Corrosion Detection in Reinforced Concrete Beams Using Piezoceramics

Transcription

1 Corrosion Detection in Reinforced Concrete Beams Using Piezoceramics Shainur Ahsan, REU Student Dr. Haichang Gu, Post-Doc Mentor Dr. Gangbing Song, Faculty Mentor Department of Civil and Environmental Engineering Department of Mechanical Engineering University of Houston Houston, TX Sponsored by the National Science Foundation July 9

2 Introduction As more civil infrastructure reach the end of their intended lifespan, they are becoming plagued by many problems as a result of regular wear and tear as well as being exposed to environmental elements. One of the problems, corrosion, occurs as chloride ions react with metals. As a result, part of the metal dissolves into a byproduct; the remaining metal has a smaller cross-sectional area. Corrosion costs the US economy roughly $78 billion dollars a year with approximately $8.3 billion dollars in damage to highway bridges (Virmani). As more and more infrastructure near the end of their service life, this number will only continue to increase. Corrosion can be hard to detect until major damage has already been done. Because steel rebar is encased inside of a concrete cover, it is hard to observe damage from corrosion directly. Existing methods have used ultrasound technology to detect damage within the concrete. However, this is an inefficient method because of the high cost of equipment as well as high training cost of personnel. The equipment is usually very bulky which makes many areas inaccessible to monitoring. Recently, more emphasis has been placed on embedding sensors inside of the structure to improve monitoring. Though many sensors have been developed to monitor corrosion detection in steel reinforcement, the search for an efficient and functional sensor is ongoing. The majority of existing methods employ electrochemical methods to monitor factors such as ph, oxygen content, and chloride and carbon dioxide concentration. However, many researchers have identified interference from environmental factors as problems to these methods. A method that is immune to environmental effects would be very beneficial. Piezoceramics have been studied for structural health monitoring (SHM) for a number of years. The relationship between electrical and mechanical impedance is used to detect stress. Because a mechanical stress is induced with an electrical current, the ceramic patches also have an application in vibration dampening. Because of their versatility, they are being used in more and more monitoring scenarios. Their use for corrosion detection in rebar has not been thoroughly examined. Corrosion monitoring in aircraft has garnered the attention of many researchers. The declining state of KC-35 refueling tankers in the US Air Force s fleet is one example. Most of these models have well exceeded their intended service life though they continue to be in use today. Corrosion is one of the problems the aircraft are suffering from (Groner). A number of researchers have devised ways to monitor corrosion including the use of piezoceramics. A number of studies have experimented with using piezoceramics on aluminum beams and plates to test their applicability for SHM on aircraft. Not only were the researchers able to detect the corrosion damage, but they were also able to locate and quantify it. Steel rebar in beams are much thicker than the specimens previously used in corrosion detection. The rebar is also encased in concrete and is not able to transfer vibration freely as was the case in Simmer s study. However, a study conducted by Wang et. al showed that guided waves can be used to detect debonding damage. Their study did not specifically deal with corrosion, but their methods may still be highly

3 applicable. As the rebar corrodes, the cross sectional area of the rebar will decrease. As a result, there should be corresponding debonding. In their study, piezoceramics were used to generated a guided wave signal at one section and detect the signal at other sections. The wave signal was altered as it passed through debonded sections. They was able to determine the length and location of the debonded sections. The wave signal travels faster through debonded sections. The location can be determined by analyzing the reflected wave signals. Wang found relationships between the amplitude and phase difference and the debonding location. Their results should be applicable to corroded rebar, and it is expected similar patterns will emerge. Corrosion Process Corrosion is an electrochemical process that degrades steel and generates rust byproducts. Steel is a man-made material that stores energy as a result of its manufacturing process; the stored energy makes the steel very reactive with the environment to return to its natural state as iron ore (Corrosion Protection: Concrete Bridges CP:CB). When steel rebar is placed in the concrete, the alkalinity causes the steel to form a thin protective layer. This layer makes the steel inactive and resistant to corrosion (CP:CB). Cracks commonly form in reinforced concrete; chloride ions can enter through these cracks and accumulate on the rebar. After a certain concentration of chloride accumulates, the protective layer of steel is destroyed (CP:CB). The cracks form a channel for moisture and oxygen to reach and react with the steel. Corrosion can also be induced by elements besides chloride, but chloride is the main catalyst. The concrete and steel rebar form a closed loop for a circuit to form. Parts of the steel become anodic while others become cathodic. The concrete acts as an electrolyte to complete the electrochemical circuit (CP:CB). At the cathode, the iron is oxidized, and then further reacts to become rust products. The follow equations and comments from the Corrosion Protection: Concrete Bridges document on the Federal Highway Administration s (FHWA) website explain the reactions that take place on the steel rebar.. At the anode, iron is oxidized to the ferrous state, releasing electrons. Fe Fe + + e - (). At the cathode, these electrons combine with oxygen and moisture to form hydroxide ions. ½ O + H O + e - OH - () 3. The ferrous ions combine with hydroxyl ions to produce ferrous hydroxide. Then the latter is further oxidized in the presence of moisture to form ferric oxide.

4 Fe + + OH - Fe(OH) (3) Fe(OH) + H O + O Fe(OH) 3 () Fe(OH) 3 Fe O H O (5) Effects of Corrosion in Reinforced Concrete After corrosion has started, the resulting reactions can affect the structural integrity of reinforced concrete. As the reaction proceeds, steel is consumed. Accordingly, the diameter and mass of the rebar are decreased. The reduction of diameter leads to the loss of bond with the surrounding concrete (CB:CP). This can lead to major problems in pre-stressed concrete where the interface with the steel is important. The corrosion process also creates rust by-products. The by-products can eventually occupy more volume than the initial steel rebar (CB:CP). The by-products then cause stress on the surrounding concrete. As the by-products and resulting stress increase, delamination and spalling of concrete occur (CB:CP). The rebar could then be more exposed to the environment, and the corrosion rate can increase. Accelerated Corrosion Testing Corrosion takes place over a number of years. As a result, researchers have found different ways to accelerate corrosion for testing purposes. The Florida Depart of Transportation has a standardized method for evaluating corrosion resistant coatings. The setup involves casting a No. rebar within a. cm x. cm concrete cylinder so that the rebar sticks out of the cylinder at one end (Brown). After proper curing, the sample is placed in a tank with 5% NaCl solution at a height of 7.5 cm in the tank for twenty-eight days. A bare piece of No. 5 rebar is placed in the tank and connected to the negative terminal of a DC power supply. The positive terminal is connected to the rebar protruding from the concrete cylinder (Brown). The test is initiated by turning on the power supply and setting it to V. The NaCl solution is filtered, circulated, and maintained at a concentration of 5%. The current is measured by using a multimeter and using Ohm s law (Brown). The accumulation of corrosion products will increase stress in the concrete and cause it to crack. When this happens, the current will increase sharply (Brown). The resistance and time to cracking can then be plotted to compare different test specimens. Another method, used by Simmers et. al., corroded aluminum samples in a very short period of time. Hydrochloric acid was used to induce mass loss in the test samples. The method also involves the simplest setup. The acid is simply applied to the material to be tested until the desired level of corrosion is reached (Simmers). Existing Corrosion Sensors and Preventive Measures Corrosion has been a problem in reinforced concrete for a number of years. As a result a number of solutions have been found to help reduce the effect of corrosion. Earlier, the correlation between the use of de-icing salts and the appearance of corrosion

5 in bridges was mentioned. Since the two have been linked, the use of de-icing salts has not decreased. There has been a lack of feasible alternatives to alleviate its use (CB:CP). The FHWA also recognize corrosion as a High Priority Area and has conducted numerous research. Most corrosion sensors currently being examined consist of electrochemical sensors. The sensors consist of different electrodes used to detect different properties such as ph and chloride content. In a few reported cases, some of the electrodes did not show good stability over time (Duffo, Characterization ). The change in environmental conditions affects many materials used in these studies which made it difficult for researchers to find a reference electrode. Environmental factors may impact potential reading and make it possible to misinterpret the condition of the rebar (Duffo, Characterization 9). Experimental Procedure The testing was done on a small-scale reinforced concrete beam. The dimensions of the beam are x x. The tension reinforcement consisted of No. 3 rebar while the compression and shear reinforcement consisted of No. rebar. The below diagrams outlines the design for the RC beam:

6 Once two rebar cages were manufactured, piezoceramic sensors were attached to the rebar cages at sixteen different locations. The below diagram shows the approximate locations:

7 The ceramic patches were bonded directly to the rebar using epoxy. The piezoceramic sensors consisted of 5A piezoceramics that were cut into.5 x mm patches for the top rebar and mm x mm patches for the bottom. Liquid tape was used protect the sensors. The patches were placed so that the wires would exit the beam through the top. To protect the patches from damage during casting of the beam, the patches were covered with mortar (App. C). To prevent water damage, concrete sealant was used on the mortar cubes.

8 After mortar was placed, baseline data was taken using a D-space control box. The control box had slots for data collection from eight different sensors. For most tests fewer slots were used. The below diagram outlines the equipment arrangement: A limited number of sensors were also used as actuators to speed up the data collection process. After data collection was completed, thirty-seven data files were created for each data set. To simulate accelerated corrosion, hydrochloric acid was used to corrode the steel rebar. From initial experiments, a thirty percent mass loss was observed in the first twelve hours of exposure to the acid. After this initial period, it was observed each subsequent twelve-hour period had a much smaller mass loss. For the experiment, the acid will be

9 replaced every twelve hours to ensure corrosion is taking place. Also, at each time that acid was replaced, data was also collected from the sensors. Before the beam is cast, channels must be created to ensure there is a way to corrode the rebar inside of the beam. In this experiment, the channels are created by using traditional straws that will run from the top surface of the beam to the rebar to be corroded. The acid can be injected and replaced by using a pipette. To ensure corrosion takes place around the circumference of the rebar, a transporting material, paper, will be wrapped around the rebar to ensure equal distribution of the acid. After the necessary arrangements are made for the acid, the beam may be cast. The concrete used in the beam is listed in Appendix C. During casting care must be taken to ensure the wires protrude out of the top surface of the beam and that the channels to the rebar remain unblocked. Also, during concrete placement, the distribution of concrete should be observed to ensure there are no significant voids in the beam. The concrete was consolidated by using a vibrator. The beam should be allowed to cure for approximately one week. Because of time constraints, the beam was allowed to cure for three days. Data was collected at each hour period during the curing period. This was to observe if the strength development of the concrete would interfere with data collected in the corrosion period. For this experiment, the corrosion period was -hours. Results From the data, they seemed to be an overall decrease in signal. However, observing the plots of the voltage of the sensor revealed there was no strong evidence for a trend to be identified (App. D). Signal Change in Sensors Number of Sensors Signal Decreasing Signal Increasing No Change

10 Signal Change in Sensors 9% % 3% Signal Decreasing Signal Increasing No Change The above figures represent the observed change in the plots from the initial data set to the data set taken after sixty hours of corrosion. The pie and bar chart show that the majority of the sensors showed a decrease in voltage. The next largest group showed an increase in voltage. The smallest group showed no or negligible change in voltage. Though there is a numerical difference, the pie chart displays that there is no identifiable change due to corrosion. Nearly one-third of the signals registered no change. For the practicality of using piezoceramic materials for corrosion detection, this group must be much smaller. In addition, the percentage of sensors that showed a signal increase and decrease must be more differentiated. The pie chart identifies the two groups as only have a nine percent difference - a 5 sensor numerical difference. However, the above figures do not rule out the possibility of their use because of many possible errors in the experimental setup. The charts attempt to draw a primitive conclusion. This, however, is not necessarily indicative of the signal change as many plots had wide variation of voltage during the corrosion period. Also many showed slight changes in voltage. 8 x S S3 E F H G 7 x B B9 B7 B B B8 3 5 Tests 8 and 9 3 5

11 From figures 7-, the signal for sensor R9 shows a sharp decrease from the initial signal to a voltage of approximately zero. This sensor was located close to a corrosion site and may have been damaged by the acid. x R R9 R7 R R R8 9 x R R9 R7 R R R Tests 7 and

12 Acknowledgements Dr. Haichang Gu Dr. Gangbing Song Dr. Y. L. Mo The research study described herein was sponsored by the National Science Foundation under the Award No. EEC-93. The opinions expressed in this study are those of the authors and do not necessarily reflect the views of the sponsor.

13 References Brown, Robert P. Kessler, R. J. Florida Method of Test for An Accelerated Laboratory Method for Corrosion Testing of Reinforced Concrete Using Impressed Current. September,. Accessed June 8, 9. < ublications/fstm/methods/fm5-5.pdf> Corrosion Protection: Concrete Bridges. Turner-Fairbank Highway Research Center. Federal Highway Administration. Accessed July 3, 9. < Duffo, Gustavo S. Farina, Silvia B. Development of an embeddable sensor to monitor the corrosion process of new and existing reinforced concrete structures. Construction and Building Materials. Volume 3. Issue 8. August 9. Pages Duffo, Gustavo S. Farina, Silvia B. Giordano, C. M. Characterization of Solid Embeddable Reference Electrodes for Corrosion Monitoring in Reinforced Concrete Structures. Electrochimica Acta.Volume 5. Issue 3. January, 8. Pg.. Simmers Jr., Garnett E; Sodano, Henry A.; Park, Gyuhae; Inman, Daniel J. Detection of corrosion using piezoelectric impedance-based structural health monitoring. AIAA Journal.. Volume. Issue. Page Simmers Jr., Garnett E; Sodano, Henry A.; Park, Gyuhae; Inman, Daniel J. Impedance based corrosion detection Proceedings of SPIE, the international society for optical engineering. 5. Volume 577. Page 38. Wang, Ying; Zhu, Xinqun; Hao, Hong; Ou, Jinping. Guided Wave Propagation and Spectral Element Method for Debonding Damage Assessment in RC Structures. Journal of Sound and Vibration. Vol. 3. No July, 9. pg

14 g gram lb pound in. inches RC reinforced concrete mm millimeter Appendix A: Notation

15 Appendix B: Corrosion Test Results Steel Corrosion Results Time (hrs.) Sample Mass (g) Sample Mass (g) Sample 3 Mass (g) Steel Corrosion Rate Mass (g) 8 Sample Mass Sample Mass Sample 3 Mass 8 Time (hrs.) Percentage Lost Over Time Time (hrs.) Sample Mass (g). 3.% 37.%.3% 5.7% 7.8% 5.% 5.5% Sample Mass (g). 3.% 3.% 39.8%.% 8.% 5.% 5.% Sample 3 Mass (g). 3.7% 37.3%.8%.% 9.% 5.7% 5.% Mass Loss Percentage Over Time. Percent of Mass Lost Sample Mass Sample Mass Sample 3 Mass 8 Time (hrs.)

16 Appendix C: Mix Designs Mortar Component Parts Water.5 Sand.5 Cement Concrete Component Weight (lbs) Water +/-.5 Cement 3.5 Sand 7 Aggregate 39

17 Appendix D: Sensor Voltage Plots Note: Unless otherwise stated the ratio is set to. 8 x B B9 B7 B B B Figure Test 7; Actuator: H; Sensors: B B9 B7 B B B B B9 B7 B B B Figure Test 7; Actuator: G; Sensors: B B9 B7 B B B8

18 .5 3 x B B9 B7 B B B Figure 3 Test 8; Actuator E; Sensors: B B9 B7 B B B8 9 x B B9 B7 B B B Figure Test 89; Actuator S; Sensors: B B9 B7 B B B8; Ratio =.

19 7 x 5 5 B B9 B7 B B B Figure 5 Test 9; Actuator: S; Sensors: B B9 B7 B B B8; Ratio =. x B B9 B7 B B B Figure Test 9; Actuator: S3; Sensors: B B9 B7 B B B8; Ratio =.

20 x R R9 R7 R R R Figure 7 Test 7; Actuator: H; Sensors: R R9 R7 R R R R R9 R7 R R R Figure 8 Test 79; Actuator: F; Sensors: R R9 R7 R R R8

21 9 x 8 7 R R9 R7 R R R Figure 9 Test 83; Actuator E; Sensors: R R9 R7 R R R8 9 x R R9 R7 R R R Figure Test 99; Actuator: S; Sensors: R R9 R7 R R R8; Ratio =.

22 .5 3 x R R9 R7 R R R Figure Test Actuator: S3; Sensors: R R9 R7 R R R8; Ratio = 3.5 x S S S Figure Test 7; Actuator: H; Sensors: S S S3

23 8 S S S3 3 5 Figure 3 Test 3; Actuator: R9; Sensors: S S S3 x S S S Figure Test ; Actuator: R; Sensors: S S S3

24 9 x S S S Figure 5 Test 5; Actuator: B9; Sensors: S S S3 ; Ratio =. x 5 S S S Figure Test ; Actuator: B; Sensors: S S S3 ; Ratio =.

25 .5 3 x 5 E G F Figure 7 Test 73; Actuator: H; Sensors: E G F E F H Figure 8 Test 77 Actuator: G; Sensors: E F H

26 E G H Figure 9 Test 8; Actuator: F; Sensors: E G H 5 x F G H Figure Test 85; Actuator: E; Sensors: F G H

27 9 8 7 B B9 B7 S S S Figure Test 7; Actuator: G; Sensors: B B9 B7 S S S3 5 5 B B9 B7 S S S Figure Test 78; Actuator: F; S: B B9 B7 S S S3

28 .5 3 x B B B8 S S S Figure 3 Test 8; Actuator: E; Sensors: B B B8 S S S3 8 x S S3 E F H G 3 5 Figure Test 8; Actuator: S; Sensors: S S3 E F H G; Ratio =.

29 7 x 5 5 S S3 E F H G Figure 5 Test 87; Actuator: S; Sensors: S S3 E F H G; Ratio =. x 5 5 S S E F H G Figure Test 88; Actuator: S3; Sensors: S S E F H G; Ratio =.

30 .5 x S B8 B9 B B B Figure 7 Test 9; Actuator: B7; Sensors: S B8 B9 B B B; Ratio =. x 5 S B7 B9 B B B Figure 8 Test 93; Actuator: B8; Sensors: S B7 B9 B B B; Ratio =.

31 .5 3 x S B7 B8 B B B Figure 9 Test 9; Actuator: B9; Sensors: S B7 B8 B B B; Ratio =.;.5 3 x S B7 B8 B9 B B Figure 3 Test 95; Actuator: B; Sensors: S B7 B8 B9 B B; Ratio =.

32 x 5 S B7 B8 B9 B B Figure 3 Test 9; Actuator: B; Sensors: S B7 B8 B9 B B; Ratio =..5 x.5 S B7 B8 B9 B B Figure 3 Test 97; Actuator: B; Sensors: S B7 B8 B9 B B; Ratio =.

33 5 R R7 R R R Figure 33 Test ; Actuator: R9; Sensors: R R7 R R R8 Missing data files made the following tests incompatible with the plotting program *Test 75; Actuator: G; S: R R9 R7 R R R8 *Test 98; Actuator: S; S: R R9 R7 R R R8; Ratio =. *Test ; Actuator: R; S: R R9 R7 R R8

34 Appendix E Recommendations for Future Work Figure View of Ferrite Core Housing Figure Picture of Ferrite Cores Source: