Detection of Breaks in Reinforcing Bars with the Magnetic Flux Leakage Method

Abstract

The survey of existing structures is arbitrative to define their remaining life on one hand and a reduction of maintenance cost on the other hand. Due to this, non-destructive testing methods (NDTM) are becoming more and more important. The aim of this research is to develop a modular measurement system to detect breaks in the reinforcing bars and to gain experience interpreting experimental data. The knowledge about fatigue failures of reinforcing bars in concrete members is limited and often breaks in the reinforcement have to be assumed after a certain number of stress range cycles. Specially for this purpose, the current state of the structure can be obtained by means of the residual magnetism (RM) method. Since the early 1980s, the RM method has been used to localize breaks in prestressing tendons, where the tendons are magnetised by an external magnet. A break leads to a change of sign in the magnetic flux density which can be measured using hall sensors. Thereby the distance between tendon and sensor is crucial since the flux density decreases by the power of three with the distance. However, there are differences in the magnetic properties of reinforcing steel and those of prestressing steel. A comparison of the magnetic flux density for rebars and tendons as well as following adjustments are required to evaluate the influence of several layers and layouts of reinforcing bars. Thus, it will be necessary to know the right classification of the measured signal and the arrangement of the reinforcing bars. In experiments different specimens with predefined circumstances in the reinforcement layout are tested including breaks in the reinforcing bars. For the data interpretation a graphical solution is applied. In order to reach a data resolution that allows for promising detections of breaks in the reinforcement most of the technical components of the measurement system still have to be developed.