Capacitive Sensors Essay

Introduction

Capacitive technique is an electromagnetic method which is able to measure water content of a material. Since moisture is a common cause of the pathology of reinforced concrete, it is essential to assess the water content in concrete. Once the water content is determined, the corrosion risk of reinforcement can be estimated. The following paragraphs will first describe the theory of this technique then to its application in structural health monitoring (SHM) with a case study.

The principle

The principle of capacitive sensors is to measure the changes in capacitance. The electrodes of the sensor form a capacitor. An alternating voltage is applied to create an alternating electric current, which is detected by the sensor. In general,

With a continuous monitoring, the changes in water content are indicative of estimating corrosion risk of reinforcement. An experiment of simulating the actual SHM operation by using capacitive sensors was conducted by Daniel B.& Claus F. (2006) in France and the application is described as below.

The experiment aims to demonstrate the potential of using capacitive sensors in SHM. The experiment target was a bridge deck(fig.XX). A series of capacitive sensors was attached to the bridge deck. An oscillator was connected to the sensors and generate an alternating circuit. The frequency shift was obtained by a frequency analyzer. The data was delivered to the monitoring system for further computation and record.

Based on the analysis result, a relatively high moisture content area was detected. As shown as fig.XX, a significant high moisture content was detected. It was considered that these areas had the highest risk in corrosion or might be undergoing corrosion. To confirm the actual condition, an in-situ inspection and evaluation was conducted. Radar investigation was applied to check the corrosion of steel. It found that the reinforcement of this area was rusted and repairing work was