Field Procedures for collecting FWD data
Non-Destructive Testing Unit
** It is mandatory that you read the ModTag Manual before proceeding. This manual is also a great reference for when you have issues with any phase of the backcalculation process.
** Always double-check your file names and path, and your thicknesses and other inputs to ensure accuracy of the finished project!!!!
1)
1 Open ModTag either thru NOS (‘FWD Analysis’ / ‘Run ModTag’) or the desktop icon
Ø **(currently, you need to be on the virtual machine)
2 If a file is already loaded, either close it out to start a new one, or, continue (below) to finish it
Ø Refer to the bottom left of the ModTag screen to see if a file is open
3 If no file is loaded, select ‘New Project’
Ø **Make sure to set the correct path to the folder/file you will be working on!!
Ø Type the new .MDB file name
Ø Click ‘Save’
Ø When the ‘New Database File’ window pops up: make sure file name and path are correct, then click on ‘Create Database’.
Ø Click ‘OK’ for the US Customary units
Ø The new file has been created
2)
I. Click on the ‘Import’ button (or ‘File’ / ‘Import’) to load the FWD data
1 If you are not directed to the correct file name & path, then choose the correct path and find the DDX file from the folder you are currently working on.
Ø You should have already created this in NOS (see ‘NDT-Office_Procedures_1_NOS’)
2 Highlight the DDX file and click ‘Open’
3 A new window will pop up, enter the Project Name (same as File Name)
4 Click ‘Process’
5 Another window will pop up to ‘Verify load ranges’, click ‘OK’
Ø In this new window (‘Test Setup’), the number of drops should be correct, if not, type in the correct number. Then hit the ‘Tab’ key
Ø Enter the number of Load Levels per Location (currently it is 1 – but will depend on the type of testing required for the current project; if in doubt, check your FWD raw data). Hit the ‘Tab’ key
Ø The overall Average Load of the file will be calculated; if it looks correct, click the ‘Process’ button
Ø When it is finished processing the data, click the ‘Exit’ button
6 The FWD data is now loaded
3)
A. Under the ‘PreAnalysis’ tab:
I. Check the ‘Pre-Analysis Report’ to see if any data has been deleted or other pertinent information
II. Look at the ‘SLIC Transformation’ / ‘Station Graph’
1 Run through each station to look for potential bad data
Ø Any bad data can be deleted here (by clicking the ‘Delete’ button), or you can backcalculate first, and if the results do indeed look bad, come back here to delete the station data (if you do this, you must re-backcalculate this segment)
Ø You can confirm any suspect data when looking at the ‘Surface Modulus’ graphs
III. Look at the ‘Surface Modulus’
1 Run through each station to look for potential bad data or to confirm suspect SLIC data
Ø If the graphed line sharply changes direction more than twice, delete the data – if it only sharply changes once or twice, backcalculate the data first to verify that it is truly bad
IV. Lastly, check the ‘Cumulative Differences of Deflection’
1 Look for any MAJOR changes in slope / find the ‘Location’ number (RP) where those changes occur and write it down for future referencing (segmenting).
Ø Don’t worry about tiny/short changes – you don’t want to end up with 10 or more segments; 2 – 6 segments should be plenty.
2 Bring up NOS (should still be open) and open the ‘GPR Analysis’/‘Segment GPR Test Data’
Ø ** See the NOS Office Procedures manual to segment GPR data and obtain thicknesses in order to model your road layers.
V. If you end up with strong subgrades, you can look at the ‘Hard-Bottom’ here to see where it predicts depth to a stiff layer (hard-bottom).
4)
A. Model Layers
I. Choose the ‘Project Info.’ icon (or) the ‘Edit’ tab, then ‘Project Group Info’:
1 Choose ‘Pavement Type’ / ‘Flexible’
Ø The default Layer for a Flexible Pavement will be Asphalt Concrete/PMS
2 Change Surf Layer Thickness to the project’s overall average
3 Choose an appropriate Base Material
Ø Change the Base Thickness (to the project’s overall average)
4 If a Sub-Base is present, choose the appropriate Material type and (average) Thickness
5 *No need to choose a ‘Sub Grd1,2,3’; the computed Subgrade will appear after computing the hardbottom depth in the next step.
6 Click the ‘Compute HB Depth’ button / click ‘Exit’
7 Default Modeling is finished. If segmenting is not necessary, continue below to the back-calculation process (ModComp) outlined below.
B. Segment Project
I. You will need the segment RP’s and thicknesses chosen from your GPR analysis/segmenting (see: ‘NDT Office Procedures - GPR’).
1 Click the ‘Segment Project’ button
2 Choose the appropriate ‘End Location’ RP (for the first segment)
Ø ‘Create Segment’
· You can ‘Test Segment Creation’ first to see what the created segment will look like on the graph.
Ø ‘Segment Model To Duplicate’ window will pop up: choose ‘OK’
· This just means it will use the same Modeling Layers and Thicknesses from Segment A
Ø ‘Segment Data’ window will pop up: choose ‘OK’
3 Continue until you have created all your segments
4 Exit Segmenting window
C. Model Layers (from new segments)
I. Choose a new Segment from the ‘Project Name’ drop-down list
1 Change the ‘default’ thicknesses to the appropriate new segment thicknesses
2 Click the ‘Compute HB Depth’ button
Ø Click ‘Exit’
3 Continue until you have changed all the new segment layer thicknesses
4 Close the ‘Project Group Information’ window
5)
A. ModComp processing of data (back-calculation)
I. Choose ‘Analysis’ / ‘MODCOMP Processing’
1 Choose a segment to work on from the ‘Project Data’ drop-down menu
Ø ** do NOT choose the default Segment A
2 Select the ‘All Stations’ radio button from ‘Processing’
3 Select the ‘Computed HB’ radio button from ‘Hard Bottom’
4 Select the ‘All Stations’ radio button from ‘Processing’
II. Calculating Drop Heights
1 If the data was collected using only 1 drop height, there is no need to choose from the ‘Calc All Drop Heights?’ drop-down lists.
· Explanation: our Network-Level (and most all Project-Level) data is collected with 3 drops at one drop height (9,000lbs/9Kips). And ModTag averages these 3 to create one drop per station (the drop height is averaged as well as the resulting deflections). Since there is only 1 drop height to calculate, there is only 1 nominal load to calculate. Choosing ‘Yes’ here does not change the outcome of the back-calculation.
2 If the data was collected with multiple drop heights, you can either calculate all drop heights at once, or, choose 1 drop height at a time - less messy/confusing.
· For specific Network, Project, and Research projects – we sometimes collect FWD at 4 different drop heights: 6, 9, 12, 16 Kips.
3 Click ‘Process’
Ø A ‘DOS’ window will appear showing the iterations for each drop
Ø Then, a spreadsheet will appear showing the results for that segment
· Either enlarge this window, or, Export to Excel to view the data
Ø Scroll through the data to see if the Mr numbers look plausible for each layer
· If they are not to your liking, you may have to do more research on your thicknesses and make changes to your layers; otherwise, you must leave it as is.
** You can find information pertaining to Young’s Elastic Modulus typical values for pavement layers in the NDT office as well as on-line.
** ‘RMSE’ (Root Mean Square Error): anything under 2 is as good as it gets / from 2-10 is very good / from 10 – 15 is OK / above that is sketchy (unless pavement structure is in real poor shape).
4 Continue back-calculating (ModComp) through all your segments until finished
III. Temperature Correction
1 Choose ‘Analysis’ / ‘ModComp AC Temp. Correction’
Ø Select a segment from the drop-down box
Ø Tab to the ‘Previous Day Avg. Temp:’ box
Ø Click the ‘right arrow’ once (to move the cursor over one place)
Ø Type in the previous day’s average air temperature (date taken from the .dat file)
· NDT has a list of websites to choose from for historical weather data
Ø Click ‘Calculate’
· Click ‘OK’
2 Continue until all segments have been temperature corrected
3 Exit AC Temp. Correction window
B. Double-check Results
I. Make sure all data is reported and valid
1 Choose ‘Reports’ / ‘ModComp Modulus Results’
2 Choose the segment you wish to see
Ø Make sure all layers are accounted for and look proper
Ø Make sure the ‘Temperature Corrected’ layer shows up
Ø Not necessary but another option is to export the data to Excel to check:
· Click the ‘Data’ button / click the ‘Export to Excel’ button at the bottom of the window
3 After checking all segments, exit the ‘ModComp Modulus Results’ page
C. Backcalculation is finished
I. Close out file
1 Choose ‘File’ / ‘Close’
II. Exit ModTag
b:\ndt\ndt-field_procedures_4_fwd.docxJune 20, 20175 of 5