Bituminous pavement design methods

Some of the methods for design of bituminous airfield pavement structures are (i) Corps of Engineers (CoE) method (ii) Federal Aviation Administration method (FAA) and (iii) The Asphalt Institute method and are discussed briefly in the following:

Corps of Engineers (CoE) method

This method was developed during the World War-II, when data on runway performance was collected and analysed for various airports (Horonjeff 1975). The total thickness above a particular layer is determined by the CBR value (in percentage) of the layer. Thus, the total thickness above the subgrade, sub-base, base etc. can be found out by knowing the CBR of the respective layers. This method is simple, but purely empirical. The total thickness, T, is estimated as:

(34)

where, α is the load repetition factor, ESWL is the equivalent single wheel load of the gear load assembly (in pounds), A is the measured tyre contact area (in inch²). The values of α is suggested for various gear assembly (single wheel, dual wheel, dual wheel tandem axle etc.) used for ESWL calculation, and number of load repetitions (Horonjeff 1975). From the known CBR values of the subgrade, sub-base, and base layer the individual thickness composition are obtained. Some theoretical basis of the CBR method was proposed later on, where the thickness of the pavement is so adjusted that the shear stress developed at a given level is equal to the allowable shear stress of that layer.

U.S. Army Corps of Engineers, Naval Facilities Engineering command and Air Force Civil Engineer Support Agency have recently developed a combined manual for design of airfield pavement as Unified Facilities Criteria (UFC 2001) as a modified version of this design approach.

Federal Aviation Administration (FAA) method

The FAA uses modified CBR method for design bituminous pavement (FAA 2006). As per the FAA method, the subgrade soil is categorized based on the soil classification group, drainage and frost damage conditions. Design charts are available for design of pavements for gross aircraft weight of 30,000 lbs or more, for single, dual, and dual tandem aircrafts. It is generally assumed that 95% of the gross weight of the aircraft is carried by the main landing gear and 5% is carried by the nose gear (FAA 2006). A typical pavement design is presented in Figure 25. The design life is assumed generally assumed as 20 years.

Figure 25: A schematic bituminous pavement design chart as per FAA (2006)

The following is the design procedure followed as per the FAA method (FAA 2006):

• The subgrade CBR value, gross take-off weight, main gear weight and annual departure of the individual aircrafts are used as the input to the design. Different types of materials can be used as base/ sub-base, and accordingly some variations are observed in the design thickness.
• The individual design charts (for various categories of aircrafts) are used to find out the total pavement thickness values for different aircrafts. The aircraft whose corresponding data results in the maximum thickness is designated as the design aircraft .
• Having obtained the design aircraft, the annual departure data of the individual air-crafts are converted to equivalent annual departure of the design aircraft. This is done by two sequential steps, as follows:
  • The departures of different aircrafts which have different gear configurations are converted to equivalent departure of the design aircraft, by multiplying with suitable factors. Various factors are suggested by the FAA (2006) using which departure of any gear configuration can be converted to departure of another gear configuration.
  • The departures of individual aircrafts adjusted for the gear configuration of the design aircraft are further adjusted for the wheel loads. This is done by using the following empirical formula:

(35)

where, R_d is the equivalent departure of the design aircraft, R_a is the departure of any aircraft adjusted for the gear configuration, W_d is the single wheel load of main gear of the design aircraft, and W_a is the single wheel load of main gear of any aircraft.

• Having obtained the equivalent departures of the design aircraft adjusted for gear configuration and wheel load, all the departures are summed up. Pavement design is performed for this total equivalent departure with the help of the specific design chart applicable for the design aircraft . From the known CBR values of the subgrade, sub-base, and base layer, the individual thickness composition are obtained. The dashed line indicates the order of progression. The asphalt thicknesses are generally specified for the critical and non-critical areas.
• To take care of the frost problem (freeze-thaw), first, the freezing index of the site is obtained. Predictive charts are developed to estimate the depth of frost penetration for various soil types and freezing index. Now, either of the following three approaches can be used for design purpose.
  • The thickness of the pavement can be made at least equal to the depth of frost penetration.
  • The frost susceptible subgrade soil may be replaced with non-frost susceptible soil.
  • Adequate pavement thickness may be provided assuming subgrade will have reduced strength during thawed condition.

A worksheet software for design of bituminous airfield pavement as per FAA can be downloaded from the FAA website

The Asphalt Institute Method

Asphalt Institute (MS-11 1987) recommends full depth asphalt pavement for air-field pavement (for gross aircraft weight higher than 60,000lbs). Mechanistic-empirical (M-E) method is used for design of pavement.

As per the M-E design method, the two modes of structural failure are considered to be governing the pavement design. These are fatigue and rutting failure. The horizontal tensile strain below the bituminous layer and vertical strain on the subgrade are considered as causative factor for fatigue and rutting respectively. The field calibrated fatigue/ rutting equations are obtained by relating the initial critical strains and the number of repetitions it take for the failure due to fatigue or rutting. The allowable fatigue and rutting strains, for a design traffic, are obtained from the field calibrated fatigue and rutting equations. In M-E approach the thicknesses of the pavement layers (asphalt surfacing and base/ sub-base) are adjusted in such a way that the developed fatigue and rutting strains are comparable to the allowable strains.