| 1. |
- Ghasemi, Yahya, 1982-, et al.
(författare)
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A method for obtaining optimum packing of aggregates for concrete at the onset of flow
- 2017
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Ingår i: Proceedings of the 23rd Nordic Concrete Research Symposium. - Oslo, Norway : Nordic Concrete Federation. - 9788282080569 ; , s. 361-365
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Konferensbidrag (refereegranskat)abstract
- Particle packing models have been studied extensively during past decades and led to development of some complex and relatively accurate predictions of packing of granular materials. While the models are capable of calculating the packing density for different volumetric share of constitutes, the concept of optimum packing remains unclear. The study aims to define optimum packing based on particle packing theory and excess water layer theory .The approach makes it also possible to calculate amount of paste that is required to put a concrete mixture at the onset of flow. Some pilot tests conducted in the laboratory showed good agreement with calculated data.
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| 2. |
- Ghasemi, Yahya, 1982-, et al.
(författare)
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A theoretical study on optimal packing in mortar and paste
- 2019
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Ingår i: Advances in Cement Research. - 0951-7197 .- 1751-7605.
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Tidskriftsartikel (refereegranskat)abstract
- Packing density of particles is regarded as a key factor affecting workability of cementitious mixtures. While the value can be easily measured, and several models exist for estimating the parameter, no generally accepted definition exist for the optimal packing. Current study aims at exploring the concept of optimal packing in mortars and paste using particle packing and excess water layer theories. A semiempirical method is used for calculating water demand of mixtures based on their specific surface area. The approach allows for estimating optimal packing considering water demand and water to cement ratio of mixtures in addition to packing density.
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| 3. |
- Ghasemi, Yahya, 1982-
(författare)
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Aggregates in Concrete Mix Design
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The importance of studying the behaviour and properties of concrete can be highlighted by considering the fact that concrete is the most used man-made material in the world. The very first step in making concrete is its mix design and deciding the type and amount of constitutes used in the production of concrete which should fulfil the requirements of the final product. Mix design models are commonly used for the purpose of proportioning concrete ingredients while anticipating the properties of the final product. The current document deals with the commonly used principals in mix design models namely particle packing theory and excess water/paste layer theories. The conducted studies includes an investigation on accuracy of particle packing models (Toufar, 4C, CPM) and also tries to address the issue with measurement of specific surface area of particles as an essential input to water/paste layer theories. It has been observed that the particle packing models can predict the packing density with acceptable margin. However, it should be mentioned that the particle packing models by themselves are not mix design models but should be rather used as a part of a mix design. In addition, it was found that the accuracy of calculating the specific surface area of particles based on their size distribution curve can be further improved by assuming angular platonic solids as uniform shape of aggregate instead of traditional approach of assuming spheres for aggregates’ shape.
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| 4. |
- Ghasemi, Yahya, 1982-, et al.
(författare)
-
Effect of water film thickness on the flow in conventional mortars and concrete
- 2019
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Ingår i: Materials and Structures. - Dordrecht : Springer. - 1359-5997 .- 1871-6873. ; 52:3
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Tidskriftsartikel (refereegranskat)abstract
- Mortar and concrete can be divided into two phases of solids and water where water fills the voids between the grains and also coats the surface of particles. The current study investigates the influence of the thickness of coating water on flow spread of mortars and concretes. The article aims at correlating consistency of concretes to consistency of mortars. It was found that the flow behavior of granular mixtures can be directly related to the average water film thickness that envelops the particles. The concept was tested on mortar and concrete mixtures with different cement types, aggregate grading, aggregate shape, fineness and proportioning; proving water film thickness to be the most critical parameter affecting the flow. The results of the study indicate the possibility of predicting the flowability of mixtures by knowing the enveloping water film thickness. In addition, the relation between flowability of mixtures measured in different sizes of slump cone is explored to enable translating flow of mortars measured in mini-slump cone to flow of concrete obtained from Abram’s cone.
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| 5. |
- Ghasemi, Yahya, 1982-, et al.
(författare)
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Estimation of specific surface area of particles based on size distribution curve
- 2018
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Ingår i: Magazine of Concrete Research. - London, UK : Institution of Civil Engineers (ICE). - 0024-9831 .- 1751-763X. ; 70:10, s. 533-540
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Tidskriftsartikel (refereegranskat)abstract
- Workability in the fresh state is one of the most important factors in design and production of concrete and can be related to the water demand of the mixture, which in addition to other factors is a function of the particle shape of aggregates and binders and their specific surface area. While it is known that the shape of fine particles has a significant effect on the water demand, there are uncertainties regarding how the various shape parameters would affect the specific surface area, mainly because up to now many of the shape parameters have not yet been clearly defined and there are no commonly accepted methods for their measurement and/or estimation. In this research, the actual particle shapes were replaced with regular convex polyhedrons to calculate the total specific surface area using the size distribution curves of the samples. The obtained results indicate that while, in some cases, the assumption of a spherical particle shape leads to an acceptable estimation of the specific surface area when compared with Blaine test results, the specific surface area of powders with more angular particles could be calculated more accurately with the assumption of a polyhedron shape rather than a sphere.
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| 6. |
- Ghasemi, Yahya, 1982-, et al.
(författare)
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Exploring the relation between the flow of mortar and specific surface area of its constituents
- 2019
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Ingår i: Construction and Building Materials. - Amsterdam : Elsevier. - 0950-0618 .- 1879-0526. ; 211, s. 492-501
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Tidskriftsartikel (refereegranskat)abstract
- Mortars can be studied as mixtures of solid and flowable phases. The volume of the flowable phase required for deformation depends on the solid phase surface area according to excess layer theories. This paper examines the relation between the specific surface area of constituents in mortars and their flow. The flowable phase volume was divided by the solid phase surface area to obtain the layer thickness surrounding the surface of the particles. The results suggested that the amount of water and paste needed to ensure flow could be estimated from the packing density and specific surface area of the particles.
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| 7. |
- 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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| 8. |
- Ghasemi, Yahya, 1982-, et al.
(författare)
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Particle packing of aggregates for concrete mix design : Models and methods
- 2014
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Ingår i: Nordic Concrete Research. - Oslo : Nordic Concrete Federation. - 0800-6377. ; :50, s. 109-112
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Tidskriftsartikel (refereegranskat)abstract
- An optimized aggregate particle packing density used as a base for concrete mix design provides economic, environmental and technical advantages. The particle packing density can be determined by many methods and predicted by different models. This paper reviews common packing and procedures and compares predictions of three common packing models to each other. It was found that the models tend to show different packing densities and percentage of ingredients for the same mixture. A test setup is proposed to determine the accuracy of each model’s prediction
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| 9. |
- Ghasemi, Yahya, 1982-, et al.
(författare)
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Shape-dependent calculation of specific surface area of aggregates versus X-ray microtomography
- 2020
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Ingår i: Magazine of Concrete Research. - London : Institution of Civil Engineers (ICE). - 0024-9831 .- 1751-763X. ; 72:2, s. 88-96
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Tidskriftsartikel (refereegranskat)abstract
- The specific surface area (SSA) of constituents in a concrete mixture has a significant effect on its workability in fresh state. Excess layer theories relate the SSA to the flow behaviour of mixtures and can be used as part of an approach to mix design. However, measurement of SSA is complex and includes several issues, and thus is commonly replaced by mathematical estimation of the parameter. The mathematical approximation of surface area is based on the assumption of a spherical shape for the particles, which leads to failure of taking into account the effect of shape and the square–cube law. The article explores the possibility of replacing the assumption of a spherical shape with that of Platonic solids as the representative shape to account for the angularity of aggregates. The calculation was conducted based on information on the particle size distribution (PSD) obtained from dry sieving method. A calculated surface area on the assumption of a dodecahedron shape for natural aggregates and a cubical shape for crushed aggregates showed good agreement with SSA measurements conducted by X-ray microtomography. Furthermore, the effect of changes in PSD on the accuracy of the approach was also studied. It was found that the estimated value of SSA was improved in comparison with the traditional way of calculation on the assumption of a spherical shape.
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| 10. |
- Sayahi, Faez, et al.
(författare)
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Experimental validation of a novel method for estimating the severity of plastic shrinkage cracking in concrete
- 2022
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Ingår i: Construction and Building Materials. - Amsterdam : Elsevier. - 0950-0618 .- 1879-0526. ; 339
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Tidskriftsartikel (refereegranskat)abstract
- Plastic shrinkage cracking in cementitious materials is caused mainly by rapid and excessive moisture loss during mixture’s early ages, before sufficient tensile strength is gained. A novel model has been previously developed by the authors to estimate the severity of plastic shrinkage cracking in concrete. This paper presents findings of a series of full-scale experiments carried out to validate the accuracy of the proposed model. The experiments included investigating the impact of cement type, water-cement ratio (w/c), and admixtures (i.e., accelerator, retarder, and superplasticizer). The tests were performed in three rounds under similar ambient conditions using 3 slabs (3 m × 2 m) and 3 ring test moulds at each round. The results confirm the accuracy of the model in anticipating/comparing the cracking severity of the tested concretes.
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