Field Test of Magnetic Methods for Corrosion Detection in Prestressing Strands in Adjacent Box-Beam Bridges

Abstract

Magnetic methods are progressing in the detection of corrosion in prestressing strands in adjacent precast, prestressed concrete box-beam bridges. This study is the first field trial of magnetic strand defect detection systems on an adjacent box-beam bridge. A bridge in Fayette County, Ohio, that was scheduled for demolition was inspected. The prestressed box beams had significant strand corrosion. The corroded strands showed discontinuities and a reduced cross-sectional area. These changes are reflected in the magnetic signatures of the prestressing steel. Corrosion in the prestressing steel was detected using two magnetic methods; namely, the magnetic flux leakage method and the induced magnetic field method. The purpose of these tests was to demonstrate the ability of the magnetic methods to detect hidden corrosion in box beams in the field and tackle the logistic problem of inspecting box beams from the bottom. The inspections were validated by dissecting the bottom of the box beams after the inspections. The results showed that the magnetic flux leakage method detects hidden corrosion and strand breaks with a sufficient amount of accuracy. Both magnetic field methods were also able to estimate corrosion by detecting the effective cross-sectional area of the strand. Thus, it was shown that the magnetic methods can be used with sufficient reliability to predict hidden corrosion in prestressing strands of box beams. The recommended actions to make magnetic inspection practical are discussed.