Full-waveform GPR inversion to assess chloride gradients in concrete Alexis Kalogeropoulos a,n , Jan van der Kruk b , Johannes Hugenschmidt c , Jutta Bikowski b , Eugen Brühwiler a a Laboratory of Maintenance and Safety of Structures (MCS), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland b Institue of Bio- and Geosciences (IBG), Forschungszentrum Jülich (JFZ), Jülich, Germany c Institute of Engineering and Environmental Sciences (IBU), Hochschule für Technik Rapperswil (HSR), Rapperswil, Switzerland article info Article history: Received 21 August 2012 Received in revised form 8 January 2013 Accepted 14 March 2013 Available online 23 March 2013 Keywords: Ground Penetrating Radar Chloride gradients Full-waveform inversion Ray-based approach Reinforced concrete Corrosion Deicing salt abstract The spreading of deicing salt on roads during winter and the consequent migration of chloride ions within concrete exposes structures reinforcement to corrosion. Reinforcement corrosion is problematic because it reduces the structure's load carrying capacity and a long-term consequence can be structural failure. The use of Ground Penetrating Radar (GPR) technology is commonly applied to reinforced concrete structural mapping. This paper provides methodologies for the calculation of the electro- magnetic parameters from GPR data accounting for chloride content and its distribution within concrete. The previous work showed that they are linked with water and chloride content. A GPR dataset was carried out with bi-static off-ground GPR antennas during an experiment recreating chloride contam- ination processes in concrete specimens. Two novel solutions are presented in this paper, their use on the dataset returned the electromagnetic parameters of the concrete specimens with knowledge of their thickness. Firstly, a ray-based approach is provided as a fast computational tool to determine average material properties. Thereafter, a novel full-waveform inversion based on a multilayered forward model is presented. It allows the identification of conductivity gradients present inside the concrete. Finally, the results are compared with additional destructive testing; they present a good correspondence. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Road bridges in reinforced concrete can be exposed to chloride induced corrosion of steel reinforcement due to salt spread in winter for deicing. Steel reinforcement corrosion reduces the bridge resis- tance and a long-term consequence may be a structural failure. The use of Ground Penetrating Radar (GPR) technology is commonly described in the non-destructive testing literature as a suitable tool for reinforced concrete structural mapping [1–5]. Meanwhile, other non-destructive testing techniques for corrosion and free chloride detection were developed [6–11]. So far, GPR technology presents a good compromise to assess this problem as it enables the continuous non-destructive evaluation of reinforced concrete elements, without traffic disruption. The GPR pulse propagation within a dielectric body is described using electromagnetic parameters. When assuming non- magnetic materials μ r ¼ μ 0 ¼ 1, the electromagnetic parameters expressing the propagation are the relative dielectric permittivity, ε r and the conductivity, s. During the last decade, several authors studied the influence of water on GPR datasets [12–20], these studies providing evidence of a strong link between water content and the electromagnetic parameters. These findings are completed by Robert [21] and Soutsos et al. [22], who showed that concrete permittivity and conductivity values, from datasets acquired with electromagnetic coaxial trans- mission devices, changed with water and chloride ion content. In Hugenschmidt and Loser [23], it was observed that amplitudes of GPR pulses reflections are influenced by both the chloride and moisture content. Recently, in Kalogeropoulos et al. [24] the Hugenschmidt and Loser [23] dataset was processed with a full- waveform inversion; it was shown that chloride ions and moisture content have an influence on both permittivity and conductivity results. However in this dataset, chlorides were introduced by dissolving NaCl in the mixing water during preparation of the concrete specimens. As a consequence, the chloride ions were homogeneously distributed inside the specimens and not concen- trated within the electrolytic solution present inside concrete pores. In reality, chlorides are distributed in gradients over the element thickness due to capillarity and diffusion processes within the pores; these phenomena are caused by variations in atmo- spheric conditions, see Conciatori [25]. This paper presents methodologies for the estimation of the electromagnetic parameters accounting for chloride content and distribution inside a concrete slab. Three concrete specimens with Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ndteint NDT&E International 0963-8695/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2013.03.003 n Corresponding author. Tel.: þ41 79 297 40 54. E-mail addresses: alexis.kalogeropoulos@epfl.ch, ak@bridgology.com (A. Kalogeropoulos). NDT&E International 57 (2013) 74–84