WRITTEN BY Rachel A. Schmidt, M.S.C.E., P.E. ON May 17, 2018
When I learned that Pie would soon begin using ground penetrating radar systems (GPR), I was excited that we would now have access to this effective, time-saving, and non-invasive imaging technology. I was also happy for our clients. “No more Swiss cheese!” was a thought that came to mind. Of course, that’s a statement that requires an explanation.
Without GPR, assessing the composition of concrete slabs or locating rebar or other items (post-tension cables etc.) embedded within it often requires that one or more holes be drilled into the material. In cases where the information is hard to obtain or the reinforcement is difficult to find, multiple “cores” must be removed. The result brings the image of Swiss cheese to mind.
Neither our clients nor our engineers relish the thought of drilling into a concrete structure repeatedly. But, in some cases, there is no other choice. That is, until now. GPR is proving to be a game-changer for Pie and the building owners with which we work.
GPR, also known as Georadar and ground probing radar, produces images of what is below the surface that the imaging unit rests on or moves across. While it is called ground penetrating radar, it can be used on any surface, including walls and ceilings. GPR works by sending electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum into a material and measuring the return time and strength of the reflected signal.
The return time gets converted by the control unit into depth. The strength or “amplitude” of a reflection is an indicator of the properties of the material or materials that the energy encounters. For example, metals are considered to be complete reflectors, not allowing any of the energy to pass through them. What is called a GPR “scan” is produced by sending a series of pulses into a particular area to produce a more comprehensive image.
There are many types of GPR units. Pie uses what is called a structure-based antenna, which provides the very detailed images needed to assess structures. It does not, however, “see” with great depth and is not designed for other tasks like locating buried utilities, for example.
One of the most common uses of GPR concrete scanning is for locating rebar, post-tension cables, and conduits prior to cutting or drilling into concrete. As one of our licensed professional engineers operating a GPR unit moves it over an area, the position of these items can be noted in real time making the assessment both fast and accurate. In addition to finding rebar, GPR can be useful in locating breaks in post-tension cables within the field of the slab. By tracing post-tension cables down their length, we may be able to identify the location where the tendon has broken to inform the repair team where to open the slab.
Another use of GPR is to check the depth of a concrete slab. The pulses sent by a GPR unit reflect off materials like rebar and are also reflected by the concrete-soil interface as well. This allows Pie to assess the thickness of a slab and check for deterioration of the concrete. GPR can also be used to identify voids below a slab, which can pose a significant risk to the stability of a slab and the integrity of a structure. With masonry, GPR can help us determine if cells are grouted or reinforced, which is a common practice with newer buildings but less so with older structures.
Another valuable use of GPR imaging is to help with code-required special inspections. Building codes require certain types of inspections during construction. If an item that should be evaluated gets covered in some way before the inspection takes place, GPR can help construction crews pinpoint the item to minimize the amount of demolition needed to expose it.
GPR imaging provides a number of benefits. The most important for Pie is the ability to obtain details on the condition and contents of a concrete slab without the need for invasive testing. However, in some cases we do recommend a much less invasive testing procedure to validate the GPR findings. This saves time, effort, and expense for building owners. GPR also increases the safety of an evaluation process, as any type of invasive testing — from drilling to cutting to chipping — comes with inherent risk to workers and nearby property. Also, GPR is safer than X-ray imaging systems and doesn’t require that a structure be evacuated during testing.
Pie’s ability to perform structural evaluations for our clients using this effective, and non-invasive approach enhances our services as an industry leader in forensic engineering. The result is a very useful addition to our technology toolkit and expanded benefits to the clients we serve.