Ground-Penetrating Radar (GPR) Concrete Imaging Technology
A Brief Overview of Ground-Penetrating Radar (GPR) Technology
During our concrete imaging and scanning surveys, we harness the same ground-penetrating radar (GPR) technology used to conduct our utility locating surveys, the only difference being that we replace the mid frequency 400 MHz antenna and large survey cart with a higher frequency antenna and small handcart. We currently use the GSSI StructureScan module, which is manufactured by Geophysical Survey Systems, Inc. of Nashua, NH. To understand the theory behind GPR technology, please visit our Ground-Penetrating Radar (GPR) Technology UtilityScan Module page.
GPR Technology With The GSSI StructureScan Module
The StructureScan module is comprised of a SIR-3000 or SIR-4000 control unit, a handcart with a survey wheel, and a high frequency (high resolution) antenna, enabling us to scan a variety of concrete structures in real-time without the need for shutting down the work area or gaining access to the bottom of the slab.
High Resolution Antenna Frequencies
We use four different antennas, each of which has a specific application purpose and depth penetration capability.
1000 MHz - Deep Antenna
- Used to scan concrete slabs-on-grade
- Penetrates to a depth of 3 feet
- Can detect utilities lying below a slab
1600 MHz - All-Purpose Antenna
- Used on a wide variety of structures
- Penetrates to a depth of 18 inches
- Resolves objects with a spacing of 2 inches or more.
2000 MHz - Palm Antenna
- Used to scan tightly spaced areas
- Penetrates to a depth of 12 inches
- Resolves objects with a spacing of 1.5 inches or more.
2600 MHz - High Resolution Antenna
- Used to scan thin concrete structures
- Best for resolving multiple small targets
- Penetrates to a depth of 8 inches
- Resolves objects with a spacing of 1 inch or more
GSSI StructureScan Mini System
In addition to the multi-purpose control unit and handcart, we have a self-contained unit that can be used to scan in areas that are difficult to access with the larger equipment.
2700 MHz - All-Purpose Antenna
- Hybrid of 1600 and 2600 MHz antenna shown above
- All-in-one unit
- Quick set up time and easy to use
- Lightweight and easy to transport
- Best for scanning walls and ceilings
Determining the Feasibility of Conducting a GPR Concrete Imaging Survey
Prior to performing test scans, a visual inspection is performed to analyze the following three factors:
- Surface Coverage - The surface must be smooth and free from dirt, debris, or detritus, ensuring that it is physically possible to move the antenna over it in a fairly smooth fashion.
- Homogeneity of Subsurface Material - The subsurface material must be homogeneous; that is, it must be compact enough to not have air spaces within it, which inhibit the signal from penetrating all the way through the substrate.
- Survey Area Size - The area must be large enough to collect a sufficient amount of continuous data to make interpretation of the data possible. This is important because the success of GPR depends on one’s ability to see contrasts in the data. If the survey area is located within a small utility closet or close to a wall, collecting sufficient data may not be possible. As a good rule of thumb, we usually need at least a 2' x 2' area to obtain useful data.
Running Test Scans & Calibrating the Depth Scale Using the Ground Truth Method
Test scans are performed by making long passes perpendicular to one another across the concrete structure in question. If the results are good, scans are then performed across a target of known depth such as an exposed rebar or conduit to calibrate the depth scale on the y-axis of the data screen profile.
Setting Up an Orthogonal GPR Survey Grid
To set up an orthogonal survey grid, the location of known objects (from an as-built or test scans) are used as a reference point. Specifically, the grid is oriented so that it allows for the greatest number of scans to be performed perpendicular to known objects, the direction they must be crossed to produce the narrowest hyperbolas possible. In most situations the orthogonal grid can be imaginary, but if documentation is required, an alphanumeric grid must be created with chalk. An example of typical GPR survey grid is shown below.
The spacing of grid line intersections is dependent upon the purpose of the survey and the size of the survey area, and this decision can only be made through an analysis of multiple factors and field experience. In most concrete imaging surveys, a 3' x 3' survey grid will be created around the proposed core drilling location in question. A brief rule of thumb to determining the spacing of an imaginary grid is to perform the scan, stop at the end of the grid, pivot the wheels of the handcart, turn 180° until the wheel overlaps the track of the previous scan, and then perform a scan parallel to the first.
Advantages of GPR Technology for Concrete Imaging
- No site hazards or need to close off work areas as with radiography (X-Ray).
- Data is obtained in real time.
- Access to the underside of the slab is not always needed.
- In optimal conditions, can locate metallic targets (rebar, mesh, post-tension cables, pre-tension cables, conduits, pipes, etc.) and non-ferrous targets (plastic conduits and pipes).
- Can see multiple layers of rebar within concrete structures.
- Measures slab thickness.
- Detects voids.
- Pinpoints deteriorating areas of concrete.
Limitations of GPR Technology for Concrete Imaging
- Sometimes it is not possible to see non-ferrous targets.
- Cannot always see lower layer of rebar in multiple layered structures that are thicker than 8 inches.
- A thick or tightly spaced upper layer of rebar or mesh can block the signal from penetrating any further than this layer.
- It is impossible to see a particular piece of rebar or conduit if it is located directly below a larger or similarly sized metallic object.
- Requires a homogeneous subsurface material.
- The signal will not penetrate very far into the concrete if it is wet from having been recently poured.