Welcome to the evolving mix gradation guidelines of Superpave in the late 1990s. 

The "restricted zone" is 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). 

But serious rethinking is being done on the efficacy of the restricted zone, that portion of aggregate fines thought to encourage rutting in asphalt mixes. 

And the rethinking primarily is the result of experience gained from two years' worth of Superpave mixes being placed in the field, along with some reasoned criticism from the aggregates industry. 

Many state and local road-owning agencies are finding that Superpave mixes that don't meet the criteria of the restricted zone still are performing well in the field. And in some cases, avoiding the restricted zone may make it difficult to meet Superpave volumetric requirements due to aggregate supply problems. 

"I call it the 'twilight zone' because I'm not personally sure what's going on inside there," said Bob Erdman, P.E., senior materials engineer, Maricopa County, Ariz., which includes the Phoenix metro area. 

"SHRP put boundaries on the zone because they said it would create 
tender mixes and rutting if you go in," Erdman said. "But people 
have told me that a year from now we'll be laughing about the boundaries of the zone." 

Field experience may prove this out. While Maricopa County has found that some of its Superpave mixes that go through the restricted zone are (as predicted) showing rutting, and those that don't aren't rutting, Erdman can't draw a conclusion on that basis alone. 

"The rutting in the restricted zone mixes have six to seven times the truck traffic than the other [conforming] mixes, which are only a few months old," Erdman said. "The ones that are rutted are two years old, with a tremendous amount of truck traffic. I don't know that they're rutting because they're in the zone." 

Nonconforming Superpave mixes in these instances were placed because plus-or-minus production tolerances permitted fines to pass through the restricted zone, even as the gradations otherwise met design specifications. 

Nonetheless, the Federal Highway Administration (FHWA) continues to recommend conformance with the restricted zone guidelines to provide rut resistance, particularly until a supplemental strength test becomes available. 
 

Rethinking the restricted zone 

Superpave is the new system of performance-related hot mix asphalt 
(HMA) mix design specifications, now in various stages of adoption by state departments of transportation throughout the United States. 

In October close to 70 percent of state DOTs had adopted Superpave 
binder specs, about 15 percent had adopted the mixture specifications for standard practice, and 90 percent had placed trial sections on the way to adoption, said John d'Angelo, P.E., Federal Highway Administration Asphalt Team leader, Office of Technology Applications. 

Three elements comprise the Superpave system of asphalt mix design. These are: 

• An asphalt binder specification geared to pavement loading and local climate
• A volumetric mix design and analysis system, and
• Mix analysis tests and a performance prediction system that include computer software, weather database, and environmental and performance models.

The restricted zone was established to limit the amount of rounded, 
natural sand in a Superpave mix. It's thought that too much rounded, natural sand can cause mix instability, which will lead to pavement rutting. 

The recommendations on use of the restricted zone made by Superpave are consensus guidelines -- derived from expert opinion -- and are not based on actual research results for a wide range of materials. 

The aggregate industry -- while generally supporting the goals of 
Superpave mixes -- is cool to the restricted zone because of production complications. 

To this end, the National Stone Association-supported International 
Center for Aggregate Research (ICAR) has launched a task force called Fine Aggregate Properties and Their Influence on End Products, which is studying performance of Superpave mixes whose gradation went through the restricted zone, but still are performing well. 

Most recently, FHWA announced National Cooperative Highway 
Research Program (NCHRP) Project No. 9-14, Investigation of [the] "Forbidden Zone" in SHRP Superpave Aggregate Gradation Specification. This work will assess the impact on mix performance of aggregate blends that violate the restricted zone. 

Specifically, NCHRP 9-14 will examine how observance of the restricted zone: 

• Controls natural sands that are often rounded and have limited particle size distribution
• Decreases the sensitivity of the mix to increases in binder content
• Increases the VMA, and
• Produces suitable mixes that are neither "tender" during construction, nor prone to rutting.

A pooled-fund study -- which includes FAA, VMA and gradation -- was being conducted to validate Superpave with accelerated testing. Because existing tests for FAA don't always identify the angularity of manufactured sands correctly, the FHWA is developing an improved test. Purdue University was working on a short-term study which will yield a method for visually verifying the results of industry tests. 

Asphalt laydown contractors themselves also have raised the issue of the restricted zone. Earlier this year some questioned the validity of the restricted zone in all instances, saying "Undoubtedly there may be problem mixtures which will pass through the restricted zone. However, the type of sand (manufactured vs. natural, rounded vs. angular) is also a critical element which may not be fully addressed by the restricted zone criteria. 

"Many areas of the country have materials that fail to meet the criteria of the restricted zone, but which have shown good mixture performance on the roadway. Such materials should not be eliminated if proven to be cost effective," they said. 
 

Gap-graded mixes 

The idea of a restricted zone for Superpave pavements has been 
developed to make sure the aggregate structure in a Superpave lift is not compromised in its load-carrying capability. 

Asphalt mixes meeting Superpave gradation bands below the restricted zone are gap-graded mixes, in which the stone-on-stone contact of the coarse particles carries the load. Sand and other fine particles, if allowed to enter in quantities that would violate the restricted zone, are thought to separate load-carrying aggregates and compromise the mix's load-carrying ability. 

Mixtures passing above the restricted zone are thought to be "sandier" with a weaker aggregate structure than those below. 

Observance of the restricted zone, the theory goes, should result in a 
higher shear strength of hot mix asphalt (HMA) because the aggregate will have a high degree of internal friction, being cubical, with particle-to-particle contact, and a rough surface texture, according to the Asphalt Institute. 

But Superpave field experience is showing that some of the theoretical assumptions about how Superpave mixes should perform and be placed have to be rethought. 

"One of the things we found [from field experience] was that 
coarse-graded mixes, which we presumed would be less-sensitive to 
construction variables, seemed to be more sensitive," d'Angelo said. How compaction technique, mix temperature and lift thickness interact in Superpave mixtures are getting closer scrutiny, he said. 

Aggregates is an area in which SHRP didn't do a lot of research, d'Angelo said, having relied mostly on past experience. "Aggregates are critical to the performance of mixtures, and emphasis on aggregates is part of the normal progression," he said. "We did a lot of work on binders, and a lot work on mixtures. Now the next step is identifying more aggregate criteria that will assure good performance." 

The restricted zone may fade away as performance tests become 
developed, d'Angelo said. "The restricted zone was put in because we knew historically that if used a lot of natural sands, we'd end up with a lot of mixes that were tender and hard to place," d'Angelo said. "The restricted zone was an attempt to reduce its use.

"Also, we've known that when you produce mixes right down the maximum density line, you get very low VMAs," d'Angelo said. "The SHRP researchers were trying to make sure that mixes always had enough VMA. The restricted zone would keep mixes from going right down the maximum density line, and improve VMA. 

"It happens that you can produce gradations with good quality, angular sand which pass right through the restricted zone -- or be near it -- and they give good performance," he said. "As we develop performance tests, it will be much easier to say which mixes will or will not perform, and we won't have to worry about the gradation controls so much." 
 

Problems with production 

The aggregates industry is concerned with what it feels is a rigid 
application of gradation specs and other Superpave aggregate issues, and is pro-actively addressing the issues with research. 

Aggregate producers want to take a second look at the restricted zone because they balk at shouldering the expense of installing the crushing equipment needed to make manufactured sand that will meet the index. Still others feel they will not be able to sell the manufactured sand at a price competitive with crushed, screened aggregates. 

On a geographic basis, some may lack existing materials meeting the 
Superpave Fine Aggregate Angularity index of 45. 

"What we're talking about is bread-and-butter to the 120,000 people of the aggregates industry," said Charlie Pryor, vice president, engineering, of the National Stone Association. "While there seems to be general satisfaction with the Superpave binder specs, there appears to be widespread concern with the aggregate specs, which we hear wherever we go." 

That's because production issues complicate adherence to the restricted zone. "We find today, in our aggregate production, that a plant will tend to produce -- when all particles are combined -- a line right along that 45-degree paracurve," Pryor said. "The density tends to be there in an aggregate plant production. As you move away from that line, it causes economic losses to the aggregate producer." 

So aggregate producers will be faced with significantly higher operating costs to produce gradations which observe the restricted zone. And it's not clear that the market will pay a price per ton for aggregates that will make such capital improvements and added processing cost-effective. 

Thus further research -- especially as to which aggregates on a local 
basis can pass through the restricted zone, and still provide quality Superpave mixes -- is indicated and is ongoing. It's just part of the natural progression of development of Superpave mix designs from consensus guidelines to a living, breathing performance specification derived from field experience. 

"We know that in some areas there have been aggregate gradations go through that restricted zone which do produce quality pavements at economical prices," Pryor said. The challenge to both aggregate producers and highway agencies is to define where it will work and make those "exceptions" part of the rules. 
 

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