Use of Magnetic Tomography Technology to Evaluate Dowel Placement
Extensive laboratory and field evaluations were conducted under this project to evaluate the effectiveness and limitations of the MIT Scan-2, 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 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 uniform rotation of +80 mm (+3.1 in.) mentioned above refers to the ability to handle skewed joints.
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 percent 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 DBI. This assumes that the dowels are insulated (epoxy coated or painted) and the transport ties are cut.
Field experience with 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.
MIT Scan-2 can also be a useful tool for contractors in identifying the adjustments needed in the paving process. For example, a consistently large misalignment at one particular bar position at consecutive joints suggested that the DBI forks at that position needed adjustment. Similarly, the ability to rapidly monitor dowel placement results at consecutive joints is very helpful in adjusting the concrete mixture proportions. For dowels in baskets, significant misalignments would immediately draw attention to the procedures used to secure the basket to the base as well as the integrity of the basket itself. 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 PCC mixture proportions makes 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. Although the presence of such metal objects is easily detectable on the signal intensity plot, the loss of information for the affected bars is a limitation. The most problematic of the metal objects may be the tie bars between lanes; however, the detection of a tie bar within the influence region of the scanned joint may itself be an indication of a problem, because most States require tie bars to be at least 500 mm (20 in.) away from transverse joints.