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Alkali-activated materials are ideal for the repair of concrete structures in harsh environmental conditions due to their high durability in chemically aggressive environments. However, slag-based mortars, in particular, are prone to shrinkage and associated cracks. In this respect, the application of steel fibres is one solution to reduce the formation of shrinkage induced cracks and to improve post cracking behaviour of these mortars. This study investigated the influence of two different types of steel fibres on the tensile properties of two alkali-activated mortars. Direct tensile tests and single fibre pull-outs were performed to analyse the determining failure modes both on macro and micro scale. Mechanical testing was accompanied by non-destructive testing methods such as digital image correlation and acoustic emission for a detailed analysis of the fracture process.

Der 60 °C-Betonversuch ist eine in Deutschland und international verbreitete Methode um die Empfindlichkeit von Gesteinskornungen und Betonen hinsichtlich einer Alkali-Kieselsaure Reaktion (AKR) zu prufen. Dieser Beitrag stellt eine neuentwickelte automatisierte Variante der Prufmethode in einem Minireaktor vor. Dabei werden die Dehnungen von Betonprismen nicht, wie beim konventionellen Test manuell, sondern automatisch in entsprechend ausgerusteten Prufbehaltern gemessen. Neben einer Reduzierung des Messaufwands bringt die automatisierte Prufung im Minireaktor weitere Vorteile. Die Dehnungssignale werden, anders als bei der konventionellen Messung, quasi-kontinuierlich aufgezeichnet und bilden das Expansionsverhalten der Prismen detailliert ab. Auserdem bleiben Unterbrechungen der AKR-provozierenden Lagerung aus, die bei der konventionellen Prufung notwendig sind. Im Beitrag werden die automatisierte und konventionelle Prufung anhand der Dehnungen von Prismen zweier Gesteinskornungen verglichen. In Ubereinstimmung mit vorhergehenden Untersuchungen sind im Minireaktor gemessene Dehnungen systematisch kleiner als manuell gemessene. Als mogliche Ursache werden Unterschiede im Auslaugverhalten experimentell untersucht. Zur Uberprufung der Einstufung der zwei Gesteinskornungen erfolgen auserdem Referenzprufungen in der Nebelkammerlagerung bei 40 °C.

Bending beams and slabs are typical examples for structural elements used for reinforced concrete structures such as bridge girders, T-beams and bridge decks. Their strength related failure modes at maximum loading can be divided into bending and shear failure. The failure of beams loaded in bending can occur with or without indication. Therefore, conventional design concepts aim on failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexural compression failure. These indicating factors can also be used for Structural Health Monitoring (SHM) of civil infrastructure systems (e.g. bridges) to identify structural changes. In this context, non-destructive testing (NDT) methods offer different techniques for measuring deflections or crack formation and opening. However, profound knowledge on the determining failure modes of bending beams and their detection by NDT methods is required for the reliable application of SHM. Different NDT methods have been used in this study for analysing the load-bearing behaviour of a reinforced concrete beam in bending. The different measuring techniques are briefly described and their applicability is discussed by means of experimental results. For this purpose, the load-bearing behaviour of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by optical NDT and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending beam was equipped with two single-mode (SM) sensor fibres. One fibre served as Distributed Optical Fibre Sensor (DOFS), whereas the other fibre contained Fibre Bragg Grating (FBG) sensors. In addition, optical deformation measurements using Digital Image Correlation (DIC) and Stereophotogrammetry (SP) were conducted.

The fatigue process of concrete under compressive cyclic loading is still not completely explored. The corresponding damage processes within the material structure are especially not entirely investigated. The application of acoustic measurement methods enables a better insight into the processes of the fatigue in concrete. Normal strength concrete was investigated under compressive cyclic loading with regard to the fatigue process by using acoustic methods in combination with other nondestructive measurement methods. Acoustic emission and ultrasonic signal measurements were applied together with measurements of strains, elastic modulus, and static strength. It was possible to determine the anisotropic character of the fatigue damage caused by uniaxial loading based on the ultrasonic measurements. Furthermore, it was observed that the fatigue damage seems to consist not exclusively of load parallel oriented crack structures. Rather, crack structures perpendicular to the load as well as local compacting are likely components of the fatigue damage. Additionally, the ultrasonic velocity appears to be a good indicator for fatigue damage beside the elastic modulus. It can be concluded that acoustic methods allow an observation of the fatigue process in concrete and a better understanding, especially in combination with further measurement methods.

Inacceptable capability and durability of joint sealing systems but also inadequate traffic performance (noise emission; overrolling comfort) up to traffic safety aspects reflect the still enormous demand for data-based description of concrete pavements performance under heavy loading conditions. Especially the deformation behavior of concrete pavement slabs in the joint region in consideration of new pavement construction types and improved concrete mixtures meanwhile established but also under the steeply rising traffic loads is not sufficiently explored. To create a data basis for advanced design rules, evaluation methods and product standards - and with it to improve quality, durability and finally sustainability of pavements - an innovative 3-D sensor system SENSO JOINT adapted to german roadworking requirements and suitable for heavy-duty operating conditions was developed. The contribution introduced describes the development of an extensive technical solution based on the analysis of decisive loads, interactions and boundary conditions. Based on calibration data, results of laboratory testing and finally field-testing on different concrete pavement construction types the outcome of a multi-level evaluation process shall introduce the potential of the new sensor system.

Grouted connections are intensively used in offshore rigs, platforms as well as jacket and monopile offshore wind turbine structures. Being located in remote offshore conditions, these connections can experience considerable adverse loading during their lifetimes. Degradation was reported inside similar connections, which were installed in the last three decades. Grouting in the offshore sites may often be proven difficult, which eventually leads to reduced load-bearing capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs. In this study, scaled grouted connections were manufactured using a novel mould, whose integrity were monitored using digital image correlation (DIC). The connections were loaded under static load to visualize the main failure pattern using distributed fibre optic sensors and acoustic emission (AE) analysis. Grouted connections were then repaired using a cementitious injectable grout. The effectiveness of the grout injection was monitored using dye penetration technique. Finally, specimens are reloaded to identify the potential of such repair for grouted connections.

Two fly ash-based geopolymer concretes with quartz aggregates or with expanded clay (lightweight) aggregates were exposed to the ISO 834-1 standard fire curve in a small-scale fire test set-up. Acoustic emission measurements during fire exposure and subsequent cooling were employed to study spalling events and cracking during the tests. Optical microscopy and additional acoustic measurements were conducted after the testing to better understand the crack propagation in the samples. The testing revealed that neither of the concretes were susceptible to spalling, which is particularly notable for the concrete with quartz aggregates, as it is a high-strength concrete. This behavior is attributed to the relatively high permeability of the concretes and their low amount of chemically bound water. Significant crack formation was detected only around the temperature of the α–β quartz transition (573 °C) and on cooling. Because of aggregate deformations at the quartz transition temperature, deterioration after heating was more significant in the geopolymer concrete with quartz aggregates. Crack formation also occurred in the concrete with expanded clay aggregates, caused by shrinkage of the geopolymer paste on cooling. Acoustic emission measurements proved to be a valuable tool to investigate processes during high temperature exposure.

The susceptibility of concrete structures due to alkali–silica reaction (ASR) can be assessed by means of ASR concrete prism testing at 60 °C, according to RILEM AAR 4.1. There, expansion of concrete prisms indicates alkali-reactivity of the examined concrete mix. This work applies in situ expansion measurement to accelerated concrete prism testing. Automated measuring facilitates both storage without the usually necessary interruptions for manual measurement and acquisition of quasi-continuous expansion data. A comparative experimental programme showed that conventional testing resulted in stronger expansion and leaching of alkalis than automated testing. Experimental simulation of interruptions, typically associated with manual measurements in conventional testing, could prove the influence of these cooling–heating cycles. Two phenomenological approaches, frequently used for describing reaction kinetics of ASR by linking it to expansion results from ASR-testing, were validated with continuous expansion data of three types of aggregate. Experimental expansion depicted s-shaped curves similar to them of the modelling approaches. However, strong swelling recorded in the beginning of the test was not covered by the model curves. Auxiliary measurement of acoustic emissions and ultrasonic velocity helped characterising mechanisms such as hydration and cracking, which also influence prism expansion. The proposed modification of the measurement procedure provides an extended basis to analyse expansion mechanisms. Regarding data for validation of ASR-expansion models, continuous expansion results are preferable to conventional test results.

Severe fires at the Friedrichshain flak bunker in May 1945 destroyed or severely damaged an important part of the collection of the Kaiser-Friedrich-Museum, Berlin, today’s Bode Museum. Fragments of two marble sculptures by Tullio Lombardo, sculptor of the Italian renaissance, have survived and are now stored in the Bode Museum. In preparation for restoration, the condition of the sculptures has been documented in detail in 2010. The study revealed that the fragments are not stable enough to exhibit them in a horizontal nor in an upright position. The intention of a further study in 2012 was to find a correlation between ultrasonic velocity and mechanical properties of burned Carrara marble as a basis for stabilization measures. The results and numerous supplementary experiments show that the current condition of the sculptural fragments can not only be explained by the exposure to fire.

This paper presents the analysis of crack propagation in strain-hardening cement-based composite (SHCC) under tensile and flexural load by using acoustic emission (AE). AE is a non-destructive technique to monitor the development of structural damage due to external forces. The main objective of this research was to characterise the cracking behaviour in SHCC in direct tensile and flexural tests by using AE. A better understanding of the development of microcracks in SHCC will lead to a better understanding of pseudo strain-hardening behaviour of SHCC and its general performance. ARAMIS optical deformation analysis was also used in direct tensile tests to observe crack propagation in SHCC materials. For the direct tensile tests, SHCC specimens were prepared with polyvinyl alcohol (PVA) fibre with three different volume percentages (1%, 1.85% and 2.5%). For the flexural test beam specimens, only a fibre dosage of 1.85% was applied. It was found that the application of AE in SHCC can be a good option to analyse the crack growth in the specimens under increasing load, the location of the cracks and most importantly the identification of matrix cracking and fibre rupture or slippage.

The tensile behavior of strain-hardening cement-based composites (SHCC) is usually investigated on macroscopic scale by means of direct tension tests or bending tests. Additionally, the micromechanical properties of the composites are often described based on single fiber tension and pull-out tests. Such investigations, performed both on macroscopic and microscopic scales, are based on ‘classical’ force and displacement measuring techniques. Advanced test methods such as digital image correlation (DIC) and acoustic emission analysis (AE) may facilitate the identification and the analysis of the failure mechanisms in SHCC, which is important for both monitoring loaded SHCC elements and further material development and optimization. In this study, these two techniques are combined to characterize the failure mechanisms of three different types of SHCC in direct tension tests. The results are related to data of stress and strain measurements. It is shown that DIC provides detailed spatially resolved and stress related strain measurements. Furthermore, it is demonstrated that AE allows for the localization of active cracks, quantification of the damage accumulation under increasing stresses, and characterization of the dominant crack bridging mechanisms and failure modes observed in the different types of SHCC.

The article at hand presents an investigation on crack development in three different types of strain-hardening cement-based composites (SHCC) subjected to uni-axial tensile loading. The aim of the work was to evaluate the applicability of acoustic emission (AE) measurements for determining the progressive damage within the material, as well as for differentiating the individual damage events by their origin and decisive mechanisms, such as matrix cracking, fiber pullout or fiber rupture. The acoustic emission method proved to be fully appropriate for recording and evaluating the fracture related processes in various types of SHCC. Valuable information on failure mechanisms and quantitative description of damage depending on SHCC composition was obtained and evaluated with respect to the measured stress-displacement curves and under consideration of fracture surfaces and crack patterns observed on the specimens.

Zur Beurteilung der Alkaliempfindlichkeit der Gesteinskornung finden normativ der Betonversuch mit Nebelkammerlagerung bei 40 °C und alternativ der 60 °C-Betonversuch Anwendung. Als Beurteilungskriterium fur den Schadigungsprozess wird bei beiden Verfahren die an Betonprismen diskontinuierlich ermittelte Dehnung herangezogen. Auch bei den zusatzlich in der Nebelkammer gelagerten Wurfeln beschrankt sich die Schadensanalyse auf eine diskontinuierliche Verfolgung der maximalen Breite der Oberflachenrisse. Im Rahmen des Beitrags wird eine neuartige, bereits erfolgreich erprobte zerstorungsfreie Prufmethodik vorgestellt. Diese ermoglicht durch eine gleichzeitige kontinuierliche Dehnungs-, Schallemissions- und Ultraschallmessung eine kontinuierliche Verfolgung der inneren Gefugeveranderungen in den Betonprufkorpern wahrend der AKR-provozierenden Lagerung. Dabei wurden drei zeitlich aufeinanderfolgende Phasen der inneren Gefugeveranderung mit einer Dominanz von Hydratation, Rissbildung sowie vermutlicher Rissverfullung mit Reaktionsprodukten identifiziert. Die ablaufenden Rissbildungsprozesse wurden zusatzlich mit diskontinuierlich durchgefuhrten rontgentomografischen Untersuchungen zeitlich und raumlich visualisiert. Die CT-Befunde wurden durch mikroskopische Untersuchungen bestatigt. Analysis of ASR-induced damage processes in concrete Application of innovative measurement techniques The alkali-reactive potential of aggregates is assessed according to the German guidelines by two different test methods. These are the 40 °C fog chamber storage and as an alternative method the 60 °C concrete prism test. In both tests, the discontinuously measured expansion of ASR-generating stored concrete prisms is used as criterion for the internal damage process. The damage analysis of the cubes additionally stored in the 40 °C fog chamber is also restricted to the discontinuous measurement of the crack width on the cubes surfaces. This paper describes an innovative, already successfully applied, non-destructive measurement methodology. This methodology enables a continuous monitoring of the internal damage processes in ASR-generating stored concrete specimens due to a continuous recording of expansion, acoustic emission and ultrasonic velocity. Thereby, three sequent phases of internal structural changes dominated by hydration, crack formation and filling of the cracks with ASR-gel and/or other reaction products can be detected. The crack formation was temporally and spatially visualised by discontinuously applied X-ray computed tomography measurements. Microscopic analyses confirm the findings of the X-ray 3D-CT.

Es werden drei Pruefverfahren verglichen, die eine Beurteilung der Wirksamkeit von Faserzugaben zum Beton zur Reduzierung der Schrumpfrisse erlauben. Anhand der Versuchsergebnisse werden dann die Vor- und Nachteile benannt und diskutiert. Die Beigabe von Fasern ist eine von mehreren Methoden, um Schrumpfrissbildungen im Beton zu vermindern. Ist diese Wirkung nachzuweisen, wird eine experimentelle Beurteilung erforderlich. Die Pruefverfahren, die sich im Hinblick auf die Durchfuehrung unterscheiden, finden sich in verschiedenen Normen und Richtlinien. Aus den moeglichen Verfahren wurden fuer den Vergleich drei Verfahren ausgewaehlt. Es handelt sich um das Pruefverfahren des Deutschen Instituts fuer Bautechnik (DIBt) zur Zulassung von Fasern als Betonzusatzstoff, das Pruefverfahren nach der Richtlinie Faserbeton der Oesterreichischen Vereinigung fuer Beton und Bautechnik (OEVBB) und das Pruefverfahren nach der Amerikanischen Pruefvorschrift ASTM C 1597. Die Versuchsanordnungen und -bedingungen sowie die Versuchsdurchfuehrung dieser drei Verfahren werden beschrieben. Fuer die durchzufuehrenden Versuche wurden zwei verschiedene Betongrundrezepturen (B1 und B2) mit verschiedenen Zementen und unterschiedlichem Wasser-Zement-Wert hergestellt. Bei den zugegebenen Fasern handelte es sich um Polypropylenfasern mit rechteckigem Querschnitt. Mit den Betonmischungen wurden sowohl unbewehrte als auch faserbewehrte Probekoerper hergestellt. Die Betongrundrezeptur B1 diente zur Herstellung der Versuchsplatten fuer die Pruefung mit dem DIBt-Verfahren, waehrend die Pruefung nach der OEVBB-Richtlinie an Probekoerpern aus der Betonmischung B2 erfolgte. Fuer die Pruefungen der mit der Betonmischung B1 hergestellten Versuchsplatten nach ASTM wurde ein neu entwickelter Versuchsaufbau benutzt, der bei den Versuchen im Windkanal einen gleichmaessigen Luftstrom ueber die Oberflaeche der Platten gewaehrleistet. Zusammenfassend wird festgestellt, dass grundsaetzlich alle drei Verfahren zur Beurteilung der Wirksamkeit von Fasern zur Verminderung der Schrumpfrissbildung geeignet sind. Die Art des erzeugten Luftstroms kann sich jedoch nachteilig auf die Reproduzierbarkeit der Ergebnisse auswirken. Des Weiteren wird empfohlen, zusaetzlich zur visuellen Beurteilung der Rissbildung eine Quantifizierung durch Rissvermessung vorzunehmen. Die Einfuehrung eines Rissminderungsbeiwertes auf der Basis der Rissflaechen wird vorgeschlagen; einschliesslich eines minimalen Rissminderungsbeiwertes als Kriterium zum Nachweis der Wirksamkeit der Fasern. Weitere Empfehlungen beziehen sich auf die Mindestrissbreite in den unbewehrten Referenzplatten, die Verdunstungsrate, die Pruefdauer und die Messung des plastischen Schwindens.

Concrete pavements subjected to fast evaporation during the first hours after concrete placing are prone to plastic shrinkage cracking. In this paper the mechanisms by which evaporation of water leads to capillary pressure, and finally to shrinkage and cracking, are briefly discussed. In the main part, the paper presents results of a study on the evaluation of three different measurement setups. With two of these setups usually the efficiency of fibres in reducing plastic shrinkage cracking of concrete is assessed in Germany and Austria. The third one is an advanced setup that was developed in collaboration of BAM and Empa. It allows performing measurements of parameters that are important for the evaluation of the cracking risk.

In diesem Beitrag werden Untersuchungsergebnisse vorgestellt, die den Einfluss von Mikrosilica auf die Selbstaustrocknung und das damit verbundene autogene Schwinden der Bindemittelmatrix von Hochleistungsbetonen beschreiben. Die dadurch hervorgerufenen Schadigungsprozesse wurden mit Hilfe von zerstorungsfreien Prufmethoden verfolgt. Dabei zeigte sich, dass eine durch auseren Zwang bedingte Mikrorissbildung durch Schallemissionsanalyse detektiert werden kann.

The present study documents the results of an inter-disciplinary model project that was planned with the aim of developing an innovative winter covering system for marble statuaries located on the Schlossbrücke (Berlin). Such a system would need to fulfil the various requirements for structural stability, aesthetics, climate and practical use. This applied research represents the first complex scientific study of the sustainability of a winter covering system. The study is characterised by the use of complex scientific instruments such as special laboratory analysis and numerical simulation tools. The interaction between the environment and the artefacts in connection with the innovative winter covering structures were studied by extensive climatic monitoring. peerReviewed

The influence of the mix composition on the early-age autogenous shrinkage of high-strength cement paste with and without silica fume and the resulting crack formation were investigated. The mechanisms of autogenous shrinkage were studied systematically by linking the measured deformations to hydration kinetics and to pore structure data. The time t0 which assigns the transition from the plastic state to a solid structure was determined by analysis of the measured strain rate. It was demonstrated that the autogenous shrinkage correlates reasonably well with the self-desiccation and the thereby expected capillary stresses. The crack formation was detected using acoustic emission (AE) analysis. AE measurements were performed simultaneously on hardening silica fume cement pastes with and without external restraint of deformation. The results showed that the intense autogenous shrinkage during the acceleration period of cement hydration was associated with the begin of micro cracking shortly after t0, irrespective of the existence of external restraint. The cracking was more pronounced in the case of restraint.

Joint sealants influence decisively the performance and service life of pavements although they account for only a small fraction of the total investment. Motivated by the damages observed and the resulting, increasing maintenance efforts, the Federal German Government recognizes the need for performance-evaluated joint sealing systems with improved capability (fitness-for-purpose) and durability. A literature study showed that an identification of the actual mechanical system behavior under realistic loads as well as a prediction regarding the durability (fatigue, climatic effects) of joint sealing systems are either completely lacking in most of the relevant evaluation methods or have only been incompletely addressed previously. Furthermore an imbalance between commonly used test methodologies and the actual development status of modern modified sealing materials exists, i.e., the current test methods are not effective in evaluating the performance of tailor-made products. In this paper, the authors suggest a methodology to overcome the present situation. In contrast to the existing, predominantly empirical evaluation and selection of joint sealing materials and systems for pavements, the new approach is defined by verified performance under relevant and superimposed loads. This new approach is expected to allow a more engineered joint design. In addition to the adaptation of performance-oriented material identification tests, a special focus was placed on the development and installation of a complex test facility for the investigation of the service capability and durability of joint sealing systems in building constructions in general. This paper presents an attempt at the realization of this approach for pavement joints with the help of our new joint sealant test equipment utilizing a specific, adapted load function, which comprises cyclic movements (slow and fast acting), as well as crucial climatic exposures. The test data and its interpretation are discussed. For example, the actual mechanical behavior of the various joint sealing systems as well as the relevant maximum loading of cohesive and adhesive bonds can be deduced and used to differentiate between systems. Furthermore, information gained allows discrimination of products within the various joint sealing systems. The test results will also enable numerical simulations, e.g., of different joint designs or materials by finite element analysis. The fatigue behavior is detected by analysis of cycle-dependent changes of the mechanical system characteristics. The evaluation methodology further allows investigation of the degradation mechanisms of specific system failures and, thus, enables service life prediction by reproducing the performance of the complete system under realistic conditions. Constructional defects and material flaws can be activated and detected by the performance-related test methodology, thus identifying possible corrections to material selection and application procedures. The potential of the proposed evaluation methodology is discussed for several thermoplastic and reactive joint sealing systems.