ADVANCED MONITORING SENSOR AND SELF-REPAIRING SYSTEM FOR CRACKS IN CONCRETE STRUCTURES

Transcription

1 ADVANCED MONITORING SENSOR AND SELF-REPAIRING SYSTEM FOR CRACKS IN CONCRETE STRUCTURES Hirozo Mihashi(1), Tomoya Nishiwaki(2), Kazuaki Miura(1) and Yoshiki Okuhara(3) (1) Dept. of Architecture and Building Science, Tohoku University, Japan (2) Faculty of Education, Art and Science, Yamagata University, Japan (3) Japan Fine Ceramics Center, Japan Abstract Cracking in concrete structures is one of the main sources to cause deterioration of the structures. Crack detection at an early stage of damage in the structure is important for effective repairing and retaining the structure. Usually visible observation and/or monitoring by various types of sensors are adopted to assess existing structures but repairing is afterwards separately carried out. If the repairing is too late to be executed, it becomes less effective and even causes a lot of difficulties to be repaired. Furthermore, for very special structures to which workers cannot approach for repairing, it is essential to apply a self-repairing system to the structure. In this study, a self-repairing system for concrete structures has been developed, which is composed of self-diagnosis composite bars and of pipes made with heat-plasticity film. The self-diagnosis composite is a kind of crack monitoring sensor that has a function of heating device. Once crack occurs crossing the sensor, electric resistance locally increases and it can selectively heat around the generated crack by electrification. Then the pipe is melted to release the repair agent for filling the crack. In this paper, an advanced crack monitoring sensor is proposed and the outline of the selfrepairing system is presented. Keywords Monitoring, crack sensor, self-repairing, thermography 401

2 1. INTRODUCTION While prolonging the service life of structures has become more and more important from the viewpoint of global ecology and the sustainable environment, one of the main sources to accelerate deterioration of concrete structures is cracks in concrete members. For assessing and effective repairing of existing concrete structures, crack detection and monitoring of its extension are essential. Currently in practice, visual inspection of cracks is most commonly adopted. However, it is available only for cracks on the surface of concrete members and it is not always reliable. On the other hand, various kinds of non-destructive testing methods have been proposed, which include acoustic emission [1], impact echo method [2], fiber optic crack sensors [3] and so on. Leung et al. [4] developed a distributed crack sensor based on optical time domain reflectometry and demonstrated to catch quantitative information on crack opening, which is important for durability consideration. Most of these technologies and methods, however, are capable only to detect and monitor cracks in concrete structures but don t directly contribute to repairing the cracks. Not only crack detection but also repair at the early stage of the deterioration is more effective to prevent the serious damage of the structure. For that purpose, self-repairing systems of concrete structures have been developed [5], [6], [7]. Dry [5] used liquid core hollow glass capillary fibers. For practical application, however, hollow glass fiber may cause a lot of difficulties in execution of concrete mixing and placing. On the other hand, Nishiwaki et al. [6] proposed to employ a strain monitoring sensor as a heating device which melts a plastic pipe containing repair agent. Once crack occurred crossing the sensor, it can selectively heat around the generated crack by electrification. Then the heat-plasticity organic film pipe beside the heating device is melted only in the heated zone. Repair agent released from the melted surface of the pipe fills the crack and hardens in the crack. In this way, it is expected that the self-repairing system will work well against the generated crack. The proposed system is free from the constraints of handling very fragile pipes such as hollow glass capillary fibers. In this paper, further developed self-diagnosis composite, that is, an advanced crack monitoring sensor is proposed which composes an important part of the self-repairing system of concrete structures. 2. SELF-REPAIRING SYSTEM In this paper, the self-repairing system is defined as follows; a system whereby, when a crack occurs in the concrete, the inherent function of the concrete material can repair the crack without human intervention using the system including remote control (delivery system of repair agent and/or heating devices, etc.) embedded in the concrete beforehand. A conceptual figure of the proposed self-repairing system based on this definition is schematically shown in Figure 1. Both the heating device and organic film pipe containing the repair agent are embedded in the concrete. A strain monitoring sensor fabricated with a type of reinforcing fibre and conductive matrix can work as the heating device. The organic film pipe is made of a thermoplastic film and filled with the repair agent. When a crack occurs crossing the sensor, the strain monitoring sensor reduces the conductive path, and it can partially increase its electrical resistance around the crack. This allows selective heating for the damaged part, and then the surface of the pipe is melt. As a result, the repair agent 402

3 released from the melted surface of the pipe fills the crack and hardens. In this manner, it is expected that the self-repairing system works to the crack generated in the concrete member. Current Crack Selective heating around the crack melts the pipe and releases the repair agent into the crack Organic film pipe containing repair Supply of repair agent Heating device (Strain monitoring sensor) Figure 1: Schematic description of proposed self-repairing system 3. DEVELOPMENT OF CRACK MONITORING SENSOR 3.1 Strain monitoring sensor A strain monitoring sensor is not only a strain gauge but also both structural reinforcement and functional material for measuring the damaged area, recording damage history and so on. In this study, a strain monitoring sensor fabricated with fibre-reinforced composites containing conductive particles was employed as a heating device. Figure 2 shows a schematic diagram of the structure of this sensor. The matrix of the composites is made of conductive paste, i.e. carbonated epoxy resin containing carbon particles [7]. In the absence of any damage, the sensor has an electrical conduction path with dispersed carbon particles. When the sensor detects a local increase of strain due to e.g. a crack formed in the concrete, its electrical resistance increases since a part of the electrical conduction path is cut off around the crack. This sensor can sensitively increase the electrical resistance even with rather small strain due to the dispersive structure of the conductive particles. By means of electrification in this sensor, a partial increase of the resistance can achieve selective heating around the crack. Cutting of conduction path by locally increased strain Continuous glass fibre Current Carbonized epoxy resin Conduction path containing dispersed carbon particles Continuous glass fibre Carbonized epoxy resin Carbon particles Carbon particles Figure 2: Schematic diagram of the structure of strain monitoring sensor 403

4 Figure 3 shows the relationship between strain and resistance change ratio under a cyclic tensile loading to the strain monitoring sensor. It is clearly shown that the previous maximum strain is memorized even after the load is released. Furthermore the resistance increases significantly as the strain increases. If the strain monitoring sensor is embeded directly in concrete, the sensor is cut off by the stress concentration once a crack passes through the sensor since the strain monitoring sensor itself has a high bond strength in concrete. For preventing such a situation, the sensor is covered with a kind of silicon polymer coating to reduce the strain concentarion at the crack. However, it causes another problem when the crack width needs to be related to the strain with higher accuracy. Figure 3: Strain vs. resistance change ratio relationship of strain monitoring sensor under tensile cyclic load 3.2 Improvement of strain monitoring sensor. Crack monitoring sensor In order to avoid the cutting off of the sensor due to cracking, knots made with prepreg sheets were settled on the sensor with a certain distance and the surface of the rod between the knots was covered with polyethylene film to keep free from the bond. Figure 4 shows a schematic diagram of the improved strain monitoring sensor, that is a crack monitoring sensor. The local strain can be given by the ratio of crack width to the distance of knots. Figure 5 shows the detail of the prepreg knot and the surface of the rod of strain monitoring sensor coated by polyethylene film. 404

5 Prepreg knot Distance between knots PE coating Crack Figure 4: Schematic description of crack monitoring sensor Prepreg knot Prepreg sheet: glass sheet soaked with epoxy resin 4 mm PE coating to unbond with concrete 3 mm Prepreg sheet is winded to a set thickness, and hardened by heating Figure 5: Schematic diagram of prepreg knot 3.3 Experimental study on crack monitoring sensor Experimental study was carried out to prove the effectiveness of the developed crack monitoring sensor. Figure 6 (a) shows the outline of the tested specimen and (b) shows the test set-up for the tensile loading. The specimen was made with mortar (W/C=0.45, S/C=2.0) and the loading test was executed by Instron Figure 7 (a) and (b) show the temperature distribution measured by thermography (a) before and (b) after cracking. It is clearly shown that the cracked part is heated up to over 80 C (30 minites after the electrification) by the crack monitoring sensor. 405

6 Figure 8 shows a comparison of two relatinships between strain and resistance change ratio. The solid line represents the relationship for the strain monitoring sensor itself which is shown in Figure 2. Circles show experimental results obtained from the test on a specimen (shown in Figure 6) in which a crack monitoring sensor is embeded in mortar. The broken line is the extrapolation curve of the experimental results. The crack monitoring sensor needs a larger strain to get the same value of the resistance change ratio than the strain monitoring sensor. The less sensitivity of crack monitoring sensor might be caused by the elastic deformation of mortar in the specimen and the porous microstructure of the interface between knot and mortar, though the equivalent strain was calculated from the ratio between the crack width and the distance of knots on the assumption of rigid body displacement. In spite of the less sensitivity, the shape of the power function to relate the strain and the resistance change ratio is almost same between the strain monitoring sensor and the crack monitoring sensor. It shows that the crack monitoring sensor can detect a crack whose width is over 0.08 mm since the minimum strain is 0.2% and the knot distance is 40 mm. Thus the obtained curve can be employed to design the self-repairing system if the minimum value of the allowable crack width is e.g. 0.1 mm. Anchor bolt (M6) Tensile load Pregregknot 320mm 40mm 60mm Crack Notch Electric wire (a) Tested specimen (b) Test set-up for tensile loading Figure 6: Experimental study to prove the effectiveness of the crack monitoring sensor 406

7 (a) (b) [ C] 80 crack (a) Before cracking (b) After cracking Figure 7: Temperature distribution measured by thermography Resistance change ratio [%] Strain [%] y = x y = x Strain monitoring sensor ( ) Crack monitoring sensor embedded in the mortar specimen Figure 8: Comparison of two relationships between strain and resistance change ratio 407

8 4. HEAT-PLASTICITY ORGANIC FILM PIPE CONTAINING REPAIR AGENT The repair agent is contained in a pipe covered with a heat-plasticity organic film. The heat-plasticity film pipe seals off the repair agent from the concrete in order to prevent its hardening reaction. On the other hand, this pipe is required to melt easily by heating around a crack once it is generated. Therefore, the melting point of the film should be lower than the upper limit of temperature that causes the concrete deterioration, and higher than the range of temperature corresponding to normal heating conditions such as hydration heat, direct rays of the sun and so on. In this study, ethylene vinyl acetate (EVA) polymer film with a melting point of 93 C was employed. The fabricated EVA film pipe has an outer diameter of 3.4 mm and an inner diameter of 2.0 mm. When the temperature around the pipe rises to over 93 C due to heating by the selfdiagnosis composite, the EVA film on the surface of the pipe melts. The repair agent is then released into the crack from the pipe. Moreover, the bond strength between the EVA film and concrete is very low. Therefore, the pipe covered with the film does retain the repair agent and doesn t release it even when a crack is formed in the concrete unless the calorific value from the sensor in the area around the crack becomes sufficiently high to melt the pipe. 5. CONCLUSIONS The self-diagnosis composite is a kind of crack monitoring sensor in which the electrical resistance increases partially around the crack. Since the electrification to the sensor generates a local heating to increase the temperature around the crack, the pipe melts to release the repair agent into the crack. In such a way, self-repairing system for cracks in concrete structures can be realized. Moreover, the thermographic observation of the selective heating by the self-diagnosis composite shows that this technique can be used as a monitoring sensor to detect cracks in concrete members. AKNOWLEDGEMENT This study was carried out as a part of the research project on Development of self-healing concrete with strain sensors which can heat around generated cracks supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Exploratory Research (Grant No ). The authors would like to express their thanks to Dr. Hideaki Matsubara of the Japan Ceramics Center for valuable advice. REFERENCES [1] Ohtsu, M., Okamoto, T. and Yuyama, S.: Moment tensor analysis of acoustic emission for cracking mechanisms in concrete, ACI Structural Journal, 95(2) (1998) [2] Rossi, P. and LaMaou, F., New method for detecting cracks in concrete using fiber optics, Mater. Struct., 22 (132) (1989) [3] Sansalone, M. and Carino, N.J., Transient impact response of plates containing flaws, J. Res. Natl. Bureau Stand. 92 (6) (1987) [4] Leung, C.K.Y., Elvin, N., Olson, N., Morse, T.F. and He, Y., A novel distributed optical crack sensor for concrete structures, Eng. Fract. Mech. 65 (2-3) (2000)

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