Using geoelectrical methods to assess corrosion of rebar and preferential flow paths in dams

Author(s): André Revil
Paper category: Proceedings
Book title: Proceedings of the International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Electrochemistry in Civil Engineering
Editor(s): Lisbeth M. Ottosen
ISBN: 978-2-35158-176-6
e-ISBN: 978-2-35158-177-3
Publisher: RILEM Publications SARL
Publication year: 2016
Pages: 176-177
Total Pages: 2
Language : English

Abstract: The first part of the talk is related to the use of a passive geophysical method to non-intrusively assess the corrosion of buried metallic objects. Indeed, large amplitude (>100 mV) negative electrical potential anomalies are generally observed close to buried metallic objects. In order to explain the mechanisms generating such signals, a controlled laboratory experiment has been carried out involving two metallic cylinders buried with vertical and horizontal orientations in the capillary fringe within a sandbox. 2D and 3D self-potential data were collected at several time steps along with collocated pH and redox potential measurements. Large dipolar selfpotential and redox potential anomalies developed in association with the progressive corrosion of the vertical metallic bar, while no anomalies were observed in the vicinity of the horizontal bar. This difference is due to the orientation of the metallic bars with the vertical bar subjected to a significantly larger redox potential gradient. The self-potential data were inverted to recover the causative source current density field using a deterministic least-squares 4D (timelapse) finite element modeling approach. These results were then used to retrieve the 3D distribution of the redox potential along the vertical metallic cylinder and associated with the corrosion. The results of the inversion were found to be in excellent agreement with the measured distribution of the redox potential. This experiment indicates that passively recorded electrical signals can be used to non-intrusively monitor corrosion processes [1].
The second part of this talk is dealing with the use of the self-potential technique to assess preferential flow path in Earth dams using the electrokinetic approach. The electrokinetic approach is based on the streaming current density associated with the drag of the excess of electrical charges existing in the diffuse layer coating the surface of the grains and by the flow of the pore water. This current density generates an electrical field that can remotely measured using a set of non-polarizing electrodes connected to a multichannel voltmeter. A field experiment is here used to show how passive seismic and electrical data can be combined together to detect a preferential flow path associated with internal erosion in an Earth dam. Continuous monitoring was performed during a 7-day full-scale earthen embankment failure test. Spatially coherent acoustic emissions events and the development of a self-potential anomaly associated with induced concentrated seepage and internal erosion phenomena were identified and imaged near the downstream toe of the embankment, in an area that subsequently developed a series of concentrated water flows and sand boils, and where liquefaction of the embankment toe eventually developed. I will discuss a novel 4D grid-search algorithm for acoustic emissions localization in both time and space, and the application of the localization results to add spatially varying constraints to time-lapse three-dimensional modeling of selfpotential data in the terms of electrokinetic source current localization (see [2] for details). Seismic signal localization results are utilized to build a set of time-invariant yet spatially varying model weights used for the inversion of the self-potential data. This approach to geophysical monitoring of earthen embankments provides an improved approach for early detection and imaging of the development of embankment defects associated with concentrated seepage and internal erosion phenomena [2, 3]. The same approach can be used to detect various types of hydromechanical disturbances in civil engineering.

Online publication : 2016
Publication type : full_text
Public price (Euros) : 0.00