An existing rural 4-lane highway is to be replaced by a 6-lane divided expressway (3 lanes in each direction). Traffic volume data on the highway indicate that the AADT (both directions) during the first year of operation is 24,000 with the following vehicle mix and axle loads. The vehicle mix is expected to remain the same throughout the design life of 20 years, although traffic is expected to grow at a rate of 3.5 percent annually. Using the AASHTO design procedure, determine the minimum depth of concrete pavement required for the design period of 20 years.

Question

Passenger cars = 50 percent
2-axle single-unit trucks (12,000 lb/axle) = 40 percent
3-axle single-unit trucks (16,000 lb/axle) = 10 percent

Pi = 4.5 J = 3.2
Pt = 2.5 Cd = 1.0
S'c = 650 lb/in2 So = 0.3
Ec = 5 × 106 lb/in2 R = 95%
k = 130 lb/in3

What will be an ideal response?

Answer 1

First, determine the growth factor for 20 years at 3.5%.
Growth factor = [(1 + 0.035)20 – 1] / 0.035
Growth factor = 28.28
Next, assume a pavement thickness and estimate the ESALs for the 20 year
period.
Assume depth of 9 inches
From Table 20.7:
ESAL of 12 kips = 0.176
ESAL of 16 kips = 0.604
ESAL for 2-axle single unit trucks:
ESAL = 0.176(2)(0.4)(28.28)(24,000)(365)(0.40)
ESAL = 13.95 × 106
ESAL for 3-axle single unit trucks:
ESAL = 0.604(3)(0.1)(28.28)(24,000)(365)(0.40)
ESAL = 17.96 × 106
Total ESAL = 31.91 × 106
Next, determine the design serviceability loss
design serviceability loss = 4.5 – 2.5
design serviceability loss = 2.0
From the nomographs provided in Figures 20.13 and 20.14 and the factors above,
the required thickness of the concrete slab is 12.3 inches, which is far from the
assumed value of 9 inches. Hence, 9 inches cannot be used as the design depth.
Assume depth of 12 inches
From Table 20.7:
ESAL of 12 kips = 0.174
ESAL of 16 kips = 0.599
ESAL for 2-axle single unit trucks:
ESAL = 0.174(2)(0.4)(28.28)(24,000)(365)(0.40)
ESAL = 13.79 × 106
ESAL for 3-axle single unit trucks:
ESAL = 0.599(3)(0.1)(28.28)(24,000)(365)(0.40)
ESAL = 17.81 × 106
Total ESAL = 31.60 × 106
From the nomographs provided in Figures 20.13 and 20.14 and the factors above,
the required thickness of the concrete slab is 12.2 inches, which is close to the
assumed value of 12 inches. Hence, the rounded value of 12 inches will be used
as the design depth.