Filtern
Erscheinungsjahr
Dokumenttyp
- Vortrag (107) (entfernen)
Referierte Publikation
- nein (107) (entfernen)
Schlagworte
- Concrete (107) (entfernen)
Organisationseinheit der BAM
- 8 Zerstörungsfreie Prüfung (52)
- 8.2 Zerstörungsfreie Prüfmethoden für das Bauwesen (48)
- 7 Bauwerkssicherheit (16)
- 8.0 Abteilungsleitung und andere (6)
- 7.1 Baustoffe (5)
- 7.6 Korrosion und Korrosionsschutz (4)
- 7.7 Modellierung und Simulation (3)
- 5 Werkstofftechnik (2)
- 5.5 Materialmodellierung (2)
- 7.0 Abteilungsleitung und andere (2)
Eingeladener Vortrag
- nein (107)
Simulations of concrete on the mesoscale require complex geometry models that resolve single aggregates. Since they fill up to 60-80% of the virtual specimen, efficient algorithms for their placement are necessary.
Event-driven molecular dynamics (EDMD) algorithms using growing spheres are proven to create dense polydisperse sphere packings. In this paper, a modified EDMD algorithm is presented and compared to the widely used random sequential addition algorithm. It reaches denser aggregate packings and saves computation time at high volume fractions.
When meshing the geometry, a minimal distance between the aggregates ensures undistorted elements. It is obtained by increasing the aggregate size during the placement process, which makes the packing problem more challenging. For a given aggregate volume fraction, the presented algorithm maximizes this distance and meshable mesoscale geometries for concrete specimens with more than 70% aggregate content are produced.
Carbon-based conductive coatings are complex composites, consisting of an organic or inorganic binder and conductive carbon components, for application as anodes in impressed current cathodic protection systems of reinforced concrete structures. The electrochemical properties of three coatings at different humidity and in saturated calcium hydroxide solution were studied by electrochemical methods, such as electrochemical impedance measurement, measuring of open circuit potential over time and galvanostatic polarization.
Thermo-mechanical and spalling behavior of normal weight and lightweight geopolymer concretes
(2016)
The mechanical and microstructural properties of two geopolymer concretes, produced with either quartz aggregate or expanded clay aggregate, were assessed before, during and after high-temperature exposure up to 750 °C in order to better understand the engineering properties of the material. SEM investigations were also undertaken to better understand the observed changes in the mechanical properties. It was found that dehydration of capillary water caused micro-cracking and strength losses at temperatures ≤ 300 °C. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases, leading significant strength advantages over conventional concretes. Stress-mechanical strain curves, which are the basis of the fire design of concrete structures, were determined. In addition, the two geopolymer concretes where exposed to the ISO 834-1 standard fire curve in a small-scale spalling test set-up. Acoustic emission measurements during, and acoustic measurements and optical microscopy after heat exposure were employed to investigate crack formation during the tests. Both concretes did not spall, which is attributed to their comparatively high permeability 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 occurred also in the concrete with expanded clay aggregates, presumably caused by shrinkage of the geopolymer paste on cooling.
One of the more difficult tasks for structural health monitoring is the continuous evaluation of the stability and load capacity of the building materials. This knowledge can be won, e.g., by taking material samples at the examining place with the drawback of partly destroying the structure. To avoid this, modern sensor and communication technologies offer promising methods for non-destructive testing. To address the tasks for monitoring of concrete structures, in the presented study, different sensors were combined with RFID transponders and embedded in concrete components.
Presentation on recent progress in ultrasinic testing and monitoring of concrete for massive structures. First, a new instrument (LAUS) for ultrasonic echo testing of thicknesses up to 5 m is shown. A new method to provide better images of the concrete interior, Reverse Mitem Migration (RTM)is presented. Second, the use of embedded ultrasonic transducers and data processing methods borrowed from seismology to detect subtle changes in concrete are documented.
The current knowledge about fatigue behavior of concrete is still incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Therefore, the process of fatigue itself under cyclic compressive loading was investigated in a systematic and comprehensive way. The aim of this investigation was to obtain a deeper insight and to provide a better understanding of the damage process occurring within the material during fatigue loading. Concrete cylinders were tested with a number of cycles to failure between 1E6 and 1E7. To investigate macroscopic and microscopic changes in the material, various methods of non-destructive and destructive testing were used. One main result was, in contrast to other authors, that the investigated changes in macroscopic material behavior could not be explained only by a development of micro-cracks. The results indicated rather, that the related changes in the fatigue behavior are mainly a result of viscous processes in the hardened cement paste, similar to the processes of creep. Based on the experimental results a description of the possible processes was derived which take place in the material structure during fatigue loading and also leads to the observed macroscopic changes in the material behavior. In this context, the results have shown that in case of isotropic material behavior a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain-relationship caused by fatigue.
The presentation will give an overview on non-destructive testing techniques being developed at the Federal Institute for Materials Research and Testing, Germany (BAM). This includes ultrasonic methods, ground penetration radar, Laser-Induced Breakdown Spectroscopy (LIBS), infrared thermography, pile testing, sensor technology and building scanner. A focus of the talk will be ultrasonic methods, which are well established in various aspects of concrete testing. They are used for imaging the interior geometry of constructions, estimation of concrete strength or monitoring lab investigations. However, so far the detection of distributed damages, especially in an early stage, has been almost impossible. The arrival of new technologies as embedded transducers and sensitive data processing techniques adopted from seismology has opened new field of work. Recent research has been focused to detect changes in concrete elements induced by stress, temperature, moisture or chemical attacks by permanent monitoring. Techniques as Coda Wave Interferometry can resolve changes in ultrasonic velocity in the order of 1*10-5. In addition, many researchers believe that the investigation of nonlinear effects can be used to characterize damages. The presentation will give a wrap up of ultrasonic techniques currently used in practice. This will include echo based methods as multi-channel/multi-offset imaging of structural elements using commercial and prototype devices. Imaging methods as SAFT and RTM will be shortly discussed. The focus will be on the emerging techniques used for monitoring. New types of sensors will be presented as well as the devices used in laboratory and field applications. Insight will be given on the various influence factors on ultrasonic signals and various ways of feature extraction and data processing. The results of lab experiments will be shown to demonstrate the detection of various kind of damages from mechanical load, ASR, corrosion to fatigue. The experiences with our first installations in real constructions (bridges, tunnel) will also be presented.
Fatigue is of relevance not only for metals but also for concrete. The current knowledge about fatigue behaviour of concrete is, however, incomplete. This concerns especially the progress of fatigue which precedes the fatigue failure. Some macroscopic effects of this process are well known but the governing material changes behind them are still less studied. The focus of the present contribution lay on a systematic and comprehensive investigation of the fatigue process under cyclic compressive loading. The aim was to obtain a deeper insight and to provide a better understanding of the damage process in the material. Cylindrical specimens were tested with a number of cycles to failure between 106 and 107. Various methods of non-destructive and destructive testing were used to investigate macroscopic and microscopic changes in the material. One main result was that the changes in the macroscopic material behaviour could not be explained only by the development of micro cracks. The results indicated that the related changes in the fatigue behaviour originate rather from viscous processes in the cement stone, similar to the process of creep. A sound description of the possible processes was derived from the obtained experimental results which concerns material structure during fatigue loading. It has been shown that in case of isotropic material behaviour a stiffness reduction related to a scalar value could not capture the damage effect on the stress-strain relationship caused by fatigue.
Corrosion of galvanized steel in carbonated concrete is different from the corrosion of reinforcement made of carbon steel. This is based on the corrosion mechanism of galvanized steel in dependence of the pH-value of carbonated concrete. To describe the influence of a lower pHvalue on the corrosion activity of galvanized reinforcement, different electrochemical measurements were made. The anodic and cathodic partial reaction were investigated by galvanostatic pulse measurements and potentiodynamic measurements. Especially the galvanostatic pulse measurements can describe the influence of the lower pH-value by the anodic polarization resistance with pt = 20 sec. (polarization time). Additionally, corrosion current measurements were performed, coupled with potential measurements, between galvanized steel in carbonated and non-carbonated concrete. Normally, the potential measurements are an indicator to localize the anodic and cathodic parts of a macro system. In this case the cathode was more negative than the anode before they were coupled. After the coupling of the two specimens the mixed potential was much more positive than the single potential of the galvanized steel in carbonated concrete. It increased up to the potential range of the non-carbonated specimens. This means that in case of carbonated outer areas, the embedded galvanized steel forms the cathodic part of a macro element.