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- Ghasemi, Yahya, 1982-
(författare)
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Flowability and proportioning of cementitious mixtures
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the role of constituents of cementitious mixtures as the most globally used human-made material and their effect on the flowability of the blends is of great importance. A comprehensive understanding of the ingredients of mixtures allows for optimized proportioning of constituents and can lead to a reduction in cement and water demand of the blends.The thesis focuses on relating the flow of mixtures to the specific surface area of the particles through the concepts of excess water layer theory by assuming that the particles are enveloped by a thin film layer that separates the grains and lubricates their surfaces. However, in order to study the film thickness, it is inevitable to consider packing density and specific surface area of the particles. Both of the mentioned parameters and their influence on water requirement of mixtures were investigated as a part of the project.The theoretical part of the thesis includes background and explanation of the concepts and theories used in conducting the research including particle packing theory, specific surface area, and excess layer theories. In addition, the thesis attempts at defining and formulating terms and parameters such as representative shape, mixer efficiency, and optimal packing.The experimental part of the thesis consists of laboratory measurements of packing density in the loose state, estimation of specific surface area using microtomography and slump tests for mortar and concrete.The results of the thesis indicate that the available packing models can estimate the packing density with acceptable accuracy. In addition, it was shown that it is possible to estimate flowability of mixtures based on information about the specific surface area of the constituents. A mix design approach is introduced which predicts flow spread of slump test, a measure that is often used in laboratories and at the building sites.Moreover, the research revealed that the estimation of the specific surface area of particles can be improved by assuming a platonic solid shape for the particles instead of spheres. Furthermore, the mixer efficiency was quantified and optimization of mixtures against packing density and water requirement was explained.The finding of the project lays a foundation for a simple workability based mix design approach.
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| 2. |
- Nilsson, Martin
(författare)
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Restraint factors and partial coefficients for crack risk analyses of early age concrete structures
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is well known for contractors that due to volume change during the hydration phase in concrete structures, large stresses can arise if the structures are restrained, stresses that may cause extensive cracking. Crack risk estimation of early age concrete structures can be based on five steps. Firstly, the type of structure, the material proportions and possible measures to avoid cracking have to be chosen. Secondly, the temperature development has to be determined, and thirdly the restraint situation. Fourthly, structural calculation of the stress or strain ratios follows, which in the fifth step are compared to stated partial coefficients that should not be exceeded. The restraint situation is one of the crucial factors in the crack risk analyses. A semi-analytical method has been derived for the determination of the restraint variation in early age concrete structures and especially for the case wall on slab. The method is derived using compensated line theory. The model depends on the geometry of the structure, the boundary restraint situation, and the location of the young parts on the old parts. The model is supplemented with the effects of high walls and the effects of short structures and/or possible slip failure in the ends of the joint between the young and the old concrete. The model is by regression technique compared to almost 3000 3D elastic FEM calculations of the restraint variation in walls on slabs with different dimensions and base restraint situations. The effective width of the slab is introduced as the only adjustment parameter to get the model to correspond with the FEM calculations. Partial coefficients for thermal cracking problems of young concrete have been calculated and compared with the values stated in the Swedish building code for bridges. The code values are only based on experiences and logical reasoning, whereas the calculated values form a more theoretical base for their determi-nation. The coefficients are calculated with a probabilistic method. Various possible variations of the used variables have been studied showing the wide range of possible results depending on the input. However, with use of material properties and reasonable assumptions related to thermal cracking problems, fairly good agreement has been found between the stated values in the Swedish code and the values obtained through the probabilistic method.
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