Electromagnetic Field Testing
Non Destructive Testing that relies on the application of an electromagnetic field provides useful data on building assemblies.
A Magnetometer (or Pachometer) can be used to detect the presence and location of metal elements such as wall ties, studs, and reinforcing bar. A Covermeter is a more sophisticated device which induces eddy currents in metal and then records the relaxation field. Covermeters can be used to estimate the size of reinforcing bar and the distance of the bar from the surface (known as the "depth of [concrete] cover"). Similar devices are used to detect the thickness of coatings on metal substrates. Using such devices to determine the extent of corrosion of embedded metal is known as the Mass Magnetic Probe method.
Electrical Impedance (EI) meters place two conductive plates on a surface and detect phase changes due to difference in the dielectric permittivity of the assembly. Since moisture produces dramatic changes in dielectric permittivity, EI meters are commonly sold as "moisture meters," but EI meters can also detect voids, changes in materials, and even act as metal detectors. Because the technology uses two plates there are older literature references to "electrical capacitance" meters but industry practice and an ASTM standard (D7954) have obsoleted the older terminology.
Ground Penetrating Radar (GPR) (sometimes referred to as "impulse radar") uses an antenna to transmit a high-frequency electromagnetic pulse into a material or assembly; the echoes are typically received with a second antenna. Short duration pulses result in a broad frequency spectrum; the frequency of the antenna is the center frequency of the broadband pulse spectrum. Lower frequency pulses can travel miles, higher frequency pulses can only travel inches; lower frequency pulses are longer and have less resolution, higher frequency pulses are shorter and have higher resolution. GPR can be used to detect geological formations as well as a host of embedded objects and conditions including conduits, metal, water, and air. Changes in echoes from objects at a known distance can also reveal information about the condition of the soil, stone, or concrete that the pulse is travelling through.
Other NDT devices that utilize electromagnetic fields include Concrete Resistivity meters, Half-Cell Potential probes, and DC Pinhole (or "holiday") detectors.
In Electromagnetic Field Testing, electromagnetic waves are produced from a source and picked up by a transducer for display or recording.
Electromagnetic waves are produced by changing magnetic and electric fields, as described by Maxwell's equations. Constant, static fields are called DC (from "direct current'), and varying, dynamic fields are called AC (from "alternating current").
AC waves are sometimes transformed from the time domain to the frequency domain, where they are viewed as components of the electromagnetic spectrum. The electromagnetic spectrum spans from x-rays, to ultraviolet light, to visible light, to infrared, to microwaves. Portions of the electromagnetic spectrum are used for other NDT techniques, such as Radiographic Testing, Eddy Current Testing, and Infrared Thermographic Testing. The tests described in this section use either DC or microwaves.