Lecture for Week 4

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Lecture for Week 4

• Environmental Considerations

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Environmental Considerations

• References WSDOT Pavement Guide, Volume 2,
  Section 3.0, Environment PGI, Module 4-Section 4
• Topics
• Climate information and temperature variations
• Frost action

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EnvironmentA Few Climate Stats
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EnvironmentTemperature Variation
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EnvironmentTemperature Variation
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Temperature and Pavements

• HMA and PG binders
• PCCP
• Expansion and contraction
• Slab curling

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PCCP and HMA
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PCCP Expansion and Contraction
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PCC Slab Curling
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PCC Slab CurlingEdge Stress
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PCC Slab CurlingInterior Stress
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PCC Slab CurlingCorner Stress
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PCC Slab CurlingRadius of Relative Stiffness
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PCC Slab CurlingC coefficients
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Freezing and Thawing Effects

• Frost heave and thaw weakening
• Frost heave
• Caused by the creation of ice lenses and largely
  driven through capillary rise in frost
  susceptible soils.
• Three conditions necessary for ice lenses
• Frost susceptible soil
• Subfreezing temperatures
• Water

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In addition

• Remove any of the 3 conditions and frost effects
  are eliminated. If the 3 conditions occur
  uniformly, heaving will be uniformotherwise
  differential heaving will occur resulting in
  pavement cracking and roughness. This is more
  likely to occur where
• Subgrades change from clean (NFS) sands to silty
  frost susceptible materials.
• Abrupt transitions from cut to fill with
  groundwater close to the surface.
• Drains, culverts, etc, often result in
  differential heaving due to different backfill
  material or compaction. Pipes that can change the
  thermal conditions (ie, that add or remove heat).

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Thaw Weakening
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Thaw weakening can lead to this
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Sources of Water
Source WSDOT Pavement Guide, Volume 2, Figure
2.27 (pdf)
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Frost Heave Process and Related Damage
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County road in Ferry County which shows frost
heavesnote location of cut and fill sections.
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SR 20 Okanogan CountyLongitudinal cracks due to
ice lenses growing parallel to shoulder slope.
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Capillary Rise
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Casagrande Criterion

• In 1932, Arthur Casagrande noted following
  rule-of-thumb
• "Under natural freezing conditions and with
  sufficient water supply one should expect
  considerable ice segregation in non-uniform soils
  containing more than 3 of grains smaller than
  0.02 mm, and in very uniform soils containing
  more than 10 percent smaller than 0.02 mm. No ice
  segregation was observed in soils containing less
  than 1 percent of grains smaller than 0.02 mm,
  even if the groundwater level is as high as the
  frost line."

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USACE Frost Design Soil Classification and
Associated USC Types
Source WSDOT Pavement Guide, Volume 2, Table 2.9
(pdf)
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WSDOT Granular Base Courses

• WSDOT uses crushed surfacing base course (CSBC)
  as a frost resistant crushed aggregate because it
  has a maximum of only 7.5 passing the 0.075 mm
  (No. 200) sieve.

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Mitigating Frost Action

• Limit the depth of frost into the subgrade soils.
• Removing and replacing frost-susceptible
  subgrade.
• Design the pavement structure based on reduced
  subgrade support.
• Provide a capillary break.

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But how do you estimate depth of freeze under a
new or existing pavement?
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Calculation of Freezing Index
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Mean Freezing Index Contours
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Design Freezing Index Contours
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Depth of Freeze Based on Design Freezing Index
for Fine-Grain Soil
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Depth of Freeze Based on Design Freezing Index
for Coarse-Grain Soil
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WSDOT Design for Freezing Conditions
Source WSDOT Pavement Guide, Volume 2, p. 2-106
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Depth of Freeze Based on WSDOT Field
MeasurementsWinters 1949 and 1950
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Modified Berggren Equationor how to calculate
the expected depth of freeze or thaw
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Stefan Formula

• Assumes latent heat of the soil is the only heat
  which must be removed when freezing the soil.
• This is an over simplification of the actual
  conditions.

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Stefan Formula
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Stefan Formula
Source WSDOT Pavement Guide, Volume 2, p. 2-118
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Stefan Formula

• Refer to Para 3.2.2.2 and Equation 2.1
• ?(?T)(dt) surface freezing index (F-hr).
  Freezing index normally expressed as F-days.
  Thus expression is (2)(24)(n)(FI) 48nFI

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Modified Berggren Equation

• Best reference for Assignment No. 3 is PGI
  (1995), Volume 2, Section 2.0, Paragraph 3.2.2
• Modified Berggren Equation

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Inputs

• k thermal conductivity BTU/hr-ft2-F/ft
• 
  BTU/hr-ft-F
• k for pavement material\s f(density, mc)
• k for HMA?

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Inputs

• kavg
• Average thermal conductivity of each layer.
  First, you must estimate the layer moisture
  content and dry density (see Para 3.2). If you
  want to use figures for determination of k, refer
  to Figs 2.28-2.30. Get k values for frozen and
  unfrozen cases, then average.

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Inputs

• Sources for equation inputs
• FI Average FI is given for most Washington State
  cities in Table 2.10. Use the contour map (Fig
  2.33) for Design FI. Units are F-days.
• n Adjusts air FI (which is what you get from
  Table 2.10 or Fig 2.33) to surface FI. Refer to
  Typical Values in Para 3.2.1.5(c)(i).
• n surface freezing index/air
  freezing index

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Inputs

• Sources for equation inputs
• L Latent heat is the heat that must be removed
  to convert an unfrozen soil to frozen at 32F.
  Function of the layer density and moisture
  content (refer to Para 3.2.1.4).
• L (144 BTU/lb)(w)(?d) BTU/ft3

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Modified Berggren Equation

• Must deal with multiple layers for most
  conditions (and Assignment 3)
• Use the following

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Use of Modified Berggren Equation

• How do you calculate (L/k)eff?
• First, assume a depth of freeze, x
• Second, use

This is the assumed total depth of freeze (ft)