Handbook of nondestructive testing of concrete：Magnetic Electrical Method.pdf

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 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 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 indicated. 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 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 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; they 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 ferromagnetic 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 nondestructive 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, 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 in 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 connected 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 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 shown 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 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 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 shaped 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 probe 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, calibrated 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, magnetic 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 within 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 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 inspection 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 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 developed as a result of the interaction of a steady current fl owing in a steady magnetic fi eld). Magnetic diodes and transistors, whose output current or gain change with magnetic fi eld intensity, can also be used. To a lesser extent, magnetic tape systems have also been used. The more highly magnetized the ferromagnetic object, the higher its leakage fl ux intensity from a given defect. The amount of leakage fl ux produced also depends on defect geometry. Broad, shallow defects will not produce a large outward component of leakage fl ux; neither will a defect whose long axis is parallel to the lines of fl ux. The latter are more easily detected with circular magnetic fi elds. Internal defects in thick parts may not be detected because the magnetic lines of fl ux nearly bypass the defect with little leakage. Defects oriented so that they are perpendicular to the surface and at right angles to the lines of fl ux will be more easily detected than defects laying at an angle with respect to the surface or fl ux lines. Defects lying at a shallow angle to the surface and oriented in the direction of the fl ux lines produce the weakest lines of leakage fl ux. 10.2.2.3Nuclear Magnetic Resonance (NMR) This technique is based on the interaction between nuclear magnetic dipole moments and a magnetic fi eld. This interaction can be used as a basis for determining the amount of moisture present in a material by detection of a signal from the hydrogen nuclei in water molecules. The term resonance is used because the frequency of gyroscopic precession of the magnetic moments is detected in an applied magnetic fi eld. FIGURE 10.1 Principle of operation of induction meter used to locate reinforcement. (From Malhotra, V.M., Testing Hardened Concrete: Nondestructive Methods, ACI Monogr. No. 9, The Iowa State University Press, Ames, and American Concrete Institute, Detroit, 1976. With permission.) FIGURE 10.2 Effect of defects on fl ux pattern and measurement. CALIBRATING VOLTAGE POWER SOURCE PROBE AMPLIFIER FLUX GENERATOR COIL E-DIFF METER READOUT BATTERIES-GENERATORS POWER SOURCE PROBE MAGNETIC FLUX LINKAGE CHANGE IN MAGNETIC FLUX FERROUS MATERIAL 10-4Handbook on Nondestructive Testing of Concrete: Second Edition Several methods of generating and detecting NMR signals are available. The method preferred for most practical applications is the transient or pulsed method because measurements can be made rapidly and the data obtained provide a maximum amount of information about the investigated species. Detailed information on this method is available in several texts.57 10.2.3Test Methods 10.2.3.1Depth of Concrete Cover The induction principle resulted in the development of equipment for determining the location, sizes, and depth of reinforcement.8,9 In 1951 an apparatus called the “covermeter” was developed in England by the Cement and Concrete Association in conjunction with the Cast Stone and Cast Concrete Products Industry.8 Their reports indicate the effectiveness of this type of equipment for both plastic and hardened concrete. Refi ned versions are now available, using more sophisticated electronic circuits, which can detect reinforcement at depths of 12 in. A typical meter is shown in Figure 10.3. Meters must be recalibrated for different probes. The probes are highly directional. A distinct maximum in induced current is observed when the long axis of the probe and reinforcement are aligned and when the probe is directly above the reinforcement. By using spacers of known thickness, the size of reinforcing bars between 3/8 and 2 in. (9.52 mm and 57.33 mm) can be estimated. British Standard 4408 pt 1 suggests a basic calibration procedure involving a cube of concrete of given proportions with reinforcing bars at specifi ed distances from the surface.10 These meters can be used to estimate the thickness of concrete members accessible from both sides. If a steel plate is aligned on one side with the probe on the other side, the measured induced current will indicate the thickness of the slab. The equipment must be especially calibrated for this use. Commercial reinforcement bar locaters are portable, inexpensive instruments that can be easily used. Accuracy of ±2% or 0.1 in. up to depths of 6 in. for any bar size has been claimed. A bar size accuracy of ±10% to a depth of 8 in. is also indicated. The latest equipment utilizes headphones that can detect by tone, a 3/4-in. bar at 12-in. depth. Tone trigger levels can be preset for depths less than 6 in. If cover determinations are carried out on a grid system over the concrete surface, equi-depth contours can be constructed which clearly illustrate the variability in depth of cover and any regions where it is less than satisfactory. An example of this type of map is illustrated in Figure 10.4. The use of a hand-held instrument in mapping the cover of reinforcement in a bridge deck is very time consuming. The U.S. Federal Highway Administration developed a “Rolling Pachometer,” which FIGURE 10.3 A meter used to locate reinforcement. Instrument includes a standard probe, a special probe for magnetic concrete, headphones and a spacer block for rebar measurement. (Courtesy of NDT James Instrument Inc.) Magnetic/Electrical Methods10-5 proved to be accurate, reliable, and capable of gathering data at a rate 20 times that of conventional hand-held methods.11 The second generation system contains a modifi ed hand-held meter, a battery operated, two channel, pressurized ink recorder; a speedometer; and associated electronics. The electron- ics include an amplifi er, fi lters, voltage regulators, adjustable high and low reinforcing bar limit controls, a magnetic sensor, and counters for processing and displaying distance marks on the chart graph. A constant scan speed of 1 mph is required. The speedometer works in conjunction with the magnetic sensor which is located on one of the wheels. Pulses from the sensors, which are processed and used to indicate speed, are also fed into counters which trigger a distance mark on the left side of the chart paper (Figure 10.5). These distance marks occur every 18 in. of travel and, when used with the manual event FIGURE 10.4 Depth of concrete cover on a reinforced concrete bridge deck. FIGURE 10.5 Typical chart recorder presentation of “Rolling Pachometer.” (Adapted from Reference 11.) 1“ 11/2“ 11/4“ 1“ 11/4“ 1“ 1“ 11/4“ Event mark activated by pushbutton located in handle. Each peak represents a reinforcing bar. Depth can be determined by the distance from each peak to edge of graph. Distance marks occur every 18 in (457 mm) of travel. Indicates rebars lower than designed depth tolerance. When no spikes occur, rebars are within designed depth tolerance. Indicates rebars higher than designed depth tolerance. 10-6Handbook on Nondestructive Testing of Concrete: Second Edition marker switch, can be correlated to a particular area of interest on the bridge. The manual event mark is displayed on the left side of the chart paper. Data, representing variations of the signal as the probe passes over the reinforcing bars, are displayed on the left channel of the chart paper. The sinusoidal nature of the recorded data represents the peaks associated with a reinforcing bar and, by measuring the distance of each peak to the edge of the chart paper graph, depth of cover can be determined from a calibration curve. High and low limit controls permit identifi cation of reinforcing bars either higher or lower than the allowed tolerances for the specifi ed depth. This is indicated on the right-hand channel. A spike to the left of center indicates a rebar that is lower than the preset limit. A spike to the right of center indicates a rebar that is higher than the preset limit. Calibration curves for