Civil Engineering Reference
In-Depth Information
The most common type of flexible pavement consists of the following:
●
Asphalt concrete:
The uppermost layer (surface course) is typically
asphalt concrete
that distributes the vehicle load in a cone-shaped area under the wheel and acts as the
wearing surface. The ingredients in asphalt concrete are asphalt (the cementing agent),
coarse and fine aggregates, mineral filler (i.e., fines such as limestone dust), and air.
Asphalt concrete is usually hot-mixed in an asphalt plant and then hot-laid and com-
pacted by using smooth-wheeled rollers. Other common names for asphalt concrete are
black-top, hot mix,
or simply
asphalt
(Atkins 1983).
●
Base:
Although not always a requirement, in many cases there is a base material that
supports the asphalt concrete. The base typically consists of aggregates that are well
graded, hard, and resistant to degradation from traffic loads. The base material is com-
pacted into a dense layer that has a high frictional resistance and good load distribution
qualities. The base can be mixed with up to 6 percent Portland cement to give it greater
strength, and this is termed a
cement-treated base
(CTB).
●
Subbase:
In some cases, a subbase is used to support the base and asphalt concrete layers.
The subbase often consists of a lesser-quality aggregate that is lower-priced than the base
material.
●
Subgrade:
The subgrade supports the pavement section (i.e., the overlying subbase,
base, and asphalt concrete). The subgrade could be native soil or rock, a compacted fill,
or soil that has been strengthened by the addition of lime or other cementing agents.
Instead of strengthening the subgrade, a geotextile could be placed on top of the subgrade
to improve its load-carrying capacity.
Many different types of methods can be used for the design of the pavements. For
example, empirical equations and charts have been developed based on the performance of
pavements in actual service. For the design of flexible pavements in California, an empirical
equation is utilized that relates the required pavement thickness to the anticipated traffic
loads, shear strength of the materials (
R
value), and gravel equivalent factor (California
Division of Highways 1973; ASTM Standard No. D 2844). Instead of using the
R
value, some methods utilize the
California bearing ratio
(CBR) as a measure of the shear
strength of the base and subgrade. Numerous charts have also been developed that relate the
shear strength of the subgrade and the traffic loads to a recommended pavement thickness
(e.g., Asphalt Institute 1984). When designing pavements, the geotechnical engineer should
always check with the local transportation authority for design requirements as well as the
local building department or governing agency for possible specifications on the type of
method that must be used for the design.
11.3.3 Earthquake Design
The design of an asphalt concrete road typically does not include any factors to account for
earthquake conditions. The reason is that usually the surface course, base, and subbase are
in a compacted state and are not affected by the ground shaking. In addition, the cumula-
tive impact and vibration effect of cars and trucks tends to have greater impact than the
shaking due to earthquakes.
Concrete pavement and concrete median barriers are often damaged at their joints, or
they are literally buckled upward. This damage frequently develops because the concrete
sections are so rigid and there are insufficient joint openings to allow for lateral movement
during the earthquake. For example, Fig. 11.4 shows compressional damage to the roadway
and at the median barrier caused by the Northridge earthquake, in California, on January
17, 1994. In additional to rigid pavements, flexible pavements can be damaged by localized
compression, such as shown in Fig. 11.5.
