Non-destructive evaluation of concrete using a capacitive imaging technique: Preliminary modelling and experiments

Abstract

This paper describes the application of capacitive imaging to the inspection of concrete. A two-dimensional finite-element method is employed to model the electric field distribution from capacitive imaging probe, and how it interacts with concrete samples. The results indicate that the capacitive imaging technique could be used to detect micro-cracks on the surface of concrete samples, as well as sub-surface air voids and steel reinforcement bars.

Detecting surface features on conducting specimens through an insulation layer using a capacitive imaging technique

“…For example, the CI technique is sensitive to features caused by corrosion on conducting surfaces over a significant air gap and/or insulation layer and the lift-off limit encountered in conventional eddy current technique is absent. Previous work has confirmed that the CI technique is useful in detecting both surface and hidden features in insulators and materials with very low conductivity such as composites and concrete [23,24], while this work is focussed on the detection of surface features on conductors. Related techniques using capacitive sensors have also been described, in which the capacitive sensors were used to characterise the properties of a specimen [25,26], to precisely define the edges of surface slots in conductors [27], to image buried object (e.g.…”

“…If the testing specimen is a dielectric material or possesses very low conductivity, as has been described in previous work [23,24,29], the electric field generated from the capacitive imaging probe will have a certain volume of influence in the testing specimen. Any defect presented in the volume will distort the electric field, and change the amount of induced charges on the sensing electrode.…”

Further capacitive imaging experiments using modified probes

“…The equipment described in previous work 5 was used to scan the probe in plane above the surface of the rear side of a two pence coin, as show in Fig. 2 (a).…”

“…For NDE purposes, capacitive sensors also has its unique advantages, for instance, it does not require a surface preparation, only requires one-side access to the specimen, and can sense property change on conducting surface over a significant lift-off [4]. One of the examples of the capacitive sensors used in the NDE field is the capacitive imaging technique, which has been successfully used on a various types of materials, including concrete [5], glass/carbon fibre composite [6], steel [7], etc. In previous work, the CI probes are with metal electrodes etched onto Printed Circuit Board (PCB), and the design principle [8,9], influencing factors for imaging performance [10] and measurement sensitivity distribution determined primarily by the electrode geometry [11] were studied.…”

Positive and negative variations in capacitive images for given defects under varying experimental conditions

“…In previous work, the feasibility of the CI technique on various materials and structures, e.g., concrete [3,4] composite [5][6][7][8][9], Corrosion under Insulation (CUI) [10,11], and adhesive layer [12] has been demonstrated. Normally, a decreased overall permittivity (e.g., presence of air void in glass fiber composite) within the volume of influence of the CI probe at a given scanning position will result in a decreased capacitance value and produce a "darker point" in the capacitive image.…”