A section of a two-lane rural highway is to be realigned and replaced by a four-lane highway with a full-depth asphalt pavement. The AADT (both ways) on the existing section can be represented by 500 ESAL. It is expected that construction will be completed 5 years from now. If the traffic growth rate is 5% and the effective CBR of the subgrade on the new alignment is 8.5, determine a suitable depth of the asphalt pavement using the AASHTO method. Take the design life of the pavement as 20 years. The resilient modulus of the asphalt (EAC) is 400,000 lb/in2. Assume mi for the subgrade is 1 and the percent of traffic on the design lane is 45%. Use a reliability level of 90%, a standard deviation of 0.45, and a design serviceability loss of 2.0.
What will be an ideal response?
First, determine design ESAL
From Table 19.6, the growth factor, Gjt, for 25 years (5 years of construction plus
20 year design life) = 47.73 and the growth factor, Gjt, for 5 years (to subtract the
ESAL for the construction period) = 5.53
Design ESAL = (500)(47.73 – 5.53)(365)(0.45)
Design ESAL = 3.47 × 106
Determine Mr from Equation 19.3,
Mr = 12,750 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 3.47 × 106 and extend to second turning
line to create “B”
Step 3: Connect “B” to Mr of 12.75 × 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 3.4
Determine structural coefficients for each layer
From Figure 19.7, with EAC = 400,000 lb/in2
a1= 0.40
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 = 3.4 = (0.40)D1
D1 = 8.5 inches.
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