Considerations for Producing
Modified Asphalt Binders

To meet tomorrow's performance requirements for asphalt binders, these binders must be modified. Contractors are faced with the decision of purchasing modified product or investing in a plant to produce the products. The type of plant and peripherals are key considerations, and questions like these are easier to answer if a contractor considers the business situation and the technical requirements of the asphalt modification process.

By Bernardo Hinojosa, Reichel & Drews, Inc.

Over the next two years, some 45 states will implement the "Superpave" asphalt binder specification program. Binder users may find that asphalt suppliers cannot deliver product that meets the "performance-based" specifications of the Superpave program. In that case, contractors will need to modify the binders they purchase. The choice is to outsource this modification or make the investment in a modification plant. There are three major benefits to making the investment: profit potential, product availability and control over product quality. However, the key to success in producing modified asphalt binders is research and planning.

Understanding the Need for Modified Asphalt Binders

State agencies are specifying performance-based asphalts with capabilities far beyond those of the traditional product.

The Superpave program rates each binder with a "performance grade" that consists of "PG" and a pair of numbers. A binder rated "PG58-22," for instance, has acceptable physical properties at temperatures as high as 58° C (136° F) and as low as -22° C (-7.6° F). Three new tests will be used to measure those physical properties: Dynamic Shear, Bending Beam, and Direct Tension. (See Appendix A for descriptions of these tests.)

"Polymers" (plastics made from complex chains of hydrocarbon compounds) are the key to remedying the shortcomings of asphalt. At low temperatures, asphalt modified with polymers is much more elastic than the conventional product; and at high temperatures it will not deform nearly as readily.

Key Issues to Consider before Investing in a Plant First, evaluate the potential market, by quantifying the projected demand for modified asphalt binders in the planned area. That area should be limited to about three hours' trucking time from the point of loading. Modified binders must be agitated continuously and kept at constant temperatures until shortly before they are used. Defining a market in terms of "truckable" distances minimizes the risk of arriving at a delivery site with a load of lukewarm asphalt "jelly" that not only cannot be reheated in the truck but in fact cannot be removed from the truck.

Then, consider the commitment level necessary and ongoing quality control issues associated with the modifying business. Polymers are expensive. Production costs for modified binders can go up sharply if more than the necessary amount of polymer is mixed with the asphalt. Also, precise temperature levels and rates of agitation must be maintained in both processing and storage of modified asphalt binders.

Next, determine which polymers would be in use. (See Appendix B for descriptions of the polymers that are used most frequently in the paving industry.) Each state has its own specifications for the use of polymers in modifying binders. Some states have reduced the number of polymers they allow producers to use, which may narrow the options.

Identifying the polymers to be used is a major step in reaching a decision to modify asphalt binders since it has a major impact on the cost of a modification plant.

Some polymers can be blended with asphalt in a nearly continuous process; others must be blended in relatively small batches; still others can be blended in both types of processes. Usually, the products of continuous processing must be sent to storage tanks before they are used. In those tanks, the polymers are "digested" by the asphalt while the mix is agitated constantly and kept at a uniform temperature. Most batch-processed binder solutions, on the other hand, can be used as soon as they are mixed.

With so many possible combinations of raw materials, equipment, and processes, a choice of even a few polymers can present the would-be modifier with a wide variety of system options.

In considering polymers, it is critical to talk to the people who produce them. They are excellent sources of information, and they can also provide valuable help in the final step of pre-decision considerations: testing the asphalt for "compatibility."

Compatibility testing is vital. When a polymer is mixed with asphalt, it must dissolve completely, and stay in solution. Undissolved concentrations of polymers will produce an off-quality binder. If a supplier's asphalts won't work with the polymers a contractor must use, it will be necessary to find other suppliers - and keep testing for compatibility until the results are satisfactory.

Plant Design and Process Requirements

The goal for any modification system is to blend polymer and asphalt into a stable, homogeneous solution. That's a matter of introducing the polymer (in powder or pellet form) into a mixing tank containing asphalt, agitating the mixture to "wet" the polymer (make it soluble) and blend it with the asphalt, and transporting the modified binder to storage tanks.

Processing is usually done at 193° to 199° C (380°- 390° F). A heating medium (like hot oil) is needed, to keep pipes, valves, and tanks at the right temperature.

Processing equipment must be designed to create an effective "mixing path," that delivers the polymer to the mixing blades. A mixing path is produced by the shape of the mixing tank, the design of the mixing blades, and the rotation speed of the blades. The mixing path must form a liquid vortex that draws everything - asphalt and polymer - toward the blade. If undissolved polymers collect on top of the asphalt, they cannot be made to dissolve later.

Storage is an important part of the production of modified asphalt binders. The product must be agitated during storage, to prevent localized high concentrations of polymer. Most storage tanks for modified asphalt are equipped with side-mounted mixers.

If the polymers are supplied in pellet form, special equipment is needed to reduce the size of the pellets. A widely used example of this type of equipment is the homogenizer mill. It reduces polymer pellets that have already been mixed with asphalt. Asphalt/polymer mix can be passed through the homogenizer mill more than once, to progressively reduce polymer pellets to soluble size.

Powdered polymers need only appropriate mixing and agitation to dissolve, but cost considerably more than pellet-form polymers. In the long run, the cost of equipment that reduces the size of pellets can be recovered in savings on raw materials.

An asphalt modification plant changes patterns of energy use. Consumption of electricity is affected by the motors used to transport, mix, and mill the polymer. The high temperatures required for the process increases consumption of heating fuel.

Also, modification plants need a reliable source of compressed air for operating the asphalt and hot oil valves Disruptions in air supply can compromise production.

Finally, local labor conditions will determine the degree of automation that you build into the plant. Just just one person can operate an asphalt modification plant. The most efficient "automated" setups, however, use two people: one person to load the polymers, the other to operate the controls. Where labor is not scarce, it can be practical to build a plant that runs almost completely manually. A principal concern in designing a manually operated plant lies in providing enough safety interlocks to make a consistently safe working environment.

By defining the requirements and identifying good sources for asphalt and polymers, the contractors will be much better able to decide on the best approach to provide support to achieving business goals.

 

Appendix A

Superpave Tests

Appendix B

Types of Polymers Used in Modifying Asphalt Binders