Choosing alternatives to coal tar-based pavement sealcoats: Guidance for property owners, associations and managers
When you are considering a surface seal for short-term pavement maintenance, the Minnesota Pollution Control Agency (MPCA) urges you to seek out alternatives to coal tar-based sealcoats which are safer for human health and the environment. Protect your environment AND your pavement.
Please send information on new products or suggestions on this guidance to Al Innes at firstname.lastname@example.org.
Disclaimer (click on bar to expand)
The MPCA is not positioned to evaluate, recommend or provide a list of specific products. Product attributes and best practices provided here come from the input of pavement experts, but their individual recommendations — and those of other experts and contractors — might vary somewhat from this Guidance.
It is the buyer’s responsibility to be informed and ask good questions about products and application practices.
Sealcoat definitions (click on bar to expand)
“Sealcoats” are mixtures of liquids and solid additives which are brushed or sprayed on asphalt pavement. They may include a layer of gravel but most homeowners understand the term as liquid-only. Properly applied, they cure into a layer which seals out moisture and oxygen which break down the underlying asphalt. They are a lower-cost treatment which can last two to four years, depending on the product, and extend the life of the pavement. Some owners use them to maintain a uniform black appearance to their paved surfaces.
You could hear several terms used for maintenance products which are applied on top of asphalt pavement and generally fall under the heading “sealcoat”. Most now on the market are emulsions, a mixture of liquids that would separate without an additive (emulsifier) that allows them to mix and stay mixed. Pavement experts in some regions of North America use the term “surface seal” instead of “sealcoat.” This guidance will use “sealcoat.”
The following are binder oil and water emulsions, in order of increasing upfront cost and life:
- Fog seal — the sealcoat option used most by homeowners or others with a tighter budget is typically a form of fog seal emulsion of about 30% binder and 50% or more water, which may have polymers added to enhance product performance:
- Asphalt (petroleum-based) emulsion
- Coal tar pitch emulsion
- Gilsonite© emulsion
- Acrylic emulsion
- Slurry seal — an asphalt emulsion with aggregates, mineral fillers and additives which bonds to existing pavement and cures to form a new wearing surface.
- Chip seal — combines a layer of asphalt emulsion (about 60% asphalt and 40% water) and a layer of fine aggregate on top of the existing pavement layer.
For sealcoat emulsions with water, the water makes them easier and safer to spread, and when it evaporates, the thorough mixing allows the remaining binder oils and solids to cure in a uniform layer which seals out moisture and oxygen which break down the underlying asphalt.
Some newer types of sealcoats contain no water, with the liquids being agricultural, plant-based oil and various types of hydrocarbon solvents, usually with some polymers. Some of these are designed to replace lost or oxidized oils and restore flexing and binding properties of the asphalt pavement.
Many people use “sealant” and “sealcoat” to mean the same thing, but the industry uses “sealant” materials to fill cracks ¼ to 1 inch wide.
More on why to move away from coal tar (click on bar to expand)
Based on the pitch residue from the coking of coal for steel production, coal tar sealcoats can cause cancer and developmental impacts in aquatic organisms and people due to chemicals they contain: polycyclic aromatic hydrocarbons or PAHs. These PAHs escape from the sealcoat layer as it dries and later, as it breaks down.
PAHs escape from coal tar sealcoat and other pavement materials when they are scraped, washed or blown into stormwater holding ponds and other waters receiving rain and snow runoff. The PAHs build up in the sediments that drop to the bottom of these water bodies.
Minnesota researchers have traced about 54% of the PAHs in Twin Cities stormwater pond sediment to coal tar-based sealcoats.
Cleaning out and properly managing highly-contaminated stormwater sediments in Minnesota is expensive: up to three times the cost of low-contamination, unregulated sediments.
While there are other active sources of PAH contamination such as wood fires and vehicle emissions, this is one of the more easily-preventable sources. In addition, safer substitutes are now available which are comparable in performance and price.
Most highway departments and road agencies have not used coal tar products on asphalt pavement for many years, and many experts say they reduce asphalt life in the long run.
Minnesota lawmakers banned State agency purchases of coal tar-based products in 2009, and many Minnesota cities have banned sealcoat with coal tar-based content since then. See the latest list of cities.
More detail on each of these main points is presented below.
- Monitor your pavement’s condition and invest in treatments before significant deterioration occurs.
- With your pavement condition in mind, talk to two or more sealcoat contractors or pavement engineering consultants with experience using safer alternatives to coal tar like petroleum asphalt-based products.
- Work only with providers who can show you complete product data sheets so you can avoid coal tar content and make sure you are choosing safer alternatives.
- Be aware of best application practices and make sure they are used.
- Thoroughly repair and prepare the surface.
- Follow the manufacturer’s recommendations for mixing, application rate, and number of coats needed for your conditions.
- Apply sealcoat only under the right weather and temperature conditions.
- Understand what the final result should be: when fully cured, the sealcoat should have uniform coverage and appearance, with all aggregate and spaces between covered without gaps or pinholes, as shown in the photo below.
The first step to protecting your asphalt pavement is monitoring it closely, year-in and year-out. Watch for signs of loss of asphalt binder between the small stones (“aggregate”), and loss of the aggregate itself - see examples in Reference 2, pages 47-49 (4.11-4.13). Also look for the beginnings of fine cracking and take action before they widen. If you have new pavement, treat-ments can start any time after the first year.
While a sealcoat is a less-expensive, short-term treatment option, it may not be the most protective and cost-effective in the long run for your pavement’s condition. If your pavement is older or has already deteriorated, a different treatment or total replacement with asphalt or other materials may be the best option.
Talk with paving engineers or contractors about the history of damage (fuel or oil spills or drips, extreme heat and rutting, severe freeze/thaw conditions, overhanging vegetation, etc.) and what future threats you need to protect against. Consider also whether your primary concern is the visual appearance versus preservation.
If a sealcoat is determined to be the best option, then you need to work with your professional(s) to choose the lowest PAH sealcoat possible which will perform well for protection and preservation. They can also help set up the timing and process of application.
More on information needed (click on bar to expand)
If your primary concern is appearance, you may be applying sealcoat every 1 to 2 years regardless of performance or need. However, applications this frequent reduce adhesion to the pavement, meaning sealcoat will wear off and track away more easily (including indoors).
For commercial owners or managers
(Or others with larger budgets), a consulting engineer can provide independent analysis of your pavement’s condition, and what products will work best for its preservation needs and your budget.
Consultants providing this service for commercial customers can be found at the American Council of Engineering Companies of Minnesota directory (with the directory open, type “pavement” into your browser’s “Find” function).
(Or others with a tighter budget) consultant services may be too costly. If this is the case, then be sure to interview two or more contractors for an analysis of pavement condition, products, and costs.
As you do this, be aware of local requirements. In some cities with coal tar bans, violations of coal tar bans can be enforced against the property owner and the contractor, if one is involved. See the latest list of cities.
You should request a safety data sheet with clear ingredient identification and percentages. If a safety data sheet is not available, ask for more information or move to a product or supplier who offers one.
It is the buyer’s responsibility to look at data sheets and talk to providers about key ingredients for protection and manufacturer’s recommendations for optimal application.
Do not select products if their safety data sheets, container labels, or technical bulletins show the Chemical Abstracts Service (CAS) numbers 65996–93–2, 65996-89-6, or 8007-45-2, or the words “coal tar,” “refined coal tar,” “refined tar,” “refined coal tar pitch,” “coal tar pitch volatiles,” "RT-12," "tar" or similar terms.
You may see coal tar/asphalt blends offered with as low as 10% coal tar content, but even at that level, PAH content is around 100 times higher than asphalt-based sealcoat. These blends should be avoided.
The most common and cheapest alternative to coal tar now on the market is petroleum asphalt-based sealcoat (CAS number 8052-42-4). Asphalt sealcoats contain PAHs, but at as little as 1/1000th the PAH level of coal tar sealcoats. Good asphalt sealcoat emulsions are very affordable, will provide a black appearance for 1-2 years, and can provide less-visible protection for 2-4 years if properly applied. They are permitted in locations with bans on coal tar.
Asphalt sealcoat data sheets may give ranges such as 10-30% for asphalt content, but ask for 30%. Polymer content of 2.5% to 5% aids drying, adds flexibility, and helps retain aggregate (chip or gravel) if you are applying a chip seal.
Other alternatives such as Gilsonite®, acrylic and agricultural oil-based seals contain few or no PAHs, but they tend to be higher-priced and they have less of a performance track record than asphalt seals. Even so, you should ask about these products and their effectiveness and cost as research and market conditions evolve.
More on safer alternatives (click on bar to expand)
It is the buyer’s responsibility to look at data sheets and talk to providers about key ingredients for protection and manufacturer’s recommendations for optimal application:
- For asphalt fog seal-sealcoat products, look for the binder component (asphalt) to be 25-30%. Many data sheets will give wide ranges such as 10-30%, but be persistent in asking for better information and higher asphalt content;
- The asphalt emulsion for chip seals (with a layer of gravel) should be around 65% residual asphalt;
- For asphalt-based sealcoat emulsions, water content as supplied will be on the data sheet, and the manufacturer may also provide guidance on limits to the final dilution an applicator can do to aid in spraying or spreading. Be aware of these guidelines but recognize that applicators may still vary dilution. The bottom line is that when cured, the sealcoat should have uniform coverage and appearance, with all aggregate and spaces between covered without gaps or pinholes;
- Polymers are commonly used in sealcoats but not essential. Rubbery polymers like styrene/butadiene, isoprene, or neoprene increase flexibility to stand up to extremes of heat and cold, and they help the sealcoat set faster which is essential when rain, cold or darkness are impending or in commercial parking lots where traffic needs to get back on the pavement as quickly as possible. If you need these properties, your asphalt sealcoat should contain 2.5 to 5% polymers. Note that many highway and road agency experts and purchasers specify a polymer-modified binder in fog and chip seals;
- Clay, mineral, quartz or similar materials add strength and are typically around 20% by weight;
- Carbon black, slate and other mineral additives will darken the coating (if desired);
- Not listed on data sheets will be sand, often added by a contractor at 3 to 6 pounds per gallon to fill a rough surface, or to provide traction to newer or smoother pavement. However, be aware that as the sealcoat binder starts to wear away, the sand released will cause greater abrasion under tires and accelerate the wear rate of the coating.
For larger commercial parking lots, chip seals with gravel are slightly more expensive than sealcoat with no stone but can last twice as long as an asphalt sealcoat with no gravel.
Gilsonite® sealcoats typically have some petroleum asphalt content, so will be another low-PAH option. However, they may also include naptha, mineral spirits or other solvents which may be a local regulatory (ground-level ozone or smog formation) concern. It is often recommended they only be applied on unsealed or asphalt-sealed pavement; not onto coal tar or acrylic-sealed surfaces. Gilsonite-based sealcoats may be more than twice the cost of asphalt-based sealcoats, but experiences vary on how much extra life they offer.
The no-PAH options include acrylic-based and agricultural oil-based sealcoats, and cement-based micro-layers. These are not as readily available as asphalt-based sealcoats, and there is less long-term experience or research showing how they protect and preserve pavement, especially for retail acrylics applied by homeowners. Even though somewhat higher in initial cost than asphalt-based sealcoats, professionally-formulated and -applied non-asphalt sealcoats may be worth considering in some applications.
There are links to a small sample of studies of no-PAH products at the end of this guidance, but with all the products and emerging research, look for pavement experts to help you stay current. Suppliers should contact the Minnesota Department of Transportation Office of Materials & Road Research for recommendations on testing new products.
With zero-PAH products, it is important that you accept some uncertainty about the longevity of the treatment. Get advice from more than one source with experience using these newer products, and make sure to follow manufacturer’s guidance on compatibility with the surface and its condition, and rates of application. Improperly applied, they might ruin your pavement or reduce its life.
If one is offered at all, the length of product warranty can vary widely for sealcoats, whether store-bought or supplied by a contractor. For instance, one survey of asphalt-based retail products found that warranties varied from 1 to 10 years. Ask if a warranty is offered, get it in writing, and compare product claims whenever possible.
Best application practices
If you are going with sealcoat for preservation and have chosen your safer sealcoat product, you must determine if you are equipped to do the job well yourself or whether a contractor is needed for some tasks (for example, cleaning, routing and hot-sealing cracks). There are many products which can work, but good surface preparation and proper weather conditions during application and drying are essential.
- Clean cracks under 1 inch wide thoroughly and fill them with a durable and flexible rubberized sealant which will adhere tightly to the asphalt. Cracks larger than 1 inch wide should be filled with new asphalt pavement;
- Remove and clean loose and foreign material including past sealcoat layers, fuel and oil leaks, and spill spots, then prime or seal;
- Follow the manufacturer's mixing, water dilution, and application recommendations closely;
- If more than one coat is required, follow the manufacturer’s recommendations for time or dryness required before applying a second coat. This will vary depending on weather conditions;
- Application of asphalt sealcoat should take place only if the air temperature is at least 55°F and rising, approximately May through September;
- Asphalt and other sealcoats should not be applied at night or when rain is expected in less than 3 hours, in foggy conditions, or when the pavement surface has standing water. Turn off or cover all sprinkler irrigation systems.
More on best application practices (click on bar to expand)
While a contractor may suggest sealcoating every two to three years, doing it too often can cause build-up, flaking or peeling. Waiting five years or more means most of the old application is worn off high areas and what remains will stay in place underneath a new coat. If you’re worried about whether a new coating will stick, pre-test a small area. Here’s additional information on applying sealcoat:
- Make sure the applicator follows the manufacturer's mixing, water dilution and application recommendations closely. Let them know you expect full and unbroken coverage when the coating is fully cured (as much as 90 days after application) or you will expect them to return, prepare the surface properly, and apply more sealcoat at no extra cost;
- If more than one coat is required, be sure you or the contractor follows the manufacturer’s recommendations for time and surface condition required before applying a second coat. If drying conditions are good and the surface area is big enough, contractors can work around the entire surface with the first coat and have it be dry enough that they can immediately start applying a second coat where they began. This may not be possible in other conditions, to the extent that if a return trip is required, it may increase a contractor’s quote. Make sure this is clear before accepting a bid;
- Application should get under way only if the weather radar and forecast are indicating more than three hours before rain. If it does rain unexpectedly, work should stop as soon as showers begin. Before accepting a contractor’s bid, make it clear that if rain does fall on fresh sealcoat, they must contain it and clean it up, and return under dry conditions to prepare the surface again and re-apply at no extra cost;
- Asphalt and other seal coats should not be applied at night. Not only will temperature, wind, and humidity be less ideal for drying, but there will be no ultraviolet light to help cure the sealcoat. Commercial parking lot owners who have sealcoat applied at night to avoid parking disruptions take a risk that the coating will fail or wear out quickly..
Working with a contractor
- Start first with the interactive map of contractors who have pledged not to use coal tar sealcoats;
- Or use the electronic Yellow Pages, searching for “sealcoating” or "driveway contractors" near your city or town;
- Look for ones with established local listings and check the Better Business Bureau;
- Get proof of commercial (for any damage) and workers compensation insurance;
- Reputable contractors who are experienced in applying alternatives to coal tar will stand behind their application for at least a year.
More on working with a contractor (click on bar to expand)
If you contracted for a product other than coal tar but suspect coal tar has been applied, call the contractor and if not satisfied, lodge complaints with city staff and the Better Business Bureau. If you believe a contractor on the interactive map or list of contractors has applied coal tar, call MPCA at 651-296-6300 or 800-657-3864 and ask for Pollution Prevention staff.
- Remember that the interactive map of contractors only reflects the contractors’ commitment not to apply coal tar. It says nothing about the quality of their work;
- Be wary of quotes less than 20 cents per square foot.
- Be aware that the sealcoat will shrink in thickness over 50% by the time it’s fully cured (which could be up to 3 months if conditions aren’t very good). Let your contractor know you will make your final assessment of their work then.
Ask about the contractor’s guarantee and get it in writing. This will protect you should final curing not show the complete coverage suggested earlier, if weather conditions prevent proper curing or if some other early failure should occur. Reputable contractors who are experienced in applying alternatives to coal tar will stand behind their application for at least a year. Note that if you insist on overnight, early-season or late-season application, they may refuse to offer their guarantee since adhesion and curing is much less certain.
Some data sheets for products used in the Upper Midwest will cite California or Asphalt Sealcoat Manufacturers Association specifications or application recommendations. However, pavements, materials used, and weather conditions in this area are different than in the West, so take care to make sure materials and practices appropriate for this area of the country are used (see References for Minnesota advice more applicable to the Upper Midwest and Great Lakes regions).
If you believe a contractor has applied coal tar, there will be a strong odor like mothballs for 1-2 days. Call city offices if you live where a coal tar ban is in place.
If you’re not in a ban city and you contracted with someone not on the MPCA pledge list for a product other than coal tar but suspect coal tar has been applied, your only recourse is to call the contractor and if not satisfied, lodge complaints with city staff and the Better Business Bureau.
Be aware that the condition of your pavement will dictate how much sealcoat is required. Make sure all estimates are based on the same assessment of condition, and the intended application rates and number of coats. Do not accept a lower price estimate based on under-applying the product.
A 1,000 square foot area with a smooth surface will cost about 25 cents per square foot ($2.25 per square yard) or $250; a rough surface adds up to 10 cents more per square foot, or $100 in total.
Larger jobs get a lower per-square-yard price since the basic cost of mobilizing a crew, equipment and materials is spread over a larger area. Quotes for large parking lots are in the $1.00 to $1.50/square yard range for sealcoat; $1.50 to $2.25 for chip seal.
More on application costs (click on bar to expand)
For homeowners with smaller surfaces
Comparison of 2012 retail prices shows asphalt-based sealcoats averaging about $3.60/gallon; acrylic-based at an average of $8.00/gallon, and; Gilsonite®-based at an $8.40/gallon average. MPCA analysis suggests that sealcoat cost to contractors is slightly under retail.
Contractors may quote prices per square yard. To convert your pavement area in square feet to square yards, divide by 9: for example, 1000 square feet divided by 9 equals 111 square yards. To convert square foot price to square yard price, multiply by 9: 25 cents per square foot becomes $2.25 per square yard.
Costs for owners with larger surfaces
Here’s a comparison of large-surface costs for good condition pavement (remember that costs per square yard will be higher for smaller surfaces or poor condition pavement):
- Asphalt fog seal/sealcoat jobs can be in the $1.00 to $1.50/square yard range;
- Chip seal (asphalt emulsion with fine aggregate embedded into it), in the $1.50 to $2.25/square yard range;
- Gilsonite® and acrylic sealcoat, in the $3.00 to $7.00/square yard range (smaller jobs will cost more, towards the high end of the range);
- Agricultural oil seals, in the $1.70 to $2.00/square yard range.
- Moving to Safer Alternatives to Coal Tar Sealcoats
- Minnesota Seal Coat Handbook 2006, Minnesota Local Road Research Board, MnDOT Research Services Section
- Minnesota Department of Transportation, Standard S-142: Bituminous Seal Coat (replaces MnDOT 2356)
- Minnesota Department of Transportation, Standard 3151: Bituminous Material (commonly referenced by other state DOTs)
- The Asphalt Institute, A Basic Asphalt Emulsion Manual (2012)
Sample Studies of No-PAH Alternatives
- Minnesota Local Technical Assistance Program http://www.mnltap.umn.edu/about/programs/opera/fact/documents/hutchinson.pdf
- Engineering Brief No. 62: Polymer Composite Micro-Overlay (E-Krete™ TOL 3000) For Fuel-Resistant Wearing Surfaces, October 2002 http://www.faa.gov/airports/engineering/engineering_briefs/media/EB_62.pdf
- Fuel Resistant Sealers and Binders for HMA Airfield Pavements; Airfield Asphalt Pavement Technology Program (AAPTP) http://www.aaptp.us/Report.Final.05-02.pdf
- Minnesota Department of Transportation Office of Materials and Road Research
- Inspec Inc.
- Larson Engineering Inc.
- Gale Tec