Illinois I-70
Asphalt Technology Showcase
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The I-70 project near Casey, Illinois, incorporates many of the latest advances in asphalt pavement technology. The Illinois DOT (IDOT) website describes the work as "…reconstruction consisting primarily of the rubblization of the existing continuously reinforced concrete pavement (CRCP) and the construction of a 17.5-inch 30-year Extended Life Bituminous Concrete Pavement."
In addition to rubblization and long-life asphalt pavement, the project includes both Superpave and stone matrix asphalt (SMA) in the asphalt overlay. The project also serves as a head-to-head performance comparison of asphalt and concrete.
Dr. Marvin Traylor, Director of Engineering and Research for the Illinois Asphalt Pavement Association, sees the project as having an impact on many other projects. IDOT has committed to build several Perpetual Pavements in the future. As a result, Traylor thinks that IDOT and the asphalt industry need to gain as much as they can from the I-70 project as quickly as possible. "We do not want to have to wait 30 years to see how well this pavement performs before we build the next one," says Traylor.
The original I-70 pavement was constructed in 1969 and consisted of 8 inches CRCP on 4 inches of Bituminous Asphalt Mix base (BAM) that was subsequently overlaid with 3.5 inches of hot mix asphalt (HMA). After 34 years, the project was carrying an average of 24,000 vehicles per day (vpd), of which 49 percent were heavy trucks.
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| Project Design |
The work is the second phase of a two-year special demonstration project conducted by IDOT and the FHWA. The project originated when the Illinois General Assembly passed legislation requiring IDOT to complete a project with a 30-year life pavement.
IDOT decided to evaluate the service-life capabilities of both asphalt and concrete on the 21-mile project. Nine miles of the job were rehabilitated with concrete and the remaining 12 miles with asphalt. Both pavement types were required to provide a five-year warranty.
The 550,000-ton overlay project was only one facet of a huge effort. The total project cost exceeded $144 million and included 40 miles of underdrains, nearly 200,000 square yards of rubblized concrete, raising two overhead bridges, and re-decking a third viaduct bridge.
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| Rubblization |
Antigo Construction of Antigo, Wisconsin, rubblized the existing concrete using a multihead breaking machine. IDOT uses a special design chart in conjunction with information from the Dynamic Cone Penetrometer to select the equipment used for the rubblization process. IDOT's specification required that the maximum particle size of the fractured concrete above the reinforcing steel be 3 inches, and no larger than 9 inches below the steel.
A tracked paver, rather than a rubber-tire paver, was required to prevent "spin-out" when placing HMA directly on the rubblized base. After completing the first HMA layer, the contractor was permitted to switch back to a rubber-tired paver.
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A multihead breaking machine rubblized
the old concrete.
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| Superpave and SMA |
The HMA overlay of the rubblized concrete consisted of 10 inches of a 90-gyration, 3/4-inch nominal size Superpave base course with a polymer-modified PG 70-22. The intermediate course consisted of 5.5 inches of a 105-gyration, 1/2-inch nominal size Superpave mix with a polymer-modified PG 76-28 asphalt binder. The surface course was 2 inches of heavy-duty, 105-gyration, 1/2-inch nominal size SMA surface with an SBS modified, PG 76-28 binder. The SMA contained steel-slag aggregate for extra skid resistance.
The thickness and composition of the shoulder was slightly different than the mainline pavement. The thickness of the HMA in the shoulder areas was reduced to 14.5 inches. The lift composition of the shoulder areas consisted of 9.75 inches of a 70-gyration, 3/4-inch nominal size Superpave mix using a PG 64-22 binder, 2.5 inches of a 90-gyration, 1/2-inch nominal size Superpave mix with 15 percent RAP using a PG 70-28 binder, and two inches of a 90-gyration, IDOT "D-mix" using a PG 70-28 binder.
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A modified roller is used to break up the surface
pieces and densify the fractured concrete.
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All mixtures were required to use 1 percent hydrated lime as an anti-stripping additive. In order to promote bonding between adjacent paving lanes and to minimize moisture intrusion, a double application of a polymer-modified tack was required to be applied to all longitudinal joints.
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| Mix Design |
The mix design for all mixtures was developed by the contractors and approved by IDOT Mix design criteria required that the asphalt content for all mixtures be established at 4 percent air voids and that the voids filled with asphalt are limited to 65 to 75 percent. The voids in the mineral aggregate criteria for the Superpave mixtures varied in accordance with the nominal aggregate size and the required compaction level (number of gyrations). The required in-place density of the various mixtures was established based upon the position of the mixture within the pavement section.
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The Superpave mixture typically weighed an average of 112 pounds per square yard per inch of thickness while the steel-slag SMA surface mixture weighed 132 pounds per square yard per inch of thickness.
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| Perpetual Pavement Variation |
Although IDOT wanted to build an extended-life pavement, they did not adopt the total Perpetual Pavement approach. The pavement design was intended to arrive at the overlay thickness needed to carry a total of 78 million ESALs over the 20-year design life of the pavement. The eventual 17.5-inch overlay thickness was a compromise between the 28 inches determined from IDOT's mechanistic design procedure and the 11 inches calculated using the Perpetual Pavement methodology.
Additionally IDOT, after much internal discussion, decided to diverge from the "rich" bottom-layer concept of Perpetual Pavements by excluding this layer because the potentially higher permeability of Superpave mixtures might permit infiltrating moisture to become entrapped within the layer, thus promoting premature stripping of the HMA.
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A materials transfer vehicle is used to assist in providing mix uniformly to the paver.
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| Asphalt and Concrete Performance Comparison |
The nine miles of reconstructed concrete was completed in 2002. The new concrete pavement consisted of 13 inches of continuously reinforced concrete over 6 inches of asphalt base course and 12 inches of aggregate base. Work on the asphalt section started in April 2003 and was completed in the early fall of 2003.
The ultimate comparison of the two paving materials will be revealing. The asphalt section was completed in less time and at less cost and resulted in a smoother ride. Some time in the future, the asphalt section will be restored to like-new condition with a simple mill-and-replace treatment of the wearing course, while the concrete portion will have to be completely rebuilt.
Source: The Asphalt Institute. Spring, 2004
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