·Table of Contents
·Materials Characterization and testing
Study of Reinforcing Bars Detection Buried in Concrete Structures Using Eddy Current MethodOsamu YOKOTA
College of Engineering, Nihon University
Tamura, Koriyama, Fukushima-ken, Japan
|Fig 1: Outline drawing of probe coils|
|Type||Symbol||Ferrite core||Coil diameter||Coil turn||Coil distance|
|A0||Nothing||80 mm||80||2 mm|
|A1||Nothing||100 mm||270||2 mm|
|A||A2||Nothing||100 mm||140||2 mm|
|A3||Nothing||100 mm||50||2 mm|
|A4||Nothing||80 mm||270||2 mm|
|A5||Nothing||60 mm||270||2 mm|
|C||C||Existence||2 mm||200||20 mm|
|Table 1: Specification of probe coils used in the experiment|
|Fig 2: Marks for amplitude and phase of Lissajou's figure||Fig 3: Change of Iy (V) in Lissajou's figure received when various types of probe coils were used|
3.1 Change of the eddy current signal due to various test coils
5 kinds of test coil A were produced and measurement result showed in Fig.3. The output voltage becomes rapidly small, when the covering depth is increased. Compared with probe coils A1, A2 and A3 that the diameter of the coil is same and that the winding number is different, the signal amplitude from A1 was the biggest. Compared with probe coils A1, A4 and A5 that the diameter of the coil is different and that the winding number is same, the signal amplitude of A1 was most increased. That is to say, it is because the attenuation of the magnetic field of the X-axis direction becomes gentle, as the coil diameter is bigger. Therefore, the signal amplitude of output voltage should be big as much as possible in order to quantify information from the reinforcement. It is desirable that the reinforcements in the concrete are detected by using probe coils A1 and A4. However, the resolution becomes bad, when the coil diameter increases. It becomes difficult that it is distinguishing detected approaching reinforcement bars buried.
3.2 Eddy current signal on the voltage - phase plane of round bar and deformed bar
Fig.4 shows the relationship between the cover depth and reinforcement diameter of round bars SR235. When the cover depth increases, the signal amplitude decreases, but the phase does not change. In the meantime, when the reinforcement diameter increases, the amplitude increases and the phase also changes in the rotation's direction of the clock. When the covering depth consists over 100 mm, the signal amplitude considerably decreases, and the measurement becomes difficult.
|Fig 4: Change of I (V) in Lissajou's figure received when prove coil was fed over difference of covering depth and diameter of reinforcement bars||Fig 5: Change of I (V) in Lissajou's figure received when prove coil was fed over difference of covering depth and diameter of reinforcement bars|
3.3 Eddy current signal on voltage - phase plane due to the test frequency
The relationship between of the signal of the amplitude and phase by the test frequency was shown in Fig.6. The round bar SR235 with diameter 16 mm buried in the covering depth 57 mm was measured using test coil A0. It is known that when the test frequency changes, the phase and the amplitude of the signal waveform change simultaneously. That is to say, when the test frequency gets higher, the phase advances to the clockwise direction and the signal amplitude becomes smaller. And the signal waveform changes linearly.
|Fig 6: Changes of I (V) and phase in Lissajou's figure obtained test frequencies when probe coil was fed over reinforcement bar||Fig 7: Change of Iy (V) in Lissajou's figure received when probe coils A and B were fed over two reinforcement bars|
3.4 Separation limit of the reinforcement bars
Separation limit of the reinforcement bars in concrete would arise a problem. In this study, minimum distance between two steel bars that are laid very close each other at which these could be detected separately is investigated in this experiment. The result is shown in Fig.7. The probe was made to move along the direction perpendicular to the bars at the constant speed. The bars are placed at the distance of 0, 80 and 220 mm. When tested by Probe coil A shown in Fig.7 (a), it had perfectly divided the signal waveform into two mountains at reinforcement interval 220 mm. When the reinforcement interval becomes 80 mm, two waveforms overlapped each other, but the height of waveforms remained unchanged. On the other hand, it becomes 0 mm, the signal waveform presents one mountain and the detection of two reinforcement bars became impossible. In this time, the height of the signal waveform has greatly appeared further.
On the other hand, the signal waveform by using probe B is shown in Fig.7 (b). In the reinforcement interval of 220 mm, it is measured two sine waves and perfectly separation. It becomes the synthesis of two sine waves in two reinforcements in order to show in sine wave. The waveforms of the inside and outside cancels each other at the reinforcement interval of 80 mm. Further the reinforcement interval narrows, it is shown one sine wave, and the amplitude has greatly appeared further than the case of one reinforcement bar.
3.5 Lissajou's figure due to corrosion reinforcement bar
The reinforcement is made to corrode by the autoclave test, and the measurement result was shown in Fig.8. The output signal from did not obtain, when Probe coil C was located in the site A without the rust fouling. When the coil moves from the site A to B with the rust fouling, the amplitude of the signal becomes large. The signal becomes zero that it is moved to the site C site. Therefore, it is possible to detect the rust fouling by the electromagnetic testing.
|Fig 8: Change of Iy (V) in Lissajou's figure received when probe coil was fed over contaminated zone on concrete|
|Fig 9: Macrostructure in concrete with contaminated mortar zone||Fig 10: Comparison of Lissajou's figure received when probe coil was fed over points (A) to (C) in Fig.9|
3.6 Lissajou's figure due to rib direction of the deformed bar
Fig.11 shows the feeding direction of probe coil when the rib of deformed bar was tilted in any desired direction. Fig.12 was shown the difference of Lissajou's figure as tilting angle a of the rib of deformed bar is made to change to 0÷ to 360÷ in the 30÷ interval. Lissajou's figure is greatly different by rib angle a. That is to say, Lissajou's figure is shown in one line in ƒ¿ =0÷, 90÷, 180÷, 270÷ and 360÷. Moreover, Lissajou's figure from a =0÷, 180÷ and 360÷got the same amplitude, phase and shape. Lissajou's figure from a=90° and 270÷ becomes also an identical signal. With the exception of their angle, Lissajou's figure described the loop. The loop in Lissajou's figure was described that the rib of deformed bar was not located for the right and left object for the probe coil.
|Fig 11: Feeding direction of prove coil when rib in deformed bar was tiled in any desired direction.||Fig 12: Change of Lissajou's figures received when probe coil was fed over rib direction of deformed bars|
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