SCAN BEFORE YOU CUT (OR CORE)
By JESSE HODGINSConcrete Construction Equipment Technology
Many troubles can be avoided when modern scanning methods are combined to provide a clear picture of what’s happening below the surface.
The most common reason to perform a concrete scan is to detect objects embedded within the concrete slab, and more often than not, these scans are done because there is a need to cut, drill or core into the concrete. And this is not an unusual task; it happens every day on a vast variety of construction projects.
There might be a need to install utilities like water pipes or sewer lines, or phone and computer lines may be needed for a specific area. In these instances, a concrete scanning technician can be brought in to image the area to identify which objects are buried within the slab, and at what depths.
THE BENEFITS OF SCANNING BEFOREHAND
Existing concrete infrastructure can be riddled with embedded objects. These are often planned, as is the case with electrical conduits, radiant heating lines, post-tension cables, rebar and wire mesh, but knowledge of their exact, precise positioning is rarely available, and even with a layout in hand, there is a possibility that something may have shifted during the concrete pour.
Electrical conduits usually measure between 20 and 50 mm in diameter, and are commonly found within a concrete structure. If one of these conduits is severed, power outages can occur, internet access can go down, and coring technicians run the risk of being electrocuted.
Cutting into post-tension cables, rebar, or wire mesh can cause significant structural damage to a building. In addition to potentially compromising building safety, repairing the damage can result in project delays and rising costs.
Thankfully, all of the above scenarios can be avoided by simply scanning before any cutting is performed. With a detailed report in hand, coring technicians will be able to cut confidently without risking their own health and safety of the structure.
Safety is one thing; efficient project planning is another. In addition to preventing damage to objects beneath the surface, concrete scans can help guide next steps at various stages of construction.
Professional companies, like the one I work for, can produce accurate and detailed surveys, along with accurate as-built drawings. These comprehensive reports can detail slab thickness, reinforcement cover, spacing, rebar diameter and foundations. Structural drawings provide valuable information that allow engineers to make data-driven decisions for planning and construction phases.
Raw data, maps and CAD-ready files can easily incorporate existing as-built slab conditions into structural design and analysis.
The benefits of concrete scanning are far-reaching. Having a scan done prior to cutting and coring concrete can save time, help make informed decisions, prevent costly mistakes, and ensure safe measures are being taken.
METHODS OF SCANNING AND MAPPING
There are several methods used to scan concrete, each with its own set of advantages and limitations. Known throughout the industry as non-destructive testing, the most widely used methods are Ground Penetrating Radar (GPR), Ultrasonic Pulse Echo (UPE), X-ray, Half-Cell Potential (HCP), and Ultrasonic Pulse Velocity (UPV).
Ground Penetrating Radar
The most established method for concrete scanning and imaging, GPR works by transmitting high-frequency electromagnetic waves into the structure via an antenna. It’s quick, efficient and relatively affordable, but its findings require a high level of interpretation.
Ultrasonic Pulse Echo
UPE is able to provide accurate readings at larger depths than GPR. It is an advanced imaging technique that can vividly map out honeycombing, large cracks and voids. This method can determine the thickness of reinforced concrete structures up to eight feet in depth. It runs up against issues when testing on rough or unfinished concrete, however.
Using X-ray to scan concrete results in a direct image of the subsurface. It provides a great snapshot of the objects within the slab but is restrictive, expensive and can be very time-consuming.
The two main limitations to this method of scanning are that X-rays are unable to provide the depth of embedded objects, and access to both sides of the slab is required. X-ray cannot be used with concrete slab on grade.
Half Cell Potential
This type of measurement can gauge the potential for corrosion, and thus, the durability of concrete reinforcement. HCP is advantageous when working with concrete structures like parking lots, concrete piers, docks and tunnels.
Ultrasonic Pulse Velocity
UPV can be used to assess the quality of concrete structures, such as beams, columns, walls and slabs. With this technology, ideally there is access to both sides of the structure, but even if access to the structure is one-sided, indirect transmission can be used.
COMBINING THE POWERS
Although conventional GPR scanning is a proven method for concrete scanning, it has its shortcomings. Take slab-on-grade for example—the bottom layer is often quite weak or can be laying on a range of base materials. When scanning slab-ongrade with GPR, a technician will likely have difficulty determining the depth of the slab or getting a reliable read.
By making use of a combination of the varying methods, based on the project assigned and deliverables required, it is possible to retrieve an accurate reading for slab-on-grade that more conventional single-method scanning cannot match.
When methods are combined, it becomes possible to “see” through wire mesh, and the technician is able to distinguish between specific objects. The postscan report will show if the slab contains electrical conduits, radiant heating lines, rebar, or post-tension cables. It might even have them all!
Jesse Hodgins, C.Tech, has worked with Xradar, a leading provider of concrete scanning services in Canada, since 2017. He recently spearheaded the company’s expansion into Ottawa.
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