When a business traveler has to fly to a meeting, arriving safely and on time are certainly higher priorities than the structural integrity of the airport pavement. However, to the individuals running the airports and the airlines using the airports, pavement integrity is critically important.
In fact, delays associated with closures due to airside pavement rehabilitation and reconstruction can have a significant economic impact on both the airport and the airlines in terms of lost revenue from commercial and cargo operations. For that reason, accelerated airfield rehabilitation and reconstruction techniques are becoming increasingly important when minimizing closure times is critical.
Fixing concrete with concrete
In the past, pavement rehabilitation and reconstruction solutions for existing airfield concrete pavements often relied on the use of hot-mix asphalt because it could be placed quickly. However, in many instances where concrete pavement was used for original construction, the reasons for its use are still valid when rehabilitation and reconstruction are considered (for example, resistance to heavy loads, no deformation under static loads, resistance to fuel spills, longer performance periods without the need for maintenance and rehabilitation and so on). These have led to widespread interest in a group of strategies that are referred to as accelerated rigid pavement techniques, which allows the use of portland cement concrete (PCC)-based strategies where they would not have been considered in the past.
Accelerating PCC pavement opening times is not limited to the selection of appropriate PCC and other materials. In a project conducted for the IPRF, 16 airport projects covering a range of facilities, environments and operational challenges were selected to illustrate the many methods available to accelerate airfield PCC projects during the planning and coordination, design and construction phases. Several other specific issues, such as safety, security, electrical and navigational aids, were addressed separately. This article focuses on methods that were identified from the case studies for consideration during a project’s design phase to accelerate airfield PCC construction.
The design strategies considered during the project are divided into several categories: alternative designs, performance and risk assessment, innovative materials, available closure times, opening requirements, mix design and development of plans and specifications. Several of these are summarized in this article.
Work with what you have
Project case studies show the use of alternative pavement designs is a proven way to accelerate construction. Several factors can be considered in looking at alternate designs:
- Need for accelerated construction: It may be possible to differentiate between areas that require accelerated construction because of their impact on operations (such as runway tie-ins) and areas where more conventional methods can be used because they are not on the project’s critical path. Both areas do not have the same opening requirements;
- Time available for construction: Available closure time greatly influences the design approach. Determining and using alternative designs can make the most of the available construction window; and
- Existing pavement conditions: The types of pavement layers (stabilized or aggregate) and condition of the layers—whether or not they can be salvaged—should be carefully evaluated. If the base can be left in place, construction is almost always accelerated.
Several projects used alternative pavement designs in which the number of pavement layers was reduced to accelerate construction. This was accomplished by evaluating and reusing existing layers and by designing alternative cross sections. For example, in both the reconstruction of Runway 9R-27L at Atlanta-Hartsfield and the reconstruction of the intersection of Runways 12R-30L and 4-22 at Houston Hobby, the existing stabilized base layers were found to be in good condition and the bond breaker used during the previous construction allowed the stabilized base to be retained. This meant that only the PCC surface needed reconstruction, saving considerable time, materials and costs.
Two runway intersection reconstruction projects in South Carolina at Charleston International and Savannah-Hilton Head also involved reconstructing only a new PCC layer. The initial design process for these projects evaluated a conventional design using a stabilized base layer, but designers decided to increase the PCC thickness to eliminate the use of a stabilized or aggregate base. This approach can be validated theoretically by calculating equivalent strains in the different pavement systems.
Design alternatives don’t have to originate with the designer. Upon the contractor’s recommendations, the design for the partial reconstruction of Airborne Airpark’s runway in Ohio was simplified to include only two layers—an aggregate base and the PCC surface layer—because the contractor determined the weekend closure did not allow sufficient time to construct a stabilized base. To account for the elimination of the stabilized layer, both the aggregate layer and PCC layer thicknesses were increased by 2 in. The aggregate base layer remained in the design due to subgrade drainage concerns. However, the Airborne project was not FAA funded and didn’t require FAA approval of the design.
The taxiway project at Cleveland Hopkins International Airport provided a pavement section with an asphalt-stabilized base for time-critical runway tie-ins and a design with cement-treated base elsewhere. They felt the asphalt-treated base would allow construction to continue sooner after placing the layer than would the use of a cement-treated base (note that the contractor used a cement-treated base for tie-ins after problems occurred with the first tie-in areas constructed using the asphalt-treated base).
Quick with quality
As noted, accelerated paving is often associated with the use of high-early-strength or rapid-setting materials; however, such materials are sometimes associated with poor long-term performance. There also is, perhaps, a general mindset that work done quickly sacrifices quality. The case studies showed that high-quality, long-lasting concrete pavements can be constructed using accelerated techniques. In any case, in making design decisions for accelerated construction, designers should evaluate the anticipated (or required) pavement performance and balance this with project constraints.
For example, at the Seattle-Tacoma International Airport, the owner had undertaken a process of slab replacement on Runway 16R-34L during nighttime closures over a number of construction seasons. The entire closure was limited to 7.5 hours per night. Because it had already been determined that the repairs would eventually be replaced as part of major, more permanent rehabilitation or reconstruction, the long-term performance of the repairs was not a critical project requirement and the risk of early deterioration associated with the use of a particular high-early-strength, rapid-setting material was acceptable.
Clearly, risk and performance are key considerations in the design of accelerated airfield projects. On the Memphis runway reconstruction project, conventional materials and construction methods were used to reduce the chance of surprises during construction: Materials and methods were specified with which the contractor would be familiar.
The apron design used for the Detroit Metropolitan deicing pad project was based on a standard used elsewhere at the airport. This not only helped accelerate the design phase, but also minimized the risk of poor performance from trying something new.
The majority of accelerated projects included in the case studies used what would be considered conventional materials. Most of the projects used Type I cement and readily available admixtures to meet project requirements. However, to meet construction time constraints in several projects, proprietary materials were specified during the design to accelerate construction, in particular the Charleston and Savannah runway intersection projects. The use of innovative materials allowed the reconstruction of runway intersections during overnight closures, whose closure for conventional construction methods and materials would have essentially closed the airport.
Innovative materials also have a place in overnight repair operations. Colorado Springs selected a patching material that set quickly, but also retained flexibility for a longer service life. The material’s rapid set time allows maintenance crews to work up to 30 minutes before reopening facilities to traffic, allowing more work to be conducted during an overnight closure.
The available closure time greatly dictates the type of work that can typically be done and how much can be accomplished. Three general categories of closure times, and work that is possible within those closure times, were identified in the IPRF project: overnight, weekend and longer closures.
Overnight closures are generally less than 12 hours long, with six to eight hours being common. The short time available in an overnight closure restricts the extent of work that can be completed, but does not make repair work impossible. Work is generally limited to slab replacement and slab patching.
For these shorter closures, the time for the PCC to gain sufficient strength to reopen is a significant factor, but construction methods can be just as critical since a good portion of the closure is necessary for the required curing time. For example, Charleston’s runway closure started at 10 p.m. and ended at 6:45 a.m. With the material requiring four to five hours to reach sufficient strength, all placement work had to be completed by 1:45 to 2:45 a.m. As such, there were only approximately four hours for removal, preparation and placement of the PCC.
Some of the lessons learned on overnight closures, including at Charleston, Colorado Springs, Phoenix, Savannah and Seattle, were:
- Use rapid-setting, rapid-strength PCC materials to minimize curing time and maximize preparation time;
- Early saw cutting (such as during a previous closure) allows slab removal to occur earlier in the closure period;
- Reducing the number of layers to be constructed or reconstructed shortens the overall construction time;
- Using temporary, precast slabs provides some flexibility to extend preparation and placement work over multiple closures;
- Providing sufficient equipment and labor to perform the work and providing backup equipment can avoid possible problems; and
- Evaluating the opening requirements to maximize the work allowed in the construction window (such as not requiring full pavement strength in areas that are outside of the main traffic area).
Weekend closures also provide a relatively short construction window and restrict the type of work that can be completed, but are obviously not as restrictive as overnight closures. Weekend closures allow areas of slabs to be replaced (instead of individual slabs) and allow time for additional items of work to be performed, such as drainage improvements. As such, projects that can be completed during weekend closures include reconstruction of large sections of a runway, such as Airborne Airpark and Columbia International. These same projects could take as long as two to four weeks under night closures, could have a much greater impact on operations and could ultimately be much more expensive.
Many of the considerations for overnight closures can be applied to weekend closures, but there is greater flexibility with the longer closure. For example, relatively quick PCC strength gain is still essential, but it does not have to be as aggressive as the few hours needed for overnight closures and may only need to be used for areas paved immediately prior to reopening. Airborne required opening strength within 24 hours, while Charleston required strength gain in four to five hours.
With longer closures, more conventional designs, construction methods and materials can be used and the types of projects that can be considered are scarcely limited. For example, although the Memphis International Runway 18R-36L reconstruction was on an accelerated schedule, the paving portion of the work was performed in 12-hour shifts, six days a week, using conventional paving techniques and PCC materials. The emphasis during longer closures begins to focus on keeping more tasks on track for the accelerated schedule and on planning to minimize possible delays. Key considerations during longer closures include:
- A high level of communication needs to be maintained, decisions are addressed in a timely manner and phases are coordinated with stakeholders; and
- Weather delays can be minimized by using stabilized subgrade or stabilized base layer. Weather is more likely to be an issue with longer closures because grades can be open for a longer period. Short closures can be cancelled if poor weather is in the forecast.
Finding an opening
Opening requirements are related to the PCC obtaining sufficient strength to support aircraft (or construction) traffic without damage. For example, the FAA P-501 specification sets opening requirements as a PCC flexural strength of 550 psi (compressive strength of 3,500 psi if specified) or 14 days after the PCC has been placed if strength testing is not available. However, with accelerated projects some repairs are completed and reopened in less than 14 hours, not 14 days.
Although most projects could not afford 14 days for curing, the majority of projects required opening strengths of 550 psi (or very close to it). The opening requirements included opening for construction traffic. What generally varied between projects was the time available to achieve that strength. Some examples of varying opening requirements are:
- The runway intersection reconstruction at Savannah-Hilton Head required 500-psi flexural strength in four hours;
- The runway slab replacements at Seattle-Tacoma required 550-psi flexural strength in five hours;
- Airborne Airpark required 650-psi flexural strength in 24 hours on the runway reconstruction;
- In the reconstruction of Taxiway M at Cincinnati/Northern Kentucky International, 700-psi flexural strength was required in three days;
- The reconstruction and extension of Runway 8-26 at Phoenix Sky Harbor required 750-psi compressive strength for opening during construction of the middle section of the runway. This lower strength was considered acceptable because the area was included as part of the overrun for the reduced runway length. Although the area had to be reopened to traffic every morning, the pavement was only required to support an aircraft in case of emergency; should an overrun occur the pavement would likely be damaged and it was required that the pavement be replaced, but it would support an aircraft; and
- Similarly, the PCC on the cross-runway construction at Savannah that was outside of the intersection but still within the active runway safety area was only required to achieve initial set. Because the pavement was not directly in the active intersection, it only needed to support an aircraft in case of an emergency. As with the Phoenix project, it was anticipated that the pavement would be damaged and would need to be replaced if used in an emergency, but it would have sufficient strength to support an aircraft.
Thus, as the above examples illustrate, there is often some flexibility in determining opening requirements for PCC. The advantage of a flexible approach to opening strength is that the construction window is maximized by requiring less curing time prior to reopening the active pavement area.
And the advantage to the business traveler is the peace of mind associated with uninterrupted travel due to construction delays.
The case studies showed there are many opportunities to accelerate airfield PCC paving projects. While this article primarily focuses on the design phase, providing some of the examples and lessons learned from the study, the final report addresses steps that can be taken during project planning and construction to further accelerate construction. The report also introduces decision tools and checklists that allow the designer, owner or contractor to identify the types of airfield PCC projects that can be accelerated, provides guidance on how that can be done and directs the reader to sources for additional guidance (the case studies themselves). The two-volume final report (Project 02-02) may be downloaded from IPRF’s website at www.iprf.org/products/main.html.