Corrosion Detection in Concrete Rebars using a Spectroscopic Technique
Presented by: Edward Garboczi Affiliation: Natl Inst of Stands Tech Description: Detecting the early corrosion of steel in reinforced concrete is a goal that has been much pursued. Since 2010, NIST has been pursuing a large project to develop an electromagnetic (EM) probe that detects the actual corrosion products via spectroscopic means. Several principal iron
corrosion products, such as hematite and goethite, are antiferromagnetic at field temperatures. At a given applied EM frequency, which depends on temperature, these compounds undergo a resonance that can be detected spectroscopically, which pinpoints the presence of these particular
iron corrosion products. The electromagnetic waves tend to be of higher frequency, on order of 100 GHz, so getting them through the cover concrete and back out again to the detector has been challenging. We have successfully detected these two iron corrosion products, and are
developing equipment and methodologies that will be capable of penetrating the typical 50 mm of cover concrete in the field. The novel part of this project is that we actually detect specific compounds, rather than just look at changes in rebar cross-section. This method has the potential
of providing an early-corrosion probe for steel in reinforced concrete, and for other applications where steel is covered by various layers and coatings.
Applications of Electrical Impedance based Methods as Microstructure and Property Predictive Tools in Cementitious Systems
Presented by: Narayanan Neithalath Affiliation: Arizona State University Description: Electrical impedance spectroscopy (EIS) has been used to understand the evolving microstructure, and thus to provide indications of the mechanical and durability performance of cementitious systems. This paper presents three applications where EIS is used: (a) in conjunction in effective media theories to predict the porosity of hydrating cementing systems, (b) along with microstructural models to extract the relative influence of pore structure features and pore solution conductivity on the measured rapid
chloride permeability (RCP – ASTM C 1202) values, and (c) along with equivalent circuit models to quantify the chloride binding during non-steady state chloride migration tests. A generalized effective medium (GEM) theory is used to predict the porosity of cement pastes and concretes containing several cement replacement materials. The thermal signature of hydrating cementitious systems, represented using the equivalent age maturity index, is related to a microstructural parameter obtained from electrical impedance. A unique relationship is observed between the equivalent age and the microstructural parameter thereby providing a crucial link between maturity and microstructure
development. In the second part, EIS and associated equivalent circuit modeling are used to extract the microstructural features of the plain concrete as well as concretes modified with varying amounts of
Class F fly ash or silica fume. A methodology is developed in this paper that utilizes the ratios of RCP values and the ratios of effective conductivities to pore solution conductivities of plain and modified
concretes, to quantify the relative influence of pore solution conductivity and pore structure on the RCP values. Further, electrical circuit models, and especially changes in the simulated interfacial capacitances are used to predict the changes in the microstructure of conventional and alkali activated concretes during accelerated chloride transport tests.
Characterization of Steel Fiber Content in Cement-based Composite Materials using Electric/Magnetic Methods
Presented by: Stephen Garrett Affiliation: The University of Illinios Description: Material developments have paved the way for steel fiber-reinforced concrete (SFRC) to be a viable building material for structural applications. However material variability concerns, which are inherent to the addition of fibers in the concrete matrix, like the fiber content,
dispersion and orientation within an SFRC element must be addressed. Nondestructive techniques can be employed to compliment, or replace, the current destructive methods of quality control to address this issue. Electrical- and magnetic-based methods show promise for
nondestructive characterization of fiber properties in SFRC, because these techniques utilize the conducting nature of the steel fibers. In this study, we apply several nondestructive tests to SFCR samples with varying fiber content (20, 45, and 70 kg/m3) and orientation, where the fiber orientations in the samples were controlled using flow characteristics of the mixture during casting. Two different geometries (beams and panels) are considered. Results from tests using surface resistivity and eddy current response on the samples are reported and compared to results from non-electrical test methods such as resonance vibration and ultrasonic pulse velocity. The results show that the electrical- and magnetic-based methods are most appropriate for fiber characterization of SFRC, and that reasonable estimation of fiber volume and orientation are obtained when those tests are properly applied and interpreted.
Numerical Investigation of the Influence of Concrete Cracks on Electrical Resistivity Measurements using Four-Point Wenner Probe
Presented by: O Burkan Isgor Affiliation: Oregon State University Description: It is widely reported that electrical resistivity of concrete plays a major role in controlling the corrosion rate of embedded reinforcement in concrete. In addition, many recent studies have shown that resistivity and micro-structural changes in concrete are highly correlated. In the past, a number of studies have identified the factors affecting the resistivity measurements using four-point Wenner probe and provided guidelines for minimizing their confounding effects on the readings. However, the effects of existing surface cracks in concrete on the measurements have not been studied systematically. In this numerical study, finite element method has been used to investigate this issue by carrying out a parametric investigation.
Some of the analysis parameters of the study involved (1) crack dimensions, (2) crack locations and orientations with respect to the probe position, (3) density and location of embedded reinforcement, (4) orientation of the probe with respect to reinforcement, and (5) moisture
content of concrete. It will be demonstrated that depending on the location and geometrical properties of the crack as well as its angle with respect to the Wenner probe, concrete resistivity measurements can contain considerable errors. The presence of rebar mesh and its misalignment with respect to the crack and the measurement direction will exacerbate the errors - in some cases by over 100%. Suggestions will be provided to help minimize the effects of the cracks on electrical resistivity measurements.
A Painted Sensing Skin for EIT-based Damage Detection in Concrete Elements
Presented by: Mohammad Pour-Ghaz Affiliation: NC State University Description: This presentation presents findings of an experimental and numerical study on the potential use of conductive materials that are applied to the surface of concrete in two dimensions as a "sensing skin" for characterizing damage (i.e., detecting, locating, and quantifying) in the
concrete substrate. The presence of defects will locally change (increase) the electrical resistivity of the sensing skin, Electrical Resistance Tomography is used to detect this change in electrical resistivity through reconstruction of electrical resistivity distribution image of the entire sensing skin. Results indicated that the two-dimensional sensing skin is capable to characterize the damage in concrete and reinforced concrete elements. Several experiments with different damage patterns were carried out on the sensing skin applied to the surface of polymeric and
cementitious-based substrate. The sensing skin was successfully used to detect, and locate the damage in two-dimensional settings. When applied to concrete-based substrate, the sensing skin was used to detect damage due to impact loading, tensile stresses, and bending. In all cases
sensing skin successfully detected cracking. When the sensing skin was used to detect cracking due to tensile stresses, crack branching was also detected. The results of bending test on notched beam confirmed that the sensing skin provides a promising method to detect cracking pattern of
a structural member even when the cracks are not easily detected by naked eye.
Non-Destructive Evaluation of Cracks in Mass Concrete using Normal Resistivity Borehole Logging
Presented by: Elodie Taillet Description: Conventional electrical resistivity methods are effective for the detection of cracks and their localization close to the surface of concrete structures. However, these methods are not effective for deep investigations of massive structure i.e. the dams and the locks. In this paper, we propose to
perform resistivity measurements in pre-existing boreholes, using a classic probe used for borehole logging. The normal resistivity probe is used to non-destructively characterize the discontinuities (cracks, damaged zones, construction joints or else the rock/concrete contact).
Four electrode spacings of the probe were used to obtain information from different volumes of investigation. The measurements combination allows having a radial exploration along the borehole. The aim is to use the apparent resistivity and inversed data to estimate the position, the aperture and the horizontal extension of cracks. The study is first based on a 2D - axisymmetric modeling and inversion simulation to understand the
tool response in a cracked concrete. Results show that the normal resistivity can detect a horizontal crack with an infinite extension, a high electrical contrast, and an aperture between 1 mm and 2 cm.
For each spacing, a specific equation was developed and a data coupling was established. If the resistivity contrast between the crack and the concrete is known, the crack apertures of more than 1 mm are determinate. An in-situ campaign was carried out on a large concrete hydraulic structure. Field results show the method capacities to detect damaged zones like thin conductive layers and to characterize isolated cracks or interfaces.
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