SESSION 6 Thickness Design
Jan 17, 2016
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