Superpave is here to stay. But, the aggregate consensus properties that have been a point of contention between Superpave purists and many aggregate producers may change. Ongoing research is refining aggregate characterization and establishing or refuting relationships between the properties and pavement performance. That was the indication of presentations at two back-to-back conferences in Denver in April.

Superpave: Building Roads for the 21st Century, sponsored by The Asphalt Institute, the Federal Highways Administration and the American Association of State Highway and Transportation Officials, preceded the International Center for Aggregates Research's (ICAR) 8th Annual Symposium. Following are summaries of some of the presentations at these two meetings.

Superpave for the 21st Century In a keynote address, Douglas Rose, chief engineer for the Maryland State Highway Administration, noted the rapid acceptance of the new design system: in 1992 less than 1,000 tons of Superpave asphalt mixes were placed; in 2000 more than 5,000 projects are planned. Nevertheless, he said, there are several Superpave-user needs:

* simple performance test;

* improved guidance for using recycled asphalt pavement (RAP);

* refined aggregate standards; and

* improved moisture sensitivity test.

But better mix design will not solve all asphalt pavement problems. "We will not see the total value of Superpave if we don't improve construction techniques," Rose said.

Improving aggregate plant production to meet Superpave specifications is the challenge for many aggregate producers, said Kim Snyder, president of Eastern Industries. His company's 1940s-vintage plant equipment that has been upgraded over time is often inadequate to produce aggregate that meets more stringent cubicity requirements, he said.

In addition, the transition to Superpave specifications can be difficult because local markets may lag state implementation. Commercial customers may not recognize the value of improved aggregate quality, Snyder said, making it difficult for producers to the recover higher production costs.

John Cheever, quality control manager for Aggregate Industries, also cited production cost increases as a problem in meeting Superpave specifications. Some operations, he said, are making 2 tons of crusher fines for every ton of 11/42-in. chips. Significant time and money expenditures may be necessary to make aggregate for Superpave mixes. In addition, producers must balance production to continue serving other markets, such as concrete.

Successfully serving Superpave markets requires timely quality control testing and consistent production, Cheever said, including consistent gradation and specific gravity.

Superpave mixes react drastically to subtle changes in aggregate properties, according to Jack Weigel, corporate technical/QC manager for Payne & Dolan. Specific gravity must be checked from different benches in the quarry to ensure consistency. It is also important, he said, to make sure that material in aggregate stockpiles is the same as the material used in mix design.

In some cases, it might be necessary to blend aggregate to obtain uniform specific gravity, said Mike Worischeck, plant operations manager for Staker Paving. One key to success with Superpave mixes is to split the aggregate into as many size fractions as possible to be able to better control volumetrics, said Worischeck. This requires a greater number of aggregate bins on asphalt plants and use of fractionated aggregate plants. He also recommends use of telescoping stacking conveyors to decrease aggregate degradation and segregation in stockpiles (see page 46).

Aggregates research symposium Superpave mixes containing RAP should perform as well as mixes with only virgin aggregate, concluded Rebecca McDaniel of the North Central Superpave Center. At low contents (about 10%), effects of the RAP are not significant. At higher contents (20% to 40%), RAP contributes to a more rut-resistant mix, based on lab tests, due to increased binder stiffness, she said.

Rut resistance through selective aggregate grading was the intended purpose of one of the most controversial components of Superpave-the restricted zone. A study by Joe Button, senior research engineer, and Arif Chowdhury, engineering research associate, both with the Texas Transportation Institute, evaluated the effect of the restricted zone on pavement rutting using three types of aggregate: crushed granite, crushed limestone and crushed river gravel. Three asphalt mixes were tested for each aggregate-above, through and below the restricted zone. Based on lab shear tests, the researchers concluded that there is no relationship between the restricted zone and permanent deformation.

Particle shape is another aggregate property addressed by Superpave specifications by limiting the amount of flat and elongated particles. But, according to David Jahn, product development manager for Martin Marietta, the Superpave procedure for determining and describing a sample's particle shape provides limited detail about the variety of particle shapes within the sample.

Jahn introduced a concept he called Multiple Ratio Analysis (MRA) that evaluates an aggregate sample based on five maximum-to-minimum-dimension ratios: less than 2:1, 2:1 to 3:1, 3:1 to 4:1, 4:1 to 5:1 and greater than 5:1. Martin Marietta developed a low-cost device (patent pending) using a digital caliper interfaced with an Excel spreadsheet to quickly and accurately measure and tabulate the particle measurements and to determine into which ratio group each particle falls. After a sample is separated into the five ratio groups, the operator weighs and records in the spreadsheet the total mass for each group. The spreadsheet calculates the weighted average for the total sample.

"MRA will allow mixes to be designed by first determining the various particle shapes involved," said Jahn, "and then using the correct combined gradation to fit the particle shapes for optimum [pavement] performance."

MRA also can be used to provide a complete picture of particle shapes at each stage in the aggregate-production process, Jahn said. In addition, the ability to carefully evaluate product consistency could be used to monitor the condition of crusher wear liners.

Selection and operation of crushing equipment is key to either improving or diminishing particle shape during processing, said Don Powell, Vulcan Materials Co. As part of an ICAR research project on increased use of fines in concrete, a wide range of rock types were crushed in a Barmac B3000 VSI to make a minus-#4-mesh product. Three feed sizes were used: a clean graded coarse aggregate (19 Yen 2 mm), screenings and a washed manufactured sand. For two rock types-limestone and granite-the three feed sizes were processed at eight rotor speeds.

Based on the limestone and granite processing, Powell concluded that:

* fines generation is related to mineralogy, feed size, grading and rotor speed;

* the finer the feed size, the smaller the increase of minus-200-mesh material in the product;

* increasing rotor speed tends to reduce the uncompacted voids of the fine aggregate product; and

* increasing rotor speed tends to increase generation of minus-200-mesh material.

Using fines in asphalt pavements was the subject of a presentation by Ronald Collins, president of Pavement Technology. High-frequency screens and special crushers allow production and separation of fines into fractions, such as 31/48 in. Yen #4, #4 Yen #8, #8 Yen #16, #16 Yen #50 and minus #50. Reblending these fines to specific proportions can produce highly rut-resistant mixes, Collins said.

Telescoping conveyor reduces segregation Material consistency is one of the keys to producing high-quality asphalt pavement. It is especially important with Superpave mixes. Aggregate segregation and degradation in stockpiles, therefore, can be a significant problem.

One of Michigan's largest asphalt contractors, Thompson & McCully Construction Co. added a portable radial telescoping stacking conveyor to its recycling division operation. Previously, the company stockpiled recycled asphalt pavement (RAP) with a 100-ft fold-out portable stacker and then pushed the material with a dozer to blend it.

Blending with a bulldozer, however, was expensive. The company spent $100 a day in dozer maintenance, about $150 a day for fuel and $80,000 per year (including benefits) for an operator, according to John Thompson, manager of the recycling division.

Thompson & McCully purchased a Superior Equipment TS-150 portable telescoping stacking conveyor for about the same price as a dozer. The unit has a programmable logic controller that moves the stacker in any direction, extending and retracting it to build stockpiles in layers. Multiple piles are built in windrow fashion, blending fine and coarse material and reducing segregation. Thompson estimated that the telescopic stacker solves 95% to 99% of RAP pile segregation problems.

Conveyors are more reliable than dozers, according to Thompson. "We own conveyors that have been in inventory for 15 to 20 years," he said. "As long as you maintain them, they're the cheapest way to move material."

VSI solves stone-shape problem When New Mexico began implementing the Superpave mix-design system in 1997, many aggregate producers had to adapt their plants to continue supplying state projects.

"We knew it was coming, so we started looking for a crusher that could produce material to meet the new specifications," said Richard Cook, owner of Associated Asphalt Materials located in Espanola, N.M. "We decided an impactor would solve the problems we were having with our cones, which were producing too many elongated particles in small rock and not giving us enough production."

Following a presentation of the Pioneer 2500 Ultra-Spec VSI by Paul Miller, general manager of Aggregate & Mining Supply, Cook decided to rent one of the machines for a trial. After a six-month rental period the company purchased and installed an Ultra-Spec.

Associated feeds 1-in. stone to the VSI at 300 tph. Using a 6- Yen 20-ft screen, it makes three products: 11/48 Yen 31/48 in., 31/48 Yen 11/42 in. and fines. The crusher is self-contained with its own diesel engine and operates at variable rotor speed.

Pioneer's Ultra-Spec 2500 uses a hybrid rock shelf. Approximately two-thirds of the stone exiting the rotor strikes rock built up on the shelf in front of the anvil; the other third strikes the anvil faces, the manufacturer said. Some stone is driven upward past the slope of material on the shelf to the anvil face where it is impacted by other stone exiting the rotor. The hybrid design helps increase anvil wear life to more than 1,500 hours in tests in a variety of abrasive applications, according to Kolberg-Pioneer.

Proceedings of the 8th Annual ICAR Symposium are available in printed form ($95 plus shipping) or on CD in a searchable pdf format ($45 including postage). Contact: ICAR, The University of Texas at Austin, ECJ 5.200, Austin, Texas 78712-1076; (512) 471-4498; fax: (512) 471-3191; www.ce.utexas.edu/org/icar