Fwd thinking
description
Transcript of Fwd thinking
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David P. Orr, PE – Cornell Local Roads Program
Goals for this Session
What is an FWD?
Fundamentals of FWD operations and testing
Learn about some typical applications for an FWD
Pavement Deflection
Benkelman beam Isada, 1966
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JILSCarl Bro
DynatestKUAB
FWD Loading System
What Can a Typical FWD Measure?
DeflectionsLoad or PressureTemperature
• Tow Vehicle
• Data Acquisition System
• Testing Device
• Operator
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FWD with GPS
• Deflections
• Load or Pressure
• Temperature
• (Surface and Air)
‐Dwight D. Eisenhower
FWD Testing ProtocolTravel to siteCoordinate personnel Inspect test sectionRecord temperature & other measurementsPrepare FWD for testingWarm up FWD & check FWD operationCollect FWD dataPerform data checks & basic analysisPrepare FWD for travelReturn to baseProvide data to engineerFinish analysis of FWD data
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Travel to SiteTravel to Site
Test Section Limits
Coordinate Personnel
Traffic control crew
FWD operator
Coordinate Personnel
Sampling and geotechnical testing crew (as appropriate)
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Inspect Test SectionInspect Test Section
General pavement condition
Pavement distress
Who records?
Record Pavement Temperature?
Prepare FWD for Testing
Perform check of equipment
Prep for testing
Computer software
Warm up FWD and Check FWD Operation
Collect Deflection Data
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Testing ConsiderationsType of FWD TestingResearch LevelProject LevelNetwork LevelType of Pavement StructureFlexibleRigid (Jointed or Continuously Reinforced)CompositeOther (aggregate, surface treatment on aggregate, etc.)
Test spacing
Number of drops per point
Number of load levels
Project Level FWD Testing
Spacing 25‐100 ft
Network Level FWD Testing
250‐500 ft spacing
Maximize use of traffic control
Determine average test points per day
Collect Deflection Data
Testing Program
An Effective Testing Program Should:
Minimize errors
Maximize data collection
Be flexible
Perform Data Checks and Basic Analysis
Before or after leaving traveled way?
What about the FWD software?
Automation
Perform Data Checks and Basic Analysis Prepare FWD for Travel
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Prepare FWD for Travel
Prepare for high‐speed travel
Create backup of data
Complete hand written notes
Return to Base
Missing data will always be missing
Provide Data to Engineer
Data files
Other documentation
Photographs
Finish Analysis of FWD Data
Finish Analysis of FWD Data
Is the FWD data good?
Data checks in the field
What is the goal of the testing?• The testing company, the engineer,
and the County working in collaboration
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• Geotechnical investigation
• Test pits
• Core holes
• GPR?
• Pavement temperature
• Pictures
• Distress investigation
• ….
• NO!
• Depends upon data needs and length of section
• Take advantage of a full day of testing
Flexible PavementsDesign of rehab/overlaySubgrade response Identify weak areasForensicsRigid PavementsLayer moduliSubgrade supportLocate bad joints or voidsNetwork EvaluationPavement parametersSubgrade strength
Typical Applications of an FWD
Landon Road
Where does the poor base area begin and end?
What are the best locations for the test pits?
Will cement stabilization work?
Did the rehab work?
Landon Road
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Visual inspection
Interview with Town Highway Superintendent
FWD testing in Outer Wheel Path
2 test pits
Soil samples
Testing Program Plots of FWD Deflection
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0.5
1
1.5
2
2.5
3
0 1000 2000 3000 4000 5000 6000
Def
lect
ion
(m
ils)
Station (feet)
Dropt Height 4 DeflectionsOuter Deflection 72"
April 02, NB Sept 02, SB
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10
20
30
40
50
60
70
80
0 1000 2000 3000 4000 5000 6000
Def
lect
ion
(m
ils)
Station (feet)
Dropt Height 4 DeflectionsCenter Deflection
April 02, NB Sept 02, SB
Cement Stabilization Demo After a Chip Seal
• FWD Testing
• Test pits are sited using FWD data Eden‐Evans Center Road – Eric County
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Why does the road heave so badly?
What are the rehabilitation options?
Eden‐Evans Center Road
Visual inspection
Interview with crew leaders
FWD testing in Outer Wheel Path
2 test pits
Soil samples
Testing Program
Material AASHTO Unified % Fines PI SE
Brown Gravel
A‐2‐4 GM‐GC 30.6% 7 11
Grey Slag
A‐1‐a GP*2.6‐6.5%
NP27‐29
Subgrade A‐4 SM‐SC 44.1% 6 13
Laboratory Tests Brown Gravel
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0.001 0.01 0.1 1 10
Per
cent
Pas
ing
Grain size (inches)
Sieve Test - Upper Brown Gravel - Test Pit 1Erie County - Eden-Evans Center Road
Cornell Local Roads Program
Variation in Deflection Data
Overlay
Reconstruction
Recycling the Surface
Full Depth Rehabilitation
Rehabilitation Choices
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Reconstruction or at least removal of the brown gravel is a minimum needed at this site
• Visual inspection
• Test pits
• Laboratory analysis
County Route 5, Lyme, NY – Jefferson County
How good was the construction in 2011?
Are there any weak spots?
What is the expected lifespan?
County Rt 5
Visual inspection
Discussion with County
Road is also in Seasonal Research Study
FWD testing in Outer Wheel Path
Use data from research geotechnical investigation
Soil samples
Testing Program
Can see exposed bedrock
~6 feet deep
Bedrock at Station 1+50
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Bedrock at Station 2+80
9.9 feet deep
Bedrock Depth (from FWD data)
0.0
5.0
10.0
15.0
20.0
25.0
0 100 200 300 400 500
Bedro
ck D
epth (feet)
Distance (feet)
Bedrock Depth (from FWD data)
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5
10
15
20
25
0 2,000 4,000 6,000 8,000
Bedro
ck D
epth (feet)
Distance (feet) Layer Thickness (inches)AC 7.5
Gravel 11.0
Upper Subgrade 24
Lower Subgrade Calculated
Bedrock ∞
Backcalculation Model
Weak Spots
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20,000
40,000
60,000
80,000
100,000
120,000
0 2,000 4,000 6,000 8,000
Modulus (p
si)
Distance (feet)
Lower Subgrade Modulus
Terminal ESALs
0.E+00
1.E+09
2.E+09
3.E+09
4.E+09
0 2,000 4,000 6,000 8,000
ESALs
Distance (feet)
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Terminal ESALs
0
20,000,000
40,000,000
60,000,000
80,000,000
100,000,000
0 2,000 4,000 6,000 8,000
ESALs
Distance (feet)
5,000 AADT
• Bedrock (within ~25 feet)
• Weak spots (only a relative value)
• Can also be used with thickness information to determine the strength of the pavement
Conclusions
Non destructive testing device
Lots of data quickly
Not always the right choice
Backcalculation?• An FWD is a non‐destructive testing (NDT) device
for pavement that ‘simulates’ the loading of a truck traveling at about 40 mph