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Use of Magnetic Tomography Technology to Evaluate Dowel Placement
LABORATORY EVALUATION (continued)
A comprehensive series of tests were conducted to verify the accuracy of MIT Scan-2 results. Several factors are known to affect the accuracy of MIT Scan-2 results, including the bar depth, magnitude of position error (side shift), and amount of misalignment. The effects of these factors on MIT Scan-2 results were evaluated systematically to verify the accuracy of MIT Scan-2 and to determine the operating range of MIT Scan-2. Within the operating range, the device is expected to provide the specified level of accuracy. The measurement errors were determined by comparing MIT Scan-2 results to manual measurements.
The range of values of the test parameters for the testing program was selected in consideration of both the MIT Scan-2 specifications and the application requirements. According to manufacturer specifications, the device provides the specified accuracy under the following conditions:
When the above conditions are satisfied, the manufacturer-specified accuracy is within 4 mm (0.16 in.) on rotation and bar depth.
The ranges of values of the key parameters specified for MIT Scan-2 are adequate to cover most situations for highway applications, except for the following:
During the course of this study, MIT GmbH made software enhancements to allow testing on skewed joints. To verify that the results for skewed joints are accurate to the same degree as the results for square joints, the evaluation range for horizontal misalignment was extended to +120 mm (4.7 in.).
The limits on the range of depths, however, are physical limitations (relating to the signal to noise ratio), which cannot be overcome without a hardware modification. The sensitivity of measurement errors to depth is shown in Figure 12 (supplied by MIT GmbH). The error is less than 4 mm (0.2 in.) for depths up to about 200 mm (7.9 in.). At depths above 200 mm (7.9 in.), the measurement error increases rapidly. This is only a limitation for the devices that were optimized for highway applications, where the typical bar depths range from about 100 to 165 mm (4 to 6.5 in.). For testing thicker pavements, the signal strengths can be increased to shift the zone of accurate results upward. In fact, the signal strength of the devices shipped to the United States was reduced by half from the original design to accommodate testing that included dowel baskets. Excessively high signal strength causes the response signal to saturate the sensors, making it impossible to obtain any information about the dowel position.
The accuracy of MIT Scan-2 and effects of various factors affecting the results were evaluated primarily based on tests on a single dowel bar. The only exception is the limited testing conducted to verify the minimum allowable bar spacing. MIT specifies a minimum bar spacing of 250 mm (9.8 in.). Limited testing conducted using multiple dowel bars has shown that a bar placed within 200 mm (7.9 in.) of the testing sample does increase the measurement error, but a bar placed 255 mm (10 in.) or farther away has no effect on the test results. The key parameter is the d/z (bar spacing over depth) ratio. The error increases with increasing depth. At shallower depths, a closer bar spacing can be accommodated. In typical highway applications, the 250-mm (9.8-in.) limit on bar spacing does not pose a problem.
The nominal ranges of the test parameters evaluated in the laboratory testing are as follows:
The evaluation results are shown in Figures 13 through 22. The following can be observed from the test results:
The laboratory testing results confirm that MIT Scan-2 provides accuracy that is both reasonable and useful for horizontal and vertical misalignments within the following limits:
Note that the operating range for horizontal misalignment is a range of misalignment (+40 mm [+1.6 in.]) plus a uniform rotation. Although the laboratory evaluation covered the full range of horizontal misalignment (+120 mm [+4.7 in]), the tests were conducted using a single dowel bar. In an actual joint, extreme horizontal misalignments can cause the ends of neighboring bars to come very close to each other, which in turn can cause additional error due to overlapping response signal. On skewed joints, the magnitude of apparent horizontal misalignment is large, but a major portion of the registered horizontal misalignment is due to a uniform rotation caused by joint skew, which does not cause a problem for neighboring dowel bars. For the actual misalignment, the range originally specified for horizontal misalignment (+40 mm [+1.6 in.]) is more than adequate to cover all practical cases. If the actual horizontal misalignment exceeds +40 mm (+1.6 in.), the exact quantitative results are not important; it is sufficient to know that the horizontal misalignment is large.
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