The U.S. military pioneered airfield design methods as it coped with designing for bombers of unprecedented weight in World War II. This pioneering work has continued to evolve with the active involvement and collaboration of the tri-services airfield working team. Airfield pavements require a high level of technology to design, build, and operate efficiently—their cost is substantial. The U.S. Army Corps of Engineers pioneered the rigid airfield pavement design methods. Current methods use layered elastic procedures, and, the future looks toward finite element based procedures. This page presents the philosophical approaches to hangar pavement design that exist between structural and pavement engineers.
Philosophy Comparison: Structural and Pavement Engineers
Pavement engineers and structural engineers approach pavement design with different philosophy, perspectives and tools; however, their objectives remain the same: safely supporting the imposed loadings and economically maintaining pavement life. Structural engineers rely on steel reinforcement to compensate for the inherent weakness of concrete to carry tensile, flexural stresses. Military pavement engineers have found, through extensive testing sponsored by the Corps of Engineers at the Lockborne trials in the 1940s, that steel reinforcing delays the deterioration of the crack but not the formation of the crack, and, thus prefer to use thicker, non-reinforced pavement sections. When used to slow deterioration and control Foreign Object Damage (FOD) hazards, pavement engineers locate steel at the slab mid-depth to hold the cracks shut; not to carry tensile forces as the structural engineer would do. Pavement engineers prefer to use a fatigue design method that considers a ratio of the actual strength to calculated stress (adjusted for load transfer conditions). Pavement designers vary in the approach to the amount of load transfer provided at joints ranging from aggregate interlock (25%) to the shear capacity available in a steel dowel. Both approaches rely on the continuous support of the subgrade and distribution of the applied loadings through a derivative of the traditional Westergaard or Boussinesq methods. Stresses in rigid pavements result from wheel loads; cyclic changes in temperature (warping and shrinkage or expansion); changes in moisture; and volumetric changes in the subgrade. Concrete pavements can be modeled as a beam on an elastic foundation (first proposed by E. Winkler in 1867 for one-way bending) and subsequently refined by Westergaard to capture two-way bending allowing a more effective and economical design. The reactive pressure is proportional to the deflection; that is p= k x y where the term k is the modulus of sub-grade reaction and y represents the slab deflection. Both pavement and structural engineers recognize that proper placement of steel in pavements or concrete slabs on ground is essential.
Pavement engineers focus on joint designs to control slab behavior and stresses; they include construction joints, expansion joints, and crack control joints. To reduce stresses from the volumetric changes, pavement sections perform best by setting the length to width ratio (aspect ratio) as close to 1 as possible. Pavement engineers mandate the use of steel for odd shaped slabs and oversized slabs where cracking would be expected, and steel would help slow deterioration.
The Navy has recently given significant scrutiny to using reinforcement in a new concrete airfield planned in Japan. Deviation from traditional military design procedures seems warranted due to poor soil conditions and high tendency toward differential settlement. The new Japanese airfield is unique in that the subgrade consists of land reclaimed from the sea. To create more residential and industrial land, the Japanese are removing the top of a mountain and crushing the material into fill. They are building the airfield subgrade using sand compaction piles, sand drains, and surcharging the site. The Japanese first applied this technique at Kinsai Airport, a commercial airfield facility near Osaka. Kinsai experienced an initial settlement of 11 meters that had to be compensated for by additional fill and surcharging. Kinsai is built as a flexible pavement system to be more forgiving with changes in the sub-grade support. Due to the heat signature of U.S. military aircraft, a concrete airfield is required for the new joint use Japanese/U.S. military airfield ($10 billion dollars, 10 year design construction effort by the Government of Japan administered by the Corps of Engineers with an expert Navy review team). A test plan evaluation section is planned for late summer 2004 to compare the performance of a continuous reinforced section with the traditional U.S. military procedures. Pavements subject to heavy-duty loads on reclaimed land are not new (Hamilton Army Airfield on San Francisco bay mud in the 1940s, Logan Airport, and many port facilities). Controlling deflections has always been a challenge.
The Navy has recently used design/build acquisition procedures to construct a $50 million, 3–module hangar for P–3s, a marine patrol aircraft in Brunswick, Maine. See Figure 1 for a visual comparison of the typical airfield parking apron spatial relationship to a typical Navy hangar. Both hangar and apron pavement types are required to support similar loads; however, the project structural engineer normally designs the hangar pavement, and the project civil engineer designs the aircraft parking apron. I think greater attention is given to life-cycle costs for the parking apron where normal traffic presents a greater FOD potential. The parking apron pavement is designed in accordance with UFC 3-260-02 providing a mix of aircraft traffic; the Hangar pavement is designed in accordance with MIL-HDBK 1028/1 soon to be replaced with UFC 4-211-01 (60% of the aircraft wartime weight). Navy/Marine Corps hangars are designed for flexibility in mission with the likely potential that aircraft loading will change frequently in the future.
Navy hangars would not use the initial economy provided by the Special Air Force hanger as shown in Figure 2, where the hangar pavement was thickened along the hangar centerline where the aircraft wheel path is located, and a reduced pavement section is used below the aircraft wings and maintenance/storage area within the hangar bay. The hangar pavement normally doesn't receive the maintenance attention the apron pavement receives, especially regarding pavement condition index inspections. PAVER software has been widely adopted by the Navy as the backbone of the preventive maintenance airfield program for taxiways, aprons, airfields, and miscellaneous airfield pavements. Hangar pavement is designed for lowest initial cost with, perhaps some consideration for FOD control, in preventing any cracks that form from experiencing additional deterioration. Previous Navy hangar pavement designs used reinforced pavement based on ACI 360 methods with minimal reinforcement for shrinkage and temperature stresses as well as friction drag forces where the subgrade restrains the pavement's tendency to expand and contract.
Due to the high concentrated aircraft gear loadings, the Navy recently revised its hangar design criteria in UFC 4-211-01 "Aircraft Maintenance Facilities" to encourage the design methods reflected in UFC 3-260-02 "Pavement Design for Airfields" and Pavement Computer Aided Structural Engineering (PCASE). Rationally, hangar pavement should be treated in the same manner as apron pavement. The hangar shop and equipment area adjacent to the enclosed space to park aircraft undergoing maintenance should be designed with a different approach due to the different applied loading conditions. The American Concrete Institute (ACI) 360, the National Precast Concrete Association (NPCA), and the Concrete Reinforcing Steel Institute (CRSI) all provide guidance on uniform live loading conditions and varying the amount of reinforcing and slab thickness to provide additional load carrying capacity. Table 1 provides a summary of comparison of the required pavement thickness as determined by a pavement engineer for the exterior portion of a hangar and as determined by a structural engineer for the interior portion of a hangar for the same aircraft loading with slightly different methods and programs.
Pavement engineers focus on FOD control by striving for zero cracks in the pavement surface. Structural engineers recognize that concrete will crack, and they focus in minimizing and controlling the cracks. Although highly concerned with minimizing cracks, the structural engineer is more prone to tolerating some cracking in the normal facility design objectives unless jet aircraft are involved with the facility. Jet engine aircraft is especially susceptible to engine damage by ingesting loose debris on the pavement surface causing the pavement engineer to have zero tolerance for concrete cracks. The tremendously large surface area and repetitive construction procedures used in airfield construction lend themselves to "doing it right the first time". Pavement engineers also strive to prevent the intrusion of incompressible material at the joints by giving greater attention to design and maintenance of the joint seals. Accelerated concrete spalling and cracking results where the joint material fails, allowing water to intrude and pump the subgrade away. Pavement engineers control stresses by limiting the spacing of the pavement joints to approximately 12.5' x 15'. Pavement engineers are highly focused on minimizing downtime for airfield repairs and, for that reason, prefer to avoid reinforcement. Working around the steel or removing the steel tends to prolong the repair process leaving airfields unavailable for normal aircraft movements over a greater period of time.
Table 1: Hangar Slab vs. Apron Airfield Pavement Thickness Comparison
|Navy Hangar Project||Apron Pavement||Interior Hangar Pavement|
|Hangar 6 (P-3 Aircraft) Brunswick, Maine||13" PCC||10.5" over 12" crushed stone (non-reinforced)|
|Corrosion Control Hangar (Type 1) Kaneohe Bay, Hawaii||9" thick Slab-on-Grade PCC (SoG), #4@12" o.c.||9" thick PCC Slab-on-Grade (SoG), #4@12" o.c.|
|Norfolk, Virginia P-519||12" thick PCC||10" SoG, 4 x 4 W2.9 x W2.9 (hangar)
6" SoG (shop areas)
|Norfolk, Virginia P-522 & 524||9-5/8" PCC w/ WWF T & B on
9-5/8" compacted aggregate
|Norfolk, Virginia P-523||11" PCC on 6" compacted aggregate base||9-7/8" PCC on 9-7/8" compacted aggregate base|
|Norfolk, Virginia P-525||11" PCC||7-7/8" PCC, 9-7/8" aggregate base w/ WWF|
|NAS Oceana P141U||11" PCC||7" PCC w/#16(M) @ 20" o.c.e.w.|
|NS Norfolk P633||12" PCC||10" slab w/4 x 4-W2.9 x W2.9|
|30" PCC hangar space, 15" PCC shop/admin|
|New River, NC
|6" PCC on 6" new aggregate||7-5/8" w/ #13(M) @ 12" o.c.e.w.|
|NAS Oceana Bldg. 500||8" PCC on 6" Base||9" slab hangar; 6" slab shop|
|Guam Air Force B-52 Hangar-Existing||16" PCC||16" PCC reinforced|
|Guam Air Force Special Hangar-Under Design||17-3/4" PCC||17-3/4" PCC within wheel path; 12" PCC everywhere else|
As part of the annual tri-service airfield pavement meeting hosted by the Naval Facilities Engineering Service Center in Port Hueneme, California, in December 2003, a pavement sensitivity analysis was done and discussed by the engineers in attendance. The Navy and Army/Air Force pavement design programs have differed in the past. The primary difference between the Corps of Engineers/Air Force, and FAA method and the Navy/Portland Cement Association (PCA) method is the Westergaard edge-load (CoE, AF, & FAA) versus the interior load application (Navy & PCA). The Navy and the Air Force have significantly different traffic loading requirements and pass levels, also. The pavement sensitivity analysis showed that the Navy pavement design program has served the Navy well for many years and correlates well with the Corps of Engineers PCASE design program when the Navy loads/traffic are used; however, the Navy design program gives unrealistically conservative answers when the Air Force traffic is applied to in the Navy design program. Table 2 provides a summary of comparison of the calculated thickness varying with the concrete flexural stress and modulus of subgrade. For compelling administrative reasons (future resource constraints) and technical considerations, the Navy pavement engineers agreed to base future airfield pavement designs/evaluations primarily on the CoE methods (PCASE). The structural engineer normally selects a conservative value for the modulus of subgrade and designs the pavement support system accordingly. Pavement engineers are also accustomed to designing for very conservative modulus of sub-grade values.
In Table 1 the comparison of design pavement thickness required for aircraft parking apron versus the hangar floor slab would indicate at first glance that the civil engineer, likely the apron designer, has different methods and requirements than the hangar floor slab, a structural engineer. One must recognize that a hangar slab-on-grade with the transformed area of the reinforcing steel to the concrete used will approach an equivalent thickness of the heavy-duty, non-reinforced concrete. The apron pavement is designed and maintained using the PCASE and PAVER programs, respectively with a greater emphasis on life-cycle costs and on FOD control. The hangar pavement is designed for 60% of the maximum wartime aircraft weight (UFC 4-211-01) as the aircraft is stripped of fuel, cargo, and ordinance when undergoing maintenance. In general, the structural engineer has minimal information on the sub-grade modulus and often resorts to very conservative values for design. The airfield pavement engineer rarely has the luxury of an improved sub-grade condition so he also is likely in the less than 300 pci range. The pavement sensitivity analysis shown in Table 2 indicates that sub-grade modulus has minimal effect on the computed pavement thickness unless the design falls below 200 pci. Both the pavement engineer and the structural engineer recognize the purpose of good drainage practices in the subgrade. The structural engineer is often faced with the decision to use a vapor barrier or a vapor retarder. ACI 302.1 has recently provided a flowchart to assist the designer in making this controversial decision. The pavement engineer has recognized that it is virtually impossible to prevent water from entering the pavement profile so he has resorted to rapidly removing the drainage with a drainage layer. The structural designer must decide if the slab will have a vapor sensitive floor treatment above or simple concrete finish to determine if the vapor barrier goes below the slab, the slab dry granular base, or perhaps no vapor barrier is required at all.
Table 2: Pavement Sensitivity Analysis, Varying Concrete Flexural Strength and Modulus of Subgrade
|Pavement Profile Condition||Loading||Navy Cal. Thickness,
E=6.333 x MoR
|PCASE 2.06 Cal. Thickness,
E=6.333 x MoR
|FLEX 650,SG 100||Navy||15.0||15.7|
|FLEX 650,SG 200||Navy||13.8||14.5|
|FLEX 650,SG 300||Navy||13.2||13.2|
|FLEX 650,SG 500||Navy||13.0||11.2|
|FLEX 650,SG 100||Air Force||27.2||20.3|
|FLEX 650,SG 200||Air Force||25.6||16.8|
|FLEX 650,SG 300||Air Force||24.7||13.8|
|FLEX 650,SG 500||Air Force||24.6||11.8|
|FLEX 726,SG 100||Navy||14.1||14.8|
|FLEX 726,SG 200||Navy||12.8||12.5|
|FLEX 726,SG 300||Navy||12.3||10.6|
|FLEX 726,SG 500||Navy||12.2||10.7|
|FLEX 726,SG 100||Air Force||25.6||19.0|
|FLEX 726,SG 200||Air Force||24.1||15.6|
|FLEX 726,SG 300||Air Force||23.3||12.9|
|FLEX 726,SG 500||Air Force||23.1||11.1|
Slabs are reinforced to influence crack width and location; maintain slab surface tolerances; compensate for poor soil conditions; provide impact and surface tolerances; provide shear load transfer; and insure against crack deterioration, and provide for extended joint spacing. Reinforcing does not prevent crack formation!
Relevant Codes and Standards
American Concrete Institute (ACI)
- ACI SCM-25(03)—Concrete Slabs on Ground
- ACI One-Day Seminar—Concrete Slabs on Ground, Baltimore, Maryland, December 2, 2003, Richard Smith and Bruce Suprenant.
- ACI 302.1R Guide to Concrete Floor and Slab Construction
Unified Facility Criteria
- Principles of Pavement Design, 2nd Edition by Yoder & Witczak. John Wiley & Sons, Inc., 1975.
- "Stresses in Concrete Pavements Computed by Theoretical Analysis" by Westergaard. H.M. Public Roads, V. 7, No. 2, Apr. 1926.