Abstract
This State-of-the-art report has been compiled through extensive search of relevant literature and through information and experience provided by international experts. The report deals with the production and use of manufactured sand, which is defined as aggregate material less than 4 mm, processed from crushed rock or gravel, intended for construction use. The motivation for this project is the increased miss balance between the need for aggregates in the society and the need to develop concept solutions for the use of manufactured sand as concrete aggregate. The intension of this project is based upon a holistic approach, looking into the whole concept including; environmental issues, mineral properties, sampling and testing, production processes, specifications and new development in concrete mix design involving the latest generation of admixtures. Environmental issues Aggregate production is, by the strictest definition, non-sustainable, since aggregate resources are non-renewable. However, sustainability could be achieved by optimizing the whole production process, leading to a maximum of added value to the society, without causing a need for re-deposition or pollution. The real challenge will be to merge the environmental issues with the industrial ones; to create industrial plants, which are at the same time environmentally friendly and economically profitable, which integrate quarrying and industrial production, and finally – for which there exist plans for restoration and area use after completed quarrying period. As natural aggregate resources near urban centres terminate, the transport distances increase. This is already the situation in urban areas in Norway. Even though production of manufactured sand requires more energy than corresponding production of natural sand, the vicinity to the market, with less transport, will make manufactured sand environmentally favourable. Mineralogical properties – sampling and testing When producing manufactured sand, it is possible to select the raw material, i.e. the parent rock. Properties of the parent rock are determined by various petrological parameters that have an important influence, both upon the blasting and crushing of manufactured sand, e.g. energy consumption, fines production and shape, but also upon the quality of fresh and hardened concrete. In order to tailoring the end product for specific purposes, it is important to know how these properties are influencing the end product. It is e.g. experienced that lithology has not so much impact on geometric properties for the sand fraction, i.e. 63 μm - 4 mm; however it may govern these properties for the fines. The effects of secondary minerals on properties and quality of the fines, for use in concrete, are only partly known. This needs to be examined further. A variety of test methods exist, but the industry requires development of sufficient and accurate test methods for fine aggregate. It is in particular important to examine the interaction of properties of fines and the effects of the new generation of concrete admixtures. In addition, it is necessary to define procedures for sampling, handling and testing for quality control purposes to ensure that the “right” material is being tested. Accurate classification of manufactured sand, including fines, will assist the whole industry to e.g. select proper raw material, suitable production equipments and a suitable concrete mix design procedures. Production processes In order to reach a high-quality final result, each crusher stage needs to be optimized – it is not a good approach to try to repair an insignificantly crushed product by the final crusher stage alone. The installation of Vertical Shaft Impactors (VSI) has proved to be an effective way of producing cubical (even rounded) particles in the small and medium size fractions (< approx. 5 mm). It is however a challenge to avoid generating of a high percentage of fines. The latest generation of dry screening equipment combined with the latest development of air classification have, however, enabled to govern the grading curve very precisely, including the finest part. Configurations of machinery from e.g. Metso/Buell or the V7 concept from Kemco in Japan are good examples, where manufactured sand has been produced for 100% use in concrete. However, it is important to realise that high quality aggregates could be degraded by insufficient procedures of handling and storage. Specifications Current specifications in many countries still are based on the use of natural sands, where several specifications do not allow high percentages of fines to be used in concrete. A new understanding of the properties of manufactured sand, and the need to treat it differently is required. Application in concrete – Design of concrete mixes The difference in surface texture, shape properties and particle surface texture indicates that natural and manufactured sands are two different types of material and must be treated accordingly. These facts require development of new concrete mix designs, and knowledge for the application of this material. Experiences of traditional concrete mixed design based on natural sand should not be automatically transferred into this new material. The R&D and tradition of using manufactured sand in concrete has been driven by need in different countries. This implicates that the practice differs in various parts of the world. Japan is an example of a country that early started developing and applying new technologies, since their natural sand resources got depleted many years ago. On the opposite side, North American – especially Canadian – resource conditions have been (and still are) of an order that do not call for alternatives to glaciofluvial sand/gravel as aggregate. In other countries, such as Australia, manufactured sand is seen as an appropriate substitute for natural sand, but it is claimed that it appears difficult to only depend on 100% manufactured sand. In Norway manufactured sand has both been applied in blends with natural sand and as 100% fine aggregate. It must be our ambitions for the future to enable 100% use of manufactured aggregate in concrete, producing high quality concrete, both in the fresh and hardened state.