1. Goals for this Session
 What is an FWD?
 Fundamentals of FWD operations and testing
 Learn about some typical applications for an FWD
David P. Orr, PE – Cornell Local Roads Program
Pavement Deflection
Isada, 1966
Benkelman beam
1

2. FWD Loading System
KUAB Dynatest
Carl Bro JILS
What Can a Typical FWD Measure?
 Deflections
 Load or Pressure
 Temperature
• Tow Vehicle
• Data Acquisition System
• Testing Device
• Operator
2

3. FWD with GPS
• Deflections
• Load or Pressure
• Temperature
• (Surface and Air)
‐Dwight D. Eisenhower
FWD Testing Protocol
 Travel to site
 Coordinate personnel
 Inspect test section
 Record temperature & other measurements
 Prepare FWD for testing
 Warm up FWD & check FWD operation
 Collect FWD data
 Perform data checks & basic analysis
 Prepare FWD for travel
 Return to base
 Provide data to engineer
 Finish analysis of FWD data
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4. Travel to Site
 Test Section Limits
Travel to Site
Coordinate Personnel Coordinate Personnel
 Traffic control crew  Sampling and geotechnical testing crew
 FWD operator (as appropriate)
4

5. Inspect Test Section
Inspect Test Section  General pavement condition
 Pavement distress
Record Pavement
Prepare FWD for Testing
Temperature?
 Who records?  Perform check of equipment
 Prep for testing
 Computer software
Warm up FWD and
Check FWD Operation
Collect Deflection Data
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6. Testing Considerations Collect Deflection Data
 Type of FWD Testing  Test spacing
 Research Level  Number of drops per point
 Project Level
 Number of load levels
 Network Level
 Type of Pavement Structure  Project Level FWD Testing
 Flexible  Spacing 25‐100 ft
 Rigid (Jointed or Continuously Reinforced)  Network Level FWD Testing
 Composite  250‐500 ft spacing
 Other
(aggregate, surface treatment on aggregate, etc.)  Maximize use of traffic control
 Determine average test points per day
Testing Program Perform Data Checks and Basic
 An Effective Testing Program Should: Analysis
 Minimize errors
 Maximize data collection
 Be flexible
Perform Data Checks and Basic
Analysis Prepare FWD for Travel
 Before or after leaving traveled way?
 What about the FWD software?
 Automation
6

7. Prepare FWD for Travel Return to Base
 Prepare for high‐speed travel  Missing data will always be missing
 Create backup of data
 Complete hand written notes
Provide Data to Engineer
 Data files Finish Analysis of FWD Data
 Other documentation
 Photographs
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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8. • Geotechnical investigation • NO!
• Test pits • Depends upon data needs
• Core holes and length of section
• GPR? • Take advantage of a full day of testing
• Pavement temperature
• Pictures
• Distress investigation
• ….
Typical Applications of an FWD
 Flexible Pavements
 Design of rehab/overlay
 Subgrade response
 Identify weak areas
 Forensics
 Rigid Pavements
 Layer moduli
 Subgrade support
 Locate bad joints or voids
 Network Evaluation
 Pavement parameters
 Subgrade strength
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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9. Testing Program Plots of FWD Deflection
Dropt Height 4 Deflections
 Visual inspection 80 Center Deflection
70
 Interview with Town Highway Superintendent 60
50
Deflection (mils)
 FWD testing in Outer Wheel Path 40
30
20
 2 test pits 10
0
 Soil samples 0 1000 2000
April 02, NB
3000
Station (feet)
4000
Sept 02, SB
5000 6000
Dropt Height 4 Deflections
Outer Deflection 72"
3
2.5
2
Deflection (mils)
1.5
1
0.5
0
0 1000 2000 3000 4000 5000 6000
Station (feet)
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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10. Eden‐Evans Center Road Testing Program
 Why does the road heave so badly?  Visual inspection
 What are the rehabilitation options?  Interview with crew leaders
 FWD testing in Outer Wheel Path
 2 test pits
 Soil samples
Laboratory Tests Brown Gravel
Sieve Test - Upper Brown Gravel - Test Pit 1
Erie County - Eden-Evans Center Road
100%
90%
80%
Material AASHTO Unified % Fines PI SE
Percent Pasing
70%
Brown 60%
A‐2‐4 GM‐GC 30.6% 7 11
Gravel 50%
Grey 2.6‐ 27‐ 40%
A‐1‐a GP* NP
Slag 6.5% 29 30%
20%
Subgrade A‐4 SM‐SC 44.1% 6 13 10%
0%
0.001 0.01 0.1 1 10
Cornell Local Roads Program Grain size (inches)
Variation in Deflection Data Rehabilitation Choices
 Overlay
 Reconstruction
 Recycling the Surface
 Full Depth Rehabilitation
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11. Reconstruction or at least removal of the brown • Visual inspection
gravel is a minimum needed at this site • Test pits
• Laboratory analysis
County Rt 5
 How good was the construction in 2011?
 Are there any weak spots?
 What is the expected lifespan?
County Route 5, Lyme, NY – Jefferson County
Testing Program Bedrock at Station 1+50
 Visual inspection Can see exposed bedrock
 Discussion with County
 Road is also in Seasonal Research Study ~6 feet deep
 FWD testing in Outer Wheel Path
 Use data from research geotechnical investigation
 Soil samples
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12. Bedrock Depth
Bedrock at Station 2+80 (from FWD data)
Distance (feet)
9.9 feet deep 0 100 200 300 400 500
0.0
Bedrock Depth (feet)
5.0
10.0
15.0
20.0
25.0
Bedrock Depth
(from FWD data)
Backcalculation Model
Distance (feet) Layer Thickness (inches)
0 2,000 4,000 6,000 8,000 AC 7.5
0
Gravel 11.0
Bedrock Depth (feet)
5
10 Upper Subgrade 24
15
20 Lower Subgrade Calculated
25
Bedrock ∞
Weak Spots Terminal ESALs
Lower Subgrade Modulus 4.E+09
120,000
3.E+09
Modulus (psi)
100,000
ESALs
80,000 2.E+09
60,000
40,000 1.E+09
20,000
0 0.E+00
0 2,000 4,000 6,000 8,000 0 2,000 4,000 6,000 8,000
Distance (feet) Distance (feet)
12

13. Terminal ESALs
100,000,000
80,000,000
• Bedrock (within ~25 feet)
ESALs
60,000,000
• Weak spots (only a relative value)
40,000,000
5,000
20,000,000 AADT • Can also be used with thickness information to
0 determine the strength of the pavement
0 2,000 4,000 6,000 8,000
Distance (feet)
Conclusions
Non destructive testing device
 Lots of data quickly
 Not always the right choice • An FWD is a non‐destructive testing (NDT) device
 Backcalculation? for pavement that ‘simulates’ the loading of a
truck traveling at about 40 mph
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