Pavement Design

CGN 4503/6905 Pavement Design Homework 2 Spring 2018

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Due Monday 19th Name:__________________________________

1. (10 points) Select the most appropriate answer for the following:

Typical thickness of a pavement structure

a. 1 in (2.5 cm)

b. 10 in (25 cm)

c. 30 in (75 cm)

d. 100 in (2.5 m)

Typical granular layer thickness a. 1 in (2.5 cm)

b. 10 in (25 cm)

c. 30 in (75 cm)

d. 100 in (2.5 m)

Typical tire pressure of a heavy truck a. 1 psi (2.5 cm)

b. 10 psi (25 cm)

c. 100 psi (75 cm)

d. 1000 psi (2.5 m)

Typical maximum deflection of a flexible pavement

a. 0.5 mils (0.01 mm)

b. 2 mils (0.05 mm)

c. 20 mils (0.5 mm)

d. 100 mils (2.5 mm)

Maximum compressive strain to which a subgrade may be subjected

a. 5 με b. 50 με c. 500 με d. 5000 με

2. (8 points) Sketch the approximate trends of the critical tensile strain (εt) and the critical compressive strain (εc) with the thickness of the base for: a. A pavement with a thin asphalt layer (2-4 in) b. A pavement with a thick asphalt layer (8-10 in)

Base thickness

ε t

Base thickness

ε c

CGN 4503/6905 Pavement Design Homework 2 Spring 2018

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3. (8 points) Determine the minimum thickness of asphalt concrete requited to withstand 5,000,000 repetitions of an 18-kip single axle load with a tire pressure of 80 psi. The asphalt concrete to be used has the following properties:

 Modulus at the highest expected temperature = 200,000 psi.  Modulus at the lowest expected temperature = 1,000,000 psi.

Additional information:

 You need to check both cracking and rutting distress modes  If needed, assume that lateral stresses in the subgrade are negligible compared to vertical stress.  Remember to properly round layer thickness.

, 0.0796 ∙ 1 .

∙ 1 .

, 1.365 ∙ 10 ∙ 1 .

E=10,000 psi ν=0.5

h1= ? E=see above ν=0.5

80 psi

CGN 4503/6905 Pavement Design Homework 2 Spring 2018

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4. The following figure represents a 3-layer pavement system subjected to a uniform circular load:

Based on the information provided in the figure, use the KENPAVE computer program to complete the following:

a. (6 points) Plot the deflection basin (surface vertical deflections along the transverse axis) for this problem and for the Example 1 discussed in class. Note: For a proper illustration of the deflection basin, determined deflections at the following distances from load center: 0, 5, 12, 24, 36, 48, 60, and 72 inches. Note that you must run KENPAVE for example 1 as well as for the pavement above.

b. (4 points) Compute the critical tensile strain and compare it to that obtained in Example 1. What was the primary reason for the difference?

c. (4 points) Compute the critical compressive strain and compare it to that obtained in Example 1. What was the primary reason for the difference?

d. (4 points) Estimate the number of cycles to failure (NF,c & NF,d) for both sections using the equations developed by the Asphalt Institute.

e. (2 points) Identify the dominant failure mode (fatigue cracking or rutting) of this section and compare it to that of Example 1.

Notes:

 You are expected not only to report values but also to comment on the reasonableness of the results obtained at each step.

 Include your four KENPAVE output files (*.txt) in your solution.

, 0.0796 ∙ 1 .

∙ 1 .

, 1.365 ∙ 10 ∙ 1 .

h1=6 in

q=100 psi

a=5 in

E3=10,000 psi ν3=0.45

E1=500,000 psi ν1=0.35

h2=12 in E2=20,000 psi ν2=0.40

∞