Asphalt emulsion bond coats glue layers of pavement together, give pavements durability by helping them resist shear forces. Here we’ll talk about why you need to tack before paving.

By Tom Kuennen

Asphalt emulsion bond coats stop delamination and movement of conventional or thin asphalt overlays when they are placed over bituminous, Portland cement concrete or cold-milled pavements.

Also called “tack” coats, bond coats adhere a fresh layer of asphalt to a lower layer of pavement.

Today, according to the Asphalt Pavement Alliance, some 94 percent of America’s roads are surfaced with asphalt. But if a strong bond is not created between the layer on which the asphalt driving course is placed, be it asphalt or concrete, the new asphalt surface is at risk of failure.

Thin veneers of wood are glued to each other to form a strong sheet of plywood. Like plywood, asphalt pavements are built of layers, and their bonding is critical to good performance. “Building asphalt pavements in layers provides a number of advantages, [as] each layer can be engineered to have specific characteristics and properties,” the National Center for Asphalt Technology (NCAT) reported in Spring 2015.

For example, surface layers may be designed to provide high levels of friction, be porous to drain water and eliminate hydroplaning, and optimized to provide low levels of tire/pavement noise, NCAT said. “Intermediate layers can be designed to be highly crack-resistant if placed over an existing pavement with joints or cracks, or they can be designed as high-modulus layers to provide structural stiffness to the pavement to minimize tensile strains that lead to fatigue cracking, and limit high compressive strains that lead to permanent deformation in lower unbound layers.”

Regardless of function, if the layers aren’t bonded together, the interface between layers becomes the weak link in the system, and the pavement will not perform as intended. “Although we can design, construct and rehabilitate each layer independently, the layers must be bonded together for the pavement to respond to loads as intended,” NCAT said. “That is the purpose of tack coats.”

So-called ‘zebra’ tack pattern will achieve less than 50 percent available adhesion,
leading to potential overlay failure. Photos courtesy Asphalt Institute.

Can’t Afford Not to Bond

If layers are insufficiently bonded, traffic load-driven shear forces within the driving or friction course may cause it to begin disintegrating. A commonly observed problem related to poor bond between layers is slippage failure, which may occur at locations where traffic accelerates, decelerates or turns, NCAT reported. Other ills include delamination and a type of fatigue distress sometimes referred to as alligator cracking.

Yet, the cost of a bond coat represents a very inexpensive kind of insurance against much more costly fixes on delaminated or slipped friction or riding courses. Sadly, bond coat application is inconsistent and varies widely among both contractors and road agencies.

Shear strength is highest in top bituminous layer, suggesting need for firm bond between layers provided by properly executed asphalt emulsion bond coat Image courtesy Louisiana Transportation Research Center

Some local road agencies don’t provide detailed bond coat specs. In its own way, at least one DOT doesn’t require them on milled surfaces. If tack coats are not a contract pay item, their application may be at the mercy of the contractor, who might consider bond coats an additional expense.

“An analysis of bid tabs shows that the cost of tack coat is relatively minor compared to other components of a typical paving project,” consulting engineer Dale Decker said. “Therefore, the risk of a poor or a compromised bond from not applying tack should be deemed too great when one considers the ramifications of the loss of fatigue life from this condition. Tack is such a cheap component that it makes little sense to shortchange this critical step and introduce so much risk.”

Importance of Bond Coats

The need for asphalt emulsion-based bond coats to be used consistently and placed full lane-width was emphasized in a technical presentation late in 2016.

Interlayer bonding is essential to optimize bituminous overlay performance, said Louay N. Mohammed, Ph.D., Louisiana Transportation Research Center of Louisiana State University, in his paper The Effect of Tack Coats on Asphalt Pavement Performance, presented at the International Symposium on Asphalt Emulsion Technology, Arlington, Virginia, Nov. 1 through 4.

In the paper co-authored by Mostafa Elseifi, Ph.D., and Ramendra Das, Mohammed spells out the whys of optimal bond coat placement and the problems of inadequate placements.

“Tack coats are used to ensure a bond between the surface being paved and underlying course,” Mohammed said in November. “Interfacial bonding is necessary to transmit traffic loads down through the whole pavement structure. If layers are not properly bonded, their tendency toward debonding—delamination, slippage or sliding—or fatigue cracking is increased, leading to failure in the new overlay.”

Mohammed’s work at Louisiana Transportation Research Council culminated in the publishing of the document, National Cooperative Highway Research Council (NCHRP) Report 712, Optimization of Tack Coat for HMA Placement (download your copy here).

Executed within NCHRP Project 9-40, NCHRP 712 looks at how bond coats are applied today, and establishes best practice guidelines.

In the past, application of tack or bond coats mostly has been a subjective event, but things are changing, the report says. “In general, selection of tack coats has been mainly based on experience, convenience, and empirical judgment,” NCHRP 712 says. “In addition, quality-control and quality-assurance testing of the tack coat construction process is rarely conducted, resulting in the possibility of unacceptable performance at the interface and even premature pavement failure.”

NCHRP 712 suggests the best application methods, equipment types and calibration procedures, application rates, and asphalt binder materials for the application of bond coats, and recommends revisions to AASHTO methods and practices.

It also describes two new devices for testing of bond coats, the Louisiana Tack Coat Quality Tester (LTCQT) for field use, and the Louisiana Interlayer Shear Strength Tester (LISST), for laboratory use.

Recommended tack (bond) coat application rates from NCHRP Project 9-40 Image courtesy Louisiana Transportation Research Center

The LTCQT evaluates the quality of tack coat spray application in the field, and can serve as a valuable tool for highway agencies to perform comparative evaluations of various tack coat materials and application methods and rates in the field. To complement this device, the LISST was developed to characterize interface shear strength of cylindrical specimens in the laboratory.

Why Asphalt Emulsion Bond Coats?

For bonding of asphalt layers, asphalt emulsions are preferred over both “neat” liquid asphalt, and “cutback” asphalts.

In a thorough, worldwide survey conducted for NCHRP Report 712, the authors found that 100 percent of the respondents—including 46 of the 50 states—allow the use of asphalt emulsions for bond coats. By comparison, only 27 and 20 percent of respondents allow paving grades and cutback asphalts, respectively. Worldwide, the survey found some 92 percent of bond coats are of asphalt emulsions.

An asphalt emulsion for bond coats or surface treatments is a homogeneous mixture of two insoluble substances, oil and water. In it particles of liquid asphalt (the dispersed phase) are surrounded by molecules of water (the continuous phase). Compared to hot liquid asphalt, asphalt emulsions have greatly reduced viscosity, are safe to use at lower temperatures, and allow expensive liquid asphalt to be spread more thinly, saving money in most cases.

Emulsified asphalt is produced by dispersing tiny globules of asphalt cement into water treated with a small quantity of emulsifying agent. The dispersion takes place in a powerful blender, called a colloid mill, where spinning blades break or shear the liquid asphalt into suspended microdroplets.

The emulsifier—commonly a surfactant (detergent) or surface-active agent—maintains the microscopic asphalt droplets in a stable suspension within the water, keeping them from recombining. Emulsifiers also help to control the “break” time following placement on a road, in which the water evaporates, leaving the asphalt behind. These are categorized as rapid setting (RS), medium setting (MS), slow setting (SS) and quick setting (QS) emulsions.

So-called ‘zebra’ tack pattern will achieve less than 50 percent available adhesion,
leading to potential overlay failure. Photos courtesy Asphalt Institute.

Temperature Variations

Application temperature differences between pre-placement asphalt emulsion bond coats exist, but don’t play much of a role in placement or performance, so long as the temperatures remain in the specified range.

“Different formulations for bond coats may be required for different applications,” Mark Ishee said. He’s the vice president, Ergon Asphalt & Emulsions Inc. “Especially when you get into trackless materials with their varied formulations. As well, the raw materials used to manufacture bond coats can be different. Bond coats are no different from chip seal emulsions obtained from two suppliers; they are usually not exactly the same, as different suppliers may use different asphalt bases, emulsifiers and different formulations. As it relates to comparing application temperatures, there is usually no real difference between suppliers,” Ishee told AsphaltPro.

“No matter which company manufactured the emulsion, it’s going to start to lose water above 180 degrees F and it needs to be above 120 degrees F or so to allow the material to flow freely through the pumps and distributor nozzles.”

“You may have slightly different recommendations between manufacturers for the application range; one may suggest the range be between 130 and 150 degrees F; another may recommend a range of 140 to 160 degrees F,” Ishee said. “The rule-of-thumb from the Asphalt Emulsion Manufacturers Association is not to exceed 180 degrees F. But the suggestions and recommendations of the manufacturer should be considered.”

Additives Less Significant

Will emulsions—and potential additives or modifiers—be different for use ahead of a mill-and-fill project, as opposed to a Thinlay asphalt overlay on a diamond-ground PCC pavement?

“Ultimately, asphalt emulsions are all about moving asphalt from Point A to Point B,” Ishee said. “The emulsifiers used in a bond coat emulsion are not usually specifically designed for performance of the residual asphalt itself, but to keep the asphalt in emulsion form long enough to get from the plant to the job site. Once applied and the water’s evaporated, the residual asphalt can perform its intended task. There may be a difference in the amount of residual asphalt required for different paving techniques, but for a diamond-grind, versus a regular milling operation versus a non-milled project, the standard bond coat formulation is going to remain the same.”

Louay Mohammed, Ph.D., presents rationale for best practice bond coats at International Symposium on Asphalt Emulsion Technology November 2016. Photo courtesy Tom Kuennen.

When you get into the trackless tacks, and specialty bond coats, that may change, but for conventional bond coats there’s little difference.

Because the liquid asphalt in asphalt emulsion bond coats serve as the glue that bonds an asphalt overlay to the underlying substrate, does this stickiness not pose a problem for placement and distribution, for example, through the nozzles of a spray bar?

“The ‘stickiness’ is a property of the asphalt, not the emulsion itself, and consider that the equipment is designed to apply asphalt binder,” Ishee told AsphaltPro. “Typically, the specification related to application will be included in the project plans, and the contractor will follow those requirements on behalf of the agency or client. As long as the emulsion is within the application temperature range and reasonable handling practices are followed, the liquid asphalt in the emulsion won’t gum-up the distributor pump or nozzles.”

Once the emulsion breaks—as the water component of the emulsion leaves the system—stickiness becomes an issue, Ishee said. “Stickiness or viscosity of the residual asphalt is not a consideration so long as you still are working with an emulsion,” he said. “But once the emulsion breaks and the system begins reverting back to a liquid asphalt state, it will retain the properties of the base asphalt used to make the emulsion. Usually, that means it will become sticky, a desirable trait for a conventional bond coat.”

For more information about bond or tack coats, or about asphalt emulsions, visit the Asphalt Emulsion Manufacturers Association at www.aema.org.