Monitoring Damage and Unsaturated Moisture Flow in Concrete with Electrical Resistance Tomography (ERT)

Cracking adversely affects the long-term performance of reinforced concrete structures since cracking accelerates the ingress of moisture and aggressive ions (e.g., chlorides) into concrete. Detecting cracking and quantifying the rate of moisture movement in concrete, therefore, are essential for predicting the long-term performance of concrete structures. This study investigates whether Electrical Resistance Tomography (ERT) is a feasible and reliable nondestructive testing technique for crack detection and monitoring unsaturated moisture movement in concrete structures.;For crack detection in concrete, ERT-based sensing skin was developed. Sensing skin is a thin layer of conductive material which is applied to the surface of concrete elements. Cracking of the concrete substrate ruptures the sensing skin, locally changing its electrical conductivity. The local change of electrical conductivity is detected with ERT.;For crack detection as well as detection of ingress of chloride ions, a multi-layer multifunctional sensing skin is developed. This sensing skin is made of two individual sensing skins where the material for each layer is selected based on its functionality: The electrical conductivity of one layer changes due to cracking and the presence of corrosive ions, while the electrical conductivity of the other layer changes only due to cracking. ERT is employed to monitor this change in electrical conductivity. Results indicate that the ERT-based sensing skin can capture cracking patterns as well as the presence of chloride ions.;Two ERT reconstruction schemes (difference and absolute imaging) are used in this study and advantages and limitations of each method are discussed completely. Application-specific models and computational methods were utilized in absolute imaging including Total Variation (TV) prior model for the damage and an approximate correction of the modeling errors caused by the inhomogeneity of the painted sensing skin. The developed absolute imaging method was tested against difference imaging. The results indicate that the developed application specific imaging method had a higher resolution and provided quantitative results.;To monitor the unsaturated flow, ERT measurement are performed directly on the cement based materials. An experiment was carried out in which simultaneous ERT and neutron radiography measurements were carried out during the water ingress in cement paste. The neutron radiographs served as quantitative high resolution two-dimensional moisture distribution images, against which the ERT reconstructions were compared. The results of this experiment strongly support the feasibility of using ERT for imaging moisture distributions in cementitious materials.