Poor roads and not enough funds to fix them. "Not in my back yard" responses to requests for new permits for quarries and asphalt plants. Dwindling supplies of virgin aggregates. Expensive freight costs for paving remote locations. Environmental concerns. Cracked, rough pavements.
What's the answer for these problems? Cold in-place recycling. Many agencies, including the Montana Department of Transportation (MDOT), are using new recycling technology as the solution. On Jan. 1, 2001, MDOT fully implemented a maintenance program for 5,300 lane miles of secondary roads. In the past they have found that traditional hot-mix asphalt thin overlays do little to prevent the recurrence of thermal and reflective cracking. In 2001, MDOT did one of the first projects using this technology near Red Lodge and recently completed another project on a 61/2-mile section of secondary state highway 279 northwest of Helena. The project was chosen as a trial area to determine if the partial depth cold in-place recycling would reduce the amount of thermal and reflective cracking and be a cost-effective pavement maintenance technique.
They thought of it first
Approximately 50% of U.S. rural interstate mileage and almost 60% of urban interstate mileage is rated in fair to poor condition. Highway funding just can't keep up in maintaining the current state of our roads, let alone upgrading those roads and highways to an acceptable condition. Montana is finding that emulsion cold in-place recycling (CIR) is a cost-effective solution.
The Federal Highway Administration recently issued a policy statement, stating, "Recycling and reuse can offer engineering, economic and environmental benefits. Recycled materials should get first consideration in materials selection. Determination of the use of recycled materials should include an initial review of engineering and environmental suitability. An assessment of economic benefits should follow in the selection process."
CIR can be used to remove thermal and reflective cracks, maintain clearances, improve poor aggregate gradations, reuse existing materials and minimize the need for new materials, as well as strengthen the pavement.
The innovative process selected by MDOT combines a defined sampling protocol, an engineered design protocol with performance-related testing of laboratory prepared samples, quicker field compaction and construction specifications and a new chemistry (ReFlex) emulsion. Rather than taking the approach of rejuvenating the existing pavement, this process identifies the final mixture properties through performance testing.
The new chemistry allows a better coating and higher asphalt content than conventional CIR. The performance-related specifications include tests for low-temperature cracking, raveling, strength and stripping resistance. The specifications also include requirements for the construction equipment and practices, as well as quality control and quality assurance.
Situated in the mountains and a climate which experiences temperature extremes, Rte. 279 had severe transverse thermal cracking and the early signs of intermittent fatigue cracking, but came with a structurally sound base. Prior to the project, cores were taken from the road following the defined sampling protocol. These cores were crushed into two gradations (medium and coarse) and the resultant reclaimed asphalt pavement (RAP) was used for mix designs in the Koch Pavement Solutions Terre Haute, Ind., laboratory. The design included formulating an emulsion specifically for the RAP to meet performance-related specifications for strength and resistance to thermal cracking, rutting, raveling and moisture damage.
Designs were completed for both the medium and coarse gradations, so that the design could be matched for the actual gradations achieved in the field once construction started. The specifications and the design test results for this project are given in the table above. The design emulsion content for the medium gradation was 3%, 2.5% for the coarse. Design water content for both gradations was 2-4%.
Quicker with a kocal
Valentine Surfacing Co., Vancouver, Wash., was the main contractor for the project, with Century Cos., Lewiston, Mont., as a sub for the laydown. Prior to construction, the shoulders were bladed back to prevent their incorporation into the recycled mix.
The project was completed Aug. 1-8, 2003, with little disruption of traffic (another advantage of in-place recycling). The temperatures during construction ranged from the mid 80s to the high 90s. In the partial depth CIR process, 3 in. of the distressed pavement was milled, leaving sufficient base to support the recycling train. The existing pavement was milled using a PR-1000 Rotomill, and the millings were processed with an El Jay Screen Deck and a Cedarapids Hammermill Crusher.
An Eagle Pugmill and an Asphalt Equipment Service Co. (AESCO) Blending System were used to blend the emulsion with the millings. In the past, most CIR used the same off-the-shelf emulsions as used for tack coats and a multitude of other uses. The emulsion used for this project was engineered specifically to give a chemical break and early strength to the CIR process, as well as meet the performance specifications.
The ReFlex emulsion was supplied by Koch's Billings, Mont., plant. The early strength allows a quicker return to traffic and earlier placement of a wearing course. According to Chuck Valentine, president of Valentine Surfacing Co., "The early strength of the ReFlex material is a very positive aspect. Traffic can get on the recycled mat much earlier (within 1-2 hours) and the potential damage due to traffic is virtually eliminated. Also, in the states that we CIR, the necessity to reroll is eliminated using the ReFlex product.
"The key to ReFlex CIR is primarily the laydown and compaction operation. We have found that the windrow elevator (kocal) is a key component. The larger, highest-horsepower machine is most desirable because the mix sets up faster than conventional CIR. Therefore, it is very important to have a large, high-horsepower kocal to pick the processed material up and deposit it in the hopper of the paver."
A Lincoln 660 Windrow Elevator fed the recycled mixture into a Blaw Knox PF 5510 paver which laid the mat. The mat was compacted using two steel-wheel rollers and one 25-ton rubber-tire roller. Valentine said, "We have had little difficulty achieving target densities with the ReFlex product."
The CIR maintained the current profile of the roadway, and MDOT measurements showed improved rideability. A seal and cover was placed on the CIR 10 days after the job was completed. Because of a relatively high void content, a fog seal was considered. However, traffic during the construction period tightened the mat surface, which allowed the design recommended seal and cover application rates.
According to Koch technical marketing representative Sean Pellersels, "Close communication and cooperation between the prime and subcontractors played a major role in the success of the project, which had a very smooth mat, high production and limited traffic delays."