Understand the Limitations of Ground Penetrating Radar.
High Resolution Concrete and Structural Imaging is a Geophysical method that uses radar pulses to image concrete floors, walls and ceilings.
The radar is used to locate rebar, plastic and metal conduits, locate and estimate depth of post-tension cables, determine cover depth, detect deterioration, discover voids, detect current carrying cables, etc.
Design and Construction professionals involved in concrete/structural coring, cutting and chipping now have a safe and reliable means of working in these environments: High resolution Ground Penetrating Radar (GPR). Construction records for many structures are not readily available and construction often differs from design, resulting in GPR being the only way to assure what is there. GPR can identify both metallic and non-metallic features making it a versatile imaging tool. GPR is now widely used for assessing the interior of concrete structures. When cutting and coring for renovation and repair, avoiding reinforcing materials, post-tension cables and embedded conduits is a priority. Knowing their precise location is critical for the operator and public safety. It also ensures structural integrity.
GPR equipment transmits electromagnetic waves into the subsurface of concrete and detects interfaces between differing materials. The interfaces are identified by changes which occur in the electromagnetic waves. These changes can be voids, reinforcing bars, underground utilities (metal or plastic), conduits, or various other items. The antenna transmits and receives a high-frequency electromagnetic pulse into the study material and records the travel time and amplitude of these impulses. The GPR system records these deflections and digitally processes them. Data output is typically read and interpreted by the use of a color video screen. The data can also be displayed in three dimensional imaging digitally recorded and downloaded to a computer for further processing and interpretation.
GPR scanning, performed by our Certified Technicians, is safe in that it uses no radiation and does not require the evacuation of the public and other personnel. High resolution GPR offers a detailed view of “what is within” and works in a fast reliable manner. This process yields real time results which are not possible with traditional radiographic (x-ray) methods. The GPR system is highly portable and can be set up within minutes, even in locations with limited access. This non-destructive testing technology benefits our clients as it allows them to execute their projects efficiently and effectively, saving them time and money.
The Concrete/Structural Imaging System offers a quick and efficient inspection of slabs, floors, ceilings, roofs, columns, beams, decks, encasements, pylons, balconies, bridges, tunnels and more. It is useful in penetrating and detecting the precise location of utility lines, pipes, conduit, rebar, post- tension cables, and other buried objects as well as determining the thickness of the material examined. The depth range of GPR is limited by the electrical conductivity of the subject material, and the transmitting frequency. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Optimal depth penetration is achieved in dry, cured concrete where the depth of penetration is up to 18 inches. In moist and uncured concrete with high electrical conductivity, penetration is sometimes only a few inches. Data collected is processed quickly and easily. The results produce and display a real time 2-D image and or a 3-D image for a more intuitive approach to data analysis and interpretation.
Multiple viewing options allow the user to move around in the data, many times revealing features not visible in traditional vertical data profiles. A full report of a survey can be provided in two modes of acquisition: Line Scan and Grid Scan. Line scan displays section view data in real time mode. Grid Scan provides a rapid, intuitive collection of grid data that gets processed onboard to create plan maps. Data from both modes can be saved then copied to a compact flash memory card. Depth slice imaging can be provided at one inch intervals and all data can be viewed on a LCD color monitor, capable of displaying data in multiple color palettes.
When Should Concrete Be Scanned?
- New Construction
- Tenant Improvements
- Research and Investigations
- Seismic Modifications
- Surveying Prior to Design
|Locating Rebar||Locating Radiant Heat Pipe||Locating Pipes||Alterations|
|Locating Post Tension Cables||Locating Plastic Pipes||Locating Concrete Footings||Renovations|
|Locating Voids||Locating Conduits||Locating Grade Beams||New Construction|
|Determining Slab, Wall and Ceiling Thickness||Locating A Clear Place to Core||Locating Concrete and asphalt thickness||Tenant Improvements|
|Locating Concrete Deterioration||Locating Fiber Optics||Roadways||Research and Investigations|
|Runways and Tarmacs||Bridge Decks||Tunnels||Seismic Modifications|
|Garages||Retrofitting||Balconies||Surveying Prior to Design|
|Walls and Columns||Towers||Monuments|
|3D Mapping of Interior Obstructions||Architectural Facade Inspection||Hydronic Lines|
Many people refer to Concrete and Structural Scanning as concrete x-ray, but this is incorrect. Concrete structural scanning and concrete x-ray are very different technologies. X-ray technology poses serious health risks, requires stopping work in the area, and is not as fast, accurate, or efficient as concrete and structural scanning.
Concrete and Structural scanning radar technology is non-destructive, non-invasive, and additionally, it is more cost effective compared to the older x-ray technology. This radar technology can also scan slab on grade because only single sided access is necessary. Due to its portability factor, a GPR Concrete/Structure Scan allows us to collect large amounts of excellent onsite real time data in a relatively short period of time with color display and 3D imaging capabilities.
Advantages of GPR Over X-Ray
|Access Required||2 sides||1 side|
|Date Collection Speed||slow||Fast|
|Real-time Inspection Results||No||yes|
|Data Storage Medium||film||digital|
|Large Area Data Collection Method||Step and repeat||continuous|
|Must Process to Get Results||yes||Optional|
|Can Determine X-Y location||with calculations||direct readout|
|Can determine Exact Z (depth) Location||with calculation||direct readout|
GPR is a valuable tool used to penetrate non-conductive surfaces and find varying material compositions. Some benefits of radar include:
- Non-Destructive: Frequency waves inflict absolutely no damage on the subsurface, environment or surrounding people.
- Disturbance-Free: Makes little noise and doesn’t bother surrounding people during use.
- Easily Deployed: Multiple size options that are user friendly, so it can be stored, moved and utilized almost anywhere with few limitations.
- Rugged: Entire construction is extremely durable and designed to endure the everyday riggers associated with its suitable applications.
- Multiple Applications: Ideal in most utility locating, underground locating, structural, archaeological, and concrete applications.
How Does It Work?
Simply put, the GPR shows you what is on the other side. Slowly move the unit over the medium you want to investigate, like a wall, concrete floor, road or any other non-conductive surface. The antenna sends safe ultra wide spectrum RF energy pulses through that medium and back to the antenna to create an image of the subsurface on the operator interface. It’s that simple.
The GPR has the same basic principles as a metal detector. A metal detector sends energy into the earth in up to 17 frequencies. When that energy meets a metallic object, it is translated into a recognizable tone. The GPR sends out thousands of frequencies that return to the antenna and translate material composition definition in the subsurface.
Radar is sensitive to changes in material composition. Detecting these changes requires movement. In the case of air traffic control radar, the targets are moving, so a stationary transmitter works. In the case of GPR, we are looking for stationary targets, so it is necessary to move the radar to detect the target.
Can GPR See Through Everything?
Almost. Radar is the only remote sensing technology that can detect both conductive and non-conductive materials. Although radar can easily see conductive materials such as metal and salt water, it cannot see through them. Also, concrete is conductive when it is fresh, but becomes non- conductive as it cures.
What Can I Find With GPR?
GPR is designed to display differences in material composition. It can be used to locate any object that has a different composition than its surrounding materials. For example, a PVC pipe will have a different composition than the surrounding soil. Voids and excavations that have been filled in will also have different compositions than the surrounding soil. However, it does not know what the actual materials are that it is imaging. For this reason, it is not suited to locating gold, precious gems, and treasures.
Is GPR Safe?
Yes, GPR is extremely safe. It emits less power than a cell phone.
How Deep Does It Go?
The depth of your findings will be determined by three factors:
- Soil Type
- Antenna Frequency
- Size of the Target
The radar signal is attenuated or absorbed differently in various soil conditions. Dense wet clays are the most difficult material to penetrate whereas clean dry sand is the easiest.
Lower frequency antennas will yield greater depth penetration, however, the minimum size of object which is visible to the radar increases as the antenna frequency decreases.
|Antenna||Approx. Penetration in Dense Wet Clay||Approx. Penetration in Clean Dry Sand||Example of smallest visible object|
|100 MHz||20ft (6m)||60ft+ (18m+)||Tunnel @ 60ft (18m) Depth2ft (60cm) Pipe @ 20ft (6m) Depth|
|250 MHz||13ft (4m)||40ft (12m)||3ft. (90cm) Pipe @ 12m6in. (15cm) Pipe @ 13ft (4m)|
|500 MHz||6ft. (1.8m)||14.5ft. (4.4m)||4in. (10cm) pipe @ 4m3/16 in. (0.5 cm) Hose 1.8m & Less|
|1000 MHz||3ft (90cm)||6ft (1.8m)||3/16 in. (0.5 cm) Hose @ 3ft. (90cm)Wire mesh, Shallow|
|2000 MHz||.5 ft. (15cm)||2ft. (60cm)||Monofilament Fishing Line|
The 300-800 MHz antenna’s are most widely used for locating subsurface utilities.
The 1000-2000 MHz antenna’s are most widely used for locating rebar and utilities in walls and floors.
NOTE: In many cases if it is not possible to penetrate to the depth of a buried utility due to soil conditions, but it is still often possible to detect the disturbed soil from the original excavation.
How Accurate Is It?
Generally, GPR will reveal the horizontal positioning of targets in their exact locations; however, there are a number of factors which can affect the accuracy of the depth measurements.
The speed of the radar signal is dependent upon the composition of the material being penetrated. The depth to a target is calculated based on the amount of time it takes for the radar signal to be reflected back to the antenna. Radar signals travel at different velocities through different types of materials. The moisture content of the material also affects the velocity of the signal.
It is usually not possible to know the exact velocity that the radar signal travels through a material, however it is usually possible to estimate this to within +/- 10%. It is possible to use a depth to a known object to determine a precise velocity and thus calibrate the depth calculations. This technique only works well however, when the material being investigated has a consistent composition such as concrete.
When investigating underground, the inescapable limitation is that due to natural differences in the composition of the geological layers, the exact velocity will vary from one point to the next. There are some techniques for modeling the variations in velocity along the path of a survey, however, ultimately these are all estimations and none are completely precise.
What Do You See?
There are three basic types of data that can be generated by operating ground penetrating radar. 2D, 3D, or point data.
A survey always starts with raw 2D data. It is almost always necessary to work with the raw data to at least ensure that data is being collected properly and any processing algorithms are configured appropriately for the medium being investigated. 3D data can be generated by combining multiple sets of 2D data which has been collected in a perpendicular grid pattern and processed with one of several different techniques to make it appropriate for 3D viewing.
Understanding GPR Data
First, it is essential to understand that the radar signal spreads in a fan shape when it is transmitted. Because of this, an object will be visible to the radar before and after the radar is directly over it. This is the reason that a point-shaped object will show up as a hyperbola (arc shape). Since the radar signal will always have the shortest time to travel when the antenna is directly over the target, the centerline of the target will always be at the highest point of the hyperbola in the data.
In the case of tanks and larger targets, the edges can be located in a similar fashion. This type of display is always the starting point with any GPR. There are various types of processing and display techniques which can be applied to this data to tailor it to specific needs.
3D data can be represented in one of 3 ways: Either a 3D alignment of 2D traces, one or more depth slices, or isosurfaces. A 3D alignment of 2D traces requires almost no post-processing thus requiring less time to produce and the least amount of assumptions regarding velocity variation. Depth slices require an accurate model of the velocity of the medium being investigated. The more consistent the material is (i.e. concrete), the quicker and easier it is to achieve this. In some cases, velocity can be measured and sometimes, this can be worked out through a combination of educated guesses and trial and error. Depth slices tend to be good for modeling linear features such as rebar and conduit. Isosurfaces require the most amount of post-processing and filtering. The result can be good for modeling more complex features, but also have a tendency to filter out smaller and fainter features. Sometimes this is desirable, and sometimes it isn’t.
Planning A GPR Survey
3D data can be useful for more complex sites with many targets. The generation of 3D data requires that data be collected on a regular grid in perpendicular directions and also usually requires some degree of post-processing. The amount of post processing required increases as the uniformity of the medium being investigated decreases. Also, there is a practical correlation between the uniformity of the medium being investigated and the clarity of the images which can be expected to be produced through post-processing. Furthermore, usable 3D presentations usually require that data be collected on a much denser grid than is necessary with 2D data presentation. In many cases, the number of survey lines is doubled or quadrupled. For these reasons, 3D data tends to be used more often on smaller scale surveys of concrete floors and walls than it is on large scale ground surveys.
A common misconception is that the size of the antenna affects the amount of area covered. This is not the case. The size of the antenna relates to the frequency of the antenna and subsequently, the depth that it can penetrate. While the signal from a GPR antenna does spread in the direction of travel, the lateral width which it scans per pass is razor thin regardless of the antenna used. Furthermore, targets are most easily identified with GPR when the survey path is perpendicular to the orientation of the target. For this reason, surveys are usually conducted on a grid in two perpendicular directions:
The spacing of the grid is determined based on the size of the targets that need to be identified and what sort of results are going to be produced from the survey. Typical grid spacing’s can be 1ft, 3ft, 5ft, 10ft, 20ft for ground surveys and 1in-12in’s for concrete/structural surveys of walls and floors.
GPR is capable of capturing data at highway speeds, so the speed at which data can be collected along a survey line is limited by only two factors: 1) any time spent interpreting real-time data and/or spent doing on the spot mark out. 2) keeping the antenna in smooth contact with the ground.
For more information and details on how to put CNI Locates concrete and structural imaging to work, call (877) 826-1177