Recycled concrete aggregate is aggregate made from used concrete from demolition sites. The quantity of old concrete from a demolition site depends on the type of building. For example, the list below shows the percentage of concrete for various building types.
The concrete may go through a jawbreaker first, then an impactor and finally sorted using a screener. The screener will stockpile the crushed concrete into aggregate size, for example, 25mm 12mm 5mm to dust.
As the name implies, Impact crushes use impacts to crush material. Instead of applying compression to break the concrete, they use the impact of hammers. A hammer mill or horizontal shaft impactor, break the concrete by hitting it with heavy hammers that are fixed to a spinning shaft. An impact crusher is good to crush softer, nonabrasive materials.
Once the concrete has been crushed, it is a mixture of lots of aggregate sizes. To allow the aggregate to be mixed into concrete they need to be separated. Once the material is separated a concrete technician can design the concrete mix.
The material is passed over a vibrating screen box that has several different gauges. The material passes through the various screens to separate the aggregates. A belt conveyor then transports the aggregate to individual stockpiles for use.
Power screen equipment is either track or wheel mounted to make them mobile. This means the Power screen machine can move to the raw material rather than the expensive movement of the material to the screen. Click on the link for more information on concrete recycling production.
Recycling demolition concrete is eco-friendly as it preserves natural stone resources. The process uses less energy than mining and reduces the carbon footprint. When concrete is produced on the site where the material has been processed, the reduction in transport costs is significant.
Using recycled concrete gives the contractor LEED or BREEAM points. When tendering for work, the more LEED points that can be provided for a good recycling plan can be the difference between winning or losing the project.
The crushed concrete aggregate has a faster initial set time than virgin material, is lighter in weight, and simpler to place. When it has the right proportions, it has the same shrinkage properties as other concrete.
The French construction industry generates almost 250million tons of mineral waste per year. This waste can be used, after preparation, to replace natural materials used for construction work. This can preserve natural resources by contributing to a circular economy. A better understanding is necessary with regards to the deconstruction waste behaviour when used in redevelopment project or construction work, as backfill or embankments. This article intends to better understand how crushing influences recycled concrete aggregates leaching behaviour. Four concrete samples were studied from French power plant deconstructions. The leaching behaviour, of different grain size classifications, was tested with standard (i.e. with size reduction<4mm) and non-standard leaching tests (i.e. without size reduction). The samples were crushed with different industrial tools. The results showed that the final products maximum grain-size (Dmax) for crushing operation seems to have an influence on fine grain production. The role of the crushing technique used still remains uncertain. The analytical results from the leaching tests showed that the major elements leached are calcium, sulphates, carbonates, potassium, aluminium and silica. The trace elements were usually found in the smallest grain-size classification (06mm). For the standard leaching tests, the total dissolved solids rose for the larger grain size classifications, whereas for non-standard leaching tests (performed without size reduction) the opposite occurred. All samples followed the environmental acceptability requirements. It seems essential to control the crushing parameters to sustain and strengthen continued concrete waste recovery to help improve French and European waste recovery objectives.
Commissioner-General for Sustainable Development, Figures and statistics n164 254Million tons of waste produced by construction sector in France in 2008 (in French: http://www.statistiques.developpement-durable.gouv.fr/publications/p/130/856/254-millions-tonnes-dechets-produits-lactivite-construction.html, http://www.statistiques.developpementdurable.gouv.fr/) (October 2010). Accessed 9 June 2016
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Galvn, A. P., Ayuso, J., Garca, I., Jimenez, J. R., Gutierrez, F.: The effect of compaction on the leaching and pollutant emission time of recycled aggregates from construction and demolition waste. J. Cleaner Prod. 83, 294304 (2014). doi:10.1016/j.jclepro.2014.07.074
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The authors thank the Ministre de lEnvironnement, de lnergie et de la Mer (the French Ministry of Environment, Energy and Sea) for their financial support. The authors are grateful for EDF CIT and EDF CIDEN for providing access to the industrial sites during the deconstruction. Many thanks to Nicolas Lambert, Catherine Ollivier, Violaine Brochier for technical support and assistance in the field. The authors thank the recovery companies CLAMENS and YPREMA for their collaboration on the recovery platforms.
Coudray, C., Amant, V., Cantegrit, L. et al. Influence of Crushing Conditions on Recycled Concrete Aggregates (RCA) Leaching Behaviour. Waste Biomass Valor 8, 28672880 (2017). https://doi.org/10.1007/s12649-017-9868-2
This paper compares the use of jaw and impact secondary crushing for producing coarse recycled aggregates from concrete wastes, obtained from road pavement and demolished building materials. The crushing mechanism interferes directly with recycled aggregate properties at different levels: particle size distribution, aggregate shape, generation of micro-fractures, as well as regarding the detachment of porous hardened cement paste from particle surface in order to recover pure, non-porous natural aggregates. However, crusher selection in the recycling industry is mostly carried out by acquisition and maintenance costs, industry and manufacturer traditional habits, low cost associated with second hand equipment. It also does not consider essential parameters such as the final properties of the desired end-product. Representative samples from two recycling plants were collected after primary impact crusher and secondary crushing were performed in a controlled laboratory condition through jaw and impact crushers. The aggregates attained were characterized, demonstrating similar density, porosity, particle size distribution and content of attached cement paste. Minor observed differences do not justify the common belief in the industry that impact crushers provide an improvement in the quality of recycled aggregates due to the higher detachment of cement paste from aggregates.
The aim of this paper is to evaluate the influence of the crushing process used to obtain recycled concrete aggregates on the performance of concrete made with those aggregates. Two crushing methods were considered: primary crushing, using a jaw crusher, and primary plus secondary crushing (PSC), using a jaw crusher followed by a hammer mill. Besides natural aggregates (NA), these two processes were also used to crush three types of concrete made in laboratory (L20, L45 e L65) and three more others from the precast industry (P20, P45 e P65). The coarse natural aggregates were totally replaced by coarse recycled concrete aggregates. The recycled aggregates concrete mixes were compared with reference concrete mixes made using only NA, and the following properties related to the mechanical and durability performance were tested: compressive strength; splitting tensile strength; modulus of elasticity; carbonation resistance; chloride penetration resistance; water absorption by capillarity; water absorption by immersion; and shrinkage. The results show that the PSC process leads to better performances, especially in the durability properties.
Florea MVA, Brouwers HJH (2013) The influence of crushing method on recycled concrete properties. In: Presentation at the international conference on advances in cement and concrete technology in Africa, Johannesburg, pp 10411050
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Kong D, Lei T, Zheng J, Ma C, Jiang J, Jiang J (2010) Effect and mechanism of surface-coating pozzolanic materials around aggregate on properties and ITZ microstructure of recycled aggregate concrete. Constr Build Mater 24(5):701708
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Pedro, D., de Brito, J. & Evangelista, L. Performance of concrete made with aggregates recycled from precasting industry waste: influence of the crushing process. Mater Struct 48, 39653978 (2015). https://doi.org/10.1617/s11527-014-0456-7
The research progress of the concrete with fine recycled concrete aggregates (fRCA) is reviewed.fRCA material properties and their (non)structural applications are systematically presented.Future challenges in the development and applications of the fRCA in concrete are discussed.
This paper discusses the state-of-the-art of the fine recycled concrete aggregates (fRCA), focusing on their physical and chemical properties, engineering properties and durability of concretes with fRCA. Based on the systematic review of the published literature, it is impossible to deduce without any further research the guidelines and tools to introduce the widespread application of the fRCA in new concrete whilst keeping the cement contents at least the same or preferably lower. Namely, what is still missing is knowledge on key physico-chemical properties and their relation to the quality of the concrete mix and the concrete performance. This paper sets the foundations for better understanding the quality of fRCA obtained either from parent concrete specifically produced in the laboratory, with controlled crushing and sieving of the recycled aggregates or from field structures. By comparing properties of fRCA with properties of fine natural aggregates, the key limiting properties of fRCA are identified as the high water absorption of fRCA, moisture state of fRCA, agglomeration of particles and adhered mortar. As such, continuous quality of fRCA is hard to be obtained, even though they may be more continuous in terms of chemistry. Advanced characterization techniques and concrete technology tools are needed to account for limiting properties of fRCA in concrete mix design.
A study was recently conducted at the Hong Kong Polytechnic University to investigate the possibility of using recycled concrete aggregates and crushed clay brick as aggregates in unbound subbase materials. The results showed that the use of 100% recycled concrete aggregates increased the optimum moisture content and decreased the maximum dry density of the subbase materials compared to those of natural subbase materials. Moreover, the replacement of recycled concrete aggregates by crushed clay brick further increased the optimum moisture content and decreased the maximum dry density. This was mainly attributed to the lower particle density and higher water absorption of crushed clay brick compared to those of recycled concrete aggregates. The CBR values (unsoaked and soaked) of the subbase materials prepared with 100% recycled concrete aggregates were lower than those of natural subbase materials. The CBR values further decreased as the replacement level of recycled concrete aggregates by crushed clay brick increased. Nevertheless, the soaked CBR values for all recycled subbases were greater than 30%, which is a minimum strength requirement in Hong Kong. Furthermore, the recorded percentage swells for all subbases were less than 0.13% which can be considered negligible.