Conventional chip seals and slurry seals — and lesser-seen variants like fog seals and sandwich seals — are part of an arsenal of lower-cost pavement preservation strategies that are available to state, municipal, town, and county agencies. 

These long-standing surface treatments are ideal solutions for pavement preservation and maintenance when neither agency cash flow nor traffic loadings indicate use of a new concrete or asphalt lift.

Following decades of applications, today’s basic, conventional chip-seal application and structure are relatively unchanged from yesterday’s.

What’s changing in the industry is a new wave of polymer-modified and rubber-modified liquid asphalts and asphalt emulsions that offer faster placement of longer-lived chip and slurry seals, albeit at a higher price.

We will explore the world of higher-performance modified chip seals, slurry seals and surface treatments — in which the modifiers enhance the elasticity and adhesion characteristics of the binder — in Part II of this study next month.

Many benefits of seals

Asphalt chip seals have been used for many years on lower volume roads (less than 2,000 vehicles per day) as the wearing surface on untreated granular roadbeds.

The Asphalt Institute/Asphalt Emulsion Manufacturer Association’s Basic Asphalt Emulsion Handbook says chip seals and surface treatments provide a low-cost, all-weather surface for light to medium traffic, and a waterproof layer to prevent the intrusion of moisture into the underlying courses.

Other applications and benefits listed by the handbook include:

Skid-resistant surface. Pavements that have become slippery because of bleeding or wear and polishing of surface aggregates may be treated with sharp, hard aggregate to restore skid resistance.

New life to weathered surfaces. A pavement that has become weathered to the point where raveling might occur can be restored to useful service by application of single-surface or multiple-surface treatments.

Temporary cover. Surface treatments can protect a new base course that is to be carried through a winter or for planned stage construction. The surface treatment makes an excellent temporary surface until the final asphalt courses are placed.

Salvage old pavements. Surface treatments can provide a stop-gap measure for pavements that have deteriorated because of aging, shrinkage cracking, or stress cracking. Although the surface treatment has little or no structural strength, it can serve until a more permanent upgrading can be funded and completed.

Define shoulders. Surface treatments can define shoulders so they won’t be mistaken as traffic lanes.

Provide rumble strips for safety.

“Seals can form an interlayer or under-seal between pavement layers, seal existing pavements, and enhance the bond between an overlay and existing pavement,” said Bill O’Leary of Prime Materials and Supply in Houston. “What it can’t do is increase the strength of a roadway, or restore a failed roadway.”

What is a chip seal?

While everyone seems to know what a chip seal is, it’s just one point on a spectrum of surface treatments, all of which are tools for the knowledgeable pavement manager.

Defining these techniques can tell a road manager where his program will fall within this continuum. The Foundation for Pavement Preservation defines the chip seal as a surface treatment in which the pavement is sprayed with asphalt (generally emulsified) and then immediately covered with aggregate (ranging in size from 0.375 to 0.25 of an inch) and rolled, usually with a pneumatic (rubber-tire) roller. Chip seals commonly are used as a wearing course on low-volume roads.

With a double chip seal, a second coat is placed immediately after the first. This treatment waterproofs the surface, seals small cracks, reduces oxidation of the pavement surface, and improves friction, according to a Best Practice publication of the Minnesota Department of Transportation.

Some, not all, experts suggest application of sand to blind the surface after a chip seal is placed. Another variant is that after giving the surface a week or two to cure, a fog seal may be applied, which reduces the amount of loose rock and helps smooth the roadway. This procedure seals the surface of a pavement afflicted with non-load-associated cracks, and is said to improve surface friction.

Spectrum of surfacings

At the most simple end of the surface treatment continuum is the fog seal, an application of diluted emulsion (normally 1:1) to enrich the pavement surface and hinder raveling and oxidation. “This is considered a temporary application,” said Dr. R. Gary Hicks, P.E., professor emeritus of Civil Engineering, Oregon State University, in his guide, Selecting a Preventive Maintenance Treatment for Flexible Pavements, published by FP2.

Next, come the chip seals. Although typically used on low volume roads and streets, Hicks says they can also be used on high volume highways and expressways but only with high-performance binders.

Next, come thin cold-mix seals, into which Hicks groups slurry seals, cape seals, and micro-surfacings. “[These] are used on all types of facilities to fill cracks, improve friction, and improve ride quality,” Hicks said. Today such seals utilize polymer modifiers to enhance performance, so they will be discussed in detail next month.

Finally, at the far end of performance and initial cost are thin asphalt overlays, which include dense-, open-, and gap-graded mixes, as well as surface recycling. These approaches are used to improve ride quality, provide surface drainage and friction, and correct surface irregularities, Hicks said. Thin-lift overlays are generally 37 mm in thickness and almost always are of hot-mix asphalt.

“The longer maintenance is delayed the more it will cost to repair the pavement,” Hicks said. “Alternatively, if a pavement is maintained too soon [similar to painting your house more frequently than needed], you spend money unnecessarily. Early maintenance results in higher annual costs.”

When the costs of delayed maintenance are compared to those of early maintenance, the optimum timing to fix pavements can be determined, Hicks said. Generally, he said, the optimum time for applying the various treatments is:

• Fog seals, every 1-3 years.

• Crack seals, every 2-4 years.

• Chip seals, every 5-7 years.

• Slurry seals, every 5-7 years.

• Thin overlays, every 7-10 years.

“The actual timing for the various treatments may vary depending on traffic level and environment,” Hicks said. “Each agency is encouraged to develop their own optimal timing for maintenance treatments to minimize life-cycle costs.”

Chip seals

There is nothing new about chip seals. In 1913 Caddo Parish, Louisiana, was reputed to be the first government body in that state to use heavy asphaltic oil as a surface treatment for clay gravel roads.

“Within 30- to 60-days’ time, travel by vehicles had ironed out the oil into an almost perfect surface, resembling an asphaltic concrete pavement,” said J.T. Bullen, Caddo Parish engineer in 1928.

By 1920, coal tar sealants and light oils were used extensively for enhancing the driving surface of gravel roads that were experiencing heavier traffic loads.

“The only satisfactory method we have developed for maintaining gravel roads under a volume of traffic exceeding 500 vehicles per 12-hour day is to surface-coat with bituminous material,” said Paul Sargent, chief engineer of the Maine State Highway Commission in 1928. “We have used refined tar products and various grades of asphaltic oil, but for the last six of eight years we have used, almost exclusively, refined tar.” Coal tar products no longer are used in road surfacing applications.

Surface treatments around the nation

A 1999 study by the American Association of State Highway and Transportation Officials found thin asphalt overlays are the most frequently cited surface treatments, followed by single-course chip seals, crack treatments, and single-course micro-surfacings. Slurry seals, fog seals, paver-placed surface seals, scrub seals, and cape seal are least widely used.

All 41 state transportation agencies responding to the team’s questionnaire used some preventive maintenance techniques. Some 38 used mill and overlay, 37 used asphalt overlay (under 1.5 inches), and 33 used single-course chip seal.

At the low end, 18 states used multiple-course chip seals, 14 used ultra-thin HMA overlay (under 0.75 inch), 14 used hot in-place recycling (under 1.5 inches), 14 used slurry seal, 12 used fog seal, 11 used a paver-placed surface seal such as Koch Pavement Solution’s NovaChip, seven used the scrub seal, and two the cape seal.

Michigan has been using chip seals to great success, said Larry Galehouse, P.E., executive director, National Center for Pavement Preservation. “From the beginning, Michigan’s emphasis has been on targeting pavement surface defects caused by the environment and by deficiencies in materials, not on deficiencies in the pavement structure caused by traffic loading,” Galehouse said.

“Surface treatments for flexible pavement surfaces [in Michigan] include micro-surfacing, chip seals, slurry seals, crack sealing, 0.75-inch overlays of ultrathin hot-mix asphalt, and 1.5-inch hot-mix asphalt overlays,” Galehouse said. “In some situations, it was cost-effective to treat curb and gutter pavement sections by cold-milling and resurfacing with a 1.5-inch hot-mix asphalt overlay.”

Asphalts for surfacings

Three varieties of liquid asphalts are used in conventional surface treatments, and are described by the FHWA in its discontinued NHI course (now undergoing revision and expansion).

Asphalt cement (straight) is used to construct chip seals in regions that have very hot weather. “The asphalt cement is shot at high temperatures where it flows well and accepts chips readily,” NHI said. “A fairly warm pavement surface is required so the asphalt does not cool off too quickly before placement of the chips.”

Cutback asphalts are blends of asphalt cement with solvents, which make the asphalt cement fluid for spraying or mixing. “The solvents then evaporate, leaving the base asphalt cement in place to bind the rock,” NHI said. “The solvents used include gasoline for rapid-curing, kerosene for medium-curing, and diesel fuel for slow-curing cutbacks. The rapid cure cutbacks are normally used for single-sized chip seals, while the medium-cure cutbacks are used for graded chip seals.”

The use of cutback asphalts, once common for chip seals, has declined considerably because of environmental restrictions on hydrocarbon emissions from evaporating solvents in specific regions around the country, NHI said.

Emulsified asphalts are an emulsion of very small asphalt cement particles or globules held in suspension in water with the use of an emulsifying agent.

Asphalt emulsion distributor truck precedes chip spreader.	Chip spreader places clean aggregate in chip seal project.

“Like cutback asphalts, emulsified asphalts come in rapid-, medium-, and slow-setting grades for different uses,” NHI said. “The rapid-, medium-, and slow-setting grades are developed through the use of different emulsifying agents and the addition of some solvents.” These asphalt particles are either anionic (negatively charged) or cationic (positively charged).

The rapid-setting emulsions are used mostly for chip sealing, while the medium- and slow-setting grades are used for the construction of emulsion mixes or recycling and fog or tack seals.

“The emulsified asphalt sets or breaks when the asphalt particles precipitate or fall out of the water suspension and coat the aggregates,” NHI said. “The emulsion turns color from brown to black during this process. For rapid-setting emulsions, this process starts on contact with the chips.”

For medium-setting emulsions, the break starts some time after mixing with the aggregate, depending upon the emulsion and the amount of fines in the aggregate. Slow-setting emulsions are very stable and normally break with the evaporation of the water.

“Rapid-setting asphalt emulsions are normally used for surface treatments, to react quickly with the aggregate and cure rapidly,” says the Asphalt Institute in its Basic Asphalt Emulsion Manual. “High-volume traffic roads may require a polymer-modified, rapid-setting emulsion.”

Construction of chip seals

Construction of a chip seal project begins with calibration of asphalt distributor and aggregate spreader. The existing pavement surface is thoroughly broomed and the liquid asphalt is applied at the required rate.

Using a dedicated chip spreader, aggregate is placed at the desired rate before the emulsion breaks. This aggregate is rolled gently, immediately after being spread and before the binder cures, the rate of which depending on ambient temperature and relative humidity.   

After the binder is cured — from 15 to 24 hours — the chip seal is broomed to get rid of loose aggregate that will result in broken windshields and complaints from motorists; some agencies reckon the initial performance of a chip seal by the number of broken windshield claims they get.

“The existing pavement surface should be dry, unless asphalt emulsions are used in the chip seal,” NHI said. “The work should be conducted in warm weather, and should not be attempted when air temperatures are below 60 degrees Farenheit.”

Usually, 4.5-metric-ton pneumatic-tired rollers are used, although rollers with weights of 2.7 to 7.3 metric tons also are used. Steel-wheeled rollers are not generally used because they may crush the aggregate and have a tendency to bridge over low spots so that no compaction occurs there.

Enough compactors have to be on hand to make sure the aggregate is compacted into the seal before the liquid asphalt sets or cures. “Once the asphalt begins to harden, the aggregate cannot be adequately seated into the asphalt,” NHI said. “Failure to achieve this seating can lead to the aggregate being pulled out by traffic. The maximum amount of rolling should be determined by economics, while the minimum amount should be no fewer than three passes.”

Tips for successful seals

Prime’s O’Leary said that for a successful chip seal, an existing pavement must exhibit minimal structural distress.

“Areas of structural distress, such as alligator cracking or potholes, should be patched or repaired well in advance,” O’Leary said. “Rutted or corrugated areas should be corrected by leveling or milling. New patching (less than six-months old) will likely cause problems. Excessive crack fill material (overfilled cracks) will likely cause problems.”

Flushed or bleeding roads — seeping with existing liquid asphalt — require special attention, he said. “Inconsistent surfaces (patches or flushed wheel paths) present a challenge. Each roadway surface must be considered and application rates may need adjustment.”

Project failures — that is, aggregate loss — on the first day of construction may stem from too little asphalt binder, too much aggregate, unacceptable weather conditions, or poor traffic control.

“The aggregate may be applied too late,” O’Leary said. “The chip spreader and roller may travel too fast. The rock may be dirty, or the asphalt may be the wrong product or just plain bad.”

After the first winter, aggregate loss may be the result of too little asphalt binder, a too stiff asphalt binder, or the chip seal placed too late in season, O’Leary told Better Roads.

He added that between placements of chip seals, longitudinal joints should not be in a wheel path. Instead, let 4 to 6 inches of asphalt remain uncovered on the first shot. Then overlap the joint on the second shot and cover with stone. “Always check the application rate of the chip spreader,” O’Leary said. “Excess aggregate will dislodge or crush newly bound chips.”

Bond of aggregate, asphalt

The aggregate is held to the asphalt by both mechanical and chemical bonds, O’Leary said. “The mechanical bond is accomplished when the fluid binder penetrates the pores of the cover stone and solidifies,” he said. “A second type of mechanical bond depends on the interlocking, or frictional resistance, of the aggregate particles. The mechanical bond is developed by effective wetting of the aggregate, the surface texture of the aggregate and the adhesive qualities of the asphalt binder.”

Chemical bonding at the interface is developed by wetting the aggregate surface with the liquid emulsion, he said. The chemical bond can be controlled based on the emulsifier used. “The asphalt, emulsifier and aggregate must be considered as a system,” O’Leary said.

Aggregate moisture content is important when using emulsions, O’Leary added. “Asphalt emulsion is water-constant,” he said. “Water is the delivery vehicle for the asphalt. Dry aggregate has such high surface tension that the emulsion may not wet the aggregate. Excess dry fines also will not allow good adhesion. Pre-wetting the aggregate stockpile will help the emulsion to make an adhesive contact with the cover stone. But if the aggregate is too wet, the emulsion will cure very slowly.”

For More Information

A tremendous amount of information on pavement preservation and surface treatments is available:

A 128-page Best Practices for Asphalt Maintenance guidebook of the Minnesota DOT — packed with guidance on preventive maintenance for roads that are still in decent condition — is available for download from the Minnesota LTAP Center at www.mnltap.umn.edu/pdf/asphalt.pdf.

An 18-page Guide to Paving Fabric Interlayer Systems in .pdf format is available from the Asphalt Interlayer Association at no charge from www.aia-us.org/docs/Interlayer_Guide.pdf.

A 15-page glossary of pavement preservation terminology may be downloaded from the Foundation for Pavement Preservation Web site, www.fp2.org/ pdffiles/GlossaryOfTerms.pdf.

California has been diligent in framing its chip and surface sealing specifications for decades. Read Caltrans specs for these seals on the California Chip Seal Association Web site, www.chipseal.org/docs/ 37-1_Document-1.pdf.

Larry Galehouse’s breakthrough article Pavement Preventive Strategic Maintenance, as published in TR News of March-April 2002, may be downloaded from the California Chip Seal Association at www.chipseal.org/docs/TRN_article.pdf.

The 87-page guide, Selecting a Preventive Maintenance Treatment for Flexible Pavements, by Dr. R. Gary Hicks, P.E., Professor Emeritus of Civil Engineering, Oregon State University, may be downloaded from the National Center for Pavement Preservation Web site, www.pavementpreservation.org/publications/index.php.

The 28-page special report, Insights into Pavement Preservation: A Compendium, by the Federal Highway Administration (FHWA, January 2000), provides a compilation of articles on pavement preservation from 1997 to 2000 that highlight state experiences with preventive maintenance programs. Download it at www.fhwa.dot.gov/infrastructure/asstmgmt/compend.pdf.

A report on the type of surface treatments used throughout the U.S., Pavement Preservation in the United States Survey, by the Lead States Team on Pavement Preservation, American Association of State Highway and Transportation Officials, may be downloaded at http://leadstates.transportation.org /pp/survey/survey_report.pdf

The FHWA and National Center for Pavement Preservation offer more reference guides on pavement preservation techniques. Pavement Preservation 2: State of the Practice, draws on experience from California, Delaware, Michigan, Minnesota, Montana, North Carolina, Ohio, and South Dakota. National Pavement Preservation Forum II: Investing in the Future (FHWA-IF-03-019), includes papers and presentations from the 2001 Forum hosted by Caltrans and FP2 in San Diego.

Spiral-bound pocket checklists on pavement preservation produced by the FHWA and FP2 take users through such steps as project review, material checks, surface preparation, equipment inspections, weather requirements, and common problems and solutions. They are: Crack Seal Application (FHWA-IF-02-005); Chip Seal Application (FHWA-IF-02-046); Thin Hot-Mix Asphalt Overlay (FHWA-IF-02-049); Fog Seal Application (FHWA-IF-03-001); Microsurfacing Application (FHWA-IF-03-002); and Joint Sealing of Portland Cement Concrete Pavements (FHWA-IF-03-003).

To obtain copies, contact Steve Mueller in the FHWA’s Office of Asset Management (202-366-1557 or e-mail him at steve.mueller@fhwa.dot.gov), or contact Patti Hahn at the National Center for Pavement Preservation (517-432-8220 or e-mail her at hahnp@egr.msu.edu.)

Consider browsing the Web sites of FHWA Pavement Preservation at www.fhwa.dot.gov/preservation, the Foundation for Pavement Preservation at www.fp2.org, and the new National Center for Pavement Preservation at www.pavementpreservation.org.

The Basic Asphalt Emulsion Manual of the Asphalt Institute and the Asphalt Emulsion Manufacturers Association, is available at nominal charge from www.aema.org, or call AEMA at 410-267-0023. A print copy is available, as well as a CD-ROM which contains all the info in the hard copy, plus video clips and other enhancements.

Reprinted from Better Roads Magazine
October 2004

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