Filtern
Erscheinungsjahr
Dokumenttyp
- Beitrag zu einem Tagungsband (105)
- Zeitschriftenartikel (63)
- Vortrag (59)
- Beitrag zu einem Sammelband (16)
- Posterpräsentation (7)
- Forschungsbericht (2)
- Buchkapitel (1)
- Sonstiges (1)
Sprache
- Englisch (128)
- Deutsch (119)
- Mehrsprachig (6)
- Russisch (1)
Schlagworte
- Rheology (31)
- Repair mortar (12)
- Rheologie (11)
- Betoninstandsetzung (10)
- Cement (10)
- Concrete (10)
- Geopolymers (9)
- Self-compacting concrete (9)
- Instandsetzung (8)
- Instandsetzungsmörtel (8)
Organisationseinheit der BAM
Paper des Monats
- ja (2)
Eingeladener Vortrag
- nein (59)
The SPIN project
(2011)
Globally, cement and concrete experts are at the cutting-edge to sustainable, green, healthy but nonetheless high-performance concrete. In the present age, concrete is not yet well established in Africa, which öfters the unique opportunity to build up a cement and concrete market based on the highest available state of technology. As this industry needs high level expertise, a central issue in implementation of skilled technology is cross-linking research institutions and laboratories. It should not be neglected that concrete is a product with comparably low transport ranges, which means that an improved concrete market will mainly support the local economy without exceeding financial drains to the international market, thus fostering the fight against poverty, which is an urgent need in most African countries.
The SPIN project highlights recent developments in the field of cement and concrete research with impact on the local and global economy. Challenges, future developments and opportunities for the African construction industry are in the focus.
The SPIN project is funded by the European Commission (EC) and supported by the African, Caribbean and Pacific (ACP) Group of States under the project body of the ACP Science and Technology Programme. SPIN is acronym for “Spearhead network for Innovative, Clean and Safe Cement and Concrete Technologies”. The project aims to cross-link experts with industry and policy making bodies, aiming to establish sustainable cement and concrete construction in Africa.
Selbstverdichtender Beton verhält sich unter Temperatureinfluss anders als Normalbeton, da die Rheologie neben der fortschreitenden Hydratation zusätzlich durch die von der Zeit und dem Hydratationsfortschritt abhängige Adsorption von Fließmitteln beeinflusst wird. Anhand rheometrischer Betonversuche an SVB unterschiedlicher Entwurfskonzepte mit variierter anionischer Ladungsdichte im Fließmittel wird verdeutlicht, dass mehlkornreiche SVB bei niedrigen Temperaturen sehr robust sind, während bei hohen Temperaturen mehlkornärmere Entwürfe zu bevorzugen sind. Darüber hinaus wird gezeigt, wie sich unterschiedliche Fließmittelmodifikationen in bestimmten Temperaturbereichen verhalten. Anhand des Wasser-Feststoff-Verhältnisses und des Adsorptionsverhaltens von Fließmitteln werden die maßgeblichen Prozesse erläutert und Möglichkeiten für die Entwicklung robuster Mischungen für individuelle Temperaturbereiche aufgeführt.--------------------------------------------------------------------------------------------------------------------------------------------------------------
Regarding the temperature dependent performance, self-compacting concrete (SCC) distinguishes from normal concrete, since its rheology does not only depend upon the hydration itself but supplementary upon the adsorption of superplasticizers, which is affected by the time and the hydration progress. Based on rheometric concrete investigations with different SCC mixture compositions and varied anionic charge densities of the superplasticizers, it is shown that SCC, which is rich in powder components, shows robust performance at low temperatures, while compositions with lower powder contents are favourable at high temperatures. Furthermore, the performance of different superplasticizer modifications at different temperature ranges is demonstrated. The relevant processes are explained by means of the water to powder ratio as well as the adsorption behaviour of superplasticizers, and options for the development of robust mixture compositions for individual temperature ranges are itemised.
As soon as cement is mixed with water, the hydration reaction starts to set of. The progression of the hydration process is governed by the amount and the availability of water and its possibility to access the unhydrated cement grain surfaces. For a total hydration cement theoretically requires water amounting approximately 0.38 of its own mass. Nevertheless, due to the cement’s highly hygroscopic nature, already the moisture in the ambient air can cause first hydration reactions to set in upon the cement grain surfaces. Such pre-hydration processes have an effect on the cement's properties. Dpending on the specific conditions during the storage the significance of the impact on certain properties can vary.
If cement is consumed soon after production and is transported only short distances in a silo wagon to its final destination for immediate use without further stock transfer, the effects are rather negligible. However, if cement is delivered in bags, transported over long distances, shifted serval times and stored for longer time periods until it is finally consumend, these effects can be quite severe. Due to the actual logistical situation with comparably few cement plants, large delivery distances and partially less developed infrastructures, the latter scenario pictures the situation for the most cases in Africa quite well.
In order to verify the relevance of the pre-hydation for practical application, a series of tests was conducted at the German Federal Institute for Materials Research and Testing (BAM). The influence of the cement storage on gresh and hardened concrete properties was investigated for different concrete and mortar types. The investigations show that the impact of the storage conditions is more pronounced for specialized concretes with high sophisticated optimized mixture compositions containing admixtures. Nevertheless, the effects also occur for ordinary concrete and should not be ignored.
Although being a very promising area of concrete technology, computational modeling of fresh concrete flow is a comprehensive and time consuming task. The complexity and required computation time are additionally increased when simulating casting of heavily reinforced sections, where each single reinforcement bar has to be modeled. In order to improve the computation speed and to get closer to a practical tool for simulation of casting processes, an innovative approach to model reinforced sections is proposed here. The basic idea of this approach is to treat the reinforcement zone as a porous medium in which a concrete is propagating. In the present paper, the numerical implementation of this concept is described. A methodology allowing for the computation of the equivalent permeability of the steel bars network is suggested. Finally, this numerical technique efficiency is evaluated by a comparison with experimental results of model fluids casting in model formworks.
This report is based on a three-year experience as coordinator of the ACP-EU funded SPIN project. The project involved eight African and three European partners. Practical and unforeseeable problems are addressed that occurred during the project implementation and ways how they were successfully solved are reported. The major problems in many African institutions were administrations that are not used to work in multi-national consortia and a lack of institutional support for the African researchers. In Europe a misjudgement of the African boundary conditions as well as inflexible administrations caused problems. Finally the funding scheme did not always allow to easily overcome the high number of unexpected practical problems. A major deficit identified during the project was a general trend in Africa to underestimate the role of research for the development of the continent. Most universities focus on teaching applied sciences, which is without doubt important to cover the market need for qualified staff in the rapidly growing African industries. However, this makes it unattractive for highly qualified performers to conduct research on the continent. Furthermore this leads to the situation that the curricula can only contain second hand knowledge. In order to attract international high level researchers, it is suggested to install multi-national regional excellence research centres that only promote PhD and post-doc research on a level that can cope with highest international excellence. High level research is often perceived as not capable of focusing on immediate actual problems that many African economies have to cope with.. However, it is considered to be very sustainable in terms of autonomy and mastering future challenges.
Protection and repair of concrete structures is one of the outstanding topics of the last 20 years of research and development in the concrete research. Reinforced concrete is spread technically relevant for approximate 100 years, as a new construction technology. For decades, concrete as a building material was established and known as a building material of unlimited possibilities and durability, to resist all exposures. The concrete structures were considered untouchable by environmental influences. But at least since the 70s of the 20th Century was obvious due to increased structural damage that concrete components and structures require special protection and rehabilitation measures. Otherwise they will not achieve their planned life time. In this period were also the development of the first regulations for the protection and repair of concrete structures, which were crucially shaped by the former senior public officials in Berlin, Germany. Since that time, for the different application areas: structural engineering, bridge engineering, power plant construction, drinking water construction, wastewater constructions, road and railway constructions, etc partly independent partly closely linked sets of rules were developed.
These national Standards have been superposed by the European harmonization of construction products in the late 90s. CEN as a European Standards Institution has published since 2009 in the EN 1504 series of six product Standards and 4 additional supporting Standards that should govern the repair area. On a world scale the ISO works in TC 71/SC7 on “Maintenance and repair of concrete structures” to develop regulations.
Currently in the whole African subcontinent no binding regulations for concrete repair are available, although a variety of concrete structures are present. There is thus the opportunity to adapt the rules as far as necessary and adapt to African conditions. The contribution would give a brief overview of the Standard techniques and the applicability in Africa.
Rheological optimisation for flowable mixture compositions specified for African boundary conditions
(2013)
To date concrete technology is not yet well established in sub-Saharan Africa but considering the construction technological challenges of the region, without doubt, concrete will play a major role in future. The social, economic, and geographic boundary conditions distinguish greatly from those that can be found in many other regions of the world, where concrete is considerably well established. Düring the last decades, concrete technology underwent a rapid evolution process, but in most countries, where concrete is already well established, innovations are difficult to be brought into practice, Africa, currently has the opportunity and potential, of directly establishing the best available practice in concrete technology.
The conclusion is drawn that self-compacting concrete (SCC) or highly flowable concrete is a reasonable solution for African construction sites. Since on most African construction sites the disadvantageous concreting environment is the most crucial factor for the concrete quality, the outstanding workability properties outweigh any disadvantages of SCC.
Based on this consideration, concepts are developed, how to bring about robust SCC cost-efficiently and reliably into practice as pre-mixed dry compound under consideration of local materials such as rice husk ashes, bagasse ashes, natural pozzolans, cassava starch and lignosulphonate.
For the protection of the world’s cultural heritage the restoration of listed buildings and monuments is an important task. In many cases the need to repair and reprofile natural stone elements occurs. Repair mortars for such applications have to match historico-cultural interests as well as durability related and constructive interests. Currently a research project dealing with the development and optimisation of such mortars is set up at the German Federal Institute of Materials Research and Testing (BAM). To provide the conservatorwith the best possible options to match the colour, texture and technical requirements of the stone, the repair mortar System will be setup on a modular basis. Different binder Systems are planned to be combined with different aggregate compositions and different pigments. By the modular setup the possibility of independent adjustment of mechanical properties and appearance is aimed. In order to guaranty the reversibility and to ensure that the repair mortar does not induce any damage to the original substance, strength and stiffness of the repair mortar have to be well controlled. The use of polymer dispersions is helpful here, as they allow the production of low stiffness mortars with at the same time relatively high flexural strength values. Also investigations on “alternative” modifications known from historical applications such as the use of casein are carried out.
Despite the urgency of seemingly more important building tasks, there is also a great demand for natural stone repair in cultural heritage on the African continent. There is a wide ränge of different objects reaching from prehistoric monuments up to colonial architecture, all in different säte of preservations and subjected to a variety of individual environmental ant climatic conditions, which demand individual and adjustable restoration techniques and materials.
Nachdem vor ca. 20 Jahren in Japan die Entwicklung der selbstverdichtenden Betone begann, dauerte es weitere 10 Jahre, bis diese Entwicklung auch Europa erreichte. Obwohl im Fertigteilsektor bereits deutlich mehr SVB zum Einsatz kommt als im Transportbetonbereich, bleiben die Produktionsmengen hinter dem zurück, was die Potenziale und Chancen dieser Technologie eigentlich erwarten ließen. Eine Ursache für die geringe Marktverbreitung sind die stark schwankenden und teilweise nur schwer beherrschbaren Frischbetoneigenschaften dieser Betone.
Im Rahmen des europäischen Forschungsprojekts Microcon wurden wirtschaftliche und ökologische Ansätze zur Verwendung von Feinstfüllstoffen so genannten Microfillern in Bauprodukten, die auf zementgebundenen Systemen basieren, untersucht. Hauptzielsetzung war die Entwicklung von modifizierten Bindemitteln und Zusätzen für die Transportbeton- und Fertigteilindustrie bis hin zur Marktreife. Eine der Neuentwicklungen war eine leicht handhabbare Werktrockenmischung für selbstverdichtenden Beton (SVB), die alle Schlüsselkomponenten bis auf das Bindemittel und die grobe Gesteinskörnung enthält und eine hohe Robustheit in der Anwendung gewährleistet.
In Produktionspausen können in Betonwerken nennenswerte Lagerungsintervalle entstehen. Auf Baustellen und in den Instandsetzungen werden Zement- und Trockenmörtelsäcke teilweise über längere Zeiträume gelagert. Auch im Zusammenhang mit Zementimporten und zunehmend filigran abgestimmten Betonrezepturen die bereits bei kleinen Änderungen der Ausgangsstoffe sensibel reagieren, nimmt die Relevanz der Zementlagerung zu. Aus diesem Grund wurde der Einfluss der Zementlagerung auf die Eigenschaften von Betonen in einer Versuchsreihe an der Bundesanstalt für Materialforschung und -prüfung (BAM) ermittelt.
Varying ambient temperatures in plants or on construction sites during casting of SCC can cause serious problems affecting the fresh concrete performances, such as rheological properties and setting time, with consequences for the hardened properties in the structure. By sensible choices of the components, the robustness of SCC mixtures against temperature variations can be improved. However, this aspect has not been the focus of intensive research yet. In this study the effects of varied ambient temperatures on the early performance of differently composed SCC mixtures are investigated, with emphasis on changes in rheology, early deformations, heat evolution and setting. Focus is placed on the study of the influence of the polycarboxylate ether molecule type as well as different viscositymodifying agents on the temperature robustness. Effects on the relevance for practical concrete applications are evaluated, which will provide a reliable framework of possible actions on the appropriate use of admixtures for SCC.
A major requirement on self-compacting concrete (SCC) is the resistance to any kind of separation. In the presented studies the segregation behavior of aggregates was analyzed systematically. The rheology of selfcompacting mortars was dealt with at first. Following, the segregation of particles of different size, shape and density in various mortars was examined. The aim was to find an analytical relation to estimate the risk of sedimentation, using the characteristics of the particles and those of the mortars. The classification of the aggregates according to their potential segregation is rather simple. But the description of the segregation resistance of the mortar is much more difficult. It is not possible to evaluate a mortar solely based on the rheological properties. The mixture composition must always be considered as well. Tests on concrete samples were conducted additionally to determine the influence of the overall system of SCC on the sedimentation of the aggregates.
Computational modeling of fresh SCC flow is a comprehensive and time consuming task. The computational time is additionally increased when simulating casting of reinforced sections, where each single reinforcement bar has to be modeled. In order to deal with this issue and to decrease the computational time, an innovative approach of treating a reinforcement network as a porous medium is applied. This contribution presents the model for concrete flow through reinforced sections, based on Computational Fluid Dynamics (CFD), coupling a single-phase flow model for SCC and a continuum macroscopic model for porous medium. In the last part of this paper, numerical simulations are compared with experimental results obtained on model fluids.