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Alkali-aggregate reaction (AAR) is a prominent degradation mechanism of concrete structures, which results from the dissolution of reactive silicate aggregates and the associated formation of damaging, expansive AAR gels. Due to the complex nature of AAR reactions, it has remained ambiguous which factors contribute the most to its occurrence on a global scale. Similarly, concrete monitoring in Finland has only recently begun to adapt to the reality that AAR often occurs concomitantly with other degradation mechanisms such as freeze-thaw damage, highlighting the need for critical evaluation of current methods to identify AAR occurrence and distinguish it from these other mechanisms. Machine learning (ML) provides a data-driven framework for both evaluating the relative “importance” of various data features, as well as predicting their consequent influence on AAR damage to concrete. Building on the success of previous such data-driven learning for the prediction of well-defined concrete properties such as compressive strength and setting time, the current work evaluates the feasibility of extending ML methods to AAR-relevant predictions. Results provide new insights into several of the most relevant concrete characteristics linked with AAR occurrence, and establish a basis for future work to extend and enhance such predictions to supplement monitoring and risk management of concrete structures.

International audience; This paper summarizes the work performed in Task 3.6, Reliability and Qualification of components, of the EU H2020 project Modern2020. It synthezises progress made by eight expert organisations on a common multi-stage methodology for qualifying monitoring components (MC) of the measurement chain (sensor, connecting cable and/or wireless system/controller) at a Deep Geological Repository (DGR). This study was reported as document D3.6 [1] in the Modern2020 project and results from a multi-stage analysis including: i) the study of transferable experience gained from the energy and space fields, ii) the feed back of long-term and demonstration experiments operating in conditions close to those expected in repositories at Underground Research Laboratories (URLs) or at large mock-up, iii) the initiatives for the development of a qualification process for selecting and testing the monitoring components and at last, iv) the proposal for a global protocol appropriate to all monitoring contexts.

Non-invasive monitoring of radioactive waste repositories is one of the key objectives addressed in the MODERN2020 project. For this task, geophysical techniques offer excellent means. Previous studies have identified seismic full waveform inversion (FWI) to be the most promising option for delineating subtle changes within a repository using data acquired outside of the repository. Significant anisotropy of the host rock, particularly in clay environments, precluded so far application of FWI technology for repository monitoring. With the development of a novel model parameterization, this problem could be resolved. Moreover, incorporation of structural constraints further improved the quality and reliability of our FWI algorithms. This was demonstrated with a field data set acquired in the Mont Terri rock laboratory. For a better characterization of small differential changes between two consecutive experiments, a novel differential tomography methodology was developed. It was tested with field data sets, with which differential traveltime inversions were performed. It is expected that this new method can be transferred in a straightforward manner to FWI problems. FWI technologies require extensive data analyses and substantial computer resources. Therefore, it was checked, if it is possible to employ quick and inexpensive tools, with which temporal changes in a repository can be detected, but not necessarily imaged. For that purpose, an anomaly detection algorithm was developed, and it will be tested with field data. In addition to seismic methods, geoelectrical techniques can provide valuable information for repository monitoring. For that purpose, tomographic algorithms for geoelectrical and induced polarization data were established and tested with laboratory data. For transferring the electrical parameters, obtained from these tomographic inversions, into relevant physical parameters, such as temperature and moisture content, calibration measurements were performed, and constitutive relationships between these parameters were established.

Construction methods, environmental stressors, and aging factors are the main causes for defects of reinforced concrete in nuclear power plants (NPP). These defects are typically occurred as corrosion of reinforcement steel, delamination, cracks, malfunction of post-tensioning or steel composite systems etc. Some of the challenges for assessing the performance of these structures by nondestructive testing methods (NDT) are that the assessment could be performed only during the annual overhauls when testing is time-limited and uncertainty of the accuracy and reliability of the available NDT testing devices combined with the lack of the international uniformity of the methods used for NDT tests.To overcome these challenges, a mock-up wall representing a section of the concrete containment of the NPP. The mock-up wall included simulated defects, which mimic the most common types of defects in NPP concrete structures such as dimensional errors, honeycombing, delamination, defects adjacent to the steel liner and voids in grouted tendon ducts for the posttensionedstructures. This paper introduce the design and construction of the wall including the concrete properties, reinforcement, tendon ducts and the types of the simulated defects. The paper also introduces the NDT methods and techniques that are suitable for assessing the condition of the mock-up wall under the real environmental conditions. These techniques include mechanicalmethods as rebound hammer, ultrasonic and electromagnetic methods. This mock-up wall will enable to investigate reliably available NDT methods and experts’ skills providing also an important and very much need educational platform for future NDE experts.

Wireless energy transfer by applying low-frequency (LF) magnetic fields through the repository barriers has been proposed as one long-term power sourcing option of geological repository monitoring sensors. The general goal of this research has been to investigate the applicability and basic limitations of the wireless energy transfer for the powering of nuclear repository monitoring sensors. Another goal has been a feasibility study and pilot implementation of a wireless bi-directional data transfer add-on in the wireless energy transfer system without compromising the energy transfer performance. The main research methods have been performance analysis with 10 m operation distance by applying existing theory of inductive coupling and power transfer, and a 125 kHz pilot system design and experiments through medium representing repository barriers consisting of host rock with relatively low conductivity. The research until now has demonstrated that the wireless energy transfer through repository barriers is a relevant powering technology of wireless repository monitoring sensors. Tests of the pilot system also indicated that the wireless data transfer add-on can be implemented without compromising the power transfer performance significantly. Further elaboration of the pilot system towards the final repository monitoring implementations is needed for maintaining the antenna resonance during the long-term use, including an interim energy storage for sensor payload supply power conditioning, improving the immunity of the data uplink to the external noise, and the design of the TDM based protocol for powering, data uplink and data downlink. This paper gives an overview of the research carried out in this topic by VTT as a part of the European project Modern2020 Task 3.3. A more detailed description of the results can be found in Modern2020 deliverable D3.3.

SKB and Posiva have two different reference methods for production of buffer blocks: uniaxial and isostatic compression, respectively. Both methods have been shown to fulfil current requirements on buffer blocks. Yet the comparisons of performance have not been made by blocks produced of the exact same material. The goal of the project was to test, compare and evaluate the properties of the uniaxial and isostatic full-scale buffer blocks with a diameter of 1800 mm and manufactured from the same bentonite batch by the same predefined parameters and technical properties. The same batch of MX-80 bentonite with a water content of w = 17%, was used for the comparative study. It was the intention to perform the evaluation based on similar conditions, i.e. at the same age, on the same size samples and using the same test methods for both blocks. After all, a full-scale block, manufactured by isostatic compression, could not be made available. Therefore, the testing was done on an isostatic block with a height of 1250 mm and a diameter of 1200 mm, which is 2/3rd of the diameter of the full-scale uniaxial block having a diameter of 1800 mm and a height of 500 mm. From both blocks, several core samples were taken in the vertical and horizontal directions. The core samples were cut to prepare further specimens. The specimens were used to determine their bulk density, water content, compressive and splitting tensile strength and modulus of elasticity. Further tests were performed to determine the humidity adsorption, change in density and cracking behaviour of the bentonite samples when stored in ambient air with different relative humidity. Special samples in the form of a plate were cut from both blocks. The thermal conductivity of the bentonite plates was measured with a heat flow meter. Based on the testing results the properties and the behaviour of both blocks were compared and evaluated in order to provide the basis for a final decision on the future manufacturing technique for buffer blocks.

The aim of this preliminary study is to identify and compile information regarding existing acceptance criteria that trigger maintenance actions of reinforced concrete infrastructure of nuclear facilities. This study focuses on the acceptance criteria for all aspects of safe and reliable long-term operation, i.e. structural performance, durability and ageing of reinforced concrete structures. The study covers a literature review of current standards and guidelines used in several fields of civil engineering concrete infrastructure, experience gained from relevant research projects and own practical experience, and shared expertise from international technical committees addressing all aspects of concrete structures. In order to assess the effects of age-related deterioration of reinforced concrete structures, structural condition assessment of concrete structures must be performed. This includes visual inspections, special inspections and analytical methods. The study reveals that a significant proportion of condition assessment is limited to the use of visual inspection. Since inspection based on observation is subjective, and the assessment is in great part qualitative, a lot of effort has been put into developing acceptance criteria for the assessment of reinforced concrete structures. This has resulted in extensive descriptions of condition damage and defects, including image/pictorial guides. The use of special inspection is strongly dependant on the experience and qualification of the structural inspector, and is qualitative in nature. There are no comprehensive guides with procedures to specify the need for destructive and non-destructive testing methods. Furthermore, there are no guidelines how to interpret the results of both destructive and non-destructive tests. Concrete durability performance is a function of its constituents, its quality, its location and environmental loading conditions. This makes concrete durability a unique evaluation for every structure, and even for the same structure for different structural elements. For this reason, it is based on nowadays expertise and knowledge level, and it is not possible to generalise acceptance criteria for all levels of condition assessment, and for all structures. This study also includes a list of R&D needs.

The service life design of reinforced concrete structures requires models capable of reliably describing both the mechanisms of damage and its progression over time. For concrete exposed to chlorides, service life design typically disregards the onset of damage, i.e. the progress of corrosion of the reinforcing steel. Common service life design practice typically considers the end of the initiation phase of the degradation process as the design limit state. A method is proposed where a model for estimating the time to corrosion initiation is coupled with a model for estimating the time to corrosion induced cracking into a single limit state determination. An example is provided of a structural element that has been designed for the serviceability limit state of corrosion induced concrete cover cracking. The results show that the corrosion rate is a major factor for the coupled service life determination. Where fast corrosion is expected, service life extension is insignificant, and where slow corrosion rates are expected, service life extension can be significant.

The service life design of reinforced concrete structures requires material models capable of reliably describing both mechanisms of damage and the general progression of damage over time. However, most models that are currently being used only capture the process of carbonation and chloride penetration into the uncracked concrete that is at the initial phase of degradation. Typically, these models disregard the actual damage, i.e. the corrosion of the reinforcing steel. As a result, the service life design established to date only considers the end of the initiation phase of the degradation process, or, in other words, the onset of damage (time of depassivation or onset of corrosion) as a critical limit state. The corrosion of the reinforcement and its consequences, i.e. the crack formation and spalling of concrete, are not considered, which may lead to a substantially shorter estimated service life of the structures. Comprehensive investigations were recently undertaken on the depassivation of steel reinforcement and on crack formation in concrete which have resulted in an analytical prediction model for corrosion-induced cracking occurring in the surface zones of structural components. This paper presents a holistic approach which combines two models used for determining the time to depassivation (initiation phase) and the time to cover cracking as a result of reinforcement corrosion (propagation phase). An example is provided of a structural element which has been designed for the serviceability limit state of concrete cover cracking.

The feasibility of reliable wireless data transmissions over distances of several metres through dense materials was investigated within a trial project on wireless monitoring systems. By means of the project it could be shown that specially developed wireless sender units were able to transmit a signal provided by a temperature sensor through rock mass, compacted bentonite and reinforced concrete in the range of decimetres to several metres. Latest developments by VTT in the field of wireless sensor and network systems offer a large potential for sensor-based performance and condition monitoring of (infra)structures.

Cracking due to reinforcement corrosion defines an important limit state of durability and structural stability of concrete structures. By means of a comprehensive research work the influence of concrete porosity on the corrosion morphology and the mechanical behaviour of the concrete cover affected by the corrosion process could be investigated. The investigations allowed for the detailed analysis of the stresses, strains and the crack formation within the concrete cover. An analytical prediction model for concrete cover cracking was derived. This model enables the prediction of the time dependent damage process under conditions of practical relevance.

Rissbildungen und Abplatzungen über korrodierender Bewehrung kennzeichnen wichtige Grenzzustände der Dauerhaftigkeit von Stahlbetonbauwerken. Mithilfe eines umfassenden Versuchsprogramms konnte das Verhalten der Betonrandzone unter dem Einfluss korrodierender Bewehrung detailliert untersucht werden. Die systematische Analyse der Schädigungsprozesse ermöglichte die Herleitung eines analytischen Prognosemodells für die korrosionsinduzierte Rissbildung in Stahlbetonbauteilen.

Der Bereich der Betonbautechnik ist durch bemerkenswerte Innovationen der vergangenen Jahre gekennzeichnet. Einzelne Entwicklungen haben bereits Eingang in aktuelle Richtlinien gefunden. Das in der Praxis nutzbare Potential neuer wissenschaftlicher Erkenntnisse im Bereich der Betonbautechnik ist aber bei Weitem noch nicht ausgeschöpft. Man darf also gespannt auf die kommenden Jahre blicken. Der zu diesem Symposium "Innovationen in der Betonbautechnik" begleitend erschienene Tagungsband fasst hierbei alle Beiträge zu den einzelnen Vorträgen zusammen.

In some nuclear power plants, the containment liners are embedded in concrete. The purpose of the containment liner is to minimize the risk of radioactive leakage by granting a leak-tight containment building. In the presence of flaws in the surrounding concrete, such as voids, or foreign objects embedded in the concrete, there is a risk that the liner plate might corrode. This could degrade the leak tightness of the containment building, leading to an increased risk of radioactive leakage. A project is being carried out at Lund University whose purpose is to assess the capability of non-linear ultrasound to detect corroded steel plates embedded in concrete. Second harmonic analysis has been used to determine if general corrosion induced an increase in acoustical non-linearity. This was done by inspecting a steel plate with varying grades of corrosion immersed in water. Results from this study indicated that severe corrosion increases the relative parameter of quadratic non-linearity ( Beta’). Beta' is given by the ratio between the second harmonic amplitude and the fundamental amplitudesquared. Building on this result, initial findings from modulation experiments on concrete cylinders with embedded steel plates with varying grades of corrosion are presented.