. These two time values establish the base
slab rate of deterioration between the SCI=100 and an SCI=0. Figure 19-10 shows the base slab
performance curve for the 356-millimeter (14-inch) overlay. Similar plots are generated for the 305- and
the 406-millimeter (12- and the 16-inch) overlay trials. Finally, the time from the placement of the overlay
(mo) to initial cracking of the base slab is computed in column 12. Since 90 percent of the base slab life
has been consumed, the remaining life is then 10 percent (Bo = 0.10). For the 356-millimeter (14-inch)
overlay, mo = 0.10/0.0158 = 6.33 years.
b. Subdivide Base Slab Performance Curve. The next step is to subdivide the base slab
performance curve for a trial overlay thickness into time intervals. For the 356-millimeter (14-inch)
overlay (Figure 19-10), the curve is divided into six intervals. The first interval will be from the time when
the overlay is placed (mo) to initial cracking of the base slab (To). This first interval then corresponds to
6.33 years. Between To and Tf, the performance curve is then divided into time intervals to account for
the deterioration of the base slab with time (the base slab modulus of elasticity decreases). The slope
100. To illustrate this, the time between To and Tf in Figure 19-10 is divided in four equal time intervals
(on a logarithm scale) and the magnitude of each time interval recorded. The last time period
corresponds to the time Tf and beyond (SCI=0). Table 19-9 summarizes this procedure and
Figure 19-11 illustrates the actual base slab performance curve when the 356-millimeter (14-inch)
overlay is in place. The magnitude of these time intervals will be used in the calculation of the
cumulative damage in the overlay within a time interval.
19-17
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