Calculating Dimensions for a Typical Pavement Section using 1993 AASHTO Flexible Pavement Design Guide

The AASHTO Guide for Design of Pavement Structures (AASHTO, 1993) is the primary document used to design new and rehabilitated highway pavements. Approximately 80% of all states use the AASHTO pavement design procedures, with the majority using the 1993 version. All versions of the AASHTO Design Guide are empirical design methods based on field performance data measured at the AASHO Road Test in 1958-60.

Design of a flexible pavement section requires the developers to establish a baseline scenario for the location where the pavement is going to be placed The steps in the 1993 AASHTO flexible pavement design procedure summarized below and are in the context of the baseline scenario presented by Licon Engineering Co. (LEC):

Step 1: Determine the analysis period. For the design scenario, a 20-year design life is specified.

Step 2: Evaluate the design traffic: Street Classification: Residential Sub-Collector From recommendations from the City of El Paso, W18 =269,000 EALs. EAL=Equivalent Axle Loading

Step 3: Determine the design reliability factors: Reliability = 80% (local urban traffic), ZR = standard normal deviate = -0.841, S0 = standard deviation = 0.45 (most commonly used).

Step 4: Determine the allowable serviceability loss due to traffic (ΔPSI) ΔPSI = 2.2 given by equation: ΔPSI = pt− po=4.2−2.0=2.2 Values most commonly used for pt and po

Step 5: Evaluate the seasonally averaged subgrade resilient modulus MR using the formula: MR = 2555*(CBR) 0.64 MR = 6204.5 psi.

Step 6: Determine the layer properties: Values for design: Structural layer coefficients ai for all bound layers. Recommendations for appropriate ai values for design are given by 1993 AASHTO Design Guide. Values for design: a1 = 0.44, a2 = 0.14, and a3=0.11  Drainage coefficients mi for all unbound layers are set to 1.0

Step 7: Solving Equation:   for the required overall structural number (SN) using software or excel solver: SN = 2.71.

Step 8: Determine the design layer thicknesses for the pavement section to apply to equation:   From the City of El Paso guidelines, minimum HMA section (wearing surface) should not be less than 1.5 inches. Therefore, D1 is equal to 2.0 inches.  From the City of El Paso and TxDOT guidelines, recommended base range 4-10 inches,  Therefore D2 is equal to 5.5 inches Solving equation for subbase layer: 2.7= .44(2.0) + 0.14(5.5) + 0.11(D3) D3 = 7.73 inches Use D3 = 8.0 inches for construction.1 1After rounding to the nearest half-inch, per the recommendations in the 1993 AASHTO Design Guide. Unbound layer thicknesses are rounded to the nearest inch.

Pavement Section