Construction-Related%20Variability%20in%20Mat%20Density%20Due%20To%20Temperature%20Differentials

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Infrared Thermography Revolutionizes
How?
Asphalt Paving
Significant aving for Federal, State and
Municipal DOTs
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Infrared Thermography Revolutionizes
AsphaltPaving
Leonard Phillips, FLIR SystemsKim Willoughby,
Washington State DOTProf. Joe Mahoney,
University of Washington
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Pavement Pioneers
Kim WilloughbyPavement Structure
EngineerWashington State Dept. of Transportation
Prof. Joe MahoneyProfessor of Civil
Environmental EngineeringUniversity of
Washington
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Typical HMA Highway Structure
Direction of Travel
Direction of Travel
Shoulder
Shoulder
Surface Course (HMA)
HMA thickness ranges from 2 inches to over 12
inches. Crushed Surfacing Base Course (CSBC).
Base Course
Subgrade Existing Soil(s)
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The Basic Story

• Subgrade
• Base
• Crushed Surfacing Base Course (CSBS)
• Compacted unbound aggregate base (UAB)
• 6 to 16-inch lifts
• Milled pre-existing roadway
• Cold planermills surface
• Sweeperpicks up debris
• Distributor truckapplies tack coat

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What is hot-mix asphalt (HMA)?

• Mixture of asphalt binder and aggregate (stone)
• Combined in a batch plant at 330F
• Temperature monitored at plant
• Higher temperature causes asphalt drain-down
  liquefied asphalt washes off the aggregate
• Lower temperatures can cause mix to be too stiff
  for compaction

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HMA transport to the work site

• Truck haulage
• Cooling
• Atmosphere
• Truck bed contact
• 175F (79C) cessation temperature
• Cooler HMA is too stiff to be compacted has
  higher air voids lower density than adjacent,
  warmer HMA
• Temperature differentials can lead to localized
  rapid wear

Bottom photos courtesy Gary Orlove, Infrared
Training Center
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Paver (Roadtec RP-185-10)

• Tracked paver accepting HMA directly from truck.
• Direction of travel is to the right.

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Paver Operation

• Paver accepting HMA directly from truck.

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Material Transfer Vehicle (Roadtec SB-2500)
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Material Transfer Vehicle RoadTec SB-2500C

• Stores and transfers HMA from truck to paver
• Anti-segregation auger remixes materials
• 25-ton (22.7 MT) surge capacity

OutgoingHMA
Incoming HMA
Hopper
Paver
Paver
Dump truck
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More on MTVs

• Allow continuous in-line paving
• Reblend HMA
• Defeat thermal segregation
• Defeat aggregate segregation
• Transfer HMA to paver while reblending
• Can accept HMA from haul truck while moving OR
• Can be equipped to pick up HMA from windrows
  dropped by belly dump trucks and transfer it to
  a haul truck.

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Compactor Hamm HD 120

• 78-inch-wide, double-drum roller
• Operating weight 26,675 lbs.
• Centrifugal forces up to 38,700 lbs.
• Vibration frequency range 2,520 to 3,000
  vibrations per minute
• Roller constantly sprayed with water

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Thermal effect of roller on HMA
Cooling water
Photo courtesy Gary Orlove, Infrared Training
Center
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The Problem

• Localized areas of coarse surface texture
• Premature failure due to raveling, moisture
  damage, and fatigue cracking

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Types of Damage
Aggregate Segregation
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Research Began in 1996

• Masters thesis
• University of Washington
• Former road construction worker

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1998 WSDOT/UW Study Program

• Collaboration Washington State DOT and
  University of Washington
• Four projects chosen to maximize the occurrence
  of temperature differentials (1) early or (2)
  late season or night operations
• FLIR PM280 used to identify temperature
  differentials in HMA mat after laydown to
  sample loose mix from truck
• In-place nuclear density testing performed on
  finished pavement in normal and cool test
  temperature areas
• Tests aggregate segregation, asphalt/aggregate
  segregation, and density differentials

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FINDINGCyclic pattern of temperature anomalies
Correlation with premature failures
End dumps
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Damage Mechanism

• HMA cools during transport below cessation
  temp about 79C (175F)
• When dumped directly into paver, cool crust is
  not sublimated.
• Paver extrudes cool, stiff material that cant be
  compacted.

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1998 Study Results

• All 4 projects experienced temperature
  differentials
• Differential measured vs. normal temperature
  areas
• From 739 C (1370 F) cooler
• Mean of 21 C (38 F)
• No significant aggregate segregation
• Good rolling and paver practices can minimize
  compaction deficiencies

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1998 Study Results (cont.)

• Temperature differentials correspond to low
  density areas
• Air voids increased over normal temperature
  areas from 1.6 to 7.8 (average of 3.9)
• 5 density readings taken per LOT (max 400 T of
  HMA ? 0.6 mile of 10-foot lane, 2 inches thick)
• Values evaluated as average and standard
  deviation
• What density is acceptable?
• Target air void percentage is 7 (93 density)
• Up to 9 air voids (91 density) can be
  acceptable
• Long term WSDOT average density 93.1

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Effect on Service Life

• RULE OF THUMB There is a 10 reduction in HMA
  pavement life for every 1 increase in air voids
  over 7.
• (NOTE Youll see this material again!)

John Q. You-Know-Who!
EXTRA CREDIT PROBLEM Who pays to repair
or reconstruct pavement that fails
prematurely?
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1999 Study Program

• Determine temperature differentials with respect
  to different material transfer devices/vehicles,
  haul times, environmental conditions, and other
  equipment, etc.
• 36 projects studied throughout entire paving
  season
• Infrared camera used to detect temperature
  differentials and locate test areas
• In-place density testing performed (nuclear
  densitometer) on finished pavement in specified
  normal and cool temperature areas

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1999 Study Results

• Temperature differentials 338 C (5.468.4F)
• Localized air voids increase with
• Increasing temperature differentials (gt 14 C
  25 F)
• Increasing haul time
• No remixing prior to placement
• Localized air voids decrease with
• Remixing of the mix prior to placement (decrease
  in temperature differentials)
• An increase in air and/or mix temperatures (more
  time to compact)

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Anomaly Pattern During Laydown

• ?T 30 C (54 F)
• Note twinning of anomalies

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Extent of Pavement Affected

• Area affected per truckload
• Width (across mat) can range from 1 meter up to
  the entire width of the mat
• Length can range from 1 to 6 meters or more
• Typical size of low-temperature area is
  approximately 1.2 meters by 3 meters
• Frequently occurs along parallel tracks due to
  paver extrusion pattern

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1999 Study Results MTV performance
The Material Transfer Vehicle (MTV) accepts HMA
from the truck (left), remixes it, and offloads
it into the paver (right), which is followed by a
compactor. Shuttle Buggy shown below. Movement of
the paving train is toward the left.
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REMEMBER

• RULE OF THUMB There is a 10 reduction in HMA
  pavement life for every 1 increase in air voids
  over 7.

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2000 Study Program

• Conduct infrared imaging of unconsolidated mat in
  17 projects
• Select longitudinal density profile locations
  for nuclear densitometry
• Differential anomaly ?T ? 25 F (17 C)
• ? density range 6 lbs/ft3
• ? density drop (mean min.) 3 lbs/ft3
• Procedure
• Minimum 3 to 4 profiles per paving project
  (nuclear gauge)
• Begin nuclear densitometer readings 10 feet
  behind anomaly
• Take readings through differential area every 5
  feet for 50 feet
• Calculate density differences for each profile

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Density Profile Testing Example
Thermography-detected temperature differential
area
Edge of pavement
5 ft
AnomalyOffset
3.7 m(12 ft)
Longitudinal profile line
Nuclear density tests
Anomaly Differential
Test section 50 ft
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Example Failing Profile

• DT 33 Co (59.4 Fo)
• Density Results
• Mean 2058 kg/m3
• Max 2138 kg/m3
• Min 1953 kg/m3
• Density Criteria
• Range 185 kg/m3
• Drop 105 kg/m3
• kg/m3 x 0.0623 lbs/ft3

Approx. 5 air voids over min.
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Roadway Condition -- Failed Profile

• Only 1 year after construction
• Premature wear in the mat surface from traffic

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Example Passing Criteria

• DT 3 Co (5 Fo)
• Density Results
• Mean 2205 kg/m3
• Max 2247 kg/m3
• Min 2179 kg/m3
• Density Criteria
• Range 69 kg/m3
• Drop 27 kg/m3

5908
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Roadway Condition Passing Profile

• Roadway condition 1 year after construction.
• Surface shows no visible wear.

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Example Aggressive Rolling

• DT 30 Co (54 Fo)
• Density Results
• Mean 2436 kg/m3
• Max 2494 kg/m3
• Min 2387 kg/m3
• Density Criteria--better than expected
• Range 107 kg/m3
• Drop 49 kg/m3

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2000 Study Results

• Density criteria
• Range lt 96 kg/m3 (6 lbs/ft3)
• Drop lt 48 kg/m3
• Note the pass/fail pattern vs. ?T

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Summary of 19992000 Study Findings

• Temperature and density differentials are a
  significant issue.
• Approximately ½ of the projects (28 out of 53)
  studied regularly exhibited temperature
  differentials gt14 C (25 F)
• Differential densities resulting from cooler than
  desirable mix shorten pavement life.
• When differential gt14 C (25 F) air voids
  typically increase 2 or more.

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Rule of Thumb and Implications

• There is a 10 reduction in HMA pavement life for
  every 1 increase in air voids over 7.
• Therefore, when differential gt14 C (25 F) and
  air voids increase 2, pavement life may be
  reduced by approximately 20.
• Without an MTV and during cool ambient conditions
  and a long trucking trip from the batch plant,
  differentials can be MUCH larger than this!

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TYPICAL SOLUTION Reconstruction

• U.S. 32 billion in 2002 to build and maintain
  highways to meet growing traffic volumes and
  loads.
• Low density areas fail prematurely due to
  raveling, cracking, and moisture damage
• Failure can occur within one year of paving
• Failure becomes a maintenance issue with
  potential safety implicationsand costs!

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TEST METHODS Random Sampling

• Random sampling procedure assumptions
• All mix is uniform (within specified tolerances
  and risks) within a lot
• All mix within a lot has an equal chance to be
  compacted to a specified density
• BUT low-temperature differential areas are
  anomalies with different material properties
• THEREFORE Random sampling cannot assess the
  occurrence or severity of density differentials

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EXAMPLE Random Test
1/5 segment of an 890 meter-long QA lot. Note
cyclic end-dump thermal anomalies (ovals) and
required Random QA test (red dot).

• Typical WSDOT overlay one lot 400 Tons
• 3.7 meter (?10 ft) wide lane, 45 mm (?1 ¾ in.)
  thick, 890 meters (? 0.55 mile) long
• Note anomalies
• 5 random QA density tests per lot required
• Only one random test would be taken (dot) in
  section
• 10 out-of-spec areas/lane would be missed
  completely
• Worst-case scenario miss 50 out-of-spec
  locations per lot

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Systematic Density Specification

• WSDOT is implementing a specification to locate
  and test density differentials
• Disincentive of 15 of the ACP unit price
  possible if density differentials are located
• Performed on 10 projects in 2002
• Performed on 10 projects in 2003

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Testing Procedure

• Use handheld IR camera or temperature gun to
  locate temperature differentials
• 4 or more anomalously cool locations per lot will
  trigger pay disincentive based on these potential
  low-density areas
• If the densities are less than the minimum
  allowable density or exceed density profile
  criteria, then the contractor is penalized

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Conclusions

• A tour of pavements in Washington State shows
  that density differentials are a significant
  problem
• Research results
• Temperature differentials lead to density
  differentials that reduce pavement life
• ? pavement life f (density differentials)
• Temperature differentials occur in a cyclic
  pattern
• Random density testing alone does not capture the
  severity of density differentials

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Conclusions (continued)

• Density profiles taken through anomalous areas
  can be used to evaluate the effects of
  temperature differentials
• WA State at this time allows the use of either
• A systematic density specification (one density
  test in a temperature differential area)
• A density profile specification
• Performance specification (based on density) was
  implemented in 2002 on 10 projects and has been
  used on 10 projects in 2003

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Other States

• Alaska
• California
• Connecticut
• Georgia
• Kansas
• Maryland
• Massachusetts
• Minnesota
• Texas
• Utah.

Photos courtesy Simon Howell, Alaska DOT
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Questions?

• A full research report, tech note, and infrared
  imagebase can be found at the following website
• http//www.wsdot.wa.gov/biz/mats/pavement
• Click on Pavement Research for report and tech
  note.