Olsson Associates

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Olsson adds ground penetrating radar to its NDT services

Thursday, October 10, 2013

Michael Sullivan, Non-destructive Testing

Count being able to “see through” concrete as another skill Olsson’s Non-destructive Testing (NDT) technicians have acquired with the company’s recent purchase of ground penetrating radar (GPR).

GPR is an exceptional tool when conducting core drilling and installing anchor bolts within concrete objects, such as concrete floors, concrete walls, and CIP concrete columns. It’s very helpful to technicians, as it allows them to avoid rebar, tension cables, conduit, and voids. In addition to locating, the scanner can also be used as an analytical tool to determine the spacing of objects, their approximate sizes, and depth. It can also be used to assess gaps in bars, non-metallic inclusions, voids, the splice length of objects, and even radiant heating in floors.

For eight years, Olsson staff members have practiced several NDT methods that have included visual, ultrasound, phased-array, magnetic particle, and liquid dye testing. For the past four years, Olsson has conducted radiography testing. Adding GPR not only provides another option to locate objects in concrete, it has numerous advantages over common methods and allows Olsson to offer cost-effective, non-destructive, and accurate ways to investigate concrete. 

Olsson's primary goal in adding GPR was to find a safe, efficient, accurate, and cost-effective method to locate objects in concrete before drilling or coring. Olsson developed the method under the guidance of supplier GSSI.

Advantages of GPR over other testing options include the following:

  • Using GPR over x-ray for locating or investigating purposes is much faster and cost effective.
  • GPR only requires access to one side of the object versus x-ray that needs both sides.
  • GPR is more versatile. It only weighs about three pounds and measures 12 inches by six inches.
  • The results obtained by GPR can be archived and reported using an interfacing program on the operator’s computer.
  • GPR has the capabilities of real-time 2-D scanning and 3-D High Resolution viewing.
  • Using GPR is much safer than using x-rays, as there’s no need to evacuate areas while it’s in use. Therefore, job sites do not need to be shut down, thus reducing down time and odd hours.

The instrument Olsson technicians use is, at this time, the most advanced GPR unit on the market. This unit has the capability of scanning in a high-resolution 3-D mode with up to 16-inch penetration depth. The unit itself has a real-time digital readout on a screen that can be easily viewed while in use.

How Ground Penetrating Radar Works

Basically, each GPR system is made up of three essential components:

  • Control unit
  • Antenna
  • Battery supply

The control unit contains the electronics that trigger the pulse of radar energy that the antenna sends into the ground. It also has a built-in computer and hard disk/solid state memory to store data for examination after fieldwork.

The antenna receives the electrical pulse produced by the control unit, amplifies it, and transmits it into the concrete at a particular frequency. Antenna frequency is one major factor in depth penetration. The lower the frequency of the antenna, the less it will penetrate. A higher frequency antenna will have a higher sensitivity and have the ability of locating smaller targets. Antenna choice is one of the most important factors in survey design.

The instrument Olsson uses has all three components combined with the bonus of a real-time 2-D viewing screen built in for convenience. The antenna uses 2,600 MHz, which is the highest frequency and sensitivity on the market, which allows for the 16-inch penetration depth.

The GPR method works by sending tiny pulses of energy into a material over a given time and recording the strength (after attenuation) and the time required for the return of any reflected signal. A series of pulses over a given time while passing over a single area make up what is referred to as a scan. Reflections, which are referred to as hyperbolas, are produced whenever the energy pulse enters into a material with different conductivity or dielectric permeability from the material it was initially induced to.

The strength, or amplitude, of the reflection is determined by the contrast in the dielectric constants and conductivities of the two materials. This means that a pulse that moves from concrete (dielectric of five to nine, depending on moisture and age) to steel (dielectric of more than 40) will produce a very strong reflection. A pulse that moves from concrete (again, dielectric of five to nine, depending on moisture and age) to conduit (dielectric of three) will produce a relatively weak reflection. The operator will view these signals, or hyperbola, and interpret the findings.

If you have questions about ground penetrating radar, please contact Michael Sullivan at 402.659.4165 ormsullivan@olssonassociates.com.

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