High-Performance Emulsion Boosts Urban Chip Seals
Better Roads Staff
Use of a new high-performance asphalt emulsion for seal coats is providing an urban Texas county more durable chip seals, less disruption to traffic in an urban area, and significantly fewer broken windshields.
“Chip seals are economical and are one of the better treatments you can use to preserve pavements,” said Tony Vasquez, public works operations manager, Bexar County, Tex. “With chip seals we get a thick coat of asphalt with a durable Grade 5 trap rock. With that combination our roads stay in great shape, which minimizes potholes, and the aggregate lasts from eight to twelve years.”
Bexar County applies a fundamental tenet of pavement preservation, that is, for the lowest-cost, long-term performance, treat roads before they show distress. “When we go back after that initial period the chip seal will still be in good condition, and we will apply another chip seal,” Vasquez said. “We are proactive in our pavement preservation and chip seals are part of that.”
Bexar (pronounced “bear”) County includes urban as well as rural pavements, as the City of San Antonio is located there. Vasquez is responsible for approximately 2,400 lane miles, nearly all of which are paved. “We try to chip seal about 100 centerline miles of pavement per year, between 1.5 and 2.0 million square yards,” Vasquez said. Chip seals are done by county forces.
Use of igneous trap rock is an important part of the mix, he said. Despite an abundance of high-quality limestone deposits in Bexar County – the San Antonio metro area sits at the foot of the Balcones Escarpment and is the center of an active cement manufacturing industry – the county uses 3/8-in. crushed basalt instead of limestone.
“We prefer the trap rock because it’s a harder rock,” Vasquez said. “When it’s put down it’s a light gray, but after about six months it turns black and looks like hot mix asphalt. We’ve learned constituents find that to be aesthetically pleasing. When you combine that with its superior skid resistance trap rock is the right choice.”
In the field Bexar County uses the Grade 5 trap rock with 0.3 gal/sq yd of emulsion, applying 15 to 16 lb./sq. yd. of aggregate. The placement is followed by seating of chips with a pneumatic roller. Bexar County typically does not sweep immediately after placement but does the next day.
“The emulsion breaks in minutes,” Vasquez said. “We shoot it and lay the chips after a minute. It doesn’t take long for the emulsion to break and you can walk, or drive on it in a forward and backward motion, right after the chips go down.” Turning motions will displace the aggregate, though, until the emulsion is properly cured. Traffic is permitted about a half hour after the chip seal has been rolled.
Even though the seal isn’t swept until the next day, windshield complaints are minimal, Vasquez said. “We calibrate our equipment, and when you strictly control the amount of aggregate that we put down, and use an emulsion with high chip-retention, you minimize claims. I don’t recall a claim in all 2011.”
While chip seals have been a long-time choice for preserving Bexar County pavements, last year the county enhanced chip seal performance by adopting use of a new cationic, high-float, rapid-set emulsion.
“We used it for the first time in 2011, and it’s become the only emulsion we will use,” Vasquez said. “Our paving foreman Sam Joiner commented that he has seen an improvement in chip retention, that our sweepings are a lot fewer as he does not see those chips coming off, and we are able to get traffic back on the pavement a lot sooner.”
Bexar County was using a CHFRS-2P asphalt emulsion from Ergon Asphalt & Emulsions, Inc. That language denotes a cationic, high-float, rapid-set emulsion in the No. 2 viscosity range with polymer added. In this case the polymer is Butonal NX 1122 from BASF Corporation. Previously the county used a CRS-2P emulsion.
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