UFC 3-260-02

30 June 2001

moisture content lower than saturation would normally consist of field moisture content measurements

under similar pavements located in the area. These measurements should be made during the most

critical period of the year when the water table is at its highest elevation. Extreme caution should be

exercised when the design is based on other than the saturated condition.

b. Traffic Data. The traffic parameters to be considered are the type of design aircraft, aircraft

loading, traffic volume, and traffic area.

(1) Traffic volume. The design traffic volume is expressed in terms of total operations of the

design aircraft expected during the life of the pavement. This traffic volume must be converted to a

number of expected strain repetitions. In converting operations to strain repetitions, the concept of

effective gear print is introduced. The effective gear print is the width of pavement that sustains an

effective strain repetition at a given depth in the pavement. The effective gear print is a function of the

number of tires in a transverse line, the transverse spacing, the width of the contact area, and the

effective thickness of pavement above the location of strain. The effective thickness of the pavement is

the sum of the thickness of unbound material plus twice the thickness of bound material where a bound

material is an asphalt concrete or stabilized layer. Thus, for a pavement having 76 millimeters (3 inches)

of asphalt and 381 millimeters (15 inches) of unbound gravel, the effective thickness with reference to

the strain at the top of the subgrade would be 381+(276) (15+(23)), or 533 millimeters (21 inches),

and with respect to the strain at the bottom of the asphalt, the effective thickness would be 276 (23),

or 152 millimeters (6 inches). With the determination of the effective thickness, the gear print is

computed as illustrated in Figures 11-2 and 11-3. If the gear is composed of tracking tires such as

tandem gear, then the number of strain repetitions may be somewhat greater than if the gear were not

tandem. When the tracking tires are located far enough apart, two distinct strain pulses will occur and

the multiplication factor for the tandem gear is 2. When the tires are sufficiently close, the strain pulses

merge into a single pulse and the multiplication factor is 1. The computation of F is shown in

Figure 11-4. In the figure, B is the spacing between tandem tires in the gear; te is the effective

pavement thickness; and Tw is the length of the ellipse that is formed by the tire imprint. When te is

less than B - Tw , F is 2. When te is greater than twice the difference between B and Tw , F is 1. For

values of te between the two conditions, F is computed based on the equation:

&

&

'

(11-1)

&

(a) The concept for conversion of aircraft operations to effective strain repetitions involves

assuming that traffic distribution on the pavement can be represented by a normal distribution. For traffic

on taxiways and runway ends (first 305 meters (1,000 feet)), the distribution has a wander width of

approximately 178 millimeters (70 inches), and traffic on runway interiors has a wander width of

approximately 355 millimeters (140 inches). (Note that wander width is defined as the width that

contains 75 percent of the applied traffic.) From the normal distribution, the fraction of traffic for which

the effective gear print will encompass a given point in the pavement can be computed. This fraction

times F gives the number or fraction of the effective strain repetitions at a point in the pavement for each

aircraft operation.

(b) The number of effective strain repetitions the pavement sustains at a point for every

aircraft operation is the pass-to-strain conversion percentage. For an effective thickness of

0.00 millimeters (0 inches), the percentage is the inverse of the pass-to-coverage ratio multiplied by 100.

11-3

Integrated Publishing, Inc. |