The biggest change the Colorado Department of Transportation (CDOT) will pocket came in the form of quarters.
Coring the pavement more frequently led to excellence on the SH 133 job between Carbondale and Redstone, Colo., so much so that it helped win a 2016 Roads & Bridges/Asphalt Reclaiming and Recycling Association Recycling Award.
In the state of Colorado, the rule of thumb when executing a hot in-place recycling (HIR) project is to core every mile to check the asphalt mix. However, HIR has delivered some challenges statewide.
“We are working through those challenges,” Damian Leyba, project manager for CDOT, told Roads & Bridges. “It has been a learning process for all involved. You need to get the oil content right and you need to reach the right densities.”
CDOT does not spec when a contractor has to core on an HIR job. It is up to the road builder’s discretion, so when United Cos. wanted to check every quarter-mile, Leyba was intrigued.
“I asked them why and they said they would get a better mix design,” said Lebya.
It also would burn another couple of traffic control days, but United Cos. did not receive any resistance from CDOT. A contractor can always go off of historical data in Colorado, but sometimes those numbers are outdated. If maintenance crews come in and throw down a chip seal, the formula would be skewed for the HIR work.
“So coring it saved a lot of brain damage,” said Leyba. “We had an accurate reading of what was out there and the contractor then submitted its mix design to CDOT and we were able to go forward.”
CDOT contracts will read differently in the future, all thanks to the success on the SH 133 project. Contractors will now be required to core every quarter-mile when executing HIR.
Keep applying the heat
Before being rescued, SH 133 from mile markers 44 to 61 was losing its battle to the elements. Alligator and longitudinal cracking were prominent, and rutting had occurred in both the northbound and southbound lanes. Due to budget constraints, crews did some temporary patching and a chip seal to help hold it together, according to Leyba.
It was time to do something more extensive, which is why CDOT went with a $3.9 million HIR project. Not only was it more affordable than virgin construction, it also would take up far less time on a route that is only two lanes and is a recreational draw during the warmer months.
The quarter-mile coring created what Leyba believed to be one of the best mix designs in Colorado. It was a Superpave mix, HIP medium that called for 75 gyrations. The average AC content was 6.6% and the contractor used a PG 58-28 binder. The mix also contained 97% RAP, and the contractor was allowed to use 3% of his own RAP. The average aggregate size was 3⁄8 in. to a 1⁄2 in.
The Superpave mix employed on SH 133 contained 97% RAP. The average asphalt content was 6.6% and the contractor used a PG 58-28 binder.
After the mix design was finalized it was time to send in the milling machines, and that’s when another contractor had a different way of doing things. CDOT specs called for grinding 2 in. of pavement in at least two passes. Dustrol wanted to use a propane heater and take off a 1⁄2 in. in four passes. So the propane heater was followed by a Caterpillar milling machine (Dustrol used four of them), which took off the 1⁄2 in. When contractors chew 1 in. at a time the process can be very slow, but heating the existing pavement prior to milling accelerated productivity and significantly reduced downtime due to teeth replacement. CDOT called for 1 mile to be completed every day, and Dustrol was rolling through 2 to 21⁄2 miles daily. The increased production rates saved the project $25,000 in traffic-control management and inspection.
The extra heat also helped assuage the effects of the higher elevation and the lack of sunlight on the project. Because a good portion of the 17-mile stretch was in a canyon, crews would be lucky to see the sun for two hours on any given day.
“The sun hardly ever hit it,” said Leyba. “The constant heating of the mat, it helped a lot.”
Coming in behind the milling machines was a Cedarapids MS-5 asphalt windrow elevator, a Cedarapids CR 426R track paver with a 10/20 screed (paving 12 ft wide) and two Ingersoll Rand DD158 steel double-drum rollers, one serving as the breakdown and the other as the finishing roller. The temperature of the mat was 280°F to 330°F at laydown. A water truck also was used to cool down the new pavement for traffic.
“If you are heating that mat four times it’s up to the contractor to determine how creative they want to get with their densities. They could have gotten any density they wanted.”
According to Dustrol, because there were different types of asphalt layers used, the rolling pattern varied throughout. A total of 51 density readings were taken with a Troxler 3440 nuclear soil moist density gauge, with densities falling between 92% and 96%. In total the contractor laid 251,000 sq yd of HIR. Gradation tests were run every 2,000 tons on-site, and at every 10,000 tons a sample was sent to the CDOT region lab to cross-check the readings at the project site.
The contract called for two months for paving. Crews started on June 15 and finished July 10. Total savings on the job was $250,000.
A profilograph was used to check for smoothness when the job was complete, and there were no sections that had to be grinded out. Before the project, the existing pavement was averaging 102 in./mile. When work was complete the average was 48 in./mile.
“That road is going to last a very long time,” said Leyba. “It is supposed to last eight to 10 years, but now because of the densities they were reaching it should last a lot longer than that.”
Because the contractor had to close one of the two lanes to traffic, a pilot car was used to safely guide cars through the work zone.
Rock fall mitigation was conducted months prior to paving. A contractor scaled three areas of the canyon and checked for loose debris and rock.
“This project is ground-breaking for the state of Colorado. Right now we are still in the stage where we are still trying to figure things out. We encourage the use of RAP. If we can recycle asphalt, it is more environmentally friendly and we don’t have to dig into a new pit for aggregate,” said Leyba. R&B