Use of Magnetic Tomography Technology to Evaluate Dowel Placement

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Figure 1. MIT Scan-2, consisting of the sensor unit (a rectangular green box), onboard computer, and glass-fiber-reinforced plastic rail system.
Figure 2. Scanning a joint using MIT Scan-2.
Figure 3. Example field output of MIT Scan-2.
Figure 4. Example graphical output of MagnoProof.
Figure 5. Calibration measurements being taken at MIT GmbH laboratory.
Figure 6. Test track at MIT GmbH for validation of calibration results.
Figure 7. Saw cut being made on test slab to simulate a joint at the MnRoad facility.
Figure 8. Dowel bar wedged in a slot of the test slab at the MnRoad facility.
Figure 9. Dowel covered with aggregate for the evaluation of the effects of cover material.
Figure 10. Exposed dowel bars for validation of MIT Scan-2 results.
Figure 11. Apparent horizontal misalignment due to joint skew.
Figure 12. Sensitivity of measurement to bar depth for 32-mm (1.25-in.) dowel bar (MIT GmbH).
Figure 13. Measurement error on horizontal misalignment as a function of side shift.
Figure 14. Measurement error on vertical misalignment as a function of side shift.
Figure 15. Measurement error on horizontal misalignment as a function of bar depth.
Figure 16. Measurement error on vertical misalignment as a function of bar depth.
Figure 17. Measurement error on horizontal misalignment as a function of horizontal misalignment.
Figure 18. Measurement error on vertical misalignment as a function of horizontal misalignment.
Figure 19. Measurement error on horizontal misalignment as a function of vertical misalignment.
Figure 20. Measurement error on vertical misalignment as a function of vertical misalignment.
Figure 21. Measurement error on bar depth as a function of side shift.
Figure 22. Measurement error on side shift as a function of side shift.
Figure 23. Presence of tie bar directly over the outer dowel bars.
Figure 24. Influence of the presence of foreign metal: (a) tie bar at the lane-shoulder joint affecting the results for bars 23 and 24; (b) tie bar at the longitudinal joint affecting the results for bars 11-13.
Figure 25. Measurement error on horizontal misalignment as a function of side shift (MIT test track).
Figure 26. Measurement error on vertical misalignment as a function of side shift (MIT test track).
Figure 27. Measurement error on horizontal misalignment as a function of depth (MIT test track).
Figure 28. Measurement error on vertical misalignment as a function of depth (MIT test track).
Figure 29. Measurement error on horizontal misalignment as a function of horizontal misalignment (MIT test track).
Figure 30. Measurement error on vertical misalignment as a function of horizontal misalignment (MIT test track).
Figure 31. Measurement error on horizontal misalignment as a function of vertical misalignment (MIT test track).
Figure 32. Measurement error on vertical misalignment as a function of vertical misalignment (MIT test track).
Figure 33. Measurement error on depth as a function of side shift (MIT test track).
Figure 34. Measurement error on side shift as a function of side shift (MIT test track).
Figure 35. Examples of problem baskets: (a) basket is deformed, (b) basket is pulled apart, and (c) basket is severely deformed and rotated almost completely off of the joint.
Figure 36. Comparison of GPR and MIT Scan-2 results from Kansas.
Figure 37. Signal intensity plot showing uncut ties between location 110 and 150 (cm).
Figure 38. Comparison of dowel alignment for baskets anchored using 4 pins per basket and 10 pins per basket.
Figure 39. Joint-by-joint evaluation results for the baskets anchored using 4 pins per basket, showing a significant number of potentially locked joints (Joint Score > 10).
Figure 40. Joint-by-joint evaluation results for the baskets anchored using 10 pins per basket, showing outstanding dowel alignment.
Figure 41. The effects of mix optimization on dowel alignment on the US-64 Bypass project.
Figure 42. Distribution of dowel misalignment in the I-81 project in Pennsylvania (maximum of either horizontal or vertical misalignment).
Figure 43. Joint Scores for the I-81 project in Pennsylvania.

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Technical Report Documentation Page

1. Report No. FHWA-IF-06-006	2. Government Accession No.			3. Recipient's Catalog No.
4. Title and Subtitle Use of Magnetic Tomography Technology to Evaluate Dowel Placement				5. Report Date November 2005
				6. Performing Organization Code
7. Author(s) H. Thomas Yu and L. Khazanovich				8. Performing Organization Report No.
9. Performing Organization Name and Address ERES Consultants 505 West University Ave Champaign, IL 61820				10. Work Unit No.
				11. Contract or Grant No. DTFH61-03-C-00120
12. Sponsoring Agency Name and Address Federal Highway Administration Office of Pavement Technology 1200 New Jersey Avenue SE Washington, DC 20590				13. Type of Report and Period Covered Final Report
				14. Sponsoring Agency Code
15. Supplementary Notes Samuel S. Tyson, P.E., Contracting Officer's Technical Representative Mail Code: HIPT-20 E-mail: sam.tyson@fhwa.dot.gov
16. Abstract Extensive laboratory and field evaluations were conducted under this project to evaluate the effectiveness and limitations of the MIT Scan-2 device, which uses magnetic tomography technology to evaluate the placement of metal dowel bars in concrete pavements. The laboratory testing results confirm that the MIT Scan-2 device provides accuracy that is both reasonable and useful for horizontal and vertical misalignments within the following limits: Depth - 100 to 190 mm (3.9 to 7.5 in.) Side shift - +100 mm (+4 in.) Horizontal misalignment - +40 (+1.6 in.) plus a uniform rotation of +80 mm (+3.1 in.) Vertical misalignment - +40 mm (+1.6 in.) The estimated overall standard deviation of measurement error is 3.0 mm (0.12 in.), which means that the device can provide measurement accuracy of +5 mm (0.20 in.) with 95% reliability. With proper calibration to account for dowel baskets, the device can provide similar levels of accuracy for dowel bars placed either in dowel baskets or by a dowel bar inserter. This assumes that the dowels are insulated (epoxy coated or painted) and that the transport ties are cut. Field experience with the MIT Scan-2 showed that the device is reliable and easy to use. Up to 400 or more joints can be tested in an 8-hour period using a single charge of the battery. An exception is that testing during cold weather greatly reduces the battery life. The ability to assess MIT Scan-2 results and focus, in real time, on potentially needed adjustments both in the paving equipment and hardware and in the portland cement concrete mixture proportions make the MIT Scan-2 a unique and valuable tool. The principal limitation of MIT Scan-2 is that the presence of other metal objects (such as tie bars, nails in the joint, coins, pieces of wire, or any other metal item) near the measurement region can introduce significant errors, effectively invalidating the results.
17. Key Words concrete pavement, dowel bar alignment, joints, load transfer, magnetic tomography, MIT Scan-2, non-destructive testing.			18. Distribution Statement
19. Security Classif. (of this report)		20. Security Classif. (of this page)			21. No of Pages 80	22. Price

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Use of Magnetic Tomography Technology to Evaluate Dowel Placement

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