Use of Magnetic Tomography Technology to Evaluate Dowel Placement
Appendix A MIT Scan-2 Operations Guide
MIT Scan-2 is a magnetic imaging tool specifically developed for measuring dowel and tie bar alignments in PCC pavements. The device consists of the scan unit, onboard computer (Casio IT2000), and rail system as shown in Figure A1.
MIT Scan-2 emits electromagnetic pulse and detects the induced magnetic field. Consequently, any metallic objects within proximity of the scan unit influence the measurement results. To obtain reliable results, the surface of the joint to be scanned must be free of any metallic objects (e.g., coins, pocket knife, keys). If the inspectors are wearing steel-toed boots, the boots must be kept a minimum 3 ft (0.9 m) away from the scan unit during measurements.
Preparation consists mainly of charging batteries and management of flash memory on IT2000.
The scan unit and onboard computer have batteries that require charging:
- Scan unit - The scan unit contains a maintenance-free, lead-acid battery. This battery should only be charged using the charger supplied with the unit. The charger connects to the power/data port near the handle. The charger should be protected from moisture, dust, and excessive heat. The cooling vents on the charger should not be covered, and charging should be stopped for a minimum of 2 hours if condensate forms on the charger. When the battery is fully charged, the indicator on the charger turns green. The battery charges in about 4 hours.
- Casio IT2000 - The onboard computer contains a lithium-ion battery. This battery is compatible with Sony F-550 (camcorder battery). The supplied charger fully charges the battery in about 10 hours. The power indicator on IT2000 turns green when the battery is fully charged. The IT2000 battery can also be removed and charged using an external charger, which fully charges the battery in about 2 hours.
MIT Scan-2 is designed to function for a minimum of 8 hours on a single charge of battery.
The MIT Scan-2 testing data are stored in a flash memory card on IT2000. The unit is supplied with a 32-megabyte (MB) flash memory card, which can hold data for about 600 joints (one lane). The memory card plugs into the PCMCIA port at the bottom of IT2000. This card can be removed and plugged into any laptop computer for managing data files. The testing data should be periodically downloaded onto a laptop or other permanent storage device, and the flash card cleared to provide room for data from future testing. MIT Scan-2 uses the following conventions for data storage:
- All scan data are stored in the flash memory card (PCMCIA card).
- Data for each day's testing are stored in a single folder that is named as follows:
yy_mm_dd (year_month_day) - for example, data from March 31, 2003 would be stored in a folder named 03_03_31.
- Each scan produces the following binary file:
ddMMhhmm.HDF (day, month, hour, minute) - for example, data from 1:16 p.m. on March 31 would be stored under file 31031316.HDF.
- If the scan results are successfully analyzed by the onboard computer (IT2000), the following text file is produced:
ddMMhhmm.TXT (day, month, hour, minute) - for example, data from 1:16 p.m. on March 31 would be stored under file 31031316.TXT.
IMPORTANT: The flash card also contains the calibration files (*.CPF), which must be present in the root directory of the flash card for the unit to collect meaningful data. There is a calibration file for each type of bar that can be scanned as well as the default calibration file spf.cpf, which is required for the unit to function.
After the data are downloaded from the flash memory card, the memory card should be firmly reseated and locked into the PCMCIA port of IT2000. A loosely placed memory card can cause IT2000 to shut down during testing.
The setup activities for MIT Scan-2 consist of the following:
- Powering on the scan unit.
- Assembling and placing the rail system.
- Placing the IT2000 on the scan unit.
- Connecting the data cable and switching on IT2000.
Powering on the Scan Unit
The power switch on the scan unit is located to the right of the handle. When the power is on, the switch is illuminated. This should be the first task in setting up MIT Scan-2, because the unit takes 5 to 10 minutes to warm up. If the measurements are taken while the unit is not warmed up, they will be inaccurate. The readiness of the unit can be verified by setting up, and then testing, the same joint 3 times without moving the rail. If the unit is properly warmed up, the maximum difference in measurements for any dowel should be 2 mm (0.08 in.) or less.
Assembling and Placing the Rail System
The rail system for MIT Scan comes in 3-ft (0.9-m) tube sections connected by tie sections. The outside edge of the first tie should be placed flush with the pavement edge, and the rail assembly should be placed centered across the joint being scanned, as shown in Figure A2.
Placing the IT2000 on the Scan Unit
After the rail system is assembled, the scan unit should be placed on the rail and IT2000 placed on the scan unit at the designated place. The location of IT2000 is marked with rubber stoppers, as shown in Figure A3, and Velcro is used to hold the onboard computer in place.
Connecting the Data Cable
A special cable connects the scan unit and IT2000 for data communication. Both ends of the connectors have notches for proper alignment of the connectors. If the connectors are properly aligned, they should slip in without much resistance. Do not force the connectors. At the scan-unit end, the connector is threaded for secure connection. To connect:
- Connect the cable to the scan unit first, and lock it in place by turning the rotating sleeve clockwise (Figure A4-a).
- Connect the IT2000 end, flat side up (Figure A4-b).
Switch on the IT2000 after installing the data cable. Figure A5 shows the screen that displays if all sensors and communication between the scan unit and IT2000 are working properly.
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