Accounting of Strain Cycles Sustained by Airfield Rigid Pavements

The so-called 72-in. rule, employed in U.S. Department of Defense rigid pavement design for establishing the number of strain cycles arising under a pass of any aircraft on a particular pavement system, is re-examined using mechanistic tools, particularly layer elastic theory and dimensional analysis. Field data collected at Denver International Airport are reproduced using analytical simulations, which permit the generation of analogous synthetic results pertaining to different pavement systems and aircraft gear configurations. The analysis affirms the expectation that the criterion for establishing the number of strain cycles cannot be simply a fixed value, defined exclusively by the tandem wheel spacing. Rather, the dual wheel spacing and the radius of each tire-print must also be taken into consideration. In addition, the radius of relative stiffness of the pavement system needs to be accounted for. In this study, these variables are accommodated in the form of three dimensionless independent input parameters. The single dependent variable is the ratio (trough strain / maximum strain), denoted herein as υ. A process is formulated to ascertain whether υ is positive or negative: if υ>0, then one strain cycle may be expected; if υ<0, then two strain cycles may be expected. Comparisons of the process outcomes to those from the 72-in. rule show excellent agreement for the Denver conditions, testifying to the admirable simplicity and laudable wisdom of the latter. The process may be further refined for application to more complex gear configurations, e.g., tridems.