Non Destructive Testing (NDT) refers to a wide group of techniques used in science and industry to determine properties of materials without significantly damaging the material.  Most NDT methods are based on sound or electromagnetic radiation principles.  At TEC, we typically use NDT to determine the as-built construction of structural and non structural concrete and masonry.  Our techniques are used to measure the following: 

  • Location, size, and quantity of pre-stressed, post-tensioned, and conventional reinforcing steel embedded within concrete  
  • Concrete cover associated with steel reinforcing
  • Corrosion of reinforcing steel
  • Member thickness
  • Concrete strength
  • Density
  • Discontinuities with concrete such as honey combing and cracking
Our Primary NDT Techniques Include The Following:

GPR

The ground penetrating radar (GPR) method uses electromagnetic energy, emitted at regular intervals by an antenna to map subsurface features and discrete objects.  The electromagnetic pulses are reflected where changes in electrical properties (e.g. dielectric constants) occur.  In the case of collecting GPR data on concrete structures, these reflections may occur due to the presence of rebar, cracking, voids, or the far side of the structure in question. 
 

Cover Meter

A cover meter (sometimes called a pachometer) is a nondestructive testing device for locating steel in concrete and masonry structures.  The principle of the meter is that the field of an electromagnet is affected by the presence of steel.  The meter is based on a pulse-induction eddy current design and consists of two parts, a locating probe and the meter itself that has a display giving the dimension of the cover over the steel or a signal strength reading.  The locating probe of the cover meter is a rectangular encapsulated unit containing a directional search coil.  An audible indication is also provided by the meter that gives a guide to signal strength and location of embedded steel.

Impact Echo

Impact-echo testing is a special form of nondestructive stress wave testing of concrete.  The usual method of nondestructive stress wave testing is the ultrasonic pulse velocity technique described in ASTM C 597.  ASTM has voted approval on a standard test procedure for the impact-echo technique but it has not yet been published.  The test procedure utilizes stress waves which propagate through the concrete with the initiation and sensing of the stress wave being on the same surface of the concrete.  The method can be used to determine the thickness of a concrete section, or if there are internal defects or cracks in the concrete.  The uniformity of the concrete quality can be evaluated by this procedure.

Ultrasonic Pulse Velocity

The nondestructive ultrasonic pulse velocity technique measures the velocity of stress waves traveling through concrete.  Ultrasonic stress waves are initiated at one face of a concrete member when a transducer is excited by high voltage pulses.  This ultrasonic stress wave pulse travels through the concrete and is received at the opposite face of the concrete member by a similar transducer.  This signal is amplified and can be displayed on an oscilloscope.  Time measurement circuitry in the V-meter determines the elapsed time for the stress wave pulse to travel through the concrete.  The pulse velocity of the concrete being tested is determined dividing the measured path length by the elapsed time measured by the V-meter.

For an investigation, the velocities of stress waves pulses propagating in the compressional mode are measured.  The compression wave velocity through the concrete is related to the elastic modulus and the mass density of the concrete as well as other factors.  The elastic modulus is related to the compressive strength of the concrete and is dependent on the constituents of the concrete as well as the general in-place quality.  The calculated pulse velocity is dependent on the actual wave pulse path length traveled through the concrete.  If the path length is interrupted by a crack, zone of honeycombing or some other discontinuity, the wave energy will attempt to circumvent the discontinuity.  The resulting time delay will indicate lower velocities.

In summary pulse velocity data, when properly analyzed, are indices of the strength, density and quality of the concrete.  This technique is described in ASTM Method C 597-83 (Reapproved 1991), “Standard Method of Test for the Pulse Velocity Through Concrete”.

Half-Cell Potential Testing

Half-cell potential testing is a method used for determining the corrosion activity of reinforcing steel in hardened concrete.  The standard test procedure is presented in ASTM C 876, “Half-Cell Potentials for Uncoated Reinforcing Steel in Concrete.”  The test method is based on the principal that during the electrochemical corrosion reaction an electric potential difference is generated in the reinforcing steel.  The potential difference between the reinforcing steel (anode) and a copper sulfate half cell (cathode) gives an indication of the presence or absence of corrosion activity in uncoated reinforcing steel in concrete.
Cover Meter Cover Meter
Ground Penetrating Radar (GPR) Ground Penetrating Radar (GPR)
Impact Echo Impact Echo
Ultrasonic Ultrasonic