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Eingeladener Vortrag
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Todays superplasticizers for self-compacting concrete and high Performance cementitious materials are very versatile. Typically superplasticizers are composed of a polycarboxylic backbone equipped with polyethylene oxide graft chains of variable length and grafting degree. The mode of operation of these admixtures is based on the steric repulsion of the particles upon adsorption. The adsorption is strongly depending upon the charge density of a superplasticizer and the time dependent Adsorption processes control the retention of the flow performance – an important issue particularly for ready-mix and construction site concrete. However, the properties of polycarboxylic superplasticizers typically remain a black box for users. The paper suggests a simple and rapid test method, which can be conducted without sophisticated equipment, to qualitatively distinguish between two types of superplasticizers based on the adsorption mechanism. Interpreting the results in the right way provides a powerful tool to choose the right admixture for individual time dependent flow specifications.
Organic/inorganic mixtures were prepared from ordinary Portland cement (OPC), water (w/c 0.22), a fluorescent dye in aqueous solution (stable at alkaline pH; BAM-I), and two different comb shape polycarboxylates (PCEs), i.e., high charge (PCE-HC) and low charge (PCE-LC), respectively. Rheological and calorimetric measurements were performed prior to optical studies in order to select PCE concentrations. Absorption and fluorescence spectroscopy of the system OPC + BAM-I (CBAM-I) revealed maxima of dye BAM-I located at 645 nm and 663 nm, respectively. In presence of PCE-HC and PCE-LC, these mixtures displayed a small red shift in reflectance and a faster decrease in intensity compared to studies with CBAM-I; however, only slight differences were observed between the different PCEs. With time, all systems exhibited a decrease in intensity of BAM-I in absorption/reflectance and emission. This could be caused by dye adsorption and possibly decomposition when in contact with cement particles or hydration products.
Organic/inorganic mixtures were prepared from ordinary Portland cement (OPC), water (w/c 0.22), a fluorescent dye in aqueous solution (stable at alkaline pH; BAM-I), and two different comb shape polycarboxylates (PCEs), i.e., high charge (PCE-HC) and low charge (PCE-LC), respectively. Rheological and calorimetric measurements were performed prior to optical studies in order to select PCE concentrations. Absorption and fluorescence spectroscopy of the system OPC + BAM-I (CBAM-I) revealed maxima of dye BAM-I located at 645 nm and 663 nm, respectively. In presence of PCE-HC and PCE-LC, these mixtures displayed a small red shift in reflectance and a faster decrease in intensity compared to studies with CBAM-I; however, only slight differences were observed between the different PCEs. With time, all systems exhibited a decrease in intensity of BAM-I in absorption/reflectance and emission. This could be caused by dye adsorption and possibly decomposition when in contact with cement particles or hydration products.
Rwanda is a landlocked country in the East Africa. It is surrounded by Uganda, Tanzania, Congo and Burundi. Rwanda is a fast developing country and it spends most of its revenues to import fossil fuels from either through Mombasa Port in Kenya or Dar es Salaam port in Tanzania because the energy production in Rwanda is not sufficient for its development.
Transporting the fuels from these ports, add on to the cost of all materials, cement industry being no exception. There are three cement companies in Rwanda. The cement industries could not run in full production due to the shortage of fuel. Moreover, Rwanda is importing all the construction materials such as steel, roofing materials, etc from its neighbouring countries. This increases the cost of construction and the common man find difficult to own a house. In order to sustain the energy needs of Rwanda, different sources of energies should be focused. They are Peat, Geothermal, Methane gas, solar, wind, waste materials and Municipal wastes. Without affecting the environment, there is an urgent need to find a solution on sustainable energy in Rwanda. This paper discusses about the possible sources of energy in Rwanda which will improve the energy sustainability and turn the economy of Rwanda.
South Africa and recently Nigeria are the only net exporter of cement in Africa, yet cements are imported into the country mainly on the basis of price competitiveness. This poses potential for scatter in the properties of the cements in the South African market as pricing seems to be the only determining factor that affects Portland cements imported. A survey of cements of grade 42.5N in the South African cement market was carried out to identify major players in the industry. The identified cements, which included four locally produced and one imported cement were then analysed for both chemical composition and physical properties in line with the Eurocode EN 196. Tests carried out include loss on ignition, chloride content, sulphate content, specific surface (Blaine), standard consistence, initial setting time, final setting time, soundness and flexural and compressive strength at 2, 7 and 28 days. The testing program was initiated as part of on-going Africa-wide cement testing competency program in partnership with BAM and PTB in Germany. The performance of the identified cements from the different local manufacturers and the imported cement were analysed and are presented in this paper. These results reveal some similarities and differences in the properties. Particularly of interest is the marked difference in the 28 days compressive strength of the cements. This paper shows that there is need for further tests across the cement industry as some of the cements failed to meet certain requirement as set by EN 196. The paper also recommends the use of proficiency testing schemes in the cement industry of the country to ensure the cement laboratories are providing results of high quality and at the same time act as a check to make sure these laboratories are not failing to meet standard requirements.
In many regions of the world the urbanisation process is accelerating dramatically. This puts pressure on urban planners but also politics to develop strategies for sustainable city growth. With the rapidly increasing demand for living space in urban areas, cities typically grow vertically. This is largely driven by real estate markets and sometimes also by the desire for status symbols.
Certainly, vertical urban growth makes sense, when horizontal growth destroys important flora and Fauna (e.g. in rain forest regions), but in many cases vertical growth is result of real-estate business and Expansion limitation due to state or country borders. However, economics and borders are made by humans. They follow human-made rules. Gravity does not. Therefore, from a point of view of sustainable materials and resourceuse, the trending vertical growth of cities may come under scrutiny.
The following aspects should be considered, when a decision is taken between a new quarter with limited number of storeys or a new skyscraper.
The economic use of chemical admixtures depends on supply chains. Therefore, in most regions ins sub-Saharan Africa (SSA), the use of admixtures is not common practice. This amplifies the unfavorable framework for concrete construction such as fragmentary supply chains, high local cement prices, and unfavorable construction site facilities in this region significantly. The use of superplasticizer (SP) and stabilizing agents (STA) can enhance the concrete technology in SSA, since they can disassociate the concrete quality from external boundary influences. After providing a general overview of the peculiarities of the SSA boundary framework, economic concepts are provided, how existing material solutions can be significantly improved by the use of SPs and STAs based on locally available materials such as lignosulphonates and cassava starch. Finally a three step optimization process is described that helps developing flowable concrete based on materials that can be accessed in most locations in SSA.
Hochduktiler Beton eröffnet neue Möglichkeiten bei der Instandsetzung von Bauwerken, der Herstellung dünnwandiger Bauelemente oder dem Einsatz von Dämpfungselementen in stoßartig beanspruchten Bauwerken (z.B. in Erdbebengebieten).
Die Steigerung der Duktilität kann durch die Zugabe von Kurzfasern erreicht werden. Im Falle einer Rissbildung in der Zementsteinmatrix überbrücken die Fasern den Riss, nehmen die Spannung vollständig auf und stoppen lokal das Risswachstum. Bei weiterer Steigerung der Last reißt die Matrix an anderer Stelle. Auf diese Weise wird die Rissbildung fein verteilt und es werden große Dehnungen erreicht, bevor das Bauteil versagt. Zur Optimierung der Festbetoneigenschaften müssen die Matrixfestigkeit und die Faserart sowie deren Gehalt aufeinander abgestimmt werden. Dabei sind auch die Verarbeitungseigenschaften des Frischbetons zu berücksichtigen.
Anhand der Schallemissionen kann die Rissbildung in der Matrix und die Interaktion mit den Fasern analysiert werden. So konnte in ersten Zugversuchen gezeigt werden, dass sich Schallereignisse dem Versagen der Zementsteinmatrix bzw. dem Auszug der Fasern aus der Matrix zuordnen lassen. Damit steht zur Beurteilung des mechanischen Verhaltens von hochduktilem Beton neben der Spannungs-Dehnungs-Linie ein weiteres Verfahren zur Verfügung.
Control chart systems are used in quality management systems to keep production processes stable. In concrete production they are often applied for the control of the compressive strength. The most efficient control chart systems are cumulated sums (cusum), which help to observe deviations from a targeted value. Unlike alternative observation chart systems that focus on process data evaluation, cusum observations react very rapidly on systematic process changes in processes. Therefore, the cusum method is particularly suitable if immediate counteractions have to be taken, like in the case of fresh concrete properties of self-compacting concrete (SCC), which can have severe influences on the durability and mechanical strength of concrete. The rheology of SCC can be affected by an enormous variety of influencing factors, from raw material properties to handling or environmental temperature. The present study shows that regardless of the material based influencing parameter for a rheology change, a steady slump flow can be achieved only by adding supplementary superplasticizer in case of loss of flow and adding stabilising agent (ST) in case of increased flow if the respective V-mask indicates a systematic change. A reference SCC was artificially manipulated in order to achieve realistic experimental data for flowability and flowability changes due to changes in the raw material properties. The results point out that productions with applied cusum methods were capable of keeping the target slump flow deviation within approximately 1% only, while productions without applied cusum method deviated up to 14% and more from the target.
In the last years flowable concrete has become increasingly important in applications such as, for systems with highly reinforced concrete with a complicated formwork, or sprayed concrete and 3D-printing of concrete. For all these applications it is necessary to have tailored rheological properties. Rheology can be described by values of yield stress and plastic viscosity, which can be determined for example by evaluation of rheometer measurements. But for different materials various rheometers with different geometries and stirrers are being used. To see the effects from paste in concrete, it is necessary to investigate mixes of paste, mortar and concrete, stepwise. But currently there exists no device, which is calibrated for these different systems at once. Due to this fact, conventional tests such as V-funnel efflux-time and flow diameter were determined, as well as rheometer data in different cell sizes and geometries.
In this study the assessment of rheological effects in the binder on the rheology of mortar and concrete was investigated by using combination of two rheometers.
Baupraxis und Infrastrukturentwicklung im Hinblick auf die Betonindustrie in Ost- und Südafrika
(2011)
Der Bausektor stellt in Afrika traditionell eine der stabileren Quellen für den Arbeitsmarkt dar. Durch ständige Wechsel in der Politik wurden jedoch in zahlreichen Ländern Einschnitte bei den öffentlichen Investitionen sowie ein Rückgang der Beschäftigtenzahlen im Privatsektor beobachtet. Darüber hinaus konnte eine Verlagerung von offiziell tätigen und registrierten hin zu nicht registrierten Bauunternehmen festgestellt werden. Zahlreiche Bauunternehmen bieten Dienstleistungen an, die sie nicht ständig ausführen oder für die sie sich das erforderliche Gerät leihen müssen. Einer im Jahre 2000 hinsichtlich der Situation in Tansania veröffentlichten Studie zufolge verfügen nur etwa 15 % der Unternehmen, die Betonbauarbeiten anbieten, über einen eigenen Betonmischer. Die auf dem afrikanischen Kontinent angewandten Baupraktiken im Betonbau weichen erheblich voneinander ab. Während Südafrika über eine fortgeschrittene Technologie und eine hoch entwickelte Infrastruktur im Hinblick auf die Zement- und Betonindustrie verfügt, sind Zement und Beton in den ostafrikanischen Ländern, insbesondere in jenen, die in den 90er Jahren des vergangenen Jahrhunderts von schweren Bürgerkriegen erschüttert wurden, kaum verfügbar und zählen keinesfalls zu den in der täglichen Baupraxis verwendeten Baustoffen. Hier wird auf einheimische Baumethoden, bei denen vorwiegend Holz verwendet wird, zurückgegriffen. Dieses wird durch Abholzung gewonnen, was schließlich zur Rodung ganzer Waldgebiete führt. Die ohne entsprechende Planung, Genehmigung und Erschließung errichteten Siedlungen in Südafrika und Kenia veranschaulichen erschreckend, wie einstmals von grüner Vegetation bedeckte Landstriche in künstliche, wüstenartige Landschaften verwandelt wurden.