The assessment of ageing concrete structures is a major technical challenge faced by the Civil Engineers of today. Concrete bridges are exposed to corrosion from the spreading of de-icing salts on roads during winter. Corrosion is problematic because it reduces the reinforcement's load carrying capacity and a long-term consequence can be structural failure. This paper describes an experiment that consisted in exposing a concrete specimen to a controlled wetting-drying cycle using a saline solution. This setup enabled the concentration of chlorides to be distributed in gradients inside the specimen. A novel methodology to estimate material properties, based on ground penetrating radar (GPR) full-waveform inversion, was used to process GPR traces. In this paper, each reflection present in the trace was regarded independently. This procedure was applied to a dataset acquired with bistatic off-ground GPR antennas. Measurements were carried out above the test specimen at regular time lapses. This allowed monitoring of the electromagnetic parameters of the top and bottom concrete specimen surfaces during the wetting-drying cycle.