Use of Magnetic Tomography Technology to Evaluate Dowel Placement
A comprehensive literature search was conducted to identify and evaluate the available devices for determining dowel bar alignment. Of the available devices, MIT Scan-2 is the only one developed specifically for detecting dowel bar alignment and optimized for that application. Other devices consist of various types of cover meter and GPR. An Internet search was performed to collect information about all the devices identified. The equipment manufacturers and distributors were also contacted to obtain additional information.
Cover meters work on the same basic principle as the MIT Scan-2. The devices emit an electromagnetic pulse and detect the magnetic field induced in metal objects. Various types of cover meters are available, including the following:
- Micro Covermeter
- Rebar locator R-HR-7000
- Fisher MODEL M-101 Rebar Locator
- Refor 3
Cover meters are mainly designed for locating reinforcement in concrete structures and determining the depth of concrete cover. Many of the older devices are capable only of detecting metals close to the concrete surface (e.g., concrete cover of 75 mm [3 in.] or less) and cannot be used for determining dowel alignment at all. The newer devices listed above, however, have a detection range (range of depths) similar to that of MIT Scan-2.
All of the devices have a similar configuration, with one or more sensors for detecting the induced magnetic field. Based on the duration or intensity of the induced magnetic field, the location of embedded metal is determined. All devices provide the concrete cover and horizontal distance to the bar. To determine dowel alignment, the ends of the dowel bar have to be found and marked manually. The ends of the bar are found by finding the location where the signal drops off abruptly. The alignment is determined from the marked positions (for the horizontal alignment) and the depths measured from those locations (for vertical alignment). This process is slow and is subject to the errors introduced during marking and taking the readings precisely at the bar ends.
Because cover meters work on the same principles as the MIT Scan-2, they are subject to the same advantages and limitations as MIT Scan-2; however, some key advantages of the MIT Scan-2 method of dowel bar detection include the following:
- The weather conditions (dry vs. wet) do not affect measurement results. This is a significant limitation for GPR.
- Testing can be conducted on fresh concrete. The other devices will actually work on fresh concrete, but there are no practical means of taking measurements without marring the surface.
The main disadvantage of these devices (and MIT Scan-2) is that the presence of other metal will affect the measurement results. For accurate results using all devices, the bar diameter must be known. For highly accurate results, MIT Scan-2 requires that both the diameter and length of the bars be known and that a calibration be performed using the specific dowel bar.
Profometer, Micro Covermeter, CoverMaster, Rebar locator R-HR-7000, Fisher MODEL M-101 Rebar Locator, Refor 3, and Ferroscan all have similar features. They are user-friendly (automatically display concrete cover thickness, ability to determine horizontal location, etc.). However, locating a large number of individual dowel bars is time consuming. In addition, many of these devices do not have effective data storage systems, so the information has to be recoded manually. These devices may be effective for random checks of dowel alignments, but they are not practical for evaluating the alignment of all bars in a joint, which is needed to assess whether improperly placed dowels will interfere with the proper functioning of the joint.
GPR systems operate by transmitting polarized pulses of electromagnetic energy into the ground and then recording the energy that is reflected back to the surface. The GPR signal responds to variations in the electrical properties of subsurface materials (dielectric constant and conductivity) that are a function of material type, moisture content, and pore fluid type. Where a contrast in dielectric properties exists between adjacent materials, a proportion of the electromagnetic pulse will be reflected back. Subsurface structures are mapped by measuring the amplitude and travel time of this reflected energy.
A recent study conducted by the Missouri DOT (MDOT 2003) demonstrated that GPR can be used to assess dowel bar alignment accurately. The researchers reported the measurement accuracy of +3 mm (0.1 in.) on vertical alignment. The accuracy of detecting lateral dowel position is believed to be within 10 mm (0.4 in.). However, this method of detecting dowel alignment is sensitive to the dielectric constant of concrete, which is a function of moisture content, temperature, and antenna frequency, among others. This method cannot be used on fresh concrete or when the concrete surface is wet. Another drawback for this method is that the data processing is quite involved.
Over the past few years, significant advances have been made in GPR technology to overcome many of the past difficulties of using GPR. Signal calibration techniques have been developed that enable scan-by-scan calibration of the data obtained without requiring physical testing for material properties (core testing). The dependence on the accuracy of the material property (dielectric constant) of the test results had been a critical limitation of GPR technology in the past. The data analysis process has also been streamlined for certain applications to provide real-time results (e.g., detection of rebar location and depth on structures and bridge decks). The main advantage of the GPR technology (over the magnetic) is that the results are not affected by the presence of foreign metal. The presence of tie bars, metal covers for drainage inlets, or any other metal objects close to dowel bars poses a problem for measurements using the magnetic technology. Because of the insensitivity of the test results to the presence of foreign metal, the GPR technology is better suited for testing dowels placed in baskets. However, for testing bare bars (inserted bars), the magnetic technology may be more reliable, because the results are based on direct measurements, rather than correlation to a calibration.
MIT Scan-2 was developed specifically for locating dowel bars placed by DBI. The device has features that make it superior to all other devices:
- In one scan, the device determines the location and alignment of all dowels along the entire joint (up to three lanes wide).
- Immediately after the measurements, preliminary results can be printed. More comprehensive analysis can be performed later.
- Multiple sensors and innovative data interpretation software make the device extremely accurate.
- The device can be used on fresh concrete.
Developed by MIT GmbH, Dresden, Germany, MIT Scan-2 was created specifically for locating steel dowel and tie bars in concrete pavements. If operated by a two-person crew, 200 or more joints can be tested using MIT Scan-2 in an 8-hour shift. Up to three lanes can be scanned at one time, and the productivity is similar for scanning one or two lanes because the measuring task takes minimal time. The device was designed to work continuously for at least 8 hours on one battery charge. The onboard computer allows the crew to perform a preliminary analysis of the test data and print the results after testing in real time. The test data are stored on a flash memory card and can be analyzed later on a more powerful computer, using more sophisticated software.