Warm Mix Asphalt Takes Flight at O’Hare International
O'Hare International is the first airport in the Great Lakes Region and among the first in the United States to produce and place warm mix asphalt as part of the construction of new taxiways.
The CDA worked in conjunction with the FAA for approximately two years before receiving FAA concurrence to use WMA on the specified taxiway projects. This achievement was the result of the CDA’s extensive and vigorous testing program coupled with assurances to the FAA that WMA is a suitable substitute for the traditional HMA material.
To ensure the WMA is properly mixed and installed the OMP conducts a very rigorous quality control and assurance program. This includes preparation and approval of a mix design, placement and diligent monitoring of a WMA test strip, and laboratory and additional field testing.
Numerous factors are considered and evaluated when designing and engineering pavement sections for runways and taxiways. The weight of an aircraft is one consideration taken into account during the design phase.
Other factors include the weights of the design aircraft, estimated repetition of arrivals and departures during the desired design life, underlying soil conditions, typical depth of frost, and cost.
Larger aircraft factor in
Over the past several years there has been the introduction of the next generation of NLA such as the Airbus A-380 and the Boeing 747-8. The A-380 is currently the world’s largest passenger aircraft at 240 feet long and a wingspan of 262 feet. This behemoth in various cabin configurations can carry more than 850 passengers.
O’Hare sees a large volume of heavy cargo planes including various models of the 747. One of the considerations of the OMP was to design portions of the new infrastructure with the accommodation of large aircraft as a major consideration.
Available runway length for arrivals and departures is extremely important for the airlines, both passenger and cargo. An example at O’Hare is Runway 10L-28R which is 13,000 feet long and provides sufficient departure length for large aircraft departing nonstop to the Far East. Shorter runway lengths force airlines to absorb weight penalties. These include reducing the number of passengers on a flight, the amount of cargo carried, or the amount of fuel onboard which can affect range of the aircraft.
In addition to runway and taxiway widths, the separation distances between them dictate the type of aircraft allowed to use those pavements. Two of the OMP runways, 10C-28C and 9C-27C are designed to accommodate Group VI aircraft such as the 747-800s and the A-380. Building the infrastructure to accommodate large planes and/or to reduce the incidence of weight penalties is expensive. The cost savings of WMA is therefore significant in the process of the OMP.
Anatomy of the runway
The anatomy of the runway and taxiway components and materials will be discussed starting with the deepest layer which is the lime stabilized sub-grade. 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 sub-grade 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.
Runway designs
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.
