Concrete Scanning – Slab-On-Grade
Ground penetrating radar’s ability to visualize what’s inside and under concrete slab-on-grade is one of its biggest advantages over X-Ray scanning technology.
As the name suggests, slab-on-grade is a type of concrete slab poured directly onto the ground. A stone base is often placed between the slab and the native soil.
GPR is a non-destructive detection and imaging method which identifies subsurface elements either underground or within a surface such as concrete. It functions both along horizontal surfaces as well as on walls and other vertical structures.
GPR works by sending a radio signal into a structure. This radio wave bounces off any material it encounters and creates a reading that displays those “bounces” as hyperbolas. An experienced GPR technician interprets this reading to determine the type of material located.
GPR can detect both metallic and non-metallic objects, making it an excellent tool for precision concrete scanning. Because GPR only needs access to one side of a concrete slab or structure to scan the material for anomalies, it can evaluate slab-on-grade. X-Ray scanning, however, requires access to both sides of a concrete structure – one side to place the radioactive material, and the other to place the receiver.
If the slab-on-grade does contain rebar, it will usually have just one or two layers. Less rebar is needed than with an elevated slab. The rebar placement is usually very consistent throughout the entire slab.
If the slab-on-grade does contain rebar, it will usually have just one or two layers. Less rebar is needed than with an elevated slab. The rebar placement is usually very consistent throughout the entire slab.
Conducting a GPR Scan on Slab-on-Grade
When conducting a GPR scan on slab-on-grade construction, a professional GPR technician’s first goal will be to diagnose the slab. The biggest challenge in this process is locating the bottom of the slab.
Locating the bottom of a slab-on-grade can be tricky because there is a stone base and native soil below the slab rather than air. There will be a low level of contrast between these materials on the GPR display.
Sometimes the contrast between the stone base and the native soil below can be a higher contrast than the bottom of the slab, making it even trickier to find the bottom of the slab.
A rule of thumb to follow is that slab-on-grade is typically3”-6” thick. While some examples could fall outside this range, this should bea red flag that triggers further investigation and questioning.
While difficult, finding the bottom of a slab-on-grade could be vital to the success and safety of your project.
- When saw cutting concrete, you need to know how deep to set your blade.
- Voids will typically be located at the bottom of the slab, so identifying the bottom will help ensure that voids are correctly identified.
- It is essential to know whether a conduit is in or below the slab, and have that information marked and communicated to anyone who may be working with or around the slab.
- Thicker portions of the slab can be an indicator that a grade beam or footing is present. A change in thickness can also indicate that a trench was previously made in that area for a utility to be fed through.
These are items that need to be marked out to avoid potentially catastrophic subsurface damage.
Locating Conduits
Main feeds and high voltage conduits will often be, and should be, buried or trenched into the soil, as shown in the photo below. However, they are not always this deep and could be directly below the slab or in danger of being damaged by a saw-cutting blade. All conduits should be marked on the surface, even if they are beyond the depth of the saw blade. In addition, well-trained GPR scanning technicians will always verify with you any additional excavation activities that will take place.
Conduits most commonly lie directly on the grade with the mesh placed on top. This makes them challenging to locate below the mesh. Additionally, they can also blend in with the “bounce” from the bottom of the slab.
Electromagnetic (EM) locators can be used in concert with GPR for concrete scanning. These devices can passively detect live AC power or radio signals traveling along conductive utilities. They can also be used in conjunction with a transmitter to connect directly to accessible metallic pipes, risers, or tracer wires. A current is sent through the pipe or tracer wire at a specific frequency and the resulting signal can then be detected by the receiver.
Using a locator in Power mode will often find main feeds, but it should not be assumed that Power mode will locate all conduits, especially low voltage conduits that are not actively drawing power at the scanning time. These conduits would still be considered live but may not have enough current to be detectable.
Anything that doesn’t fit the pattern should be marked as a conduit, including something at a different spacing, depth, or angle than the reinforcing or if it has a different reflection.
Many PT slabs on grade are stamped, as shown below. If this cannot be found or the grout pockets cannot be seen, the best clue is the spacing being greater than 2’. Most rebar will be more closely spaced than the PT.
Post Tension Slab-On-Grade
Slab-on-grade can also be post-tensioned (PT). These types of slabs are usually found in certain parts of the country with expansive soils. These have single cables in an evenly spaced grid in two directions. These cables do not drape and are often anywhere from 2’-5’ on center.
Many PT slabs-on-grade are stamped, as shown below. If this cannot be found or the grout pockets cannot be seen, the best clue is the spacing being greater than 2’. Most rebar will be more closely spaced than the PT.
With over 400 Project Managers strategically stationed in every major market across the U.S., GPRS is a concrete scanning and imaging company that you can trust. Our suite of infrastructure visualization services ensure we can help you Intelligently Visualize The Built World™ to keep your project on time, on budget, and safe.
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