Survey of concrete blocks submitted to artificial sea water tide during one year by electrical resistivity tomography

Electrical resistivity methods are very sensitive to the evolution of water and chloride contents of concrete. The multi-electrode device, used in this study, is applied on the concrete surface to measure the apparent resistivities. After data inversion, we get ‘true’ resistivity profiles function of depth. In this study, we present the survey methodology based on this electrical resistivity tomography method. It was implemented to monitor the penetration of salt water in concrete blocks placed in a tidal basin. Three concrete formulations are studied: the first named C100-0 mixed with ordinary Portland cement CEM I, the second, named C70-30, containing 30% of replacement blast furnace slag and the third named C50-50, containing 50% of replacement slag. The artificial tidal cycles reproduce natural tides and the salt water contains 35 g/L of sodium chloride like seawater. The blocks were inspected at short intervals at the start of exposure and then at more spaced monthly intervals. The method for inversion of apparent resistivities is briefly summarized. The results corresponding to the three concretes are presented for one year, firstly as apparent resistivity values and then as ‘true’ resistivity profiles with respect to depth. The influence of slag is discussed: it induces a higher resistivity but the results do not show a mitigation of the penetration kinetics of the salted water in the actual studied conditions.