Use of Magnetic Tomography Technology to Evaluate Dowel Placement
The use of large-diameter dowel bars has been a long-standing recommendation for all jointed concrete pavements (JCP) subjected to high volumes of heavy truck traffic to prevent roughness caused by faulting. Implicit in this recommendation is the assumption that the dowel bars will be placed in proper position and alignment. Inadequate concrete cover can lead to steel corrosion and spalling. If the bars are not adequately centered under the joint saw cut, the bars may not be effective in providing load transfer. More critical, in terms of margin of error, is the dowel bar orientation. Misaligned dowel bars can interfere with the proper functioning of the joint, which in turn, can lead to spalling or cracking of the concrete. Severely misaligned dowel bars can also cause looseness around the dowel bars, greatly reducing their effectiveness. While the importance of achieving good dowel alignment is widely recognized, the ability to monitor the placement accuracy of dowel bars effectively has been limited by the lack of practical means of measuring the position and orientation of dowel bars embedded in concrete.
The past difficulties in measuring dowel alignment have several important consequences on concrete pavement construction:
- Limited validation testing for dowel alignment-Most agencies conduct only a limited amount of coring to evaluate dowel alignment. The obvious limitation of this approach is that dowel alignment is evaluated based on an extremely small sample. To determine whether misaligned dowels will interfere with proper functioning of a joint, the alignment of every bar in the joint must be known. To determine the quality of dowel alignment in a section of pavement (e.g., one day of paving) numerous joints must be tested. This amount of testing is not practical by coring.
- Possibility of excessively strict dowel placement tolerance-Most agencies have fairly strict tolerances on dowel placement accuracy, but those standards are based on limited laboratory and field data. In some cases, the fabrication tolerances for dowel baskets are adopted directly and used as the tolerance on dowel placement accuracy, which leaves no room for any placement error during construction. The actual dowel bar alignment needed to ensure good pavement performance is largely unknown.
- Limited usage of dowel bar inserters (DBIs)-Because of the concern over the dowel alignment and the lack of practical means of verifying dowel alignment in the past, DBI usage has not been widespread in the United States, and many highway agencies specifically prohibit the use of a DBI.
The placement accuracy of dowel bars embedded in jointed plain concrete pavements (JPCP) can now be evaluated with unparalleled accuracy and efficiency using MIT Scan-2. MIT Scan-2 is a state-of-the-art, nondestructive testing (NDT) device for measuring the position and alignment of dowel bars embedded in concrete. The device is simple to operate, is efficient, and provides accurate, real-time results in the field. This device holds the promise of greatly improving the quality of concrete pavement construction, as well as significant cost savings by preventing costly errors. The California Department of Transportation (Caltrans) was instrumental in bringing this technology to the United States (Khazanovich et al. 2003).
This study was initiated to evaluate the effectiveness and limitation of MIT Scan-2, especially as a tool for monitoring dowel placement accuracy during construction. The specific objectives include the following:
- Conduct laboratory and field evaluation of MIT Scan-2 to assess accuracy and repeatability of measurements of dowel position.
- Compare MIT Scan-2 measurements with measurements of other devices such as cover meter and ground penetrating radar (GPR).
- Demonstrate MIT Scan-2 to contractors and State department of transportation (DOT) personnel and collect their comments on the usability of this device.
- Develop recommendations for the use of MIT Scan-2 in quality assurance/quality control (QA/QC) procedures by contractors and State DOTs, and develop comprehensive training materials.