T Visalakshi

USING PZT PATCHES AS A NEW SENSOR FOR CORROSION MONITORING BASED ON ELECTRO-MECHANICAL IMPEDANCE (EMI) TECHNIQUE

MOTIVATION As more and more civil infrastructure is developing, they are becoming plagued by many problems as a result of regular wear and tear as well as being exposed to environmental elements in their intended life span. One of the problems, corrosion, occurs as chloride ions react with metals. Though many sensors have been developed to monitor corrosion detection in steel reinforcement, the search for an efficient and functional sensor is ongoing. The advent of ‘smart’ or ‘intelligent’ materials and structures concept and technologies has ushered a new avenue for the development of monitoring systems. Smart piezoelectric-ceramic (PZT) materials, for example belong to the category of “Smart Materials” which can be used for effective corrosion sensor technology in order to greatly reduce the cost and provide safer structures.

OBJECTIVES This research aims for using PZT patches as a new sensor for corrosion monitoring based on Electro-Mechanical Impedance (EMI) technique. The EMI technique utilizes the self sensing capabilities of PZT patches to non-destructively evaluate and inspect the health of the structures.

METHODOLOGY In this investigative study, high yield deformed (HYD) steel specimens, one embedded in concrete cube (150x150x150mm) and the other as a bare bar were instrumented with square PZT patches (10x10x0.3mm, PIC 151). The electrodes were soldered to the PZT patch and attached to LCR meter (Agilent E4980A). In this manner the electro mechanical admittance signature, consisting of real part (conductance G) and imaginary part (susceptance B) was acquired in a frequency range of 100-300 kHz. A frequency interval of 100 Hz is used for each impedance measurement. All tests were performed under controlled laboratory conditions so that the temperature fluctuations were controlled to within a few degrees. Under normal environmental conditions, corrosion of a rebar is a relatively slow process, often taking many years to occur. In order to obtain data in a reasonable time frame for a laboratory-based study the cathodic method of accelerating corrosion was adopted. After the baseline signatures were acquired, the specimens were placed in a beaker containing “brine” solution (Salinity 35 parts per thousand), the corrosion was instigated by the application of an electric potential and the signatures were taken at frequent intervals.

RESULTS AND KEY ACHIEVEMENTS As clearly visible in Figure (a&b) substantial corrosion has occurred after application of the constant voltage (to accelerate corrosion). The signatures obtained from the sensor are shown in Figure (c) the variation of G (the real part of admittance) of the PZT patches, bonded to the rebar specimens is considerable and the conductance signature can be observed to change with increase in duration of the application of constant voltage. A marked difference in the signature of the specimens before and after corrosion can be observed, which indicates that mechanical impedance has changed due to corrosion. The results of the conductance signature indicate clear difference between the pre and post corrosion readings. This happens because the corrosion changes the mass, stiffness, and/or damping properties of the structure, which in turn causes the shape of the conductance signature to change. With corrosion progress, the mass (identified by PZT patch) is found to reduce as shown in Figure (d). Although the values of m so determined by the PZT patches differed from the actual measured mass, the loss of the PZT identified ‘m’ reasonably correlates with the loss of the actual mass of the specimens, as exemplified by Figure (e), for purpose of quantification, mass loss can be identified in non-dimensional form as In the first step of project we are trying to monitor the hydration process of lime and want to differentiate, different type of chemical reaction which provide strength to lime. This part of the project is collaborated under the UKIERI and same work is going on in BATH university. After lime we will try to monitor the strength, gain and curing process of concrete with the help of EMI technique which will help in strength determination damage detection and level of damage in concrete.

"where m0 is the original mass of the specimen and is the loss of mass. Here, both the actual and the PZT identified mass can be substituted. This index defines the severity of corrosion in the present scenario. The two are related by the following equation

PUBLICATION

    • T. Visalakshi and Bhalla. S. “Detection and quantification of Rebar corrosion using Impedance method” International Journal of Earth Sciences and Engineering, ISSN0974-5904, Volume 04, No 06SPL, October 2011, pp 889-891.

    • T. Visalakshi and Bhalla. S. (Working) “Assessment of Corrosion In Reinforcing Steel Bars of Reinforced Concrete Structures Using Piezo-Impedance Transducers.

   • T. Visalakshi and Bhalla. S. “Review of Impedance based Structural Health Monitoring” Proceedings of the international conference on Vibration engineering and Technology of Machines, IIT Delhi 13-15 December 2010, Pg 550-557 "