Welcome to another blog about Ground Penetrating Radar. This post is from GSSI Academy and provides GPR theory basics and key concepts of ground penetrating radar. In this post, we discuss how to determine different material types when conducting underground utility locating surveys.
GPR Theory: Determining Material Types
One of the most critical tasks in using Ground Penetrating Radar, or GPR, technology is that of determining materials types for materials in the ground. Given that GPR technology works by transmitting and receiving a high frequency electromagnetic wave through the ground, distinguishing between different materials – typically air, water, and metal – is possible by analyzing the degree of difference in the dielectric constant of the material. Dielectric constant measures how easily radar waves move through a material; for more in-depth information about it, check out this blog post.
Air, Water, or Metal?
For underground utility locating, the three main material types encountered are metal, air, and water. Determining material type is possible by analyzing dielectric change, or the change in how easily radar waves penetrate the ground as compared with the material in the ground. The larger the dielectric change, the stronger the reflection and the brighter the image produced on the GPR control unit. Greater change produces brighter targets on screen, and smaller changes produce dimmer targets. The color of the target onscreen indicates whether the dielectric change is positive or negative.
Metal has an infinite dielectric constant – GPR cannot pass through metal because it’s a perfect reflector for GPR energy – and so it will be shown onscreen as the brightest possible positive dielectric change.
Whereas an air-filled PVC pipe, through which the radar energy moves easily, will demonstrate a negative change, since the dielectric constant of air is less than that of ground materials. The data image below shows a water-filled PVC pipe that clearly identifies the top and bottom of the pipe.
The same is true when identifying underground voids. Below is an example of a well-defined void underneath a reinforced concrete slab with an asphalt overlay.
Finally, in the case of a water-filled PVC target, the screen will show a positive change like that of metal because water has a higher dielectric constant than any ground materials. Determining water targets compared to metal, however, is possible because while water has a high dielectric constant, it is not impermeable to GPR and thus will show up as a dimmer positive target on the screen of a GPR unit.
In this new blog we present a new chapter from GSSI Academy. It is a basic introduction to some of the key concepts of ground penetrating radar. To begin, we discuss the importance of the dielectric constant and methods to determine the right dielectric. Just as Ground Penetrating Radar is also known as GPR, Georadar, and ground probing radar, the term dielectric constant can also be known as velocity and medium type, depending on specific GPR manufacturers.
GPR Theory: Understanding the Dielectric Constant:
GPR works by transmitting and receiving a high frequency electromagnetic wave through the ground, whether it’s soil, concrete, gravel, or other material. Radar waves travel at different velocities depending on what material it’s traveling through; anything ‘different’ that it interacts with will produce a reflection, to be received by the GPR device. Upon receiving the reflection, a GPR device will take note of the amount of time it takes for the signal to return and the strength of that reflection. The system will take these two pieces of information and convert them into a depth reading; in order to do so, they must be programmed with what’s known as the dielectric constant. This constant describes the speed at which electromagnetic waves move through a particular material.
The Importance of the Dielectric Constant:
The dielectric constant is critically important to getting accurate depth readings with GPR systems. Dielectric constants, also known as relative dielectric permittivity, are measured on a scale of 1 to 81, where 1 is the dielectric constant for air (through which radar waves travel most quickly) and 81 the constant for water (through which radar waves travel most slowly). Metallic objects exist outside the scale, since radar waves cannot penetrate them at all; they are described as having an infinite dielectric constant.
In order to convert the variable that is produced by a radar reading – time – into the desired product of the reading – depth – GPR systems must be accurately programmed with the correct dielectric constant for the ground material in question. This enables GPR systems to produce meaningful depth readings, instead of timed reflection readings; these time reflection readings are transformed in an equation with the proper dielectric constant. As a result, depth readings from GPR systems are only as accurate as the dielectric constant with which they are programmed for each particular ground material. There are three key methods for determining an accurate dielectric constant, each with their own benefits and drawbacks.
Methods of Determining the Dielectric Constant:
One way of determining the dielectric constant is by utilizing a published reference, available from GSSI in manuals and products documentation, as well as online. Using a published reference is the quickest and easiest way to obtain a dielectric constant because it doesn’t require field analysis if ground material type is known. It’s not the most accurate, though, because published references are averages and not site specific.
A more accurate way to determine the constant can be with utilizing a method known as migration, or hyperbola fitting. Hyperbola fitting relies on data gathered from pipe-like targets in the ground; individual GPR units are capable of calculating dielectric constants for different soils simply by fitting a hyperbola tool to hyperbola data gathered in real-time from one of these pipe-like targets. This is the most consistent way to determine the dielectric constant in the field.
The final method for determining the dielectric constant is through what’s known as ground truth. In this case, a GPR unit is positioned over something for which the actual depth is known; by programming the unit with this known depth (known from a prior dig or a chart), the unit can calculate the dielectric constant and effectively gauge depths for other objects in the field.
Load testing is a common practice among bridge engineers for the assessment of bridge safety and serviceability. Diagnostic load testing is one type of load test methods that can be used either as a means for estimating the load carrying capacity of an in-service bridge or as an acceptance test before the bridge is put into service [Olaszek et al, 2014]. This article provides a quick review to Diagnostic Load Testing of Bridges and its application to bridge condition assessment.
Diagnostic Load Testing
Bridge Load Testing-Diagnostic Load Tests can be used to evaluate specific response characteristics of the bridge such as lateral load distribution and secondary stiffening effects and to validate the load rating analytical models.
Diagnostic load testing usually involves installation of a variety of sensors, including:
strain gauges, acceleration sensors, displacement sensors and highly sensitive rotation devices.
Sensors are normally installed on structural components. The test is generally performed with controlled load situations. Applied loads are typically limited to legal load levels or load levels that are known to be safe for a particular structure (historical data).
The main goal of the diagnostic load testing of bridges is to identify structural response (deformations, strains, etc.) for a given load condition. The measured responses are compared with those that are derived from the analysis of the theoretical models, for the same loading condition. Head to head comparison is then used as the basis for validating the theoretical model and defining how accurately the model simulates actual load paths [Santini Bell et al, 2013; Shahsavari et al, 2019].
The field portion of diagnostic load tests can be executed with low-cost because they can be done quickly and with minimal impact on traffic. The load testing vehicle is typically a legally loaded dump-truck and the instrumentation is installed in temporary fashion which can usually be installed in a day with a small crew. Actual tests are conducted with brief road closures or moving blockades to minimize conflicts with the traveling public.
Diagnostic Load Testing for Structural Model Calibration
The ability to calibrate a structural model is the primary concept behind diagnostic load testing. The basis of comparison can be static and/or dynamic structural responses generated by a known loading condition. Static measurements are often global responses such as midspan displacement and girder rotation at an abutment or local member cross-section responses such as axial or flexural deformation obtained from strain measurements. Dynamic measurements usually consist of acceleration, which are further processed to generate structural mode shapes and natural frequencies.
The calibration of the Finite Element (FE) models using the monitoring data helps reduce possible errors induced by simplifications or inaccurate assumptions made in model development. Wrong assumptions are mostly due to insufficient knowledge about structural details, materials properties, inevitable simplifications of the details or ignorance of the non-structural components, and boundary conditions. In the model calibration procedure, prior to refining the wrong assumptions made in the model developments, it is essential to determine the important parameters causing a deviant response in the model. The refinement attempts are required to change the recognized influential parameters and minimize the errors until the desired accuracy is achieved.
Performing load tests with moving loads can be very efficient and minimize impact on traffic, but it requires that truck position be monitored and recorded along with all the structural responses. The key part of diagnostic load testing is an apples-to-apples comparison of data. This requires a convenient method for extracting field data for specific truck positions corresponding to the simulated load cases from the model. The procedure is based on direct comparison of responses for many sensor locations and many different load cases. The load test results are a series of response histories as a function of load position.
Relative Services to Bridge Condition Assessment
When a bridge experiences an unexpected occurrence of accidental events, such as truck accident, a major concern for bridge managers is effective and informed operational decision-making with respect to the remaining capacity of the bridge. Given the load rating reduction estimated by the bridge analytical model, an integrated decision-making protocol combining different approaches will be beneficial to bridge managers for making decisions on the damaged (pre-repair) state of the structure with respect to different damage scenarios. A calibrated structural model helps establish a baseline to determine the structural system response for performance prediction and investigate the bridge decreased load carrying capacity under different damage scenarios [Shahsavari et al, 2020].
Monitoring a bridge’s structural response has the potential to:
detect the presence of structural changes for condition assessment,
inform the bridge manger to assist in daily operational decision-making,
validate the structural design assumptions,
refine a structural model of the bridge to be used for performance prediction [Read More].
Unlike conventional load test practices which have the potential to induce damage into bridge components (e.g., PS/C girders) and reduce its serviceability, diagnostic load testing of bridges appears to be promising for determining the bridge maximum safe load capacity through analytical investigations. The structural models calibrated through controlled load tests would be beneficial for operational decisions such as those relating to maintenance scheduling and overweight vehicle permitting.
Diagnostic load testing would measure structural performance at the applied test loads but not provide any indication as to how the bridge would behave at higher loads or what the effective factor of safety would be for legal post loads. Creating a calibrated structural model that can predict the impact of operational and environmental variations on bridge performance provides support for better management strategies such as fatigue performance prediction, load rating deterioration and real-time condition assessment.
To obtain a realistic capacity, an alternative approach would be a combination of diagnostic and traditional load testing procedures in such a way that loading a bridge up to its serviceability limit without inducing any damage to the structure or cause any reduction to service life. The use of Structural Health Monitoring (SHM) sensors being deployed at critical locations may be warranted to examine bridge performance over time. The measured parameters include, but not limited to, changes in stress in critical regions, changes in deflection, identification of crack activity, etc.
Moisture testing devices are an integral tool for the restoration industry. Though a team of professionals can employ every piece of equipment on a job site, unless the damaged or dampened building materials are free from moisture to a required standard, the job will not be satisfactory for the long term.
Tramex moisture meters make it possible to identify and track the source and extent of moisture problems without destroying the materials being tested. We design our products to fit your needs because we know working conditions on the job site, such as tight spaces, difficult to reach places, corners, curves, and textures, are challenging. These areas can only be correctly measured with the precise moisture meter.
We also know, the sheer variety of materials required to be tested demands using the right restoration moisture meter. How do you know you have the moisture meter for the restoration project? Here are a couple of things to consider:
How quickly you can get a measurement
The type of scale(s) on the meter
The depth moisture can be detected
How do you determine the best moisture meter for your next restoration project? Here's a quick overview.
The Moisture Encounter ME5 provides an instant measurement and evaluation for a wide range of building materials
The Concrete Moisture Encounter CME5 is a non-destructive meter for measuring moisture content instantly in concrete slabs
A digital version of the CME4 handheld the CMEX II provides instant and precise measurements in concrete and other floorings (incorporating the optional Hygro-i plug-in ports transforms it into an exemplary all-in-one instrument)
The MRH III is a handheld digital moisture meter calibrated for most building materials (the optional plug-in Hygro-i2 makes it suitable for water damage restoration, flooring, checking indoor air quality, inspectors and pest companies)
A mobile and non-destructive impedance device, the Dec Scanner is excellent for surveying instant moisture of roofing and waterproofing, checking for water leaks, and integrity tests
For a pocket-sized meter, the Skipper Plus is non-destructive and is a comprehensive, safe method for detecting excess moisture in boat hulls and fittings
The handheld PTM 2.0 digital, pin-type meter takes exact measurements in wood, drywall products, and comparative WME (Wood Moisture Equivalent) values in wood by-products as well as more than 500 wood species
The options of different moisture measurement meters from TRAMEX make it a versatile tool for restoration experts. It is precisely what professionals need to shore up the exactness required in many contracts.
With in-built quality you can trust, you can count on an investment in TRAMEX by having ownership of a quality product to bring you serviceability for years to come.
Because of the versatility of the GEM-2 instrument, our customers are constantly finding new ways to use it. One application is the measurement of sea ice thickness in the polar regions. Both airborne and ground-based EMI (ElectroMagnetic Induction) have been used before to estimate sea ice thickness. Because of its moderate length (<2 metres) the GEM-2 can easily be mounted in a kayak or sled for towing.
There many parallels between ice measurements and a traditional ground conductivity survey. The simplest assumption for data analysis is a layered model, with the seawater conductivity much greater than that of the sea ice, snow and air above it. (> 2,000 mS/m vs. < 100 mS/m). But the picture is more complicated when ice-platelet clusters form under the ice. These are clumps of crystalline ice which float in the seawater on the underside of the ice layer; their density and depth, and hence bulk conductivity, are highly variable. They will cause errors in ice thickness measurements if not taken into account.
Scientists from the Alfred Wegener Institute of the Helmholtz Centre for Polar and Marine Research in Germany have developed calibration and deployment methods for characterizing sea ice shelves in the Antarctic. In particular, they have been able to exploit the GEM-2's advanced capabilities to perform more complex characterizations than simply measuring ice thickness.
Scientists conducted expeditions to various locations with and without platelet formations. They used the GEM-2 to record response at several frequencies, for different heights and correlated these results with physical measurements at that location, the 'ground truth'. They found that having independent measurements for both in-phase and quadrature at each frequency reduced the ambiguity of the recorded signal, leading to more accurate calibrations. Also they found the higher frequencies (63 kHz and 93 kHz) most useful in characterizing the platelet layer. These types of measurements are not available from more primitive single-frequency/fixed frequency instruments.
Many free-air calibrations were carried out to estimate any long-term drift and temperature dependency of the GEM-2; temperatures ranged between -10 C and -24 C . They did not observe a strong temperature dependence of the zero-level offsets for any frequency, and the instrument performed well. The cold affected the capacity of GEM-2 batteries; this was solved by keeping a spare battery inside someone's coat while the other battery was in use. The GEM-2 comes with a spare battery, and a battery change takes less than a minute, so near-continuous surveying is easy.
The GEM-2 itself is made of industrial-grade components rated from -40 C to +85 C, so operating temperatures are not normally an issue.
This post is based partly on the paper "Towards an estimation of sub-sea-ice platelet-layer volume with multi-frequency electromagnetic induction sounding" by Priska A. Hunkeler, Stefan Hendricks, Mario Hoppmann, Stephan Paul And Rüdiger Gerdes. The paper is at https://epic.awi.de/id/eprint/36936/
Other photos courtesy of Alfred Wegener Institute ( www.awi.de )
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When drying concrete after water intrusions it is important to monitor and measure the moisture content of the concrete in two phases: First during the drying phase; and again after the drying is complete. This allows the restorer to establish valuable knowledge of how the drying process is progressing and, once the drying process is complete and the concrete has been brought back to pre-loss conditions, decide what mitigation, if any, will be required before reinstalling a floor covering. In order to monitor and measure the moisture content of the concrete in a meaningful way the restorer needs to understand the different test methods, the meaning of their results and how they relate to the restoration industry compared to the flooring industry for which these test methods have been developed. This is an important point to keep in mind because the testing methods prescribed were designed for the flooring industry to test the drying process of newly poured concrete and establish when it is dry enough to receive a floor coating or covering. This is different to the restoration industry as the goal here is to dry the concrete back to pre loss conditions. Equilibrium Relative Humidity as per ASTM F2170, Calcium Chloride vapor emission testing as per ASTM F1869 and non-destructive Electrical Impedance measurement as per ASTM F2659 are the most commonly specified tests for measuring the moisture in concrete in the United States.
F2170 and F1869 are both considered quantitative tests whereas F2659 is considered qualitative and while the differences are about how the tests are perceived rather than what they are actually measuring, this will mean that most flooring manufacturers will require that either F2170 or F1869 are carried out before a floor can be installed. In fact, both F2170 and F1869 both measure the water vapor, not the moisture content, in concrete and as such they are very impractical tests to carry out during the drying phase as they require the building to be in service condition for at least 48 hours before the testing can begin. This is due to the fact that changing environmental conditions will affect the relationship between the water vapor and the water. Most tests which have been developed for the flooring industry are designed to measure the construction moisture within the concrete and not the moisture from intrusion or other external sources.
As such it is important to be able to distinguish between the different sources when inspecting concrete after water damage, and the best way to do this is to look at the results from a variety of test methods. The restorer needs to be able to identify background moisture which could either be from construction moisture still in the slab, or moisture which is still entering a slab from beneath if there is an insufficient sub floor sealer, or from above if there are dew point issues or leaks in the building. Without this understanding it is common for restorers to attempt to dry against nature and waste a lot of energy drying moisture which will possibly return after the drying has been completed. This is especially important if the moisture is coming from beneath the concrete due to the absence of a sealer.
By using an electrical impedance device, which gives instant and repeatable results, to map and monitor the moisture during the drying phase, a restorer is able to focus on the changes in readings rather than the readings themselves. At the beginning of the drying process the moisture content readings will be high and should rapidly reduce as the drying progresses. As the drying progresses the difference between readings over time will decrease and this will help determine when the drying is complete. The mapping of these readings will also indicate where to test further once the building is back in an in-service condition, using either Relative Humidity testing as per ASTM F2170 or Vapor Emission testing as per ASTM F1869. If the impedance device used indicates a percentage moisture content value (MC%) of the slab then this information can be very useful when further testing with Equilibrium Relative Humidity testing.
For example. Relative Humidity testing per F2170, when used as a stand-alone test, is prone to giving false positive readings and possibly false negative readings due to the quality of the concrete;- false positive readings due to the concrete having less air movement when it is of a high quality; and false negative readings due to uncured materials such as salts lowering the equilibrium relative humidity. If the results of the non-destructive impedance test and the Relative Humidity test do not concur then it is possible to do further simple testing which can complete the information needed. The combination of ambient testing, surface concrete temperature testing, in situ RH testing and non-destructive impedance testing will cover the majority of the testing needs, with calcium chloride testing only used when the results of the others do not concur. The use of multiple, yet simple testing methods allows for a complete picture of the moisture condition of the slab. The approach of marketing one test method over another has, in my view, done a disservice to the industry. The combination of tests helps draw a much better and more complete picture, as long as it is clearly understood what exactly is being measured with each test method and how they can be affected by the different ambient conditions that arise during the entire restoration process.
Obtaining as much information about the roof as possible in advance is invaluable. A set of roof drawings or plans make moisture mapping much easier and a knowledge of the construction will make the job of calibration much faster.
If a set of drawings or roof plan is not available, prepare a plan and report sheets for each section being surveyed or, better still, use the Tramex Moisture Mapping App. To perform a non-destructive flat roof moisture inspection to ASTM D7954 with the Dec Scanner, first ensure that the surface is free of debris and is dry from rain or dew. Aggregates may be left in place but should be dry and of uniform thickness.
Once calibration and range selection is complete, proceed by moving the Dec Scanner along the imaginary gridlines in a continuous, systematic manner. The Dec Scanner is designed to cover the width of an average roll of roofing felt, making it simple to follow a systematic row-by-row methodology. Mark areas of concern onto the roof plan/Tramex Moisture Mapping App as well as directly onto the roof surface if required. Marking the surface directly can be helpful for finding the precise location for core samples later. Areas where the roof has non-uniform composition or thickness, such as areas which have been recovered or seams, should be tested and noted separately as they may provide different results. By continuously checking this way, a complete picture of the moisture condition can be built up quickly. An area of up to 100,000 sqft can be reasonably covered in one day.
Selected suspect wet areas should be confirmed by core sampling using gravimetric analysis in accordance with ASTM C1616. It is permitted to check core samples immediately after extraction with a pin-type resistance meter such as the Tramex PTM. An identified area of high moisture may also be checked with extended insulated resistance pins before core sampling, by first puncturing the surface of the roofing membrane with the Tramex Hole Punch and then with the Tramex hand held resistance probe – together with 7" or 15" insulated pins – insert the pins into the insulation for a further relative reading. These readings should be recorded on the report sheets to correlate with gravimetric measurements at the verification stage.
The Crosshole Seismic (CS) system and method determine shear and compressional wave velocity versus depth profiles. From these measurements, parameters, such as Poisson’s ratios and moduli, can be easily determined. In addition, the material damping can be determined from CS tests. These dynamic soil and rock properties are often utilized for earthquake design analyses necessary for certain structures, liquefaction potential studies, site development, and dynamic machine foundation design. The most complete version of this downhole system, as manufactured by Olson Instruments, is comprised of a borehole source capable of generating shear and compressional waves and a pair of matching three component triaxial geophone receivers. These instruments are lowered to the same depth in boreholes set at ~ 10 ft (3 m) apart in a line. The instruments are coupled to the side of the grouted borehole inclinometer casing, allowing for the detection of shear and compressional waves as they pass between the receivers.
The Downhole Seismic (DS) investigations are similar to CS investigations, but require only one borehole to provide shear and compressional velocity wave profiles. The DS method uses a hammer source at the surface to impact a wood plank and generate shear and compressional waves. This is typically accomplished by coupling a plank to the ground near the borehole and then impacting the plank in the vertical and horizontal directions. The energy from these impacts is then received by a pair of matching three component geophone receivers, which have been lowered downhole and are spaced 5 to 10 ft (1.5 to 3 m) apart.
■ Real-time waveform display while testing
■ Thin layers, which are often invisible to surface methods, can be detected with CS/DS investigations
■ Acquisition and processing software are easy to use, yielding fast and accurate results
■ CS method is the most accurate method for determining material properties of rock and soil sites
■ Accuracy and resolution for the CS test method are constant for all test depths, whereas the accuracy and resolution for
the DS surface method decreases with depth
■ Sources and receivers can be oriented with inclinometer casing dummy probes
■ P-SV source used in CS tests can impact in the up, down, and radial directions
■ Correlation between CS and Spectral Analysis of Surface Waves (SASW) tests on soil sites showed that the values from both tests typically compare within a 10-15% difference
The CS investigation requires drilling of two or more (ideally three) boreholes cased with PVC or slope inclinometer casing
for deeper borings up to 328 ft (100 m), and grouted in accordance with ASTM standards to ensure good transmission of wave energy. The boreholes are typically 4-6 inches in diameter cased with 2.32 to 3 inch (59 to 76 mm) I.D. casing. The testing is simplified if inclinometer casing is used rather than normal PVC pipe. Typical distances between adjacent in-line boreholes are on the order of 10 ft (3 m). The testing is performed by lowering both the source and receiver(s) to an investigation depth, firing the source, and recording the energy with the receivers.
The DS investigation requires drilling a single borehole with similar specifications as listed above, except that only a single grouted 2 inch (50 mm) to 3 inch (76 mm) I.D. PVC casing is needed. The testing is performed by lowering the receiver(s) to an investigation depth, impacting the coupled surface plank, and recording the energy with the receivers.