Detection and reconstruction of complex structural cracking patterns with electrical imaging

The ability to detect cracks in structural elements is an integral component in the assessment of structural heath and integrity. Recently, Electrical Resistance Tomography (ERT) -based sensing skins have been shown to reliably image progressive surface damage on structural members. However, so far the approach has only been tested in cases of relatively simple crack patterns. Because the spatial resolution of ERT is generally low, it is an open question whether the ERT-based sensing skins are able to image complex structural cracking patterns. In this paper, we test the accuracy of ERT for reconstructing cracking patterns experimentally and computationally. In the computational study, we use a set of numerical simulations that model progressive cracking in a rectangular beam geometry. We also investigate the effect of image reconstruction methods on the crack pattern estimates: In addition to the contemporary image reconstruction method used in the recent sensing skin studies, we test the feasibility of a novel approach where model-based structural prior information on the cracking probability is accounted for in the image reconstruction. The results of this study indicate that ERT-based sensing skins are able to detect and reconstruct complex structural cracking patterns, especially when structural prior information is utilized in the image reconstruction

Three-dimensional electrical impedance tomography to monitor unsaturated moisture ingress in cement-based materials

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

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

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

Investigating Crack Initiation and Propagation of Concrete in Restrained Shrinkage Circular/Elliptical Ring Test

The restrained ring test, which is recommended by AASHTO and ASTM, has been used for assessing the potential of early-age cracking of concrete and other cement-based materials. Recently, a novel elliptical ring test method has been proposed to replace the circular ring test method for the purpose of shortening ring test duration and observing crack initiation and propagation more conveniently. In order to explore the mechanism of this novel test method, a numerical model is developed to analyze crack initiation and propagation process in restrained concrete rings, in which the effect of concrete shrinkage is simulated by a fictitious temperature drop applied on concrete causing the same strain as that induced by shrinkage. First, an elastic analysis is conducted to obtain the circumferential stress contour of a concrete ring subject to restrained shrinkage. Combined with the fictitious crack model, a fracture mechanics method is introduced to determine crack initiation and propagation, in which crack resistance caused by cohesive force acting on fracture process zone is considered. Finite element analysis is carried out to simulate the evolution of stress intensity factor in restrained concrete rings subject to circumferential drying. Cracking age and position of a series of circular/elliptical concrete rings are obtained from numerical analyses which agree reasonably well with experimental results. It is found that the sudden drop of steel strain observed in the restrained ring test represents the onset of unstable crack propagation rather than crack initiation. The results given by the AASHTO/ASTM restrained ring test actually reflects the response of a concrete ring as a structure to external stimulation, in this case restrained concrete shrinkage.The financial support from the National Natural Science Foundation of China under the grants of NSFC 51478083 & 51421064, Engineering and Physical Sciences Research Council under the grant of EP/I031952/1, and the National Basic Research Program of China (973 Program, Grant No. 2015CB057703) is gratefully acknowledged

Imaging of reactive transport in fractured cement-based materials with X-ray CT

The need to improve the understanding of the properties of cement-based materials calls for the development of tools for visualizing and quantifying chemical reactions and flows of fluids within them. In this paper, we report the results of an experimental study where a sample of fractured cement-paste was subjected to injection of fluids (krypton, CO2 and water) and imaged simultaneously by X-ray computed tomography (CT). Initial porosity of the sample was estimated using a subtraction method based on CT scans taken initially and during krypton injection. The CT reconstructions were segmented to visualize crack patterns and fluid flow in three-dimensions and to quantify the evolution of porosity during the experiment. The results show that CT captures the formation of a carbonate phase in the sample during CO2 injection, and the flow of water in the fractured media. We quantify the reduction of porosity resulting from the carbonation reaction. We observe that the newly formed carbonated layer impedes water flow and, locally, can lead to crack healing. The results demonstrate the ability of CT to image reactive transport in cement-based materials, and support the feasibility of this imaging tool for their characterization

Assessment of the behavior of buried concrete pipelines subjected to ground rupture: Experimental study

Rapid assessment of damage to buried pipelines from earthquake-induced ground deformation is a crucial component to recovery efforts. This paper reports on the first year of a four-year study aimed at developing rapid, reliable, and cost-effective sensing systems for health monitoring and damage detection for buried concrete pipelines subjected to ground deformation. A custom-designed sensing strategy was implemented in a ground rupture experiment with a scaled-down concrete pipeline. The behavior of the pipeline, including the failure modes and damage inflicted to the pipe segments, was monitored during the test. Two modes of failure were identified in the test: (1) compression associated with telescoping-type deformation and (2) bending at the pipeline joints closest to the fault plane. Consequently, future research toward advancing sensing technology for concrete pipelines will likely focus on the behavior of the joints. © 2012 American Society of Civil Engineers

Underground Sensing Strategies for the Health Assessment of Buried Pipelines

Buried lifeline infrastructure including pipelines, tunnels, power and communication lines, among others, are vital to ensuring the operation of the national economy. Permanent ground displacement (PGD) from earthquakes and landslides is the most serious hazard to buried pipelines, prompting often slow and expensive methods of damage localization before repairs can be made. Due to the importance of these buried lifelines, it is critical that low-cost and rapid methodologies for damage detection and localization be developed. Monitoring systems embedded in and around the pipeline are an obvious approach but typically suffer from the cost and obtrusiveness of long cable requirements. The primary goal of this chapter is to illustrate novel sensing methods that can serve as the basis for monitoring buried pipelines exposed to PGD. In particular, the chapter focuses on the monitoring of segmented concrete pipelines, which typically experience damage at their joints due to PGD. Wireless telemetry is evaluated to validate wireless sensors for buried applications, thus reducing greatly the cost of dense sensor systems in regions of high PGD risk. An overview of current buried pipeline sensing technology is made and three experimental full-scale PGD tests are conducted to evaluate pipeline motion and damage detection methodologies in segmented concrete pipelines. Real-time monitoring of joint rotations and translations by potentiometers as well as direct damage measures of joint regions by acoustic emission and conductive surface sensors were made. Strain gages were used to successfully portray global load transfer throughout the pipeline, validated by load cell measurements at the pipe ends. The combined sensor information is successfully used to create a hypothesis for the damage evolution process of buried segmented concrete pipelines under PGD and to validate the use of wireless sensors for buried pipeline monitoring

Damage detection and health monitoring of buried, segmental concrete pipes

Presented herein is a summary of the results from the first two years of a four-year study on damage detection and health monitoring of buried, segmental concrete pipes, such as those used in urban water distribution networks. The rapid assessment of damage to such networks from earthquake-induced permanent ground deformations is an essential component to recovery efforts. To identify the failure modes that occur in the pipelines subjected to permanent ground deformation and to assess the performance of sensor technologies for detecting these modes and for potential use in situ, large-scale tests were performed at the NEES Lifeline Experimental and Testing Facility at Cornell University. The predominant failure modes identified in the tests performed to date include compression at all joints and combined compression and bending at the pipe joints closest to the fault. Accurate measurements of the pipeline displacements and strains were recorded up to the compressive and flexural failure of the pipeline joints. © 2011 ASCE