Concrete Scanning – Slab-On-Grade
GPRS is the nation’s largest private subsurface locating company. GPRS is the company provides concrete scanning, utility locating, leak detection, and video pipe inspection services. Our dedication to safety has helped us achieve an over 99.8% subsurface damage prevention scanning rate on hundreds of thousands of scanning and location projects. GPRS has unparalleled accuracy, and as a company, we want to raise the industry standard. To help push the industry forward, GPRS is releasing articles about our training methodology.This article will review our training documentation and scanning methods in as lab-on-grade scenario.
Slab-On-Grade Construction
A slab on grade is a concrete slab poured directly onto the ground. A stone base is often placed on the ground below the slab, as shown here.
Since the slab is usually well supported by the soil orstone base, there is not usually a need for structural reinforcing. Concrete isstrong when in compression, it is well supported by the ground and does notneed steel reinforcing. Therefore, itcan sometimes contain no steel but often includes welded wire mesh. Wire meshis not structural and serves only to prevent cracking.
Most slabs on grade contain wire mesh but will sometimes bereinforced with rebar. They will usually have just one layer, sometimestwo. Less rebar would be needed than anelevated slab, so the spacing will commonly be 12”-24”. The rebar placement will tend to be veryconsistent throughout the entire slab.
Diagnosing Slab-On-Grade
The first goal of a slab on grade scanning project will be to diagnose the slab. What type of slab is it, how is it reinforced, can the bottom be detected, and how thick is it? Finding the bottom of the slab can be tricky with a slab on grade because there is soil below the slab rather than air. The amplitude (strength) of the reflection at the bottom of the slab depends on the contrast between the concrete and whatever is below. There will always be high contrast between concrete and air for an elevated slab, but concrete and soil or concrete and a crushed stone base can be very similar. To make it even more complex, the contrast between the stone base and the native soil below can be a higher contrast than the bottom of the slab.
While the numbers below can vary, these are some common examples:
The slab below would be diagnosed as a slab on the grade based on at least these 4 things:
- The only reinforcing that can be seen is wire mesh. Wire mesh is not structural and cannot alone support an elevated slab.
- The thickness of a slab on grade will always have some variance. It is poured on dirt/stone, which will never be a perfectly flat surface. Up and down movement in a layer is a clue to slab on grade.
- The thickness lands within the expected depths of 3”-6”. There is a negative reflection in the data image below the mesh.
- The layer that appears to be the bottom of the slab is close to the depth of the mesh. The mesh will have to be shallower than the slab, but the mesh will typically be close to the bottom, at least in some parts, although it can move up and down.
Slab on grade will typically be 3”-6” thick. These slabs can certainly be outside these ranges, but if a thickness is found outside these ranges, it should be a red flag that triggers further investigation and questions. Is there is a reason for this slab to be very thin or very thick? Am I sure that I’m finding the thickness correctly? ShouldI get a second opinion?
How to find the bottom of the slab is essential for multiple reasons:
- Informing the client. For a saw cutting job, it may be necessary to inform the client, so they know what they are dealing with and how deep to set their blade. For a concrete analysis job, the project may depend on the accuracy of the slab thickness.
2. Voids. Voids will typically be located at the bottom of the slab, so identifying the bottom will help ensure that voids are correctly identified
3. Conduit depths. It is essential to know whether a conduit is in the slab or below the slab and mark and communicate accordingly.
4. Thicker portions of the slab can be a clue to a grade beam or footing that would need to be marked out, or a change in thickness can indicate a trench that has been previously made that would indicate a utility below.
In the example below, the slab does not contain any steel reinforcing. The bottom of the slab should be assumed to be the first layer of reflection that has movement from being poured on soil. The reflection from the bottom of the slab may not be as bright as the bottom of the stone base.
Below, the slab contains rebar, not mesh. The bottom of the stone base gives a powerful reflection, and the bottom of the slab is subtle. If the slab appears to be very thick and the bottom seems to be more than about 2” below the reinforcing, red flag!Investigate further, look closer, and get a second opinion.
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 to the right. 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. Also, always verify with the client whether there will be excavation beyond the saw cutting depth.
Conduits are most common to lie directly on the grade with the mesh placed on top. This makes them challenging to locate below the mesh, and they can also blend in with the reflection from the bottom of the slab.
Using a locator in Power mode will often find main feeds, but it should not be assumed that Power 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. See utility locating training documents for further training on using passive modes and other locating tips.
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.
This should be an easy one. There is something that breaks the pattern below the mesh. The tricky part is knowing how wide to mark it or how many conduits might be there. There may be multiple conduits here and would need to be marked wide. A cross-polarized scan may give a better view of these.
In this example below, the deeper items are known to be conduits. Slabs on grade rarely have two layers of rebar, and if they did, the two layers would be consistent spacing. Also, the reflection of the items on the left is negative (black) rather than positive and looks different than the rebar. These would be marked as conduits regardless of whether a power reading could be detected or whether these run on angles or in line with the rebar. These conduits are near the bottom of the slab but are in the slab.
Like the previous image, most slabs on grade do not have two layers of rebar. If it did, then the lower layer should be a reasonably even spacing. The spacing in slab on grade tends to be much more consistent than an elevated slab since the load should be spread evenly through out. The spacing and depths of the lower items change, and the reflections are different on some of them means that they should all be marked as a conduit. In addition, if there was a bottom layer of rebar, it would need to be running in both directions. A scan in the opposite direction would reveal that these are not rebar in a grid/mat, and again, these would all need to be marked as conduits.
Verifying Mesh Overlap
Wire mesh never needs to be marked out as long as it can be confirmed to be wire mesh. Every sheet of wire mesh will overlap with the next sheet. We typically refer to this as mesh overlap. Seeing overlap in an evenly spaced pattern is one of the clues to confirming wire mesh reinforcement. However, overlap can be problematic because it is tough for the GPR to penetrate through the tight spacing at the overlapped area, and conduits can be hard to see below the overlap. Overlap can also appear to be a conduit due to the extra reflections and could give a false positive.
Overlap can appear to be a different spacing and reflection, so, when in doubt, it should be marked as a conduit. However, marking mesh overlap as a conduit every 5’ throughout a building would cause a problem for the client, who would have to stop saw cutting and chip out the concrete around a conduit marking, only to find wire mesh overlap rather than a conduit.
The overlap should not be marked as a conduit as long as it can be confirmed as an overlap. For overlap to be confirmed, these 4 conditions should all be met:
Each time you encounter what might be overlap:
- Scan the opposite direction to confirm the overlap is running in both directions.
- The spacing should still be a perfect 6” for each item. The overlap makes this difficult to determine, but each item should fall into the spacing for one of the mesh sheets from one side of the overlap or the other.
- Depth should match in each direction.
Mesh should disappear.
Step 2 is to confirm overlap is to verify that each hyperbola lands perfectly in the mesh spacing (usually 6”). If not, it would need to be marked as a conduit regardless of the results of the other steps.
Step 3 is to confirm that the depths match in both directions. The end of a mesh sheet could rise and appear to be a conduit. Even if it meets the conditions for Steps 1and 2, it would need to be marked as a conduit if it is not at the same depth as the mesh in both directions. Since the mesh is welded together, the depths must match in both directions.
Step 4 is to cross polarize. All of the mesh should disappear, even those that are double and brighter from overlap. Anything that still appears when cross-polarized should be marked as a conduit even if it has already passed steps 1-3.
Post-Tension Slab-On-Grade
Slab on grade can also be post-tensioned. These are usually found in certain parts of the country with expansive soils. The most common types are shown below. 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.
Slab on grade can also be post-tensioned. These are usually found in certain parts of the country with expansive soils. The most common types are shown below. 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.
Standard Operating Procedure and Markings
Saw Cutting SOP
Pre-Scan
1. Complete work authorization and JHA in Infor
2. Request as-built drawings from the client
3. Walk the site with the client
4. Confirm the depth of the saw cutting and the total depth of excavation
5. Determine whether a report is needed and whether GPR data will need to be saved
6. Update JHA
Scanning
1. Collect long scans to evaluate the slab
2. Identify the bottom of the slab within the GPR data
3. Collect scans at 90° angles to the building
4. Draw all findings on the surface
5. Check findings with 45° angled scans
6. Cross polarized scans
7. Check accuracy with reference dashes
8. Use a pipe locator to trace conduits/utilities
9. Passive sweep
10. Locate utilities if necessary
11. Mark scan boundaries
12. Document findings
Post-Scan
1. Walk the site with the client
2. Job Summary Report
Final markings for saw cutting jobs should look similar to this. Spray paint is preferred when ever it is allowed by the client. Single spray paint lines can be used for anything that appears to be a small, single conduit. If the size appears more prominent or unknown, a wider marking should be used. The lines can be dashed, as shown here, or solid. Depths should be clearly labeled when possible. Depths should be rounded up (shallower) for a safety buffer for saw cutting applications. They can be written as a range for an added buffer and to help to communicate that the depth may not be perfectly accurate, such as 2”-3” or ∼2”-3”. The depth should also be remarked wherever there is a depth change.
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GPRS specializes in ground penetrating radar, video pipe inspection, and mapping and modeling services. Our Project Managers have the equipment and expertise to handle all subsurface challenges presented. GPRS does this by utilizing various equipment paired with their industry-leading SIM process.
