The humble chip seal and other road surface treatments that use asphalt emulsions are being taken to new heights by a variety of modifiers that enhance performance, albeit at a higher price.

The old saw says that oil and water don’t mix, but the truth is, it will under the right circumstances. An asphalt emulsion is a suspension of asphalt cement droplets held in a continuous water or aqueous phase, which contains a small amount of an emulsifying agent, also called a surfactant. What’s important is that the chemistry of the aqueous phase be specific to the field application at hand.

An asphalt chip seal, also referred to as a seal coat or a bituminous surface treatment, consists of an asphalt emulsion and stone chips, applied either singly or in layers, which builds up structure that can approach 1-inch thick. Chip seals are an inexpensive solution for an oxidized or raveling asphalt pavement.

“Based on the type of emulsifier used, you can cause the asphalt to have a greater affinity for the aggregate you want to retain,” explained Bill O’Leary, president of Prime Materials & Supply, a manufacturer of asphalt emulsions.

“Antistrips commonly are used in hot mixes so the liquid asphalt will have an affinity for the aggregate,” O’Leary told Better Roads. “But the surfactants and the emulsifier are closely related to the antistrip chemistry, so the emulsion itself will cause the asphalt to adhere strongly to certain aggregates.”

Better Roads looked at construction and composition of conventional chip seals last month (see Surface Treatments: When Seals Make Sense, October 2004, pp 24-37). This month we review how plastic (polymer) and rubber additives are allowing the plain-vanilla chip seal to last longer and stand up to heavy traffic.

Tough, modified surface treatments

Polymer modifiers are taking the chip seal well beyond its classic application on low-volume roads. For example, polymer modification allows the use of emulsions in new, robust applications such as Ralumac microsurfacing, NovaChip ultrathin bonded wearing course, RoadArmor macrosurfacing, and Encore emulsion hot in-place recycling and chip seals on high-traffic-volume highways, all brands of Koch Pavement Solutions.

Thus polymer-modified asphalt emulsions can improve performance to the point that with the right design, they can challenge hot-mix asphalt as an overlay option for high-level, high-traffic pavements.

In response, the asphalt industry has countered with a new concept, the HMA thin surfacing (see Pavement Preservation with Thin Overlays, June 2003, Better Roads). An HMA thin surfacing consists of a single layer of hot-mix asphalt (minimum of 1-inch thick, but often 2-inches thick) used to level, waterproof, and restore the original street shape and ride.

Another type of modified chip seal is the rubberized asphalt chip seal. Rather than utilizing an asphalt emulsion, this approach blends rubber (ground-rubber tires) with hot liquid-asphalt cement. This type of seal has been used both as a stress-absorbing membrane and a stress-absorbing membrane interlayer to help reduce reflection cracking associated with HMA overlays. And it has also been used without overlays.

Fog seals — not necessarily polymer-modified — are very light applications of an asphalt emulsion to the pavement surface with no aggregate. These applications seal the surface and provide a small amount of rejuvenation, depending on the type of emulsion used and the condition of the existing pavement surface.

Modified slurry surfacings

A slurry surfacing, also known as a slurry seal, is a mixture of aggregates dispersed in an asphalt emulsion and applied in a slurry state. It’s a mix of polymer-modified emulsion and crushed rock aggregate that is spread simultaneously in one pass over the street at a particular thickness. The slurry cures as the water evaporates, leaving only the asphalt to coat the aggregate.

Slurry surfacings are more than just a shot of emulsion followed by a shot of chips. California-based Valley Slurry Seal, a leader in the field, makes the point that slurry seals are designed in a lab, proportioned by the slurry machine, and laid down and cured so the asphalt-to-aggregate ratio is maintained at the optimum value to assure uniform aggregate coating and adhesion. Such friction courses use very large fractions of fines material, giving a very high surface area and a lot of microstructure. This translated into a smooth finish that has a sandpaper surface and high skid resistance.

Cape seals are a combination of chip seal and slurry surfacing or seal. For paved roads, the chip seal is applied first, then, four to 10 days later, the slurry seal is applied. For unsurfaced roads, an application of penetration oil (MC-70 or SC-70) is applied first as a prime coat, followed about two days later by a chip seal and about two weeks later by a slurry seal.

Microsurfacing is a more advanced extension of the slurry surfacing concept. Microsurfacing is a polymer-modified, cold-mix paving system that can remedy a broad range of problems on today’s streets, highways, and airfields.

“Like its parent product, slurry seal, microsurfacing begins as a mixture of dense-graded aggregate, asphalt emulsion, water, and mineral fillers,” says the International Slurry Surfacing Association. “While conventional slurry seal is used around the world as an economical treatment for sealing and extending the service life of both urban and rural roads, microsurfacing has added capabilities, thanks to the use of high-quality, carefully monitored materials, including advanced polymers and other modern additives.”

Introduced in the United States in 1980, microsurfacing is applied to existing pavements by a specialized machine that mixes all components of the system on-site, and spreads the mixture onto the road surface. These materials are continuously and accurately measured, and then thoroughly combined in the microsurfacer’s mixer.

Emulsions and polymer modifiers

Polymer modifers are extremely common as binder modifiers because they enhance aggregate/binder bonding and other benefits. The tradeoff is a higher price, typically about 30% over non-modified emulsions.

Chip-seal aggregate and asphalt-emulsion binder are bound to each other through chemical, mechanical, and even electrostatic means. Aggregate characteristics that influence bonding include porosity, surface texture, mineralogy, and surface chemistry. The best polymer-modified chip seal designs optimize the chemical, mechanical, and electrostatic variables to best weld aggregate to asphalt cement as the emulsion water carrier evaporates or breaks.

The term polymer simply refers to a very large molecule made by chemically reacting many (poly) smaller molecules (monomers) to one another in long chains or clusters, according to Koch Pavement Solutions. Koch says the properties of the modified asphalt depend on the polymer system used, and the compatibility of the polymer with the asphalt cement.

“Such polymers in emulsions will increase early stiffness of the binder, which leads to a better early aggregate-chip retention,” reported the Iowa Highway Research Board. “When compared with non-polymer-modified binders, the flexibility of the treated surface is increased in cold weather and over time as a result of the emulsion being modified with the addition of polymers.”

Also, bleeding and flushing of surfaces treated with polymer-modified emulsions is reduced in warm weather because polymers enhance binder stiffness at high temperatures.

“Depending on the roadway and the circumstances for the road, the benefits of the polymer-modified emulsion may warrant its use,” Iowa said. “Some roads that may warrant their use are high-volume roads and areas where more turning, starting, and stopping occurs, such as roads in municipalities.” Thus, use of polymer-modified emulsions broadens the application of chip seals to more demanding pavements, putting it in direct competition with hot-mix asphalt.

Styrene, butadiene modifiers

Polymer modifiers for asphalt can be either elastomeric or plastomeric.

Specific binder and mix properties can be engineered by choosing the right polymer for a given application, and making sure it is compatible with the asphalt, says Koch Pavement Solutions. “In general, elastomers are chosen to give a more resilient, flexible pavement, while plastomers result in mixes with higher stabilities and stiffness values.

“Elastomers include copolymers of styrene and butadiene (such as, styrene butadiene diblock, styrene butadiene triblock or radial, styrene isoprene, and styrene ethylbutylene),” according to the essay Current and Future Use of Nonbituminous Components of Bituminous Paving Mixtures, by Kent Hansen, Bob McGennis, Brian Prowell, and Anne Stonex. Their work was produced by the Transportation Research Board as a forward-looking Millennium project in January 2000. “These products are normally milled into the asphalt binder at temperatures above 160 degrees C by a high shear mixer,” they said.

Other elastomers, they added, include styrene butadiene rubber latex, polychloroprene latex, polyisoprene, and crumb rubber modifier. The most common, SBR, is normally introduced as a latex emulsion and is flashed into the asphalt, they said.

Crumb rubber traditionally has been introduced and reacted in agitated tanks of hot (177 to 196 deg C) asphalt, although now some organizations are milling the crumb rubber into the binder like other elastomeric modifiers.

Modified chip seals using crumb rubber use ground rubber from tires, blending it with the asphalt cement. The rubber adds stiffness and resiliency to the asphalt, and also improves bonding with the aggregate. The added stiffness and resiliency may also enable the seal to bridge existing cracks better.

Plastomers listed by the authors include ethylene vinyl acetate, polyethylene (unstabilized and stabilized), and various compounds based on polypropylene.

Cationic or anioic emulsions

Whether polymer-modified or not, asphalt emulsions for roads almost always have slight electrical charge, either positive (cationic) or negative (anionic). Some aggregates work better with a cationic emulsion, others with an anionic. It’s incumbent on the mix designer to match the right emulsion with the aggregates at hand, although most road agencies already have a feel for what works locally.

Make sure asphalt-emulsion spray nozzles work in overlapping pattern.

Macrosurfacing is a single-pass surface treatment which utilizes polymer modified asphalt emulsion and single-sized aggregate.

“Emulsions are predominantly cationic, and you want to select the emulsion chemistry that will have the best affinity for the aggregate being used,” O’Leary said. “But that chemistry resides on the surface of each of the asphalt droplets, so the droplets themselves have the affinity for the right aggregates employed for the project. It’s important to match them up.”

The charge is not applied at the plant; instead, it’s based on the chemistry of the emulsion. “The charge of the asphalt droplets is electrochemical,” O’Leary told Better Roads. “It’s based on the chemistry; anionic or negatively charged materials generally are wood-derived, such as lignins, and have a high pH.”

The cationic, or positively charged materials, have a low pH, and are of a wide variety of chemistry, including the fatty amines, O’Leary said. “Today, the more sophisticated binders are cationic,” he said, “because there seems to be more control over their rate of cure; they tend to be more rapid curing than their anionic counterparts.” Cationics also tend to be more expensive.

There is a lot of emphasis on the charge of the emulsion and the charge of the aggregate, and those are important factors, but they’re not the only factors, he said.

“The surface texture and microporosity of the aggregate will help the asphalt bond mechanically, whereas the chemical bond imparted by the negative/positive attributes seems to be more important when slick, siliceous gravels are used,” O’Leary said. “Those are best held with a cationic emulsion. Cationics tend to work with both good and bad aggregates; the anionics tend to work best with good limestone aggregates or those with lots of surface texture. When you get to the problematic aggregates, nine times out of 10 your cationics will be the solution.”

Good results can be obtained with both cationic and anionic emulsions, O’Leary cautioned. But there are special situations where one would want one or the other. “The agency should check with the emulsion manufacturer, who often will be familiar with local aggregates and will be able to make a recommendation,” he said.

High float emulsions

Yet another variety of polymer-modified emulsion is the high float emulsion.

“High float emulsions are made with a special family of emulsifying agents that leaves a gel structure behind in the asphalt residue,” writes the Iowa Highway Research Board. “High float emulsions were developed for low-volume roads in areas where a graded cover aggregate is to be used.”

High float emulsions are also quite effective when used with somewhat dusty aggregates because they provide a thicker asphalt film on the aggregate and the aggregate can penetrate into the emulsion much more uniformly, the IHRB says.

“This is because high float emulsions are slightly anionic (sets slower than most cationic emulsions), and there is a small amount of solvents in them that act as a cutter in penetrating the dust,” Iowa said. “A thicker asphalt film coats the aggregate; therefore, high float emulsions do not flow and drain as readily as conventional emulsions.”

New testing protocol revealed

The incompatibility of aggregate and binder can cause failures of chip seals, and although most local road agencies know which of their local aggregates will work with locally available asphalt emulsions, unanticipated changes are always possible.

But a new test protocol developed at Texas Tech University in Lubbock may make chip seal performance more predictable through simple lab tests which incorporate field conditions.

“Aggregate loss (or raveling) from a seal coat is a problem commonly experienced by highway agencies, and it can be traced to a lack of compatibility between the aggregate and binder used in seal coats,” wrote Baris Yazgan, research assistant, and Sanjaya Senadheera, Ph.D, assistant professor, in their 2004 TRB presentation, A New Testing Protocol for Seal Coat (Chip Seal) Material Selection.

“Aggregate-binder incompatibility could be due to a number of reasons, including chemical incompatibility as well as construction-related factors, [such as] aggregate dust, moisture content, binder temperature when aggregate is spread, and binder temperature when rolling is performed,” they say.

This echoes some of O’Leary’s hints for a successful chip seal. “Because the continuous phase of the asphalt emulsion is water, that phase is continued into the aggregate if the aggregate has moisture present,” O’Leary told Better Roads. “If the aggregate is moist, it breaks the surface tension and actually helps the asphalt find its way into the surface pores. They blend together a little more nicely if there’s a dampness on the aggregate. It’s a great optimizer.”

Likewise, dusty aggregate will hinder asphalt adhesion. “We just spoke with a state engineer who had to stop a job because there was too much dust in the stockpile, and the aggregate was not holding to the road,” O’Leary said. “The dust acts like flour on a countertop, it’s a bond-breaker. Very simply, clean rock is best, but if you’re faced with aggregate with a lot of surface dust, it will work if you prewet it.”

Low viscosity equals better penetration

Asphalt viscosity is a function of its temperature. “When the binder is less viscous, it will wet the aggregate and penetrate into the pores more effectively, resulting in a better aggregate-binder bond,” Yazgan and Senadheera say. “Moreover, the ambient temperature and the temperature of existing pavement surface affect the cooling rate of the binder after it is sprayed on the pavement. Therefore, the time lag between asphalt spray and the aggregate spread becomes a key factor that determines the effectiveness of aggregate-binder bond.”

Time elapsed between aggregate spreading and rolling is another issue, they say. “This is of particular significance when hot asphalts are used,” they say. “Rollers apply the energy needed to embed aggregates into the binder, and help to seat each aggregate particle such that its center of gravity is at the lowest possible position, thus giving it more stability. The longer it takes for the rolling operation to begin, the stiffer the binder will get, and as a result, the aggregate may end up having a lower embedment depth than the design value.”

High-level chip seals possible

State transportation departments in Kansas, Michigan, Louisiana, Arkansas, Wyoming, and New Mexico are among those that routinely use polymer modified emulsion chip seals on state highways and Interstates, reports Koch Pavement Solutions. These are specified for pavements exceeding 7,600 vehicles per day and are federal-funds eligible.

Another user is the South Dakota DOT, which recently studied their performance after some prominent failures.

Polymer-modified chip seals have great application for use on high-level highways, so long as they are done correctly, and that may require polymer modification, said Daris Ormesher, P.E., South Dakota DOT Office of Research, and Monty J. Wade, P.E., and David G. Peshkin, P.E., Applied Pavement Technology, in their January 2002 TRB presentation, Evaluation of Chip Seals on High-Speed Roadways.

Despite the visible failures, the authors conclude that the investigation shows chip seals can be effectively used on high-volume roadways in South Dakota, and their performance can be enhanced through special considerations, such as the use of polymer-modified emulsions, precoated aggregates, or a fog seal cover.

Among their recommendations:

Use a polymer-modified emulsion to obtain better adhesion, especially on high-volume roadways.

Like California specs, apply a fog seal over the chip seal to help with retention.

Develop a design procedure to determine application rates for each specific project.

Use a higher emulsion application rate to achieve greater aggregate embedment.

Develop a tighter and more gap-graded gradation to ensure uniformity and provide a single layer of chips.

Limit the amount of fines (material passing the 0.075-mm [No. 200] sieve) in the chips.

Conduct testing to limit the amount of flat and elongated particles (ASTM D 4791 or flakiness index).

Conduct testing to determine the adhesion between the aggregate chips and the emulsion.

Sweep the pavement surface approximately two hours following placement (before opening to traffic)

Run the pilot vehicle on the chip seal to help with chip embedment and orientation.

Enforce the speed restriction on the rollers, or provide more rollers on a project.

Limit the amount of paving per day or limit the speed of the operation to meet rolling requirements.

Wet the aggregate stockpile the morning of construction and rewet in the field as needed.

Prevent late season paving by changing seasonal restriction to June 1 through August 31.

Conduct an embedment check to ensure adequate embedment of the aggregate.

Conduct a sweeping test to limit the amount of excess aggregate to between 5 and 10% of the total.

Limit the amount of paving that can be conducted each day.

Apply a choke stone layer of small chips over the chip seal to lock in the larger aggregate particles.

Precoat the aggregate chips to reduce dust and improve adhesion.

Develop a Surface Treatment Manual for design and construction personnel.

Conduct training for staff and contractors.

Best practice report in 2005

The ubiquitous chip seal is used with success year after year by many road agencies, but often these road agencies follow existing practice with little knowledge of what constitutes best practice.

To bridge that gap, the National Cooperative Highway Research Program has been working on a new “synthesis of best practice” — NCHRP 35-02: Chip Seal Best Practices.

Launched in August 2003, the synthesis will summarize research both here and overseas on the materials, design, construction techniques, and effectiveness of chip seals in practice. The topic consultant is Doug Gransberg, Oklahoma State University, and work is expected to be completed by the end of the year.

A comprehensive literature review, including international experience, will be abetted by a survey of industry associations, state DOTs via the American Association of State Highway & Transportation Officials’ Highway Subcommittee on Maintenance, and a representative sample of local governments.

The report will identify best practices, problems solved, and lessons learned. Emerging trends of practice will be identified. Case studies may be included. The resulting work with synthesis will incorporate the state of art in chip-seal lore.

Slurry, microsurfacing study

Slurry seals and microsurfacings rely on modified asphalt emulsions for performance. So as NCHRP studies chip-seal best practices, a 4.5-year Slurry Seal and Microsurfacing Pooled Fund Mix Design Study also is underway.

Fourteen states are pooling their money to sponsor this analysis of slurry-seal and microsurfacing designs in most of the climate zones of the United States. The work is being undertaken by a consortium of engineering firms and Caltrans.

Now, during Phase 2, design and laboratory test methods will be evaluated over a 24-month period, including worldwide assessment of existing test methods and design procedures. That will lead to Phase 3, which will validate design procedures with field trials. This phase will include the development of guidelines, specifications, and a training program for agencies, contractors, and material suppliers.

In the meantime, the Foundation for Pavement Preservation, in conjunction with the Federal Highway Administration, continues its evaluation of emulsified sealer/binders for extending the life of asphalt pavements. This multi-year program is being executed by Arizona DOT’s Larry Scofield in Arizona, Minnesota, Michigan, and California.

A second phase of the program has started which includes non-destructive testing to measure friction, texture, and roadway profile. The University of Wyoming in Laramie is providing technical assistance in this project to evaluate properties of the binders being studied.

Eller New Executive Director of FP2

Gerald L. “Gerry” Eller, P.E., has been appointed executive director of the Foundation for Pavement Preservation.

Eller — who served in various positions with the Federal Highway Administration for 34 years, culminating as director, Office of Engineering — is president, GLE Technical Services, Inc., where he provides consulting services to a variety of clients.

Pavement preservation is a planned system of treating pavements at the optimum time to maximize their useful life, thus enhancing pavement longevity at the lowest cost. Experience shows that spending $1 on pavement preservation eliminates or delays spending $6 to $10 on rehabilitation or reconstruction costs.

In addition to Eller being appointed FP2 executive director, the foundation announced that Fugro Consultants, L.P., of Austin, will provide administrative and financial services to FP2. William E. “Bill” Ballou remains FP2 president.

The new address for FP2 is Foundation for Pavement Preservation, 8613 Cross Park Drive, Austin, Tex., 78754, phone (866) 862-4587, fax (512) 973-9565, Web http://fp2.org.

For More Information

More information about polymer modified asphalt emulsions is available from a variety of sources. Here are some places to start:

Better Roads. The October 2004 issue of Better Roads carried the opening article of this two-part report on chip seals and asphalt emulsions, including an extensive glossary developed by the Foundation for Pavement Preservation. See pp 24 to 37 of last month’s issue.

Valley Slurry Seal. This West Sacramento, California-based company, a leader in slurry seal applications utilizing polymer-modified asphalt emulsions, offers a variety of technical information, articles, and model specs at www.slurry.com.

BASF Corporation. This supplier of polymer modifiers has an abundance of technical articles and presentations for download on its Web site: www.basf.com/businesses/consumer/dispersions/usa/asphalt/articles/.

The Basic Asphalt Emulsion Manual. Published by the Asphalt Institute and the Asphalt Emulsion Manufacturers Association, this text is available at nominal charge from www.aema.org, or call AEMA at (410) 267-0023. A print copy is available, and a CD-ROM also is available which contains all the information in the hard copy, plus video clips and other enhancements.

Polymer Modified Asphalt Binders. This compilation textbook, edited by Kenneth R. Wardlaw and Scott Shuler (ASTM, 1992), is available as a hardcover book through a variety of resellers, including Amazon.com.

Ohio DOT. The Ohio DOT has developed a common sense specification for chip seals using polymer-modified asphalt emulsions. Read it at www.dot.state.oh.us/construction/OCA/Specs/2002CMS/2003_Manual_for_web/422.htm.

The Asphalt Institute. Their handy glossary of asphalt industry terms includes many relating to asphalt emulsions. Download it at www.asphaltinstitute.org/ai_pages/FAQs/PDFs/Asphalt_Industry_Glossary_of_Terms.pdf.

National Center for Pavement Preservation. The 87-page report, Selecting a Preventive Maintenance Treatment for Flexible Pavements by Dr. R. Gary Hicks, P.E. et. al., is available for download at the NCPP Web site: www.pavementpreservation.org/publications/Hicks_Paper.pdf.

Douglas Gransberg. A Seal Coat Inspectors Guide, developed in 2000 for the Texas DOT by Doug Gransberg is available on the author’s Web site:  www.cns.ou.edu/ResearchDoug/sealcoat.htm.

Two reports, Thin Maintenance Surfaces, Phase I and II, done for the Iowa DOT and the Iowa Highway Research Board, are available for download from the same Web site. Visit www.pavementpreservation.org/publications/index.php.

Caltrans. The complete Caltrans Maintenance Technical Advisory Guide that covers all pavement maintenance techniques can be downloaded from a Caltrans Web site dedicated to pavement preservation and maintenance: www.dot.ca.gov/hq/maint/roadway.htm.

Rubber Pavements Association. Hicks and Epps paper, Life Cycle Cost Analysis of Asphalt-Rubber Paving Materials, may be downloaded from the Web site of the Rubber Pavements Association. View it and other technical papers at www.rubberpavements.org/library/index.asp.

Reprinted from Better Roads Magazine
November 2004

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