Two well-known classification systems are the
American Association of State Highway and
Transportation Officials (AASHTO) Classification
System and the Unified Soil Classification System, used
by the Department of Defense.
PREPARING ASPHALT FOR
Paving grade asphalt (asphalt cement), which at
normal atmospheric temperatures is semisolid and
highly viscous, must be made temporarily fluid
(liquefied) for handling during construction operations,
such as pumping through pipes, transporting in tanks,
spraying through nozzles, and mixing with aggregate.
When pavement construction operations are finished,
the asphalt cement reverts to its normal condition and
functions as the cementing (or binding) and
waterproofing agent that makes the pavement stable and
Asphalt cement can be made temporarily fluid
(liquefied) for construction operations in three ways:
1. By heating the asphalt. After construction
operations, the hot liquid asphalt cement cools and
changes from a fluid to its normal, semisolid condition.
2. By dissolving the asphalt in selected petroleum
solvents. This process is called cutting back; the
diluted asphalt is called cutback asphalt. After
construction the solvent evaporates, leaving the asphalt
cement in place.
NOTE: The use of cutback asphalt in the United
States has declined because of the petroleum shortage
and government environmental regulations. It is being
superseded by emulsified asphalt, which contains little
or no solvent, and can be used for almost any purpose
that cutbacks can.
3. By emulsifying the asphalt with an emul-
sifying agent and water.
While asphalt and water
ordinarily do not mix, they can be made to mix by
churning asphalt in a colloid mill. The resulting
product, called emulsified asphalt, is a fluid and is
ready for construction operations. During construction
the water and asphalt separate. The asphalt particles
merge into a continuous film that cements the aggregate
particles together as the water evaporates. When the
water and asphalt separate, it is said that the emulsion
breaks or sets.
A hot-mixed or hot-load paving mixture is the best
type of pavement; the aggregate and binder should be
heated to approximately 310°F and laid no colder than
250°F. Determining the exact temperature(s) to use will
depend upon the weather and the distance that the
material is hauled. Some clues that indicate the
condition of the asphalt are as follows:
. Overheated asphalt loses some of its binding
qualities. Blue smoke, rising from the spreader hopper,
is sometimes an indicator this condition exists.
. A generally stiff appearance and improper
coating of aggregate indicates the mix is too cold.
l Material laying flat in the bed of the truck with a
shiny appearance means the mix is too rich in asphalt
. When it is too lean, the mix will look dry and dull.
Priming consists of the initial treatment on a
granular base before surfacing with a bituminous
material or pavement. The purpose of a prime coat is to
penetrate the base (about 1/4-inch minimum penetration
is desired), fill most of the voids, promote adhesion
between the base and the bituminous applications placed
on top of it, and waterproof the base. Surfaces must be
as clean as possible, and where and conditions exist
(dried-out surfaces), a light fog spray with water should
be considered before priming actually begins.
The priming material may be either a low-
viscosity tar, a low-viscosity asphalt, or a diluted
asphalt emulsion. The bituminous materials, used for
the prime coat, should be applied in quantities known
as rate of application (ROA) of not less than 0.2
gallon or more than 0.5 gallon per square yard.
Normally, the construction project specifications
denote the ROA for the prime coat application;
however, when the ROA is not included in the project
specifications, the NCF uses an ROA of .3 for
planning purposes. When the base absorbs all of the
prime material within 1 to 3 hours or when penetration
is too shallow, the base is underpriced. Underprim-
ing may be corrected by applying a second coating of
the prime material.
An overprimed base may fail to cure or set and may
contribute to failure of the pavement or bleed up
through the asphalt mat. A free film of prime material
remaining on the base after a 45-hour curing period
indicates that the base is overpriced. This condition
may be corrected by spreading a light, uniform layer of
clean, dry sand over the prime coat to absorb the excess
material. Application of the sand is usually followed by