Foamed Asphalt And WR 2500 S Make Ideal Match

"Foamed" or "expanded" asphalt is a road base recycling process relatively new to the United States.
With foamed asphalt, a stabilized road base is created by carefully injecting a predetermined amount of cold water into hot penetration-grade asphalt in the mixing chamber of a pavement remixing unit, such as the Wirtgen WR 2500 S.

When a Wirtgen WR 2500 S remixer equipped for foam asphalt is used to make a foamed asphalt-stabilized base, a high-surface-area asphalt froth is created by precisely injecting cold water into hot liquid asphalt as it enters the machine's mixing chamber.

There, air bubbles in the expanded liquid asphalt froth act as the carrier of liquid asphalt to fines in a reclaimed asphalt pavement (RAP) aggregate mix.

In less than 15 seconds the froth subsides and the dispersion of asphalt is achieved, eliminating time waiting for the "break" required when expensive asphalt emulsions are used. The technology also sidesteps use of costly cutback solvents. The liquid asphalt cement is pure, with nothing added to it to change its properties. That makes it more economical to use than emulsions, which are a processed oil. 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.

With emulsions, full-particle coating of all materials in-situ is taking place. With foam, asphalt is being attached to fines in the roadway, material finer than sand. Optimum size is under-200 sieve size, which is material so fine that if tossed in the air, it would blow in the wind. If such fines are not available, adjustments will have to be made for that in the mix.

Corings and analysis are needed to determine what's underneath the driving surface. An operations manual will guide contractors through the mix design process.

The rapid strength gain from use of foamed emulsion means that traffic may be introduced onto the recycled road as soon as compaction is complete. A binder-based, flexible roadbed allows the HMA surface course thickness to be reduced by an inch or inch-and-a-half, one contractor said.

Aggregate or asphalt road millings can be treated with foamed asphalt in a stationary plant. However, use of a suitably equipped mobile remixing machine like the Wirtgen WR 2500 S offers firm benefits to plant mixing.

Inside the WR 2500 S

Inside the WR 2500 S are 16 expansion chambers, where a very small amount of water is introduced to the hot asphalt liquid. Air also can be introduced if a harder penetration-grade asphalt cement is used. A froth is developed as the viscosity of the asphalt cement is changed to that of shaving cream. The air bubbles serve as carriers to incorporate the asphalt to the fines in the WR 2500 S mixer. Asphalt/water emulsions work similarly, in which the water serves as carrier. But emulsions are expensive due to the manufacturing process and additives required.

Foamed asphalt saves time, too, because the time waiting for the emulsion to "break" is eliminated. The actual period of foaming is less than 15 seconds.

Following mixing of base materials with the liquid asphalt froth, a large rubber-tired compactor performs breakdown rolling right behind the remixer, prior to initial grading of the road using a motor grader. The final compaction in the high 90s is achieved by a steel-wheeled roller following the grader.
Foamed asphalt now can take its place with the three other forms of asphalt in road construction: as hot mix asphalt, as a warm "cutback" product, mixed with a volatile solvent, and as a cold product emulsified by chemicals in a water suspension.

The WR 2500 S also can perform base recycling with emulsions where appropriate, base pulverizing only, or even volumetrically controlled soil or base stabilization using lime or cement in dry or slurry form.

Every WR 2500 S that comes equipped for foam also has the capability to inject emulsion, which is a great tool as well. Foamed asphalt is just another tool for contractors, governments and engineers to use to make our roads better and use in-situ materials.

Recycling, contractors and cash flow

As far as a contractor is concerned, if full-depth reclamation and recycling represents less asphalt produced, moved and placed on behalf of a contractor -- thus less profit -- a paving contractor who expands into milling and foamed asphalt stabilization has a chance to reclaim some of that cash flow. The ability to do the entire foamed asphalt project -- by use of the WR 2500 S -- enables a contractor to recapture much of that income while providing a great value for the government agency customer due to the reuse of existing materials.

This leveraging of construction funds through recycling may enable the customer to do even road recycling work in a given season.

That's because with recycling, contractor and owner are using a material that's already been mined, processed, bought and paid for by the government agency. This is becoming critical because virgin aggregate sites are getting harder and harder to find and put into operation. Thus asphalt recycling will continue to play a larger role in the roadbuilding industry.

Also, the elimination of the need to truck in virgin aggregates and HMA is a big cost saver with recycling, as is the speed of completion and the ability to keep the road open for residents, school buses and public safety purposes.

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