Tunnel - Ultrasonic Tomography (UST)
Target of Investigation
Ultrasonic pulse echo (UPE) testing and ultrasonic tomography (UST) are primarily used to inspect interiors of concrete structural members and tunnel linings. These technologies have the following applications:
- Measuring thickness of concrete members.
- Detecting poor bonding or debonding of overlays and repairs.
- Identifying delamination.
- Detecting voids and honeycombing in concrete members.
- Locating ducts and reinforcement in concrete structures, such as rebars, strands, and tendons.
- Quality control and quality assurance during construction of reinforced and prestressed concrete elements.
- Detecting voids behind tunnel linings and below slabs on grade.
UPE testing involves using dry-point contact transmitting and receiving ultrasonic transducers. The transducers are arranged in a pitch–catch configuration, in which shear waves are emitted into the concrete and reflections from objects of different acoustic impedances are recorded. UPE equipment is available in different sizes, varying from two transducers to a large number of transducers in an array (figure 1).
Some systems use multiple transducers as transmitters and receivers to accelerate the ultrasonic data acquisition, enhance the signal-to-noise ratio, and minimize the effects of wave scattering from aggregate particles and similar heterogeneities in concrete structures. Such systems are used for UST.(1) A device with an array of 48 low-frequency broadband shear transducers is shown in figure 2. While one row of transducers acts as a receiver, the remaining rows act as transmitters. Each transducer is mechanically isolated and dampened independently, enabling the instrument to fit the profile of a rough concrete testing surface.
UPE testing involves generating and recording ultrasonic compression and shear waves being propagated through the surveyed element and reflected from objects of different acoustic impedances. As illustrated in figure 3, the transducer probe generates the wave field while the receiving probe records the reflected waves. UPE testing defines the depth of the reflector (internal flaws such as cracking, voids, delamination, or horizontal cracking) by measuring the round-trip travel time (Δt) of the pulse and using the wave propagation velocity (C) in the investigated material.(2) The calculation of depth (d) is illustrated in figure 4.
The 66 ray paths for the measured transit time are shown in figure 5. The tomographer then uses the synthetic aperture focusing technique (SAFT) to reconstruct the medium based on the shear‑wave reflections.(3,4)
In general, it is critical to have good coupling between the surface and transducers. In the past, coupling was achieved using a coupling medium. Some newer systems spring-load individual transducers so they conform to the surface and, thus, do not require any couplants. The nominal center frequency for units using shear waves is usually between 20 and 200 kHz. For most concrete applications, a center frequency of 50 kHz will yield optimal results.
The UPE system consists of transducers, a portable computer with analysis software, and a data cable. Figure 6 shows data collection with an ultrasonic echo device and a BAM scanner, a device designed by BAM (the acronym for the German Federal Institute for Materials Research and Testing). For the UST system, transducers and electronics are mounted in a handheld box that is placed on concrete surfaces.
Adapting test procedures to the needs of the desired application is necessary, but the different collection modes have common basic steps. Before the start of the actual testing, the instrument settings should be checked by examining a few locations with known conditions. This presetting is called the review or explore mode. The actual testing should be conducted on a predefined grid to obtain a complete dataset for the element surveyed. This actual setting is called the map or scan mode.
Data processing for the UPE system is relatively simple. Data collected from each test location constitute an A-scan. The depth of the reflector can be calculated based on A-scan data (figure 4). For the UST system, collected and stored data are reconstructed using SAFT, which results in a three-dimensional (3D) representation of the surveyed elements with features in the image indicating reflections from objects and discontinuities. In the 3D volume, the position and strength of reflections from identified reflectors are presented by a color scheme, which can be used to obtain the actual position and shape of the reflector.
The 3D volume can be examined by cutting vertical or horizontal planes. Images of detected features in three such planes, termed B-scan, C-scan, and D-scan, are shown in figure 7. The B‑scan is an image slice showing the depth of the specimen on the vertical (z) axis and the width of scan on the horizontal (x) axis. This slice is a plane perpendicular to the scanning surface and parallel to the length of the device. The C-scan is an image slice showing the plan view of the tested area, with the vertical (y) axis of the scan depicting the width parallel to the scanning direction and the x-axis of the scan representing the length perpendicular to the scanning direction. The D-scan is like the B-scan, which images a plane perpendicular to the testing surface, but the D-scan is oriented parallel to the scanning direction.
Interpretation of UPE testing results is usually simple. The calculated depth may indicate an existing defect if it is less than the thickness of the structural element. For UST, the 3D visualization along with different plane sections (B-, C-, and D-scans) can be used to interpret the data. A color template is used to describe features depending on the strength of the reflection of ultrasonic waves. In most cases, low reflectivity is described using cold colors (blues and greens), while high reflectivity is described using hot colors (reds and yellows). High reflectivity, in most cases, indicates the presence of objects and discontinuities, such as reinforcements, cracks, delaminations, and voids. Low reflectivity, in most cases, indicates a sound condition. Figure 8 shows color-coded B- and C-scan images of a tunnel survey by UST. The color scheme is scaled based on the intensity variations of the reflected shear waves received by the transducer array. Discontinuities, indicated by hot colors, are readily apparent.
Guidelines for interpreting UST data for common defects and features are summarized as follows:
- Backwall (element thickness)—Measuring the thickness of an element is based on the evaluation of the travel time of an ultrasonic wave reflected from the backwall. To get a clear reflection from the backwall, it is necessary to have a strong contrast in acoustic impedances of the element material and material behind the backwall. The backwall is identified by a high-amplitude reflection plotted in red. The horizontal reflections from a B-scan indicate a constant thickness of the element.
- Reinforcement, conduit, tendons, etc.—Cable-like objects are identified as localized high-intensity reflections. Similar to backwall detection, there must be a contrast in acoustic impedances of the concrete and objects. The most effective way to detect objects is by scanning perpendicularly to the direction of their layout. A single B-scan will identify a linear object as a circular or oval high-amplitude region when the scan is perpendicular to the direction of the object. The position and orientation of a linear object is defined from numerous parallel scans across the object and may be seen from a C-scan. It is generally not possible to identify the material type. Examining the phase of the reflection can provide some insight on whether the reflector is of an acoustic impedance higher or lower than the surrounding concrete. Rebars are most commonly recognized by a regular spacing between detected objects.
- Delamination—Similarly to the backwall, delamination is identified as a high-amplitude linear reflection shallower than the backwall. Therefore, it is necessary to establish the position of the backwall. Since delamination is often a curved and inclined surface, it may appear as a linear object of a variable depth. Since a delamination prevents propagation of an ultrasonic wave below it, the backwall is usually not visible at delamination locations. Instead, ultrasonic waves get confined between the delamination and top surface, which may result in multiple equally spaced reflections with depth in a B-scan.
- Air- and water-filled voids—Voids appear as high-amplitude reflections of a finite size. Because both air- and water-filled voids are of a lower acoustic impedance than concrete, it is nearly impossible to determine whether a void is filled with water.
Advantages of UPE testing and UST include the following:
- Reliable, repeatable, and consistent results.
- Easy field operation.
- Real-time data collection and processing.
Additional benefits of UST include the following:
- Tomographic data present volumetric and informative images of the surveyed element.
- Low to medium level of expertise required for equipment setup and data collection with modern equipment.
- Surface preparation not required because of dry-point contact transducers.
Limitations of UPE testing and UST include the following:
- Time consuming and labor intensive.
- Slow data collection.
- No discernible information deeper than the first layer’s interface.
- Difficult to detect additional reinforcement below two layers of reinforcement.
- Considerable engineering judgment required to properly evaluate a measurement.
- Possible misinterpretation when poor contact is made.
- Kozlov, V.N., Samokrutov, V.V., Shevaldykin, V.G. (2006). “Ultrasonic Equipment for Evaluation of Concrete Structures Based on Transducers with Dry Point Contact.” Al-Quadi, I. and Washer, G. (eds.), Proceedings of the NDE Conference on Civil Engineering,
pp. 496–498, American Society for Nondestructive Testing, Columbus, OH.
- American Concrete Institute Committee 228. (1998). Nondestructive Test Methods for Evaluation of Concrete in Structures, ACI, Farmington Hills, MI.
- Langenberg, K.J., Brandfaß, M., Hannemann, R., Kaczorowski, T., Kostka, J., Hofmann, C., Marklein, R., Mayer, K., Pitsch, A., (2001). “Inverse Scattering with Acoustic, Electromagnetic, and Elastic Waves as Applied in Nondestructive Evaluation.” Wavefield Inversion, pp. 59–118, Springer, Basingstoke, England.
- Marklein, R., Mayer, K., Hannemann, R., Krylow, T., Balasubramanian, K., Langenberg, K.J., Schmitz, V. (2002). “Linear and Nonlinear Inversion Algorithms Applied in Nondestructive Evaluation.” Inverse Problems, 18(6), pp. 1,733–1,759, IOP Science, Bristol, England.
- Wimsatt, A., White, J., Leung, C., Scullion, T., Hurlebaus, S., Zollinger, D., Grasley, Z., et al. (2014). Mapping Voids, Debonding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings, Report No. S2-R06G-RR-1, Transportation Research Board, Washington, DC.