How Subgrade Construction Requirements Relate to Surfacing Design

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How Subgrade Construction Requirements Relate to
Surfacing Design
Highways and Engineering Conference February 7,
2006
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Presentation Outline

• Pavement Design Procedure
• Pavement Basics
• Subgrade Characteristics
• Special Borrow
• Importance of Constructing a Strong Subgrade
• Monitoring R-Value During Construction

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Surfacing Design Unit Overview

• What does Surfacing Design do?
• Pavement Design
• project selection
• pavement type
• Pavement layer thicknesses
• Pavement materials
• Investigate New Pavement Technologies
• Serve as a Technical Resource

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The Pavement Design Process
Climate
Traffic
Reliability
Subgrade Properties
Pavement Structure
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Pavement Basics
120 psi
15 psi
3.7 psi
Special Borrow
0.8 psi
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Pavement Basics
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Subgrade Characteristics
Subgrade
Special Borrow (Subgrade)
Fill
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Subgrade Characteristics

• Resilient Modulus
• Key measurement for pavement design
• Measure of soil strength and resistance to
  deformation
• Range of Values
• A-7 clays 3,750 psi
• A-1-a 30,000 psi

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Resilient Modulus
10,000 lbs.
10,000 lbs.
Pavement
Sandy Gravel
Subgrade
Subgrade
Clay
Sandy Gravel
Clay
MR3,750
MR30,000
Deformation or Subgrade Rutting
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How do we Measure Resilient Modulus

• NDT Testing
• R-Value

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NDT Testing
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R-Value Testing

• Samples gathered during the soil survey
• Cores drilled at ½ mile intervals
• Subgrade and Base Course samples taken
• Sent to Helena Laboratory for R-Value
• A-6 and A-7 Soils Automatically given R-Value5

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R-Value Testing
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R-Value Testing

• Measure of how much a soil sample deforms under a
  vertical load
• Measured under saturated conditions
• Spring Thaw
• R-Value Range
• A-7 5
• A-1-a 60

2000 lb.
Deformation
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Conversion of R-Value to Resilient Modulus
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85th Percentile R-Value
Soil Survey R-Values
Soil Survey R-Values
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24
23
14
25
38
27
28
22
22
44
31
46
28
26
65
29
25
34
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85th Percentile R-Value
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SURFACING SECTION DESIGN BASED ON THE TOP 0.6
METERS OF SUBGRADE HAVING AN R-VALUE OF 23
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Special Borrow

• The purpose is two-fold
• Thinner Pavement Section
• Base Pavement Design on Special Borrow R-Value
  rather than the Subgrade
• Ease of Construction

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Special Borrow
Subgrade R-Value5 Special Borrow R-Value 30
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Why is Special Borrow 2 Deep?

• 2 Special Borrow is an Old Geotechnical Rule of
  Thumb

120 psi
15 psi
3.7 psi
Special Borrow
0.8 psi
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What if Substandard Subgrade is placed?
20-year Pavement Life 11,000,000 ESALs Subgrade
R-Value5 Special Borrow R-Value 30
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What if Substandard Subgrade Material is Placed?

• Subgrade Rutting
• Base Contamination

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Subgrade Rutting
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Subgrade Rutting

• Virtually Non-existant in Montana
• Why?
• Good Construction Practices
• Proper Compaction
• Specifying R-Value
• Good Pavement Design
• Limit Subgrade Strain

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Base Contamination
Thin Pavement with Wet, Fine Subgrade
Repeated Loads
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Base Contamination
Thick Pavement with Wet, Fine Subgrade OR Thinner
Pavement with a Coarse Subgrade
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How to Avoid Base Contamination

• Good Pavement Design
• Proper Subgrade or Special Borrow R-Value
• Good Construction
• Proper Compaction
• Dig-outs

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How to Avoid Base Contamination

• Provide Drainage
• Cross Slope
• Daylight Base Course
• Daylight Special Borrow
• Ditches

• Impervious Pavement
• Seal Cover
• Cross Slope

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Monitoring R-Value During Construction

• A-1, A-2, A-3 and A-4 Soils
• 3 or more cuts or fills per mile
• Sample every 1000
• Less than 3 cuts or fills per mile
• Sample every 2000
• A-6 and A-7 Soils
• For typical sections designed on R-Value 5
• No R-Value sampling
• Special Borrow
• R-value testing done to qualify special borrow
  source
• No R-Value sampling during construction
• Keep an Eye Out!

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Presentation Outline

• Special Borrow
• Specifying Soils Classes
• R-Value Testing
• Subgrade Construction
• Density Control
• Importance of Moisture Control
• Mixed Soils

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Special Borrow

• The purpose is two-fold
• Thinner Pavement Section
• Base Pavement Design on Special Borrow R-Value
  rather than the Subgrade
• Ease of Construction

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Subgrade Characteristics
Subgrade
Special Borrow (Subgrade)
Fill
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Special Borrow

• Special Borrow Bid item used when specific
  requirements are specified for a borrow material.
• R-Value
• Soils classification
• Gradation

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Why would you want to use Soils Classifications
for Special Borrow?
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R-Value Limitations

• Rocky materials vary to some degree because
  larger rock is removed for the testing.
• Surfacing Design has recently implemented a
  policy where the maximum design R-Value allowed
  will be 60.
• Highly variable for certain soil types
• A-4 materials can vary dramatically.

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Advantages of Soils Classifications

• Quick turnaround
• Most labs can do the testing
• Less variability in test results

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Disadvantages of Soils Classifications

• Generally does not correlate to the strength of
  the material.

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Soils Classifications R-Value

• A-1-a(0) materials can be automatically assigned
  an R-Value of 30 or less.
• No written policy at this time.
• Based on historic information.
• A-6 and A-7 soils classes are not tested for
  R-Value.
• Automatically assigned a R-Value 5.

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Why are we still using it?
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Subgrade Rutting

• Virtually Non-existant in Montana
• Why?
• Good Construction Practices
• Proper Compaction
• Specifying R-Value
• Good Pavement Design
• Limit Subgrade Strain

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Other Tools

• MDT has looked at using other tests
• Would lose historic data
• California Bearing Ratio (CBR)
• Commonly used by other states
• Occasionally used by consultants on MDT projects
• Dynamic Modulus

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85th Percentile R-Value
23
SURFACING SECTION DESIGN BASED ON THE TOP 0.6
METERS OF SUBGRADE HAVING AN R-VALUE OF 23
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Construction Quality Control

• Finished Subgrade Testing
• MT 201 Sampling Roadway Materials, Subsection 28
• 28 Field Construction Sampling
• 28.1 This procedure applies to material used for
  cut and fill sections within the R/W limits of
  the project.
• 28.2 Samples for soil classification and R
  value shall be obtained from the top 2 feet (0.6
  meters) of the sub-grade. Sample frequency will
  be one sample every 1000 feet (305 meters) for
  projects with 3 or more cuts or fills per mile.
  If the project has fewer than 3 cuts or fills per
  mile the sample distance is extended to 2000
  feet (610 meters).
• 28.3 District/Area lab personnel will determine
  the soil classification. If the soil class is not
  equal to or better than that used by the
  Surfacing Design Unit to determine the typical
  section, then samples for R value
  determination are to be submitted to the
  Materials Bureau in Helena.
• NOTE These samples are as a design check only.
  Sampling every 1000 feet (305 meters) is a
  general guide and some discretion should be used.
  For example, it may be advisable to take more
  samples in fill sections than cut sections, or if
  there is an obvious change in the soil conditions
  or soil class, etc. No samples will be required
  for typical sections with a design R of 5 or
  less or for soils classified as A-6 or A-7.

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R-Value Testing

• Samples gathered during the soil survey
• Cores drilled at ½ mile intervals
• Subgrade and Base Course samples taken
• Sent to Helena Laboratory for R-Value
• A-6 and A-7 Soils Automatically given R-Value5

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Monitoring R-Value During Construction

• A-6 and A-7 Soils
• For typical sections designed on R-Value 5
• No R-Value sampling
• Special Borrow
• R-value testing done to qualify special borrow
  source
• No R-Value sampling during construction

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Subgrade Construction

• How do we control subgrade construction?

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Construction Quality Control

• Outlined in the Standard Specifications and
  MT-218 Determining Relative Compaction and
  Percent Moisture and MT-601 under density
  control embankment
• Compaction
• Proctors
• Zero Air Voids
• Control Strips (Exception only)
• Moisture
• Lift thicknesses
• R-Value (Special Borrow)

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Standard vs Modified Proctor

• 5.5 lb Hammer
• 12 inch drop
• 4 inch or 6 inch molds
• 3 layers of material w/25 blows per layer (Total
  depth of 5 in)

• 10 lb hammer
• 18 inch drop
• 4 inch or 6 inch molds
• 5 layers of material w/25 blows per layer (total
  depth of 5 in)

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Why have two Methods?

• Modified proctor is used in two cases.
• Standard Specification 203.03.3 Moisture and
  Density Requirements requires its use on A-1
  embankment material.
• Crushed Aggregate Course (CAC).
• Standard proctor is used for all other soils.

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Moisture Density Requirements
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Sampling Testing Requirements
DENSITY CONTROL CULVERT BACKFILL STD. SPEC. 603 SAMPLE MT201 TEST MT218 MT229 MT201 1 TEST PER 6" LIFT ON EACH SIDE OF THE PIPE FOR EACH 100', A MINIMUM OF 1 TEST PER SITE.
DENSITY CONTROL EMBANKMENT STD. SPEC. 203 SAMPLE MT201 TEST MT204 MT218 MT229 MT201 1 TEST FOR EACH 4,000 CUBIC YARDS FOR A-1 THRU A-3 MATERIAL. 1 TEST FOR EACH 2,000 CUBIC YARDS A-4 THRU A-7 MATERIAL. IA TESTS - 1 FOR EACH 3 MILES OR 200,000 CUBIC YARDS.
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Pretty easy, Right?
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Too Rocky to Test

• MT 218 Method of Test for Determining Relative
  Compaction and Percent Moisture Subsection 4.1.5

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Too Rocky to Test

• 4.1.5 When the material under test is a soil or
  aggregate with a maximum size larger than 2
  inches (50 mm) and more than 50 of the material
  under test is retained on the 4 mesh (4.75 mm),
  the material will not be required to meet 95
  density within 2 optimum moisture but
• 4.1.5.1 A screen analysis must be provided on
  representative samples from each lift of the
  embankment area to prove that more than 50 of
  the material is retained on the 4 mesh (4.75 mm)
  sieve.
• 4.1.5.2 Nuclear M/D readings must be taken on
  each lift in the embankment area to demonstrate
  that uniform relative density has been achieved.
• 4.1.5.3 Notes must be made on the Summary of
  Compaction Data (Form 1006) showing the results
  obtained in paragraphs 4.1.5.1 and 4.1.5.2.

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Compaction
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Moisture

• /- 2 moisture required for subgrade.
• Compacting at lower moisture content is allowable
  by Standard Specification 203.03.3
• If proctors are used for density control, the
  Contractor may make a written request to the
  Engineer to compact the soils at a lower moisture
  content. Identify the soil class in the request.
• The Engineer may approve the request provided a
  Department investigation ensures the lower
  moisture content is not detrimental to compaction
  of the soil class.

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Pros and Cons

• Lubricates the soil particles. Minimizes the
  required compactive effort.
• Swells clays
• Dust abatement

• Excess moisture causes the soil to lose its
  strength resulting in soft spots.

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Challenges

• Soft areas that meet density and moisture
  requirements.
• Areas with greater than 100 compaction
• Selecting the proper proctor.
• Mixed soils.

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Mixed Soils
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Mixed Soils
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One Point Proctors

• 7 One Point Proctors
• 7.1 A "one point Proctor" is an abbreviated
  standard Proctor compaction test and is used in
  conjunction with the family of curves. Rather
  than determining the moisture and density points
  for an entire curve, a single point is determined
  for the purpose of selecting the curve, which
  represents the soil being compacted, from the
  family of curves.
• 7.1.1 One-point Proctors shall be run whenever
  there is any doubt that the soil being compacted
  is from a location on the project, which is
  represented, by one of the curves in the family
  of curves.
• 7.1.2 Frequently soils may be mixed by heavy
  equipment excavating and hauling to the
  embankment site and a one-point Proctor may not
  fit any of the established curves. In these
  cases a new curve will have to be prepared from
  the mixture and added to the family of curves.
• 7.1.3 It is necessary to run the one-point
  Proctor as close to optimum moisture as possible.
  The point should be within plus or minus three
  percent of optimum on most curves and within plus
  or minus two percent of optimum on sharp breaking
  curves. If the point is established on either
  side of optimum and some distance from the peak
  of the curve, it may very well fit more than one
  curve in the family of curves, or none at all and
  it will be impossible to select the proper
  curve.
• 7.1.4 The moisture and density results obtained
  by the one-point Proctor are plotted on the
  family of curves and, when obtained near optimum,
  will fall near one of the curves in the family of
  curves, provided that particular type of soil or
  mixture of soils has been tested for optimum
  moisture and maximum density. The peak of the
  curve selected shall be considered the optimum
  moisture and maximum density of the material
  represented by the one-point Proctor.
• Note 2 If the one-point plotted within or on
  the family of curves does not fall in the 80 to
  100 percent of optimum moisture range, compact
  another specimen, using the same material, at an
  adjusted moisture content that will place the
  one-point within this range.

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Moisture Problems!
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Sportsmans Campground Test ResultsEmulsified
Asphalt Treated Aggregate (EATA)
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Sportsmans Campground Test ResultsEATA

• Emulsified Asphalt Treated Aggregate (EATA)
• Top Lift of Base Aggregate Pug Mill Blended
• Control Section did not include emulsion
• Test Sections included 1.5, 2, and 2.5 AC
  Residue

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Sportsmans Campground Emulsified Asphalt
Treated Aggregate
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Sportsmans Campground Emulsified Asphalt
Treated Aggregate
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Soil Stiffness GaugeBob Weber and Bill Henning
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Soil Stiffness Gauge

• The Stiffness gauge was developed from military
• technology used to locate non metallic buried
• land mines.
• About the soil stiffness gauge
• Non Nuclear
• Non Destructive
• Does not measure density
• Measures 6-12 inches below the gauge
• Is a miniature plate load test
• Quantifies the age old boot test

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How the Stiffness Gauge Works

• Very basic concept the gauge calculates the
• Ratio of force to displacement.
• The gauge applies Youngs modulus and Poissons
  ratio of soils.
• Basically the process is the dynamic equivalent
  to the
• Plate Load test. Meaning
• The soil deflects in an amount that is
  proportional to the foot geometry, and force
  applied in a given time.

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What is the Gauge Doing?

• The gauge imparts very small displacements at 25
• steady state frequencies between 100 and 196 Hz.
• The stiffness is determined at each frequency and
  
• averaged.
• The stiffness is determined by the ratio of force
• to displacement and is proportional to the size
  of the
• foot.

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Sportsmans Campground Test ResultsEATA
Stiffness Gauge, Density Data Sheet Stiffness Gauge, Density Data Sheet Stiffness Gauge, Density Data Sheet Stiffness Gauge, Density Data Sheet
                     
Test No Station Distance Cure Time Percent Percent Lbs 3 Stiffness    
    from Time/Days Density Moisture   Value Remarks  
    Center Line                
1 4250   fresh (new) 98.2 7.3 136.1 4.5 time 1230 time 1230  
2 4250   2hr. 15min       5.5 time 1445 time 1445  
                 
3 295   7 days 100.8 2.4 139.7 30.5      
4 2231   5days 101.7 3.3 140.9 30.3      
5 3730   36 hr. 101.7 3.5 141 29.7      
                     
6 4465   fresh (new) 97.1 7.9 134.5 2.1 placed 9-11-06 placed 9-11-06  
6A 4363   1 hr 99.9 6.6 138.5 5.6 1 hr plus more compaction 1 hr plus more compaction 1 hr plus more compaction
7 4465   3days n/a n/a n/a 27 follow up test 9-14-06 follow up test 9-14-06 rain/wet
                     
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Sportsmans Campground
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Columbus-South CTB
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Columbus-South CTB
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Columbus-South Test ResultsCTB
Stiffness Gauge, Density Data Sheet Stiffness Gauge, Density Data Sheet Stiffness Gauge, Density Data Sheet Stiffness Gauge, Density Data Sheet
                         
Test No Soil Class Station Distance Cure Time Percent Percent Kg/m3 Stiffness      
      from Time/Days Density Moisture   Value Remarks Remarks  
      Center Line                  
1 CTB 23505 Rt 1 hr 97 6.9 2097 19.66 Tested 9/26/2006 Tested 9/26/2006 Tested 9/26/2006  
2               20.31 1100 AM 1100 AM    
                     
3   23675 1.9m Rt .5 hr 94.5 6.5 2043 13.36 Tested 1200 PM Tested 1200 PM Tested 1200 PM  
          95.8 6.7 2071 13.87        
                         
4   23780 3.3m Rt .45 hr 98.3 6.8 2136 15.29 Tested 1230 PM Tested 1230 PM Tested 1230 PM  
          100.6 6.4 2152 15.78      
                         
5   23675 Lt 1 day 100 6.6 2152 45.42 Tested 9/26/2006 Tested 9/26/2006 Tested 9/26/2006  
                46.25 Laid down 9/25/2006 Laid down 9/25/2006 Laid down 9/25/2006  
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Soil Stiffness Gauge

• MDT will be researching with the intent to
  develop
• Cure Specification for CBC and CTS
• Develop Cure Specifications for other materials
  such as
• Pulverized Plant Mix
• Cold in place recycled plant mix
• CTB, Micro Cracking CTB, and EATA

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Next Generation of Stiffness Gauge(Work begins
next year for the following capabilities)

• Measurement of in place moisture
• Measure Asphalt Stiffness
• Integrated with compaction equipment
• Graphical data-processing software
• On-board GPS
• Will measure to depths greater than 12 inches

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Further Down the Road

• Developing Research is on going
• Develop Stiffness Parameters for Mechanistic
  Design
• Compare to dynamic cone penetrometer, Falling
  Weight Deflectomer, and laboratory resilient
  modulus (R-value)

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The End

• Any Questions?