The projected AADT during the first year of operation is 3900 (both directions). If the traffic growth rate is estimated at 3% and the CBR of the subgrade is 7.5, determine the depth of a full-asphalt pavement using the AASHTO method and n = 20 years. The resilient modulus of the asphalt (EAC) is 320,000 lb/in2. Assume mi for the subgrade is 1 and the percent of traffic on the design lane is 42%, pt = 2.5 and SN = 4. Use a reliability level of 90%, a standard deviation of 0.45, and a design serviceability loss of 2.0.
The predicted traffic mix of a proposed four-lane urban non-interstate freeway:
Passenger cars = 78%
Single-unit trucks
2-axle, 5,000 lb/axle = 12%
2-axle, 9,000 lb/axle = 4%
3-axle or more, 23,000 lb/axle = 3%
Tractor semitrailers and combinations
3-axle, 20,000 lb/axle = 3%
What will be an ideal response?
Use Equation 19.2 to determine design ESALs.
ESALi = (fd)(Gjt)(AADTi)(365)(Ni)(fEi)
From Table 19.6, Gjt = 27.04
From Table 19.5, the following truck factors are obtained:
f1 = 0.008 (2-axle, 5,000 lb/axle)
f2 = 0.0715 (2-axle, 9,000 lb/axle)
f3 = 0.2495 (3-axle or more, 23,000 lb/axle)
f4 = 0.141 (3-axle, 20,000 lb/axle)
ESAL1 = (0.42)(27.04)(3900)(0.12)(365)(2)(0.008) = 3.10 × 104
ESAL2 = (0.42)(27.04)(3900)(0.04)(365)(2)(0.0715) = 9.25 × 104
ESAL3 = (0.42)(27.04)(3900)(0.03)(365)(3)(0.2495) = 3.63 × 105
ESAL4 = (0.42)(27.04)(3900)(0.03)(365)(3)(0.141) = 2.05 × 105
Total ESAL = 6.92 × 105
Determine Mr from Equation 19.3, for a CBR of 7.5,
Mr = 11,250 psi
Using the nomograph in Figure 19.10,
Step 1: Connect Reliability of 90% to standard deviation of 0.45 and
extend to first turning line to create “A”
Step 2: Connect “A” to ESAL of 6.92 × 105 and extend to second turning
line to create “B”
Step 3: Connect “B” to Mr of 11.25 × 103 and extend to design
serviceability loss chart to create “C”
Step 4: Connect “C” to ?PSI of 2.0
Step 5: Draw vertical line to read SN of 2.7
Determine structural coefficients for each layer
From Figure 19.5, with EAC = 320,000 lb/in2
a1= 0.36
Since this pavement is being designed with full-depth asphalt concrete, the asphalt
concrete must provide all of the structural support. Therefore, equation 19.6 can
be modified for use in this problem as:
SN = a1D1 = 2.7 = (0.36)D1
D1 = 7.5 inches
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