By: Tara Vantimmeren
The Colorado DOT has followed its trendsetting longitudinal joint density specification of 2004 with a stringent, new spec mandating condition of a milled asphalt surface. The spec determines the roughness of a milled pavement by placement of glass beads on a surface. The beads are then carefully spread out using a clear plastic disk. By the time the disk comes to rest on the peaks of the milled surface, the bead pile must have spread at least 9.5 in. in diameter, indicating a fine pattern; if they spill too quickly, not achieving the required diameter, the pavement is too rough and must be re-milled.
The new milling spec was demonstrated in early June on a night job on Denver’s I-25 expressway. There, in advance of a stone-matrix asphalt (SMA) overlay being placed by prime contractor Brannan Sand and Gravel Co., subcontractor Alpha Milling Co., Denver, was cold milling I-25 from just south of I-70 to the 23rd Street exit just north of downtown Denver.
At night, Alpha was milling four lanes of expressway both north- and southbound, while Brannan compacted the asphalt overlay using a Hamm HD O120V roller.
“We used one [Wirtgen] W 2200, three W 2000s and a W 2100,” said Larry Ware, president of Alpha Milling Co. “The 2200 had a 12.5-ft drum while the 2000 had the standard 7-ft 3-in. drum.”
In total the project involved 265,000 sq yd. of 2-in. milling, followed by placement of a 2.5-in. SMA overlay.
“At 8 p.m. they gave us one lane, at 10 p.m. they gave us a second lane and at 11 p.m. we got a third lane closure,” said Doug Jones, Alpha general manager.
When visited, Alpha began work with multiple machines on a northbound ramp onto I-25. “They began close together, but as the night went on, they got as far as a quarter-mile apart, depending on who’s taking an off-ramp and who’s taking a travel lane, for example.”
Joint compaction
“We put down a 2.5-in. SMA overlay in a 2-in. milled surface,” said Brian Knapp, project superintendent for Brannan. “Our hot-mix plant for this project was only minutes away, right at the very next exit. We were very fortunate in that respect.”
Early on, Brannan determined that oscillation compaction was the simple way to meet Colorado’s longitudinal joint density spec, to be verified by cores. Attaining adequate longitudinal joint compaction can be problematic anywhere because it usually involves placement of a lift of fresh, hot-mix asphalt against a new but cold mat. Because bonding between the longitudinal mat edges does not take place under those conditions, the longitudinal joint between the lanes becomes a focus for pavement deterioration. For that reason, Brannan was using its Hamm roller for longitudinal joint compaction on I-25.
Colorado’s longitudinal joint spec has a target 92% of theoretical maximum density, with a range of ±4%. “We put the [roller] in the middle of the paving train for our joint densities, because there are a lot of incentives to do so,” Knapp said. “With SMA it’s minimum 90% density on the joints, and we were getting 91%. The [roller’s] main focus is the longitudinal joint. For the mat, 93 to 97% is required. We were doing very well on all counts.”
Macro milling
Brannan knows the value of a good milled surface to attaining a pavement that meets smoothness specs: winning the maximum incentive payments allowed for superior smoothness on a project.
“We like the Wirtgen mills a lot,” Knapp said. “We follow Larry [Ware] and his mills all the time, and they give us a start for a smooth ride.”
The ability of Alpha’s mills to provide a perfectly planed surface for asphalt overlays is proven by their performance in meeting Colorado’s new milled surface macrotexture spec. “The macrotexture test is Colorado Procedure 77, Standard Procedure for Determination of Macrotexture of Planed Hot Mix Asphalt Pavement,” Jones recited.
“In layman’s terms, it measures the roughness of the milled pattern. It’s a CDOT spec, for the first time mandated for all federally funded CDOT projects in 2007.”
For the test, from a maximum 4-in. height, 200 ml of glass beads used for retroreflectivity of lane striping are poured onto the milled surface, then distributed evenly on the surface using a slow rotating motion with a plastic disk, until the disk rests on the points of the milled surface.
“We measure how far that 200 ml of beads spreads out,” Jones said. “It has to be within a certain range, over 9.5 inches, which is less than 0.17 macrotexture, thickness of the glass beads displaced. Anything less than 9.5 inches indicates the pavement is too rough and requires a re-mill.”
Practice makes smoothness
From the contractor’s point of view, for the smoothest pattern possible he has to make sure he has a quality mill, in good working order, and make sure he is not going too fast, Jones said.
“With our [milling machines] we have not yet failed a test,” he added. “We did a lot of practice tests leading up to the spec implementation so we know how they perform. But we hear on the grapevine that other contractors are having trouble meeting the spec.”
A copy of the spec is available from the Colorado Asphalt Pavement Association at www.co-asphalt.com/-documents/CM-MacrotextureMilledPavement-7-2005.doc.
—contributed by Tom Kuennen
