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Self-compacting concrete (SCC) differs substantially from normal concrete as regards the quantity of superplasticiser dosage. The functionality of superplasticisers, based on the polycarboxylate ether (PCE) superplasticiser customary with SCC, is nowadays very well understood at construction material research level. However, knowledge concerning pertinent correlations does not always permeate practice to an equal extent. PCEs are extremely polymorphic as opposed to earlier superplasticiser groups, which still possess a considerable proportion of the superplasticiser market. The geometry of polymers can be adjusted individually to performance characteristics required for certain uses. Yet, in selecting a superplasticiser for its specified rheological properties, a very strong influence can equally be exerted simultaneously on the subsequent hydration process. The charge density of the PCE employed also plays a key role in connection with both rheology and early hydration.
Innovative admixture technology has significantly widened up the ränge of possibilities of concrete engineers. For many decades the water to cement ratio (w/c) was the major influencing factor for the performance of concrete. Due to the need to adjust a consistency, which still allowed reasonable workability, the w/c was typically significantly higher than technologically reasonable. Rheology modifying admixtures Support adjusting the concrete consistency largely independent of the w/c. It was only after the invention of the first superplasticizers that modern concrete technology significantly evolved in terms of flowability, strength, and durability, and only due to the steady evolution of the technology modern innovations, such as Self-Compacting Concrete, Ultra-High-Performance Concrete, or Engineered Cementitious Composites were made possible. Today’s superplasticizers are extremely versatile and can be adjusted to individual technological specifications. However, the other side of the coin of versatility is that cementitious Systems incorporating superplasticizers have become more sensitive against environmental influences, such as the environmental temperature, which may cause unwanted effects or demand for supplementary admixture use such as stabilizing admixtures. Hence, concrete mixture composition with admixtures demands for a high level of expertise and offen there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technologists. The paper gives a comprehensive overview about rheology modifying admixtures such as superplasticizers or stabilizing agents, and how they can be used depending upon the application in the most favourable way. Based on experiences with the sub-Saharan African concreting boundary conditions, which exhibit many challenges in terms of environmental boundary conditions and construction site logistics, conclusions are finally drawn, how admixtures can be used in the most beneficial way to improve the concrete casting Situation.
Innovative admixture technology has significantly widened up the ränge of possibilities of concrete engineers. For many decades the water to cement ratio (w/c) was the major influencing factor for the performance of concrete. Due to the need to adjust a consistency, which still allowed reasonable workability, the w/c was typically significantly higher than technologically reasonable. Rheology modifying admixtures Support adjusting the concrete consistency largely independent of the w/c. It was only after the invention of the first superplasticizers that modern concrete technology significantly evolved in terms of flowability, strength, and durability, and only due to the steady evolution of the technology modern innovations, such as Self-Compacting Concrete, Ultra-High-Performance Concrete, or Engineered Cementitious Composites were made possible. Today’s superplasticizers are extremely versatile and can be adjusted to individual technologicalspecifications. However, the other side of the coin of versatility is that cementitious Systems incorporating superplasticizers have become more sensitive against environmental influences, such as the environmental temperature, which may cause unwanted effects or demand for supplementary admixture use such as stabilizing admixtures. Hence, concrete mixture composition with admixtures demands for a high level of expertise and offen there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technologists. The paper gives a comprehensive overview about rheology modifying admixtures such as superplasticizers or stabilizing agents, and how they can be used depending upon the application in the most favourable way. Based on experiences with the sub-Saharan African concreting boundary conditions, which exhibit many challenges in terms of environmental boundary conditions and construction site logistics, conclusions are finally drawn, how admixtures can be used in the most beneficial way to improve the concrete casting Situation.
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.
While today, engineers can choose from a wide range of rheology modifying admixtures, in some parts of the world, these are difficult to access, due to their complex processing. However, alternatives can be bio‐based polymers such as polysaccharides from various sources. These are easily accessible all over the world, do not demand for complicated processing, and typically they are more sustainable than many established materials, which are crude oil‐based.
The paper presents the effects of acacia gum, cassava starch and the gum of triumfetta pendrata A. Rich on the rheological performance of cementitious systems. It is shown that acacia gum can be as efficient as polycarboxylate based superplasticisers, cassava starch can reduce the yield stress slightly with little effect on the plastic viscosity, and the gum of triumfet ta pendrata A. Rich increases the thixotropy of cement pastes with plasticizing polymers significantly.
There is a wide range of research worldwide on supplementary cementitious materials (SCMs) such as fly ash and slag for substituting pure cement. Such materials are suitable to be considered in a cementitious system with ordinary Portland cement (OPC) due to their high pozzolanic properties. In addition, majority of the SCMs are said to significantly improve concrete properties especially in terms of increased strength and durability. Unfortunately, the production of such SCMs is not entirely eco-friendly and also limited to certain parts of the world, hence one has to look at alternative options.
The issue of availability of resources is a strong concept that is ever-increasing, and the use of more eco-friendly SCMs in a cementitious system is furthermore attractive. Hence this paper addresses the use of eco-friendly SCMs in concrete such as rice husk ash (RHA). Despite the fact that extensive research has been done on this material, its application in a cementitious system to obtain sufficient concrete properties is still rather limited. In a country like Tanzania, high strength concrete construction is applicable in special construction cases but certainly not a high priority. Majority of construction is still undertaken using normal strength concrete. In a ternary cementitious system consisting of OPC, RHA and other readily available resources such as limestone filler (LSF), normal strength concrete can still be produced having good performance and suitable for regular on-site construction. This paper explains a possible application of obtaining sufficient concrete properties from the available resources.
The paper addresses numerical modelling of fresh self-compacting concrete. In the previous studies, a numerical tool for casting prediction is built based on the assumption that fresh concrete behaves as a yield-stress fluid and treating zones with rebars as porous media. The present contribution discusses the determination of material parameters, which are required as an input for numerical simulations of casting processes.
This study focuses on concrete flow in presence of obstacles and develops a mathematical model and a computational approach for SCC flow through reinforced formworks. In order to decrease high computational times needed to simulate castings through reinforced elements, an innovative approach to model the reinforced sections as porous media is proposed here. In the previous work, this numerical model is proved able to simulate the free-surface flow of non-Newtonian fluids through the reinforcement networks. In the present study, the applicability of the model on the concrete flow will finally be proved. The large-scale form-filling experiments with SCC will be conducted and the experiments will be simulated using the proposed numerical model. The numerical model will then be validated through the comparison of the experimental results and the results of the numerical simulations.
Interactions between waste paper sludge ashes and superplasticizers based on polycarboxylates
(2016)
In many industrial nations, about two third of the paper demand is covered by recovered paper. A major process step within the treatment of waste paper is the de-inking. It is a floating process yielding paper sludge as a waste product. About 50 % of this residue is used as a fuel. In several cases it is burnt at temperature of about 850 °C and thereafter the accrued ashes are collected in the flue gas filter. During the combustion, kaolinite and calcium oxide generate gehlenite and larnite. Calcite is the main component of waste paper sludge ash (PA).The chemical and mineralogical composition of PA suggests using it as a supplementary cementitious material.
In modern construction materials technology, workability aspects gain importance, since for most modern materials the rheology and compaction ability are relevant for the operation at a hardened state. It was observed that PA significantly increases the water demand of powder systems, which can cause serious problems during the casting of mineral binder systems containing PA. It is therefore obvious that binder systems containing PA might demand for the use of superplasticizers. Superplasticizers are polymers with anionic backbone that cause electrostatic and steric repulsion effects upon adsorption on surfaces of particles and hydration phases.
In this paper interactions between superplasticizers and waste paper sludge ashes are discussed and analysed. Based on observations of changes in the zeta potential and the dispersion of the particle system, the influence of the charge density of superplasticizers is observed and time dependent effects are demonstrated.
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.
Polysaccharides are important rheology modifying admixtures in the building material sector.
The use of starch is becoming increasingly important, due to many ecological and economic advantages. In the construction sector, starch ethers are being used as thickeners and as means to increase the yield stress. The starch ethers that are available on the market differ in
their behaviour, which can vary greatly depending upon the binder system and mortar composition, e.g. solid volume content, binder type, additional admixtures. In view of the limited knowledge about the influence of molecular modifications associated with cement based systems, some fundamental rheological functional mechanisms were analysed in this study. The differently modified starch ethers used were derived from potatoes. They varied in their charges and degrees of hydroxypropylation. The setting and the flow behaviour of all examined variations
of starch ethers were analysed in cement pastes. In order to illustrate the effects of the starch ethers that were used, the water-cement ratio (w/c) was held constant in all the mixtures [Schmidt 2012]. The results indicated significant differences in setting and flow behaviour.
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.
With ongoing innovation in process technology, the challenges of concrete technology are more and more focused on the rheological optimisation for these processes, since improper mixture stability or poor compaction ability negatively affect the concrete homogeneity and quality. However, along with the increasing complexity of today’s concrete mixture compositions, concrete becomes more prone to failure regarding the casting process. Variable properties of the raw materials typically cause changing workability. The reasons can be found among others in scattering water contents, physical or chemical properties of the cement or varying environmental temperatures. Robustness in the delicately adjusted rheology, however, is of utmost importance for modern and future process technology, from sprayed concrete over pumpable concrete towards 3D-printing, with regard to the long-term strength, the function and the durability. Typically, material induced changes cannot be identified easily due to the complex interactions of concrete constituents. Therefore, a precise and prompt counteraction is impossible. However, it is known that the yield stress can be controlled by addition of supplementary superplasticizer or stabilising agent. In combination with computerized process observation tools that can rapidly interpret and react on changes in the rheology, it is therefore thinkable, that only these two admixture types can adjust the rheology steadily and permanently, regardless of the actual root cause for observed macroscopic rheology change. The presentation will firstly give a comprehensive overview of effects at the interface between pore solution, particles and hydrates, which affect the rheology of fresh concrete. Secondly, ways are recommended how the rheology can be actively manipulated before eventually computerized methods are demonstrated that help to actively and rapidly assess and counteract performance scatter during steady casting processes.
African cement inffastructure is quite complex. Apart from Northern Africa and South Africa in particular, cement plants are scarce resulting in highly unstable cement pricing. Clinker and cement are imported from overseas, e.g. from Portugal, Turkey, Pakistan, Indonesia, and China. Imports are typically determined by the lowest price, and as a result the countries of origin of products vaiy regularly yielding large scatter of properties. Quality control and a good quality infrastructure are thus of utmost importance for the safety of the popuiace, an issue, which is actually often neglected. With funding of the German Metrology Institute (PTB) and Support of the SPIN project, a proficiency testing scheme for cement testing according to EN 196 was set up for African laboratories. Proficiency testing schemes, also called round robins. are interlaboratory performance comparisons allowing participants to evaluate themselves against pre-established criteria. Thev are a powerful tool to help laboratories improve their performance as well as demonstrate their competences to accreditation bodies or Customers. 26 laboratories from 20 nations, 18 of which from Africa, participated. The BAM Federal Institute for Materials Research and Testing acted as coordinator and provider of the scheme. The aim of the round robin was to interpret the submitted data further beyond the pure statistic analyses. The data provided a positive picture of the performance of the participants in general, but it also exhibited a number of technical fields that need improvement. The paper provides the general results of the scheme and analyses identified strengths and weak points based on the submitted and non submitted data as well as on discrepancies from the EN 196 procedures during measurements. The application of EN Standards for material testing is critically discussed and since quality infrastructure is also always an issue between industrial and political stakeholders, suggestions for the mitigation of the identified shared problems are given.
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.