The structure of an airport flexible pavement may consist of untreated or treated (stabilized) granular base overlain by an impervious or waterproofing riding course much in the same way as with flexible highway pavements.
However, it is recommended that whenever possible, flexible airport pavements must be constructed as full-depth asphalt pavements as such a construction apart from not entrapping water which tends to reduce subgrade strength and impair performance is able to spread loads over a broader area and requires less pavement structure thickness.
Several methods for the design of flexible airport pavements exist but the most comprehensive and very recent is the method by the Asphalt Institute which will be discussed here.
The Asphalt Institute Method for design of flexible Airport pavements
This method designs the flexible airfield pavement as a full-depth asphalt structure based on the multi-layer elastic theory.
The design is based on the principle that application of load to the full-depth asphalt pavement produces two critical elastic strains: a horizontal tensile strain at the bottom of the asphalt layer and a compressive vertical strain at the top of the sub-grade.
Design curves developed in terms of the maximum allowable values for both critical strains are used to select the thickness of the full-depth asphalt pavement that will meet the aircraft traffic loading condition.
The design takes into consideration the effect on the strength of asphalt concrete of temperature condition peculiar to the geographical location of the airport.
The different types of aircraft using an airport pavement impose different strains and the effect of the strain repetitions on the pavement is cumulative.
It is assumed that all aircraft movements are channelized. The strain repetitions imposed on the pavement depending on aircraft type, gear load, the number of aircraft passes, and the transverse wander characteristics of the aircraft on the design area.
The effect of strain repetitions on the pavement due to a mixture of different aircraft is obtained by equating the number of strain repetitions to a number of equivalent strain repetitions produced by an arbitrary standard aircraft (DC-8-63F) which weighs 162 ton at 100% gross weight through equivalency factors.
The aircraft equivalency factors which are functions of aircraft type, pavement thickness, and performance criteria exist also in graphical form.
Thickness design procedure
The procedure for thickness design is shown in a flow diagram in Fig. 4.1. The design requires the following inputs:
- Design sub-grade elastic modulus
- Mean annual air temperature
- Projected aircraft traffic mix up to the end of the design period
The following steps are then followed for the tensile strain and the compressive strain criteria:
Step1: Allowable traffic value analysis
1. Using the mean annual air temperature, select the appropriate graph from the family of curves relating TA to subgrade modulus and various levels of allowable repetitions that correspond to the temperature. This is to be done for each strain criterion.
2. Using the design subgrade, determine from the selected graph the thickness of pavement (TA) that will satisfy the strain criterion for the different lines of strain repetitions shown on the selected graph. The results may be set out in a tabular form as shown below.
Note that this procedure will produce two tables one for tensile strain criteria and the other for compressive strain.
3. Develop the allowable traffic, Na, curve for each criterion by plotting the data in each table on a semi-log graph paper with TA on the arithmetic scale.
Step 2: Predicted traffic value
1. Draw up a table (Worksheet No. 1) showing the forecast aircraft movements for 5-year periods up to the design period. For each period, draw up two columns (A and B) and record under A the forecast traffic movements within the period and under B the cumulative number of movements up to the end of the period for all types of aircraft that will use the facility. See the example worksheet below.
2. Transfer the data for the cumulative aircraft passes for the design year onto Worksheet No. 2 (see sample format below). On this worksheet record for each aircraft type the number of equivalent DC-8-63F strain repetitions at various distances from the centreline using the cumulative movements or passes for the design period.
These are determined for each aircraft from the appropriate aircraft equivalency charts (Fig. D1-D65). For compressive strain criteria, the thicknesses to consider are; 10, 20, 30, 40in and for tensile strain 10, 30, 50in.
3. For each assumed thickness, sum the entries under each ‘distance’ column and circle the maximum value.
4. Develop the predicted traffic, Np, curve by plotting the circled maximum value against the assumed thickness for each criterion. This curve must be developed on the same graph on which the Na curve was developed for the corresponding and each strain criterion.
5. The thickness of full-depth asphalt concrete pavement required to satisfy each strain criterion is obtained by the simultaneous graphical solution of the plots of allowable and predicted traffic values i.e. the point of intersection of the Na and Np curves for each strain criterion.
6. The simultaneous solution will yield two thickness values; one for the tensile strain criterion and the other for compressive strain. Select the greater of the two as the design thickness of the pavement.