Acoustic emission vis-à-vis electrochemical techniques for corrosion assessment in RC elements

Abstract

Reinforced concrete (RC) structures have the potential to be very durable and capable of withstanding a variety of adverse environmental conditions. However, failures in the RC structures still occur due to adverse effects of external or environmental agencies. One of the main causes leading to degradation of RC structures is the corrosion of steel reinforcement. Corrosion of steel rebar in concrete is an electrochemical process and it has been widely studied using various non-destructive techniques. Half-cell potential measurement is one of the most widely used, practical and standardised nondestructive method which provides an indication of the likelihood of corrosion activity at the time of measurement. The other electrochemical methods such as linear polarisation resistance, electrochemical impedance spectroscopy etc. provide corrosion rate, but the calculations are based on certain assumptions of degree of polarization of the embedded bar. Another non-destructive technique widely used in practice is ultra-sonic pulse velocity measurement. Ultrasonic guided waves at high frequencies can be effectively used to distinguish between the loss of bar crosssectional area and changing interface conditions. However, to produce guided waves in the rebar embedded in concrete, it is necessary to attach two transducers at the ends of the rebar projected out of concrete specimens, which is not possible in actual structural element. All these techniques cannot be called truly non-destructive techniques as these methods require either electrical or physical contact with steel embedded in concrete. Thus, to overcome all these difficulties, there is a need to develop a non-destructive methodology, which will assess the corrosion of steel v embedded in RC element effectively without physical contact with the rebar. The researchers have reported that acoustic emission (AE) technique is suitable for failure monitoring of various structures and can also identify onset of corrosion without physical contact with the rebar. AE technique has been widely used to monitor and examine the behaviour of variety of materials. This technique is unique in the sense that it literally records the signals emitted from different sources within the structure under service loads. AE technique makes it possible to measure the ultrasonic elastic waves generated within a material by local micro displacements and is a tool which allows early detection of any mechanical, physical or chemical damage, as source of energy. These elastic waves or elastic energy generated within the structure at the location of distress, propagates as a stress wave in the structure and is detected by one or more AE sensors. By separating the back-ground noise from AE data and through detailed study of various parameters of AE, the physical phenomena or damages in different materials can be evaluated. The earlier research reported that AE technique can be used for detecting onset of corrosion. AE technique is a qualitative method which can find the initiation of corrosion of steel embedded in concrete by identifying developed cracks in concrete, however it has not been used to quantify the rate of corrosion. Thus, it is necessary to develop a method to quantify the losses due to corrosion of rebar using AE technique. Hence, current research work aims to assess the corrosion of steel embedded in concrete using AE technique quantitatively. In the present work, applicability of AE technique was studied vis-à-vis electrochemical techniques (half-cell potential measurement and Tafel extrapolation vi technique) for corrosion assessment in RC elements. To achieve sufficient degree of corrosion in laboratory within limited time period, impressed current technique was used for accelerating corrosion process. Different variables used in the study include three bar diameters, two types of steel and two types of cement. A statistical tool (ANOVA) was used to study the effect of these variables on corrosion and hence on AE measurements. The result obtained from AE technique is correlated with the result of destructive technique i.e. actual gravimetric mass loss of steel embedded in concrete to develop a mathematical model. SOLVER function was used for the development of mathematical model. Further, the developed mathematical model was validated for realistic corrosion exposure conditions in the laboratory using internal chloride exposure. The results of experimental work concluded that cumulative signal strength (CSS) parameter of AE technique is a promising parameter for corrosion monitoring studies as the variation of CSS w.r.t. time has shown a specific trend indicating active corrosion which is similar to the curve of typical phenomenological corrosion loss of steel due to seawater immersion. Chloride induced corrosion of steel embedded in concrete can be classified in two different stages. Both these stages are well identified by electrochemical techniques as far as corrosion process of steel embedded in concrete is concerned, whereas AE technique can identify the corrosion process of steel reinforcement as well as damage to concrete due to subsequent cracking. The results of ANOVA using CSS values for all material variables viz. cement type, steel type and rebar diameter proved that there is not enough evidence to reject the null hypothesis which indicates that the effect of variables on magnitudes of CSS is not significant. Similarity in the results of ANOVA obtained for magnitudes of vii corrosion current densities and CSS values satisfactorily validated the results of AE measurements. The results of SOLVER confirmed that a non-linear relation exists between max CSS value and gravimetric mass loss. The relation between the two parameters is analogous to natural exponential growth function. From the results of experimental work conducted using internal chloride exposure, it is concluded that the developed mathematical model is well validated and can also be used appropriately for realistic corrosion exposure conditions in the laboratory. Thus, AE technique proves to be a powerful and truly non-destructive technique for corrosion assessment and can be successfully used for quantification of corrosion of steel embedded in concrete without having physical/electrical contact with the steel, which is required in electrochemical techniques.