Chemie und Prozesstechnik
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- Concrete (9)
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NDE of foundations
(2019)
This talk teaches the basic princioles and applications of NDT methods for deep foundations, sprecifically piles. A variety of testing methods are discussed (integrity testing, ultrasonic crosshole logging, parallel seismic and others). Capabilities and limitations are given as well as specific hints for practical use.
The propagation of ultrasonic waves in concrete is affected by its micro- and macro-structure, geometry and properties as well as external influences as stress, temperature or moisture. In addition, age and degradation have a strong influence. Therefore, Ultrasound has been used to monitor concrete samples and structures since decades. However, early applications using conventional techniques as time-of flight or changes in amplitudes have been limited to detect changes in a late stage close to serviceability or ultimate load states.
Around 2000, several new, more sensitive techniques adopted from geophysics or other field of material sciences have been introduced to research in ultrasonic monitoring of concrete. The most discussed methodologies are coda wave interferometry, a technique which allows to detect very subtle changes from repeated ultrasonic measurements. Nonlinear acoustic techniques help to identify e. g. cracks even in an inhomogeneous background. Both techniques can be combined.
This paper reviews methods and results achieved so far on the laboratory scale and with full scale models the directions for future research and application is given as well.
The propagation of ultrasonic waves in concrete is affected by its micro- and macro-structure, geometry and properties as well as external influences as stress, temperature or moisture. In addition, age and degradation have a strong influence. Therefore, Ultrasound has been used to monitor concrete samples and structures since decades. However, early applications using conventional techniques as time-of flight or changes in amplitudes have been limited to detect changes in a late stage close to serviceability or ultimate load states.
Around 2000, several new, more sensitive techniques adopted from geophysics or other field of material sciences have been introduced to research in ultrasonic monitoring of concrete. The most discussed methodologies are coda wave interferometry, a technique which allows to detect very subtle changes from repeated ultrasonic measurements. Nonlinear acoustic techniques help to identify e. g. cracks even in an inhomogeneous background. Both techniques can be combined.
This paper reviews methods and results achieved so far on the laboratory scale and with full scale models the directions for future research and application is given as well.
Ultrasonic non-destructive testing methods have found various applications in quality assurance and condition assessment of nuclear concrete structures. This includes but is not limited to the localization of construction features (thickness measurements, reinforcement, tendon ducts, and others) and damage detection (e. g. corrosion or cracks). However, there are still limitations, e. g. limited penetration depth and resolution, issues when testing hybrid steel/concrete structures as well as absence of standards and regulations.
This presentation will show progress in some of these issues and will focus on advanced instrumentation and validation of ultrasonic NDT for concrete structures.
Ultrasonic echo testing has been limited to a penetration depth of less than one meter in reinforced concrete. The development of the LAUS (Large Aperture Ultrasonic System), using a large array of ultrasonic transducers and wider offsets between transmitter and receiver, allows to reach up to 5 m in reinforced concrete and 9 m in unreinforced concrete. This capability has been demonstrated by collecting data from an engineered salt concrete barrier in an underground nuclear waste storage facility. Due to an increasingly competitive environment, commercial manufacturers will continue to develop equipment with enhanced penetration and/or increased resolution.
To quantify the capabilities and reliability of ultrasonic tests, validation mockups with engineered flaws are required. EPRI and BAM have built a large-scale mockup at a test site close to Berlin (BAM-TTS, Horstwalde) using a design which is repeated at other places. This presentation will also cover the methods deployed on engineered flaws and the repeatability of the test results.
In this study, an Impulse Response Function analysis of pile response to sine-sweep excitation by a low cost, portable Shaker was used to identify defects in piles. In straightforward impact-echo methods, echoes from the pile toe and defects are visible in the time domain measurements. However, these echoes are not present in the time domain records of piles subjected to sine-sweep excitations, due to interactions between the input and output signals. For this reason, the impulse response function in the time domain has been calculated and is able to identify the echoes from pile impedance changes. The proposed methodology has been evaluated both numerically and experimentally. A one-dimensional pile-soil interaction system was developed, and a finite difference method used to calculate the pile response to sine-sweep excitation. The numerical simulations indicate that impulse response measurements with a synthesized logarithmic, sine-sweep excitation could be an effective tool for detecting defects in piles. The methodology was further tested with field trials on 6 cast in situ concrete test piles including 1 intact pile and 5 defective piles subjected to sine-sweep excitations by a shaker. In 5 of the 6 cases the echoes from the pile toe could be identified in the deconvoluted waveforms—the impulse Response functions. Damage detection is more difficult and dependent on the selection of the optimal regularization parameter. Further research and optimization of the deconvolution process is needed to evaluate the effectiveness compared to standard pile integrity testing methods.
The necessity and demand for nondestructive testing of wood-based materials which can automatically scan huge areas of wood is increasing. Air-coupled ultrasound (ACU) is used to detect defects and damage without altering the structure permanently. Using through transmission it is possible to detect even small holes and missing adhesive. If only one side of an object is accessible the reflection mode is preferred at the expense of a reduced resolution and penetration depth. Novel ferroelectret transducers with a high signal-to-noise ratio (SNR) enable a high-precision structure recognition. The transducers made of cellular polypropylene (PP) are quite suitable for ACU testing due to their extremely low Young’s modulus and low density which result in a favorable acoustic impedance for the transmission of ultrasonic waves between the transducer and air. Thus, defects such as delamination, rot, and cracks can be detected. Promising results were obtained under laboratory conditions with frequencies from 90 kHz to 200 kHz. The advantage of these ACU transducers is that they do not require contact to the sample, are accurate, and cost effective. Ultrasonic quality assurance for Wood is an important attempt to increase the acceptance of wooden structures and towards sustainability in civil engineering in general.
Ultrasonic transmission measurements are used to monitor concrete elements mostly on a laboratory scale since decades. Recently, coda wave interferometry, a technique adapted from seismology, has been introduced to civil engineering experiments. It can be used to reveal subtle changes in concrete samples and even large construction elements without having a transducer directly at the location where the change is taking place. The methodology works best with embedded transducers to avoid coupling issues or excessive environmental influence. These transducers can be used for newly built and existing structures. Recently, large concrete beams have been equipped with a network of transducers and loaded until failure. Using code wave interferometry, it was possible to visualize stress fields and damaged areas. This paper gives an overview of the state of the art, recent results achieved at BAM and a task list for further results and development.
Ultrasonic transmission measurements are used to monitor concrete elements mostly on a laboratory scale since decades. Recently, coda wave interferometry, a technique adapted from seismology, has been introduced to civil engineering experiments. It can be used to reveal subtle changes in concrete samples and even large construction elements without having a transducer directly at the location where the change is taking place. The methodology works best with embedded transducers to avoid coupling issues or excessive environmental influence. These transducers can be used for newly built and existing structures. Recently, large concrete beams have been equipped with a network of transducers and loaded until failure. Using code wave interferometry, it was possible to visualize stress fields and damaged areas. This paper gives an overview of the state of the art, recent results achieved at BAM and a task list for further results and development.