Subgrade

1
SESSION 5
Base Considerations
2
Objectives

• Describe functions of a base
• Describe different types of bases
• Describe advantages and disadvantages of each
  base type
• Discuss structural contribution of the base to
  the pavement

3
Functions of a Base

• Reduce bending stresses in slab
• Reduce deflections at joints and cracks
• Protect subgrade from frost
• Improve drainage of pavement
• Provide a stable platform for construction
• Achieve desired smoothness

4
Granular Base

• Untreated dense-graded aggregate
• Crushed stone
• Crushed slag
• Crushed or uncrushed gravel
• Sand
• Mixtures of these materials

5
Granular Base

• Pros
• Inexpensive
• Easy to construct
• Cons
• Susceptible to erosion
• May be very slow to drain
• Not well suited to high traffic

6
Asphalt-Treated Base

• Dense-graded aggregate mixed with binder (asphalt
  cement or emulsion)
• Same aggregate as granular base
• Granular subbase beneath

7
Asphalt-Treated Base

• Pros
• Easy to construct
• Stronger and more erosion resistant than granular
  base
• Suited to medium and high traffic levels
• Contributes to structural capacity if bonded to
  slab

8
Asphalt-Treated Base

• Cons
• Costs more than granular base
• Susceptible to stripping
• May increase slab curling stresses if debonding
  occurs

9
Cement-Treated Base

• Dense-graded aggregate mixed with portland cement
• Same aggregate as granular base
• 20-25 of paving concrete strength
• Granular subbase beneath

10
Cement-Treated Base

• Pros
• Easy to construct
• Stronger and more erosion resistant than granular
  base
• Suited to medium and high traffic levels
• Contributes to structural capacity if bonded to
  slab

11
Cement-Treated Base

• Cons
• Costs more than granular base
• May be susceptible to erosion
• May cause reflection cracking in PCC slab.
• May increase slab curling stresses if debonding
  occurs

12
Lean Concrete Base

• Similar to paving concrete, but less cement and
  therefore lower strength
• Typical cement contents 120-210 kg/m3 (200-350
  lb/yd3)
• 20-50 of paving concrete strength
• Longitudinal construction joints must match
  longitudinal joints in slab
• Base and slab may be bonded or unbonded

13
PCC Slab/LCBInterface Condition

• Unbonded
• Double coating wax curing compound
• Bonded
• Rough finish texture to base
• Notch base at joint locations
• Wet cure
• Position tie bars lower in slab
• Saw longitudinal joints 0.4-0.45 D

14
LCB Bonding in Germany
Transverse joints sawed 0.3 D
Longitudinal joint sawed 0.4 to 0.45 D
Tie bar placed D/3 from slab bottom
PCC slab
D/3
Lean concrete base
Transverse joint in base sawed 0.33 D
Longitudinal joint in base sawed 0.33 D
15
Failure to Match Joints
16
Alternative Interface

• Asphalt interlayer
• Resists reflection cracking
• Provides some friction

17
Lean Concrete Base

• Pros
• Uniform support
• Strong and erosion resistant
• Suited to medium and high traffic levels
• Contributes to structural capacity if bonded to
  slab

18
Lean Concrete Base

• Cons
• Costs more than granular or other treated bases
• May cause reflection cracking in PCC slab
• May increase slab curling stresses if debonded
  from slab

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Permeable Base

• Open-graded aggregate, untreated, or treated with
  asphalt or cement
• Remove water rapidly from pavement
• Require collector/outlet system and separator
  layer
• High-quality crushed stone required

20
Permeable Base

• Pros
• Drains more rapidly than dense-graded base
• May slow development of moisture-related
  distresses (faulting, D cracking)
• Contributes to structural capacity if bonded to
  slab

21
Permeable Base

• Cons
• Costs more than granular base
• Untreated permeable base can be difficult to
  construct
• May increase slab curling stresses if debonded
  from slab

22
Structural Contribution of Base

• Past thinking base was an improvement to the
  bearing capacity of the subgrade
• AASHTO 1986/1993 Guide, PCA Procedure
• Current thinking base is an improvement to the
  structural capacity of the slab
• AASHTO 1998 Supplement

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Structural Contribution of Base

• Reduces bending stress in proportion to
• Thickness of the base
• Elastic modulus of the base
• Degree of friction between slab and base
• Stiff base and low friction may increase slab
  curling stress
• Thicker slab and/or shorter joint spacing required

24
Summary

• Base serves several useful functions
• Factors in base type selection
• Cost,
• Traffic level
• Long-term durability (stripping and erosion)
• Drainage
• Ease of construction
• Slab/base friction
• Jointing

25
Summary (continued)

• Structural contribution of base
• past thinking improvement of foundation
• current thinking improvement to slab
• Stiff bases may be beneficial or detrimental
• Thicker slab and/or shorter joint spacing may be
  needed