Comprehensive MMF–MFL inspection for corrosion detection and estimation in embedded prestressing strands

Abstract

This paper describes the development of a magnetic sensor system for the evaluation of the condition of prestressed strands inside the bottom layer of concrete box beams. The sensor consists of an electromagnet which magnetizes embedded prestressing strand and measures the magnetic field response of the strands using Hall sensors embedded at different positions on the electromagnet. The novel aspect of this comprehensive system is the integration of measuring magnetic flux leakage (MFL) and main magnetic flux (MMF) signals. A computer-aided simulation model has also been developed to use these signals in estimating corrosion. The MFL signal gives indications of the presence of corrosion and cracks or fractures in the strands. The MMF signal quantifies the amount of steel loss in the strands due to corrosion or rusting. Both these techniques have been independently evaluated using magnetic inspection systems which made them suitable to be used in a single sensor system. This comprehensive system was evaluated in a laboratory setting to identify corrosion and strand breaks in prestressing strands. It was observed that the MFL signal could identify the presence of corrosion and strand breaks, while the MMF signal could be used to estimate the loss of steel due to corrosion. This system was also evaluated using it in an inspection of decommissioned prestressed box beams removed from Ohio’s oldest prestressed box-beam bridge. The findings from this inspection revealed a good match with the dissected areas of the box beams. Steps to move the system to implementation are also discussed. With further development improving the accuracy of this inspection system, it can become a valuable tool to bridge engineers for box-beam inspection.