New Tech Uses Electricity to Track Water, ID Potential Problems in Concrete

Photograph of one of the cracked samples tested in this work. Image at the background shows the flow of water in crack. Credit: Julie Williams Dixon.

Researchers from North Carolina State University and the University of Eastern Finland have developed a new technique for tracking water in concrete structures – allowing engineers to identify potential issues before they become big problems.

“When we think about construction – from bridges and skyscrapers to nuclear plants and dams – they all rely on concrete,” says Mohammad Pour-Ghaz, an assistant professor of civil, construction and environmental engineering at North Carolina State University and lead investigator on the project. Tracking concrete degradation is essential to public safety, and the culprit behind concrete degradation is water. Water contributes to the degradation by itself, or it can carry other chemicals – like the road salt used on bridges – that can expedite corrosion of both concrete and its underlying steel reinforcement structure.

Quantitative imaging of moisture flow in concrete  after 1, 2, 4, and 22 hours of water ingress. Actual specimen is shown in far left. Credit: Danny Smyl. Click to enlarge.
Quantitative imaging of moisture flow in concrete after 1, 2, 4, and 22 hours of water ingress. Actual specimen is shown in far left. Credit: Danny Smyl. Click to enlarge.

“We have developed a technology that allows us to identify and track water movement in concrete using a small current of electricity that is faster, safer and less expensive than existing technologies – and is also more accurate when monitoring large samples, such as structures,” Pour-Ghaz says. “The technology can not only determine where and whether water is infiltrating concrete, but how fast it is moving, how much water there is, and how existing cracks or damage are influencing the movement of the water.”

Previous technologies for assessing water in concrete relied on X-rays or neutron radiation, but both have significant limitations. X-rays offer only limited penetration into concrete, making it impossible to use with large samples or on structures. Neutron radiation is more accurate, but also has limited penetration, is expensive, and poses health and safety risks.

“Our electrical imaging approach is something that you could use in the field to examine buildings or bridges, which would be difficult or impossible to do with previous technologies,” Pour-Ghaz says.

For their electrical imaging technique, researchers apply electrodes around the perimeter of a structure. A computer program then runs a small current between two of the electrodes at a time, cycling through a number of possible electrode combinations.

Every time the current runs between two electrodes, a computer monitors and records the electrical potential at all of the electrodes on the structure. The researchers then use their own customized software to compute the changes in conductivity and produce a three-dimensional image of the water in the concrete.

“By rapidly repeating this process – and we can do it even more than once per second – we can also capture the rate, and therefore the volume, of the water flow,” Pour-Ghaz says.

The researchers have already created and tested a prototype of the system in a lab, accurately capturing images of water flow in concrete samples that are too large to be analyzed using X-rays or neutron radiation. The researchers have also been able to monitor water flow through cracks in concrete, which is more difficult and time-consuming when older technologies are used.

“Our electrical imaging technology is ready to be packaged and commercialized for laboratory use, and we’d also be willing to work with the private sector to scale this up for use as an on-site tool to assess the integrity of structures,” Pour-Ghaz says.

The work is described in three papers. Lead author on all three papers is Danny Smyl, a Ph.D. student at NC State. All three papers were co-authored by Aku Seppänen, of the University of Eastern Finland, and Pour-Ghaz. “Can Electrical Impedance Tomography be used for imaging unsaturated moisture flow in cement-based materials with discrete cracks?” is published in the journal Cement and Concrete Research, and was co-authored by Reza Rashetnia, a Ph.D. student at NC State.

Quantitative electrical imaging of three-dimensional moisture flow in cement-based materials” was published in International Journal of Heat and Mass Transfer. “Three-Dimensional Electrical Impedance Tomography to Monitor Unsaturated Moisture Ingress in Cement-Based Materials” was published in the journal Transport in Porous Media. Both papers were co-authored by Milad Hallaji, a former Ph.D. student at NC State.

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Note to Editors: The study abstracts follow.

“Can Electrical Resistance Tomography be used for imaging unsaturated moisture flow in cement-based materials with discrete cracks?”

Authors: Danny Smyl, Reza Rashetnia, Mohammad Pour-Ghaz, North Carolina State University; Aku Seppänen, University of Eastern Finland

Published: Oct. 27, Cement and Concrete Research

DOI: 10.1016/j.cemconres.2016.10.009

Abstract: Previously, it has been shown that Electrical Resistance Tomography (ERT) can be used for monitoring moisture flow in undamaged cement-based materials. In this work, we investigate whether ERT could be used for imaging three-dimensional (3D) unsaturated moisture flow in cement-based materials that contain discrete cracks.  Novel computational methods based on the so-called absolute imaging framework are developed and used in ERT image reconstructions, aiming at a better tolerance of the reconstructed images with respect to the complexity of the conductivity distribution in cracked material. ERT is first tested using specimens with physically simulated cracks of known geometries, and corroborated with numerical simulations of unsaturated moisture flow. Next, specimens with loading-induced cracks are imaged; here, ERT reconstructions are evaluated qualitatively based on visual observations and known properties of unsaturated moisture flow. Results indicate that ERT is a viable method of visualizing 3D unsaturated moisture flow in cement-based materials with discrete cracks.

“Quantitative electrical imaging of three-dimensional moisture flow in cement-based materials”

Authors: Danny Smyl, Milad Hallaji, Mohammad Pour-Ghaz, North Carolina State University; Aku Seppänen, University of Eastern Finland

Published: Aug. 23, International Journal of Heat and Mass Transfer

DOI: 10.1016/j.ijheatmasstransfer.2016.08.039

Abstract: The presence of moisture significantly affects the mechanical, hydraulic, chemical, electrical, and thermal properties of cement-based and other porous materials, and therefore, methods for detecting and quantifying the moisture ingress in these materials are needed. Recent research studies have shown that the ingress of moisture in porous materials can be qualitatively imaged with Electrical Impedance Tomography (EIT) – an imaging modality which uses electrical measurements from object’s surface to reconstruct the electrical conductivity distribution inside the object. The aim of this study is to investigate whether EIT could image the three-dimensional volumetric moisture content within cement-based materials quantitatively. For this aim, we apply the so-called absolute imaging scheme to the EIT image reconstruction, and use an experimentally developed model for converting the electrical conductivity distribution to volumetric moisture content. The results of the experimental studies support the feasibility of EIT for quantitative imaging of three-dimensional moisture flows in cement-based materials.

“Three-Dimensional Electrical Impedance Tomography to Monitor Unsaturated Moisture Ingress in Cement-Based Materials”

Authors: Danny Smyl, Milad Hallaji, Mohammad Pour-Ghaz, North Carolina State University; Aku Seppänen, University of Eastern Finland

Published: Aug. 22, Transport in Porous Media

DOI: 10.1007/s11242-016-0756-1

Abstract: The development of tools to monitor unsaturated moisture flow in cement-based material is of great importance, as most degradation processes in cement-based materials take place in the presence of moisture. In this paper, the feasibility of electrical impedance tomography (EIT) to monitor three-dimensional (3D) moisture flow in mortar containing fine aggregates is investigated. In the experiments, EIT measurements are taken during moisture ingress in mortar, using electrodes attached on the outer surface of specimens. For EIT, the so-called difference imaging scheme is adopted to reconstruct the change of the 3D electrical conductivity distribution within a specimen caused by the ingress of water into mortar. To study the ability of EIT to detect differences in the rate of ingress, the experiment is performed using plain water and with water containing a viscosity-modifying agent yielding a slower flow rate. To corroborate EIT, X-ray computed tomography (CT) and simulations of unsaturated moisture flow are carried out. While X-ray CT shows contrast with respect to background only in highly saturated regions, EIT shows the conductivity change also in the regions of low degree of saturation. The results of EIT compare well with simulations of unsaturated moisture flow. Moreover, the EIT reconstructions show a clear difference between the cases of water without and with the viscosity-modifying agent and demonstrate the ability of EIT to distinguish between different flow rates.

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