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Organisationseinheit der BAM
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.
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.
Seismic interferometry (SI) deals either with the sensible detection of changes in the subsurface or with the reconstruction of virtual signals between two receivers by crosscorrelation of signals from diffuse sources. These concepts can be applied in NDT in civil engineering for various purposes, e. g. to detect changes in bridges. Here it is demonstrated using data from a reference structure on our test site. Practical applications can be expected in the very near future.
"INFRASTAR aims to develop knowledge, expertise and skills for optimal and reliable management of structures. The generic methodology is applied to bridges and wind turbines in relation to fatigue offering the opportunity to deal with complementary notions (such as old and new asset management, unique and similar structures, wind and traffic actions) while addressing 3 major challenges: 1/ advanced modelling of concrete fatigue behaviour, 2/new non destructive testing methods for early aged damage detection and 3/probabilistic approach of structure reliability under fatigue. Benefit of cross-experience and inter-disciplinary synergies creates new knowledge. INFRASTAR proposes innovative solutions for civil infrastructure asset management so that young scientists acquire a high employment profile in close dialogue between industry and academic partners. Modern engineering methods, including probabilistic approaches, risk and reliability assessment tools, will take into account the effective structural behaviour of existing bridges and wind turbines by exploiting monitored data. Existing methods and current state-of -the art is based on excessive conservatism which produces high costs and hinders sustainability. INFRASTAR improves knowledge for optimising the design of new structures, for more realistic verification of structural safety and more accurate prediction of future lifetime of the existing structures. That is a challenge for a sustainable development because it reduces building material and energy consumption as well as CO2 production. Within the global framework of optimal infrastructure asset management, INFRASTAR will result in a multi-disciplinary body of knowledge covering generic problems from the design stage process of the new civil infrastructures up to recycling after dismantlement. This approach and the proposed methods and tools are new and allow a step forward for innovative and effective process."
The legend continued: The International Symposium NonDestructive Testing in Civil Engineering (NDT-CE) had returned to Berlin. The Bundesanstalt für Materialforschung und -prüfung (BAM) and the Technical University of Berlin (TUB) hosted this prestigious event in the industrial landmark building Peter-Behrens-Halle and the Test Site Technical Safety (TTS) in Horstwalde in September 2015. Almost 300 participants from more than 35 countries gathered to present recent research, exchange knowledge and to lest the newest equipment in NDT-CE. This symposium, held regularly in various locations all over the world, was organized for the third time in Berlin and is by far the largest of its kind.
Sealing and strengthening of the subsoil by injection is a major issue in the field of geotechnical engineering. One commonly applied method is jet grouting, which allows creating columns of grouted soil by eroding and mixing the in-situ soil with a thin cement suspension. A general difficulty linked with this method is to predict the resulting column diameter and its material strength. In this paper we illustrate the application of a newly developed non-destructive quality assurance testing tool used to determine the diameter of jet grout columns. This approach incorporates standard crosshole and downhole seismic measurements. To demonstrate its effectiveness, we tested the new approach within two-dimensional finite-difference numerical simulations. Additional field tests showed that this tool is also applicable in real site conditions. For this purpose, three jet grout columns were produced with different process parameters in a depth between 3.0 and 10 m. The evaluated diameters were within 1.0 and 1.5 m, slightly deviating from the previously predicted range by the jet grouting contractor. Moreover, we were able to detect the base of the columns at 10 m depth with no significant difficulties. On the other hand, unsaturated, partly unconsolidated sands between ground water level and surface considerably affected the seismic data, hence complicating the detection of the top of the columns.
Geophysical methods have been used in civil engineering for decades. The main field of application is - to no surprise – in geotechnical projects from site characterization to foundation quality assurance. For more than 25 years, ground penetrating radar (GPR) and seismic methods have found applications in structural engineering. Recently introduced geophysical methods have been adopted to ultrasonic investigations in various fields. They help to improve the quality of structural imaging and to detect small changes in concrete. An overview of the history and current use of geophysics in civil engineering is given. Selected examples of new concepts include advances in wave based imaging, quality assurance for foundations, detecting small changes in concrete as well as moisture and corrosion detection are discussed.
Challenging new constructions and ageing infrastructure are increasing the demand for permanent monitoring of loads and condition. Various methods and sensors are used for this purpose. But the technologies available today have difficulties in detecting slowly progressing locally confined damages. Extensive investigations or instrumentations are required so far for this purpose. In this study we present new sensors and data processing methods for ultrasonic transmission, which can be used for non-destructive long term monitoring of concrete. They can be mounted during construction or thereafter. Larger volumes can be monitored by a limited number of sensors for changes of material properties. The principles of ultrasonic transmission and influencing factors are presented. This latter include load, damages as well as environmental parameters as temperature or moisture. Various methods for data processing, e. g. coda wave interferometry are introduced. They allow the detection of very small changes in the medium. The embedded sensors are shown including mounting and operation. Application examples so far include small scale laboratory freeze-thaw experiments, localizing loads in larger concrete models, monitoring load effects on real structures as well as detecting acoustic events. Some sensors are operating already for several years. The sensors can be used as transmitter or receivers or switched between both roles. While most of the previous experiments have been active (at least one sensor serving as transmitter), new studies show that the sensors are useful as well for passive measurements, e. g. in acoustic emission or time reversal experiments. Besides application in civil engineering our setups can also be used for model studies in geosciences.
Ultrasonic echo and transmission techniques are used in civil engineering on a regular basis. New sensors and data processing techniques have lead to many new applications in the structural investigation as well as quality control. But concrete structures in the nuclear sector have special features and parameters, which pose problems for the methods and instrumentation currently available, e.g. extreme thickness, dense reinforcement, steel liners or special materials. Several innovative ultrasonic techniques have been developed to deal with these issues at least partly in lab experiments and pilot studies. Modern imaging techniques as multi-offset SAFT have been used e. g. to map delaminations. Thick concrete walls have successfully been inspected, partly through a steel liner. Embedded ultrasonic sensors have been designed which will be used in monitoring networks of large concrete structures above and below ground. In addition, sensitive mathematical methods as coda wave interferometry have been successfully evaluated to detect subtle changes in material properties. Examples of measurements and data evaluation are presented.