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The reuse of existing foundations is considered in various construction Projects to save time and costs while avoiding unnecessary interference with other underground objects. Because the design and as-built drawings might not be complete and questions may arise regarding the condition of the foundations, a detailed investigation is a prerequisite for the planning process in most cases. Nondestructive testing (NDT) techniques are a core part of this endeavor.
The processes and procedure for foundation reuse planning are not yet standardized, and the possibilities and limitations of NDT methods are not known to many planners and stakeholders. The German research Project REFUND (2014–2016) has developed charts for the planning and Investigation process that consider available standards and the current state of the art in NDT.
These charts are separately available for single and strip, slab, and pile foundations. Available NDT methods have been compiled and evaluated for specific tasks at these foundation types, including their respective limitations.
The procedures have been successfully tested in two real-world projects. The results from these projects enable planners to improve the reliability of the process while potentially saving significant resources. This paper discusses the procedures for pile foundations and demonstrates the use of various NDT methods in a project on the reuse of electrical tower foundations.
Attenuation of ultrasonic signals in concrete has the potential to carry much information about the microstructure of the material. In this work a series of concrete specimens of varying porosities and pore size distributions were internally imaged with x-ray computed tomography (CT), and then subsequently examined with throughtransmission ultrasound. The CT images were used to quantify both capillary porosity of cement paste as well as internal interfaces that are likely to produce elastic wave scattering. Ultrasound signals were represented as a diffusion process such that absorption and scattering attenuation could be isolated. As implemented, the diffusion model was a poor predictor of capillary porosity, while diffusivity was a reasonable predictor of scattering interfaces. Diffusivity was found to scale extremely well with small scale porosity, which made it a good predictor of compressive strength.
This paper describes a new ultrasonic measuring device called “W-Box”. It was developed based on the requirements of the DFG Forschergruppe (research unit) CoDA for a portable device for monitoring of concrete specimens, models and actual structures using embedded ultrasonic transducers as well as temperature and humidity sensors. The W-Box can send ultrasonic pulses with a variable frequency of 50–100 kHz to one selectable transducer and records signals from up to 75 multiplexed channels with a sample rate of 1 MHz and a resolution of 14 bits. In addition, it measures temperature and humidity with high accuracy, adjustable amplification, restarts automatically after a power failure and can be fully controlled remotely. The measured data are automatically stored locally on-site data quality checks and transferred to remote servers. The comparison of the W-Box with a laboratory setup using commercial devices proves that it is equally reliable and precise, at much lower cost. The W-Box also shows that their measurement capacities, with the used embedded ultrasonic transducers, can reach above 6 m in concrete.
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.
Ultrasound measurements in concrete are a well-known technique in civil engineering and non-destructive testing.
For consistent monitoring of a concrete structure, the common techniques, using external sensors can often not provide the appropriate degree of repeatability, as the surface of structures changes, and comparable coupling conditions cannot be guaranteed when a measurement is repeated after some time. By embedding ultrasound transducers in concrete, we aim to develop a strategy for long-term monitoring of infrastructure, especially bridges, as a supplement and extension to other techniques. Applying the so-called coda wave interferometry to these measurements we can detect subtle changes in the medium far beyond the Resolution limit of traditional time of flight methods. A smart sensor layout enables cost-efficient sensing of the entire area of interest.
Embedding the transducers might remove uncertainties like coupling or positioning changes, while other challenges remain.
Temperature and moisture content influence the structure and the transducers. These drifts need to be recorded and removed and good coupling must be ensured while not being able to visually inspect the sensor. In a multidisciplinary research group funded by the German Research Foundation, we aim to solve these problems on the way towards an ultrasound monitoring system for reinforced concrete structures. In various experiments in the lab and field, we determine the influence of temperature variations on the measurements and the equipment. As the monitoring task is the detection of irreversible damages - not reversible changes - a smart system requires a smart way of discrimination between permanent damages and reversible changes. With the data collected in these experiments, we present an approach to an environmental correction to ultrasound data to avoid a
misinterpretation of these environmental changes as damage indicators.
Acoustic emission and ultrasonic monitoring of a prestressed concrete bridge in its final years
(2021)
Many European bridges built in the 1950s, 60s and 70s must be re-placed in the next decade due to structural deficiencies, insufficient load capacity or other issues. However, the existing bridges must be used for another couple of years even if flaws and damages already have been detected.
In Germany, several prestressed concrete bridges have been instrumented with acoustic emission detection systems to detect wire breaks and to provide early warning signs before failure. To evaluate and interpret the consequences of wire breaks additional instrumentation and accompanying measures as finite element modeling are required.
At a bridge in southern Germany we have complemented such a system with active ultrasonic monitoring. Repeated ultrasonic measurements are evaluated with a very sensible algorithm called coda wave interferometry. This method, inspired by seismology, has been shown to deliver early warning signs in lab experiments. Large volumes of concrete can be monitored with a limited network of ultrasonic transducers. We will report on the installation, capabilities, and lim-itations as well as first results.
Fatigue is one of the most prevalent issues, which directly influences the service life expectancy of concrete structures. Fatigue has been investigated for years for steel structures. However, recent findings suggest that concrete structures may also be significantly subjected to the fatigue phenomena that could lead to premature failure of certainfatigue prone structural elements. To date, fatigue of reinforced concrete has been given little focus. and Kknowledge on the influence factors and durability/capacity effects on this material should be improved. Current technological means to measure fatigue in civil structures like bridges and wind turbines (both onshore and offshore) are outdated, imprecise and inappropriate.
Meanwhile, this topic has got much more attention as dynamic loading on concrete structures plays an increasing role, e.g. in bridges with increasing traffic and heavier trucks, in wind energy production due to inherent vibrations e.g. in offshore wind turbine support structures affected by wind and waves.
The European Innovative Training Networks (ITN) Marie Skłodowska-Curie Actions project INFRASTAR (Innovation and Networking for Fatigue and Reliability Analysis of Structures - Training for Assessment of Risk) provides research training for 12 PhD students. The project aims to improve knowledge for optimizing the design of new structures as well as for more realistic verification of structural safety and more accurate prediction of the remaining fatigue lifetime of existing structures.
First, the INFRASTAR research framework is detailed. Then it will be exemplified through the presentation of the major results of the four PhD students involved in the work package dealing with auscultation and monitoring. This includes the development and improvement of Fiber Optics (FO) and Coda Wave Interferometry (CWI) for crack sizing and imagery, new sensor technologies and integration, information management, monitoring strategy for fatigue damage investigation and lifetime prediction.
Fatigue is one of the most prevalent issues, which directly influences the service life expectancy of concrete structures. Fatigue has been investigated for years for steel structures. However, recent findings suggest that concrete structures may also be significantly subjected to fatigue phenomena that could lead to premature failure of certain structural elements. To date, fatigue of reinforced concrete has been given little focus. Knowledge on the influence factors and durability/capacity effects on this material should be improved. Current technological means to measure fatigue in civil structures like bridges and wind turbines (both onshore and offshore) are outdated, imprecise and inappropriate.
Meanwhile, this topic has got much more attention as time-variant loading on concrete structures plays an increasing role, e.g. in bridges with increasing traffic and heavier trucks, and for wind turbines for renewable energy production, e.g. for offshore wind turbine support structures affected by wind and waves.
The European Innovative Training Networks (ITN) Marie Skłodowska-Curie Actions project INFRASTAR (Innovation and Networking for Fatigue and Reliability Analysis of Structures - Training for Assessment of Risk) provides research training for 12 PhD students. The project aims to improve knowledge for optimizing the design of new structures as well as for more realistic verification of structural safety and more accurate prediction of the remaining fatigue lifetime of existing concrete structures.
First, the INFRASTAR research framework is detailed. Then it will be exemplified through the presentation of the major results of the four PhD students involved in the work package dealing with auscultation and monitoring. This includes the development and improvement of Fiber Optics (FO) and Coda Wave Interferometry (CWI) for crack sizing and imagery, new sensor technologies and integration, information management, monitoring strategy for fatigue damage investigation and lifetime prediction.
Advanced ultrasonic imaging for concrete: Alternative imaging conditions for reverse time migration
(2018)
Ultrasound echo is a widely used NDT technique for determining the internal geometry of structures. Reverse-time migration (RTM) has been recently introduced to NDT applications, as an imaging method for ultrasound data, to overcome some of the limitations (e.g. imaging steeply dipping reflector) experienced by the Synthetic Aperture Focusing Technique (SAFT), the most commonly used imaging algorithm for these measurements.
The standard implementation of RTM also experiences some drawbacks caused by its imaging condition, which is based on the zero-lag of the cross-correlation between source and receiver wavefields and generates high-amplitude low-frequency artifacts. Three alternative imaging conditions, developed for seismic data applications, were tested for their ability to provide better images than the standard cross-correlation: illumination compensation, deconvolution and wavefield decomposition. A polyamide specimen was chosen for the simulation of a synthetic experiment and for real data acquisition. The migrations of both synthetic and real data were performed with the software Madagascar. The illumination imaging condition was able to reduce the low-frequency noise and had a good performance in terms of computing time. The deconvolution improved the resolution in the synthetic tests, but did not showed such benefit for the real experiments. Finally, as for the wavefield decomposition, although it presented some advantages in terms of attenuating the low-frequency noise and some unwanted reflections, it was not able to image the internal structure of the polyamide as well as the cross-correlation did. Suggestions on how to improve the cost-effectiveness of the implementation of the deconvolution and wavefield decomposition were presented, as well as possible investigations that could be carried out in the future, in order to obtain better results with those two imaging conditions.