As movement toward long-life asphalt and concrete pavements accelerates, attention falls on strength and configuration of aggregate particles

Liquid asphalt cement and portland cement are the glue that holds a pavement together, but aggregates form the bulk of the pavement’s volume and provide much of its strength.

For this reason, as the United States’ road management establishment pursues long-life pavements with high performance characteristics, its emphasis has focused as much on aggregate properties, dimensions, and quality as on the binder properties of cement and liquid asphalt.

New Superpave and High-Performance Concrete Pavement mix design systems put tremendous importance on the quality of aggregates used, and premium aggregates are vital to the performance of open-graded friction courses and other special asphalt surface treatments.

Concurrently, highway safety concerns are increasing demand for skid-resistant aggregates that resist polishing even after long periods of time in surface courses.

Even as demand for quality aggregates increases, supply issues are increasing, too.

   As public road agencies shift responsibility for aggregate specifications to the contractor, the contractor in turn is shifting more responsibility on the aggregate producer to provide consistent, more intensively processed materials — like aggregate with a particular shape and number of broken faces. This added refinement and testing boosts aggregate prices.

   If aggregate quality or type for a premium mix design like Superpave or HPCP is wanting locally, contractors are having to search far and wide for the aggregate specified, inflating the cost of construction. That’s particularly true for skid-resistant aggregates used in surface or friction courses.

   While there is no shortage of aggregates per se, permitting and extracting desirable aggregates is getting vastly more difficult. Many quarries and pits are being engulfed by urban growth, and even remote sites in rural areas are being constrained by comprehensive environmental laws and regulations. Resistance to the startup of new quarries and pits — even in rural areas — is putting even more pressure on existing extraction sites.

Along with other trends in the aggregates market, there is political and market pressure to use more reclaimed asphalt pavement and recycled demolition concrete in road construction, potentially negating the benefits of select aggregate in construction. But research is under way to quantify where RAP and demolition concrete will work best in pavements and bases, and new attention is being given to the fractionation (enhanced processing) and storage of RAP to make its performance more predictable.

And test methods for aggregate characterization continue to evolve, with the most significant work being the ongoing National Cooperative Highway Research Program project 4-30A, Test Methods for Characterizing Aggregate Shape, Texture, and Angularity (see below).

Aggregates and pavements

Hot-mix asphalt typically consists of 94 to 96% aggregate and 4 to 6% asphalt cement, reports the National Asphalt Pavement Association. And the Portland Cement Association observes that portland cement concrete, by volume, will be composed of 60 to 75% aggregates (including sand), with 10 to 15% portland cement, and the remainder water.

The National Stone, Sand & Gravel Association reports that, on average, every lane-mile of Interstate uses 38,000 tons of aggregate, compared to an average 400 tons of aggregate used for a new home.

Coarse aggregates generally are those pieces greater than 0.19 inch, but usually between 0.375 and 1.5 inches in size. Fine aggregates are natural sand, or manufactured sand from a quarry, which pass the 0.375-inch sieve. Aggregates are graded according to their size, and a gap-graded aggregate mix — needed more and more for high-performance pavements — is one that entirely leaves out certain unwanted gradations.

Aggregate properties or characteristics include durability, resistance to skids and abrasion, propensity to absorb water (critical in freeze-thaw resistance), particle size, shape and texture, grading, and the voids they provide in a mix of aggregate.

Considering that both asphalt and concrete pavements are built on stone bases, it’s clear that aggregates constitute by far the largest — and least expensive — component of road construction.

The properties of coarse and fine aggregates used in asphalt and concrete pavements and unbound base and subbase layers are very important to the performance of the pavement. Particle angularity, texture, and shape are among the aggregate characteristics with significant effects on performance. These properties vary widely with the type and source of aggregates and production processes.

Within an asphalt mix, good stone-on-stone aggregate structure within the asphalt lift serves as a skeleton which enhances pavement performance by boosting resistance to rutting and lowering internal strains that cause pavement fatigue. This kind of aggregate structure is made possible by extensive crusher processing, which creates multiple broken faces on each rock. The skeleton of the asphalt pavement lift is formed as these faces abut each other.

Existing local sources continue to be used for aggregates whenever possible. But the local availability of high-quality, inexpensive aggregates will be limited in the future as environmental and zoning laws complicate quarry placement and expansion.

Premium aggregates and asphalt

Both stone matrix asphalt and Superpave mixes demand aggregates with certain essential properties, such as more angularity in the coarser fractions and clean, angular fine aggregate (see Stone Matrix Asphalt is Catching on in the U.S., September 2003, pp 22-27). This increased angularity in mixes will provide extended performance under increasingly heavy traffic loads.

And open-graded friction courses by definition are gap-graded, processed stone meeting strict size and face characteristics (see Asphalt Rubber Makes Quiet Comeback, May 2004, pp 32-43, and A New Era for Permeable Pavements, April 2003, pp 28-35).

But even conventional asphalt mixes will perform better with premium, processed aggregates. Premium aggregates can attenuate rutting and moisture-induced damage to HMA. Rutting can be controlled by the use of large aggregates, with angular, rough, coarse aggregates, and angular fine aggregates. And changes in skid resistance, called polishing, can be retarded with the right choice of aggregate surface characteristics and shape qualities.

Superpave and premium aggregates

Twelve years after Superpave was introduced to North America — despite a few significant holdouts — it has become the standard of the United States asphalt paving community (see Does Superpave Have a Local Future?, July 2003, pp 22-29), and Asphalt’s Generation of Change, November 2003, pp s2-s10).

Superpave is a performance-based system of specifications for designing asphalt pavements that will hold up to the traffic loadings and weathering stresses of the future. Its performance-based system promises longer-lasting pavements that are specifically designed for local temperature ranges and traffic volumes.

The Superpave system includes three major themes: an asphalt binder specification primarily for a pavement’s predicted loading and local climate; a volumetric mix design and analysis system; and mix analysis tests and a performance prediction scheme. Superpave’s volumetric properties include the percentage of air voids, voids in the mineral aggregate and voids filled with asphalt.

Superpave incorporates a new method for selecting and grading asphalt binders. Its performance-graded system allows for selection of a binder for specific climatic and traffic conditions. It’s hoped that PG binders will result in reduced low-temperature cracking in pavements, precluding moisture percolating through cracks, thus improving pavement performance. It’s also anticipated that the PG binders will boost resistance of pavements by providing binders that are stiffer at the high surface temperatures that the pavement will endure.

Aggregate gradation also plays a big role in Superpave, and the system introduces new aggregate selection and gradation methods as an integral part of the mix. The goal is a strong, stable aggregate structure resistant to shear, and Superpave aggregates have to meet surface requirements of coarse and fine aggregate angularity.

They also must contain a minimum of flat and elongated particles. An excess of these aggregates could result in mixes that compact with difficulty, or contain aggregates that could shatter during rolling. The latter condition creates exposed rock faces that have not been covered with binder, setting the stage for stripping or raveling.

The current Superpave system describes aggregate shape using three tests: fine aggregate angularity, which is inferred from the volume of air voids in a loosely compacted aggregate sample; coarse aggregate angularity, which is inferred from the number of aggregate fractured faces; and the relative dimensions of coarse aggregates to identify flat-elongated particles.

Concrete and premium aggregates

Due to its rigidity and compressive strengths, high-performance portland cement concrete is more forgiving than Superpave when it comes to the configuration of aggregate particles. But not so when it comes to the kind of aggregates used: Alkali-silica reactivity and related ills require careful screening of aggregate for concrete, at increased cost.

ASR is a chemical reaction that occurs between alkalis contributed primarily by cement, and a reactive form of silica from reactive aggregate. Together, they can form an alkali/silica gel. Under the right conditions — in particular, enough available moisture — the gel will expand and produce stresses and damage in the concrete.

Over time, this expanding ASR gel exerts tremendous internal pressure that can lead to cracking of the concrete. This cracking can provide pathways for potentially deleterious materials such as water, sulfates and chlorides to the interior of the concrete matrix, which in turn can lead to serious durability issues such as freeze/thaw damage, sulfate attack or steel corrosion.

Traditionally, ASR was thought only to afflict western states, but Strategic Highway Research Program publication C-343 (Eliminating or Minimizing Alkali-Silica Reactivity) indicates it can be found in every state in the United States.

The best way to avoid ASR in new concrete is to take precautions in the mix design, including testing aggregates for reactivity. Use of premium, nonreactive (innocuous) aggregates can be specified, but these may not always be available locally. Aggregates can change in composition from one end of the pit or quarry to the other, making positive identification difficult. And established tests for reactivity may allow too much variability in the results. Suppression of ASR is possible through vigilance, and today admixtures such as lithium compounds are proven to fight ASR.

Tight specs raise agg prices

Ironically, today’s demanding aggregate mixes have the potential of using up aggregate resources even faster than before, due to the increased fines that are produced when making the premium aggregate product.

“To meet the need for aggregate angularity, more of the natural material will be wasted in production, further impacting supply,” said National Asphalt Pavement Association Hall of Famer Dick Stander, Mohican Construction Co., Mansfield, Ohio.

“Because the capital investment in resources and production facilities is quite large and the return rate is low, the producer needs to maximize the saleable products produced from the given reserves,” said D. Stephen Lane, Virginia Transportation Research Council, and Stephen Forster, FHWA, in their Transportation in the New Millennium essay, Mineral Aggregates, for the Transportation Research Board (January 2000).

“Current research into the properties and characteristics that enhance the performance of aggregate materials — in particular applications — is leading to refinements in specification requirements aimed at optimizing performance,” they wrote. These requirements include grading, particle shape, surface texture, durability, and abrasion resistance.

The problem for the industry is that in attempting to produce materials that meet more exacting requirements, aggregate producers must often confront a growing inventory of material that has no current market, and consequently may drive up the cost of marketable products.

“An example is the almost universal production of excess fines at aggregate plants,” Lane and Forster said. “This problem is likely to grow more acute given the trend toward more tightly specified particle shapes, because the crushing processes used to produce more equidimensional particles generate higher percentages of fines. Since current thinking on most bound aggregate applications tends to limit the fine aggregate fraction, this material represents a loss to the aggregate producer in terms of both reserve and, potentially, the cost of disposal.”

They added federal highway funding under the existing Transportation Equity Act for the 21st Century would increase demand for aggregate by approximately 16%, with most of it for clean or washed products.

“There will also be growing competition among different applications for the same sizes of materials,” they said. “Both of these factors exacerbate the fines issue. Continuing research is needed to determine the actual benefit to performance and ultimate cost of tighter aggregate specifications, and to develop innovative uses for the excess materials of production.”

Warranties build partnerships

The continuing shift to warranted pavements and end-result (or performance-related) specifications is changing the way government agencies, contractors and material suppliers work together, and it will also impact aggregate use.

Under the existing paradigm, an owning agency provides so-called recipe specifications and provides inspectors who test samples in the aggregate and hot-mix plants, as well as on the job site.

But today and in the future, state DOTs and lower government agencies will continue to shrink in terms of staff size. Trends strongly indicate that in the future, material inspection and pavement specification decisions will be made by the contractor or his engineering consultant, both of whom will guarantee that a pavement will perform over a certain period of time without a prescribed number of flaws, within an agreed framework.

This means the contractor, aggregate, and mix providers will have to cooperate in new ways to meet performance standards and warranty provisions. “Doing so will require setting performance standards for testing and acceptance that can be applied quickly and economically in both the laboratory and the field,” Lane and Forster said.

In a 2001 letter to the chairs of the Michigan House and Senate Appropriations Subcommittees, Michigan DOT director Greg Rosine said that contractor warranties would lead to increased use of premium aggregates.

“The use of materials and workmanship warranties is also an integral part of the department’s strategy to improve the quality of materials in HMA and concrete pavements,” Rosine said. “When a contractor assumes responsibility for assuring the long-range performance of a pavement, they will select the best available materials, at very little additional cost to the project. Material improvements are anticipated under a warranty scenario, such as the voluntary use of polymers to enhance HMA pavement performance or premium aggregates.”

Specter of depletion

In addition to aggregate properties, another consideration of road builders is the availability of quality, moderately priced aggregate resources needed to meet current and future demand.

The threat is twofold: depletion of existing production sites, and the difficulty of establishing new sites. “Many of the existing sites for aggregate production have been in use for some time and are running out of reserves, because either the quality or type of source material is changing, specifications for aggregate materials have changed, or the boundaries of the site are being approached,” Lane and Forster said.

“Increased demand for aggregates is creating pressure to explore and develop new sites,” they added. “The development of new sites is subject to difficulties, however, including the need to comply with environmental and zoning restrictions and regulations (particularly in urban areas) and the public perception of aggregate production facilities. These same difficulties can also impact expansion into undeveloped reserves at existing sites.”

Can RAP fill the bill?

Some in the industry think use of reclaimed asphalt pavement can be used with great success in Superpave mixes, so long as it’s been tested, processed, and stockpiled to uniformity.

Use of RAP can prolong the life of valuable virgin aggregate resources. RAP already contains existing aggregates that have already been acquired, permitted, shot, loaded, crushed, screened, stockpiled, reloaded, and hauled — plus  residual asphalt — so reuse of RAP saves time, money, and resources. But how will it perform?

That RAP used in today’s Superpave mixes has long been acknowledged. “The materials present in old asphalt pavements may have value even when the pavements themselves have reached the ends of their service lives,” said Rebecca McDaniel and co-researchers in NCHRP Project 9-12, Incorporation of Reclaimed Asphalt Pavement in the Superpave System.

“Use of RAP has proven to be economical and environmentally sound,” McDaniel wrote. “In addition, mixtures containing RAP have, for the most part, been found to perform as well as virgin mixtures.”

RAP use can be increased in premium mixes by handling it like a premium product. Its consistency can be enhanced if it’s kept in sheltered, blended RAP stockpiles (to keep moisture content low), and if it’s fractionated, or reprocessed, into individual gradations.

Fractionation of RAP — a relatively new concept which just now is getting coordinated exposure to the industry — screens RAP, with oversize elements broken into smaller fractions and stockpiled separately. Fractionated RAP may result in more uniform mixes, in which RAP fractions can be isolated, compared to general stockpiles in which large and smaller fractions may become segregated.

“Fractionating RAP doesn’t necessarily mean that we can put more RAP in a mix,” said Kent R. Hansen, P.E., NAPA director of engineering. “What it may mean is that we will have more mixes where the use of RAP is allowed because we can better control the uniformity.”

Perhaps more important, fractionating allows classification of RAP by its residual asphalt content, which in the right concentration will permit proportionately less liquid asphalt when reused in HMA, at great savings. Fine RAP, per pound, will have more residual liquid asphalt than coarse RAP.

Quest for premium aggregates

Traditionally in the United States, aggregates sales have been limited to a relatively small market area, circumscribed by hauling costs. But the quest for premium aggregates sometimes takes contractors far and wide for the one aggregate that meets spec.

Construction aggregates are the lowest priced of all mined products, but as a result, transportation costs impact their price more than any other product. “Since they are so low-priced, transportation costs from the mine to the point of use can become the major part of their cost to the consumer,” said Val Tepordei, commodity specialist, U.S. Geological Survey.

“In Wyoming, material produced for $2.00 per short ton is subject to transportation costs averaging $1.10 per ton-mile,” Tepordei said. “At transportation distances of even less than two miles the transportation cost exceeds the cost of the product at the mine mouth. Therefore, it is imperative that aggregate sources be located as close to the point of use as possible. This fact usually creates conflicts between aggregate producers and people who live in proximity to aggregate sources, since economically, aggregate sources must be located close to population.”

“In most United States markets, aggregate operations serve a relatively small market area due to the cost of transport by trucks,” said Rinker Materials Corp. of Florida. “The average U.S. quarry tends to serve a 25- to 40-mile market radius.”

But Florida’s relatively recent geology means there are only a few areas where construction grade quality aggregates can be produced, Rinker said. “As a result, aggregates tend to be transported much greater distances,” the firm said.

For example, Rinker’s FEC Quarry in Miami ships aggregates via the Florida East Coast Railway as far north as Jacksonville every day of the week to supply the entire east coast of Florida (hence its name).

“Although you might think that sand is a plentiful resource in Florida, quality silica sand is only available in a northeast to southwest ridge that represents ancient sea level highs,” Rinker said. “Approximately 80% of the construction sand mined in Florida is harvested from these narrow beach ridges. Finished sand is then transported by truck throughout the entire state.”

Maryland’s aggregate database

In its search for premium aggregates, the Maryland State Highway Administration has collected petrographic and physical test data from approximately 70 coarse aggregate suppliers in the state over a number of years. Data were collected for aggregates proposed for use in pavement surfaces.

“We are in the process of developing a computer database for the purpose of evaluating and comparing physical test data and petrographic analyses as relates to performance of coarse aggregates,” said Robert A. Kochen, Soils & Aggregates Division, Maryland SHA. Speaking at the 12th annual Symposium of the International Center for Aggregates Research last April, Kochen said, “The data collection includes both carbonate and non-carbonate aggregates, as well as insoluble residue and petrographic analysis based on thin sections.”

Dating from 1996, the data collected for this database shows promise for comparing changes in aggregate performance to changes in the petrographic analysis. Since petrographic analysis is faster and more economical than physical testing, Kochen said, the goal is to use the petrography of each aggregate source as a screening tool to guide the decisions for what key physical tests to perform.

Tennessee pairs aggregates to needs

In 1999, Tennessee researchers articulated a new aggregate testing method that in practice is helping the DOT prolong its existing aggregate reserves, and was superior to existing methods.

As Tennessee found in the early 1990s, cautious specifications defining what is an approved surface aggregate source posed no problem when a large supply of polish-resistant aggregates was available. But as the premium aggregate supply declined, safe bituminous pavement surfaces were attained at ever-increasing costs. Not only did the cost of the aggregate increase as sources of supply decreased, but transportation costs also ballooned because the state had to buy from more distant sources.

In 1992, the Tennessee DOT launched a project to match aggregate performance with the functional needs of the pavements (based on average daily traffic) so all Tennessee aggregate sources could be used most efficiently.

Tennessee’s efforts resulted in a test to characterize an aggregate’s ability to retain microtexture over time. The test ranked six of seven Tennessee DOT-proven performing limestones in the upper two probable performance categories. The test also identified several promising aggregate sources, including some in areas where approved surface aggregate sources are scarce.

Finding harder aggregates

Alaska is grappling with the use of harder aggregate in surface or friction courses to reduce wear caused by studded tires.

Researchers Douglas J. Frith, P.E. and Dennis A. Morian, P.E., Quality Engineering Solutions, Inc., Reno; Dr. Shelley M. Stoffels, P.E., Pennsylvania State University; and Dr. Steve Saboundjian, P.E., Alaska Department of Transportation & Public Facilities, said in January that Scandinavian countries found that harder aggregates have resulted in improved pavement performance.

When a soft aggregate is used in the asphalt surface on bare roads, they said at January’s Transportation Research Board meeting, the high stress load between each tire stud and the surface aggregate causes binder and surface aggregate erosion, resulting in wheel path rutting.

But there’s a problem with hard aggregate. “High quality aggregates are not readily available throughout Alaska,” they said.

After conducting a cost effectiveness study, the researchers recommended that Alaska implement hardness specifications on roadways with volumes exceeding 5,000 ADT and other roadways showing excessive wear, and utilize harder aggregate requirements for pavement surface courses only.

“This will preserve market share [for local aggregate producers] for lower volume highways, and preserve the premium aggregate for use in higher volume highways,” they said. “[R]emember that the harder aggregate material is only applicable to surface course paving. Other layers can continue to utilize locally available materials. The effect of this is to minimize the loss of market share experienced by local aggregate producers, and minimize the increased cost to ADOT&PF.”

The researchers suggested that the state work with hot-mix producers regarding the availability of harder aggregates in the Vancouver, B.C., and Pacific Northwest regions, which likely could be acquired at reasonable cost by barge. “Alaska DOT&PF should evaluate additional out-of-state aggregates to confirm [that] alternative sources are available to fill the need for material,” they said.

Automating aggregate characterization

As aggregate plants struggle to identify aggregate shapes, sizes, and gradations on a cost-effective basis, manufacturers are taking a close look at automated gradation analysis.

“As emphasis increases on production of high-quality aggregate products, automated rapid aggregate gradation systems are becoming more desirable,” said University of Texas researchers Hyoung Kwan Kim, Craig Browne, Alan Rauch, and Carl T. Haas, Department of Civil Engineering, The University of Texas at Austin, at TRB in January 2004.

“A modular system architecture that provides the flexibility needed to easily integrate an automated grading system into a wide variety of plant applications is suggested,” they said. “New technologies offer the promise of being able to rapidly determine the particle size distribution of aggregate samples acquired automatically in a production plant.”

This could lead to tighter control of the gradation of aggregate mixes and lead to new mix designs with smaller variations in quality. “For example, on-the-fly product adjustments could be accomplished by controlling the rate of release from charge bins when mixing sorted aggregates,” they said.

At least three commercial manufacturers are currently offering high-speed, automated grading devices to the aggregates industry, they said. All three devices rely on two-dimensional image analysis to determine size from the outline of a particle shape.

To be feasible, the costs associated with installing an automated gradation system in a plant must bring a favorable rate of return on the investment. Costs of a system include the initial equipment, annual maintenance and replacement, operator training, and productivity loss during the installation.

Benefits come from gains in productivity through optimized plant operations, resulting from accurate, rapid process information, and perhaps increased market share and profit due to better product quality.

“For example, reducing the amount of out-of-specification product that must be reprocessed can lead to tangible gains in productivity,” UT researchers said. “Also, reduced product variability yields better overall product quality that may demand a higher price or market share, and ultimately profit.”

AIMing for Superpave

In January 2003, a team of researchers described their concept for a unified computer-automated system to characterize the shape of fine and coarse aggregates. The system now is on display in FHWA’s Mobile Asphalt Laboratory.

The system articulated in a TRB paper by Thomas Fletcher, U.S. Army Corps of Engineers, and Chandan Chandan, Krishna Sivakumar and Eyad Masad, Washington State University-Pullman, would quantify texture, angularity and the three-dimensions of form.

“The Superpave tests for measuring coarse aggregate shape properties are laborious, and limited in their ability to test a representative sample of aggregates,” write Fletcher, et. al. However, advances in digital vision, along with the availability of software for motion control of system’s components, have given new tools to aggregate producers and hot-mix asphalt plants.

These advances provide the means for the development of automated methods for aggregate shape analysis based on measurements made directly from the individual aggregate, the authors write.

The researchers sought to develop a unified computer-automated system for measuring the fine and coarse aggregate shape properties, called the Aggregate Imaging System, including an automated mechanism for image acquisition, and software for the analysis of form, angularity, and texture; and to correlate aggregate shape properties with asphalt-mix laboratory performance.

AIMS is designed to be versatile enough to capture images at different resolutions, field of view, and using different lighting schemes in order to be able to analyze the form, angularity, and texture of fine and coarse aggregates.

NCHRP researches aggregate tests

In the meantime, new research by the National Cooperative Highway Research Program Project 4-30A — Test Methods for Characterizing Aggregate Shape, Texture, and Angularity — will identify test methods that will make it easier for contractors, owning agencies, and material suppliers to provide the aggregates needed for Superpave.

The $450,000 study by the Texas A&M Research Foundation began in August 2003 and is scheduled for completion in February 2005.

The research is identifying suitable test methods for measuring shape, texture, and angularity of aggregates used in asphalt and concrete pavements, and in base layers. Researchers will evaluate and validate the most promising test methods for use in central and field laboratories.

Protocols for the recommended test methods would be developed for consideration and adoption by the American Association of State Highway & Transportation Officials, and a final report would be prepared that includes an implementation plan for moving the results of this research into practice.

Reprinted from Better Roads Magazine
August 2004

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