Article Transportation Research Record 1–13 Ó National Academy of Sciences: Transportation Research Board 2018 Reprints and permissions: DOI: 10.1177/0361198118773920 journals.sagepub.com/home/trr Nondestructive Testing of Steel Corrosion in Prestressed Concrete Structures using the Magnetic Flux Leakage System Sadegh Shams 1 , Al Ghorbanpoor 2 , Shibin Lin 3 , and Hoda Azari 4 Abstract The Magnetic Flux Leakage (MFL) method can be nondestructively used to disclose the location and extent of corrosion or fracture in prestressed strands in concrete structures. In this study, parameters with the greatest effect on the performance of the MFL system are investigated using numerical simulations. The MFL system under study is based on two permanent magnets to magnetize embedded strands and Hall-effect sensors to detect normal magnetic flux leakage. The system is assessed using magnetostatic and transient numerical analysis to effectively simulate the MFL system. Results have been veri- fied by laboratory and field experiments. Both normal and axial Hall-effect sensors are modeled in simulations to better iden- tify magnetic signals at the corrosion zone. The sensor lift-off and the magnetic field masking by lateral reinforcements on nearby pitting corrosion are addressed as two main drawbacks of using MFL systems to detect corrosion in prestressed con- crete structures. To provide more details about the flux leakage interference between the pitting corrosion and lateral rein- forcements in prestressed concrete structures, linear and oriented magnets/sensor arrays are proposed and analyzed numerically. Magnetic flux leakage (MFL) systems are effective non- destructive evaluation means used in the transportation and energy industries. Kusenberger et al. proposed the MFL system for assessing the steel components of pre- stressed concrete structures (1, 2). The basic idea of MFL inspection is that the magnetic field emitted in a metallic object is distorted near the cross-section loss. The magnetic field distortion can be significant and detectable when the ferromagnetic materials are locally magnetized to full or near saturation. The amplitude of a flux leakage signal is generally proportional to the magnetization level and increases at cross section losses. The magnetic sources can be either permanent or electro magnets. The magnets are moved parallel to ferromag- netic strands to detect any leakage emerging from cross- sectional loss. As concrete is essentially a non-magnetic material with a relative magnetic permeability of unity, it has negligible influence on the magnetic measure- ments. However, the application of MFL systems in pre- stressed concrete structures has not been well developed because of inaccessible steel reinforcements and opera- tions on heterogeneous concrete structures. The MFL results can be significantly affected by the distance between the magnetizer/sensors and the object of inter- est known as sensor lift-off. Also, signals originating in desired objects can be interfered with by signals from nearby secondary ferromagnetic objects. In the current MFL system, two permanent magnets are used as the magnetizer to subject a prestressed concrete structure to the magnetic field (3, 4). The magnets/sensors assembly travels linearly along the length of the structure while maintaining a constant distance from the concrete sur- face. When the magnetic field reaches the corroded 1 Engineering Software Consultants Inc., Turner-Fairbank Highway Research Center, McLean, VA 2 College of Engineering & Applied Science, University of Wisconsin- Milwaukee, Milwaukee, WI 3 Center for Advanced Infrastructure and Transportation, Rutgers University, Piscataway, NJ 4 Turner-Fairbank Highway Research Center, Federal Highway Administration, McLean, VA Corresponding Author: Address correspondence to Shibin Lin: shibinlin@gmail.com