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Use of Magnetic Tomography Technology to Evaluate Dowel Placement
FIELD TESTING AND DEMONSTRATIONS
Several site visits were made to demonstrate MIT Scan-2 to State DOT and FHWA personnel, as well as to contractors and representatives of concrete paving trade organizations. These site visits were also instrumental in fulfilling various aspects of field evaluation of MIT Scan-2, including the following:
South Carolina was the first State to use MIT Scan-2 in an actual construction project to monitor dowel alignment. Reconstruction of I-95 through Florence was the first project in South Carolina that was constructed using a DBI. The dowel alignment was closely monitored using MIT Scan-2. That experience provided valuable preliminary information on the operation of MIT Scan-2 and the monitoring of dowel alignment. Based on the information obtained through field testing and demonstrations, experience from other projects involving MIT Scan-2, and the lessons learned from the South Carolina experience, guidelines for evaluating dowel alignment using MIT Scan-2 were developed and are provided in Appendix B, Guidelines for Evaluating Dowel Alignment Using the MIT Scan-2 Device.
The States visited include Iowa, Kansas, Minnesota, Missouri, Nevada, North Carolina, and Pennsylvania. The sites were selected based mainly on the agency interest and geographical distribution. In many cases, the site visits were used to provide answers to the questions that had been raised about dowel bar alignment or tie bar position.
In Iowa, the DOT had some concern over the dowel bar alignment when cracking occurred along the ends of the dowels at a few transverse joints in a newly constructed JPCP. The MIT Scan-2 results showed that the dowel bar alignments were in fact very good. Testing the four joints in question and two longitudinal joints (for tie bars) took less than 15 min, and the results conclusively showed that the dowel bars are in very good alignment. No other method of testing would have been able to provide the same information so easily in such a short time.
In Kansas, a project under construction was suspected of having problems with tie bar position. The DOT conducted GPR testing to determine the extent and severity of the problem, and this provided the opportunity to compare MIT Scan-2 and GPR results. A short section of the project was scanned using MIT Scan-2 to compare the results with those obtained using GPR. The very first joint scanned showed a significant discrepancy between GPR and MIT Scan-2 results. An open-air demonstration was conducted to verify that the MIT Scan-2 results are accurate. A scan of several joints in a row revealed that for some of the joints, the results compared very well.
Table 8 provides a summary of the comparison for 10 joints. The joints were matched by examining the bar location pattern. The variations in the tie bar position served as a fingerprint to match the joints between GPR and MIT Scan-2 testing results. Since consecutive joints were tested, only the location of the first joint tested using MIT Scan-2 needed to be located in the GPR test results. Table 8 shows that where good agreement exists in the detected bar positions, good agreement in bar depths is observed.
Because MIT Scan-2 results are a direct interpretation of the magnetic signals detected during testing, the bar location reported by the device is highly reliable. On the other hand, the GPR results depend on material properties used in the data evaluation. Since the material properties can vary significantly along the length of a project, localized errors are possible. Some of the bar positions reported by GPR appear to be in error. For example, in slab 7, GPR reported only three tie bars, and the bar spacing in slabs 8, 9, and 10 is highly variable, ranging from 0.5 to 129 cm (0.2 to 51 in.). Results are shown graphically in Figure 36.
Prior to laboratory testing at the MnRoad facility, a short presentation was made to the MnRoad and MnDOT staff, followed by a demonstration. MnDOT has a cover meter, and the results of MIT Scan-2 were compared with the data collected using the cover meter. The results were described in the Literature Review section. The depth measurements were nearly identical. To measure dowel alignment using a cover meter, the ends of the dowel bar needed to be located and marked manually by finding the location where the signal drops off abruptly. While accurate results could be obtained using a cover meter, testing a large number of bars using this device is not practical. As noted in the Literature Review, cover meters are subject to the same limitations as MIT Scan-2: presence of other metal objects will affect the measurement results.
The Missouri DOT was interested in a demonstration of MIT Scan-2, and a site visit could be conveniently coordinated with the visits to Kansas, Iowa, and Minnesota. A short presentation was made at the DOT office, followed by an open-air demonstration and a field demonstration. The field demonstration was made on an on-ramp under construction near the DOT office. Several transverse joints were scanned, along with one longitudinal joint. The dowel alignment was not a concern on this project. The scan results showed no major problem with dowel alignment. One noticeable feature on this project was that all joints had at least a few uncut ties, clearly visible on the signal intensity plot (Figure 37).
Field tests were conducted on an I-80 reconstruction project in Reno. This was an opportunity to evaluate the feasibility of using MIT Scan-2 to evaluate alignment of dowel bars placed in baskets. This series of tests demonstrated that MIT Scan-2 is a valuable tool for identifying alignment problems of dowel baskets even without the basket software. The test results showed that the problems that develop in dowel baskets tend to be more obvious, larger scale problems that are easy to detect from the graphical output of MagnoProof (Figure 35). In general, the problems appear to be results of the baskets bursting open or deforming during concrete placement due to inadequate anchoring.
On this project, the baskets were originally anchored using only four pins per basket, one at each corner. MIT Scan-2 test results showed that these were not sufficient to hold the basket in place. Many baskets burst open, causing large misalignments. The most common problems observed from this project are shown in Figure 35b. Such problems are also clearly evident in the numerical results obtained using the standard software. The numerical results obtained without the proper calibration must be considered qualitative, but the results still provide an adequate degree of accuracy to clearly delineate the baskets that have problems and those that exhibit very good alignment.
After discovering the problem with the basket anchoring practice, the process was modified to pin every other dowel on each side of the basket (10 pins per basket). This change resulted in immediate and drastic improvement in dowel alignment. A comparison of the dowel alignment trends before and after the change is shown in Figure 38. Severely misaligned dowel bars can cause a joint to lock. Figure 38 shows that anchoring the baskets using only 4 pins per basket caused 11.5 percent of dowel bars to be misaligned by 20 mm (0.78 in.) or more. This is a severe case of dowel misalignment. If uniformly distributed, the 11.5 percent would place at least one severely misaligned dowel bar in every joint. By contrast, the baskets anchored using 10 pins per basket had exceptional alignment. No bars were misaligned by more than 20 mm (0.78 in.), and only 0.2 percent of the bars were misaligned by more than 15 mm (0.59 in.). These results represent about the best dowel alignment that could possibly be achieved in the field using either baskets or DBI. All sections anchored using 10 pins per basket showed similar results on this project.
Both the magnitude of misalignment and the number of dowel bars in each range of misalignment affect the functioning of a joint. Also, if a joint is locked due to one or more severely misaligned bars, the negative effect of additional misaligned bars may be minimal. In an attempt to quantify the potential effect of dowel misalignment on pavement performance, an index referred to as the "Joint Score" was developed to reflect the risk of joint locking. The index is determined based on the magnitude of misalignment and the number of dowel bars in each range of misalignment. An index of 10 or greater may suggest a high risk of joint locking. A full description of the index is provided in Appendix B. In Figures 39 and 40 a joint-by-joint evaluation of MIT Scan-2 data from the Nevada field test sections is shown using the Joint Score concept.
Figure 39 shows a high percentage of joints (27 percent) with a Joint Score greater than 10 for the section that was anchored using 4 pins per basket. The dramatic improvement in dowel alignment obtained when more pins were used (10 per basket) to hold the baskets securely in place is clearly shown in Figure 40. On this project, anchoring the baskets using 10 pins per basket completely eliminated the dowel misalignment problem. MIT Scan-2 was instrumental in both identifying the problem and verifying the improvement achieved after modifying the basket anchoring practice.
The North Carolina DOT was interested in a demonstration of MIT Scan-2 to determine its suitability for use during construction to monitor dowel alignment. The demonstration was particularly timely, because the DOT was contemplating whether to allow the contractor to use a DBI on an upcoming construction project (US-64 Knightdale Bypass). In the past, the DOT had not allowed the use of DBI. However, with the availability of practical means of verifying dowel alignment, the DOT was open to considering the DBI option.
Similar to other demonstrations, a short slide presentation was made, followed by an open-air demonstration. Shortly after this demonstration, the DOT approved the use of DBI with the condition that the contractor document dowel positions. On that project, both the contractor and the State used MIT Scan- 2 to monitor dowel alignment.
For the contractor, MIT Scan-2 was also instrumental in refining paving operations. Problems with equipment adjustment were easily detected using the MIT Scan-2 results. For example, a consistently large misalignment at one particular bar position suggested that the DBI forks at that position needed adjustment. For construction using a DBI, concrete mixture proportions have a significant effect on dowel placement. The portland cement concrete (PCC) mixture must be stable enough to hold the dowel bars in place after the insertion, and the mixture must be workable enough to ensure that no voids are left behind as the dowel bars pass through the concrete to their final positions. The ability to rapidly monitor dowel placement results at consecutive joints was helpful in adjusting the mixture proportions (Figure 41). This project demonstrated the usefulness of the MIT Scan-2 for process control in real time.
MIT Scan-2 was demonstrated at a project site in Pennsylvania where cracking had developed on an I-81 reconstruction project. Because the cracking initiated at the ends of the outermost dowel, dowel misalignment was one of the suspect causes. Fifty joints were scanned using MIT Scan-2 in about an hour. The results showed that while the dowel alignment on this project is not exemplary, dowel misalignment is not likely to have been a contributing factor on the observed cracking.
The results are shown in Figures 42 and 43. As shown in Figure 42, this project contains what would appear to be a significant percentage of misaligned bars (misalignment > 20 mm [0.78 in.]). The Joint Scores for this project show that several joints on this project may be locked, but field experience shows that occasional locked joints can be tolerated (Yu 2005). Also, the joint locations on this project where cracking occurred did not have a high Joint Score.