Latest FAA P-401 Specification takes off

By John Crane, PE

Entering its second paving season, the Federal Aviation Administration’s (FAA) latest revision (10H) of their Standard Specification for Airport Construction, AC 150/5370, packs quite a punch in the constant battle with change. Those who have worked with the FAA likely know AC 150/5370 and its Item P-401, however for those that do not, Item P-401 is the specification that governs asphaltic surfaces for airfield pavements. The 10H revision updates criteria and adds items that will likely affect producers, designers and contractors in the asphalt industry. This article will outline a few of these changes.

APPLICATION OF THE SPECIFICATION

Right at the beginning of P-401, the FAA updates the requirements on when to apply the Item. This constraint, based on an aircraft’s gross weight, was increased to 30,000 pounds where previously it was limited to 12,500 pounds. In addition, the FAA also stipulated that nonprimary airports have a 60,000 pound threshold. Below these thresholds, given FAA approval, it may be possible for contractors to utilize state Department of Transportation specifications and mixtures. The use of state DOT mixtures makes sense since the increase to 30,000 pounds brings axle loads and tire pressures into a similar scenario as a standard tractor trailer; 20,000 pounds and 100 psi respectively. This allows more flexibility when it comes to paving areas within airfields that do not see high loading rates and allows for contractors to better economize their asphalt mixtures.

The FAA chose to use a Loaded Wheel Tracking test, the Asphalt Pavement Analyzer (APA) in particular, as their standard test to determine this property.

BINDER GRADE SELECTION

Grade bumping, the increase in the high-temperature binder grade due to higher traffic loading or slow-moving traffic, has been used for some time and the FAA, along with the majority of state DOTs, have applied this method historically. The FAA has now added new criteria in their binder-bumping procedure, which will now distinguish between pavements being subjected to slow and/or stationary aircraft and those that will not. Those pavements subjected to slow and/or stationary conditions will have an additional grade bump over the standard areas. This additional grade bump is important because asphalt binders act more like a viscous fluid rather than a solid under extended loading times, and by increasing the binder grade the stiffness is being increased to better carry the extended loading times.

Aerial view of DeSilva Gates Construction paving Runway 12-30 at the Oakland International Airport, which serves around 13 million passengers each year.

In addition to the binder bumping, the FAA revised the selection of the base asphalt binder for a mixture. Previously the specification outlined that the base binder would be what the state DOT used on interstate pavements. This can prove to be intricate as many states have various rules on the selection of binders, including the use of multiple binder grades for interstate pavements based on traffic levels and not strictly on roadway type. With the current revision, the FAA has simplified the situation by adjusting the base binder to be selected utilizing the environmental conditions within the state in which the work is being performed.

AGGREGATE BLENDING

P-401 specifies three different gradation bands for surface courses and, while they have retained their original nomenclature, Gradation 1, 2 and 3, adjustments to sieve tolerances suggest finer-graded asphalt mixtures. For those more familiar with the Superpave system, Gradation 1, 2 and 3 loosely resemble a 19mm, 12.5mm and 9.5mm, respectively. The update mainly adjusts Gradations 1 and 2 with all sieves from the No. 50 to their nominal maximum aggerate size, 19mm and 12.5mm, respectively, having an increase of 5 percent passing on average to the lower and upper limits. The increase to the percent passing brings the bottom of the gradation range near the maximum density line, which forces a fine-graded asphalt mixture. While this may slightly increase material costs, it will potentially aid contractors in achieving the strict density requirements outlined in the specification as finer mixtures with higher asphalt contents tend to compact easier.

PERMANENT DEFORMATION CRITERIA

Permanent deformation or rutting is an important distress to minimize on any surface, especially runways, to prevent pooling of water in the wheel paths, resulting in hydroplaning. Aircraft place extremely heavy loads on the pavement ,which can cause damage to an unstable pavement structure. To minimize this risk, the FAA has added a requirement to analyze a mixture for its resistance to rutting. The FAA has chosen to use a Loaded Wheel Tracking test, the Asphalt Pavement Analyzer (APA) in particular, as their standard test to determine this property. The APA test utilizes concave wheels that load pressurized rubber hoses laid upon an asphalt sample. The wheels then traverse over the samples for a given number of passes at a given temperature to simulate rutting.

The FAA is using an altered version of AASHTO T340, which includes an increase to hose pressure, 250psi, rather than the standard 100psi and a fixed testing temperature, 64°C, in lieu of the pavements environmental temperature. The FAA design criteria for all surfaces is a 10mm maximum deformation at 4,000 passes. If an APA is not capable of achieving the higher contact pressures, a standard APA test may be performed with a 5mm maximum deformation at 8,000 passes or a Hamburg Wheel Tracking device may be utilized following AASHTO T324 at 50 °C with a 10mm deformation at 20,000 passes.

DENSITY ANALYSIS

Often mat and joint densities are expressed in a percent. This is understood in the asphalt industry as referring to relative density, which means that in-place pavement density is really expressed as the percentage of a laboratory measured density. With the recent revision, the from a publication, “California Airport Land Use Planning Handbook” (rev. October 2011), produced by the California Department of Transportation, Division of Aeronautics. Caltrans counts 240 public-use airports in California: California has a diverse variety of airport types, ranging from large hub commercial airports to small, privately owned airstrips. Additionally, California supports a large number of facilities in a wide range of categories. Although commercial service airports handle most of the public’s air travel needs, the most common type of airport in California is the calculation for in-place density will no longer be based on the average bulk specific gravity of laboratory compacted specimens for the lot. Instead, the calculation is based on the theoretical maximum density (TMD) or Rice value, determined within the corresponding sublot of material. This change aligns the FAA with many state DOTs on how in-place density is calculated.

With the updated calculation of relative in-place density based on TMD, the FAA also revised the acceptance criteria for mat and joint density to reflect the change. It is important to note, however, that this revision in criteria is not equivalent to the previous criteria. Working through the math shows an increase in the lower-density limits for mat- and joint-density of roughly 0.4 percent and 0.9

While the FAA’s latest revision (10H) to the AC 150/5370 -“Standard Specification for Construction of Airports” and its enclosed item P-401 released on Dec. 21, 2018, has some large changes, the implications of those changes being made will lend to higher quality and longer-lasting asphalt airfields in the end. CA

John Crane, PE, is a pavements, materials and field applications engineer for Flexible Pavements of Ohio (FPO), the association that represents the asphalt pavement industry in Ohio. This article originally appeared in Ohio Asphalt, Vol. 17, Issue 2 (Summer 2020), the official magazine of the association, and is reprinted with permission of FPO.

Airfields in California

The following are excerpts percent, respectively. general aviation airport. General aviation airports offer a wide variety of services, ranging from flight instruction and recreation, to air cargo, emergency medical transportation, law enforcement and firefighting operations.

The economic importance of airports in California aviation is a vital link in the local, national and global transportation system. Air cargo, consisting mainly of high-value, time-sensitive documents and goods, plays a significant role in the vitality of the state’s economy. In today’s international and technologyoriented economy, businesses use the speed and reliability of air service to achieve operating efficiency. California’s airports are critical for providing services such as business travel, tourism, emergency response, fire suppression and law enforcement. Airports, airlines and businesses that support airports provide direct and indirect jobs and income throughout the state. The vital role that airports play in economic development and as a means of passenger and cargo transportation cannot be understated. CA