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FHWA Resource Center

PAVEMENT AND MATERIALS TEAM

Innovative Technology

What is Foamed Asphalt?
Michael Smith, Materials Engineer

Foamed Asphalt is a stabilization technique used widely throughout Europe. However,
the European process is relatively new here in the United States. The scanning team
found significant advances have occurred in the construction equipment used to apply
foamed asphalt. These advances allow for deep road base stabilization at low cost
compared to other stabilization techniques.

The European foamed asphalt stabilization technique involves the following construction
activities:

1) pulverization of the existing road surface,
2) shaping the surface to near final grade,
3) mixing and injection of foamed asphalt cement,
4) compaction to final grade.

The preferred method is to initially pulverize the existing road bed without injection of
asphalt cement. This allows the contractor the opportunity to adjust the grade to its near
final profile prior to mixing and injection. During the mixing and injection process hot
paving grade asphalt cement is foamed or expanded and injected into the chamber of
the remixing unit. A schematic of the mixing chamber is shown in Figure 1. An asphalt
nurse truck, shown in Figure 2, is required to feed hot asphalt cement to the remixing
unit.

Foamed asphalt depends on the forceful expansion of asphalt cement. As shown in
Figure 3, expansion of the asphalt cement is obtained by the addition of a small amount
of water. Air may be used if working with harder paving grade asphalts. The hot
reaction is similar to the addition of water to hot oil. However, in the foamed process this
reaction is controlled. Precisely added amounts of water control the rate and amount
foamed asphalt. In the expansion process small droplets of asphalt are forced into the
pulverized materials. As opposed to conventional paving techniques, the purpose is not
to coat the particles but to bind the pulverized materials together. The process has been
compared to spot welding. The foamed asphalt has an attraction to the fines due to the
high surface area. This creates a mortar-like paste that provides the strength and
cohesion of the mix after compaction. The trick is to use the correct amount of water to
achieve optimum expansion into the pulverized material. Expansion of the asphalt occurs
in specially designed spray nozzles.

Following the mixing process, a rubber tired roller may be used as the breakdown roller
followed by a steel wheel roller as the finish roller. The result is a cost effective deep
strength road base.

Some of the benefits of foamed asphalt over more traditional stabilization techniques
include:

· stockpile material for longer periods of time (as long as 1 month)
· achieve deeper road stabilization (as deep as 14 inches)
· open roads to traffic faster

Three manufacturers have been identified that produce foamed asphalt equipment. These
include CMI, Catepillar Paving, and Wirtgen. The team scanning team visited the
Wirtgen Facilities in Germany. Wirtgen has made 147 machines in the last five years. 5
are in the U.S., 2 in Canada, and the others around the world.

Scan Findings

Carlton we need your notes here. We were introduced to the process in Germany at the
Wirtgen facilities. However, I do not have records of use in the other countries we
visited. Perhaps the other scan members do. Perhaps the Wirtgen officials can supply
European useage information.

Contrast with United States

Foamed asphalt has been used sporadically since the late 1960s in the United States.
Research has been conducted at Iowa State University and the Waterways Experiment
Station in Mississippi. Later, Conoco owned the patent to the process. The process did
not catch on due to difficulties in foaming the asphalt cement. Limited demonstrations
have been conducted in this country. In the 1980s projects were placed in Colorado and
Wyoming. Demonstration projects have been constructed using the newer European
process as recently as 1998 in Jefferson County Wisconsin. There is a need to evaluate
the effectiveness of these projects.

(Photos and Figures are below)

Illustration Showing How Foamed Asphalt is Applied Inside


Figure 1. Illustration Showing How Foamed Asphalt is Applied Inside
the Mixing Chamber.

Illustration Showing How Foamed Asphalt Works.
Figure 3. Illustration Showing How Foamed Asphalt Works.>

The Wirtgen 2500 Is Used to Both Pulverize the Road Bed and Apply the Foamed Asphalt.

Figure 1. The Wirtgen 2500 Is Used to Both Pulverize the Road Bed and Apply the Foamed Asphalt.



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