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Each engineering decision is based on a number of more or less accurate information. In assessment of existing structures, additional relevant information collected with on-site inspections facilitate better decisions. However, observed data basically represents the physical characteristic of interest with an uncertainty. This uncertainty is a measure of the inspection quality and can be quantified by expressing the measurement uncertainty. The internationally accepted rules for calculating measurement uncertainty are well established and can be applied straightforwardly in many practical cases. Nevertheless, the calculations require the occasionally time-consuming development of an individually suitable measurement model. This contribution attempts to emphasize proposals for modelling the non-destructive depth measurement of tendons in concrete using the ultrasonic echo technique. The proposed model can serve as guideline for the determination of the quality of the measured information in future comparable inspection scenarios.
Information about an existing structure can be collected at certain costs to evaluate the reliability and condition as realistically as necessary. This information can be relevant or irrelevant, true or biased, precise or imprecise. The incorporation of relevant and quality-assessed measured information into reliability reassessment offers the chance to extend remaining lifetimes and support decision making about optimal actions or maintenance strategies. This paper shows recent developments in a national research project that aims to produce a guideline on the NDT-based, structure-specific modification of partial safety factors. The general methodology, results from recalculations according to the Eurocodes and metrologically solvable testing tasks relevant in the recalculation of the concrete bridges are shown and compared with the non-destructive testing methods applicable to concrete bridges. A case study is used to demonstrate that as-built drawings, in this case of the positions of tendons and shear reinforcement, can be verified using the radar method.
Recently, non-destructive testing in civil engineering (NDT-CE), in particular of concrete components, has successfully mastered the leap from research to practice. Several methods have been established for field inspections to determine the concrete cover of reinforcement or to estimate the compressive strength as well as other parameters related to the concrete material. In addition, the application of nondestructive testing is indispensable, if information about the inner structure - such as the location of rebars and tendon ducts or the damage-related condition assessment to detect grouting defects, honeycombs, delamination, or corrosion - is required. Besides the selection of a suitable NDT method and an appropriate inspection system, the reliability of the results depends largely on the person who applies the non-destructive inspection technique and evaluates the inspection results. To ensure a high quality of non-destructive concrete evaluation as well as to keep the uncertainty caused by the inspection personnel to a minimum, structured, consistent and regulated theoretical as well as practical training of inspection personnel is essential. To close this gap, the subcommittee of education (UA-A) within the committee for NDT-CE of the German Society for Nondestructive Testing (DGZfP) has been reactivated in 2018 to establish uniform training standards for nondestructive concrete inspections in the long term. The subcommittee consists of scientists, practitioners, authorities, and clients. So far, the national standard DIN 4871 “Non-destructive testing - Qualification and Certification of NDT personnel in Civil Engineering (NDT-CE)” was developed and is currently under review. This standard considers the civil-industry-specifics, for example, that standards for NDT of concrete, as well as related product standards with a few exceptions, still do not exist at the moment. Within this presentation, the concept, the connection to ISO 9712 and other standards as well as an overview of the developed German standard DIN 4871 will be presented.
Moisture and salt loads of concrete can significantly change its microstructure and consequently lead to chemical and mechanical degradation. However, the non-destructive investigation of moisture and salt present in concrete is still difficult. In order to address and compare the sensitivity of different methods concrete samples with different pore systems realized by varying the w/z ratios have been fabricated. The focus of this study was put on the analysis of ultrasonic long and trans waves measured on the surface and in transmission mode. The results show clear dependencies of all applied methods. With the radar and microwave methods predominantly changes in the concrete moisture could be detected, whereas the different pore systems were not observed to alter the signals. In contrast for the ultrasound method also the ongoing hydration as well as the nature of the pore system strongly influenced the signals. As a consequence in a subsequent drying experiment it was also tried to delineate the effects of moisture and hydration. All specimens have been re-saturated under pressure and the drying experiment was repeated using the same multi-sensor approach.
This contribution summarizes actual developments and draft fundamental teaching topics in the field of nondestructive testing in civil engineering (NDT-CE). It is based on the first memorandum on teaching and research in the field of NDT-CE at German speaking universities and provides an overview of the academic education and highlights possible focuses, especially in teaching but also takes into account noteworthy developments and topics in research in the field of NDT-CE.
Suggestions are given for the development and advancement of the teaching curricula in regards to a comprehensive and sound professional education of students in civil engineering and adjacent disciplines.
The acquisition and appropriate processing of relevant information about the considered system remains a major challenge in assessment of existing structures. Both the values and the validity of computed results such as failure probabilities essentially depend on the quantity and quality of the incorporated knowledge. One source of information are onsite measurements of structural or material characteristics to be modeled as basic variables in reliability assessment. The explicit use of (quantitative) measurement results in assessment requires the quantification of the quality of the measured information, i.e., the uncertainty associated with the information acquisition and processing. This uncertainty can be referred to as measurement uncertainty. Another crucial aspect is to ensure the comparability of the measurement results.This contribution attempts to outline the necessity and the advantages of measurement uncertainty calculations in modeling of measurement data-based random variables to be included in reliability assessment. It is shown, how measured data representing time-invariant characteristics, in this case non-destructively measured inner geometrical dimensions, can be transferred into measurement results that are both comparable and quality-evaluated. The calculations are based on the rules provided in the guide to the expression of uncertainty in measurement (GUM). The GUM-framework is internationally accepted in metrology and can serve as starting point for the appropriate processing of measured data to be used in assessment. In conclusion, the effects of incorporating the non-destructively measured data into reliability analysis are presented using a prestressed concrete bridge as case-study.
Properties of alkali-activated mortars with salt aggregate for sealing structures in evaporite rock
(2021)
Concrete structures for sealing of tunnels in the host rock are an essential part of systems for nuclear waste storage. However, concretes based on blended cements or magnesium oxychloride cements, which are commonly considered for this application, can deteriorate severely due to a significant heat of hydration and associated deformation and cracking. Alkali-activated materials (AAMs) offer a potential solution to this problem because of their low heat release during hardening. To explore their suitability for the construction of sealing structures in evaporite rock, various AAMs with salt aggregate were studied regarding fresh properties, heat release, mechanical properties and microstructure. The heat of reaction of the AAMs was up to 55% lower than that of a blended cement designed for sealing structures, indicating significant benefits for the intended application. Other relevant properties such as mechanical strength and permeability depended strongly on the mix-design of the AAMs and curing conditions.
Stahlbetontragwerke sind eine wichtige Konstruktionsweise im Bauwesen und ein wachsendes Anwendungsfeld für die Zerstörungsfreie Prüfung (ZfP).
Zur Sicherung der Tragsicherheit, strukturellen Integrität, Bestimmung von Geometrien und zur Detektion konstruktiver Elemente wie z.B. Bewehrung, Verankerungen und Spannkanälen besitzen zerstörungsfreie Prüfverfahren in diesem Bereich inzwischen große Relevanz. Durch den bislang erreichten Fortschritt in der Forschung und Entwicklung haben moderne akustische und elektromagnetische Verfahren eine beachtliche Leistungsfähigkeit erreicht. Von Geräteherstellern sind diese in kommerziellen Produktserien umgesetzt worden und finden eine zunehmend breite Anwendung in der Praxis. In einigen anderen Anwendungsgebieten, in denen die ZfP bereits fester Bestandteil von Überwachungen ist, wie z.B. im Maschinenbau und insbesondere in der Kerntechnik und der Luftfahrt, existieren umfangreiche Qualitätssicherungssysteme für die konforme Anwendung und zuverlässige Einbeziehung der ZfP. Dem relativ jungen Entwicklungsstadium moderner ZfP für Stahlbetonbauteile entsprechend müssen solche Konzepte im Bauwesen erst noch entwickelt bzw. implementiert werden.
Dementsprechend hat sich der Unterausschuss Qualitätssicherung (UA- S) des Fachausschusses ZfP im Bauwesen (ZfPBau) die Definition und Verbesserung der Qualität von ZfP-Anwendungen sowie insbesondere die verfahrensunabhängige Standardisierung von Prüfprozeduren und statistische Absicherung der Prüfergebnisse zum Ziel gemacht.
Die aktuellen Arbeitsfelder beinhalten die Entwicklung praxisgerechter Konzepte für Prüfprozeduren und deren Erprobung in einem Ringversuch sowie die Konzeption von Testkörpern hinsichtlich unterschiedlicher Aspekte wie Parameterstudien, Validierung, Kalibrierung und Simulation. Dabei werden Ansätze anderer Branchen durchaus einbezogen, die Anpassung an die spezifischen Bedürfnisse und Gegebenheiten des Bauwesens wird jedoch als wesentlich angesehen und stellt somit eine wichtige Vorgabe dar.
Da die Tätigkeiten nicht auf einzelne Verfahren begrenzt sind, müssen generelle Vorgehensweisen entwickelt werden, die sich auf die jeweiligen Verfahren konkretisieren lassen. Der Vortrag gibt hierzu erste Einblicke in die Arbeit des Ausschusses.
A major advantage in the reassessment of existing structures is the possibility of including measured data that describe the actual properties and the current condition of the structure to be reassessed. Currently, the incorporation of such measured information is mostly unregulated. However, the use of measurement results is vitally important, since a measured data-based improvement of the computation models level of approximation can lead at least to more meaningful results, possibly to extended remaining life times of the structure and in the best case to a saving of resources. Conversely, not appreciating well measurable and relevant information can be equated with a waste of resources. In this paper, a concept for the comparable use of non-destructively measured data as basic variables in probabilistic reliability assessments is outlined and examined using a typical prestressed concrete road bridge as a case-study. An essential requirement is the calculation of measurement uncertainties in order to evaluate the quality of the measurement results comparably. In conclusion, the example of ultrasonic and radar measurement data is used to demonstrate the effects that the incorporation of the measured information has on the reliability of the structure.
ZfPStatik bezeichnet die Verwendung von statistisch bewerteten Messergebnissen, die mit zerstörungsfreien Prüfverfahren im Bauwesen (ZfPBau) gewonnen werden und in statischen Berechnungen zum Nachweis der Tragfähigkeit verwendet werden. Dazu ist die Ermittlung der Messunsicherheit der Versuchsdaten auf der Basis des GUM (Guide to the Expression of Uncertainty in Measurement) der erste Schritt. Der statische Nachweis erfolgt durch Grenzzustandsgleichungen, die im Bauwesen angewandt werden. Diese müssen modifiziert werden, so dass jede Messgröße als Zufallsvariable in der Grenzzustandsgleichung vorhanden ist. Der statische Nachweis erfolgt mit probabilistischen Berechnungsmethoden aus der Zuverlässigkeitstheorie. Diese Vorgehensweise wird exemplarisch für den Standsicherheitsnachweis eines Plattenbalkenquerschnitts einer Stahlbetonbrücke auf Biegung angewandt. Auf die möglichen Schlussfolgerungen aus den probabilistischen Berechnungen wird abschließend kurz eingegangen.