A Field Verification Instrument to Assess the Placement Accuracy of Dowel Bars and Tie Bars in PCCP
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2005-10-01
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TRIS Online Accession Number:1019025
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Edition:Final Report
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Abstract:This report describes the design and construction of a prototype instrument for location of steel dowel bars and tie bars in highway concrete. The instrument consists of a non-metallic (wooden) wheeled platform which carries a commercially available metal detector known as a covermeter, specially modified for this project. The covermeter is designed to be held in the hand and swept over a concrete wall or road, displaying the distance to steel within the concrete. The modified covermeter, mounted on the platform together with a notebook computer, outputs a serial data stream which is converted by the included software into information about the location and orientation of an array of dowel bars. Operation of the instrument consists of pushing it slowly across the road surface along a saw cut. The output display consists of three-dimensional color depictions of the dowel array, showing the calculated positions and displacements. The instrument is able to detect dowel bars to a depth of about 30 cm. It is able to resolve a 10-mm horizontal and a 5-mm vertical displacement at a depth of 15 cm. Bar orientation is described in pitch (rotation about a horizontal axis normal to the bar) and yaw (rotation about a vertical axis). The instrument can readily resolve a 7-degree misalignment in both pitch and yaw. This represents a one-diameter rotation about the center of a bar. Although these resolutions can be improved, the cart would have to be moved at an impractically slow rate over the road. In 2001, a German firm introduced a device called MITScan (marketed in the US in late 2002), which it claims would scan roadbed automatically and output three-dimensional position information about the dowels in pavement. Initial tests of this device have been conducted by CalTran and Gomaco; results to this date, while promising, are not conclusive. With our experience, the KSU team could design a device equivalent to MITScan. This would require the use of multiple sensors, a stronger excitation signal, and custom-designed electronics and control circuitry. Of course, this would defeat the purpose of using an off-the-shelf sensor unit. If the manufacturer’s performance claims can be substantiated, and if this resolution is necessary to the application, we would recommend the purchase of the MITScan for high-resolution measurements. The present apparatus may be useful for lower resolution measurements. Applications might include surveys of known good pavement for the purpose of determining acceptable limits for displacements and misalignments of dowel bars and tie bars. The data collected could be used to support or to revise the established dowel bar placement standards. Additionally, the unit provides a low-cost approach to timely monitoring of contractor performance, and may be used to troubleshoot suspect pavement sections. If these applications are of interest, we would recommend that the instrument we have developed based on an off-the shelf covermeter be hardened for field use and refined with software enhancements as described in the Future Work section of this report
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