Material Equivalence for Flexible Aircraft Pavement Thickness Design

Material equivalence is the thickness of one pavement material which could be replaced by a different thickness of another pavement material to result in the same structural pavement capacity. The Federal Aviation Administration of the USA (FAA) pavement design guide provides ranges for equivalence factors for crushed rock base course, uncrushed gravel sub-base, asphalt and other pavement materials. These factors are commonly used to convert thicknesses of `standard' pavement structures to equivalent thicknesses of alternate pavement compositions. The FAA provided ranges of equivalence are broad and most designers tend to utilise the mid-ranges values. When converted pavement thicknesses (using the mid-range FAA values) are compared to pavement thicknesses derived directly from the layered elastic design tool Aircraft Pavement Structural Design System (APSDS) there is a significant discrepancy, especially as the pavement structure moves further away from the `standard' pavement composition. For medium to large aircraft, the equivalence factors which would be required to provide consistence with APSDS lie at the lower end of the FAA ranges. They are 1.3 for Asphalt in place of Crushed Rock, 1.2 for Crushed Rock in place of Uncrushed Gravel and 1.6 for Asphalt in place of Uncrushed Gravel. The FAA should consider revising the material equivalence values within their design guide in light of these findings. Some variability exists within the APSDS implied equivalences. Both the location of the replacement material in the pavement structure and the portion of the pavement thickness being replaced, significantly affect the equivalence implied by the layered elastic tool.