SESSION 6SESSION 6

Thickness DesignThickness Design

Objectives

• Identify key design parameters in concrete pavement design

• Describe the principal concrete pavement design procedures

1986/1993 AASHTO Guide

1998 AASHTO Supplement

Portland Cement Association

Key Design Parameters

• Traffic

• Subgrade

• Climate

• Concrete properties

• Base

• Performance

• Reliability

Key Parameter: Traffic

• Traffic over design period

Axle load spectrum (PCA) numbers and weights of axles expected over design period

ESALs (AASHTO)

axle load spectrum converted to number of equivalent 18-kip [80 kN]

single-axle loads

Key Parameter: Subgrade

• Subgrade characterization

modulus of subgrade reaction (k value)

natural soil, embankment, rigid substrate

Embankment

Natural soil

Rigid layer} Subgrade

Key Parameter: Climate

• Environmental effects

joint opening and closing

slab curling

erosion of base and foundation

freeze-thaw weakening of soils

freeze-thaw damage to concrete

corrosion of dowels, reinforcement

Key Parameter: Concrete

• Concrete strength

28-day modulus of rupture (flexural strength) used in thickness design

• Concrete stiffness

28-day modulus of elasticity

Key Parameter: Base

• Base characteristics

type

thickness

stiffness

erodibility

drainability

slab/base friction

PCC Slab

Base

Key Parameter: Performance

• Performance criteria

One or more performance criteria used to define the end of the performance life of the pavement

AASHTO: loss of serviceability

PCA: fatigue cracking, erosion

Key Parameter: Reliability

• Design reliability

margin of safety against premature failure

higher functional classes and traffic volumes warrant higher reliability

AASHTO: adjustment to ESALs

PCA: adjustment to strength

Evolution of the AASHTO Method

• Original AASHO Road Test model (1961)

applicable to Road Test conditions only

• 1962 extended AASHO model

strength, elastic modulus, k value, ESALs

• 1972 extended AASHO model

J factor

• 1981 modification

modulus of rupture safety factor

Evolution of the AASHTO Method (continued)

• 1986 AASHTO Guide drainage factors, revised J, reliability

• 1993 AASHTO Guide

overlay chapters revised

• 1998 AASHTO Supplement revised model, improved k guidelines, curling/warping, structural effects of base

Effect of Subgrade k and Base Stiffness

0 5 10 15 20 25 30 35 40 45 50

100

200

400

Su

bg

rad

e k-

valu

e (p

si/i

n)

Allowable ESALs (millions)

Lean concrete base (E = 1 Mpsi, friction = 35)Asphalt-treated base (E = 500 ksi, friction = 6) Granular base (E = 25 ksi, friction = 1.5)

Effect of Climate on Slab Thickness

8.0 8.5 9.0 9.5 10.0 10.5 11.0

Miami, FL

Las Vegas, NV

Raleigh, NC

Baltimore, MD

Chicago, IL

Albany, NY

Required Slab Thickness (in)

Effect of Climate on Joint Spacing

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Miami, FL

Las Vegas, NV

Raleigh, NC

Baltimore, MD

Chicago, IL

Albany, NY

Allowable Joint Spacing (ft)

20 ft maxrecommended

12 ft minrecommended

PCA Method

• Axle load spectrum

• Total damage due to fatigue and erosion

• Joint, edge, and corner loading stresses

• Dowels or aggregate interlock

PCA Method (continued)

• Asphalt or tied concrete shoulder

• Composite k

• Safety factor on concrete strength

• Safety factor on axle loads for high traffic conditions

Other Methods

• Customized AASHTO methods: empirical adaptations, calibration to local conditions

• Mechanistic-empirical methods: mechanistic stress calculation + empirical cracking model

Zero-Maintenance, NCHRP 1-26

• Design catalogs: guidelines on thickness and other design details, formatted for ease of use

NCHRP 1-32, other countries

Summary

Modern concrete pavement design procedures consider not only slab thickness and traffic loading, but also:

• multilayer foundations

• structural contribution of base

• interaction between thickness and joint spacing

Summary (continued)

• climatic effects (curling, warping, joint opening)

• load transfer and edge support

• cracking, faulting, corner break distresses