Three Characteristics of Quality Chip Seals
Chip seals are a valuable tool in the pavement preservation toolbox where an existing pavement is sprayed with an asphalt binder, followed by application of aggregate and compaction to seat the aggregate in the binder. When performed correctly, this treatment seals and protects the underlying pavement while improving skid resistance, sealing minor cracks and surface imperfections, and inhibiting raveling.
During a recent National Center for Pavement Preservation webinar, Shawn Brost, vice president of Vance Brothers Inc., Lees Summit, Missouri, shared what he considers to be the three characteristics that must be considered to achieve a quality chip seal.
Characteristic #1: Site Selection
“The first step in creating a successful chip seal project is selecting the right road,” Brost said. He explained that chip seals can be placed virtually anywhere, but the result will only be as good as the road it is placed on.
He’s worked primarily in Missouri and Kansas, both of which use chip seals on roads ranging from interstates with up to 40,000 average annual daily traffic to subdivisions. “You can’t hardly go anywhere without seeing [chip seals] in those states,” Brost said. “The good news is you can place a chip seal anywhere, the bad news is you may not want to.”
According to Brost, much of a chip seal’s success depends on the pavement upon which it’s placed. He gave an example of a road with significant cracking and one without, stating that both could potentially be candidates for chip seals. “That’s where expectations come into play,” Brost said.
“We hear all the time, ‘I just need something to get a few more years out of this road before I do something else to it,’” Brost said. “If that’s all you’re looking for, you could probably design a chip seal to get a cracked up road through a few years.” But to maximize the life of a chip seal, the underlying pavement should still be in good condition, with only minor surface problems.
Brost said he commonly hears that roads with curb and gutter cannot be chip sealed, but that isn’t necessarily true. “We chip seal in a lot of residential settings in the Midwest,” he said. “It just involves sweeping and collecting any excess [aggregate].” Brost added that some agencies will sweep up to four times in the first 60 days following a chip seal just to make sure any loose chips are collected and taken off the roadway.
Characteristic #2: Equipment Calibration
“A successful chip seal requires that the correct materials are placed at the correct rates,” Brost said. “Your mix design isn’t going to be any good if you haven’t calibrated that equipment to know what you’re supposed to be placing is what you are placing.”
It needs to be ensured that both the distributor and chip spreader are placing materials evenly across the entire width of application. To calibrate a chip spreader, Brost said run the chip spreader over a 1-square-yard tarp, then weigh the tarp to ensure it matches the pounds per square yard stated in the mix design. “That [process] will be repeated all the way across the width of the chipper,” Brost said. For example, a 24-foot chip spreader would need to be tested eight times to ensure calibration of the entire width of the spreader. If any section of the spreader isn’t hitting the target application rate, adjustments must be made. “You wouldn’t want a situation where the right side is heavier than the left side or the gates in the middle aren’t quite opening all the way.”
A similar process can be used to calibrate the distributor. “Lay out a cookie sheet,” Brost said—or really anything flat with a determined area—and then run the distributor across it to see gallons per the given area. “Then repeat that all the way across the bar to make sure it’s coming out evenly.”
Characteristic #3: Mix Design
“The mix design will determine what quantity of materials are placed and at what rates,” Brost said, adding that the mix design can change within the project itself as conditions change. “There’s a difference between a long straight highway and an intersection.”
The core of every mix design is the median particle size of the aggregate, he said, adding that binder rates are established to achieve 60 to 70% embedment of the aggregate. “This can be a bit of a back and forth [process],” he said. For example, one might choose Aggregate A for a project, see the recommended binder rate for that particular aggregate and decide a heavier binder rate would be more appropriate. “Then, you’ll want to potentially look at a larger aggregate that would have a larger binder rate with it.”
Surface conditions should also be considered. “The relative amount of cracks and porousness of the existing road determine the rate of binder,” Brost said. For example, if applying a chip seal over a road with an existing chip seal from three or four years ago, “you need to know that road will not absorb any of that emulsion or hot rubber asphalt.” The amount of binder must be decreased to prevent flushing.
Traffic volume and type should also be considered. “It’s a bit counter intuitive, but the less traffic, the more asphalt binder you need to hold [the aggregate] in place,” Brost said. This is because in heavy traffic conditions, the traffic will continually drive the aggregate into the binder over the life of the chip seal.
According to Brost, chip seals can be used in signalized intersections so long as the proper adjustments are made. “One of the things we’ve done here for culs-de-sac and busy intersections is switch to smaller aggregate,” he said. “That aggregate is less likely to roll over than a large aggregate. We’ve had tremendous success using that smaller aggregate versus something more robust that you might want on the mainline where people are driving straight.”
For this reason, it is likely necessary to opt for a different mix design on full-width chip sealing. “The shoulder will require a different mix design from the mainline, even if it’s the exact same road surface to begin with, because the traffic count on the shoulder is extremely low and traffic on the mainline can be extremely high,” Brost said. “We can’t treat those the same.”
Ultimately, there’s more to chip seal success than meets the eye. “Where you place it matters and how you place it matters,” Brost said, “and so does how [those factors] affect the mix design process.”
Binder Options for Hot Applied Chip Seal
During a chip seal, the existing pavement is sprayed with an asphalt binder, followed by application of aggregate and compaction to seat the aggregate in the binder. Here’s where the process diverges, based on the type of binder. In the event of a water-based asphalt emulsion, the chip seal must be given sufficient time to cure before it can be broomed to remove any loose aggregate. For hot applied chip seals, sweeping follows immediately behind the rolling operation, said Robert Betsold of All States Materials Group during a recent National Center for Pavement Preservation webinar.
This is because without the emulsion’s curing time, hot applied chip seals are a thermal set application similar to hot-mix asphalt. “Everything is driven by the temperature,” Betsold said. “Once that binder cools, that chip seal is set, which allows for immediate sweeping.”
According to Betsold, hot applied chip seal binder options include performance graded asphalt (modified or unmodified), rubber modified asphalt (terminal blend, with around 5% rubber and 2% Styrene Butadiene Styrene), or asphalt-rubber binder (wet process, with around 15% rubber).
“The second and third option can get confused, but there are differences between them,” Betsold said, adding that in nearly all cases, the aggregate will be pre-coated with PG asphalt. “The main reason for that is with hot-applied, we really need to have aggregate that is both clean and dry because of the temperatures that are being applied out there.”
A construction specification for hot applied chip seals was published in the American Association of State Highway and Transportation Officials (AASHTO) 2022 Interim Revisions, section 407, which outlines the following application rates:
- PG asphalt: 0.30 ± 0.10 gal/SY; 20 to 30 lb/SY
- Rubber modified asphalt: 0.50 ± 0.10 gal/SY; 25 to 35 lb/SY
- Asphalt-rubber binder: 0.60 ± 0.10 gal/SY; 30 to 40 lb/SY
The binder types also require different minimum application temperatures (in degrees F), according to AASHTO: 375 for asphalt rubber, 350 for rubber modified asphalt, 325 for polymer modified PG asphalt and 275 for non-modified PG asphalt.