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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