But it's one that represents real progress for the aggregates industry in meeting the rigorous demands of Superpave mixes. Based more on speculation than science, the restricted zone has long posed a dilemma to the aggregate industry.

Work also progresses in reconciliation of the industry's position on the ratio of flat and elongated aggregate particles to rounded particles in Superpave mixes, as well as the allowable voids-to-mineral aggregates within Superpave. Successful outcomes will benefit the aggregates industry.

Superpave -- an acronym for SUperior PERforming Asphalt PAVEments -- is a performance-based system of specifications for designing asphalt pavements to hold up to the meet the demanding roadway needs of the next century.

This performance-based approach offers more durable pavements that are specifically designed with local temperature extremes and traffic loads in the equation. Superpave designs are thought to provide longer-lived asphalt pavements that will stand up to local climate and traffic volumes at lower costs.

Final Superpave mix design system specs, including performance based quality control, could be fully developed by 2005, according to a long range plan released by the Transportation Research Board (TRB) Superpave Committee.

While the final, definitive statement on the restricted zone may be part of the new American Association of State Highway & Transportation Officials (AASHTO) Pavement Design Guide 2002 later this year (see below), enough peer-reviewed, substantive work on the ineffectiveness of the restricted zone has been published that the concept is on its last legs.

Annually, the New York State DOT and the Federal Highway Administration (FHWA) conduct a survey of Superpave usage among state DOTs. For the 2000 construction season -- the most recent year for which reliable information is available -- 14 state DOTs reported that all hot mix asphalt (HMA) paving projects had Superpave mixture designs. Another seven DOTs reported that 50 percent or more of all HMA projects were Superpave designs.

The benefits of Superpave is available at incrementally marginal higher prices. The nationwide cost premium for Superpave mixes was less than $1/ton, or 2.6 percent of the total purchase price, reported TRB's Superpave Committee in November 2001.

Among states with large usage, the cost of Superpave mixes is frequently lower than the cost of "conventional" HMA, the committee said. "As overall economics is ceasing to be a barrier to implementation, Superpave will quickly become the dominant or only materials and mix design system used by most state DOTs." They added this will put pressure on local road agencies -- cities, counties and townships -- to adopt Superpave as well.
 

Zone: Consensus, not science

The restricted zone has been a prime component of Superpave hot mix asphalt pavement design, as articulated in the mix designs derived from the five-year, $150 million Strategic Highway Research Program (SHRP) completed in 1993.

The restricted zone was established in initial Superpave guidelines to limit the amount of rounded, natural sand in a Superpave mix, which conventional wisdom held contributed to mix instability and premature rutting. The restricted zone lies along the maximum density gradation between the intermediate size (either 4.75 or 2.36 mm, depending on the nominal maximum size of the aggregate) and the 300-micrometer size, forming a band through which experts felt it was usually undesirable for gradations to pass.

The restricted zone guidelines never were based on actual lab testing or science. Instead, when the Superpave system of asphalt mix design specifications was developed -- in the absence of formal mix design guidelines -- the Superpave guidelines were derived by expert consensus, and adopted by vote of the experts who developed the Superpave system.

In lieu of this technical research, "consensus" properties of aggregates for Superpave were established by discussion. These so-called consensus properties include coarse aggregate angularity, fine aggregate angularity, flat and elongated particles, and clay content. Source properties include toughness, soundness and deleterious materials.

"SHRP researchers were required to produce an aggregate gradation specification, without the benefit of experimentation to support or verify its formulation," said the National Center for Asphalt Technology (NCAT) at Auburn University, in its recent research for the National Cooperative Highway Research Program, Investigation of Restricted Zone in the Superpave Aggregate Gradation Specification (NCHRP 9-14).

"In lieu of a formal research program, a group of acknowledged experts in the areas of aggregate production and behavior and paving mix design developed ... the set of recommended aggregate properties and criteria that appear in the Superpave mix design method," NCAT said.

While it always was considered a guideline, and never a compulsory spec, its adherence was strongly promoted at the federal level, and many states included it as a compulsory spec. But despite the proscription, many state and local agencies continued to place Superpave mixes, the gradations of which passed through the restricted zone.

In fact, while a tremendous amount of SHRP funding went to characterize different asphalt binders, very little work or research was done on aggregates, which comprise some 95 percent by mass of an asphalt mix, said Vulcan Materials' Chuck Marek in his White Paper on Superpave Issues of Concern to the Aggregates Industry.
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Research refutes restricted zone

The NCAT research indicates that the restricted zone is superfluous and can be disregarded, so long as other spec guidelines are observed, such as volumetric parameters and fine aggregate angularity (FAA) requirements. Other science supporting abandonment of the restricted zone was articulated by the Heritage Research Group in Indianapolis, and the International Center for Aggregates Research (ICAR) at Texas A&M University.

"The [Superpave] mixes having gradations that violated the restricted zone performed similarly or better than the mixes having gradations passing outside the restricted zone," said NCAT in its overview of NCHRP Project 9-14.  "The restricted zone is intended to eliminate poor performing humped gradings that contain too much round natural sand in relation to total sand," NCAT said. "Unfortunately, the restricted zone also eliminates many successful heavy duty, rut-resistant mixtures produced with 100 percent crushed aggregate from sources that have been used successfully for many years ... The restricted zone is not needed, and its use in Superpave specifications should be eliminated."

And more work is on the way via NCHRP Project 4-19(2), Validation of Performance-Related Tests of Aggregates for Use in Hot-Mix Asphalt Pavements, which began at Purdue University in 2000, and scheduled to complete in 2003.

This $400,000 study will determine via accelerated load tests or in-service pavement studies the validity of the aggregate tests identified in NCHRP Project 4-19 as predictors of field performance of HMA pavements. This research will deal with dense-graded virgin hot-mix asphalt concrete used in the top layers of high-load, high-volume pavements.

Yet another study, the ICAR-sponsored Effects of Superpave Restricted Zone on Permanent Deformation, by Arif Chowdhury, Joe W. Button and Jose D.C. Grau, Texas Transportation Institute at Texas A&M University, evaluated the restricted zone effect using four different aggregates.

Those were crushed granite, crushed limestone, crushed river gravel, and a mixture of crushed river gravel as coarse aggregate with natural fines.

"As the restricted zone is a component of Superpave, the blends prepared met most of the Superpave criteria, except the restricted zone in selected mixtures, and fine aggregate angularity in three mixtures," they wrote.

Each type of aggregate was used for mixture design of three gradations: above, through, and below the restricted zone as it appears on a graph. The 12 mixtures designed were tested in the laboratory to evaluate their relative resistance to permanent deformation, through four types of tests using Superpave equipment:

o Simple shear at constant height
o Frequency sweep at constant height
o Repeated shear at constant stress ratio, and
o Repeated shear at constant height.

Then, rutting resistance of the mixtures was measured. "Researchers found that there is no relationship between the restricted zone and permanent deformation when crushed aggregates are used in the mixture design," Chowdhury, Button and Grau wrote.

"Superpave mixtures with gradations below the restricted zone were generally most susceptible to permanent deformation while mixtures above the restricted zone were least susceptible to permanent deformation," they wrote, recommending elimination of the restricted zone from HMA design specifications.
 

Zone restricted producers

The adoption of Superpave specs has meant new challenges to the providers of raw materials for the new hot mix asphalt (HMA) mix designs.

Refiners and suppliers of asphalt cement have had their own inventory problems having to manufacture and stock a variety of performance-graded (PG) liquid asphalts.

But with the restricted zone, aggregates industry saw the disqualification of entire gradations of aggregates from Superpave mixes that had performed admirably in the past.

"The restricted zone was problematic for the aggregates industry because it limited the gradations of stone from a plant which could be used in Superpave mixes," said Charlie Pryor, P.E., vice president, engineering, National Stone, Sand & Gravel Association (NSSGA).

"The restricted zone precluded using certain proven gradations in asphalt mixes," Pryor said. "If you preclude the use of those aggregates there is a real impact on the aggregate producer. The restricted zone was not based on any sound science," Pryor said. "Now that we've brought science back into the gradation control process for Superpave, those aggregates can compete based on their quality, and not precluded because of unsound science.

AASHTO has been looking at formally withdrawing the restricted zone from the Superpave spec, Pryor said. "There are many aggregate blends that have been producing excellent highways, but were precluded from Superpave gradations because of the restricted zone problem. Those materials no longer will be specified out, which is an improvement."

And this will mean, for aggregate producers, an easier time selling stone for Superpave. "It gives the aggregate producer a fairer, more competitive marketplace," Pryor said. "It allows competition to occur based on science, not on supposition."

The initial approach to Superpave was that it would be implemented as promulgated, without any variation, Pryor said. "That is something that we as an industry took great exception to," Pryor said, "because it did not allow for -- as far as aggregates are concerned -- local variations due to weather, geology and conditions. It originally was promoted as a national specification, but we know that aggregates are different all across the country, and we can't preclude performing aggregates simply because of a national specification."
 

Flat and elongated particles, fines

The aggregates industry concerns are not limited to the restricted zone. "There is a continuing concern over the flat and elongated specification issue, and over the voids to mineral aggregate specifications," Pryor said. "Those issues are being looked at by a large selection of people."

The Asphalt Institute defines the flat and elongated (F&E) issue as the percentage by mass of coarse aggregates that have a maximum to minimum dimension ratio greater than five. "Flat and elongated particles are undesirable because they have a tendency to break during construction and under traffic," Asphalt Institute says in its guide, Superpave Mix Design (SP-2).

The current specification is a 10 percent limit of any aggregate 5:1 or greater. "There was a move to reduce that to 3:1, but the industry's position is that there is no justification to go from 5:1 to 3:1, because there is no science that supports that as a enhancement to performance."

Regarding voids-to-mineral-aggregate (VMA), the industry states there needs to be a specification which considers gradation as well as maximum size.  "These are issues that we need to continue work on," Pryor said. "It's through a collective effort, from industry-sponsored research, through joint research projects such as flat and elongated, through work at centers such as NCAT, Florida and Purdue."

New research continues to support lower VMA contents in Superpave. For example, at the 2001 annual meeting of the Association of Asphalt Paving Technologists, Anderson, in his presentation, Influence of Voids in the Mineral Aggregate (VMA) on the Mechanical Properties of Coarse and Fine Asphalt Mixtures, found that "increasing the VMA from 13 percent to 15 percent in the coarse mixture appears detrimental to its performance properties.

"This result can support the industry concerns that higher VMA in coarse mixtures may be unnecessary and can lead to poor performance," Anderson found. "The fine mixtures in this study did exhibit less sensitivity to changes in VMA."

And another recent ICAR-sponsored paper, Evaluation of Superpave Fine Aggregate Angularity Specification, Chowdhury, Button, Vipin Kohale of TTI, and David W. Jahn, Martin Marietta Technologies, questioned the validity of the test for FAA in Superpave mixes.

The validity of the Superpave fine aggregate angularity (FAA) requirement is questioned by owner agencies and the paving and aggregate industries, they write. "The FAA test is based on the assumption that more fractured faces will result in higher void content in the loosely compacted sample," they said. "However, this assumption is not always true."

Some agencies have found that cubical shaped particles, even with 100 percent fractured faces, may not meet the FAA requirement for high-volume traffic, they said.

In response, researchers evaluated angularity of 23 fine aggregates representing most types of paving aggregates used in the United States, using seven different procedures: FAA test, direct shear test, compacted aggregate resistance (CAR) test, three different image analyses, and visual inspection.

A small study was performed to evaluate relative rutting resistance of HMA containing fines with different particle shape parameters using the Asphalt Pavement Analyzer (APA). "The FAA test method does not consistently identify angular, cubical aggregates as high quality materials," they said, while other tests appeared to serve better.

Also, "a statistical analysis of the SHRP-LTPP (Strategic Highway Research Program-Long-Term Pavement Performance) database revealed no significant evidence relationship between FAA and rutting," they wrote. "This lack of relationship is not surprising since many uncontrolled factors contribute to pavement rutting."

The APA study revealed that FAA is not sensitive to rut resistance of HMA mixtures, they said. Until a suitable replacement method(s) for FAA can be identified, the authors recommended that the FAA criteria be lowered from 45 to 43 for 100 percent crushed aggregate.
 

More support for versatility

Further versatility in aggregate gradations in Superpave is indicated in new 2002 related research from NCAT, presented at the 81st annual meeting of the Transportation Research Board in Washington, D.C. in January.
 In Coarse vs. Fine-Graded Superpave Mixtures: Comparative Evaluation of Resistance to Rutting, NCAT's Ken Kandhal and L. Allen Cooley, Jr., said there is no significant difference between the rutting resistance of coarse- and fine-graded Superpave mixtures.

Both coarse and fine-graded hot mix asphalt mixtures can be designed within the gradation control points recommended within the Superpave mix design system, they wrote. "However, some states have begun to specify only coarse-graded mixtures (below the restricted zone) and other states are specifying only fine-graded mixtures (above the restricted zone)," they said.

This study compared coarse-graded Superpave mixtures with fine-graded mixtures as to resistance to rutting, to determine whether restrictions on gradations -- either coarse- or fine- graded mixtures -- are justified.

Fourteen mixtures comprising two nominal maximum aggregate sizes: 9.5 and 19.0 mm; two coarse aggregates: granite and crushed gravel; and four fine aggregates: sandstone, limestone, granite, and diabase, were tested.

Resistance to rutting of both coarse- and fine-graded mixtures was evaluated using three test methods:

o Asphalt Pavement Analyzer
o Superpave shear tester, and
o Repeated load confined creep test.

"Statistical analyses of the test data obtained by the three performance tests indicate no significant difference between the rutting resistance of coarse- and fine-graded Superpave mixtures," they said. "[M]ix designs should not be limited to designing mixes on the coarse or fine side of the restricted zone."
 

Superpave Plus

Another growing topic in Superpave is the completion of specs for modified Superpave binders, dubbed Superpave Plus.

Modifiers can improve binder performance, but can increase the cost of the asphalt binder 30 to 100 percent, while boosting the overall cost of hot mix 10 to 40 percent, said John D'Angelo, asphalt pavement engineer, FHWA Office of Pavement Technology.

"The Superpave binder specification uses various tests performed at multiple temperatures to characterize the asphalt binder," D'Angelo said. "The relationship to performance, for these tests, has been validated in numerous studies."

However, he said, the current Superpave binder spec doesn't adequately determine the performance of modified binders. "Because of this," he said, "many highway agencies have included additional tests to the existing Superpave binder specification to assure a desired modifier is included in the binder."

The problem that arises from the use of these Superpave Plus tests is that they do not relate to performance, but only indicate the presence of a particular modifier in the binder, D'Angelo said, adding if used in the proper way and in the right place, modified binders can be a very cost-effective way to minimize pavement distress.

He said a great deal of work is under way to improve the ability of the Superpave binder specs to identify the performance of modified binders. "NCHRP 9-10 research has identified several promising criteria for both high temperature and fatigue properties of the binder," D'Angelo said. Work on developing new specification criteria for these aspects of the asphalt binder is being done through task groups of TRB's Superpave Binder Expert Task Group.

In the near future, new specification parameters should be available for inclusion in Superpave. "We will have the answers to many of the questions about the performance characteristics of binders," D'Angelo said. "Until that time, the highway agencies will have to use engineering judgment in specifying modified asphalt binders."

And in 2002, another major NCHRP project is to be awarded, Project 9-33, A Mix Design Manual for Hot Mix Asphalt. This $500,000 project has been tentatively selected for fulfillment and a project statement and request for proposals is expected in August 2002 or later.

The objective of this research is to develop a complete working version of a Superpave mix design method for preparing project-specific mix designs that are durable and well-performing. The method will take advantage of existing AASHTO volumetric design methods, simple performance tests developed in NCHRP research, and the HMA materials characterization tests and performance models from the coming 2002 Pavement Design Guide.
 

Industry awaits new design guide

The road construction industry is eagerly awaiting the release of the new 2002 Guide for the Design of New and Rehabilitated Pavement Structures.

Upon release, the NCHRP-sponsored guide is anticipated to become the latest -- and most significant -- revision of the AASHTO Design Guide. Its pavement design methodology is based on mechanistic principles, which is thought to allow more efficient use of paving materials, improve pavement performance, and decrease life cycle costs.

The objective of NCHRP Project 1-37A is to develop and deliver the 2002 Guide, accompanied by the necessary software, for adoption and distribution by AASHTO.

The research team will validate existing pavement performance models using data from the Long-Term Pavement Performance (LTPP) program and other sources. The 2002 Guide will allow users to calibrate these models for various local conditions. More information is available off the Internet at http://www.2002designguide.com.