Handbook of nondestructive testing of concrete:Magnetic Electrical Method.pdf
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1、10-1 0-8493-1485-2/04/$0.00+$1.50 2004 by CRC Press LLC 10 Magnetic/Electrical Methods 10.1Introduction 10-1 10.2Magnetic Methods. 10-1 Introduction Theory Test Methods 10.3Electrical Methods. 10-9 Introduction Theory Electrical Properties of Concrete Test Methods The initial portion of the chapter
2、briefl y describes the theory of magnetic induction, magnetic fl ux leakage, and nuclear magnetic resonance to facilitate an understanding of equipment used to locate reinforcement and determine the moisture content of concrete. The remaining portion of the chapter discusses the electrical nature of
3、 concrete and the mechanism of reinforcement corrosion as a preliminary to understanding the use of electrical capacitance and resistance to measure moisture content, pavement thickness, and corrosion of reinforcement. Where possible, the accuracy of current magnetic and electrical apparatus is indi
4、cated. 10.1Introduction Magnetic and electrical methods are used in a number of ways to evaluate concrete structures. These methods are used to (1) locate reinforcement and measure member thickness by inductance; (2) measure the moisture content of concrete by means of its electrical properties and
5、the nuclear magnetic resonance of hydrogen atoms; (3) measure the corrosion potential of reinforcement; (4) determine pavement thickness by electrical resistivity; and (5) locate defects and corrosion in reinforcement by measuring magnetic fl ux leakage. Magnetic and electrical methods have received
6、 considerable attention in recent years. Their underlying principles range in complexity as do their practical applications in the fi eld. 10.2Magnetic Methods 10.2.1Introduction Materials containing iron, nickel, and cobalt are strongly attracted to themselves and to each other when magnetized; the
7、y are called ferromagnetic materials. Other materials, such as oxygen, which are weakly attracted by magnetic fi elds, are called paramagnetic materials. In 1905, the magazine Revue de Met mentioned for the fi rst time the possibility of detecting defects such as cracks, laminations, etc. in ferroma
8、gnetic materials by means of magnetic fi elds. In 1919, Kenneth R. Lauer University of Notre Dame 10-2Handbook on Nondestructive Testing of Concrete: Second Edition E.W. Hoke applied for the fi rst patent in the United States on a magnetic inspection method, which was granted in 1922. Magnetic nonde
9、structive testing techniques used in conjunction with concrete involve the magnetic properties of the reinforcement and the response of the hydrogen nuclei to such fi elds. Because of the need to control the magnetic fi eld, electromagnets are used in most instances. 10.2.2Theory At the present time
10、, three different aspects of magnetic fi eld phenomena are used in the nondestructive testing of reinforced concrete: (1) alternating current excitation of conducting materials and their mag- netic inductance; (2) direct current excitation resulting in magnetic fl ux leakage fi elds around defects i
11、n ferromagnetic materials; and (3) nuclear magnetic resonance. 10.2.2.1Magnetic Induction This technique is only applicable to ferromagnetic materials. Test equipment circuitry resembles a simple transformer in which the test object acts as a core (Figure 10.1). There is a primary coil, which is con
12、nected to a power supply delivering a low frequency (10 to 50 Hz) alternating current, and a secondary coil, which feeds into an amplifi er circuit. In the absence of a test object, the primary coil induces a small voltage in the secondary coil, but when a ferromagnetic object is introduced near the
13、 coils, a much higher secondary voltage is induced. The amplitude of the induced signal in the secondary coil is a function of the magnetization characteristics, location, and geometry of the object. The inductance of a coil can be reduced by bringing a conducting surface near the coil. It can be sh
14、own that the effect of a conducting plate on the coil is the same as the effect of a second coil, identical to the fi rst, carrying a current equal and opposite to the coil current and located on the coil axis at a distance 2d from the original coil, where d is the coil-to-plate distance. The second
15、 coil is said to be the image of the fi rst. The voltage induced in the fi rst coil is seen to have two components. The fi rst of these is due to the self-inductance of the coil in space, and the second is due to the mutual inductance between the coil and the plate. Thus the induced voltage is seen
16、to be the sum of two components, one a constant and the other a function of coil-to plate spacing. As a result the inductance of the coil can be used to measure coil-to place distance, d, if the relationship between mutual inductance and d is known. The probe unit consists of a highly permeable U sh
17、aped magnetic core on which two coils are mounted. An alternating current is passed through one of these coils and the current induced in the other coil is measured. The induced current depends upon the mutual inductance of the coil and upon the presence of the steel reinforcing bars. For a given pr
18、obe the induced current is controlled by the distance between the reinforcement and the probe. This relationship between induced current and distance from the probe to the reinforcement is not linear because the magnetic fl ux intensity decreases with the square of the distance. As a result, calibra
19、ted scales on commercial equipment are nonlinear. The magnetic permeability of concrete, even though low, will have some effect on the reading. 10.2.2.2Flux Leaking Theory Fundamentals of this theory have been explained in detail in a number of texts.24 When ferromagnetic materials are magnetized, m
20、agnetic lines of force (or fl ux) fl ow through the material and complete a magnetic path between the poles. These magnetic lines of fl ux increase from zero at the center of the specimen and increase in density and strength toward the outer surface. When the magnetic lines of fl ux are contained wi
21、thin the ferromagnetic object, it is diffi cult, if not impossible, to detect them in air space surrounding the member. However, if the surface is disrupted by a crack or defect, its magnetic perme- ability is drastically changed and leakage fl ux will emanate from the discontinuity. Measurement of
22、the intensity of this leakage fl ux provides a basis for nondestructive identifi cation of such discontinuities. Figure 10.2 illustrates how a notch or defect distorts the magnetic lines of fl ux causing leakage fl ux to exist in the surface of the ferromagnetic material. Automatic fl ux leakage ins
23、pection systems use magnetic fi eld sensors to detect and measure fl ux leakage signals. Flux leakage sensors usually have small diameters in order to have adequate sensitivity for Magnetic/Electrical Methods10-3 detecting short length defects. Probes are typically spring loaded to provide constant
24、lift-off (distance between probe and surface). Signals from probes are transmitted to the electronics unit where they can be fi ltered and analyzed by a continuous spectrum analyzer. A majority of the sensors are inductive coil sensors or solid-state Hall effect sensors (electromotive forces develop
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