Browsing by Subject "Aggregates (Building materials)"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item 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.Item Comparative analysis between the magnesium sulfate soundness and micro-deval tests in the evaluation of bituminous aggregates(Texas Tech University, 2000-05) Phillips, William FrankIn the past ten years, several factors have led the Texas Department of Transportation (TxDOT) to reevaluate the way it designs bituminous pavements. Foremost has been the increase in the volume of heavy truck traffic and the permitted loads they are allowed to carry. Among the highway pavements designed and constructed by the Department, Asphaltic Concrete Pavement (ACP) is the most commonly specified type of bituminous pavement. Changes in the design of ACP have been seemingly small and incremental but if we step back and consider the wider view of what has evolved during this time, the changes are significant, particularly in the quality of aggregates needed to develop an acceptable bituminous mix. The quality of aggregates used in bituminous mixes is a primary factor that determines in the performance of these pavements. New and innovative designs have raised the bituminous pavement construction process to higher levels. These designs provide us with pavements that are structurally superior to traditional ACP. CMHB, SMA, and Superpave designs are coarse, open graded mixes with fewer fines and higher asphalt contents. These bituminous mixes demand even higher quality in aggregates due to their rock on rock contact that exist in the mix. These changes will provide Texas with highways that will have longer design lives, require less maintenance and will be more cost effective in the long term. Establishing specification requirements that meet the design criteria is of utmost importance. Having the means to insure compliance with the specifications is equally important. As the Department has modified both the procedures and the requirements for bituminous mixes, the contracting industry has had to adapt. This has introduced a level of uncertainty, which lends itself to an increased risk that the contractors must account for. One of the greatest risks that a contractor is faced with when bidding a job is the selection of an aggregate source. This is one of the factors that led to higher unit bid prices for bituminous mixes. To meet this challenge, the Department must have quality control tests that are accurate, timely, and repeatable. The majority of the aggregate test methods used today were developed in the 1930's and 1940's. The relationship between the performance of aggregates in the laboratory and in the field was not well defined when these tests were developed and remains ambiguous today. One of the primary indicators of durability and resistance to degradation has been the Magnesium Sulfate Soundness (MSS) Test.Item 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.Item The effect of the aggregates characteristics on the performance of Portland cement concrete(2003) Quiroga, Pedro Nel; Fowler, David W.Item Influence of aggregate surface characteristics on pavement performance(Texas Tech University, 2003-12) Khan, Jabed Riyad; Senadheera, Sanjaya; Hossain, ShabbirAggregate surface characteristics such as texture, shape, surface electrical charge, and mineralogy affect the performance of Portland cement concrete. Negative performance of concrete pavement such as spalling is greatly influenced by material characteristics and environmental factors. Surface characteristics of aggregate play a major role in occurrence of spalling in the concrete pavement. Previous studies show that concrete pavements with siliceous gravel aggregates develop more spalling problem than that of with limestone aggregate as siliceous aggregates show more affinity to water due to a preexisting electronegative charge on their surfaces. Better aggregate-cement bonding at the interfacial region will reduce the spalling problem. Several researches have been taken places in the past and are still going on to improve the aggregate-cement paste bonding by improving the interfacial microstructure. Use of surface treatment agent on aggregate surface might produce positive result in this regard. In this research, numbers of specimens were prepared for different types of surface treatment agents on siliceous aggregate surface. Elemental compositions of the specimens at the interfacial region were analyzed using Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). Hydrated lime as a surface treatment showed improvements in the microstructural composition at the interfacial region. Silica fume also showed similar results. Fracture toughness test on the beam with lime treated aggregate showed better performance than normal concrete beam and the beam with silica fume as admixture only. Lime treatment on aggregate surface produce more fatigue resistant and durable concrete against spalling.