Increasing quality for your customers starts with well-maintained equipment that can function the way it’s meant to

As Eric Swan pointed out in his preventive maintenance article on page xx, performing in any sector of the asphalt industry requires all the moving parts be in sync—right down to a well-oiled engine. If the broom you intended to use behind your cold planer suddenly seizes, you can’t get the fine millings up, thus the tack isn’t going to stick properly, thus the asphalt isn’t going to adhere uniformly. If this takes place on a parking lot job, you might not see an immediate financial hit from an inspector docking dollars from the project, but you might have an angry customer calling you to come back and repair potholes in a couple months.

If the striping equipment arrives at 4 a.m. to prepare the lot for the day’s traffic, but someone didn’t tuck it away correctly last night and you must replace clogged nozzles before you get started, guess whose customer is about to get mad at a delay.

If you allow material to build up on the paver’s tires, the tractor will have a bumpy ride. This means you’re not paving a smooth mat, and you could risk cooled, caked material falling from other areas onto your new mat, resulting in non-compactible areas that turn into potholes later. Photo courtesy of John Ball

What Swan said holds true: paving is a high-stakes performance of people, equipment, traffic, schedules and materials. You need your equipment to be well-maintained. That takes training of invested workers who understand the importance of clean, working machines. John Ball, the proprietor of Top Quality Paving, Manchester, New Hampshire, has preached this gospel for years. When consulting from week to week, he sees asphalt professionals of all skill levels, and he commented on what he sees in the newcomers to the industry.

“These guys out there on the crew aren’t regimented,” Ball said. “There’s a certain procedure for cleaning and maintaining the equipment, but these guys aren’t following a procedure. They’ve got to learn to care. That attitude has to be homegrown. They’ve got to be taught so they know why they’re taking time at the end of the shift to clean the paver.”

Members of this paving crew get to work with putty knives, screwdrivers and release agent to remove built up material from different sensitive areas of the endgate to ensure it can function properly during the day’s shift. Without a smoothly moving endgate, you can’t expect to have a smooth mat. Photo courtesy of John Ball

When the distributor truck driver sprays disappointing strings of tack on the surface to be paved, someone must care enough to speak up. As seen in the image above, the operator didn’t have the right pressure dialed in and had clogged nozzles on the right side of the spray bar. “You can’t have spaghetti strands,” Ball reminded readers. (See the article titled “Prevent Callbacks for Pavement Slippage” for best practices in tack applications.) But someone has to train the operator on this, and the crew—or mechanic—must know how to solve the problem.

The most important machine to check over is the paver.

No matter the size of machine you’re working with—whether that’s a sidewalk paver for a last-minute walking trail, a commercial paver handling a parking lot mill-and-fill or a highway class paver taking care of 1,200 tons on the mainline today—that machine is your lifeblood on the paving project. Its care and maintenance is the difference between achieving a quality job that gets full pay from the customer and a problematic job that gets a costly call-back to fix something later.

This distributor truck driver needs help from the mechanic to get the spray bar leveled and the nozzles cleaned properly. A member of the crew needs to be empowered to call an audible to have this area re-sprayed or the tack isn’t going to do its job. Photo courtesy of John Ball

“The paver is your bread and butter,” Ball said. “If the endgates aren’t flush with the screed, you can’t pave a good joint. If you wrap your tires with asphalt, the tractor’s ride is going to be bumpy and the screed, which is your hot iron, won’t be able to smooth it out. Good cleaning and routine maintenance of the paver will make or break you.”

Due to the importance of this topic for a contractor’s pay, we’ll return to it soon. Use the tips offered with these pictures to guide your cleaning and maintenance routine and watch for upcoming articles on this topic from Ball.

Process Heating Company (PHCo) Lo-Density® Rigid Tubular Drywell Tank Heaters provide an efficient and safe form of heat for asphalt and emulsion distributor truck tanks, according to the manufacturer. With the Lo-Density heating system, the operator simply plugs the heater in at the end of the day, and it maintains the emulsion at the correct temperature overnight or over a weekend. The result is that the truck is ready to begin work immediately in the morning, without waiting on burners to heat the asphalt at the beginning of the shift.

With no open flame, these heaters eliminate concerns associated with lighting and operating burners. PHCo’s unique drywell-style elements reside inside the heater’s sheath. The low-watt-density heaters dissipate controlled heat on the sheath, providing even heat distribution throughout the reservoir to prevent coking or damage of temperature-sensitive asphalt and emulsion material. The drywell-style heating elements also are easily accessible from outside of the tank, eliminating the need to drain the tank to service them.

The tank heaters can be installed as new or retrofit equipment. In fact, in just the past few years, distributor truck manufacturers have increasingly promoted their ability to provide an electric heat option on their units. PHCo also offers a drop-in heater option for current truck owners who are interested in converting their trucks to either all-electric heat, or a combination of electric and fossil-fuel heat.

UL-listed controls located on the distributor truck fender offer easy-to-use complete automation of temperature regulation. The main indicating temperature control regulates the product temperature and includes a high-limit control for safety. The main disconnect and individually fused heat circuits provide overcurrent protection. All components are mounted and prewired in a UL-listed, weatherproof enclosure.

Each heating unit is designed for the individual application, and can be customized to meet the specific needs of the manufacturer or user for sheath diameter, number of elements and sheath length, or applications. Rigid Heaters can be made from mild steel or various grades of stainless steel, depending on the application. The sheath wall thickness ranges up to 0.3 inch for durability and corrosion resistance. The heating units are suitable for a kilowatt rating of up to 1,000 kW; voltages up to 600 VAC, in single- or 3-phase; and watt densities of 3 watts per square inch of heater surface being standard. Terminal enclosures come standard with moisture resistance. Process Heating Company stands behind its equipment, offering an industry-exclusive five-year element replacement guarantee.

For more information, visit www.processheating.com.

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.

Distributor trucks may easily be viewed as unglamorous workhorses of paving fleets. Their typical functions are to provide heated asphalt or emulsion spray as tack coating in front of a hot mix paver or to spread heated binder for chip seal projects. In simplest terms, a distributor truck comprises a cab, a tank, a form of heat and a spray system for applying the binder.

The heating elements may be dropped in through the tank’s manhole. Holes may already exist in the baffles, or they may need to be drilled to allow the heaters to rest fully along the length of the tank. A hole may be drilled for the conduit from the top of the tank, or the conduit can exit the tank through the manhole.

Accurate heat is one real key to the binder’s success, whether the material is emulsion or asphalt. Traditionally, heat is provided to the distributor material via burners that operate on #2 fuel oil, diesel or LP gas. The operator lights the burner or burners, and heat begins to circulate through flues in the bottom of the tank to bring the binder up to the right temperature. Although the process is relatively simple, there are some inherent issues with this form of distributor truck heat that include safety concerns, reduced productivity, compromised asphalt or emulsion quality and higher operating costs.

For these reasons, one asphalt/emulsion heat option for distributor trucks that is gaining interest amongst contractors and municipalities is electric heat. In fact, electric heat can reduce or fully eliminate many of the problems associated with fossil-fuel-fired burners.

Why Electric?

Traditionally, heat is provided to the distributor material via burners that operate on #2 fuel oil, diesel or LP gas.

An electric distributor truck heating system allows the operator to plug in the heater at the end of the shift and walk away. The heater maintains the asphalt binder or emulsion at the correct temperature overnight or over a weekend, so that the truck is ready to begin work immediately as its next shift begins. Traditional distributor truck heaters require the operator to light the burners, and then the crew must wait for up to an hour as the material in the tank reaches the correct heat.

Electric heaters operate without an open flame, and do not require the presence of personnel as they maintain asphalt/emulsion heat, alleviating safety concerns. Fossil-fuel burners require strict conditions for lighting, including absence of water or condensation in the tank and the presence of at least 8 inches of material covering the heat flues in the tank. The truck must be positioned on level ground and upwind, to keep flammable vapors from igniting. With LP gas burners, most manufacturers recommend that two operators be present to light the burners in order to reduce risk of flashback. Additionally, the material must be circulated with a pump or it may create explosive fumes, and it must not be removed within 20 minutes after heating. Failure to follow safety precautions could result in an explosion.

With electric heat, drywell-style heating elements reside inside a sheath, allowing heat to be dissipated in a controlled manner, without the need to circulate the material. Especially with the increasing use of emulsions instead of asphalt for applications, controlled distribution of heat is ideal, as traditional burners and flues are hard on emulsions and often cause the water from the emulsion to separate and bake out of the material, damaging the material.

How It Works

Electric heating systems that are spec’d and installed into new distributor trucks or retrofitted into existing trucks use sheaths that create a drywell, inside which the actual heating elements reside. These sheaths are located toward the bottom of the tank, running along its length through the tank baffles. Controls for the heater are installed over the rear wheel fender.

In a new or full retrofit installation, the drywell sheaths are accessible from the outside of the tank, allowing the heating elements to be easily removed and serviced from the rear without draining the tank.

Another retrofit option available is that of a drop-in heater. This type of electric heater retrofit allows the heating elements to be dropped in through the tank’s manhole. Holes may already exist in the baffles, or they may need to be drilled to allow the heaters to rest fully along the length of the tank. A hole may be drilled for the conduit from the top of the tank, or the conduit can exit the tank through the manhole. As with a new installation, the controller can be located on the rear fender of the truck.

Additional Fuel for Thought

With electric heat, drywell-style heating elements reside inside a sheath, allowing heat to be dissipated in a controlled manner, without the need to circulate the material.

In addition to improving productivity, safety and asphalt/emulsion quality, electric heat can reduce operating costs for distributor trucks. The amount of electricity required to maintain asphalt/emulsion temperatures overnight is minimal – about $4 to $5 per night – certainly less than the amount of fuel required using burners to raise the material to the correct temperature, which can range from 5 gph to as high as 11 gph, according to manufacturer specs. Factor in the cost of just one crew member waiting on the burners to heat the material and the savings become even more apparent.

Heater sheaths create a drywell, inside which the actual heating elements reside. These sheaths are located toward the bottom of the tank, running along its length through the tank baffles. Controls for the heater are installed over the rear wheel fender.

Electric heat also offers 100 percent energy efficiency because all of the energy is used to heat. No heat or emissions are exhausted into the air. Over the lifetime of the heater, that efficiency never drops. Most fossil-fuel-fired burners, however, operate at a much lower efficiency when new, resulting in a percentage of the burner’s heat being wasted – going into the air as exhaust, along with burner emissions. The burner efficiency continues to drop over time.

Finally, because electricity heats without combustion or emissions, electric heat is friendly to the environment. It eliminates a common source of fuel spills and also eliminates stacks that must be monitored for emissions.

Whether you lay down tack prior to paving or you spread a pretty sealcoat to lengthen the life of a community’s roadways, the spraying equipment you use requires routine maintenance to stay in good condition. Any buildup of tack, emulsion, sealant, paint or other material used on the job can turn into a bad day next time you put the machine in service.

It’s self-evident that a caked up spray nozzle spewing 300-degree tack at all the wrong angles could severely burn ground laborers nearby. Sealer sprayed at ambient temperatures might not burn a worker who’s too close to the spray wand, but it can get on a pricy car that your company gets to repaint.

The reasons for keeping spray bars and nozzles clean and in good repair could go on for pages. Let’s get down to brass tacks. To keep the crew from wasting material or having an incident in the work zone, make sure routine cleaning and maintenance of spray bars is part of each shift. In fact, by making the activity of cleaning the spray bar and nozzles part of the end-of-shift routine, you make this type of maintenance easier for the crew to accomplish successfully.

“Heat helps clean things up, so waiting may make it more difficult to clean,” Jarrett Welch said. He’s the proprietor of Quality Paving Consultants, Denver. “I see a lot of distributor drivers carry a roll of wire, and they use that to poke and clean the nozzles. I have seen a few carry a coffee can with replacement nozzles, so they take it off and drop it in the can and take a clean one out to replace it.”

In other words, keep a small bucket of release agent—Welch suggested the orange cleaners work well for cleaning up tack—with the distributor truck or pavement maintenance equipment, and keep one or two nozzles “soaking” in the liquid. If a nozzle becomes clogged during the shift, take the time to stop and change out the obstructed nozzle for a clean one that won’t mess up your spray rate, coverage and pattern.

What Welch described is a practice John Ball teaches the crew to perform. He’s the proprietor of Top Quality Paving, Manchester, New Hampshire, and reminded readers to treat nozzles with care. “You can use a wire brush to clean the nozzles; it doesn’t have to be anything special. The only thing you have to watch out for is the plastic tubing. You always clean the spray bar when it’s hot. To clean the brass tips, we usually use brake cleaner out of an aerosol can.”

Don Etnyre of E.D. Etnyre shared that there are operator manuals on the company’s website that give instruction on cleanout procedures for its distributors. Specifically, check out pages 29 and 30 of the Centennial Asphalt Distributor Booklet at www.Etnyre.com. He also shared that the nozzles on the Etnyre equipment don’t require a brush or wire for cleaning under normal operations.

“During normal operations we do not clean out the nozzles,” he said. “There are two functions called ‘suck back’ and ‘flush.’ First, the operator pulls as much asphalt as possible out of the bar and circulating system using pump suction, and then flushes the system with release agent. The spray system is clean enough to be used again; hot oil is circulated, which loosens any residual asphalt. Unless there is foreign matter stuck in the nozzle, they self-clean and spray.”

Once again, temperature topped the list of priorities. “The ‘suck-back—flush’ functions are performed immediately after shooting material while everything is still hot,” Etnyre said.