The asphalt industry is continually looking for ways to improve the performance of the pavement they put down. The process of spray paving initially emerged in the marketplace to help improve bonding between the existing pavement and an ultra-thin lift of asphalt. This is important because a poor bond between layers can lead to premature pavement failure.
“Subpar tack, whether through improper application or damage after application, can cause issues with the lift bonding to the underlying surface causing potholes, delamintation and cracking,” Eric Baker, vice president of sales and marketing at Roadtec says. “This is especially problematic with thin lifts where a proper bond is more critical.”
While these machines were initially designed for ultra-thin bonding courses, their use is increasing and state DOT’s are starting to notice how they can improve pavement performance. Spray paver manufacturers weigh in on the growing importance of this application method.
Create a Better Bond
When sprayed onto pavement, tack coats functions as a “glue” to hold existing and new pavement layers together. This enhances the bonding of individual pavement layers so they function as a single, unified pavement for improved strength and durability.
“A flexible pavement is essentially a laminate of multiple layers of asphalt. Tack bonds each layer together creating a uniform laminate with better loadbearing and stiffness qualities than a single layer of equal thickness could provide,” Jon Anderson, global sales consultant at Caterpillar, Inc. says. “When there are areas with insufficient or no tack, those non-uniform areas can allow layers to slip, resulting in an area of lowered stiffness that can lead to premature pavement failure.”
However, there are some issues with the traditional application of tack coats, one area where tack seems to particularly suffer is in the path of the support vehicles and traffic on the jobsite.
“When you spray tack, you need to allow a time for it to break which could be between an hour and a day depending on what part of the world you’re in,” Jim Holland, vice president of sales, Vogele, at Wirtgen America says. “During that break time and during paving, you have trucks driving on it and people walking on the job site, picking up some of that tack which can potentially cause bonding issues.”
A spray paver eliminates all that as you’re essentially combining the function of a paver and a tack truck into one by adding tack spraying equipment to the paver tractor. In most cases, an emulsion tank is placed on the paver and the spray bar is located about one foot in front of the head of HMA material in front of the paver screed. Because of the spray bar location, there is no time for the emulsion to break before the new hot mix asphalt is deposited on top of it. Also due to the location of the spray bar, no vehicle tires or paver wheels or tracks come in contact with the tack coat material and pick up the tack coat.
“With spray paving, tack coat is applied to base immediately before the hot mix such that no trucks nor equipment contact that tack coat and damage it before the mix is applied,” Kyle Neisen, product manager, pavers & MTVs at Roadtec adds. “On many traditional jobsites it’s easy to see where the trucks and other equipment have eroded the tack before hot mix is applied. This damage is concentrated in the wheel tracks where it could have been the most valuable to helping extend the longevity of pavement.”
While tracking of the tack coat decreases the bond strength, too much tack can also lead to many issues throughout the life of the pavement.
“Typically there is too little tack when it’s picked up by support vehicles, but there is also an issue where there is too much tack,” Holland adds. “When you have too much tack, the layer may not set up and that can potentially lead to asphalt layer movement. This can lead to asphalt shoving, particularly in wheel paths.”
Since a spray paver combines two pieces of equipment into one to accomplish the same job, that decreases the amount of tack being tracked on the new pavement. This can improve public perception and also streamline operations.
“With a Spray Paver there is virtually no tack outside of where it is supposed to be,” Neisen says. “Trucks and other equipment don’t drag it onto the adjacent roadways and in residential and commercial areas, neighbors and customers are not pulling tack onto the concrete driveways.”
Some manufacturers have the oil tank onboard and some have designed a module that can be placed on a paver as a module attachment. Both the paver and the modules feature an on-board product storage tank that can help contractors eliminate the need to have a tack truck on site throughout the entire day.
“This way the truck can fill the paver in the morning and then head to the next job without leaving it in an already congested area,” Neisen adds. “Because you don’t have the tack truck in the process, you can greatly reduce the length of lane closures.”
There is also the cost savings to consider by reducing the need for a tack truck.
“The cost of a spray paver is similar to the cost of a conventional paver plus a tack truck,” Baker says. “There is cost savings when you consider the fact that both the tack spraying and paving process can be done with one crew eliminating some labor costs.”
The technology on the modules can also help improve tack performance.
“The auxiliary tank is ideal for high production jobs where an insert is used to hold over 1300-gal of emulsion in addition to what is in the module,” Holland says. “Because the material is very temperature specific, we heat and circulate the emulsion inside the system to maintain material integrity. As the level drops, the sensors can turn the heat off to prevent coking and burning of this perishable product.”
However, there is a complexity to the operation of these machines so training and maintenance are always considerations before using.
“Spray pavers essentially combine two complex processes into one even more complex process,” Anderson cautions. “In many cases, they do not eliminate the tack truck completely as it is still required for refills and in some cases to maintain tack temperatures. Spray pavers can increase process complexity and increase maintenance/cleaning.”
When to Buy
Despite the complexity and with state DOTs starting to spec these machines for some projects, contractors should start considering becoming familiar with these machines.
“When considering a spray paver it is important for contractors to consider the size of jobs they hope to be working with this machinery and to make sure they choose a machine that has the capacity to meet that need,” Neisen says. “Many contractors start by subbing out new jobs to learn more about spray paving. They decide it is time to buy their own machine when they are confident the state or local agencies are going to regularly release enough work to keep that machine busy with spray paver specific jobs.”
To help increase the use of these machines, many manufacturers are designing the machines to be even more versatile the module attachment allows the machine to be used as a spray paver or in traditional paving.
“As the number of users of this equipment increases, we are evolving the module to match,” Holland says. “These machines are now easier to set up, easier to clean, easier to calibrate and easier to switch between traditional and spray paving.”
Holland says he’s also seen these machines expanded to jobs where 2-1/2-in. to 3-in. lifts are being paved proving the use of this equipment is only going to continue to grow.
“DOTs are starting to let enough work where the contractor can start to justify owning this piece of equipment,” Holland concludes. “Especially when they are getting better results.”