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

Recommended Tools & Equipment

Tested hardware and components for high reliability

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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.