Browsing by Subject "Aggregates (Building materials)"

Now showing 1 - 5 of 5
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    Aggregates in self-consolidating concrete
    (2007) Koehler, Eric Patrick; Fowler, David W.
    Self-consolidating concrete (SCC) is an advanced type of concrete that can flow under its own mass without vibration, pass through intricate geometrical configurations, and resist segregation. SCC constituent materials and mixture proportions must be properly selected to achieve these flow properties. The effects of any changes in materials or mixture proportions on hardened concrete performance must be considered in evaluating SCC. A research project was conducted to investigate the role of aggregates in SCC. The objectives of this research were to evaluate the effects of aggregate characteristics and mixture proportions on the workability and hardened properties of SCC, to identify favorable aggregate characteristics for SCC, and to develop guidelines for proportioning SCC with any set of aggregates. The research indicated that although SCC can be proportioned with a wide range of aggregates, the selection of favorable aggregates can significantly enhance the economy and performance of SCC. The effects of aggregate grading; maximum size; shape, angularity, and texture; apparent clay content; and packing density were evaluated. The main effect of aggregates larger than approximately 75 [mu]m was found to be on the minimum required paste volume for achieving SCC workability. It was found that dust-of fracture microfines, defined as mineral material finer than approximately 75 [mu]m produced during the crushing of aggregates, could be an economical choice to comprise part of the paste volume. Based on the results of this research, a mixture proportioning procedure for SCC was developed. The procedure is based on a consistent, rheology-based framework and was designed and written to be accessible and comprehensible for routine use. In the procedure, SCC is represented as a suspension of aggregates in paste. Aggregates are selected on the basis of grading, maximum size, and shape and angularity. The paste volume is set based on the aggregate characteristics in order to achieve workability requirements. The paste composition is established to achieve workability and hardened property requirements.
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    Determination of aggregate shape properties using X-ray tomographic methods and the effect of shape on concrete rheology
    (2005) Erdoğan, Sinan Turhan; Fowler, David W.; Garboczi, Edward J.
    The shape of aggregate particles can significantly influence certain properties of concrete, both in its fresh and hardened states. Therefore, there is a need to be able to completely characterize the shape of aggregate particles, in three-dimensions, in order to develop computational models which accurately predict properties. In the past, numerous methods have been suggested for this task; however these methods are often only applicable to two-dimensional images of particles, they output a single or a few values, and fail to characterize the true shape of the particle. X-ray tomographic techniques allow the capturing of the true shape of particles and have been applied to concrete aggregates. Computed tomography has been used to characterize coarse and fine aggregate particles, while X-ray microtomography has been used to characterize particles passing the 75µm sieve. Sample preparation methods and scanning parameters applicable to concrete aggregates have been developed. The spherical harmonic method was used to efficiently store shape information, and to calculate useful parameters for individual particles, such as volume and surface area. Comparisons of the results to properties determined using other techniques were made and it was determined that the results of indirect or two-dimensional shape and size characterization methods can be misleading. The shapes of aggregate particles particularly influence the rheological properties of concrete mixtures. However, it is not clear to what degree different-scale shape properties (the overall shape, angularity and texture) influence flow separately. Artificial aggregates were prepared in the laboratory and simplified test cases were chosen to independently investigate the effect of overall shape and surface texture on the yield stress and plastic viscosity of mixtures and to obtain a set of results which could be used to calibrate computational models. These tests revealed that the overall shape of coarse aggregate particles significantly influences the plastic viscosity of a mixture, but does not affect the yield stress visibly. Particle surface texture does not seem to noticeably influence either viscosity or yield stress, for the cases tested. The results were also used to verify the “Dissipative Particle Dynamics” model and showed good correlation with the predictions.
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    Effect of coarse aggregate characteristics on hardened properties of concrete
    (1993) Gardner, Britt Glenn, 1964-; Carrasquillo, R. L.
    The effects of coarse aggregate properties and characteristics on the properties of hardened concrete were examined in this experimental program. Tests were performed to determine effects on modulus of elasticity, compressive strength, splitting tensile strength, modulus of rupture, shrinkage, and coefficient of thermal expansion. The effects of varying coarse aggregate type, volume of coarse aggregate, cement content, and maximum size of coarse aggregate on the hardened properties of concrete were also studied. The results of this study show that coarse aggregate properties and characteristics do have a marked effect on the properties of hardened concrete. Modulus of elasticity is influenced by type of rock and the amount of rock present. Compressive strength is affected by surface texture of the rock. Coefficient of thermal expansion is mainly influenced by mineralogical compostion of the rock. Splitting tensile strength and modulus of rupture both are affected by type and surface texture of the rock. Drying shrinkage also seems to be influenced by the type of rock and volume of rock present. Recommendations are made regarding the incorporation of aggregate characteristics in concrete mixture design.
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    The effect of the aggregates characteristics on the performance of Portland cement concrete
    (2003) Quiroga, Pedro Nel; Fowler, David W.
    Aggregate shape, texture, and grading have a significant effect on the performance of fresh concrete. Aggregate blends with well-shaped, rounded, and smooth particles require less paste for a given slump than blends with flat, elongated, angular, and rough particles. At the same time, uniform gradings with proper amounts of each size, result in aggregate blends with high packing and in concrete with low water demand. Optimized aggregate blends have high packing, requiring low amounts of paste. As a result, they are less expensive and will have less durability problems caused by the paste such as heat generation, porosity, and drying shrinkage. Current ASTM C 33 standard limits the amount of material passing the N 200 sieve (micro fines) to 7 percent. However, manufactured fine aggregate (MFA) usually has between 10 and 20 percent micro fines. These limits, intended for natural sands, force MFA producers to wash aggregate incrementing costs and generating environmental issues. Research at the University of Texas and experience in other countries show that good quality concrete can be made with MFA with high micro fines content. Many proportioning methods such as ACI 211 do not consider high amounts of micro fines, considers partially the effect of shape and texture of aggregates, and do not encourage optimization of aggregate. The effect of shape, texture and grading of aggregates on fresh concrete was evaluated experimentally, quantified by means a proportioning method based on packing density concepts, the Compressible Packing Model (CPM), and analyzed by an empirical tool suggested by Shilstone. The effect of different types and amounts of micro fines was evaluated simultaneously as well as the impact of chemical admixtures and supplementary cementing materials on concrete with high micro fines was also evaluated experimentally. And it was concluded that chemical admixtures and some supplementary cementing materials can be used to improve the workability of concrete with high micro fines without negatively affecting hardened concrete. Guidelines for proportioning and optimizing aggregate blends were made based on Shilstone’s Coarseness Chart and the 0.45 Power Chart and CPM equations and procedures.
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