Central Idaho attracts a steady flow of visitors and new residents, so the local airport in the town of Hailey is a key transportation link. Faced with the need to rebuild its single runway, airport managers relied on consulting engineers, an experienced contractor and a soil-cement specialist to complete the job in a mere 30 days.
While some local airports shut down for twice that length of time for runway work, the Friedman Memorial Airport managers selected "full-depth reclamation" with portland cement to meet a master plan objective and economically reopen for traffic in the shortest possible time span.
Project analysis began in the spring of 2006, nearly a year before actual runway construction work began. Toothman-Orton Engineering, a Boise-based civil engineering firm, discovered significant asphalt deterioration after core samples were taken. Like many flexible pavements, a number of asphalt layers had been placed throughout the years. Though the top layer was rated in decent shape, the lower layers had been subjected to a deterioration called "asphalt stripping." Stripping is a common type of asphalt damage, caused when moisture and traffic loads cause the asphalt cement to separate (or "strip") away from the aggregate.
The airport's traffic is projected to grow 44 percent by 2022, the target date of its long-term master plan. To meet this need, there is a strong likelihood that during the course of the next 10 years or so a new airport will have to be opened to replace Friedman. Thus, there was no need for a complete runway reconstruction.
The airport board, management staff and engineers initially thought that simply milling off some old asphalt and then adding an overlay would suffice for several years until the new airport could be built.
Based on airplane traffic and underlying soils, engineers at first calculated three different asphalt-based pavement options. Concrete was not considered because longevity was not required with the total airport replacement planned within the next decade. A standard FAA design was considered along with two other pavement sections. But when none of the first three could be constructed within a 30-day construction period engineers approached the FAA and requested consideration for full-depth reclamation.
FDR with cement is a pavement rehabilitation process in which a failed asphalt pavement (surface and base) is pulverized and then blended with cement and compacted. After curing, the cement-stabilized material forms a new base for the pavement, which is then surfaced to create a long-life pavement structure. After FAA engineering expressed willingness to consider FDR, an additional pavement option was developed with input from Boise-based Terracon.
Full-depth reclamation with cement is a common practice among county road maintenance departments and state transportation departments. The process has different names in different states (such as CRAB in Idaho, which refers to cement recycled-asphalt base), but the end result is the same: rebuilding the pavement structure by recycling the existing materials and stabilizing with cement. FDR is environmentally friendly since it reuses materials on-site, eliminating the need to haul them to a landfill.
Dave Mitchell, Toothman-Orton Engineers, said that "they had to find a way to perform the necessary work and get it done within the 30 days." He was aware that the FDR method to rehabilitate a failed asphalt roadway is used commonly by members of the Idaho Association of Highway Districts as well as by the Idaho Transportation Department, which has performed hundreds of miles of FDR with cement.
Toothman-Orton Engineers enlisted the engineering expertise of Terracon to consult on an FDR analysis and design. The parties also brought together a contractor and subcontractor experienced with pulverization and cement-stabilization of old asphalt pavements to gather input from those with construction experience.
The proposed FDR approach cut 18 work days off the schedules of the other pavement options. In the case of Friedman Airport, a stronger base was not the objective, but it would result in reducing the thickness of the total pavement section and accomplish the runway improvement within a short construction time period.
Additionally, FDR allowed them to attain the 30-day goal in a sustainable fashion, as it recycles resources already in use and also eliminated "probably 4,000 truck trips that would have been a huge impact on the community," according to Dave Mitchell.
The big surprise, said Toothman-Orton's Mitchell, was that the consulting group's advice to airport management to use full-depth reclamation would "cut their costs over a million dollars" when comparing all four possible pavement scenarios. Once the consultants received approvals from the FAA (federal airport funding was involved) and the five-member airport commission, the project went to bid in early 2007.
Low bidder Western Construction Inc. from Boise mobilized earthmoving equipment in order to start immediately after Friedman Memorial Airport closed the runway on April 23. Despite some late snowfall the previous week, weather conditions cooperated with the schedule, and the project was completed by the May 23 deadline.
Western selected Valentine Surfacing from Vancouver, Wash., to perform the actual FDR work on 73,440 square yards of runway. Approximately 6,900 feet of Hailey's 7,500-foot airport runway had to be reworked.
Chuck Valentine, company president, recounted that his crew took "five days to grind and five days to mix" using two CMI pulverizing machines, a PR1200 and an RS800, along with a cement spreader truck and other typical grading equipment. A nominal 2 percent of portland cement was used at the start and was adjusted to the 2.3 percent level as construction proceeded to adjust for soil moisture.
The FAA requires tight pavement surface precision on pavements, a +/-.02-foot allowance. Toothman-Orton required "robotic total station" in the specifications they wrote, using ITD specs as a starting point.
Western Construction's Jack Snyder comments that "we can build a better product without stakes" as he refers to the application of electronic grade control. Western combined dual methods on the Friedman Airport runway, electronic machine control as well as electronic grade control. Snyder said the company started using GPS about five years ago.
Knowing that the FDR process is quick, requires precision the first time and that crews "can't come back the next day," Western Construction took dual precautions. The company retained the services of Butler Engineering from Shelley, Idaho, to build terrain models with surveying data and also assigned experienced project engineer Marilyn Broek to the Hailey job along with four project superintendents.
The project was successfully completed within the required 30 days. As a result, Airport Operations Chief Pete Kramer now recommends the FDR with cement process to other airport operators who want to minimize their runway reconstruction closure times to bare minimums.
Western's Snyder said that the successful completion depended on "the right technology and a ton of experience" brought to the table by the engineers involved and the construction team.
|John Arroyo is the executive director of the Northwest Cement Producers Group.|