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.
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.