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).
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:
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:
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 firstname.lastname@example.org.