Target of Investigation

A magnetometer is an instrument used for examining electromagnetic field features of embedded steel components. It is mainly used for the following applications:

• Determining rebar size, orientation, location, and depth (concrete cover depth).
• Obtaining as-built information on embedded rebar when record drawings are not available.
• Measuring the thickness of a concrete cover.
• Detecting (indirectly) the susceptibility of steel reinforcement to corrosion; low concrete cover thickness increases the chance for concrete to deteriorate and expose steel to oxygen and moisture for corrosion.

Description

The magnetometer (also known as a covermeter, profometer, and pachometer) was first manufactured in the 1950s. The magnetometer is commonly used in conjunction with other nondestructive evaluation technologies. For example, for tests requiring a concrete core sample or contact with embedded steel reinforcement through a drilled hole, a magnetometer can help avoid hitting any reinforcement during drilling. Additionally, a magnetometer can quickly and easily perform construction quality control, for example, if there is a concern about adequate concrete cover.

Physical Principle 

Magnetometers operate based on the fact that an electromagnetic field is affected by steel reinforcement but unaffected by nonconductive materials, such as concrete, composite, and timber.

A magnetometer using only one coil operates based on the principle of eddy currents. The alternating current in a primary coil creates a magnetic field. Eddy currents are produced on the surface of any electrically conductive material in the vicinity of the magnetic field. Eddy currents induce a magnetic field in the opposite direction of the primary coil field. This opposition affects the measured impedance in the primary coil. Reinforcing bars that are closer to the magnetometer or are of larger diameters can produce a stronger magnetic field and induce a higher impedance.

The magnetic induction principle is employed when both primary and secondary coils are used. The primary coil is charged while a secondary set of coils picks up the voltage transferred by the magnetic circuit created by the primary coils and rebar. Magnetometers that operate via magnetic induction are less sensitive than those using eddy current principles. Magnetometers using induction operate at frequencies below 90 Hz.

Regardless of the principle used, the size, orientation, and location of rebar (concrete cover thickness) affect voltage measurement in terms of amplitude and phase difference.^(1,2)

Data Acquisition 

A magnetometer consists of a magnetic probe and a data processor with a display screen. In some equipment, the probe, data processor, and display screen are integrated into a single lightweight, handheld device. The probe is moved along the surface of a concrete member, such as a slab or wall. As the probe is moved, the operator can observe changes in the magnetic field on the screen in either a digital or graphic format. Rebar orientation can be determined through a planar rotation of the probe at the concrete surface. The longitudinal axis of the probe is aligned with the direction of the rebar when a maximum value of the magnetic field is indicated on the screen of the magnetometer.

Mapping rebar in concrete elements may be accomplished by scanning the concrete surface with the probe in different directions (figure 1). The operator marks the concrete surface when the location and orientation of each rebar are determined.⁽³⁾ Some commercially available equipment have the capability to map rebar in concrete decks and display a rebar grid on the screen. The data may be recorded and saved for postprocessing and documentation.

© 2015 N. Gucunski, Rutgers University.

Data Processing 

All data processing is performed internally by the data processor component of the equipment. Thus, no significant data manipulation is required. The data processor is programmed to estimate rebar size and location by comparing detected magnetic field changes with calibrated data from known rebar sizes and known depths inside the concrete being tested.

Data Interpretation 

Magnetometers are simple to operate, with minimal operator training required. However, experience is required to interpret the results in complex cases, such as dense rebar placement. To minimize the effect of the nearby ferromagnetic materials, the maximum depth of a concrete cover should be limited to about two-thirds of the distance between two adjacent bars. When both the rebar size and concrete cover thickness are unknown, the operator must assume a value for one (usually the rebar size) to estimate the other unknown parameter.

Advantages 

Advantages of magnetometers include the following:

• Rapid information about rebar size, orientation, or concrete cover thickness.
• Fieldworthy equipment.
• Easy-to-operate equipment with minimal operator training required.
• Audible output with available headset option for working in noisy environments.
• Some systems applicable to nonferrous metals (e.g., stainless steel bars).
• Relatively inexpensive and commercially available equipment.

Limitations 

Limitations of magnetometers include the following:

• Only effective within the top 4 inches (101.6 mm) of the concrete surface.
• Only the first layer of reinforcing steel may be located and sized.
• Less effective as rebar spacing decreases. (For an accurate estimate of concrete cover thickness, the parallel rebar spacing must be 1.5 times greater than the concrete cover.)
• Ineffective in regions of congested reinforcing steel.
• Difficult data interpretation when there is interference from other magnetic materials such as adjacent parallel rebar, external magnetic fields, and magnetic aggregates in concrete.
• Only effective within a depth equal to two-thirds of the parallel strand spacing for applications to prestressed concrete bridge girder applications.
• Time intensive for rebar mapping.

References 

1. Bray, D.E. and Stanley, R.K. (1989). Nondestructive Evaluation—A Tool in Design, Manufacturing and Service, CRC Press, New York, NY.
2. ASTM D7046-11. (2011). “Standard Guide for Use of the Metal Detection Method for Subsurface Exploration.” Book of Standards 04.09, ASTM International, West Conshohocken, PA.
3. Yu, H.T. and Khazanovich, L. (2015).Use of Magnetic Tomography Technology to Evaluate Dowel Placement, Report No. FHWA-IF-06-006, Federal Highway Administration, Washington, DC. Available online: https://www.fhwa.dot.gov/pavement/concrete/mitreport/mitscan.pdf, last accessed February 28, 2019.