Filtern
Dokumenttyp
Sprache
- Englisch (13) (entfernen)
Referierte Publikation
- ja (13) (entfernen)
Schlagworte
- Concrete (4)
- Acoustic emission (2)
- Alkali-silica reaction (2)
- Fire (2)
- High strength concrete (2)
- 1H-NMR (1)
- ASR-Sensitive Aggregate (1)
- Accelerated testing (1)
- Acoustic emissions (1)
- Acoustic emissions analysis (1)
Organisationseinheit der BAM
High-strength concrete (HSC) will experience thermal microcracking, explosive spalling, and undesirable chemical changes when exposed to high temperatures, such as during fire, engulfment by lava flow, or nuclear meltdown. Knowledge of the resultant changes in mechanical, physical, and chemical properties is paramount for hazard mitigation. We present a multidisciplinary study on the influence of thermal-stressing on HSC. Our study shows that thermal microcracking in HSC initiates at 180 °C, is more prevalent during cooling, and exhibits the Kaiser 'temperature-memory' effect. We show that residual compressive strength, indirect tensile strength, ultrasonic wave velocities, and Youngs modulus and Poissons ratio decrease, whilst porosity and permeability increase with increasing temperature. We discuss these data in terms of the chemical changes during thermal-stressing, provided by thermo-gravimetric analysis, differential scanning calorimetry, and X-ray diffraction, and from optical microscopic analysis of thermally-stressed samples. We provide implications for thermally-damaged HSC structures and a new method for non-destructive monitoring.
This paper presents the results of an
experimental study on the transient strain of high
strength concrete (HSC) under heating up to 750 °C
and the impact of polypropylene (PP) fibers. Concerning
this topic only few results are available in the
literature and systematic investigations are missing.
However, basic knowledge is necessary for the
understanding of the internal damage processes in
the material as well as for heated structures. The
transient strain during heating can be separated in two
basic components: the free thermal strain and the
mechanical strain. They were experimentally determined
exemplarily for one HSC. For the determination
of the mechanisms of transient strain and particularly
the influence of PP fibers different techniques were
applied. In this context the monitoring of the microcracking
was done for the first time with acoustic
emission analysis in combination with ultrasonic
measurements. This new approach helps fundamentally
to explain the impact of PP fibers on free thermal
strain and mechanical strain during heating up.
Furthermore weight loss measurements were carried
out to characterize the moisture transport. It was
shown that the PP fibers cause an acceleration of the
moisture transport in the temperature range from 200
to 250 °C which leads to drying shrinkage in opposite
direction to the free thermal strain. Hence this paper is
a contribution to the general understanding of the
impact of PP fibers in HSC at high temperatures and
emphasizes the important influence of PP fibers on the
thermal and mechanical induced strain of HSC.
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 Schlossbru¨cke (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.
Alkali-silica reaction (ASR) is a chemical reaction within concrete which can lead over time to cracking and spalling. Due to the complexity of the problem, it still causes damage to concrete constructions worldwide.
The publication aims to illustrate the interdisciplinary research of the German Federal Institute for Materials Research and Testing (BAM) within the last 20 years, considering all aspects of ASR topics from the macro to the micro level. First, methods for characterization and assessment of ASR risks and reaction products used at BAM are explained and classified in the international context. Subsequently the added value of the research approach by combining different, preferably nondestructive, methods across all scales is explained using specific examples from a variety of research projects. Aspects covered range from the development of new test-setups to assess aggregate reactivity, to analysis of microstructure and reaction products using microscopical, spectroscopical and X-ray methods, to the development of a testing methodology for existing concrete pavements including in-depth analysis of the visual damage indicator and the de-icing salt input using innovative testing techniques. Finally, research regarding a novel avoidance strategy that makes use of internal hydrophobization of the concrete mix is presented.
Though, concrete in general is a non-combustible building material, modern High Performance Concrete (HPC) is very susceptible to violent explosive spalling during a fire attack. This requires protective measures for fire safety design of concrete structures. The current most worthwhile method to prevent explosive spalling is the addition of monofilament Polypropylene fibres (PP-fibres). However, since it has become common knowledge that PP-fibres are suitable for fire safety design, a variety of theories concerning the mode of action of PP-fibres have been suggested. The present article summarizes the most important hypothesis and presents an innovative method for the analysis of micro structural processes in heated specimens. The results show that due to the thermal decomposition of PP-fibres capillary channels are created. Simultaneously, a netlike micro crack formation occurs, which connects these capillary channels. This enables the relief of internal stresses (mechanical effect) and the formation of a permeable transport system for the escaping water vapour (permeation effect).
In this work, which is part of a larger research program, a framework called “virtual data fusion“ was developed to provide an automated and consistent crack detection method that allows for the cross-comparison of results from large quantities of X-ray Computed Tomography (CT) data. A partial implementation of this method in a custom program was developed for use in research focused on crack quantification in Alkali-Silica Reaction (ASR)-sensitive concrete aggregates. During the CT image processing, a series of image analyses tailored for detecting specific, individual crack-like characteristics were completed. The results of these analyses were then “fused” in order to identify crack-like objects within the images with much higher accuracy than that yielded by any individual image analysis procedure. The results of this strategy demonstrated the success of the program in effectively identifying crack-like structures and quantifying characteristics, such as surface area and volume. The results demonstrated that the source of aggregate has a very significant impact on the amount of internal cracking, even when the mineralogical characteristics remain very similar. River gravels, for instance, were found to contain significantly higher levels of internal cracking than quarried stone aggregates of the same mineralogical type.
Thermally-induced moisture transport in high-performance concrete studied by X-ray-CT and 1H-NMR
(2019)
The thermohydraulic damage mechanism is one of the primary causes for explosive spalling of highperformance concrete. This paper presents the spatially- and temporally-resolved analysis of the thermally-induced moisture transport and reconfiguration processes by means of X-ray-CT and 1HNMR.
Thermal testing results for a high-performance concrete, which is sensitive to explosive spalling and which was prepared with and without added polypropylene fibres, are presented in this paper. These results indicate that the addition of fibres leads to a faster and deeper migration of the drying front and, thus, to a lower likelihood of vapour-pressure induced explosive spalling.
Water migration in one-side heated concrete: 4D in-situ CT monitoring of the moisture-clog-effect
(2019)
Explosive spalling due to fire exposure in concrete structures can lead severe damage and, in the worst case, to premature component failure. For this reason, an in situ investigation of water Migration in concrete due to surface heating was undertaken.
During these experiments, a miniaturized concrete specimen within a confining and insulating double-hull was subjected to surface heating during simultaneous X-ray computed tomography (CT) scanning. Through the use of subtraction-based Image analysis techniques, it was possible to observe and quantify not only drying within areas of the concrete matrix close to the heated surface, but also the migration of moisture to both pore and matrix regions deeper within the specimen. It was also discovered that the correction of CT images for specimen deformation using DVC and variable detector performance using calibrated image filters significantly improved the quality of the results. This clearly demonstrates the potential of X-ray CT for evaluation of other rapid-density-change phenomena in concrete and other building materials.
The primary aim of this paper is to analyze the impact of mechanical pre-damage and alkali–silica reaction (ASR) on the fracture mechanical properties of pavement concrete. For this purpose, a four point bending test was applied to large format beams to produce a defined level of cyclic pre-damage. The fatigue-induced concrete degradation process was simultaneously recorded using a testing procedure specifically developed for the purpose. In addition, fatigue-induced cracks on extracted drilling cores were spatially visualized and quantified using micro X-ray 3D-computed tomography (3D-CT). The storage of the small-format test specimens, with and without cyclic pre-damage, in an ASR-conducive environment showed that pre-damage leads to an increase in ASR damage processes. Subsequent structural mechanical investigations on small format specimens with and without pre-damage show that fatigue loading and ASR significantly influence fracture mechanical parameters of the concrete.