Ultrasonic Crosshole Test or Corsshole Sonic Logging (CSL) is a widely used nondestructive test for assessing Pile Integrity and quality control of drilled shafts (piles). CSL test is a variation of Ultrasonic Pulse Velocity. In theory, the pulse velocity in concrete is a function of the modulus of elasticity, density, and Poisson’s ratio. The uniformity of concrete shaft can be assessed by measuring the pulse velocity.
In crosshole sonic logging, a number of access tubes are installed inside the reinforcement cage prior to placing concrete. For the purpose of testing, the tubes are filled with water to provide acoustic coupling to the ultrasonic transducers.
The very basic format of the test involves at least two parallel tubes installed. Two transducers (a transmitter and a receiver) are lowered down to the bottom of the shaft, and are pulled up. The transit time of an ultrasonic pulse through the concrete between the tubes is measured by a data logger. As UPV transducers are pulled up, the UPV is measured and recorded versus the elevation. This provides engineers with a vertical profile of signal transit time. A modern CSL system uses an automated depth encoder to precisely record the position of the probes inside the access tubes.
Capabilities of Ultrasonic Crosshole Test
Crosshole Sonic Test is a great test for identifying Anomalies in concrete shafts such as soil inclusion, poor quality concrete (low density, low modulus), and major voids. In general, the transit time of ultrasonic pulse between every two access tubes is measured using a high precision data acquisition system. The resolution of the scan along elevation can be controlled by the rate of the withdrawal of the transducers in the tube (normally performed form the bottom to top). The resolution of the scan at each elevation depends on a number of parameters, such as the selected frequency (pulse wavelength), the number and horizontal spacing of access tubes. A modern CSL is equipped with transducers that can operate between 25 to 50 kHz, allowing detection of defects as small as 2.5” to 4” (in each horizon). It is recommended to keep the spacing of the probes to about 12’ (3.6 m).
Analysis of CSL Results
Two main method are often used by geotechnical engineers around the globe: 1) The Waterfall method, and 2) First Arrival Time (FAT). In addition, 2D/2D tomography maps can be generated to illustrate the location and extent of damage at each elevation.
Waterfall Method for Analysis of CSL Test Results
In the Waterfall method, the results are presented as a profile based on complete ultrasonic pulse time histories. As a general practice, the positive peaks are presented by a dash line, whose width matches that of the original peak, and each negative peak is illustrated as a gap, creating a dashed line. These measurements are presented as a series of dash-lines along the shaft elevation, which allows for a more detailed review of the wave train, even when the first arrival cannot be detected. Figure shows an example of the waterfall method.
First Arrival Time for Analysis of CSL Test Results
In this procedure, the arrival time of the first peak in the ultrasonic pulse wave train is (First Arrival Time, FAT), and the overall amplitude of the early part of pulse is measured. A practical challenge in the FAT method is distinguishing between peaks in the wave train, and the noise in the construction site. This is often performed through filtering out the noise by assigning a threshold value.
Crosshole Tomography or tomographic analysis is an alteration of the Crosshole sonic logging that will provide image of an anomaly withing the shaft. Such image shows the shape and position of the affected zone. If regular CSL (horizontal setup) shows there might be a defect in the shaft, additional tests can be performed with transducers offset vertically to provide angled pulse paths. This will enable geotechnical engineers in locating the area and size of the defect.
It is important to note that the Crosshole Tomography comes with all limitations related to effectively measuring the travel path of the pulse (which will not be straight line in most cases). The positioning of the access tubes might change along the pile length. If the CSL test results does not show any anomalies in the shaft, performing tomography test does not provide any extra information.
Key Advantages of Crosshole Sonic Logging
The interpretation of the test results in CSL test is relatively easy (compared to other tests, such as the low strain pile integrity test).
In theory, there is no limitations with regards to shaft length or diameter. The test results are not affected by skin friction, variation in soil stiffness, or damping characteristics.
The test can be further enhanced by implementing a diagonal positioning of the probes (in which the elevation of transmitting transducer has an offset with the receiving transducer). This would enable engineers in creating 2D and 3D maps of defects inside shaft.
Disadvantages and Practical Limitations
The main disadvantage of the test relates to the fact that most access tubes are installed inside the steel reinforcement cage. This would limit the amount of information that can be obtained from concrete area that lies beyond the the steel cage (which happens to be the most problematic area in most cases).
CSL test does not provide information about small horizontal defects.
Another practical consideration is the installation of tubes. As the shaft diameter increases, the minimum number of access tubes is also increased. This will increase the number of paths that need CSL measurement (labor intensive and time consuming).
# what is crosshole sonic logging? # non destructive testing
The StructureScan Mini XT software update, version 1.5.0, allows users to manually adjust their gain – the artificial addition of signal in order to counteract the natural effects of attenuation. Previously, the Mini XT only provided an “Auto Gain” function, which adjusts the screen brightness to either brighten under-gained (weak) images or dampen over-gained (saturated) images. Auto Gain is typically recommended for performing real-time locating, allowing the user to initialize when the signal quality changes between profiles. Manual Gain allows users to change the amount of contrast applied to the top half (shallow) and the bottom half (deep) of the screen. With this update, users can also modify the contrast over the entire data image using the level option.
New Multi-Positional Survey Wheel
The newly designed multi-positional survey wheel for the Palm XT antenna allows users to quickly change between normal, sidecar, and cross polarization mode. GSSI will also offer an upgrade kit to retrofit previous generation Palm XT antennas. The multi-positional survey wheel adapter kit includes high strength, abrasion and impact resistant thermoplastic molded parts and associated screws for installation on top of the Palm XT, along with a step by step installation guide. If a user owns the Palm XT extension pole, there is an additional piece included in the kit to allow the pole to fit on top of the Palm XT.
Ultrasonic Pulse Velocity Tester A1404 PULSAR has a software update with two new functions - Crack Depth Measurement (implemented in the instrument software) and SONREB for concrete strength evaluation. This gives additional functionality to our very successful concrete tester.
The new application software for visualization and processing data for A1410 PULSAR can be now downloaded free of charge in GooglePlay Store. Among others, the evaluation of concrete compressive strength by SonReb method according to all the valid international standards is implemented. You can buy the instrument here
The Concrete Moisture Encounter X5 – CMEX5 is a non-destructive digital multi moisture meter for concrete floors and slabs providing an instant and precise quantitative measurement of moisture content using Gravimetric testing as a baseline.
The CMEX5 also provides Carbide Method equivalent readings for concrete and other cementitious substrates as well as comparative readings as per ASTM F2659. Incorporating plug-in ports for the optional Hygro-i2® relative humidity probe testing per ASTM F2170 and heavy-duty pin-type wood probes, this moisture meter transforms into the ideal all-in-one instrument for the flooring professional.
The new CMEX5 includes:
Bluetooth connection to IOS & Android Tramex App
Non-Destructive Quantitative %MC
Built-In Ambient RH Sensor
In-Situ RH & Pin Probe connections
500+ Wood species available
Optional Extension Bracket with Telescopic Handle
In the restoration industry, the level of understanding of moisture in building materials is generally very high. However, there is a tendency to treat concrete with suspicion and it is not uncommon for waivers to be used to reduce liability for the drying contractor. This is understandable considering there are many unknown variables in concrete, partly due to it being the only building material fabricated on site. Adding to the confusion about moisture in concrete is a mistaken assumption that measuring the vapor in concrete equates to a measurement of the total moisture content in the concrete. This assumption has contributed to many mistakes being made in the flooring and restoration industries. A better understanding of the meaning of measuring both the moisture and the vapor will go a long way to removing much of the confusion and allow for better decision making.
The Importance of Temperature and Equilibrium Equilibrium Relative Humidity (ERH) measurement, as per the ASTM F2170 standard, is a way of measuring the vapor in an airspace in equilibrium with the moisture in concrete as a percentage of the maximum vapor which the airspace can hold. For the results to be meaningful, it is important that the air is in equilibrium with the moisture in the concrete, which is why it is referred to as ERH not RH. However, this does not mean ERH measurement is a measurement of the moisture content. Relative Humidity is used rather than Absolute Humidity because the temperature of the air has a dramatic effect on the amount of vapor the air is capable of holding, making the absolute humidity measurements such as Grains Per Pound meaningless for this purpose.
To explain this simply, the Equilibrium Relative Humidity and the Humidity Saturation Point will both be affected in the same way by the changing temperature. As such, the ERH remains meaningful at different temperatures whereas the absolute humidity levels change. This highlights the need for the humidity to be in equilibrium with the moisture in the concrete, again explaining why it is referred to as ERH and why the temperature must remain stable during the measurement phase. Variables when ERH testing ERH measurement per ASTM F2170 gives useful information about the condition of the concrete but due to variables including the unknown amount of un-hydrated cement present and the unknown level of air permeability of the concrete, it cannot be considered an accurate measurement of the moisture content. To understand this more clearly, the vapor saturation level needs to be considered. It is often assumed that vapor will always saturate at 100% RH but this isn’t necessarily the case.
Take a salt calibration check for example: there is humidity saturation identified by the presence of liquid water but the RH is at 75% due to the presence of salts. Combining Non-Destructive Impedance and ERH Testing Adds Value Measuring the impedance of the concrete is also commonly used to identify the moisture content of the concrete. ASTM F2659 was written to standardize the methodology for carrying out this test. This is a practical site test, which is popular due to its ease and speed of use. However, there are variables which make the moisture content readings less meaningful, especially if we don’t know the vapor level within the concrete. For example, different ambient conditions will alter the Equilibrium Moisture Content (EMC) and different aggregates can absorb and adsorb moisture differently, also affecting the EMC. Therefore, a combination of measuring both ERH and Moisture Content is advisable. The combined approach can be compared to looking down a rifle and lining up two iron sights, instead of one. Ideally, an ERH of 85% will line up to a moisture content of 4% and 90% ERH will line up to 4.5% EMC but when they do not line up there is much information which can be gained about the concrete which would otherwise be unknown if relying on one of these test methods alone.
Many things can be learned about the concrete by combining different test methods, including identifying concrete which is highly alkaline which can have a low ERH to MC level, or identifying concrete with a high Vapor to MC level, which indicates a good quality concrete. Concrete needs a minimum RH of 85% in order for hydration to continue to take place, this hydration will continue to reduce the MC while, ideally, the RH remains at 85% or above until all the hydration is complete. Consider the Ambient Environment and Age of the Concrete Further testing of the ambient conditions is important to improve the meaningfulness of these results, as there are conditions from the environment which can also affect the relationship between the MC and ERH. The age of the concrete also needs to be taken into consideration as most moisture or RH specifications are made for new concrete where construction water is still present. Older concrete, where the moisture should be in equilibrium with the average ambient conditions, is also easier to understand with a combination of testing. Achieving Faster, More Efficient and Reliable Testing With meaningful and simple training, a technician is able to make much more informed decisions based on the data from combined tests, thus removing many of the unknowns as to the condition of the concrete.
Although more testing is carried out, the combined testing should actually be more efficient and much faster. Quick tests such as impedance are used to create a moisture map and more difficult testing such as ERH testing should be used as much as necessary to confirm the vapor condition of the slab, rather than as a method of moisture mapping. This is over time-consuming and doesn’t add to the value of the data. There is little doubt that there is a lot of room for improvement when it comes to testing concrete for moisture that can lead to potential flooring issues. The current methodology has many an expert scratching their head trying to understand the rationale for it. The view of this author is that the combination of ERH and Impedance measurement along with ambient measurements is enough to answer most questions about the condition of the concrete. However it is not the only way that this can be done. There are different methods developed in different parts of the world including tests which combine both humidity and moisture testing and there are several ways to achieve more meaningful information. The first step in improving one’s knowledge is to move away from the idea that one test competes with another, - a doctor would not for example refuse to use an X-Ray because an MRI is better. He or she would use the most practical means available to achieve the information needed to treat a patient, and that’s just pragmatic and good practice.
By Andrew Rynhart
In this article, we will review an interesting category of ultrasonic test methods for concrete inspection and testing: Ultrasonic Pulse-Echo (UPE) is widely used for the inspection of concrete elements. The method has proven to be extremely useful in determining the thickness of concrete elements with one side access (i.e. tunnel linings, trunk sewer linings, abutment walls), detect sub-surface defects such as voids, honeycombing, and delamination, and to verify Location of grouting defects in tendon ducts.
Ultrasonic Pulse Echo
Ultrasonic Pulse Echo is a non-destructive testing (NDT) method for scanning sub-surface targets in concrete elements. UPE methods use acoustic stress waves to study the properties of sub-surface layers, and locate defects by identifying any anomaly of acoustical impedance that is different from concrete. The test method was developed to address practical limitations of the general Ultrasonic Pulse Velocity test, such as the need to access both sides of the concrete element.
The ACI 228.2R Section 3.2.2 provides a comprehensive review on the evolution of ultrasonic pulse echo method, and instruments over the past few decades. While traditional UPE instruments were capable of providing A-Scans and B-Scans, modern Ultrasonic Pulse Echo Tomography devices are capable of providing real-time B-Scans that would enable engineers to see sub-surface targets with further clarity. Mobile-based Applications, along with Artificial Intelligence and Modern signal processing techniques have brought superior speed and clarity, with ease of use.
How Does Ultrasonic Pulse Echo Work?
As we discussed earlier, UPE uses stress waves. The principle concept behind the test is measuring the transit time of ultrasonic wave in concrete. A modern UPE instrument consists of an array of piezoelectric transducers that are capable of exciting concrete surface through short-burst high amplitude pulse-high voltage and high current- (see Strategic Highway Research Program-SHPR2, TRB, 2013). As the pulse propagates within the concrete, it gets reflected and refracted at the interface of voids, or other internal targets. Any anomaly in acoustical impedance leads The emitted impulse and the reflected stress waves are monitored at the receiving transducer. The signals are analyzed to calculate the wave travel time.
According to the SHRP2, “Based on the transit time or velocity, this technique can also be used to indirectly detect the presence of internal flaws, such as cracking, voids, delamination or horizontal cracking, or other damages.”
Applications of UPE Methods
Ultrasonic Pulse Echo methods are widely used in concrete inspection and testing. The following section describes the main applications and Use Cases:
1. Estimate Thickness of Concrete Elements
Ultrasonic Pulse Echo is widely used by engineers to assess the thickness of concrete elements. This is specially important in concrete elements with one-side access (Single Side Access), such as Tunnel linings: Thickness measurement is critical in the QC process for tunnel linings. It is also an important parameter for structural evaluation purpose.
Trunk Sewers: In trunk sewers, UPE can help engineers estimate the thickness of existing lining. This becomes extremely challenging because intrusive methods involving hot work with core drilling is not a safe nor cost-effective solution. Moreover, there is always the risk of coring in shallow sections with high hydro static pressure.
Concrete Tanks: Testing concrete tanks that are used in industrial chemical processes is often challenging. Maintenance managers of such facilities often have very short downtime windows, and permission to get inside the tank is not always practical (unless during essential maintenance cycles). UPE enables thickness measurement and quality assessment from exterior face.
2. Grouting Defects in Tendon Ducts
Along with Ground Penetrating Radar (GPR) and Impact-Echo, UPE can provide critical information about voids and defects that might have happened during grouting process of tendon ducts in post-tensioned concrete elements.
3. Locate Sub-Surface Defects
UPE tomography can be used to assess certain defects in concrete elements. UPE can pinpoint the following defects:
Delamination: UPE methods can be used to assess the location and extent of delamination in concrete bridge decks, parking garage slabs, and concrete tanks. Honeycombing: UPE is a great tool in the Quality Control and Quality Assurance of new construction. UPE can be used to localize honeycombs in concrete. Detailed Bridge Condition Survey - Delamination of concrete in bridge decks. Honeycomb concrete - UPE Scan. Honeycomb area during construction
4. Quality Control and Quality Assurance
UPE can used as in-direct method to assess the overall quality of concrete. Through the measurement of pulse velocity, engineers can evaluate the quality of concrete materials after construction.
5. Evaluation of Fiber Reinforced Concrete
While GPR has certain pratical limitations in evaluation Fiber Reinforced Concrete (FRC) elements, UPE methods provide a reliable alternative in thickness measurement and quality control of elements. This makes them an interesting alternative in inspection and testing of concrete linings in tunnels.
Limitations of UPE and Practical Considerations
Like all other NDT methods, UPE comes with its practical challenges for certain field conditions.
Close Spacing of Test Points: In order to generate reliable and precise maps of sub-surface defects, engineers need to use close spacing between test points. This can make the test time-consuming for large test areas. A practical solution is to use another method such as GPR for rapid screening, and use UPE for high-resolution imaging of defects.
Coupling Issues: The quality of acoustic signals depend heavily on the coupling of the transducer and concrete surface. This cab be quite challenging for rough surfaces. Modern devices have tried to address the issue with spring supported mechanism at the base of transducers to allow for maneuvering around the rough areas.
Undetected Defects: Certain defects might remain undetected. This is specially true for very shallow flaws or when operators work with low frequencies.
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.
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.
It is not uncommon for concrete contractors to request the location of rebar, conduits, post-tension cables, electrical, or plumbing in order to aid in remediation risks. Additionally, savvy contractors have been using GPR technology to determine concrete slab thickness. This provides the concrete contractor with several benefits including: the proper assignment of concrete cutting and coring tools, and the ability to better quote the work required. Trained and certified GPR operators understand the benefits and limitations of the technology and how to determine the best course of action in challenging survey conditions. Yet, even the most knowledgeable operators will wonder if there’s a tip or trick that they can deploy in the field to collect better data. Here we explore the difference between suspended slab and slab-on-grade surveys, and infield processing techniques which operators can use to get the best out of their slab-on-grade data.
The detectability of the slab bottom depends on the underlying material and amount of steel within the slab. It is easier to see when a contrasting material such as water, air or metal is under the slab because they will have a stronger dielectric contrast. In Figure 1, the data is representative of an elevated concrete slab. Note the several hyperbolic reflections on the screen, this is indicative of a double rebar mat. Towards the bottom of the data image, there is a strong dielectric contrast at the concrete-to-air boundary and therefore products a clear indicator of the bottom of the slab.
In slab-on-grade situations, the bottom may be very weak or invisible if the slab rests on sand or another concrete structure (supporting beam, for instance) with similar dielectric properties. This can be challenging due to the low dielectric contrast for the concrete-to-sand boundary and intersecting hyperbolic tails from objects embedded in the slab. The former results in weak or non-existent reflections and the latter tends to mask the reflection from the bottom of the concrete interface. Figure 2 illustrates a reinforced concrete roadway with a strong reflective boundary towards the center of the roadway. This is an example of a void. The concrete-to-grade boundary is less reflective as the air-filled void.
2-D versus 3-D Data Collection: In most cases, GPR manufacturers recommend using 2D scanning for real-time locating and simple imaging services, and to use 3D data collection for complex survey sites. However, slab-on-grade surveys are an exception to this rule. It is recommended to conduct 2D scanning in these situations because the layer interface is a planer target which is more easily viewed from the side.
Depth Setting: Conducting a preliminary scan of the area will help the operator determine the appropriate GPR system settings. A common mishap is that the system is set to a depth that is less than the overall concrete depth. This inherently causes data loss at the deeper levels. Remember to always set the depth range 2-3 inches deeper than the expected slab thickness to ensure that the full slab thickness is captured.
Gain: Display Gain and System Gain can be used to brighten a weak back reflection. However, these settings should be used with caution. Display Gain and System Gain settings often influence the rest of the data. Some systems apply a correction factor to gain based on assumed material dielectric.
Migration: Migration eliminates hyperbolas by collapsing them into dots representing the actual targets. This can be helpful to make target identification more intuitive and makes the data easier to interpret. This is especially true for slab-on-grade because the tails of hyperbolic targets can sometimes intersect and hide the concrete-to-sand boundary reflection. By collapsing the hyperbolas into dots, the bottom of the slab can become more recognizable.
Cross Polarization: When detecting linear metal targets (pipes, rebar, etc.), antenna orientation relative to the target becomes important. Antenna dipoles (transmitter and receiver) are most sensitive to the metal targets that are parallel to them. In other words, if an operator is scanning across the slab with the GPR system in its normal orientation, it is sensitive to targets that are running perpendicular to the direction the operator is moving (parallel to the antenna dipoles).
Some systems can be modified to turn the antenna 90 degrees. This method is known as “cross-polarizing”. If the operator scans over a metal target that is again perpendicular to their direction of travel, the GPR system is not as sensitive to it. This “weakens” the amplitude of the metallic objects and may result in a stronger concrete bottom reflection.
In summary, GPR interpretation and survey efficiency is a skill that requires training, field experience, time, and practice. These tips are intended to help operators troubleshoot a very specific type of survey scenario. An operator may employ all of them, some of them, or only one of them in an attempt to conduct a successful survey. In some extreme cases none of the solutions may work, and only a trained operator will know what tools to use and when.