AASHTO 1993 Flexible Pavement Design: Key Concepts
The AASHTO 1993 Guide for Design of Pavement Structures remains the primary reference for flexible pavement design on local and state roadways across much of the United States. It introduced a semi-empirical, performance-based approach grounded in the AASHO Road Test data from the late 1950s, extended through regression analysis to broader conditions.
Structural Number (SN)
The structural number is an abstract composite index representing the load-carrying capacity of the pavement section. It is computed as the sum of the products of each layer's thickness, structural coefficient, and drainage coefficient: SN = a₁D₁ + a₂m₂D₂ + a₃m₃D₃. The required SN must be met or exceeded by the proposed section. A typical surface layer structural coefficient (a₁) for dense-graded HMA is 0.44.
ESALs: The Traffic Input
Traffic loading is expressed as equivalent 18-kip single-axle loads (ESALs) for the design period, typically 20 years. A single heavily loaded truck may contribute dozens of ESALs to the pavement depending on axle configuration and load. ESAL computation requires knowing the anticipated mix of truck types, their loads, and the AASHTO load equivalency factors (LEFs) for each.
Reliability and Subgrade MR
The reliability level (R) sets the probability that the designed pavement will survive the full design period without structural failure. Higher-volume urban routes typically use 90–95% reliability; lower-volume rural roads, 75–85%. The subgrade resilient modulus (MR) characterizes the stress-dependent stiffness of the subgrade under repeated loading. It is measured in the laboratory or estimated from CBR or R-value correlations. A 50% change in MR can shift the required SN by half a unit — making accurate subgrade characterization one of the most important steps in pavement design.
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