From empirical to mechanistic-empirical
The 1993 AASHTO Guide is an empirical method calibrated almost entirely from the AASHO Road Test (Ottawa, Illinois, 1958–1961). For flexible pavement it solves for a required Structural Number(SN); for rigid pavement it solves for slab thickness D. Reliability, serviceability loss, subgrade resilient modulus and traffic (ESALs) drive the design.
The Mechanistic-Empirical Pavement Design Guide (MEPDG, NCHRP 1-37A, later AASHTOWare Pavement ME) replaces those single equations with a mechanistic analysis of stresses and strains under axle loads, coupled to empirical distress models for rutting, fatigue cracking, thermal cracking and IRI. Climate and traffic spectra are modeled hour-by-hour rather than collapsed into one ESAL number.
Side-by-side
| AASHTO 93 | MEPDG / Pavement ME | |
|---|---|---|
| Method | Empirical (AASHO Road Test regression) | Mechanistic-empirical (stress/strain + distress models) |
| Traffic | Cumulative 80 kN ESALs | Axle-load spectra by hour, class, season |
| Climate | Not modelled directly (drainage coefficient mᵢ, Cd) | Hourly temperature & moisture from EICM |
| Subgrade | Effective resilient modulus Mᴿ | Seasonal Mᴿ profile |
| Flexible output | Required Structural Number → layer thicknesses | Predicted rutting, fatigue cracking, IRI vs time |
| Rigid output | PCC slab thickness D | Predicted faulting, transverse cracking, IRI vs time |
| Reliability | Single Z_R · Sₒ shift on log W₁₈ | Distress-specific reliability per limit state |
| Calibration | AASHO Road Test (1958–61), Ottawa IL | National + local calibration to in-service sections |
| Software | Closed-form; spreadsheet/web (this app) | AASHTOWare Pavement ME (licensed) |
| Typical use | Preliminary design, small agencies, education, low-volume roads | Highway agency final design, performance prediction |
When AASHTO 93 still makes sense
- Preliminary thickness sizing and feasibility studies.
- Low-volume roads, parking lots, and access roads where MEPDG inputs are not justified.
- Coursework and licensure exam preparation — most pavement design textbooks still teach the 1993 equations.
- Agencies that have not yet locally calibrated MEPDG distress models.
When to move to MEPDG
- High-volume highways where performance prediction (rutting, cracking, IRI) drives life-cycle cost.
- Projects with unusual climate, axle spectra, or PCC mixes not represented at the AASHO Road Test.
- Agencies with locally calibrated transfer functions.
Where this calculator fits
This tool implements the AASHTO 93 flexible and rigid design equations in metric units (mm, MPa, MPa/m). It is intended for the AASHTO 93 use cases above: preliminary design, low-volume roads, and teaching. For MEPDG-level performance prediction, use AASHTOWare Pavement ME with locally calibrated inputs.