Last year, Los Angeles saved $2.4 million on rebuilding 30 miles of asphalt streets.
LeSueur County, Minn., has virtually eliminated reflective cracking on reconstructed asphalt roads.
And Harford County, Md., saved $730,000 when rebuilding an 8.2-mile asphalt roadway.
What's the common denominator in these examples? It's a process known as cold in-place recycling (CIR), or a variation of it called full-depth reclamation.
With cold in-place recycling, a portion of asphalt pavement section is pulverized, then typically mixed with an asphalt emulsion, repaved, and compacted. Full-depth reclamation pulverizes the entire thickness of asphalt pavement and mixes the recycled material with the aggregate base. In both cases, roads agencies typically add a hot-mix asphalt overlay.
Both processes have been around for years. But because of institutional momentum that favors hot-mix asphalt, or lack of knowledge or experience with cold in-place recycling, CIR isn't common.
“I love cold in-place recycling. It works great,” says Darrell Pettis, highway engineer for LeSueur County, which has cold-recycled 48 of its 300 miles of asphalt road.
Pettis uses CIR on roads that are in fair to poor condition, and expects the process to give them 15 to 20 more years of life. The county's first cold-recycled road was built in 2002 and is holding up much better than a comparable road given only a milling-and-resurfacing treatment that same year.
The L.A. Way
In Los Angeles, where the Bureau of Street Services maintains 6500 centerline miles of streets, 1000 miles of residential streets are failing.
In response, the city tried out full-depth reclamation on several successful demonstration projects, and bought a 2200 CR cold-recycling machine made by Wirtgen America Inc.
The machine pulverizes asphalt pavement and injects a predetermined amount of cold water into hot penetration-grade asphalt in the mixing chamber. The resulting foamed asphalt spreads throughout the pulverized material according to the bureau's mix design. At the rear of the machine, a paving screed spreads the material 7¼ feet wide. The city typically adds 2.5% to 4% foamed bitumen and 1.5% portland cement to the mixture.
Bureau director William Robertson estimates reclamation saves the city 25% to 35%, primarily by eliminating all the trucking required for conventional removal and replacement: hauling out the pavement millings, removing the old pavement subbase, and bringing in new base stone and hot-mix asphalt.
This year, Los Angeles has budgeted $70 million for pavement preservation. Estimated savings from using cold in-place recycling: $1.9 million. The city spends the savings from cold-recycling—in the same year they are realized—on conventional milling and resurfacing. That means the city can do 10 more miles of traditional maintenance overlay projects.
Each year Los Angeles spends 80% of its funding to save streets with the conventional mill-and-overlay process. The remaining 20% goes for reconstructing streets with full-depth reclamation. “It's like triage,” says Robertson. “We're trying to save as many streets as possible before they need complete reconstruction.”