Inspection of Concrete/Steel/Concrete pipes by ultrasonic methods affected by corrosion

Concrete/Steel/concrete pipes are present in French nuclear powerplants. These pipelines are composed of three layers: an internal layer of centimeter concrete, a millimeter layer of steel and a layer of centimeter reinforced concrete. The pipelines located by the sea undergo corrosion of the steel part caused by the chlorides present in the seawater they transport. Nowadays, the diagnosis of corrosion is made with electrochemical measurements. But they provide a qualitative indicator about corrosion. In fact, the goal is to dispose of a Non-Destructive Technique capable of evaluating the extension of holes in the steel part with a minimum diameter of 1 cm and the steel thickness elsewhere with a 100 µm uncertainty. The inspection must be conducted from the outside of the pipe still working. Ultrasonic bulk waves, ultrasonic guided waves and vibration techniques are investigated. All of them are studied with a methodology from the lab to the field. In lab, specimens are manufactured and tested. The experimental results are compared to numerical results obtained with simulation tools like CIVA or Specfem2D. Then, the promising techniques are tested on a real concrete/steel/concrete pipe. For the bulk wave techniques, the major issue is to increase the frequency signal over 1 MHz to get a better resolution of the steel investigation. But at this frequency range, the multi-scattering phenomenon on concrete aggregates is prominent. First experimental results using pulse compression on lab samples are promising: holes in steel give a specific signature and compression techniques are used to get a better resolution in steel thickness after signal autocorrelation. For both guided wave and vibration techniques, simulations are used to determine modes sensitive to the steel thickness and Zero Group Velocity modes which will induce a strong characteristic frequency response. Experimental study is going on to reveal these modes on lab specimen.