CONCRETE

IMAGING

MAY 2003 Issue

by

Joel Davis & Hans J. Wonneberger   

The term radar, an acronym for radio detection and ranging, is used to denote both the method and the apparatus for implementing the technique.  Radar science has come a long way since its discovery about 100 years ago. One of its first major applications was detecting ships and planes in WW II. Since then other applications have been developed.    Meteorologists use radar to monitor precipitation and planes can land in fog at airports equipped with radar-assisted ground-controlled approach systems.  More unusual uses of radar are for probing the subsurface.  Archeologists apply this technology for detecting buried objects, caves and gravesites. For more than thirty years, geologists have been able to locate bedrock, voids and ice thickness with radar equipment. 

Radar operates by radiating electromagnetic energy from an antenna.  If the energy encounters a target, the radar detects the reflected energy.  Detection of this energy represents detection of the target.  The antenna picks up the intensity and travel time of the returning signal. The travel time of the signal determines the distance to the object. The fact that different materials reflect the waves at a higher or lower intensity may enable detecting its structure.

Two important features that apply to subsurface radar are that lower frequencies are able to penetrate deeper while higher frequencies produce a higher resolution.  The development of faster computers makes it possible to acquire a larger amount of data, thus enabling more complex applications of this technology, such as the non-destructive evaluation of concrete structures.  Using radar for scanning concrete structures usually requires only a penetration of 0-3 feet enabling the utilization of high frequency (approx. 1000 MHz).  Shorter wavelengths require smaller transmitters and antennas allowing the equipment for concrete imaging to be very compact. The necessary transmitter, antenna, computer and LCD screen can be run by standard power of 110 Volt and can fit into a suitcase-sized container.

The sizes of most concrete scans are 2x2 or 4x4 feet.  It takes approximately 30 minutes to collect and process the data for an image. Concrete scans are not limited to just slabs but can also be performed on walls, columns and ceilings.  A close contact between the transmitter and the concrete is critical for optimal introduction of the signal. This may seem to limit its application, but common materials such as carpet, roof insulation or tiles are easily penetrated by radar waves.

 Radar technology has some advantages over the common X-ray method for analysis of structures.  X-ray requires evacuation in a radius of approximately 70 feet around the work area due to normal precautions associated using X-ray. It can also damage electronic equipment as well as medical supplies, such as blood.  The X-ray requires access to both sides of the object, limiting its application.   It cannot image slab on grade or areas where mechanical equipment are located above suspended ceilings.  X-ray does have the advantage of identifying the size of reinforcements or conduits in the concrete, while radar has the benefit of depth determination.   

The image produced by radar processing software will show the 3-dimensional location of the different objects in the structure and also allow a plan map view at individual depths. This permits instantaneous analysis of the structure to determine post-tension cables and rebar pattern embedded in the concrete. It also indicates changes in the density, such as conduits, voids and thickness. The project data can be stored for future reference or printouts.

 Concrete imaging can be applied in the design phase as well as in the construction phase of various projects. Structural engineers can use this method to investigate the buildings integrity and structural analysis. Mechanical and Electrical engineers can determine locations for potential sleeves.  Contractors can eliminate cutting and damaging structural and accommodative objects.    

There are multiple benefits of utilizing this technology. In the design phase of the project it will give the engineer more accurate and reliable information resulting in a higher level of design quality and avoid costly change orders.  During the construction phase damages resulting in disruption of services and injuries are reduced.  

Radar science has developed highly specialized applications since its invention 100 years ago. Concrete imaging is one of the newest and provides the engineering community with reliable information to ensure a high standard of professionalism and quality.