A Thesis. entitled. Corrosion Detection in Reinforced Concrete Using Acoustic Emission Technique. Gowtham Penumatsa

Transcription

1 A Thesis entitled Corrosion Detection in Reinforced Concrete Using Acoustic Emission Technique by Gowtham Penumatsa Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Civil Engineering Dr. Douglas Karl Nims, Committee Chair Dr. Liangbo Hu, Committee Member Dr. Dong Shik Kim, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo August 2016

2 Copyright 2016, Gowtham Penumatsa This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

3 An Abstract of Corrosion Detection in Reinforced Concrete Using Acoustic Emission Technique by Gowtham Penumatsa Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science Degree in Civil Engineering The University of Toledo August 2016 Corrosion of reinforcing steel is the major cause for deterioration of concrete structures. Corrosion of these steel bars potentially reduces the service life and ductility of the structures causing early failure of structure, this involves significant cost for inspection and maintenance. Early detection of corrosion is necessary for the proper diagnosis and effective prevention of failure. Therefore, damage induced due to corrosion of reinforcing steel should be detected in the early stages and the severity of corrosion should be properly anticipated by means of non-destructive testing techniques for the safety of the structure. The available methods of corrosion detection in concrete structures are generally electrochemical, such as half-cell potential (HCP) measurements and linear polarization resistance (LPR). These methods are intrusive as they require a physical connection to the corroding steel. Furthermore, these methods only provide information about local corrosion and are usually used after corrosion damage is discovered visually. Acoustic emission is sensitive enough to be a feasible nondestructive testing technique to detect early corrosion. Therefore a corrosion monitoring cell to detect corrosion in reinforced concrete beams using acoustic emission is setup for the first time at The University of Toledo and experiments are conducted. This thesis presents the first use of acoustic emission to detect corrosion in reinforced concrete at The University of Toledo. The tasks accomplished includes setting iii

4 up a corrosion cell and understanding the AE hardware and software equipment. A literature review of corrosion monitoring in reinforced concrete using acoustic emission technology is provided in order to understand the AE technology advancement to date. Corrosion monitoring experiments were designed in the laboratory to initiate corrosion in reinforced concrete in a short time span and continuously monitor with an AE data acquisition system. Electrochemical half-cell potential method is used to anticipate the initiation of corrosion and to correlate AE data with potentials at different stages of the experiment. Steel rebar and two reinforced concrete beams are corroded immersing in 3.5% NaCl solution and using constant potential. The corrosion in these rebar and concrete specimens are monitored continuously using Mistras Pocket Corpac with R15α sensors. Half cell potential measurements are also conducted to understand the method and used to establish correlation with AE. The experiments conducted helped to understand the corrosion process and detect corrosion using AE. The results of the experiments using acoustic emission were found consistent with those in the literature and the conclusions were confirmed using halfcell potential measurements. iv

5 Acknowledgments This dissertation would not have been possible without the love, support, and encouragement I received from my advisor,family and friends. I do not have words to adequately describe my deep gratitude for all they have provided me, though I hope to show them my appreciation in the years to come. I could not have completed this work without the mentoring of my advisor. His tremendous energy and constant endevour is always inspiring and motivation for me. I am truly indebted to him for his guidance and advice during my years as his student. I will always be his student. v

6 Contents Abstract iii Acknowledgments v Contents vi List of Tables ix List of Figures x 1 Introduction Background Corrosion in Reinforced Concrete Acoustic Emission Half Cell Potential Method Problem Statement Objectives Organization of This Thesis Literature Review on Corrosion Introduction Mechanics of Corrosion Different Types of Corrosion Corrosion induced by chloride ions vi

7 2.3.2 Carbonation Test Methods for Detecting Corrosion in Steel Electrochemical test methods to detect corrosion are Half cell potential survey: Linear polarization resistance survey Electrical resistivity: The Various Non-Destructive Tests For Corrosion Monitoring in Reinforced Concrete Acoustic Emission Technology Introduction Theory of Acoustic Emission Components of Acoustic Emission Stimulus or stress AE sources Wave emissions Sensors Signals Acquisition system and processing software Applications of AE to Corrosion Monitoring in Reinforced Concrete Lab Experiments and Results Discussion Introduction to AE Equipment Data acquisition system Sensors AE win software Basic Experiments of AE Pencil lead break test vii

8 4.2.2 Tests to decide threshold Attenuation test Corrosion Cell Set Up and Electrochemical Half-Cell Potential Corrosion monitoring in rebar Acoustic emission monitoring in reinforced concrete Half-Cell Potential Procedure Half-Cell Potential Measurement Experiment Concrete Rupture Tests on Reinforced Concrete and Un-reinforced Concrete Discusions and Conclusion 67 References 69 viii

9 List of Tables 2.1 Corrosion rate vs resistivity Corrosion potentials for specimen 1 with respect to copper/coppersulfate reference electrode Corrosion potentials for specimen 2 with respect to copper/coppersulfate reference electrode ix

10 List of Figures 2-1 Example of electrolytic cell (adapted from Brown and LeMay, 1988) Corrosion process in concrete (Zhoa, 2011) Stages of crack propagation due to corrosion in reinforced concrete (Composites, 2016) Presence of chlorine in the left, carbonation corrosion in the right (Corrosionengineering, 2016) Linear polarization device (Carino, 1999) (a) 3D image of GPR of concrete slab,(b)irt image of concrete cylinder.this figure is reproduced from (Zaki, 2015) Process for acoustic emission ( AE sensor with preamplifier(right) and no preamplifier (left) AE signal and its parameters for one hit ( AE monitoring corrosion setup for reinforced concrete beam and steel rebar Corrosion of rebar in saline environment corrosion loss stages (Melchers, 2006) Cumulative hits vs time graph from (Kawasaki, 2014) Cumulative signal strength vs time curve (Elbatanouny, 2013) Pocket AE instrument to record acoustic emissions R15 α sensor used for conducting experiments to monitor corrosion AE win software home screen x

11 4-4 Mechanical pencil for pencil lead break Typical pencil lead break waveform Typical pencil lead break waveform from handrail experiment performed at University of Toledo Schematic of AE sources during corrosion. ( 2016) Corrosion cell for rebar Showing corrosion stages formed in rebar from left to right. rust, start of corrosion products, after complete corrosion and reduction of cross section, AE sensor assembly attached to rebar Corrosion products on rebar Cumulative hits vs time phase corrosion curve for corrosion in rebar compared with Melcher s model Signal strength vs time for corrosion in rebar Cumulative signal strength vs time and signal strength vs time for corrosion in rebar Electrochemical potential series Cumulative hits vs time for corrosion in reinforced concrete beam specimen Cumulative signal strength vs time for corrosion in reinforced concrete beam specimen Cumulative hits vs time for corrosion in reinforced concrete beam specimen Cumulative signal strength vs time for corrosion in reinforced concrete beam specimen AE set up and corrosion cell monitoring for reinforced concrete beams Corrosion in reinforced concrete beams at the end of the experiments Potentials of copper/copper sulphate and silver/silver chloride reference electrodes with respect to normal hydrogen and saturated calomel electrode 57 xi

12 4-22 Reference electrode Cole Parmer ph reference half-cell, Ag/AgCl model EW Half cell potential method as per ASTM C The half-cell potential experiment in sequential steps Picture showing the half-cell potential experiment Picture show casing the half-cell potentail experiment Concrete beams rupture experiment and sensor position Concrete beams rupture experiment and hit data for specimen Concrete beams rupture experiment and hit data for specimen xii

13 Chapter 1 Introduction 1.1 Background Economic growth in many countries during the period led to the huge increase in construction of reinforced concrete structures. Many of these structures have required repair and maintenance because of corrosion in the steel reinforcing bar. Corrosion of these steel bars potentially reduces the service life and ductility of the structures causing early failure of the structure, involving a significant cost for inspection, maintenance and risk of collapse and loss of life. Early detection of corrosion is necessary for the proper diagnosis and effective prevention of failure. Corrosion is a destructive process of metals such as steel, aluminum, zinc and copper by chemical or electrochemical reactions with its environment. If it is not monitored and treated in the early stages, it can cause unrecoverable damage. Environmental factors are the most common reasons for corrosion formation. Practically all environments are corrosive to some degree (Lamond, 2006). Moisture, air, salt, steam and other gases, such as chlorine, ammonia, and hydrogen sulfide, causes corrosion. The chemical description of corrosion in rebar is (Davis, 1998) 4F e(s) + 6H 2 O(l) + 3O 2 (g) 4F e(oh) 3 (s) (1.1) 1

14 Where (s), (l), (g) are solid, liquid and gas respectively. Since visual detection of corrosion is not useful in terms of the challenges it has or due to its inefficiency, corrosion detection using sensors is a viable option. Sensors are also very sensitive in detecting the minor changes caused by corrosion of steel in its early stages. Acoustic emission is sensitive enough to be a feasible nondestructive testing technique to detect early corrosion. From the research that was conducted recently, it can be considered that AE technique is efficient in detecting corrosion in reinforced or prestressed concrete structures. AE is a passive technique unlike other non-destructive techniques, it can be used to assess the structural integrity by testing individual components of the structure. AE technique can also be used remotely. 1.2 Corrosion in Reinforced Concrete The corrosion of RC (reinforced concrete) structures is common in marine exposed areas and areas with deicing salts. Corrosion in reinforced concrete structures is initiated when the concentration of chlorides around the reinforcement reaches a threshold and it may not display visible signs in its early stages. The alkaline environment in chloride-free concrete protects the steel embedded within from chlorides by forming a passive oxide layer. In general, the ph in concrete is around 12, however, in seawater environments or in the presence of chlorides this ph may reach a value of 8 leading to corrosion initiation. The corrosion reduces the strength of the corroded parts. The corrosion products are usually have 6-10 times more volume than the original volume of steel which exerts a force in the concrete leading to cracking or deteriorating the steel-concrete interface. Visual inspection of early corrosion in reinforced concrete and prestressed concrete structures is impractical as the steel is embedded within the concrete. Therefore, nondestructive test methods capable of 2

15 detecting and quantifying corrosion damage on a more global basis are needed. The available methods of corrosion detection in concrete structures are discussed in the later sections of this thesis. Half-cell potential (HCP) measurements and linear polarization resistance (LPR), AC impedance spectroscopy are dominant electrochemical techniques in detecting hidden corrosion in structures. These methods are invasive as they require a physical connection to the corroding steel. Furthermore, these methods only provide information about local corrosion and are used after corrosion damage is discovered visually. The various methods to detect corrosion other than acoustic emission are visual inspection, radiography, ultrasonic, eddy current, magnetic particle and liquid penetrant and magnetic flux leakage. 1.3 Acoustic Emission Acoustic emission is a phenomenon in which transient elastic waves (release of internal energy) are generated in a material when it undergoes deformation. These deformations can be either initiation of micro cracks, existing crack expansions, or plastic deformations, corrosion, and force in the structures. Sensors are used to detect the elastic waves generated from the material degradation and converts them into electrical signals. The data recorded can be waveform (complete data set) or a set of parameters that can sufficiently describe the data of the waveform. These signals have various parameters like energy, hits, signal strength, amplitude, peak frequency etc., These parameters are used to assess the damage or deformation in a localized source or when monitoring the condition of the entire structure. AE is used for real time monitoring of the bridges, and other concrete structures using the data from the sensor network at site by transferring data to a processing system using either by wireless internet or through a dial up connection. The data processing software can detect the occurrence of AE signals. 3

16 1.4 Half Cell Potential Method Half-cell potential measurement concentrates on the corrosion potentials. During the corrosion of reinforced concrete, a potential field is formed around the corrosion site. A reference electrode, otherwise known as a half-cell, and the steel in the reinforced concrete are connected to a high impedance voltmeter to find the potentials of steel with respect to reference electrode or hall-cell. ASTM C876 provides direction to detect the corrosion potentials of uncoated reinforcing steel in concrete. According to ASTM C876, a measured half-cell potential more negative than -350mV indicates 90% probability of corrosion at the test site. If the potentials measured are in-between -350mV and -200mV, the corrosion is uncertain at the test location. A potential less negative than -200mV indicates probability of corrosion is less than 10% at that location. These values are for use with copper-copper sulphate reference electrode. The results of the half-cell potentials presented in this thesis are obtained with silver-silver chloride reference electrode converted to copper-copper sulphate equivalent potentials which is accepted by the ASTM standard C876. To understand the theory and to be well versed with the process of half-cell potential measurements, two experiments are carried out which are presented in the later sections of this thesis. 1.5 Problem Statement The goal is to detect the corrosion in reinforcing steel in concrete using acoustic emission technique. To continuously monitor and evaluate corrosion in RC with an experimental setup. The ability to detect corrosion in RC with these lab experiments should lay ground work for application of this technique to future research at The University of Toledo. 4

17 1.6 Objectives The following are the objectives of this study: Review the literature of the application of AE to corrosion inspection. This objective includes review of literature on the principles and theory of acoustic emission and its applications in detecting corrosion in reinforced concrete. Understand and use the AE hardware and software. Perform laboratory experiments to successfully initiate corrosion in reinforced concrete within a short time span and continuously monitor with an AE system in a manner that provide insight into corrosion in reinforced concrete. Collect and interpret the AE data from several laboratory experiments. Conduct electrochemical half-cell potential survey to anticipate the initiation of corrosion. Correlate AE data with corrosion data. Review, interpret and analyze data obtained from laboratory experiments and reach conclusions. 5

18 1.7 Organization of This Thesis Chapter 1:- Introduction: This chapter presents an overview of this project Chapter 2:- Literature review on general corrosion: This chapter of the thesis presents the typical corrosion mechanism and the general corrosion process initiation and explains the various NDT tests that are used to detect corrosion in reinforced concrete. Chapter 3:- Literature review on acoustic emission: Literature review on the principles of AE and its applications to corrosion monitoring in concrete structures are dealt in this chapter. Chapter 4:-Laboratory experiments and results discussion: In this chapter the various acoustic emission experiments conducted in order to learn AE, sensors, acquisition system and corrosion experiments on steel rebar, reinforced concrete are discussed in a detailed manner. Also, the electrochemical standard tests conducted to correlate AE are presented. The results of all the experiments are provided in a detaiedl manner. Chapter 5:- Conclusions and future work: Conclusion of the research work is discussed and future work is suggested. 6

19 Chapter 2 Literature Review on Corrosion 2.1 Introduction Corrosion is a destructive process of metals such as steel, aluminum, zinc and copper by chemical or electrochemical reactions with its environment (Chen, 2014). If its not monitored and treated in the early stages, it can cause critical damage. Environmental factors are the most common reasons for corrosion formation. Practically all environments are corrosive to some degree. Moisture, air, salt, steam and other gasses such as chlorine, ammonia, and hydrogen sulfide causes corrosion. Damage in structures due to corrosion of reinforcing steel, which is usually well protected by high-quality concrete in structures like bridges, parking garages, and marine structure is attracting attention. World wide the enormous cost of repairing the structures and the safety aspects are driving the research for corrosion and its mechanisms. A review of the research papers on corrosion in reinforced concrete is presented in this chapter. 2.2 Mechanics of Corrosion Iron ore that is iron oxide is processed with a high amount of energy to make iron. This energy used in the process of making iron results in a material with higher energy state. At higher levels, these metals tend to convert to lower energy level, 7

20 Figure 2-1: Example of electrolytic cell (adapted from Brown and LeMay, 1988) forming oxides or hydroxides.(lamond, 2006). Corrosion is an electrochemical reaction, where electrons or charge, transfer from one electrode to another electrode for the reaction to occur forming an electrical circuit(carino, 1999). Corrosion is a process of degradation of metals by an electrochemical process, which involves chemical reactions (change in reactants) and electrical current (flow of charge through a conductor). Corrosion is similar to a galvanic or electrolytic cell. A galvanic cell or electrolytic cell is a combination of two half-cell reactions, each half-cell reaction containing an electrode and electrolytic solution of its ions. One half-cell supplies electrons, and other consumes electrons called as anodic and cathodic cells respectively. To maintain equilibrium between these two half-cells, ions transfer from one cell to another through external and internal circuits connecting the two electrodes. Electrons flow through external connection and ions flow between two half-cells using a salt bridge. This fundamental concept of an electrolytic cell is shown in Figure 2-1. The oxidation reaction takes place at the anode and the reduction reaction at the cathode. These reduction and oxidation reactions are termed as Redox reactions. The oxidation reaction is the reaction that produces electrons from the oxidation 8

21 of iron, (Fe) to form ferrous ions or ferric ions. The other reaction that is termed reduction reaction consumes electrons from the reduction of oxygen to form hydroxyl ions (OH). During corrosion, steel in reinforced concrete undergoes anodic half-cell reaction that involves oxidation. The chemical description of corrosion in steel is: 4F e(s) + 6H 2 O(l) + 3O 2 (g) 4F e(oh) 3 (s) (2.1) In reinforced concrete corrosion, the steel reinforcement where the cover of concrete is minimal is usually attacked by chlorides or carbon dioxide from the atmosphere and becomes anodic. The steel portions where the availability of oxygen and water is less behaves cathodicly. The anodic steel releases electrons which due to conductivity and metallic continuity reach the cathode. The Figure 2-2 shows the corrosion phenomenon in reinforced concrete. Figure 2-2: Corrosion process in concrete (Zhoa, 2011) 9

22 These electrons in the concrete pores combine with water and oxygen to produce hydroxyl ions. Ionic conductivity completes the circuit initiating corrosion. The existence of ambient corrosive environment corrosion process continues and corrodes the entire steel. The voltage differences between different layers of reinforcement in reinforced concrete are due to differences in availability of water, chloride ions and oxygen. The anodic reaction is M M e (2.2) Based on the availability of oxygen and ph two types of cathodic reactions occur 2H + + 2e H 2 Hydrogen bubbling (2.3) H 2 O + 1/2O 2 + 2e 2OH Corrosion formation (2.4) When the ph is lower than four the hydrogen bubbling occurs, whereas in the real structures the corrosion reaction occurs. At passive films that forms around steel reinforcement in concrete is alkaline in nature with ph above 12. This alkaline environment of concrete surrounding reinforcing steel protects it from corroding. Also, in the absence of factors causing corrosion, ferrous hydroxide layer is developed around the steel that acts as a passive protecting film to the steel rebar. Corrosion initiation occurs when the passive protecting layers around metals is dissolved due to the presence of chlorides or carbonation of concrete cover due to atmospheric conditions. Corrosion can also initiate due to a decrease in ph level. The corrosion products caused in reinforcing steel structures due to the presence of chlorides increase the volume and reduces the cross section of the steel bar which reduces the structures capability to support design loads. The corrosion process leading to cracking of concrete is shown in the Figure

23 Figure 2-3: Stages of crack propagation due to corrosion in reinforced concrete (Composites, 2016) 2.3 Different Types of Corrosion Corrosion is said to be developed if the breakdown of the passive protection layer around the steel rebar has occurred. Corrosion can happen in the entire area or localized portion depending upon various factors causing the corrosion. Localized corrosion mostly occurs due to chloride ion attack at a mechanical failure, like a crack in the surface of the concrete cover. Corrosion over the whole surface is a result of decrease in ph to a range where the protective film layer is no longer stable. Mainly the corrosion is said to be induced by the following reasons Chloride induced corrosion Corrosion due to carbonation Corrosion induced by chloride ions Corrosion due to chlorides is the most common in all types of structures damaged along the sea coast and in northern parts where deicing salts are used to remove ice. Other sources of chlorides are admixtures, contaminants, marine environments, chemicals. Salts are capable of penetrating solid concrete along with water or moisture present in the atmosphere. The cracks that are formed on the concrete surface may 11

24 Figure 2-4: Presence of chlorine in the left, carbonation corrosion in the right (Corrosionengineering, 2016) also provide a way for chlorides to penetrate to the reinforcement and cause corrosion. The source for chlorides in bridges and parking garages is usually deicing salts. Sea water is the source in the case of marine structures. Also, chemical admixtures added to concrete that contains chlorides can cause corrosion in some cases. Chloride ions transfer from the surface to the passive layer and compete with hydroxyl ions resulting in active corrosion. The chlorides and ferrous ions in steel or iron react and form soluble complex compounds causing corrosion. The initially precipitated hydroxide ions further react with oxygen to advance the corrosion. These complex compounds formed have a higher specific volume than steel from which they formed. The volume change due to corrosion products exerts force in concrete leading to cracking or deteriorating the steel-concrete interface,leading to spalling of concrete cover. The rate of corrosion in concrete can greatly change with the availability of oxygen, moisture and chlorides. The change in these parameters can change the potentials around reinforcing steel and, thus, the rate of corrosion changes. The increase in temperature also increases the rate of corrosion as it is primarily a chemical reaction. Chloride environment that causes corrosion is caused by the following reasons 12

25 (a) Chlorides added during concrete mix (b) Environmentally diffused chlorides Admixtures containing chlorides that are used as a set accelerator for concreteare a source of chlorides present in concrete mix. These chloride containing admixtures are actively discouraged for reinforced concrete as they can cause rapid corrosion. Corrosion can be initiated even before the concrete is hardened due to faster ingress of chloride from admixtures. Alternately, chloride ion diffusion can occur in sound concrete and penetrates through pore structure with capillary action. Because of this phenomenon, cracks are not necessary for chloride ingress and causing rapid corrosion activity in the concrete. The rate of corrosion is dependent on water cement ratio, the type of cement, the temperature, the maturity of concrete. The total chloride ions present in the concrete do not all contribute to active corrosion. Some of these ions react with cement paste components such as calcium aluminate to form chloroaluminate. The passive protection film layer is destroyed only if the concentration of chloride ions reaches a threshold value and it is a function of a number of parameters. The threshold value of 0.4% of chloride ions by mass of cement has been proposed.(bremne, 2001) Some researchers showed that corrosion initiation is not dependent on chloride ion concentration alone but, dependent on the chloride-to-hydroxyl ion ratio (Bremne, 2001) Carbonation Carbonation of concrete is general term given to the reaction of alkaline components of concrete with atmospheric carbon dioxide. Carbonation leads to the decrease in ph. (Theodore, 2005). If the concrete that covers the steel reinforcement is carbonated, i.e., reducing the ph of the concrete from 13 to about 9 or 8, corrosion can take 13

26 place. Corrosion caused in this manner is called carbonation. Generally, carbonation leads to film rupture on the entire area of the surface. The rate of corrosion because of carbonation is less than the corrosion rate caused by pitting corrosion. The reaction of atmospheric gasses with concrete is higher at higher humidity, but the ingress of gasses is more at relatively lower humidity. Therefore, the most aggressive environment for carbonation or corrosion initiation is alternative dry and wet cycles. High porosity, the presence of cracks, thin concrete cover, low cement factor and high water to cement ratio accelerates corrosion process in laboratory research conditions. 2.4 Test Methods for Detecting Corrosion in Steel The available methods for corrosion detection in reinforced concrete structures are discussed in this section. Half-cell potential (HCP) measurements and linear polarization resistance (LPR) and electrical resistivity are standard electrochemical practices in predicting hidden corrosion in structures(zaki, 2015). These methods are intrusive as they require a physical connection to the corroding steel. Furthermore, these methods only provide information about local corrosion and are used after corrosion damage is discovered visually. The various methods to detect corrosion other than electrochemical measurements are, visual inspection, radiography, ultrasonic, eddy current, magnetic particle, liquid penetrant, magnetic flux leakage and acoustic emission. A brief description of each of these methods is provided in the later sections of this chapter. Visual inspection of early corrosion in reinforced concrete and prestressed concrete structures is impractical as the steel is embedded within the concrete. Therefore, nondestructive test methods capable of detecting and quantifying corrosion damage on a more global basis are needed. 14

27 2.4.1 Electrochemical test methods to detect corrosion are Half Cell potential method Linear polarization resistance (LPR), Electrical resistivity method Half cell potential survey: This survey essentially maps the active corrosion areas. The electrochemical cell action of corrosion in concrete produces an electrical potential difference in steel rebar. These potentials are measured by using a voltmeter and a half-cell (reference electrode) to map an area and observed the corrosion activity. The potential or voltage more negative than -350mV on copper sulphate half-cell indicates 90% probability of corrosion activity. These potentials can be mapped as a grid pattern that is similar to contour maps with lines of an equipotential region. This test method is described in ASTM Test method for half-cell potentials of uncoated reinforcing steel in concrete (ASTM C 876) Linear polarization resistance survey Linear polarization resistance is a prominent choice for measuring the rate of corrosion in reinforced concrete. This process is instantaneous and used in the electrochemical laboratory for decades in measuring corrosion rate of metals (Song, 2007). This test provides rate of corrosion based on the rate at which reinforcing steel is oxidized. The corrosion rate is an important parameter to understand and estimate the damage and time for additional damage of the structure to occur. Linear polarization resistance technique is based on the applied current. The ratio of the change in potential (E) to the applied current (I) is known as polarization resistance (RP) and is inversely proportional to the corrosion current I corr and also to the rate of 15

28 metal corrosion. Three electrodes namely, working electrode (WE), the counter electrode (CE) and a reference electrode (RE) are used in LPR. The reinforcing steel is the working electrode, any non-reactive metal works as a counter electrode and copper-copper sulfate half-cell, silver-silver chloride half-cell or calomel can be used as reference electrode. The LPR device that applies a small voltage to the WE via CE and the current(i) and voltage response (V) are measured. The reference electrode that is isolated from electrical circuit measure change in potential with respect to working electrode int his case the reinforcing steel. These measured voltages and current data are used to calculate polarization resistance R p and are used in the Stern-Geary equation to derive the corrosion rate. The slope of the graph obtained by measuring current per unit area when applying varying potentials to the concrete gives the resistance value. The uncertainty in this test is the area of the steel rebar affected by the current from the counter electrode. Usually the current is assumed to flow perpendicular to working electrode and the counter electrode. Thus, the bar surface area below the counter electrode is taken as the steel area. The Stern-Geary Figure 2-5: Linear polarization device (Carino, 1999) 16

29 equation is: I Corr = K A I R = K R p (2.5) I Corr = corrosion current density expressed in K = proportionality constant expressed in mv ma ; A = area of reinforcing steel polarized; A ; cm 2 I = applied current required to obtain change in potential expressed in ma; E = Voltage change resulting from the applied current expressed in mv; and R P = polarization resistance expressed in ohm cm 2 The proportionality constant K is given by the following equation K = (β a β c ) (2.3 (β a + β c )) (2.6) Where β a, β c are the anodic and cathodic Tafel constants. A value of 26mV is commonly used for steel that is actively corroding in concrete Electrical resistivity: The electrical resistivity of the concrete has a direct relation to the rate of corrosion of the embedded steel. Moisture content plays a significant role in damaging concrete and is related to the resistivity of the concrete. High water content and presence of salts creates an aggressive environment for corrosion and leads to low resistivity. The higher the resistivity of concrete, the lower is the rate of corrosion.the direct correlation between concrete resistivity and rate of corrosion is presented in table 2.1. Table 2.1: Corrosion rate vs resistivity Resistivity, K-cm Corrosion Rate > 20 Low 10 to 20 Low to moderate 5 to 10 High < 5 Very high 17

30 2.5 The Various Non-Destructive Tests For Corrosion Monitoring in Reinforced Concrete The various non-destructive testing techniques for detecting active corrosion in concrete (Akhtar, 2013), (Zaki, 2015) are described below. Each NDT method falls either into surface inspection, penetrating or principles of elastic waves. 1 Visual inspection, 2 Electromagnetic testing (EM) Ground Penetration tests Eddy Current Magnetic flux leakage 3 Elastic Wave Methods (EM) Ultrasonic Acoustic emission Impact echo 4 Infrared thermography(irt) Infrared thermography 5 Optical Sensing methods Fiber Bragg grating Visual Survey: Visual inspection is the most common and simple way of inspecting corrosion in structures. Through a visual survey of the structure, corrosion stains, cracks, spalls, scaling and other visual deteriorations in the structure are identified 18

31 and documented. The degree of corrosion or structures remaining life cannot be assessed. But, the areas where the corrosion is worst or moderate can be identified. The evidence of poor drainage, temperature effects and protective systems can be noted. Visual assessment of the structure is a vital part and should be carried in an orderly manner to used for further in-depth inspection. Electromagnetic testing: Ground Penetration radiation: This method uses electromagnetic radar pulses in profiling the subsurface. The high-frequency electromagnetic radiation is passed through the material and recorded as it arrives after reflecting or refracting at the surfaces of different dielectric properties. The change in dielectric properties of the materials is characterized by the strength and time of the returned signal. The cracks, corrosion or any other defects or information such as moisture or salt contents are infered from reflected signal. Eddy Current: The structure to be tested is subjected to magnetic induction by a coil conducting AC current near to it. Moving the coil over the surface creates a time changing magnetic field as the magnetic field oscillates due to change in AC current. This time changing current results in the formation of eddy currents in the concrete. The impedance of conduction of the coil moved over the structure is measured with magnitude and phase. The damage to the structure due to corrosion, cracks and fatigue are observed with reduced current density as they impede the conduction. Elastic Wave Methods Ultrasonic Pulse Velocity: Ultrasonic is a non-destructive testing method to detect flaws such as voids, cracks and corrosion using the velocity of the sound waves 19

32 passing thorough the material or concrete. Ultrasonic experiment is conducted with a pulse generator circuit, and transducers that emit a, receives pulses and power source. Elastic waves or pulses are transmitted through transducer at one end that is in contact with the surface and are received at the other ends(aggelis, 2011). The time these waves transmitted through the material is recorded and is used in the equation below P ulsev elocity, V = L T (2.7) Where L is the length of the two ends or width of the structure at which transducers are placed and the attenuation phenomenon has a correlation with corrosion damage in concrete. The more the attenuation than normal at the steel bar, the quality of the concrete is likely to be poor. If the corrosion level on the steel rebar is high, the direct reflection in the first wave also decreases(timcakova, 2011). Acoustic Emission: AE is also classified as elastic wave method. The phenomenon, in this case, is passive. The energy is not generated and supplied to the structure but, elastic energy originated inside the structure due to deformation, crack growths, corrosion and any other degradation from a localized source is captured using piezoelectric sensors. The AE technique is capable of monitoring the entire structure or a localized component remotely through wireless connections. The signals recorded using piezoelectric sensors are analyzed to locate, detect and classify the source. A qualified technician is required for collection and post processing of this data. Infrared Thermography Infrared thermography: In recent times, this is becoming reliable and cost effective corrosion monitoring technique for RC structures. The distribution of temperature and rate of heat transfer varies due to deterioration in the RC structure such 20

33 as cracks and corrosion. Thermography camera provides the visual image of the heat distribution radiated at the structures surface and relate it to the internal defect of the material. Figure 2-6: (a) 3D image of GPR of concrete slab,(b)irt image of concrete cylinder.this figure is reproduced from (Zaki, 2015) Optical Sensing Methods Fiber Bragg Grating This NDT method involves variation of the refractive index of the core of an optical fiber. Light at a particular wavelength called Braggs wavelength is passed through the grating. Corrosion process which leads to decrease the density of steel through expanding is measured by shift in the wavelength of the FBG wavelength (Majumder, 2008). The degree of corrosion can be assessed by the relative to change in wavelength. This is a localized testing technique and less effective in measuring corrosion. 21

34 Chapter 3 Acoustic Emission Technology 3.1 Introduction The word acoustic means hearing, most failures in any type of structure emit sound. The hearing of these sounds or emissions in real time has become a science called acoustic emissions. The origination of this method is attribute to J. Kaiser (Ono, 2007). Acoustic emission (AE) is a non-destructive testing/structural health monitoring technology used to detect minute discontinuities and changes occurring in the structure and can identify the location of the structural discontinuity/cracks. The acoustic emission technique has become immensely popular because of its sensitivity to hearing emissions. Development of the technology in recent years have led to an understanding sophisticated data analysis and in the improvement of the sensors. Acoustic emission is now widely used in various applications like bridge assessing, buildings, storage tanks, pipe lines, etc. The frequency of acoustic emissions are in the range of khz, this is above audible sound. AE can detect crack growth due to fatigue, stress corrosion, hydrogen embrittlement, and creep. The advantage of AE over other non-destructive monitoring systems is it can monitor the structure over the entire load history and various stages in developing of the failures, remotely 22

35 over a wireless connectivity. AE is also used to monitor the integrity of the whole structure unlike testing the components of the structure as is the case in other NDT methods. AE is passive, where as other technologies like ultrasonic, radiographic etc., are active. The energy created inside the structure of material is the collected and no external energy is applied. 3.2 Theory of Acoustic Emission Acoustic emissions are transient elastic waves generated by the rapid release of energy from localized sources within a material. (Excerpted from ASTM E ). The stimulus that generates AE is the state of force inside the material produced by the application of external forces, temperature gradients etc. The sources that emit energy can be crack initiation, growth, rubbing of surfaces, internal forces, corrosion etc. Figure 3-1: Process for acoustic emission ( The sources in the structure or components of the structure release energy subjected to mechanical loading or force. This energy travels in all directions in the 23

36 material in the form of high frequency stress waves which are received at the sensor (Figure 3-1). Sensors or transducers convert these energy waves into a current. This current is amplified and collected by circuits and chips and is processed as signals. Analysis of these signal data and characterizing the data based on time at arrival frequency content and voltage gives the insight of the source and its location. 3.3 Components of Acoustic Emission As seen in the Figure 3-1., the components of an AE system are stimulus, source, wave emission, sensor, signal, acquisition system and processing software Stimulus or stress The applied load on the structure creates internal forces which cause stresses in the material. Forces are the main cause of elastic and plastic deformations which changes the shape of the material. The change in shape of the material due to these stresses is called strain. Stimulus is the first step in AE process AE sources Sources are movements of dislocations or permanent deformations that take place when material ruptures and new surfaces are created. The microscopic breaks of sulfide, oxide, carbide, other non-metallic materials and cracks that are formed in the materials emits energy and thus are called sources for acoustic emissions. Other sources of acoustic emissions other than cracks formed in the material are rubbing of cracks, friction, and corrosion products. Sources are also defined as the events during which energy is released from the stress field that is created due to loading. Typically sources are differentiated into three types by the nature of their generation. Acoustic emissions can be generated from a vast number of sources, but they 24

37 are categorized as below. Primary Source Secondary Source Noise Primary sources are those which are originated within the material and emitted due to permanent change in the material such as crack development. Secondary sources are externally originated and they are like friction, or grinding of debris in the cracks, etc. Any other external sources that emits stress waves and recorded by the acquisition system such as electrical disturbances, noise, and magnetic fluxes etc. are termed noise. These are irrelevant. Proper test strategy and filtering techniques are practical precautions that can eliminate noise Wave emissions Acoustic emissions or waves in the AE process are produced from the sources and travel through the material as elastic waves in all directions. Waves of wide frequency range from very low to 1000 khz and higher are emitted. The amount of energy released by an acoustic emission and the amplitude of the waveform are related to the magnitude and velocity of the source event. The amplitude of the emission is proportional to the velocity of crack propagation and the amount of surface area created Sensors Sensors are used for acquiring the energy from the AE sources and are termed as transducers. A sensor or transducer is a device which detects and converts a stimulus 25

38 or energy into an electric signal which can be measured or recorded. Selection of suitable sensors for different applications of AE is a vital part of AE technology. The different types of sensors that are widely used across the world for AE are broad band sensors and narrow band sensors. Choice of selecting the sensors should be based on source events. Broad band sensors can acquire broad range of frequencies emitted during an AE event and renders faithful actual motion of the structure surface at sensor location. Practically, narrow band sensors or resonant sensors are most sensitive and inexpensive. If the frequency of the source is well established, these sensors can optimize system performance. Sensors have inbuilt amplifiesr/filters and frequency band pass of these sensors are to match in the AE instrument for appropriate functionality. The Figure 3-2 shows the sensors with and without amplifiers. Figure 3-2: AE sensor with preamplifier(right) and no preamplifier (left) 26

39 3.3.5 Signals A signal is the recorded data of the acoustic emission. The features of a signal such as amplitude, rise time, duration, signal energy, counts with respect to threshold are discussed here. Figure 3-3: AE signal and its parameters for one hit ( Hit is the data set acquried by the AE system as shown in the Figure 3-3. Amplitude is the peak voltage of the signal data. Amplitude is often reported in db. The area of the new surface created in the material or structure is in proportion to the amplitude of the recorded data. Amplitude is also proportional to velocity of the wave. Threshold is the minimum voltage value set in the system, above which the hits are acquired by the system. The threshold also acts as a filter to reject insignificant or irrelevant data Duration is the time in microseconds from the point it first crosses the threshold 27

40 to the point it last crosses the threshold. The area under the envelop show in the above figure is signal energy. It shows the energy of the acoustic emission of a particular hit. Rise time is the measure of time from the point it first crosses the threshold to the time it reaches the peak amplitude. A count is defined as the number of times the wave crosses the threshold. It is an important parameter of AE Acquisition system and processing software. A data acquisition system that continuously acquires acoustic emissions from the structure in the form of signals coming from the sensors is required. This system is usually a electronic system with processors and data disks to acquire and store the data. A sophisticated software which can process the data graphically or provides meaningful deductions makes life easier when using AE technology. 3.4 Applications of AE to Corrosion Monitoring in Reinforced Concrete The use of AE to monitor corrosion was first demonstrated by Davis and Dunn with series of laboratory experiments (Zaki, 2015). Later on, the advancement of AE technology with corrosion monitoring research has clearly shown the capability of AE in corrosion monitoring. The approach for processing AE data is of two types 1 Parametric or the classical approach 2 Signal or waveform based Signal based AE approach is not preferable for large scale structural monitoring systems because of its sophisticated post processing, requirement of expert skill and 28

41 huge amount of storage required to perform it. In the parametric approach the signal parameters such as counts, hits, amplitude, signal strength, rise time, voltage etc., are recorded. The wave is defined by a set of parameters. In the wave form or signal based approach, the raw waveform is captured. Analyzing the waveform in frequency domain gives significant quantitative measurements of the structure. Noise can be greatly diminished by using the waveform approach. Both data analyzing approaches are capable of detecting corrosion and can be used to monitoring the corrosion process. AE classical method is highly preferred because of its simplicity in data handling and processing. Figure 3-4: AE monitoring corrosion setup for reinforced concrete beam and steel rebar 29

42 Many authors have conducted extensive research on corrosion monitoring in reinforced concrete structures using parametric approach. According to Melcher and Li (Melchers, 2006) a typical corrosion loss during corrosion process is illustrated in the Figure 3-5, there are 4 Phases of corrosion Phase 1: Onset of corrosion occurs on the surface of the rebar. Phase 2: rate of corrosion stabilizes due to reduction of oxygen flow. Phase 3 & 4: Due to anaerobic corrosion, the loss of rebar increases and expansion of corrosion products occurs. Figure 3-5: Corrosion of rebar in saline environment corrosion loss stages (Melchers, 2006) Many researches have considered AE hits, the cumulative signal strength and Improvedb value analysis (IB-value analysis) to study corrosion in its early stages and to portray degree of corrosion.the onset of corrosion initiation is understood from laboratory experiments by Ohtsu and Tomoda (Kawasaki,Y. 2010). They found that the reinforcing steel in concrete corroded due to wet and dry cycles. The onset of corrosion and nucleation of cracks were observed corresponding to two peak hit values in the 30

43 Figure 3-6: Cumulative hits vs time graph from (Kawasaki, 2014) Figure 3-6. Kawasaki compared the cumulative hits against the phase relation of steel in marine environments and differentiated the onset of corrosion and crack nucleation in two stages, He also showed that both cumulative hits and phase relation according to Melcher and Li are in good agreement. Thus, cumulative hits can provide information about corrosion initiation and crack nucleation at early stages of corrosion monitoring using with acoustic emission technology. Ziehl (Elbatanouny, 2015) has proved that signal strength, which is measure of waveform energy released during the deterioration of the structure, is an indication of damage. The signal strength and cumulative signal strength of corroded specimens were studied and suggested that CSS and SS can provide indication of corrosion cracks. 31

44 Figure 3-7: Cumulative signal strength vs time curve (Elbatanouny, 2013) Patil (Elbatanouny, 2015) conducted a cumulative signal study and his curve shows clear indication of corrosion activity. In the figure 3-7, stage 1 indicates depassivation and corrosion onset. Stage 2 clearly show cases corrosion activity with a sudden rise. The rise at the end of the stage 2 seems to be a macro crack due to corrosion. Patil showcased his model s agreement with Melcher and Li s phase relationship by removing the sudden rise in the cumulative signal strength. 32

45 Chapter 4 Lab Experiments and Results Discussion Objective of Lab Experiments These experiments were to test the ability of acoustic emission to detect corrosion of the steel reinforcing bar in reinforced concrete. Detecting potential corrosion sources and addressing the characteristics of these sources are objectives of this research. Several unique challenges must be addressed to detect corrosion in reinforced concrete. Understanding acoustic emission from hydrogen gas evolution Identifying the existence of corrosion in reinforced concrete beams monitoring with AE technology. Differentiating cracks from loading from cracks caused by corrosion. The primary objectives are achieved by performing series of experiments in three phases. These experiments were designed in a way to overcome the challenges. The first phase of experiments was conducted to understand data acquisition system. AE and the analysis software at hand. In the second phase, a corrosion monitoring cell 33

46 was set up and active surveillance of corrosion with AE is done. Also, electrochemical tests were frequently conducted to confirm the active corrosion in the cell. In the final stage of experiments, corrosion sources are differentiated from sources that occur due to concrete rupture. This chapter describes the development of lab test methods, corrosion cell setup, and description for each test. 4.1 Introduction to AE Equipment Data acquisition system The AE data acquisition system (DAQ) used in the laboratory experiments was Pocket AE-10 (Mistras, 2016). Pocket AE is a versatile high-performance portable handheld unit for field and laboratory experiments. It has dual channel capability and one parametric input channel and can perform linear location and filtering. It can be operated on an internal battery or an external power source. The Pocket AE works with the Windows operating system. Pocket CORPAC software is loaded into the Pocket AE. The Pocket CORPAC software can acquire data continuously and can display up to eight graphs (Point plots, histogram, Waveforms, FFTs etc.). The Pocket CORPAC is designed to detect active corrosion from pitting, cracking, etc. The data recorded is saved in.dta file format and can be transferred using a flash drive or a USB cadble. This.dta file format is compatible with AEwin software (Mistras, 2016) used to post processing the data acquired. 34

47 Figure 4-1: Pocket AE instrument to record acoustic emissions 35

48 4.1.2 Sensors All lab experiments were performed with R15α Mistras sensors. The R15α (Figure 4-2) is a general purpose narrow band resonant sensor. It provides high sensitivity and low-frequency rejection and comes with stainless steel casing that is tough and durable. These qualities make the sensor appropriate to structural monitoring for pipelines, vessels, bridges, concrete structures. This sensor does not preamplify the data. Figure 4-2: R15 α sensor used for conducting experiments to monitor corrosion AE win software AEwin software is a 32bit windows based program (Mistras, 2016). It can simultaneously acquire acoustic emissions, perform waveform processing, and display graphically. AEwin can replay the data stored using AE CORPAC in.dta format and graphically analyze it. It can produce 2D and 3D graphs. The graphs provided include waveforms, line plots, point plots, histograms, FFTs and multiple plots on a 36

49 single graph with coloring options. A layout customized to user s requirement can be prepared to ease the analysis process. The typical AEwin screen is presented in the figure

50 38 Figure 4-3: AE win software home screen

51 4.2 Basic Experiments of AE To understand the performance of the AE technology and to check the sensitivity of the sensors, pencil lead break tests were conducted. To eliminate noise in the lab, thresholds tests were conducted and a threshold value that is best for the laboratory conditions is decided. To gain an understanding of AE testing and the AEwin software some basic experiments, like attenuation and stress corrosion cracking, were conducted. These experiments and their results provided in this section Pencil lead break test Pencil lead break test is usually performed with Hsu pencil named after the developer of this device. The breaking of the graphite lead which is 0.5mm (or 0.3 mm) in diameter by pressing against the surface generates impulses of very short duration similar to AE sources (Genis, 2011). The pencil lead breaks should have amplitudes of at least 80dB relative to a reference voltage of 1mV. This test indicates the sensors contact with the surface being monitored is satisfactory. Pencil lead breaks were performed for five consecutive times on the surface of the concrete.the sensor in attached to the surface of the concrete with grease and electric tape. The amplitude of the signals is 100 db at a distance of 1 feet which means the sensor is in good contact with the concrete surface.the typical pencil lead break waveform is in the figure below with an initial peak and attenuating wave. 39

52 Figure 4-4: Mechanical pencil for pencil lead break Figure 4-5: Typical pencil lead break waveform 40

53 4.2.2 Tests to decide threshold This experiment is carried to set a threshold value that eliminates noise in the lab conditions. The threshold is changed from an initial value of 30db to a value until no noise is acquired by the DAQ system. The 40db threshold value was observed to be optimum in the laboratory conditions where these experiments were carried. All experiments were done with the threshold at 40dB Attenuation test The elastic waves traveling through the material attenuate, i.e., the amplitude of the signal decreases. This phenomenon is called attenuation. The sensor (R15) is attached to the hand rail with a couplant (petroleum jelly) between the surface of the handrail and sensor. Taken care of good contact between the sensor and handrail, the pencil break test (PLB) is carried at regular distances. At each location, Pencil lead is broken thrice at approximately 30 degree angle with 2-4mm length of the lead. The graph of amplitude vs time clearly shows the attenuation phenomenon (Figure 4-6). Figure 4-6: Typical pencil lead break waveform from handrail experiment performed at University of Toledo. 41

54 4.3 Corrosion Cell Set Up and Electrochemical Half- Cell Potential In corrosion in reinforced concrete as mentioned in Chapter 2, metal ions at anodic reaction move to the cathode, and hydrogen gas and OH- ions are released at the cathode. The metal ions transfer from anodic to cathodic site through the electrolyte. The rust creates a protective layer on the surface of the metal and with the availability of electrolyte and oxygen further corrosion products are formed. The significant AE signals emitted are due to hydrogen bubbling (release of hydrogen gas), metal dissolution, stress corrosion cracking and break down of the passive layer. The AE sources from corrosion mechanism of bar in are illustrated in Figure (4-7). In reinforced concrete, due to expansion of corrosion products internal forces Figure 4-7: Schematic of AE sources during corrosion. ( 2016) 42

55 are created. These forces crack the concrete and also produce sources of acoustic emission. The lab experiments are designed to study these acoustic sources individually Corrosion monitoring in rebar A corrosion cell is created with an acrylic pipe as shown in the Figure 4-8. Figure 4-8: Corrosion cell for rebar The corrosion products are initiated in the steel rebar. Aggressive corrosive environments are created inside the corrosion cell to develop corrosion products in the rebar with saline solution and by providing direct potential to it. The corrosion cell contains a 3.5% NaCl solution as electrolyte, and has a constant voltage of 10 volts is applied to it. This voltage creates potential difference thereby causing anions and cations to form a galvanic cell. Transfer of these ions through NaCl solution initiates rapid corrosion. A copper rod is used as counter electrode in this experiment. Continuous monitoring with AE is continued until corrosion products are seen. The steel 43

56 Figure 4-9: Showing corrosion stages formed in rebar from left to right. rust, start of corrosion products, after complete corrosion and reduction of cross section, AE sensor assembly attached to rebar. rebar after corrosion products are formed is shown in Figure 4-9. The steel rebar was immersed for two days and then removed from the solution and dried for two days. It is continuously monitored for AE. After 15 days, visible corrosion products were formed. The AE emission sources acquired during continuous monitoring of steel rebar are hydrogen bubbling, break down of rust layer, metallic degradation due to corrosion products formation and fracture of de-cohesive precipitates formed on the surface of rebar. The corrosion products formed on the rebar can be seen in the Figure 4-10 The AE evaluation of these experiments is conducted in the next section. This experiment helps in understanding the nature of corrosion process that happened in the reinforced concrete beams in the later experiments. The AE data acquired from the corrosion process of rebar is presented as cumulative hit vs time curve to identify its similarities with Melcher and Li model (Melchers, 2006). These two results are found to be similar. Hence stages of corrosion as mentioned by Melcher can be followed. The, three stages of the corrosion process are 44

57 Figure 4-10: Corrosion products on rebar 45

58 Figure 4-11: Cumulative hits vs time phase corrosion curve for corrosion in rebar compared with Melcher s model. initiation, stabilization and loss of rebar. From the visual observation of the experiment the hydrogen bubbling was seen from the very first minute to the end of the experiment. But, too wide because the recorded hits count was very low in the first day when the bubbling was clearly seen, it appears that the bubbling effect was not recorded by the AE system. This may be because with the threshold value set to 40db, low amplitude bubbling was not be recorded. The signal strength and cumulative signal strength of these hits are show in Figure 4-12 and Figure According to (Elbatanouny, 2015) the sudden rise in the signal strength is corrosion activity. Therefore, the concentration of this research was at the peak values and the sudden rise in the data. 46

59 Figure 4-12: Signal strength vs time for corrosion in rebar Figure 4-13: Cumulative signal strength vs time and signal strength vs time for corrosion in rebar 47

60 A total of 862 AE events were recorded during days 4 and 5, the amplitude distribution of these hits was between 40db and 80db. The interesting thing out of these 862 hits was that 763AE hits have an amplitude range of 40db to 55db. This amplitude range is considered as corrosion activity in the steel rebar because there was no external loading or other sources that cause AE hits. The total hits recorded for the corrosion process in the entire time phase of monitoring the rebar were hits out of the total hits were found to be in the range of 40 to 55. Therefore, amplitude in this range is a good indication of corrosion loss in the rebar. Hence, AE is capable of detecting corrosion that causes rebar loss. This statement can be considered valid although there were hits with this amplitude range between day 1.5 to day 4, but the hits were very few (only 94 hits). So, the assumption of corrosion stabilization during this period also seems to be good one. The hits in the corrosion initiation stage is just two hits, so comparing these 2 hits with corrosion loss of rebar data to reach further conclusions has no point because the 2 hits may not be caused by the corrosion initiation. Therefore, it is evident that corrosion can be identified with AE and also classification of corrosion stages can be done with AE hits and cumulative signal strength Acoustic emission monitoring in reinforced concrete Reinforced concrete beam specimens were casted with rebar embedded and the corrosion process was monitored using AE. The clear cover of reinforcement in the concrete that is corroded is about 2 inches. The design compressive strength of the beams was 3500psi, and 53 Grade normal cement was used for these beams. These beams were ruptured in the center of the beam to obtain a crack so that acceleration of corrosion process would occur. The crack is extended all the way to the reinforcing steel bar. The concrete beam was then immersed in 3.5% NaCl solution untill the fluid was a half inch below the center of the beam. This allows the chlorine reach 48

61 the steel rebar by through the crack. Alternate wet and dry cycles are created in the corrosion cell by removing saline solution every two days. During wet cycles chlorine from the saline solution and applied voltage breaks down the passive protective film and initiates corrosion. In dry cycles, the ingress of gasses that cause corrosion was high. Thus, the wet and dry cycles provide an aggressive environment for corrosion in reinforced concrete. The concrete beam that was immersed in saline solution was made as a galvanic cell by placing a copper rod(copper electrode) under the concrete beam in the saline electrolytic solution. To expedite the corrosion process, the concrete reinforcement is connected to positive terminal and the copper electrode is connected to negative terminal and a constant potential of 10V is applied. This circuit connection makes the rebar an anode and copper a cathode. The copper is used as the counter electrode because it is nobler than steel. This theory is same as galvanization of steel. Since zinc is less noble than steel, zinc is coated on the surface of the steel as sacrificial electrode. A more noble metal is less reactive and more resistive to chemical and acidic attack. These metals are given in an order in the electrochemical series based on their standard electrode potentials. The Figure 4-14 shows the electrochemical series. The metals at the bottom are nobler than the metals at top. The AE sensor is then placed over the reinforced concrete beam such that the sensor does not get in contact with the saline solution or corrosion products. Half cell measurements were carried by pausing the cell (the connections to the electrodes from the power suppply were removed). The AE DAQ is paused and the potential measurements were taken for usually 20 minutes. The reinforcing steel rebar in the specimen 2 was connected to positive terminal and copper rods were connected to negative terminal of the DC power supply. An aggressive environment in the form of saline solution and voltage is provided to reinforced concrete beams to initiate corrosion. The next step in this experiment is to 49

62 monitor corrosion activity using AE. AE CORPAC was set to acquire the corrosion activity in RC beams, R15 sensor was attached to the concrete beam using gel and electric tape with perfect contact to surface of the concrete. Electrochemical half-cell test was conducted periodically along with continuous monitoring of AE to detect corrosion formation using potentials and to concentrate the study of AE data at this point of time. The AE data acquired from the corrosion monitoring in specimen 1 is analyzed, Cumulative Hits vs Time and cumulative signal strength vs Time graphs are provided in the following figures. 50

63 Figure 4-14: Electrochemical potential series 51

64 Figure 4-15: Cumulative hits vs time for corrosion in reinforced concrete beam specimen 1 Figure 4-16: Cumulative signal strength vs time for corrosion in reinforced concrete beam specimen 1 52

65 The corrosion activity is classified into three stages as previously. The time phase in each of the stages is filtered and characteristics of the wave form and parameters are studied. For concrete specimen 1 during the entire span of monitoring with AE, there were 776 hits recorded. 729 AE hits with amplitude range of 40db to 55db that considered to be corrosion sources were recorded. Since there was no loading, and there was no other factors causing any other sources in this experiment, the amplitude range was considered to be the corrosion activity. In the first stage of AE data, the average amplitude, rise time, and duration of the hits were 44dB, 40µs, 130µs. In the second stage, the time between 2 to 4 days there were 295 hits which had amplitude between 40 db to 55db. The average amplitude, rise time, duration of the hits were 45.6dB, 45µs, 162µs In the loss of rebar stage, days 5 thorugh 14 there were 158 hits which were in between 40 db to 55db amplitude range. The average amplitude, rise time, duration of the hits were 45.4dB, 36µs, and 134µs. AE data of the concrete specimen 2 in the Figure 4-17 and 4-18 is graphically interpreted into three stages from AE hits vs time and CSS vs time phase and the parameters were studied to identify the similarities in specimens 1 and 2. The amplitude distribution of the corrosion in reinforced concrete in specimen 2 is also in the range of 40-55dB. The concrete specimen 2 is monitored until corrosion was visible and a crack formed in the concrete specimen due to corrosion expansion. The visible corrosion stains in the concrete specimens and crack formed in the reinforced concrete specimen due to corrosion are shown in the Figure 4-19 and Figure

66 Figure 4-17: Cumulative hits vs time for corrosion in reinforced concrete beam specimen 2 Figure 4-18: Cumulative signal strength vs time for corrosion in reinforced concrete beam specimen 2 54

67 Figure 4-19: AE set up and corrosion cell monitoring for reinforced concrete beams Figure 4-20: Corrosion in reinforced concrete beams at the end of the experiments 55

68 4.4 Half-Cell Potential Procedure ASTM Standard C describes the procedure to detect the existence of corrosion in uncoated reinforcing steel in concrete. This method measures the electrical corrosion potentials of the reinforced concrete using a reference electrode and voltmeter. To determine the electrode potentials using this approach, the concrete surface should not be Highly resistive Treated with sealers A reference electrode is an electrode whose potentials are known on a standard hydrogen electrode. Reference electrodes provide highly stable, and reproducible voltage of the working electrode over a temperature range from 32 F to 1200 F. The reference electrode Cu Cu e corresponding to the potential of the saturated copper-copper sulfate reference electrode as referenced to the hydrogen electrode is mentioned in this standard. Unlike Copper sulfate electrode, Ag Ag + + e saturated with 3.8M KCl electrodes are more resistent to chloride contamination. So, silver/silver chloride electrodes are used for experiments of this research. This reference electrode is also used to find the potentials of atmospherically exposed concrete. According to ASTM potentials measured by reference electrodes other than saturated copper-copper sulfate should be converted to Copper-copper sulfate equivalent potential. The potentials of Cu (Copper), and Ag (silver) electrodes with respect to normal hydrogen electrode and saturated Calomel Electrode (SCE) are given in the Figure The silver/silver chloride reference electrode is composed of silver wire (Ag) that has been coated with a layer of solid silver chloride (AgCl) immersed in a solution 56

69 Figure 4-21: Potentials of copper/copper sulphate and silver/silver chloride reference electrodes with respect to normal hydrogen and saturated calomel electrode that is saturated with KCl and AgCl. The half-cell reaction that takes place in the reference electrode is like in the equation below. AgCl(s) + e Ag(s) + Cl (4.1) The silver/silver chloride electrode is shown in the Figure 4-22 To measure potential of the RC structure, rebar have to be exposed to make direct connection with the impedance voltmeter. Positive terminal of the voltmeter is connected to steel rebar and negative terminal to the reference electrode. The connection with steel rebar should be done after scrapping the bar or brush it to ensure proper conduction. To decrease the electrical resistivity of the concrete, pre-wetting the surface is usually done before conducting the experiment. Solution composed of a mixture of wetting agent, water and denaturated alcohol was used to pre-wet the concrete surface to standardize potential drop. The Figure 4-23 reproduced from ASTM C shows the procedure for potential measurements. According to ASTM 57

70 Figure 4-22: Reference electrode Cole Parmer ph reference half-cell, Ag/AgCl model EW C876-09, if the potentials measured over an area are more positive than -200mV, it is considered that there is no corrosion activity. If the potentials are in between - 200mV and -350mV, then the corrosion activity in the reinforcing steel is uncertain according to the code. If the potentials are more negative than -350mV, the possibility of corrosion occurring is 90% at that time in that area. All these potentials are with respect to the copper/copper sulfate reference electrode. 4.5 Half-Cell Potential Measurement Experiment Half-cell potential measurement experiment was conducted on a corroded reinforced bar to test reliability of my experimental conditions. The potentials at 4 points were measured. As it is a corroded area, the potentials are expected to be more negative than -350mv. Also, degree of corrosion was severe in the test area. Hence, large negative potential values are expected. The exposed steel bar surface was prepared with a wire brush and the concrete surfaces used for the measurements 58

71 Figure 4-23: Half cell potential method as per ASTM C were pre-wetted. As seen in the Figure 4-24 the sequence of steps are carried and potentials measured are -998mv, -1002mv, -963mV and -940mV at locations shown in Figure 4-24 respectively. 59

72 Figure 4-24: The half-cell potential experiment in sequential steps 60

73 The half-cell potential measurements are then carried out on two concrete specimens that were being monitored with AE. The AE is paused during the time these measurements were carried out. Potentials on three points as shown in Figure 4-25 are of concrete beam 1 were recorded each day. The potentials on point 1, 2, 3 with time is provided graphically in next section. The point 1 and point 3 are at each ends of the specimen and point 2 is at the center of the specimen where a crack is created in the concrete. On the second concrete beam that was corroded by the same process, two points (each end) of the concrete beam was measured for potentials throughout the corrosion monitoring process. The graph of the potentials with time for the second concrete beam is also presented in the next section. The below figure shows the three marked points where the potentials were measured and the attachment of AE sensor to the concrete specimen1. Half-cell potential measurements on concrete Figure 4-25: Picture showing the half-cell potential experiment specimen 1 and 2 are as follows. The table 4.1 shows the corrosion potentials of the concrete at three different points in the specimen. Table 4.2 provides the corrosion 61

74 potentials for concrete specimen 2 at the ends of the specimen. The bold data points indicate the measurements were taken during wet cycles of the specimens and the data in the remaining cells indicate dry cycle. Table 4.1: Corrosion potentials for specimen 1 with respect to copper/coppersulfate reference electrode Time(days) Potentials Potentials Potentials at Point 1 (mv) at Point 2 (mv) at point 3 (mv) Table 4.2: Corrosion potentials for specimen 2 with respect to copper/coppersulfate reference electrode Time (days) Potentials at 1 (mv) Potentials at 2 (mv) The plot 1 of these potentials vs time for the concrete specimen1 is in Figure The potentials and cumulative signal strength do not fit well with each other. The potentials from day 4.2 to 6 should be the stabilization phase because the potentials were becoming less negative which is indication of no corrosion. But with cumulative 62

75 Figure 4-26: Picture show casing the half-cell potentail experiment signal strength and visual observation of this phase, the corrosion process was active. The amount hits during this phase clearly shows the activeness of corrosion. The graph of potentials vs time clearly indicate the formation of corrosion after 2.5 days from the beginning of the experiment in both specimens. Therefore, the AE hits recorded at this time span should be initiation of corrosion. The corrosion potentials at day 5 to 6 in the concrete specimen 1 indicate no corrosion activity. Therefore, there should be less AE hits, if the corrosion process has stabilized during this period. 4.6 Concrete Rupture Tests on Reinforced Concrete and Un-reinforced Concrete Reinforced and un-reinforced concrete specimens were loaded until a tensile crack was formed in the center of the concrete beams. The AE sensors were attached to 63

76 the concrete and AE data of the cracks initiation and growth while rupturing of the concrete specimens are recorded. The typical test load points and sensor attachment to the concrete are shown in the figure below. Figure 4-27: Concrete beams rupture experiment and sensor position The load vs time of the test for two reinforced specimens are provided in the below section. The AE hits during this load vs time phase is given in Figure 4-28 and Figure 4-29 below. The AE hits during the increment of loading were crack initiation in the concrete and during unloading are due to rubbing of cracks. So the data of AE is grouped using spreadsheet between loading and unloading stages and the parameters are analyzed in the next section. 64