Cover Meter – Eddy Current

Cover Meter based on Eddy current (EC) testing is a non-destructive electromagnetic method used to locate reinforcing steel in concrete and estimate cover thickness and bar diameter. The Cover Meter method is based on electromagnetic induction rather than wave propagation.

An EC probe generates an alternating magnetic field. When the probe is placed near conductive materials such as steel reinforcement, this field induces circulating electrical currents—known as eddy currents—within the rebar. These induced currents create a secondary magnetic field that alters the impedance of the probe coil. By analyzing this change, the presence, depth, and relative size of the reinforcement can be estimated.

Eddy current testing is primarily influenced by the electrical conductivity and magnetic permeability of the reinforcement rather than the concrete itself.

  • Steel reinforcement: Highly conductive and magnetic → strong response

  • Concrete: Non-conductive → minimal direct influence on EC signals

However, factors such as bar spacing, congestion, and proximity of adjacent reinforcement can affect signal interpretation and accuracy.

Cover Meter - Eddy Current
Cover Meter - Eddy Current

How Does Cover Meter Work ?

Key principles governing eddy current testing include:

  • Electromagnetic induction: Alternating magnetic fields induce currents in conductive materials.

  • Impedance change: The presence of steel affects the coil’s electrical impedance.

  • Signal attenuation with depth: Signal strength decreases as concrete cover increases.

  • Calibration-based estimation: Accurate cover and bar size estimates rely on calibration against known reinforcement configurations.

Unlike GPR, eddy current testing does not rely on wave reflections and is less sensitive to dielectric properties of concrete.

 

Relevant Standards and Guidelines

Cover Meter or Eddy current testing for reinforcement assessment is supported by several industry references:

  • BS 1881-204 – Testing concrete: recommendations on the use of electromagnetic covermeters

  • EN 1766 / EN 1504 guidance – Concrete repair and reinforcement assessment

  • RILEM TC Recommendations – Non-destructive evaluation of concrete

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