P R OJ E C T M A N AG E M E N T
Colorado Puts Experimental Strain-Tolerant Mix to the Test
Known in Colorado as BR-ST (bituminous rich, ST 3/8 minus gradation), this high strain-tolerant mix designed to mitigate cracking through the combination of high polymer binder and specific aggregate structure. BY SARAH REDOHL
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n September 2021, Coulson Excavating Company Inc., Loveland, Colorado, placed an innovative new mix on a 4-mile section of State Highway 85 from Platteville to County Road 22. BR-ST (bituminous rich, ST 3/8 minus gradation) is a high strain-tolerant mix designed to mitigate cracking through the combination of high polymer binder and specific aggregate structure. The mix design has the same binder technology used in reflective crack interlayer (RCI) mixes, only BR-ST is a surface mix. According to Jeff Weitzel, account manager at Arkema-Road Science, Tulsa, Oklahoma, the Colorado Department of Transportation (CDOT) has used RCI since 2007 but this was the state’s first time using BR-ST. 18 | JANUARY 2022
“CDOT wanted to see how far they could move the threshold for performance of a surface mix,” Weitzel said. “They wanted to see if perhaps [BR-ST] could achieve the same or better performance as RCI followed by Superpave. That’s why they wanted to try this sideby-side comparison in Colorado.” On the two southbound lanes of U.S. 85, Coulson paved a 1-inch RCI followed by 2.5 inches of SX (12.5 mm) 100 gyration Superpave mix with 70-28 binder. The two northbound lanes were paved with 2.5 inches of SX 75 gyration mix with 70-28 followed by 1.5 inches of BR-ST. U.S. 85 made for an ideal side-by-side comparison to test out BR-ST’s crack mitigating properties. Several years ago, oil drilling operations for natural gas and crude oil in Weld
County increased with the construction of several new wells. As a result, truck traffic in the area increased by 80 percent. “They needed to find a material that would mitigate cracking and be stable enough to handle the higher traffic loading,” Weitzel said. “That is one of the main reasons why they chose this stretch of roadway for the application.” Despite severe top-down cracking, general fatigue and several areas requiring patching, the 4-mile stretch of U.S. 85 didn’t have any subgrade failures, said Coulson Excavating Project Manager John Pinello. “It’s a good road to test this type of mix on,” said Ken Coulson of Coulson Excavating, based on the pavement condition and traffic demands on this stretch of U.S. 85. “CDOT has