High Performance Concrete Pavements
CHAPTER 40. Various States, MIT Scan-2
Dowel bar placement is critical for providing proper load transfer for concrete pavements. Misaligned dowels can lock up the joint and cause premature failure. However, the ability to identify the accuracy of dowel bar placement is difficult. A new device, MIT Scan-2, developed by MIT GmbH, Dresden, Germany, was created for the specific purpose of locating steel dowel and tie bars in concrete pavements. Figure 110 provides a close-up view of the device (ERES 2003). In 2003, the FHWA conducted laboratory and field evaluation of the MIT Scan-2 technology to assess its performance in determining dowel bar location. It is anticipated that MIT Scan-2 will become an important monitoring tool in QA/QC procedures.
Figure 110. Close-up view of MIT Scan-2.
The objectives of this project are the following (ERES 2003):
- Provide 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 DOT personnel and collect their comments on usability of this device.
- Develop recommendations for use of MIT Scan-2 in QA/QC procedures by contractors and State DOTs and develop comprehensive training material.
Project Design and Layout
The laboratory testing program was designed to verify the measurement accuracy, determine the overall standard deviation of measurement error, and identify factors affecting measurement results for MIT Scan-2. The following laboratory tests were identified (ERES 2003):
- Determine repeatability of measurements.
- Determine the absolute measurement error.
- Document that the measurements are not affected by cover material or water.
- Determine the effects of common factors (variability in bar length, presence of metal objects, and water) encountered in the field.
- Determine the effects of dowel baskets - properly cut baskets only.
Other than the basic objectives, no specific plans were developed for the field testing, since the feasibility of field testing depends on the availability of projects (ERES 2003).
Literature Review Summary
A comprehensive literature search was conducted to evaluate various types of cover meters, GPR, and MIT Scan-2 in determining dowel bar alignment. The cover meters and MIT Scan-2 were noted to have the following advantages over GPR (ERES 2003):
- The weather conditions (dry vs. wet) do not affect measurement results.
- Testing can be conducted on "green" concrete.
The main disadvantage of cover meters and MIT Scan-2 is that the presence of other metal affects the measurement results. In addition to the advantages mentioned above, however, MIT Scan-2 is able to scan the entire joint and determine the location and alignment of all dowels in one measurement (ERES 2003).
Laboratory Testing Results
The first series of laboratory testing was completed during August 2003. Preliminary results from the laboratory testing confirmed the MIT's claim on the accuracy of Scan-2 (ERES 2003):
- Depth: ±2 mm (0.0787 in.)
- Horizontal misalignment: ±2 mm (0.0787 in.)
- Vertical misalignment: ±2 mm (0.0787 in.)
- Side shift: ±5 mm (0.1968 in.)
- Repeatability: ±2 mm (0.0787 in.)
With the rail fixed, the measurements from one series of repeated testing had the following standard deviations (ERES 2003):
- Horizontal misalignment: 0.6 mm (0.0236 in.)
- Vertical misalignment: 0.4 mm (0.0157 in.)
- Side shift: 0.5 mm (0.0197 in.)
These results are for the bars with misalignment less than 13 mm (0.5118 in.). The variability is somewhat higher for bars with greater misalignment.
Field Testing Results
Limited field testing conducted in Reno, Nevada showed that for dowel bars placed in baskets, the presence of the metal basket interferes with the MIT Scan-2 measurement results. However, if the transport ties for the basket are cut, good results can be obtained (ERES 2003). MIT is in the process of developing software that can compensate for the presence of the basket.
Even without the basket software, however, the field testing results showed that MIT Scan-2 can be a useful tool for detecting problems with dowel placement. Figure 111a shows an example of dowel basket that is pulled apart during construction, resulting in severely misaligned bars. Figure 111b is an example of properly placed dowel basket (ERES 2003).
Figure 111. Example sectional contour map (tomography) of electro-magnetic signal detected by MIT Scan-2:
a) severely distorted basket with bars pulled out;
b) properly placed dowel basket.
Point of Contact
Federal Highway Administration
Office of Infrastructure
Office of Pavement Technology (HIPT)
1200 New Jersey Avenue SE
Washington, DC 20590
ERES Consultants, Inc. (ERES). 2003. Use of Magnetic Tomography Technology to Evaluate Dowel Placement. Quarterly Progress Report to Federal Highway Administration.