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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.
The possibility to tailor the rheology of concrete eventually opened up the path to new technologies, where design criteria are no longer limited to the compressive strength. Thus, it can be concluded that the capability to control the rheology of concrete can be considered as catalyst for many of today’s concrete innovations. In the same way rheology modifying admixtures will be key to mastering the challenges of the next decades. In many regions of the Southern hemisphere, the effective use of chemical admixtures would significantly contribute to solve problems induced by the challenging climate, but particularly in Africa there is often a lack of local supply and supply infrastructure. In Africa, concrete admixtures are largely shipped or transported from outside the continent. The long distance transportation of chemicals is not very environmentally friendly and the economic consequences are dramatic: the agents are difficult to purchase, expensive, and there is no large variety of products available in the market. Due to the enormous relevance of rheology modifying admixtures, it is worthwhile to recollect that nature provides an enormous variety of chemicals that can be used readily or with low processing and without enormous transportation distances, since these plants are typically regionally abundantly available, cheap, and they are environmental friendly. The paper presents an overview of various options for rheology modifying admixtures, that can be found in Africa, and that may become a real alternative, once their interaction within the complex cementitious system is well understood.
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.
In order to observe the effect of the aggregate phases between 2 mm and 16 mm without overlap with rheological effects induced by the cement hy-dration and without interactions with a threshold fine sand particle size that affects both, paste and aggregates, rheological experiments were conducted on a limestone filler based paste mixed with aggregates up to 16 mm. Vari-ous aggregate fractions were blended and mixed with the replacement paste in different volumetric ratios.
The dry aggregates’ flow coefficients were determined and compared to yield stress and plastic viscosity values at different aggregate volume fractions. The results indicated that the flow coefficient is not a suitable parameter to predict the performance of the aggregates in the paste. It was shown that the yield stress of pastes is largely determined by the blend of different aggregate fractions, while the plastic viscosity to large extend depends upon the coars-est aggregate fraction.
Based on the results, ideal aggregate composition ranges for minimised yield stress are presented. For the plastic viscosity no such grading curves to achieve minimum values could be found, but high viscosity curves are identified.
Portland cement, as we know it today, has its origin in Great Britain approximately 170 years ago. Since then, concrete technology has spread out to Europe, the United States, and Japan, where it became a key component for rapid industrial development. Europe, the Unites States and many Asian countries today have developed a high level of technology regarding concrete construction.
However, each of them has a unique history and as a result, different “concrete philosophy” depending upon the social, environmental and financial boundary conditions, as well as their evolution throughout the years and local construction traditions. As a result, the word concrete may refer to rather different materials in America, Europe, and Asia.
Apart from South Africa, most sub-Saharan African countries cannot look back on a similarly long cement and concrete history. Cement and concrete are rather new materials and not yet well established. This gives African engineers the unique opportunity to learn from past mistakes and to develop a concrete technology, which refers to the best available practice. However, in many sub-Saharan African countries, standards and regulations are adopted (preferably from Europe or the US) without consideration of the historical background of these standards. Although this practice helps saving resources for the implementation, it does not necessarily yield the best result in the African environment, and also from an economic point of view it might come back disadvantageously due to unnecessary overdesigning.
By comparing the differing states-of-the-art in North America, Europe, and Asia, this paper emphasizes, how regional conditions determine the practice of concrete technology in the sub-Saharan area. It is therefore important for Africa to develop a unique African concrete technology, which is perfectly fitted to the specific local conditions, even if it may vary distinctively from the established practice elsewhere. The paper concludes that African nations should effort into adapting existing principles that have proved to function well rather than adopting existing standards.
Polymers that help tailoring rheological properties during the casting process have become inevitable constituents for all kinds of high-performance concrete technologies. Due to lacking industries, these typically crude-oil based admixtures are not readily available in many parts of the world, which limits the implementation of more sustainable high-performance construction technologies in these regions. Alternative polymers, which often demand for less processing, can be derived from local plant-based resources. The paper provides experimental data of flow tests of cement pastes with polysaccharides from Triumfetta pendrata A. Rich, acacia gum and cassava without and in the presence of polycarboxylate ether superplasticizer. The flow tests are amended by observations of the zeta potentials and the hydrodynamic diameters in the presence of and without calcium ions in the dispersion medium. The results show that in the presence of and without calcium ions all polysaccharides provide negative zeta potentials, yet, they affect flowability and thixotropy in different ways. Cassava starch, acacia gum, and the gum of Triumfetta pendrata A. Rich qualified well for robustness improvement, strong stiffening, and additive manufacturing, respectively. The reason for the different effects can be found in their average sizes and size distribution. Due to the promising results, a flow chart for local value chains is derived on the example of yet unused cassava wastes, which can be converted in parallel.
Chemical admixtures like superplasticisers or stabilising agents are of ever increasing importance for modern concrete technology. They liberate the workability of concrete from its dependency on water content, and thus, open the gate towards innovative and future oriented concrete technologies such as self-compacting concrete. Today, admixture addition has become common practice in concrete technology, but the understanding of their highly complex mode of operation is extremely difficult and demands for understanding of processes within the range between nanometres and centimetres. Due to its complex time-dependent, multi-phase and multi-scale behaviour, flowable concrete systems are highly complicated and cannot be described comprehensively by simple models. It is therefore extremely challenging to identify the relevant parameters that predominantly control flow phenomena on different size scales, since these may occur on any scale between the nano scale (e.g. superplasticizer adsorption) and macro scale (e.g. grading of the aggregates). The present study discusses fundamental mechanisms at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation, and links these effects to macroscopic flow phenomena. Methods are discussed that appear promising interdisciplinary tools for enhancement of the understanding of the relevant interactions that are responsible for the macroscopic flow of flowable concrete.
Most factors acting on concrete rheology work at an extremely small-scale level. Influencing factors in the millimetre or centimetre area are essentially restricted to sand and aggregates. The latter, however, make up 50 to 70% of the total volume of most concretes – a fact often ignored in research on controlling concrete processing properties.
Whereas suitably chosen concrete admixtures and additives can influence rheology in a very targeted manner, sand and aggregates are less suitable for controlling rheology but nonetheless contribute to the rheology of the Overall system. The actions of sand and aggregate can impose themselves upon the actions of admixtures and additives
and, in unfavourable circumstances, even render them redundant. For this reason, any results concerning the processability of binding agent systems can only be transferred to concrete with great care. It is important to better understand the action of sand and aggregates in order to be able to harmonise them in such a way that they complement the action of superplasticisers positively, instead of
working against them. Savings on costs can also be made by this targeted fine-tuning.
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.