DEVELOPMENT OF CRUSHED SAND FOR CONCRETE PRODUCTION WITH MICROPROPORTIONING
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Sand and gravel are mined all over the globe and form the largest volume of solid material extracted world-wide. These materials that have been formed by erosive processes over thousands of years are now being extracted at a rate that is far greater than their possible renewal. This has led to a situation when suitable natural sand and gravel resources that previously were taken for granted and historically used for concrete production, are now depleted around many densely populated regions around the world. On the other hand, as part of every production process of crushed coarse aggregates, up to 30% (rock dependant) of the material acquired from the bedrock are reduced to sizes smaller than 4 mm. This co-generated material is of a similar grain size than natural sand. However, typically it also has different properties because of less equi-dimensional and less rounded particles, being of a different grading and consisting of much higher fines content. Due to these reasons, it generally cannot be easily placed on the aggregate market for asphalt and especially concrete, where the materials technology is adapted to natural sand. As a result, it has to be deposited or sold at a very low price, causing environmental issues and problems with the mass balance at the aggregate quarries. There have been many attempts to use this co-generated material in concrete as a substitute for natural sand. Still cases when concrete is actually produced based on 100 % crushed fine aggregate are rare, and even if they prevail, the concrete mixes typically have higher cement content and thus also higher environmental impact and cost than similar natural sand mixes. The present thesis has thus been aimed at investigating technology for producing and characterizing crushed sand and developing a new concrete mix design philosophy (microproportioning) that is adapted to crushed sands with special engineered properties. These are different from those of natural sands, and proper characterization and the new proportioning philosophy allow utilizing the best intrinsic properties of the crushed fine aggregates. The possibilities of improving properties of crushed sand for better performance in concrete by the current state-of-the art production machinery, i.e. Vertical Shaft Impact (VSI) crushers, airclassification and washing equipment has been reviewed in full-scale experiments. The direct improvement achievable for performance in fresh concrete, as a result of processing with this equipment, has been explicitly demonstrated. The investigations have shown that an acceptable level of crushed sand particle (3 μm to 8 mm) equi-dimensionality could be achieved by VSI crushing, for all the very different processed feed materials from all around Norway, regardless of the parent rock crushability or the initial feed particle shape. The studies in the thesis have also demonstrated that crushed sand fines have a pronounced effect on fresh concrete rheological properties, and that the amount of fines generated during VSI crushing is very strongly governed by the resistance of fragmentation of the processed rock materials. Thus depending on the bedrock properties, very different total volume and also particle size distribution (PSD) of the crushed fines can prevail. It is further shown that sizing and classifying, whether by wet or dry process, is possibly as important in the production of the crushed sand as the aggregate crushing process itself. This is shown to be the key for overcoming the usually reported adverse effect of fresh state rheology when crushed sand is used in concrete, and solved as the motivation why the mayor parts of the thesis have been focused on investigations of the properties of the crushed sand fines. The investigations have involved looking for the tests methods best suitable for characterisation of the fines, understanding how the fines affect rheological properties of cement paste and concrete and elaborating on a concrete mix-design and production approach that would allow engineering the composition of the fines depending on the type of the concrete produced. The present thesis has demonstrated that control of the crushed fines volume and composition very accurately for a wide variety of bedrock would allow to control the rheological properties of the cement paste matrix, and thus concrete mix through an approach that is introduced here as the concrete micro-proportioning. The concrete micro-proportioning principle is introduced as concrete mix design approach where the crushed fines in the concrete cement paste matrix phase are adjusted by taking into account total grading curve of the fines and other mineral additives smaller than about 0.125-0.250 mm, cement and supplementary cementitious materials (SCMs), to be combined with the given aggregate particles > 0.125-0.250 mm in order to obtain the desired fresh concrete properties. It has been demonstrated for 10 widely different bedrocks that the described concrete micro-proportioning approach can be enabled, if the fine part of the crushed sand (≤ 0.250 mm) is divided into several separate fractions, which can then be blended in different proportions, as needed, either at the Ready-Mix Concrete (RMC) plant or at the crushed aggregate quarry. It has also been shown how this can be technically achieved if several static air classifiers are arranged in series built around a shared external aerodynamic cycle. A wide range of crushed aggregate powder characterisation techniques for size, shape and porosity have been investigated as tools to control particle properties and their effects on concrete cement paste matrix rheology. An important result is that sedimentation analysis seems the most promising technique for determining size distribution of crushed fine aggregate particles in the range of approx. 1 μm to 100 μm. Furthermore, very accurate X-ray micro-computed tomography (μCT) and fast Dynamic Image Analysis (DIA) techniques for shape and size analysis have also been investigated, among other things helping in the understanding of suitable shape and surface area determination methods and pointing towards the future production control tools. A pilot tool for modelling and micro-proportioning of different grading curves of the crushed fines is introduced. The tool can easily be adjusted and/ or developed further in order to include also other mineral additives, cement and SCMs. A large experimental program with micro-proportioning of a wide range of concrete cement paste matrices with 10 different crushed fines (≤ 0.250 μm) has also been conducted to investigate effect of PSD, mineralogy and shape. This showed that when PSD is controlled very accurately, the total surface of the fines (by volume of fine s added or specific surface of the fines in question) is the most important factor for rheology, whereas shape and mineralogy are less important under these conditions. To facilitate the implementation of the micro-proportioning approach, it has also been demonstrated some further understanding regarding how it could be applied through the prism of the Particle-Matrix model philosophy in order to control concrete rheological properties.