The anatomy of the runway and taxiway components and materials will be discussed starting with the deepest layer which is the lime stabilized subgrade. This establishes a solid and consistent foundation for the asphalt and concrete components of both the runway and taxiway cross sections. The top 12 inches of the soil subgrade is stabilized in the OMP with a mix of lime and water.
The next component is the asphalt treated permeable base (ATPB) which is open graded asphaltic cement concrete. This section is typically placed in one or two lifts to build the 6-inch section. In addition this layer is constructed or compacted from 100 degrees F to 150 degrees F to maintain the voids between the aggregate. The ATPB allows for drainage of water through the pavement section. The Performance Grade (PG) for this layer is 58-22.
The next layer is the asphalt binder which consists of an asphaltic blended aggregate mix based on Illinois Department of Transportation (IDOT) specifications. The PG for this material was 64-22. An approved chemical additive is included in the mix which allows for the lower temperatures and creates WMA. This 6-inch section is also typically placed in two lifts similar to the ATPB. This asphalt binder has fewer air voids than the ATPB. Asphaltic cement is used in this layer in lieu of Portland cement concrete (PCC) to reduce the cost of the pavement section.
Lastly the top surface layer is reinforced PCC. The thickness of the reinforced PCC layer varies from 16 to 21 inches depending on the runway.
Airfield pavement designs are similar to the typical Interstate highway design. However, airfield cross sections are typically thicker due to the weight of aircraft and the stress of aircraft touchdowns upon landing on runways.
Taxiway ZS used approximately 7,100 tons of WMA and 7,000 square yards of PCC. Upon completion of the Runway 9C-27C Northeast project, the contractor will have used approximately 67,000 tons of WMA and 108,000 square yards of PCC.
The Chicago Department of Aviation has been and continues to be very committed to incorporating green best practices into all aspects of O’Hare International and to Chicago’s smaller but well utilized second airport, Midway International. This includes airport design, construction, operations, terminal maintenance and day-to-day management.
The CDA developed a Sustainable Airport Manual (SAM) to establish guidelines, policy and procedure for airport operations including construction and is applying this to the OMP. The exploration and research that went into the decision of using WMA follows the sustainability practices. The incorporation of WMA as a component in O’Hare’s taxiway design and construction meets the triple bottom line.
WMA is proven to be a paving method that has less of an environmental impact than HMA for example. WMA also has the potential to save time and expenses. In relation to a project with the scope of the OMP, the savings of time, labor and materials cost, as well as the lesser environmental impact, are significant.
The benefits of using WMA are related to sustainable development and improved working conditions. Included in this are environmental benefits from reduced fuel consumption, and reduced asphalt plant emissions. There are physical benefits that relate to the paving process such as increased workability for transporting and placing asphalt and potentially extending the length of the paving season. Using this innovative and more eco-friendly paving technology is one of the many industry leading sustainability initiatives that continue to be incorporated at Chicago’s airports.