Detection of reinforcement by eddy current testing

Andreas Maar, winter semester 2016/17 

Inductive measuring methods, such as the eddy current technique, are used in civil engineering to identify the location and direction of reinforced concrete structures. Eddy currents are induced in reinforcing bars (rebar), which creates a secondary magnetic field, that opposes the magnetic field that created it. By measuring the impedance changes in the coil, the concrete surface and/ or the diameter of rebar can be determined. (Figure 1: cover meter)

Physical principles of testing methods

The eddy current technique finds its roots in electromagnetic fields, described by James Clerk Maxwell in the middle of the 19th century.

When a coil of conductive wire is excited with an electrical current, an electromagnetic field is produced around itself. If any conducting metal is near this coil of conductive wire, eddy currents are induced according to the law of Faraday’s law of electromagnetic induction. The magnetic field oscillates at the same frequency (ω = 2πƒ with <10 MHz) as the current running through the coil.^[1] The electromagnetic flux density Φ is proportional to the coil current I.

$//<![CDATA[ \begin{array}{l}Φ=L*I\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}Φ\end{array} //]]>$ = electromagnetic flux density [Wb]

$//<![CDATA[ \begin{array}{l}L\end{array} //]]>$ = electrical inductance [H]

$//<![CDATA[ \begin{array}{l}I\end{array} //]]>$ = alternating current [A]

The electromagnetic induction of rebar depends on the magnetic permeability (μr), the concrete cover (c) and the diameter of the reinforcement (d). The induced eddy currents in the reinforcing bars generate a secondary electromagnetic field that has a weakening effect on the primary magnetic field according to the Lenz´s law. (Figure 2: principle of operation of eddy current testing) By approaching a reinforcing bar, a higher impedance is measured:

$//<![CDATA[ \begin{array}{l}Z=U/I=\sqrt{R^2+X^2_L}\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}U\end{array} //]]>$ = electric potential [V]

$//<![CDATA[ \begin{array}{l}I\end{array} //]]>$ = alternating current [A]

$//<![CDATA[ \begin{array}{l}X_L\end{array} //]]>$ = capacity reactance [Ω] (=ωL)

$//<![CDATA[ \begin{array}{l}R\end{array} //]]>$ = electrical resistance [Ω]

The impedance signal consists of a real part (resistance) and an imaginary part (reactance). The comparison of the electric potential, which drops in the coil with the electric potential, which drops with the Ohm resistance, results in alteration in induction of the coil which can be measured in dependence of rebar within the concrete according:^[2]

$//<![CDATA[ \begin{array}{l}\frac{U_Sp}{U_Ω}=\frac{Z_Sp}{R_Ω}\end{array} //]]>$

The impedance changes can be shown in a visual display. By plotting the real and imaginary parts, lift-off curves show the link between signal and distance to the conductive test material. In this case, the normalized impedance plane shows the electric potential of the coil nearest to the test object in relation to the electric potential of the coil when there is no test object in reach (the test frequency is assumed to be constant). (Figure 3: lift-off curve)

The figure illustrates the relationship between the conductivity and the distance to the reinforcement bar. The larger the distance (the concrete cover), the weaker the secondary field. As a result, the imaginary part increases.^[4] By knowing the electrical conductivity of the reinforcing bar, the curve can be calibrated extremely accurately (changes in the electrical conductivity are indicated in Figure 3.

Application

The visualisation of cover meters is mainly divided in surface and line scans. Large surface scans of the concrete cover to reinforcing bars are mainly done for quality control during final acceptance. Depending on the testing grid, up to 15 m² per hour can be measured.^[5]

In the utilization phase, measurements of reinforced covers are useful for source location of reinforcing bars and as input for maintenance work planning.

The detection of individual reinforcing bars is relevant as a preliminary to some other form of testing or the connection of further concrete constructions.

A reliable determination of the reinforcement diameter is only possible for small concrete covers. Measurement inaccuracies, as defined in the manufacturer´s specifications, should be considered.

Application limits

The application of cover meters using eddy currents is limited by the distance between the reinforcing bar and the concrete surface. The maximum penetration depth is about 15 cm.

The testing method allows a useable detection of the first reinforcement layer only. The measurement is interfered by the presence of other reinforcing bars, laps, metal tie wires or double bars. Furthermore, the measuring method requires the specification of input variables in form of the cover to the reinforcement bar or the diameter. At least one parameter must be known from reinforcement plans or test drillings to confirm the rebar location.

Evaluation of measurement results

The evaluation of the measurement results is based on the DBV data sheet "Concrete cover and reinforcement: 2015-12"^[6] The concrete cover is subject to unavoidable scattering, which can be statistically determined by the NEVILLE distribution. The minimum value of the concrete cover cmin, according to Eurocode DIN EN 1992-1-1^[7] and -2^[8] as well as the fire protection standard DIN 4102-4^[9], is used as a comparison value. The measurements, especially their evaluation, should only be done by qualified personnel.

Alternative methods of reinforcement location

In order to determine the position and / or the diameter of reinforcing bars in concrete, further methods are available according to^[10]

• Magnetic Methods

• Electromagnetic Methods

• Microwave-Radar-Methods

• Induction Infrared Thermography

• Radiography Testing

Magnetic inspection methods are based on the attraction force of permanent magnets to reinforcing bars in concrete. This force varies as a function of the distance to the ferromagnetic parts, as well as the rebar diameter. By adjusting the magnet, depending on the attraction force, either the cover to the reinforcement bar or the diameter is measurable.

The electromagnetic methods can be distinguished in constant and alternating current methods. Both methods use the ferromagnetic properties of the steel to measure the cover to reinforcement bars. The finding of the source location possibilities is limited to areas close to the concrete surface.^[11]

The radar technique works by picking up reflections and scatters of electromagnetic waves on material boundary layers. Depending on the signal running time and the dielectric constant of the concrete a determination of the depth and inhomogeneity (e.g. reinforcement inserts, stressing units) is possible.^[12]

In the case of induction infrared thermography, a thermal imaging of an inductively heated reinforcement bar is detected. As soon as the isothermal heat flux of the reinforcing bar reaches the concrete surface, the radiation can be recorded as infrared waves. To calibrate the measurement, the cover to reinforcement steel bars has to be checked by comparative measuring.^[13]

When radiography testing is used, the construction body is penetrated by electromagnetic radiation. It is X-rays or gamma rays. The differences in absorption, that are depending on the density and the thickness of the testing material, are recorded. This allows detailed analysis of the internal structure of a wide range of components. Due to the complex measuring arrangement with accessibility on both sides and personal safety equipment / requirements, radiography testing is only used in a few special cases.^[14]

Literature

1. Mallwitz, Regine. Analyse von Wirbelstromsignalen mit problemangepaßten Funktionen für die zerstörungsfreie Materialprüfung. Kassel : s.n., 1999.
2. Taffe, Alexander, et al. http://bast.opus.hbz-nrw.de/frontdoor.php?source_opus=1633. [Online] November 2015. http://bast.opus.hbz-nrw.de/volltexte/2015/1633/.
3. Deutsche Gesellschaft für zerstörungsfreie Prüfung. Merkblatt B2_Bewehrungsnachweis und Überdeckungsmessung bei Stahl- und Spannbeton. Berlin : s.n., 1990.
4. Meilick, Irena. Wirbelstrom. [Hrsg.] Christian Große. Vorlesungsskript: Einführung in die Zerstörungsfreie Prüfung im Ingenieuerwesen-Grundlagen und Anwendungsbeispiele. München : s.n., Oktober 2016
5. Taffe, Alexander, Stoppel, Markus und Wiggenhauser, Herbert. [Online] [Zitat vom: 25. November 2016.] http://www.betonerhaltung-nord.de/upload/dokumente/20100411taffeua.pdf.
6. Deutscher Beton- und Bautechnik Verein, [Hrsg.]. Betondeckung und Bewehrung. 2015
7. DIN EN 1992-1-1. Eurocode 2: Bemessung und Konstruktion von Stahlbeton- und Spannbetontragwerken. Berlin : Beuth Verlag GmbH, 2015
8. DIN EN 1992-1-2 . Eurocode 2: Bemessung und Konstruktion von Stahlbeton- und Spannbetontragwerken. Berlin : Beuth Verlag GmbH, Dezember 2010
9. DIN 4102-4. Brandverhalten von Baustoffen und Bauteilen - Teil 4: Zusammenstellung und Anwendugn klassifizierter Baustoffe, Bauteile und Sonderbauteile. Berlin : Beuth Verlag GmbH, Juni 2014
10. Deutsche Gesellschaft für zerstörungsfreie Prüfung. Merkblatt B2_Bewehrungsnachweis und Überdeckungsmessung bei Stahl- und Spannbeton. Berlin : s.n., 1990.
11. Deutsche Gesellschaft für zerstörungsfreie Prüfung. Merkblatt B2_Bewehrungsnachweis und Überdeckungsmessung bei Stahl- und Spannbeton. Berlin : s.n., 1990.
12. Flohrer, Claus. Messung der Betondeckung und Ortung der Bewehrung. [Hrsg.] Deutsche Gesellschaft für Zerstörungsfreie Prüfung. Deutsche Gesellschaft für Zerstörungsfreie Prüfung: Fachtagung Bauwerksdiagnose - Praktische Anwendungen Zerstörungsfreier Prüfungen. München : s.n., 1999. Bde. DGZfP-Berichtsband 66-CD, Vortrag 4
13. Flohrer, Claus. Messung der Betondeckung und Ortung der Bewehrung. [Hrsg.] Deutsche Gesellschaft für Zerstörungsfreie Prüfung. Deutsche Gesellschaft für Zerstörungsfreie Prüfung: Fachtagung Bauwerksdiagnose - Praktische Anwendungen Zerstörungsfreier Prüfungen. München : s.n., 1999. Bde. DGZfP-Berichtsband 66-CD, Vortrag 4
14. Taffe, Alexander, Stoppel, Markus und Wiggenhauser, Herbert. Zerstörungsfreie Prüfverfahren im Bauwesen - Übersicht der Verfahren. [Hrsg.] Bundesanstalt für Materialforschung und -prüfung. Betoninstandsetzung im Ingenieur- und Wohnungsbau. Filderstadt : s.n., 2010