Structural Numbers

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STRUCTURAL NUMBERS FOR FLEXIBLE PAVEMENTS

The design of a flexible pavement or surface treatment expected to carry more than 50,000 repetitions of equivalent single 18-kip axle load (SAI) requires identification of a structural number SN that is used as a measure of the ability of the pavement to withstand anticipated axle loads. In the AASHTO design method, the structural number is defined by

SN= SN₁+SN₂+SN₃

where
SN₁ structural number for the surface course=a₁D₁

a₁= layer coefficient for the surface course
D₁=actual thickness of the surface course, in
(mm)

SN₂=structural number for the base course = a₂ D₂ m₂

a₂= layer coefficient for the base course
D₂= actual thickness of the base course, in (mm)
m₂= drainage coefficient for the base course

SN₃ =structural number for the subbase course=
a₃ D₃ m₃

a₃= layer coefficient for the subbase course
D₃= actual thickness of the subbase course, in (mm)
m₃= drainage coefficient for the subbase

The layer coefficients a_n are thus given to materials used in that corresponding layer which is then used to convert structural numbers to actual thickness.

Different transportation agencies have given different values by keeping the standards.For example the layer coefficients may be 0.44 for asphaltic-concrete surface course, 0.14 for crushed-stone base course and 0.11 for sandy-gravel sub base course.

The thicknesses D₁, D₂,and D₃ are then rounded to the nearest 1/2 in (12.7 mm). The selection of layer thicknesses is based on many factors, some of these are agency standards, economic feasibility, maintainability of the pavement etc.

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