Field & Laboratory Evaluation of the Portable Falling Weight Deflectometer (PFWD) NETC Project No. 00-4
Principal Investigators: Dana N. Humphrey, Ph.D., P.E. Professor & Chairperson, Department of Civil & Environmental Engineering, University of Maine Maureen A. Kestler Geotechnical/Pavements Engineer, U.S. Forest Service Bryan C. Steinert Graduate Research Assistant, Department of Civil & Environmental Engineering, University of Maine
Goals of the Research: Use of the PFWD to evaluate thaw weakening of seasonally posted low volume roads. Use of the PFWD as an alternative to traditional compaction control devices.
Outline of the presentation: What is a PFWD, and how does it work? Importance of Research Results from Previous Studies Spring Thaw Results Compaction Control Study Summary
What is a PFWD? non-destructive, portable device determines composite elastic modulus of construction layers (pavement, basecourse, subgrade)
How does the PFWD work? Force measured with the load cell Spring loaded geophone that measures the velocity of the surface. Load cell measures the impact force due to the falling weight (10, 15, 20 kg) Load transmitted to underlying surface through 100mm, 200mm, or 300 mm diameter load plate. Geophone measures the surface velocity (up to 3 may be used).
Continued provides maximum values of force and deflection. also provides time history of layer response to applied force. used to determine Elastic Modulus (stiffness).
PFWD Demo
Importance of research The thawing of ice lenses results in an increase in water, subsequently decreasing the effective stress and shear strength. Weight restrictions are used to minimize damage during these damage susceptible periods.
Continued Study by Kestler et al. (2000) Over one half of respondents use visual inspection/observation of roads to determine when to place and remove weight restrictions. To a lesser degree, date and quantitative methods are used.
Continued A Falling Weight Deflectometer (FWD) may be used, however, high costs associated with purchase, operation, and maintenance. $250,000 (FWD) v. $13,000 (PFWD) more units = more roads that may be evaluated
Importance of Research: Problems with current compaction control techniques: costly & time consuming sand cone / rubber balloon methods extensive training & safety requirements nuclear moisture density gauge methods report density easy to correlate with engineering properties limited insight into long term performance
Results from Previous Studies: Davies (1997) Investigated similarities between PFWD, Benkelman Beam, and FWD. Thin membrane surfaced roads. Underlain by graded/compacted subgrade. 20 test locations
Continued... R 2 = 0.86 between the Loadman PFWD and both the FWD and Benkelman Beam differences associated with depth of influence of each of the devices
Continued... Gros (1993) measurements with Loadman PFWD and FWD. tests on unbound aggregate containing sand, gravel, and crushed gravel.
Continued... Whaley (1994) Loadman PFWD and FWD measurements. 80 mm A/C, 200 mm base course, 1220 mm subbase. Loadman yielded higher moduli than backcalculated FWD values. R 2 = 0.2
Continued... Summary of Literature Results PFWD compared marginally with other devices when testing on pavement layers. PFWD compared reasonably with other devices when testing on unbound layers. PFWD did adequately follow strength change through spring thaw.
Spring Thaw Study: Testing includes PFWD (Prima 100 & Loadman), and FWD. Instrumentation to monitor frost depth and porewater pressure Seven paved and three unpaved test sites in Maine, New Hampshire, and Vermont
Vermont Parking Lot 800 Modulus (MPa) 600 400 Test Point #1 Test Point #2 Test Point #3 Test Point #4 Test Point #5 Test Point #6 Test Point #7 Test Point #8 Test Point #9 Test Point #10 Average FWD subbase Average FWD subgrade 5 HMA 12 subbase 200 0 3/13/04 3/31/04 4/18/04 5/6/04 5/24/04 6/11/04
Litchfield, Maine 500 400 Modulus (MPa) 300 200 100 6 HMA 24 subbase PFWD composite modulus - STA 4+028 PFWD composite modulus - STA 4+036 PFWD composite modulus - STA 4+044 PFWD composite modulus - STA 4+052 Average FWD subgrade modulus Average FWD composite modulus 0 3/31/04 4/18/04 5/6/04 5/24/04 6/11/04 6/29/04 7/17/04
Glenburn gravel surfaced
Glenburn gravel surfaced 800 Modulus (MPa) 600 400 Prima 100 PFWD - Test Point #1 Prima 100 PFWD - Test Point #2 Prima 100 PFWD - Test Point #3 Prima 100 PFWD - Test Point #4 Average FWD Subgrade Modulus Average FWD Composite Modulus 200 0 2/24/04 3/13/04 3/31/04 4/18/04 5/6/04 5/24/04 6/11/04 6/29/04
Preliminary PFWD & FWD Comparison Route 11 Wallagrass, Maine ~ 14 yrs. old ~120 mm (5 in.) pavement ~ 750 mm (30 in.) subbase Route 167 Presque Isle/Fort Fairfield, Maine ~ 13 yrs. old ~120 mm (5 in.) pavement ~750 mm (30 in.) subbase 5 test locations @ each site PFWD and FWD measurements
PFWD v. FWD - MeDOT Rt. 11 Wallagrass 800 FWD Effective Pavement Modulus (MPa) 600 400 200 R2 = 0.91 0 0 200 400 600 800 Prima 100 PFWD Modulus (MPa)
PFWD v. FWD - MeDOT Rt. 167 Presque Isle/Fort Fairfield 1000 FWD Effective Pavement Modulus (MPa) 800 600 400 R2 = 0.76 200 0 0 200 400 600 800 1000 Prima 100 PFWD Modulus (MPa)
Spring thaw FWD vs. PFWD --- All paved data --- 4000 FWD Composite Modulus (MPa) 3000 2000 1000 Y = 0.702*X + 147.172 R-squared = 0.809 0 0 1000 2000 3000 4000 Prima 100 PFWD Composite Modulus (MPa)
Field Compaction Control Study: Testing at construction sites in CT, NH, and ME Materials that have undergone varying degrees of compaction. PFWD & NDM at multiple locations. Southington, CT site
Lab Compaction Study: 6 x 6 6 x 3 3 test box 5 different soil types Compact at optimum water content and ± 3% of optimum and 90%, 95%, and 100% of max. dry density. PFWD and NDM tests @ 5 locations.
Results Soon!
Summary PFWD and FWD have similar effectiveness in monitoring thaw weakening PFWD and FWD give similar composite moduli Effectiveness as compaction control device TBD