TABLE A: Dielectric Constants
For further technical reading.....
FIGURE 2: Red Line Graphics
Interestingly, elephants communicate utilizing compression waves in the infrasound range (less than 20 Hz), humans utilize waves in the acoustic range (between 20 Hz and 20 kHz) and bats utilize waves in the ultrasound range (greater than 20kHz).
Concrete X-Ray - Ground Penetrating Radar (GPR) - FAQs
FIGURE 3: 3D Image
Why should we consult with JEC Services for Ground Penetrating Radar (GPR) structural scanning?
1. Cost Savings and Superior Equipment. We utilize the Hilti PS 1000 scanning system, which is extremely efficient and advanced compared to alternative scanning equipment. The system can display 3D images in real time and has a six-inch scan width, exceeding most competitive equipment available. The scan width efficiency translates into saving our customers tens of thousands of dollars in labor costs while also providing superior service. The PS 1000 has the ability to show 2D and 3D images in real time, similar to the following:
What is an electromagnetic wave?
The signal that is transmitted into the ground or structure is an electromagnetic wave. A classic definition of an electromagnetic wave is a synchronized oscillation of electric and magnetic fields that propagate at the speed of light through a vacuum. Radar waves are electromagnetic waves with frequencies and wavelengths similar in scale to radio and microwaves (see FIGURE 4 below). A wide range of electromagnetic waves with different frequencies and wavelengths make up the electromagnetic spectrum. Visible light is an electromagnetic wave that falls within the spectrum. An illustration of the spectrum follows:
FIGURE 1: A-Scan and B-Scan
Concrete scanning, concrete x-ray, ground penetrating radar, GPR, 3D imaging, locate rebar, pt cables, post-tension steel, conduit
Jensen Engineering & Construction Services, LLC
What is the difference between Ground Penetrating Radar (GPR) and x-ray?
The technology is similar, but there are several differences in the type of equipment used. These include required safety precautions, cost of service, time needed for performing the service, time needed for communicating results and site restrictions based on equipment size and resolution. GPR is superior to x-ray for all of these differences except for resolution.
Common Radar Terms
- Clutter – Clutter refers to wave signal data returned to the receiver antenna by targets not intended to be investigated. For example, if an operator is locating embedded post-tensioned steel, clutter data from mild reinforcing congestion, excessive concrete moisture or concrete voids can return unintended data that decreases the clarity of the data relating to post-tensioned steel. What is considered clutter for one operator or scan could be considered an intended target for another operator or scan.
- Interference – Interference refers to unwanted return signal that can be present naturally or can be man-made such as by other radar equipment on the same frequency. Since the unwanted return signal origination is difficult to discern, the returned data is difficult to interpret.
- Jamming – Intentional transmission of electromagnetic radiation to obstruct the reception of signals by other antenna systems.
- Mirror Reflection Echoes – Multiple reflection echoes that are caused by the radar wave being reflected from a large flat surface and subsequently reflecting off other nearby targets. This can also lead to inaccurate location data.
- Multiple Reflection Echoes and Ghost Objects – Signal returned from a real target by way of reflection from some other object in the radar path. These echoes appear in the data at inaccurate locations due to the altered travel path of the radar wave. This means the returned data can show an object where no object actually exists. This is referred to as a ghost object.
- Noise – Noise is returned random variations in the radar signal. This can be from sources internal to the radar system or external to the radar system, such as signal returned from the travel medium including natural thermal radiation reflection. Returning signal noise increases as distance traveled increases, so noise introduces a radar range limitation.
- Targets – Any object that reflects sufficient signal to be evident for data interpretation. Targets can be intended or unintended.
- Total Return Signal – The total return signal is equal to the clutter signal + noise signal + intended target signal.
What is the difference between Ground Penetrating Radar (GPR) and Ultrasonic Testing (UT) or Impact Echo (IE)?
GPR waves are an electromagnetic phenomenon. As the name suggests, the waves have an electric and magnetic component and respond to the electric and magnetic properties of the materials the wave comes in contact with. These waves can travel through space and are not limited by the need of a travel medium such as the ground, water or air. For correct interpretation of GPR data, a characterization of the electromagnetic background medium is necessary. This involves some estimation of the dielectric permittivity and of the magnetic permeability which account for radar signal losses. This topic can get complicated, but for reference and basic understanding, typical material dielectric constants and velocity (meters/nanosecond) for radar waves follow:
The Hilti System can also show three dimensional (3D) images in real time that can be rotated to visualize drilling, coring and saw cutting in low risk locations. A 3D example follows:
Our ability to create and modify electromagnetic waves haves generated a large number of useful technologies improving our world immensely. Imagine a world without radio, military defense radar, cell phones, microwave ovens, light bulbs, medical x-ray machines or nuclear energy. None of the above would be possible without an understanding of electromagnetic waves. Although these waves can be created intentionally by humans, the waves occur naturally as well. Our increasing understanding of electromagnetic waves allows us to improve upon things like GPR and x-ray scanning for the construction industry. A person cannot have a commanding understanding of GPR without understanding electromagnetic waves.
Should I use Ground Penetrating Radar (GPR) on my project?
GPR can identify and locate mild reinforcing steel, pre and post-tensioning steel, metal and plastic pipe, conduit and live wires embedded in concrete and masonry columns, walls, slabs and footings. The equipment can detect embedded objects up to 18 inches deep. GPR technology is also capable of determining the size of structural components given access to a single side, identifying voids below at-grade slabs as well as distinguish between grouted and ungrouted masonry blocks.
Performing GPR on your project prior to drilling, coring and cutting reduces the risk of unintended damage to the structure. We regularly hear about damage that could have been avoided if GPR scanning was completed prior to penetration of a structure. The cost of repairing damage can easily exceed $10,000 and much more if the repairs delay a construction schedule. Contractors and engineers regularly use GPR to:
- Locate and identify embedded objects in concrete and masonry in new and existing buildings to safely accomplish modifications to structures
- Estimate demolition costs
- Determine existing structural component capacities
- Inspect structures to confirm successful project completion
- Investigate causes of distressed structures
Please contact us to discuss how GPR may benefit your project.
Some GPR equipment can convert B-Scan images into graphics such as red lines on a screen intended for inexperienced users . The graphics show a plan view and depth views on the bottom and right sides. An example follows:
FIGURE 5: Sound Spectrum
How do you select an appropriate electromagnetic wave frequency and wavelength for your project?
Selection of an appropriate electromagnetic frequency and wavelength depends on many factors such as the material scanned, distance and size of the intended targets. Low frequency waves have larger wavelengths and travel through materials more easily than high frequency waves with shorter wavelengths. However, shorter wavelengths provide higher resolution data and can therefore detect smaller targets. As such, there is a tradeoff between wave travel distance and size of detectable targets. As the distance increases, the size of the detectable target also increases. Put another way, small objects become more difficult or impossible to detect as distance from the target increases.
How does Ground Penetrating Radar (GPR) work?
GPR is essentially an electromagnetic wave (signal) that is transmitted into the ground or structure and the characteristics of the returning signal are interpreted to determine useful information. GPR data collected by a receiving antenna is characterized by polarity, amplitude and travel time. The data collected at a single transmitting location can be illustrated by a line graph representation of the data. The line graph is referred to as an A-Scan (see FIGURE 1 below). When the transmitting antenna is moved over an area, multiple line graphs are recorded for each location of transmission. This creates several A-Scans that can then be converted into an image called a B-Scan or radargram. The B-Scan image can then be interpreted for useful information related to the scanned area such as identifying embedded target types and locations. An elevation view illustration of the A and B Scans follows (A-Scan on the right and B-Scan on the left):
FIGURE 4: Electromagnetic Spectrum
What is Ground Penetrating Radar (GPR) technology?
Ground Penetrating Radar (GPR) is a phrase that describes radar waves utilized to penetrate the ground. The terminology has come to describe radar waves intended to penetrate construction materials and structures as well. Other common terms for GPR service include non-destructive testing, non-destructive examination, structural imaging and concrete scanning. Essentially, a signal is transmitted into the ground or structure and the characteristics of the returning wave are interpreted to determine useful information about the travel medium, the location and size of intended objects (targets). This is the basic premise for ground penetrating radar and concrete penetrating radar.
For many people, the most familiar application of radar is within the aerospace industry in which radar is used to locate flying objects. The physical phenomenon of GPR technology is the same. The main differences are in variations of the transmission power, wave frequency and wavelength.
What are the health risk difference between Ground Penetrating Radar (GPR) and x-ray waves?
Low frequency waves, such as radar waves and microwaves, are less hazardous to biological cells than high frequency waves such as x-rays and gamma rays. Low frequency, low energy waves do not contain enough energy to dislodge electrons from atoms and so the effect to cells is energy absorption. Although more research regarding health effects are being completed, biological cell damage from low energy waves would be in the form of heating, such as microwave ovens and overused cell phones. High frequency, high energy waves such as x-rays and gamma rays carry enough energy to dislodge electrons from atoms or molecules, thereby resulting in atomic-level damage to biological cells. Non-ionizing radiation is the terminology for electromagnetic radiation that does not carry enough energy to discharge electrons, while ionizing radiation is the terminology for electromagnetic radiation that does carry enough energy to dislodge electrons.
The topic of safety for GPR and x-ray waves is complex.
Please contact us to understand the health risks in more detail.
What deliverables will be provided following Ground Penetrating Radar (GPR) services?
Typical deliverables include marking findings (object locations) directly on the structure using chalk, crayon or tape, verbal communication of the findings while onsite using 3D images as necessary and a report that documents each scan location. The report will show the scan location in relation to the overall structure, a photograph of the markings, an electronic illustration of the GPR data collected and a brief discussion of the purpose of the scan and the objects identified. JEC will also accommodate custom requests.
Are Ground Penetrating Radar (GPR) waves and x-ray waves hazardous?
Radar waves associated with typical GPR equipment are not considered hazardous. X-rays have potential to be harmful and therefore require certain safety measures such as closing down jobsites within a clear distance in all directions, additional operator training and personal protective gear. X-rays can result in higher resolution images than GPR waves and therefore improve the quality of the data collected. However, the additional safety measures result in additional cost which is why, when feasible, the construction industry prefers GPR over x-ray.
As you can see in Table A, air has a dielectric constant of 1 and water has a constant of 81, which illustrates that radar signal losses are significant in water. This is the reason that GPR results improve as freshly poured concrete dries.
In contrast to electromagnetic waves, sound waves such as ultrasonic or impact echo waves are mechanical phenomenoa. Mechanical properties of a travel medium, such as density and elasticity affect the speed of these waves. Essentially, a compression wave propagates through a material by transmitting a vibration of the material itself similar to a sound wave traveling through the air or submarine sonar waves traveling through water. Sound waves require a travel medium with mechanical properties to propagate and therefore cannot travel through outer space like electromagnetic waves. There are similarities between GPR and sound wave propagation and data collection, but GPR is directly affected by materials with electromagnetic properties and sound waves are directly affected by materials with mechanical properties. Interpretation of GPR data based on density and elasticity will not yield accurate results. Similarly, interpretation of sound waves data based on dielectric permittivity will not yield accurate results. They are very different physical phenomena. A misunderstanding of the phenomena would result in misinterpretation, miscommunication and inaccurate conclusions. A person cannot have a commanding understanding of GPR and sound waves without understanding the differences between electromagnetic waves and mechanical waves.
For reference and contrast to the electromagnetic spectrum, an illustration of the sound spectrum follows:
Typical competitor equipment shows images similar to the following:
How much does Ground Penetrating Radar (GPR) cost?
GPR is less expensive than x-ray and less expensive than costs associated with construction project delays from unintended damage. For straightforward scanning conditions, an expert GPR operator can scan 40 locations or 100s of square feet in a single day with a single scanner. Challenging project conditions including difficult access and concrete components highly congested with steel reinforcing may reduce the scanning pace.
Request a cost estimate for your project here: Request A Quote
Some general contractors and concrete cutting companies have purchased Ground Penetrating Radar (GPR) scanning equipment and use internal staff to scan. Should we consider this?
This is a business decision that you should make as a company by reviewing the cost effectiveness of the decision. Some costs to consider:
- Employee compensation and benefits, turnover, training, continuing education and licensing
- Equipment purchase, storage, efficiency upgrades and calibration certificates
- Equipment not in continuous use
- Maintaining knowledge related to industry changes in GPR equipment, standards and licensing requirements
GPR technology has made significant improvements in recent years, which has resulted in manufacturers asserting that their equipment can be utilized by inexperienced personnel. In our experience, relying on inexperienced equipment users who are interpreting raw data and images produced by software algorithms they do not entirely understand has led to inaccurate conclusions and costly mistakes. GPR is not a simple technology. It is a highly specialized field, requiring a deep understanding of the underlying science for successful application. When used cautiously and with direct supervision of a highly qualified expert, the risk of damage to structures can be significantly reduced. Success rates of 99.9% can be achieved. Based on the difference in results achieved between utilizing a GPR expert and an untrained operator, we believe that this sales pitch by manufacturers is irresponsible.
At times, GPR evaluation alone is insufficient for a project’s need and additional data collection is necessary to significantly reduce the risk of damage. Drilling, coring or cutting near columns in flat-plate construction and through seismic resisting elements such as shear walls should not be completed without expertise and approval from an engineer. An experienced and responsible GPR service provider can inform you when GPR evaluation alone is not sufficient for your application.
Given the high-cost for misinterpreting GPR structural data, we believe this justifies the cost of utilizing highly qualified GPR experts, whether the person is internal to your company, or is hired as a sub-consultant. Contact us to have a board licensed Professional Engineer (PE) oversee or perform GPR structural scanning on your project. PEs are required by law to meet distinguished levels of professional and ethical responsibility.
A video advertisement of our x-ray equipment can be viewed here: YouTube - Hilti PS 1000 Scanning System
2. Exceptional Equipment Experience. JEC equipment operators have years of experience utilizing non-destructive testing equipment while conducting forensic engineering investigations. This includes data collection, interpretation and conclusions that are defensible in court. There is currently no standard level of training, experience, examination or licensing required for GPR technicians resulting in a wide range of quality amongst service providers. Until minimum training requirements are established, you can ensure a high-level of qualification by utilizing consultants with years of experience and a Professional Engineer license.
3. Structural Engineering Design Experience. Our engineers have completed engineering designs for several mild reinforced concrete, masonry and post-tension structures. In addition, they have excellent experience providing construction administration, structural observation and pre-pour deck-walk engineering reviews. This gives us a comprehensive understanding of structural load paths, structural components and building configurations allowing us to know when and where scanning results are the most critical to the structural integrity of a building.
4. Licensed Professional Engineers. JEC equipment operators are licensed Professional Engineers that practice in strict accordance with the laws that dictate professional and ethical responsibility.
5. Understanding of Risks and Liabilities. Our engineers have extensive experience evaluating structures where contractors were unsuccessful providing post-tension cables that met project specifications. The review of post-tension stressing reports and evaluation of post-tension structures have increased our understanding of the risks and liabilities associated with structural scanning services.
These are just a few reasons your company would benefit from having JEC engineers oversee or perform structural scanning on your project. We are available to answer questions you may have and would look forward to the opportunity to work together.
Please contact us to have a GPR structural scanning expert on your project.