"Foamed" or "expanded" asphalt (or bitumen) is a road base recycling process relatively new to the United States. The process, popular in many other countries, offers a cost-effective alternate for road base stabilization to other techniques.

With foamed asphalt, a stabilized road base is created by carefully injecting a predetermined amount of cold water into hot performance graded (PG) asphalt in a pavement remixing unit, approximately 2-percent to 3-percent water by weight of asphalt.

Hot liquid asphalt rapidly expands into millions of bubbles (foam) when it comes into contact with cold water, similar to the spattering which takes place when drops of water stray into hot cooking oil on a stove top. When injected into the hot liquid asphalt, the water evaporates abruptly, thus causing explosive foaming of the asphalt in the saturated steam.

The water is the carrier of the atomized asphalt, and within only a few seconds the asphalt can thus be expanded 15 to 30 times its original volume. Precisely added water allows control of the rate and amount of asphalt foaming or expansion. The expanded asphalt has a lower viscosity and a resulting high surface area available for bonding with aggregate fines.

The intensity and effectiveness of the foaming process can be further improved by control of the basic physical conditions, such as pressure and temperature, which is possible when in-place foamed asphalt road stabilization is accomplished with a specialized machine such as the Wirtgen WR 2500 S, its predecessor the WR 2500 or the new WR 2000.

The foamed asphalt leaves the individual expansion chambers through nozzles and is immediately injected into the machine''s cutting chamber, where it is mixed with the reclaimed asphalt pavement (RAP) and the base mineral aggregates.

While expanded asphalt doesn''t completely coat all aggregate surfaces, it does form a mortar or glue which bonds the particles together. The expanded asphalt has an affinity for finer particles, those of 75 microns or less. This effective coating of finer particles increases the available surface area of the expanded asphalt for bonding with the coarser particles of material and "spot welds" the material matrix together.

Typically, the recycling or mixing machine is coupled with an asphalt supply tanker, in front, which is propelled by the recycler. The asphalt is heated to a temperature of 350 degrees Fahrenheit. A water cart, in rear, is pulled by the recycler.

When hot liquid asphalt cement is injected with a small amount of water, the volume of most types of liquid asphalt expands greatly as it forms black foam. Two foam properties are critical to successful road base stabilization, the expansion ratio and the foam''s half-life.

The expansion ratio is the maximum volume of foam relative to the original asphalt volume and is an indicator of how well the foam will disperse and coat the RAP and soil particles in the reclamation process. The half-life is the amount of time it takes for the foam to collapse to 50 percent of its maximum expanded volume and is an indicator of the foam''s stability. As the asphalt temperature or the amount of injected water increase, the expansion ratio increases, but the half-life decreases. Typically an expansion ratio of 15 times and a half-life of 12 seconds are best for the base stabilization process.

However, each foamed asphalt mix design is different and must be undertaken based on materials that will be encountered on-site. Based on lab tests of material extracted from the existing roadway, the initial mix design called for 3.0 plus or minus 0.3-percent foamed asphalt plus 1.0-percent portland cement, and compaction of at least 97-percent modified Proctor maximum dry density.