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Safe and Smart Concrete Scanning

Ground penetrating radar (GPR) is commonly used to image the thickness of entire bridge decks from the road surface and can be used safely in public spaces

Safe and Smart Concrete Scanning

Cutting, coring or drilling into concrete on a project site is risky business. Hitting an electrical line, conduit or other buried feature within concrete is not only extremely dangerous, it can also be costly, leading to work delays, emergency repairs and a loss of production. Even when as-built drawings exist, they may not be accurate or detailed enough to pinpoint the location of hidden conduits, electrical lines, rebar and junction boxes.

Today, most project owners are aware that scanning concrete is a vital first step to ensuring safety on any sized project where cutting, coring or drilling are involved. However, the traditional method of X-ray scanning poses its own set of risks and challenges, and the method is largely being replaced by ground penetrating radar (GPR). In addition to providing a safer way to scan, GPR technology also brings many other advantages to the project site. 

Reducing risk and project delays
Concrete X-rays for construction projects first began to surface in the early 1980s and gained significant traction over the following decade as a necessary risk reduction technique. Construction operators transitioned from relying on potentially outdated records or incomplete as-builts to leveraging a clear picture of embedded features. 

However, a major challenge with the technology is that the radiation emitted poses a risk to human health, which is particularly significant when the work is being carried out in a densely populated area, such as a shopping mall or office building. An 80- to 100-foot clearance is generally required, warning signs and barriers must be erected, and the clearance area has to be policed to ensure that no one enters without authorization. Often, a building may need to be evacuated prior to performing the scan, or the work has to be performed at night. This can result in project delays and disruption to business continuity, especially in light of the fact that an X-ray scan can take up to five hours to complete. 

Unlike X-ray, ground penetrating radar (GPR) transmits harmless electromagnetic waves into the ground or structure and then analyzes the reflected energy to create a profile of the subsurface features. It can assess slab thickness and detect features deep below the surface that show the presence of utilities, rebar, mesh and post tension cables. It can also provide information on concrete conditions, such as deterioration in reinforcement materials, void spaces and delamination. No personnel evacuation is required and it can be performed during regular business hours. 

Projects show versatility of GPR
In Ottawa, a drain was being installed at the City’s Public Library, and a GPR scan was performed in the electrical room and underground garage to prepare for coring. As the library is a public space in close proximity to a sidewalk, applying X-ray technology would have required a complete evacuation of the building and surrounding area. Instead, utilizing GPR allowed the day’s activities to proceed as scheduled without disruption. 

GPR played an integral role in construction works at two large shopping centres in Ontario. At Yorkdale Mall, GPR was applied in support of a large-scale expansion. It was used to image concrete and identify rebar on top of a parking garage, as well as to locate underground plant, including a fire main, to aid the design of a canopy entrance. 3D depth slices of the data were created to show the precise location, depth and position of the buried structures. 

At Sherway Gardens, new signs were being installed at the intersection of two busy corridors and information was required on the location of rebar and conduits in the floor. A GPR scan was carried out during regular business hours without disrupting surrounding activities. 

Flexible enough for any size project
Another advantage of GPR comes down to accessibility. X-ray techniques require access to both sides of a concrete object, often limiting projects to elevated concrete slabs or surface walls. As such, the technology is ineffective for slab on grade and foundation walls, where corrosion, voids and utility placement are most common. 

In contrast, a GPR scan can be conducted on a single side of the concrete. This is a major advantage for projects that require data regarding embedded features, such as clearing a drill area for anchor bolts in a concrete floor where conduits and rebar may be present. Small areas can be scanned using a hand held device that combines a screen and antenna in a single unit allowing technicians to maneuver confined access areas with ease. 

An example of this occurred just outside of Renfrew, Ontario where GPR was used to scan a 12-foot x 3-foot surface area on three Hydro Dam towers to determine the location of structural reinforcement and utilities within the base of the towers. The towers were being replaced and the project owner needed to map out the embedded rebar as well as any active or empty conduits prior to construction. The scan identified the presence, location and position of both rebar and electrical conduits, and the information was provided to the client as a geophysical map along with selected profile sections. X-ray technology would not have been an option for this project due to the small, restricted surface areas.
 
A final thought
In light of the advantages of GPR imaging, is there ever a situation where X-ray might be the better choice? The answer is yes, but rarely. The one advantage of X-ray technology is that it is capable of yielding a very high resolution image of the embedded features in concrete. If a project requires precise visuals showing the physical condition of a buried object or a detailed comparison between similar objects, X-ray should be considered. GPR is more suited to highlighting the location, position and depth of buried objects. And since most coring or drilling projects simply require the clearest path in the slab for cutting, the images yielded by GPR generally suffice.

Another consideration is the thickness of the survey area. X-ray is only effective for concrete thickness of 20 inches or less, whereas GPR can be adjusted to accommodate various depths and is commonly used to image the thickness of entire bridge decks from just the road surface. As such, a project owner must consider the level of detail required to achieve the project’s goals in addition to access, safety, and thickness of the area being scanned.

Other things to think about include budget and project schedule. An advantage of GPR data is that it can be revealed in real-time; technicians can view the results of the scan on screen and immediately mark the concrete slab. The data can also be presented in a number of ways such as plan, profile or 3D view. 

As GPR data requires an analysis component, the key to deriving maximum value from the data always comes down to ensuring the interpretation is conducted by a certified and experienced GPR professional.

Company info

325 Matheson Blvd E
Mississauga, ON
CA, L4Z 1X8

Website:
multiview.ca

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