May 10th, 2011
YOU’RE SO COLDPreserve Infrastructure, by Catherine Higgins.
A cold process that grinds up and re-uses old asphalt is restoring some UDOT roads.
Old asphalt roads have basically the same content as new asphalt roads – aggregate and bonding agents. The difference between the old and new is that Mother Nature and traffic loads have taken their toll, and the old roads can become cracked and rutted. One budget-smart way UDOT rejuvenates old asphalt is by using a Cold In-place Recycling process to give new life to what was purchased long ago. Where appropriate, CIR can be used to re-surface roads or create a new road base.
The CIR process is accomplished on-site with a long train of equipment and a multi-step process. The exact process can vary from contractor to contractor and from project to project, but the core steps are:
- Removing and pulverizing – The old asphalt is milled off and pulverized. New aggregate can also be added if needed.
- Adding binder – Depending on the mix design, an emulsion or Portland cement binder is added to the mix.
- Spreading asphalt — The rejuvenated asphalt is placed on the road where the old material was just removed.
- Compacting – The newly placed material is compacted to the right density with a roller or other machine. CIR can also be used to create a road base for a new wearing course.
Advantages and disadvantages
The main advantage of CIR is the cost savings. Compared to a typical roto-mill and overlay process, CIR is about one-third the cost, according to UDOT’s Engineer for Asphalt Materials Kevin VanFrank. Since the process takes place on-site, the old material does not need to be hauled off and “all of that trucking is saved,” says VanFrank.
CIR is a cold process, so the energy used to heat Hot Mix Asphalt is also saved. There are also many CIR processes and uses, which gives contractors and engineers at UDOT options for bidding and designing good solutions for maintaining roads.
But, “it’s not HMA,” says Van Frank about the disadvantages of using CIR. HMA is stronger than a CIR asphalt road. CIR is more susceptible to the freeze-thaw cycle, and not a good wearing-course choice in cold climates. CIR is also not the best choice on high volume roads or roads used by trucks carrying very heavy loads.
HMA, a familiar and often used product, is also more predictable than CIR, which is subject to many variables, such as original mix design and aggregate size. Those variables make the characteristics of the final product a challenge to predict.
Maintenance of Traffic is also a problem with CIR since the process requires a long train of machines that can stop traffic. UDOT places a premium on keeping traffic moving during construction, so CIR is not used in high traffic urban areas.
UDOT will continue to use CIR. “It’s an effective way to grind up a surface that has no value and make use of it,” says VanFrank. CIR will continue to be an option when it comes to keeping UDOT roads in good shape.
For more information:
Contact an expert: UDOT Engineer for Asphalt, bituminous buff Kevin Van Frank
For a post about FDR, another way to re-use old asphalt, see URBAN UPGRADE.
Pavement Interactive is an online wiki for the pavement community that is loaded with good information that is fun to read, and includes this article about Cold In-place Recycling. Pavement professionals can also interact with each other online and collaborate to solve problems, seek more information or share knowledge by contributing to articles.
About the project in the photo above:
The project was 9 miles of 6” deep cut to remove the wide transverse cracking existing on the before recycle. The cracks were 14’ to 16’ spacing for the length of the project. The work operation consisted of numerous milling machines to cut the first 3” of recycle and haul to station mixing point for the top 3” of CIR. Behind the first cut of 3” a second 3” cut was performed and recycled with the mixing train of sizer, lime slurry tanker, solvent-less emulsion tanker, asphalt pickup machine and asphalt paver. — From a construction report written by Barry Sharp.