AASHTO Flexible Pavement Design Calculator

Highway engineering & pavement geotechnics: Compute required Structural Number ($SN$) via the AASHTO 1993 design equation and determine asphalt, base, and subbase layer thicknesses.

Traffic, Reliability & Subgrade

Design ESALs W₁₈ (millions): 18-kip equivalent single axle loads

Design Reliability R (%): Statistical certainty level

Subgrade Modulus M_R (psi): Roadbed soil resilient modulus (CBR ~5)

Standard Deviation S₀: AASHTO flexible default = 0.45

Initial Serviceability p₀: New pavement PSI (typically 4.2)

Terminal Serviceability p_t: End-of-life PSI (Major = 2.5, Local = 2.0)

Surface Layer a₁ / D₁ (in):

HMA coeff. a₁ / chosen D₁ (in)

Base Layer a₂ / m₂ / D₂ (in):

Crushed stone a₂ / chosen D₂ (in)

Structural Number & Cross-Section

Required Structural Number (SN)

AASHTO 1993 implicit equation

Provided Structural Number

Recommended Subbase (D₃)

Assuming a₃ = 0.11, m₃ = 1.0

Total Pavement Depth

Serviceability Drop (ΔPSI)

p₀ - p_t

Equivalent Subgrade CBR

CBR ≈ M_R / 1500

Frequently Asked Questions

What is an ESAL (Equivalent Single Axle Load)?

An ESAL converts the damaging fatigue effect of mixed axle configurations (steering, single, tandem, tridem) into equivalent standard 18,000-pound (80 kN) single axle passes. Damage follows the generalized "fourth-power rule" ((Load / 18,000)⁴).

How does subgrade resilient modulus (M_R) affect thickness?

Weak subgrades with low resilient modulus (e.g., wet soft clay with CBR 2-3, M_R ≈ 3,000 psi) deform excessively under wheel loads, requiring much larger Structural Numbers (SN > 5.0) and thicker aggregate layers to distribute surface stress.

Why are minimum thickness thresholds enforced for HMA?

AASHTO dictates minimum practical construction thicknesses (e.g. 2 to 3 inches for HMA, 6 inches for aggregate base) regardless of theoretical SN calculations to prevent compaction tearing, thermal shrinkage cracking, and premature rutting.