Subgrade

1
SESSION 6
Thickness Design
2
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

3
Key Design Parameters

• Traffic
• Subgrade
• Climate
• Concrete properties
• Base
• Performance
• Reliability

4
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

5
Key Parameter Subgrade

• Subgrade characterization
• modulus of subgrade reaction (k value)
• natural soil, embankment, rigid
  substrate

Embankment
Subgrade
Natural soil
Rigid layer
6
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

7
Key Parameter Concrete

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

• Concrete stiffness
• 28-day modulus of elasticity

8
Key Parameter Base

• Base characteristics
• type
• thickness
• stiffness
• erodibility
• drainability
• slab/base friction

PCC Slab
Base
9
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

10
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

11
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

12
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

13
Effect of Subgrade k and Base Stiffness
Lean concrete base (E 1 Mpsi, friction 35)
400
Asphalt-treated base (E 500 ksi, friction 6)
Granular base (E 25 ksi, friction 1.5)
Subgrade k-value (psi/in)
200
100
0
5
10
15
20
25
30
35
40
45
50
Allowable ESALs (millions)
14
Effect of Climate on Slab Thickness
Required Slab Thickness (in)
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
15
Effect of Climate on Joint Spacing
Allowable Joint Spacing (ft)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Miami, FL
20 ft max
12 ft min
recommended
recommended
Las Vegas, NV
Raleigh, NC
Baltimore, MD
Chicago, IL
Albany, NY
16
PCA Method

• Axle load spectrum
• Total damage due to fatigue and erosion
• Joint, edge, and corner loading stresses
• Dowels or aggregate interlock

17
PCA Method (continued)

• Asphalt or tied concrete shoulder
• Composite k
• Safety factor on concrete strength
• Safety factor on axle loads for high traffic
  conditions

18
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

19
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

20
Summary (continued)

• climatic effects (curling, warping, joint
  opening)
• load transfer and edge support
• cracking, faulting, corner break distresses