Pavement Design
Approximately 3.0 million miles of highways in U.S. (45% fully paved).
Basic Functions of Pavements:
•
Visual guidance to drivers
•
Support vehicle load and distribute the trafficload stresses to the soil
(subgrade) at a magnitude that will not shear or distort the soil.

soil bearing capacity
→
2 to 50 psi

3500 lb auto
→
tire pressures
≈
35 psi

80,000 lb trucks (5axles)
→
tire pressure
≈
100 psi or higher (used for design)
Pavement Types
:
•
Flexible Pavements: asphaltic cement and aggregates
24 inches
→
Wearing Surface
←
asphaltic cement and aggregates
410 inches
→
Base
←
crushed aggregates (high quality)
410 inches
→
Subbase
←
crushed aggregates (rock)
Soil
→
Subgrade
68 inches of it is blended and compacted to maximum density
NOTE:

Base is usually stabilized with Portland cement, lime, fly ash, or asphaltic cement.

Wearing surface protects the base layer from wheel abrasion and waterproof the
entire pavement structure.
It also provides a skidresistant surface.

Thickness of Layers = f(type of axle load, available material, and expected
pavement life).
•
Rigid Pavements:
Portland Cement Concrete (PCC) and aggregates
“dowels” are load
Joint
transfer device
812 inches
PCC Slab
PCC Slab
→
10 to 13 ft wide to 40 ft or more long
48 inches
Base Layer (optional)
→
depending on the properties of subgrade
Soil
→
Subgrade
68 inches of it is scarified, blended, and compacted to maximize density
•
Other Variations of Flexible and Rigid Pavements:

Composite Pavements (Rigid and Flexible)

Continuously Reinforced

Posttensioned

etc.
1
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Principles of Flexible Pavement Design:
•
Load distribution on a flexible pavement:
Wheel Load
Wearing
Base
Subbase
Subgrade
•
Stresses and deflections:

Boussinesq theory:
assumes that the pavement is one layer thick and the material is
elastic, homogeneous, and isotrophic.
P
→
point load
x
y
⇒
one layer of pavement
∞
depth
z
stress at a point in the system is
2
z
z
P
K
=
σ
EQ. 4.1
σ
z
= stress at point z in the system, lb/in
2
P = wheel load, lb
z = depth of the point in question, inches
and
2
/
5
2
z
r
1
1
2
3
K
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎠
⎞
⎜
⎝
⎛
+
⋅
π
=
EQ. 4.2
where,
r = radial distance in inches from the centerline of the point load to the
point in question.
2
A more realistic approach is to expand the point load to an elliptical area that represents a tire
footprint. The tire footprint can be defined by an equivalent circular area with a radius
calculated by
π
=
p
P
a
EQ. 4.3
P = tire load in lb
p
= tire pressure in lb/in
2
a = equivalent load radius of the tire footprint, inches
The integration of the load from a point to a circular area can be used to determine the stresses
and deflections in a onelayer pavement system.
Influence charts, graphical solutions, and other equations are developed by researchers to
calculate stresses and deflections.
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 Spring '11
 schultz
 Normal Distribution, Inch, pavement

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