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 750 ESAL. It is expected that construction will be completed 5 years from now. If the traffic growth rate is 2% and the effective CBR of the subgrade on the new alignment is 9.0, 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 450,000 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.

Question

What will be an ideal response?

Answer 1

First, determine design ESAL
From Table 19.4, the growth factor, Gjt, for 25 years (5 years of construction plus
20 year design life) = 32.03 and the growth factor, Gjt, for 5 years (to subtract the
ESAL for the construction period) = 5.20
Design ESAL = (750)(32.03 – 5.20)(365)(0.45)
Design ESAL = 3.31 × 106
Determine Mr from Equation 19.3,
Mr = 13,500 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 3.31 × 106 and extend to second turning
line to create “B”
Step 3: Connect “B” to Mr of 13.5 x 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 3.2
Determine structural coefficients for each layer
From Figure 19.5, with EAC = 450,000 lb/in2
a1= 0.43
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 = 3.2 = (0.43)D1
D1 = 7.44 inches.
Therefore, the depth of the asphalt concrete should be 7.5 inches.