AASHTO-93 flexible pavement design (structural number) on the PE Civil Transportation exam

An AASHTO-93 flexible pavement problem asks one of three things: how many 18-kip ESALs the design lane will see, whether a given structural number (SN) can carry them, or how thick each layer must be to supply it. The method is in the AASHTO Guide for Design of Pavement Structures (1993), which is supplied with the exam. The Handbook covers part of it. It has the design-lane ESAL equations, a table of typical structural layer coefficients (when a problem gives its own a-values, use those), and the correlation of subgrade resilient modulus with CBR. It does not print the flexible design equation, the drainage coefficients or the reliability table. For those, use the Guide.

What you actually need to own

• Design-lane ESALs. For each truck class, multiply daily volume × 365 × the growth factor × axles per truck × the load equivalency factor per axle. Then add the classes and take the design lane's share: ESAL_DL = D_D × D_L × ESAL. If you're reading the factors from the Guide's tables, use the flexible tables at the right SN and terminal serviceability. A tandem is one axle group, read in the tandem table.
• The design equation. It relates log W18 to SN, reliability (Z_R·S_0), the serviceability loss, and the effective roadbed resilient modulus M_R. The Guide prints it with its flexible design chart. Run it forward (SN given, find W18), or iterate on SN until W18 matches the design ESALs.
• Reliability. Z_R is the standard normal deviate for the chosen reliability, from the Guide's table. It's negative, so the Z_R·S_0 term lowers the traffic a section can carry. S_0 is given.
• Serviceability. ΔPSI = p0 − pt, the loss. The 4.2 and 1.5 inside the equation's serviceability term are fixed constants from the road test, not your project's p0 and pt.
• Effective M_R. If it's given, use it. For seasonal moduli, the Guide converts each month's modulus to a relative damage, u_f = 1.18 × 10⁸ × M_R^−2.32, averages the damage, and converts back. The softest months dominate the result.
• Structural number. SN = a1·D1 + a2·D2·m2 + a3·D3·m3. Drainage coefficients m apply to the untreated base and subbase only. They come from a table indexed by drainage quality and the percent of time the structure sits near saturation.
• Layered design. Find the SN required above the base, above the subbase and above the roadbed. Size the surface first. Round each layer up to the next ½ in. (the Guide sets thicknesses in ½-in. steps), then carry the SN it actually provides at that thickness down to the next layer.
• Overlays. The required overlay structure is SN_f − SN_eff. The Guide estimates SN_eff from deflection testing or from a remaining-life condition factor.

Where people lose points

• Entering pt as ΔPSI, or swapping p0 − pt into the slot for the equation's 4.2 − 1.5 constants.
• Estimating M_R from CBR when M_R is given. The given effective modulus is the design input.
• Averaging seasonal moduli. Average the damage, not the moduli.
• Applying a per-axle factor once per truck, or pairing one class's axles with another class's factor.
• Splitting a tandem into two single axles, reading the rigid table for a flexible pavement, or reading the load row one step off.
• Subtracting the required SN of the upper layers instead of the SN their rounded thicknesses provide.
• Borrowing the base's a or m for the subbase. Each layer has its own coefficients.
• Misreading the drainage table by one row (drainage quality) or one column (time near saturation).
• Overlay by deflection: D is the total thickness of all layers above the subgrade, and E_p goes in psi, not ksi.

How to study it

Take one section and work it in both directions. Given SN, find W18 and compare it with design-lane ESALs you built from a truck mix. Then fix the ESALs, iterate SN to two decimals, and split it into rounded layers. Change one input at a time (reliability, ΔPSI, M_R) and watch which one moves SN most. The exam also supplies the MEPDG; know its three input levels (measured, correlated, default). For the earthwork beneath the pavement, see mass haul diagrams.

Part of the Geotechnical and Pavement area of the PE Civil Transportation exam. → Start practicing free — the whole practice bank, free with an account.