Article published in
Hot Mix Magazine, Volume 11, Number 1
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This is a rubber-blending plant that was purchased from CEI Enterprises by Blue Diamond Materials. The plant is currently operating in the Los Angeles, California area and was designed to meet Caltrans Greenbook specifications. it consists of two units: a rubber-blending trailer (right foreground) and a asphalt-rubber reaction tank (left background). A detailed description of the plant's physical set-up and operation is in the text of this article.

INTRODUCTORY COURSE IN ASPHALT-RUBBER

This special binder material has been in use since the late 1960's in Arizona...and is now spreading to other states, as well as to Canada, Mexico, and Europe.

Asphalt-Rubber has been around for 40 years or so, but relatively few people in the hot-mix asphalt (HMA) industry are really familiar with what it is, how it works, and how it is made. This brief article may shed a little light on the subject. Hopefully, you will find that useful since you will probably be hearing more about asphalt-rubber in the future.

What is asphalt-rubber?
Without getting too technical, let's just say that asphalt-rubber is a modified binder product that is made by mixing liquid asphalt with crumb rubber — a material that is the result of grinding rubber automotive tires to reduce the rubber to small particles. When the two materials are mixed, the result is a binder material that some of the specification manuals refer to as crumb-rubber asphalt or CRA.

Asphalt-rubber was originally developed as a paving material in the late 1960's in Arizona. In more recent years, use of this special technology in paving applications has spread across the southwestern part of the United States. It is also beginning to be picked up by organizations in other parts of the United States, as well as Canada, Mexico, and Europe.

Most experts will tell you that the increasing acceptance of asphalt-rubber by industry professionals is because research and testing has allowed them to gain a better understanding of the benefits this binder product offers in solving certain highway problems.

Asphalt-rubber is defined by ATSM Specification D8-88. Under this specification, it must contain a minimum of 15 percent crumb rubber by weight. The specifications for most paving projects, however, will call for 18 to 22 percent crumb-rubber content. The asphalt-binder content on these projects is usually between 8 and 10 percent of the total mix.

Pavements made with asphalt-rubber binder material have reduced reflective cracking, improved rutting resistance...and they last longer than pavements made with conventional mixes.

What are the benefits of asphalt-rubber?
The ATSM specifications provide a paving material that has quite a number of distinct advantages. For example:

• Asphalt-rubber pavements experience reduced reflective cracking and improved rutting resistance;
• The thickness of an overlay can be reduced and there will be better surface adherence;
• Drivers will notice reduced noise levels and better traction;
• There will be better drainage and less tire spray;
• The surface will appear to be darker, thereby providing more visible highway markings;
• The surface will last longer than pavements made with conventional mixes. And there are usually lower maintenance costs over the long-term.

The special design of an asphalt-rubber blending plant
Unlike conventional HMA mixes that are used on virtually all roadways, asphalt-rubber mixes will have only limited applications in any given geographical area. As a result, the technology of making these special mixes has focused on specially-designed mobile equipment that is highly productive, but can be easily moved from one location to another.

Beginning in the early days of asphalt-rubber technology, CEI has manufactured a total of 27 asphalt-rubber blending plants with high capacity, quick mobility, and automated control systems.

CEI Enterprises (CEI) has its headquarters in Albuquerque, New Mexico and was therefore very conveniently located to play an active role in the early years of asphalt-rubber development. CEI's engineers planned and designed equipment that was tailored for each producer's special requirements. Over the years, CEI has designed and manufactured a total of 27 mobile asphalt-rubber blending plants.

The mixing trailer contains a number of valves and augers that are used to mix the crumb rubber and liquid asphalt. Controls for the pneumatic valves and augers are conveniently located on the main control console. This photo shows an example of one of those control panels.

 

These blending plants feature high capacity, quick mobility, and fully automated control systems. When they were designed, the engineers were called upon to give consideration to a number of different factors, such as the efficiency of the mixing apparatus; the limits on upper and lower temperatures for reaction; the need for an agitated reaction tank; a minimum reaction time prior to using the material as a mix binder; and the required viscosity of the final asphalt-rubber product.

Basic components of an asphalt-rubber blending plant
Most plants will consist of three mobile trailers similar to those shown on these pages:

The Mixing Trailer includes a crumb-rubber hopper and equipment that is used to move the crumb rubber, as well as heaters that are used to prepare the crumb rubber and the liquid asphalt for mixing.

This mixing trailer constitutes one-half of the asphalt-rubber system from CEI Enterprises. The other half is shown below. The mixing trailer is equipped with a heater (mounted on the trailer gooseneck to the right of the photo) that provides heat for both the mixing trailer and the reaction tank. To the left of the heater is a control panel for operating the blending process. And to the left of that is a horizontal booster heater that raises the temperature of the virgin asphalt prior to mixing it with the crumb rubber. This unit can produce asphalt-rubber at the rate of more than 35 tph (32 tonnes per hour).

 

The Reaction Trailer includes tanks to hold and agitate the crumb-rubber and liquid-asphalt mixture during the processing stage. It also stores this binder mix until it is used in making the paving material.

This two-compartment reaction tank was designed and manufactured by CEI Enterprises. The tank is capable of holding a total of 25,000 gal. (94,600 L) of asphalt-rubber. Each compartment of the tank is equipped with 24-in. (61-cm) diameter augers near the bottom that keep the crumb-rubber particles in suspension during the reaction process and subsequent storage. All of the asphalt valves are pneumatically actuated and controlled through the main console on the mixing trailer. There is a folding walkway on the top of the tank. And there is a blue-smoke vent on the gooseneck.

 

The reaction trailer has its own user-friendly control system. It includes a PLC (programmable-logic control) program that permits the accurate blending ratios of all the essential material ingredients: the crumb rubber, the liquid asphalt, and the extender oil.

 

The Calibration Trailer carries the plant's calibration tank and the hot-oil heating system.

In actual operation, these plant components are usually set up side by side, with flexible hoses connecting the blending trailer and the reaction trailer so asphalt and heat-transfer fluid can be moved as needed. As you can see in the photos, all of the controls that are required are included.

The crumb rubber is loaded into the hopper so it can be fed into the mixing compartment. There, it is blended with liquid asphalt and extender oil.

The blended asphalt-rubber mix is then pumped to the tanks on the reaction trailer where it will be held for a minimum of 45 minutes so it can undergo the reaction that will allow it to achieve the proper viscosity. After this reaction time, the asphalt-rubber mix is sent to the HMA plant to be blended with the aggregate.

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