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Design Methods
• Highway Pavements
AASHTO The Asphalt Institute Portland Cement Association

• Airfield Pavements
FAA The Asphalt Institute Portland Cement Association U.S. Army Corps of Engineers

Objectives of Pavement Design
To provide a surface that is:

• Strong
Surface strength Moisture control

• Smooth • Safe
Friction Drainage

• Economical
Initial construction costRecurring maintenance cost

Pavements are Designed to Fail !!

Pavement Design Methodologies
• Experience • Empirical
Statistical models from road tests

• Mechanistic-Empirical
Calculation of pavement stresses/strains/deformations Empirical pavement performance models

• Mechanistic
Calculation of pavement stresses/strains/deformations Mechanics-based pavement performance models Empirical vs. Mechanistic Design P d Wood Floor Joist

L

Empirical “Rule of 2”: d in inches= (L in feet / 2) + 2

Mechanistic: σbending =

PL ≤ σ allowable 4S

1993 Version

AASHTO Pavement Design Guide
• Empirical design methodology • Several versions:
1961 (Interim Guide) 1972 1986
Refined material characterization Version included in Huang (1993)

1993
More on rehabilitation Moreconsistency between flexible, rigid designs Current version

2002
Under development Will be based on mechanistic-empirical approach

AASHO Road Test (late 1950’s)

(AASHO, 1961)

One Rainfall Zone...

(AASHO, 1961)

One Temperature Zone...

(AASHO, 1961)

One Subgrade...

A-6 / A-7-6 (Clay) Poor Drainage
(AASHO, 1961)

Limited Set of Materials...
• One asphalt concrete3/4” surface course 1” binder course

• One Portland cement concrete (3500 psi @ 14 days) • Four base materials
Well-graded crushed limestone (main experiment) Well-graded uncrushed gravel (special studies) Bituminous-treated base (special studies) Cement-treated base (special studies)

• One uniform sand/gravel subbase

1950’s Construction Methods...

(AASHO, 1961)

(AASHO, 1961)1950’s Vehicle Loads...

Limited Traffic Volumes...
1.1M Axles 1.1M Axles

Axle Loads (Thousands)

2 Years 2 Years

Time (Months)
(AASHO, 1961)

1950’s Data Analysis...
(AASHO, 1961)

Some Failures...

(Some pavements too!)

(AASHO, 1961)

AASHTO Design Based on Serviceability Decrease

(AASHTO, 1993)

What is Serviceability?
• Based upon Present
Serviceability Rating(PSR)

• Subjective rating by
individual/panel
Initial/post-construction Various times after construction

• 0 < PSR < 5 • PSR < ~2.5: Unacceptable
(AASHO, 1961)

Present Serviceability Index (PSI)
• PSR correlated to physical pavement measures via Present
Serviceability Index (PSI):
PSI = 5.03 − 1.91log(1 + SV ) − 1.38 RD − 0.01(C + P)1/ 2 SV = slope variance (measure of roughness) RD= average rut depth (inches) C + P = area of cracking and patching per 1000 ft 2 PSI ≈ PSR
2

Empirical!

AASHTO Design Guide (1993)
Part I: Pavement Design and Management Principles • Introduction and Background • Design Related to Project Level Pavement Management • Economic Evaluation of Alternative Design Strategies • Reliability

AASHTO Design Guide (1993)
Part II: Pavement DesignProcedures for New Construction or Reconstruction • Design Requirements • Highway Pavement Structural Design • Low-Volume Road Design

AASHTO Design Guide (1993)
Part III: Pavement Design Procedures for Rehabilitation of Existing Pavements • Rehabilitation Concepts • Guides for Field Data Collection • Rehabilitation Methods Other Than Overlay • Rehabilitation Methods With Overlays

DesignScenarios Included in AASHTO Guide

(AASHTO, 1993)

AASHTO Design Based on Serviceability Decrease

(AASHTO, 1993)

Flexible Pavements

Design Equation
log10 (W18 ) = Z R So + 9.36 log10 ( SN + 1) − 0.20  ∆PSI  log10   4.2 − 1.5  + 2.32 log M − 8.07  + 10 ( R) 1094 0.40 + 5.19 ( SN + 1)
W18 = design traffic (18-kip ESALs) ZR = standard normal deviate So = combined standard error...
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