Magentic flux leakage detection of corrosion damage in prestressed concrete poles

Urban infrastructure includes large amounts of concrete poles which support power lines, communication cables, and street lighting. These poles are made of spun concrete with embedded pre-stressed reinforcement steel. The chemical environment at the ground-air interface, in combination with tensile prestressing and possibly low concrete cover, promotes corrosion of the steel rods. Stress corrosion cracking (SCC) has been observed in many concrete structures under these circumstances. Motivated by recent failures of concrete poles in Germany, a mobile, USB-powered magnetic flux leakage (MFL) based inspection system was developed. It was shown that the system allows the identification of artificial flaws starting with approximately 25% loss of cross-section in a single steel rod, through a concrete cover of 3 cm. Results indicate that the orientation of the crack towards or away from the sensor does not affect the detection. Moreover, it was observed that the gap length of a 100% rupture can vary between 0.5 mm and at least 25 mm without strong effect on the signal amplitude. Filters were implemented which suppress the magnetic field gradient of the permanent magnet yoke and emphasize either flaws or the existence and proximity of the rods. First on-site experience was gathered in Bonn, Germany.