According to the Australian government report on sustainable aggregates, approximately 9 million tonnes of aggregates were recycled from demolition in Australia between 2008 and 2009 [1]. The recycled materials were used to replace the use of virgin crushed rocks in construction

Abstract Key words: Concrete, Sustainability, Natural Aggregate, Recycled Concrete Aggregate, Concrete Strength, Concrete Costs, Emissions The use of recycled concrete aggregates in structural applications can be an important breakthrough towards sustainable construction. It presents a valuable solution to the challenge of excessive waste produced from construction and demolition activities in Australia. The successful application of recycled concrete aggregates in structural applications relies heavily on its ability to meet the quality specifications of virgin materials. The use of recycled aggregates has been limited to non-structural applications such as backfills, construction of pavements and roadways. The use of recycled aggregates in structural applications is limited in Darwin city. However, in Germany, recycled aggregates have been used in structural applications. This research is a thorough investigation of the properties of coarse and fine aggregates and the resulting concrete and mortar. Additionally, site visits are conducted to investigate the current use of recycled aggregates in Darwin and the survey indicated that the main challenge towards the use of recycled aggregates in structural applications is the lack of specifications and guidelines. The industry players also lack experience in using recycled concrete aggregates. Recycled aggregate samples were collected and experiments conducted in order to examine their properties when used to make concrete and mortar. The strength of concrete decreased with increase in the amount of recycled concrete aggregates. In order to ensure sustainable use of recycled aggregates, a balance must be attained between concrete strength and environmental protection. iii iv v Table of Contents Abstract……………………………………………………………………………………………………………………………….. ii Acknowledgements …………………………………………………………………….. Error! Bookmark not defined. List of Figures……………………………………………………………………………………………………………………. viii List of Tables………………………………………………………………………………………………………………………. ix 1. INTRODUCTION …………………………………………………………………………………………………………..1 1. Background ……………………………………………………………………………………………………………….1 1.2. Statement of the problem …………………………………………………………………………………………….5 1.3. Scope ………………………………………………………………………………………………………………………….7 1.4. Methodology ……………………………………………………………………………………………………………….7 1.4.1. Desk Research ………………………………………………………………………………………………………….7 1.4.2. Case studies………………………………………………………………………………………………………………8 1.4.3. Experimental studies ………………………………………………………………………………………………..9 2: REVIEW OF THE CURRENT USE OF USES OF CONCRETE AND DEMOLITION WASTE.10 2.1. Introduction………………………………………………………………………………………………………………10 2.1.1. Overview of current status of construction and demolition waste ………………………..10 2.2. Challenges associated with construction and demolition waste………………………………..12 2.3. Aggregates and their use ……………………………………………………………………………………….14 2.4. History of Recycled Concrete Aggregates……………………………………………………………….15 2.5. Modern use of Recycled Concrete Aggregates………………………………………………………..17 2.5.1. Production ………………………………………………………………………………………………………..20 2.5.2. Current challenges in the used of RCA……………………………………………………………….23 2.6. Environmental impacts of concrete………………………………………………………………………..23 2.6.1. Portland cement ………………………………………………………………………………………………..25 2.6.2. Aggregates ………………………………………………………………………………………………………..26 2.6.3. Water………………………………………………………………………………………………………………..27 2.6.4. Admixtures ……………………………………………………………………………………………………….27 2.7. Properties of Recycled Concrete Coarse Aggregates……………………………………………….27 2.7.1. Adhered mortar…………………………………………………………………………………………………28 2.7.2. Particle size distribution…………………………………………………………………………………….28 2.7.3. Specific Gravity and Water Absorption ……………………………………………………………..29 2.7.4. Bulk Density ……………………………………………………………………………………………………..30 2.7.5. Abrasion Index and Impact Values…………………………………………………………………….30 2.7.6. Chemical properties…………………………………………………………………………………………..31 vi 3: RESEARCH……………………………………………………………………………………………………………………..33 3.1. Introduction………………………………………………………………………………………………………………33 3.2. Design mix…………………………………………………………………………………………………………………33 3.3. Sourcing of cement, fine and coarse aggregates…………………………………………………………..33 3.4. Sourcing recycled fine aggregates ………………………………………………………………………………33 3.5. Sourcing recycled coarse aggregates…………………………………………………………………………..34 3.5.1. Washing …………………………………………………………………………………………………………………34 3.5.2. Heating…………………………………………………………………………………………………………………..35 3.5.3. Casting of cubes and cylinders…………………………………………………………………………………35 3.5.4. Test for concrete……………………………………………………………………………………………………..35 3.6. Test for fresh concrete ……………………………………………………………………………………………….36 3.7. Tests for hardened concrete ……………………………………………………………………………………….37 3.7.1. Compressive strength test ……………………………………………………………………………………….37 3.7.2. Tensile splitting test ………………………………………………………………………………………………..38 4: A CASE STUDY ON THE USE OF RECYCLED AGGREGATES CONCRETE IN DARWIN CITY …………………………………………………………………………………………………………………………………..39 4.1. Introduction………………………………………………………………………………………………………….39 4.2. Waste management and recycling in Darwin, NT, Australia …………………………………..39 4.3. Current waste management strategies in Darwin, Northern Territory, Australia…….41 4.4. Site visits ………………………………………………………………………………………………………………45 4.5. Barriers to recycling construction and demolition waste…………………………………………45 5: ANALYSIS OF PROPERTIES OF COARSE RECYCLED AGGREGATES CONCRETE……….47 5.1. Introduction………………………………………………………………………………………………………….47 5.2. Mixing proportions……………………………………………………………………………………………….47 5.3. Mixing procedure………………………………………………………………………………………………….47 5.4. Properties of recycled aggregate concrete ………………………………………………………………48 5.4.1. Workability……………………………………………………………………………………………………….48 5.4.2. Density………………………………………………………………………………………………………………50 5.4.3. Concrete Strength ……………………………………………………………………………………………..51 5.4.4. Deformation………………………………………………………………………………………………………52 5.5. Summary………………………………………………………………………………………………………………54 6: ANALYSIS OF PROPERTIES OF FINE RECYCLED AGGREGATES MORTAR ………………..55 6.1. Introduction………………………………………………………………………………………………………………55 6.2. Collection of recycled fine aggregates…………………………………………………………………………55 vii 6.3. Mix proportions ………………………………………………………………………………………………………..57 6.4. Fabrication of specimens……………………………………………………………………………………………57 6.5. Properties of fine recycled aggregates mortar……………………………………………………………..58 6.5.1. Flowability ……………………………………………………………………………………………………………..58 6.5.2. Density……………………………………………………………………………………………………………………59 6.5.3. Absorption ……………………………………………………………………………………………………………..60 6.5.4. Dry shrinkage…………………………………………………………………………………………………………61 6.5.5. Compressive strength ……………………………………………………………………………………………..62 7: CONCLUSION AND FUTURE WORKS ……………………………………………………………………………63 7.1. Introduction………………………………………………………………………………………………………….63 7.2. Conclusion ……………………………………………………………………………………………………………63 7.2.1. Use of recycled concrete aggregates in Darwin ……………………………………………………63 7.2.2. Use of coarse recycled concrete aggregates …………………………………………………………64 7.2.3. Use of fine recycled concrete aggregates……………………………………………………………..65 7.3. Future work………………………………………………………………………………………………………….65 8. REFERENCES ……………………………………………………………………………………………………………..67 viii List of Figures Figure 1: Demolition waste that can be recycled into aggregates …………………………………………………2 Figure 2: A crushing machine recycling concrete aggregates………………………………………………………..3 Figure 3: Crushed Concrete Stockpile ready for use at RUB site ……………………………………………………5 Figure 4: Carbon footprint of cement use in comparison with other human activities…………………….6 Figure 5: Recycled concrete aggregates …………………………………………………………………………………..11 Figure 6: Concrete rubbles left behind after the bombing of Pforzheim in 1945 …………………………..16 Figure 7: Recycled aggregates and manufactured sand in Australia…………………………………………….17 Figure 8: Static aggregate crushing plant………………………………………………………………………………….21 Figure 9: Mobile impact crusher……………………………………………………………………………………………..22 Figure 10: Excavations and emissions caused by production of concrete …………………………………….25 Figure 11: Concrete compressive crushing strength machine at Charles Darwin University……………38 Figure 12: Hierarchy of waste management……………………………………………………………………………..39 Figure 13: Concrete Block ready for crushing at RUB site ………………………………………………………….43 Figure 14: Recycled fine aggregates ………………………………………………………………………………………..56 Figure 15: Natural fine aggregates…………………………………………………………………………………………..56 Figure 16: Flow rate values at water to cement ratio of 0.35 ……………………………………………………..59 Figure 17: density of mortars at 28 days with different replacement ratios………………………………….60 Figure 18: Absorption of mortars at different replacement ratios at 28 days……………………………….61 ix List of Tables Table 1: Construction and demolition waste from different countries…………………………………………..13 Table 2: composition of construction and demolition waste in Australia………………………………………14 Table 3: Environmental concerns associated with manufacture of cement ……………………………………25 Table 4: Environmental issues associated with production and processing of aggregates……………….26 Table 5: A comparison of physical properties of RCA and natural aggregates………………………………30 Table 6: The average percentage composition of construction and demolition waste……………………..41 Table 7: Mixing ratios……………………………………………………………………………………………………………47 Table 8: Concrete density at different water cement ratio and RA replacement ratios…………………….50 Table 9: Compressive strength of RAC with water to cement ratio between 0.45 and 0.60 …………….51 Table 10: Tensile split strength of RAC with water to cement ratio between 0.45 and 0.60…………….52 Table 11: Flexural strength of RAC with water to cement ratio between 0.45 and 0.60………………….52 Table 12: Dry shrinkage deformation in concrete………………………………………………………………………53 Table 13: Mix proportions……………………………………………………………………………………………………..57 Page 1 of 70 Recycled Aggregate 1. INTRODUCTION 1. Background The Australian construction industry is estimated by the Australian Government (2015) to be worth $230 billion annually. This value indicates the scale of construction work that undergoes in the country annually. The industry is hence responsible for producing an approximated 19 million tonnes of waste material annually from construction and demolition activities [1]. According to the Australian government publication on sustainable aggregates (2015), the construction and demolition of structures contribute about 40% of the total waste material in the country. However, the report further notes that about 55% of the waste from demolition is being recycled or reused in many states in Australia. The Environmental Protection and Heritage Council (EPHC) have developed a policy that is geared towards reducing wastage of materials through a shift towards resource recovery and recycling. This approach encourages the waste produced from one activity to be used in another activity instead of being disposed. The reuse of variable construction waste materials can help reduce the need for extracting or manufacturing new materials from scratch and thus enhancing the social, economic and environmental benefits. The Figure 1 below shows demolition debris that can be recycled. The concept of recycling is not new to mankind. Throughout the history of mankind, people have tried to reuse their old resources in order to cut costs involved with buying or searching for new ones. However, modern recycling has evolved to include the use of complex and hightech machines in order to recycle materials in large quantities. The recycling process has become more organised and complicated over the last few years. The Australian construction industry is steadily embracing recycling as part of sustainable construction practices [2-4]. One of the practices that are gaining traction in the industry is the use of sustainable aggregates[4]. Sustainable aggregates are the aggregates that are produced though the recovery of materials Page 2 of 70 Recycled Aggregate from a demolition. According to the Australian government report on sustainable aggregates, approximately 9 million tonnes of aggregates were recycled from demolition in Australia between 2008 and 2009 [1]. The recycled materials were used to replace the use of virgin crushed rocks in construction. The use of recycled aggregates is considered to be more sustainable for several reasons [5-7]. One of the reasons is that recycled aggregates are more economical to the end users in terms of transport costs and the Figure 1: Demolition waste that can be recycled into aggregates Source:[8] cost per tonne [5]. Another reason is that the use of recycled aggregates can reduce the need to quarry or mine for virgin rocks [7] and thus help to reduce the generation of dust and waste materials on these sites [6]. The growth patterns of the Australian construction industry suggest that the use of recycled aggregates cannot be enough. Thus, it will be important to mix the virgin materials and the recycled ones depending on the needs of the projects. The Figure 2 below shows a crushing machine recycling aggregate on site. Page 3 of 70 Recycled Aggregate Figure 2: A crushing machine recycling concrete aggregates Source: [9] The major challenge facing the use of recycled aggregates is the perception by customers that they cannot be as good as the virgin ones [6, 10]. This perception has made many potential customers to fail to take advantages of benefit of using recycled materials. However, the Australian government has moved to address these perceptions by undertaking detailed research and using these materials in major construction projects[11]. The manufacturers of recycled aggregates are required to comply with strict testing standards in order to produce materials that meet or exceed the strengths of virgin materials [11]. Users of recycled materials need to ensure that the organisations they purchase their aggregates from meet the required specifications. The materials should further be tested by independent laboratories that are accredited by the government[11]. If the users can be assured that the performance of recycled materials is similar to that of virgin materials, it will be easier for them to select the materials that offer them more social, economic and environmental benefits. According to Morledge and Jackson (2001) there are very few companies in Darwin that recycle aggregates. Darwin, as the capital of the Australia’s Northern Territory, has a lot of Page 4 of 70 Recycled Aggregate construction projects taking place annually. As a home to some of the most iconic beaches and green parks, such as the Bicentennial Park, it is essential to ensure the construction activities are sustainable. Hyder Consulting (2011) indicates that there very little data relating to the recycling and reuse of construction and demolition materials in the Northern Territory. This lack of data can only be attributed to the absence of recycling activities in the territory. Waste from the Darwin is normally disposed by the MACHAHON Holdings in the Shoal Bay Landfills that are owned and operated by the City of Darwin [12]. The waste disposal company is normally paid by the volume of waste it disposes to the landfills. This arrangement does not offer any incentive to the company to divert any waste from the landfills. There are no concrete blocks or any other inert materials removed before the waste is disposed to landfills. The rate paid for disposal of waste in the fills ranges from $32 to $47 per tonne depending on the type of waste [12]. In an effort to promote recycling, the City of Darwin awarded the NT Recycling Services a contract to extract recyclable materials from the waste [12]. However, the efforts by the company are hampered by lack of incentives for landfill operators to reduce the volume of waste disposed in the fills. Noguchi and Tamura (2001) noted that RUB Group is one of the few companies that are involved in the recycling of aggregates in the City of Darwin. The Figure 3 below shows some of the crushed concrete stockpile at R.U.B. Group. Page 5 of 70 Recycled Aggregate Figure 3: Crushed Concrete Stockpile ready for use at RUB site Source:[13] The group is involves in the crushing and recycling of concrete aggregates. It operates a crusher that can produce approximately 200 tonnes of aggregates in an hour. However, when compared to the HB Group, a company that processes natural aggregates, the output by R.U.B. Group is insignificant [14]. The HB Group processes an estimated 500 tonnes of natural aggregates in an hour. In order to compete favourably, the R.U.B Group must increase its output in order to match that produced from natural aggregates[14]. That lack of proper technology and investment in recycling of aggregates in Darwin has made many construction projects to use natural aggregates. 1.2. Statement of the problem Concrete is the most important construction material toady. In order to produce concrete, fine aggregates, coarse aggregates, sand, cement and steel are required. All these components of concrete generate a lot of waste throughout their lifecycle [15]. However, very little has been done to reuse or recycle some of these components. Research shows that repeat recycling can be an effective way of reducing emissions just as it is the case with aluminium and steel [16- Page 6 of 70 Recycled Aggregate 18]. According to Meyer (2009) construction aggregates accounts for more than 70% of all the materials consumed in Australia and other developed and developing countries. Research shows that the production and the use of concrete accounts for about 5% of total carbon produced in the world [16, 18, 19]. The Figure 4 below illustrates the carbon produced in production and use of cement in comparison with other human activities Figure 4: Carbon footprint of cement use in comparison with other human activities Source: [19] Environmental pollution is becoming a major concern in the construction industry in Australia [20]. This industry is considered to be one of the biggest consumers of natural resources in the world [20]. Consequently, a lot of waste is generated from its activities. There is little progress in waste management in this industry. A lot of water, aggregates and cement are wasted in all types of construction activities. These wastes pollute the natural resources such as soil, water and forest if they are not disposed properly. Additionally, a lot of energy is wasted when resources such as cement, aggregates and water go to waste. Recent research has shown that the recycling and the reuse of materials in construction can help reduce pollution resulting from wastages. Page 7 of 70 Recycled Aggregate 1.3. Scope This research is based in Darwin, Northern Territory, Australia. The study focuses on the use of recycled fine and coarse aggregates within the City. The objective of this thesis is to determine why recycled aggregate is not used as compared to the natural aggregate in the construction work. Which aspect of recycle concrete makes it different from natural aggregate? For further research properties of natural aggregate in query is compared with recycled aggregate. 1.4. Methodology This research involves investigating the current practices in the management and recycling of construction and demolition concrete waste in Darwin and the comparison of the physical and mechanical properties of natural and recycled aggregates. In order to achieve the objectives of this study, the following research methodologies were used: 1.4.1. Desk Research Desk research was used in investigating the current management and recycling of demolition waste in the Darwin city. This research helped the researcher to determine the following: a) The current level of waste generated from construction industry in Darwin and Australia in general. b) The environmental pollution problems associated with improper management and lack of recycling of concrete waste c) The current strategies that are used to reduce construction and demolition waste in Australia and the world d) The benefits and the challenges associated with recycling of concrete demolition waste. Page 8 of 70 Recycled Aggregate 1.4.2. Case studies Cases studies are important in evaluating a research phenomenon in greater details. They were found important in this study for evaluating the current situation in the management and recycling of waste from construction industry in Darwin. Several construction companies within Darwin city were visited in order to investigate their construction practices. The visits include ones to centralised recycling plants, demolition sites, landfills and conducting interviews with managers and representative of these organisations. The visits helped in evaluating the following: a) Environmental cost of using natural aggregates and using recycled concrete aggregates are compared. Firstly, the cost of recycled to virgin aggregates was compared by taking into account the transport distances. The distance from the quarries to construction sites was used. Most quarries are located in the rural areas of the Northern Territory. The shorter the distance to the site from the quarry, the cheaper the aggregates. The cost of the aggregates was estimated by enquiring from companies such as Yebna Quarries and the HB group. The cost of recycling was enquired from a representative of the RUB Group at the East Arm crushing plant. These costs were obtained through telephone interviews with the general manager of the HB Group and the owner of the RUB demolition. Another cost that is taken into account is the cost of dumping concrete waste in landfills. This is considered as a saved cost when comparing the use of recycled concrete aggregate with the use of virgin materials. The cost of emissions from the transport facilities is also taken into consideration when calculating the total cost of using each type of aggregate. Multiple criteria were used to evaluate emissions through the consideration of the machines used to cut, sort and transport the aggregates. Emissions from transportation are estimated based on the annual averages that are Page 9 of 70 Recycled Aggregate developed by Australian Government’s Department of the Environment. However, administration costs are considered negligible and hence ignored in both cases. A similar concrete mix design was developed for both the natural and recycled concrete aggregates. 1.4.3. Experimental studies Several analyses were conducted on previous experiments on the properties of recycled fine and coarse concrete aggregates and the resulting concrete. The various experimental tests that were studied are outlined below: a) There are numerous studies conducted on the characteristics of fine and coarse recycled concrete aggregates [21-24]. These studies were reviewed in order to determine properties such as particle size distribution, particle density, water absorption, dry strength, wet strength, uncompacted and compacted bulk density, weak particles, contaminant, flakiness index, sulphate content, chloride content, and shape of particles. All the studies of interest had conducted their experiments as per the Australian Standards for sampling and testing aggregates (AS 1141). b) The physical and mechanical properties of concrete made of fine and coarse recycled concrete aggregates were examined from previous studies [25-27]. The properties investigated include the workability of fresh concrete, tensile strength, and the density, modulus of elasticity, compressive strength, flexural strength, creep and shrinkage. All these studies have conducted the tests according to Australian Standards (AS 1012: Methods of Testing Concrete). The review of these tests was done in order to compare the differences between the properties of concrete made from either recycled or natural aggregates. Page 10 of 70 Recycled Aggregate 2: REVIEW OF THE CURRENT USE OF USES OF CONCRETE AND DEMOLITION WASTE 2.1. Introduction This section provides a review of literature on previous studies conducted on the use of concrete and demolition waste, and the recycling of concrete aggregates. Firstly, the chapter provides an overview of the current status of construction and demolition waste. This section also investigates the use of aggregates in the present day Australia and more specifically in the city of Darwin, Northern Territory. Also in this section is a review of the historical development in the use of recycled concrete aggregates. The modern use of recycled concrete aggregates is also reviewed under this section. Under the modern use of recycled concrete aggregates, aspects such as the production process and the current challenges are investigated in details. Further to this, the essential properties of concrete are outlined. Previous research on the environmental impacts of concrete is also reviewed in details. This section also outlines the current uses of recycled concrete aggregates in Australia. 2.1.1. Overview of current status of construction and demolition waste Environmental conservation and sustainability has become a major issue of concern in the construction industry [28, 29]. Construction activities consume a lot of resources and hence produce a lot of waste. These activities have a lot of impacts on the environment and sustainability of entire life cycle [30]. Industry players are looking into ways they can reduce or recycle wastes resulting from construction activities in order to ensure sustainable use of resources [28, 29]. Concrete is a very essential material in the construction industry and cannot be substituted yet [30]. However, efforts can be made to conserve over exploitation of natural resources by recycling old concrete resulting from demolitions. Recycled concrete aggregates can be used in making new concrete with good strength. Recycled aggregates are stone particles that are found in concrete bonded together using mortar [30]. These particles can be separated from the concrete mass through crushing of the demolished concrete [31]. Recycled aggregates Page 11 of 70 Recycled Aggregate concrete can be made by mixing the recycled aggregates with normal mixing materials such as water, cement, fine aggregates and other concrete additives to achieve the desired strength. Repeated recycling is possible for concrete due to durability of aggregates [21]. Concrete can be recycled in a fully closed system in order to reduce the amount of emissions, landfills and concrete waste[21]. Concrete can also be recycled to produce secondary raw materials for use as road bases and sub-bases, backfills, or aggregates in the production of new concrete for applications in structures. The Figure 5 below shows recycled concrete aggregates. Figure 5: Recycled concrete aggregates Source:[21] Recycling of concrete has significant benefits to the environment such as saving of resources, avoiding pollution from landfills, and lowering costs of construction[21]. Despite the advantages of saving resources and reducing construction costs, the use of recycled concrete has its challenges. Recycled concrete aggregates form weak interfacial zones between them and the mortar leading to lower strength[28, 29]. Additionally, the presence of a layer of old mortar around aggregates prevents the development of strong bonds leading to segregation and lower quality concrete [31]. In order to encourage people to embrace the concept of recycling concrete, it is important to come up with solutions for these problems. Page 12 of 70 Recycled Aggregate Various researchers have conducted investigations to examine how demolished concrete can be processed and ensure it has adequate physical and mechanical properties [21, 28, 29, 31]. Some of these studies have found that recycled aggregate concrete has lower strength than concrete made using natural aggregates [28, 29, 32]. Due to these challenges, recycled aggregates concrete has only been used in applications such as production of pavements and grading work [28, 29]. The use of recycled aggregates in production of structural concrete is rare due to its low compressive strength and variation in quality[32]. However, improved mixed designs can help to reduce quality variations and improve the strength of this concrete [32]. Some researchers have tested the structural performance of recycled aggregates concrete and identified similar failure patterns as ones observed in natural concrete[31]. The ultimate flexural and compressive strength in recycled aggregates concrete were found to be slightly lower than ones in normal concrete [32]. Investigations in the long-term performance of recycled concrete aggregates indicate that shrinkage and creep strain increased with the increasing replacement of aggregates with recycled ones [32]. 50% replacement of aggregates was found to be optimal ratio for concrete designed to withstand seismic vibrations [28, 29]. The use of recycled aggregates concrete in the construction of buildings such as Vilbeler and Waldaspirale shows that RAC can be used effectively as the normal concrete. 2.2. Challenges associated with construction and demolition waste A lot of campaigns have been conducted around the world to enhance environmental management and sustainable development practices. Consequently, there is a growing interest on environmental issues and the problems of poor environmental practices[32]. Despite these efforts, construction has not become environmentally friendly [31]. Previous researchers have documented numerous environmental impacts of uncontrolled construction activities. Some of the identified environmental impacts include the consumption of huge tract of land, resource Page 13 of 70 Recycled Aggregate depletion, land deterioration, generation of waste, and emissions along the life cycle of construction materials[31, 32]. The debris that results from construction and demolition of concrete structures constitutes a large percentage of total solid waste produced. For instance, in the UK, approximately 70 million tonnes of solid waste come from construction and demolition activities [28, 29, 32]. Additionally, 50% of this waste is disposed of in landfills. In Australia, 17 million tonnes of solid waste is disposed of in landfills [32]. About 42% of this waste results from construction and demolition activities [32]. In the United States of America, solid waste from construction and demolition activities contributes about 30% of the total solid waste [31]. The Table 1 below provides a summary of construction and demolition waste from different countries. Table 1: Construction and demolition waste from different countries Source: [28, 29] Country Proportion of construction waste to total waste (%) Construction and demolition waste recycled (%) Australia 42 51 Brazil 15 8 Denmark 25-50 80 Finland 14 40 France 25 20-30 Germany 19 40-60 Hong Kong 38 No information Italy 30 10 Japan 36 65 Netherlands 26 75 Norway 30 7 Spain 70 17 United Kingdom Over 50 40 United States of America 29 25 Concrete is the most dominant type of construction waste. In Australia, it comprises of about 82% of the total construction and demolition waste [31]. The Table 2 below shows the composition of construction and demolition waste generated in Australia grouped according to the type of material and the source. Page 14 of 70 Recycled Aggregate Table 2: composition of construction and demolition waste in Australia Source:[31] Waste type Source, percentage Municipal C&I C&D Concrete 3 3 81.8 Food and garden 47 13 0.9 Glass 7 2 0.1 Metal 5 22 6.8 Paper 23 22 0.1 Plastic 4 2 0.2 Timber 1 9 4.1 Others 12 24 6.1 2.3. Aggregates and their use There is a growing interest in the processing, the use and the impacts of using aggregates in construction. Apart from the natural aggregates, recent research has also focused on the use of non-traditional forms of aggregates such as the industrial by-products and recycled wastes from construction and demolition[21, 31]. These developments have led the Department of the Environment and Water Resources in Australia to develop a guide for using recycled aggregates and other masonry wastes [20]. The recycled aggregates are the ones produced from industrial wastes and from the wastes resulting from demolitions [21]. These wastes would otherwise end up in landfills. Aggregates are normally classified according to their sources or usage. Some of the common classification of aggregates is explained below: a) Natural aggregate: these are construction aggregates that are sourced directly from gravels, sand, and crushing of stones[31]. These types of aggregates are also referred as virgin aggregates. b) Manufactured aggregates: these aggregates are produced artificially from selected naturally occurring materials [31]. They can also be obtained from the wastes of some industrial processes. Some examples of manufactured aggregates include shales, slates, expanded clays, polystyrene aggregates (PSA), foamed blast furnace slag (FBS), fly ash aggregate and manufactured sand. Page 15 of 70 Recycled Aggregate c) Recycled aggregates: these are produced by crushing materials that had previously been used in construction[31]. Some examples of recycled aggregates include glass cullet, used foundry sand, reclaimed asphalt pavement (RAP), reclaimed asphalt aggregate (RAA), reclaimed aggregate, recycled concrete aggregate (RCA), and recycled concrete and masonry (RCM). d) Reused by-product: these are aggregates that are produced from by-products of an industrial process[31]. Examples of reused by-products include mine tailings, organic materials, crusher fines, fly ash (FA), air-cooled BF slag (BFS), electric arc furnace (EAF), granulated BF slag (GBS), steel furnace slag (BOS), furnace bottom ash (FBA), coal washery reject (CWR), and incinerator bottom ash (IBA) 2.4. History of Recycled Concrete Aggregates Recycling of aggregates has existed since the start of civilisation. The ancient Romans have been recorded to have recycled their old masonry in construction of new buildings [31]. However, the earliest use of crushed aggregates was recorded in Germany in late nineteenth century [14, 21]. This practice gained traction after the enormous destruction of structures during the World War II. The resulting rumbles provided an opportunity to test the new technology of recycling concrete aggregates [32]. The Germans used the demolished concrete and masonry to recycle the aggregate that was used for rebuilding purpose [21]. The recycling of the rumbles helped to clean the cities and help the country from investing a lot of resources in the extraction of virgin aggregates. About 600 million cubic meters of concrete waste was available for recycling after the war [21]. The Figure 6 below shows a pile of concrete rumbles that was left behind after the bombing of Pforzheim, Germany in 1945 during the Second World War. Page 16 of 70 Recycled Aggregate Figure 6: Concrete rubbles left behind after the bombing of Pforzheim in 1945 Source:[33] In order to remove the rumbles from the cities, the Germany government established several recycling plants around the affected areas. Within the next ten years, the plants had successfully recycled more 11.5 million cubic metres of concrete [34]. The recycled concrete enabled the country to rebuild more than 175,000 units at a low cost [34]. However, the supply of waste concrete continued to decline as the rumbles that were left behind after the Second World War continued to diminish. In Australia, the practice of recycling concrete aggregates is relatively new. It is estimated that about 6.2 million tonnes of demolition waste is disposed of in landfills [35]. Recycling of aggregates in Australia is limited for low strength concrete applications [32]. This has seen major recycling projects conducted in the road industry. This practice has existed since 1960s where several asphalt recyclers were set up. Steady increase in recycling has however been observed. Recycled concrete aggregates are now available in cities such as Melbourne and Sydney [35]. Recycled aggregates are also readily available in a number of other Australian localities. Some of the popular forms of recycled aggregates are manufactured sand and aircooled blast furnace [32]. Performance data on air-cooled blast furnace are available in Page 17 of 70 Recycled Aggregate Australia[35]. However, the performance of manufactured sand has not been tested. These materials have been found to be effective in the construction of road bases, sub-bases and pavements[36]. In all these applications, the availability of a consistent supply of aggregates is essential. Figure 7: Recycled aggregates and manufactured sand in Australia Source: [36] 2.5. Modern use of Recycled Concrete Aggregates Research has demonstrated that recycled concrete aggregates can be applied in various engineering uses. Commonwealth Scientific and Industrial Research Organisation (CSIRO) have shown that Class 1A of recycled concrete aggregate is well graded and it is of good quality. The type of aggregates normally has less than 0.5% in brick content [36]. The Class 1A recycled concrete aggregates can be used in wide engineering applications [36]. However, the aggregate should be tested in order to ensure it meets the performance requirements before it can be used. This type of aggregate is commonly used in partial replacement of natural or virgin aggregates in the concrete mix. Usually the recycled aggregates replace about 30% of Page 18 of 70 Recycled Aggregate the natural aggregates[23]. The resulting concrete has to sufficient strength to be used in nonstructural applications such as the construction of gutters and kerbs [23, 32]. However, current applications of recycled aggregates in these applications are not prevalent in Australia [30, 36]. The use of recycled concrete aggregates in structural applications as not been sufficiently tested. However, some researchers have observed that Class 1A recycled concrete aggregates can be added in concrete mixes of 30-40MPa [26]. The addition of recycled concrete aggregates in can however lead to deterioration of certain properties of concrete. Some of the properties that are affected include shrinkage and permeability [32]. The use of recycled concrete also requires high cement content in order to achieve the desired strength [26]. However, the extra amount of cement used in this type of mixes is compensated by the lower cost of producing recycled concrete aggregates (RCA). RCA has also been noted to have lower specific gravity that that of natural aggregates. Studies record the specific gravity of RCA to range from 2.44 to 2.46 [21] [22]. On the other hand, the water absorption of RCA is higher than that of natural aggregates[21, 22]. The absorption rate ranges from 4.5 to 5.4% [21] [22]. According to Paranavithana and Mohajerani (2006) very fine aggregates have even lower specific gravity and the higher the water absorption rate. Experiment shows that very fine aggregates can have a specific gravity that is as low as 2.32 and a water absorption rate of as high as 6.2% [21, 22, 37] . The unit weight of recycled concrete aggregates ranges from 2240 kg/m3 to 2320 kg/m3 [38]. The higher water absorption rate of RCA implies that more water is required to mix concrete of similar volume than those using natural aggregates. The high water content lead to reduced compressive strength than concrete produced from natural concrete [38]. Additionally, the concrete made from RCA has lower flexural strength and lower elastic modulus than concrete produced from natural aggregates [38]. The use of RCA also results to higher shrinkage in concrete. The creep in RCA is also said to be bigger than when using natural aggregates [38]. The high water absorption in Page 19 of 70 Recycled Aggregate recycled concrete aggregates can be explained by the presence of mortar around the RCA. This mortar dust also lead to lower specific gravity and reduced mechanical properties in recycled concrete aggregates[32]. Swelling in concrete produced using RCA can be caused by presence of contaminants such as gypsum in the RCA [38]. These shortcomings in the concrete produced using RCA can be resolved if the mix design is adjusted. An effective mix design helps to improve workability, strength and reduce shrinkage and absorption. The RCA concrete has also been found to have higher permeability to chloride ions than similar concrete made from natural aggregates [38]. The permeability of concrete to corrosive ions can reduce it durability. The alkali silica reaction (ASR) in RCA concrete can lead to expansion of cracks in the concrete [37]. In order to prevent this reaction, 20 percent cement should be replaced using fly ash. The use of fly ash has been found to reduce the level of cracking to safe levels [37]. It has also been highlighted that the high water absorption in recycled concrete can increase permeability and shrinkage in concrete [32]. The shrinkage is caused by the movement of water from the surface of the aggregates to cement paste that surrounds the aggregates. This movement of water increase the volume of water and the number of pores in the concrete [37]. The large amount of hydrated cement dust that surrounds the RCA makes the aggregates to be more absorbent than the normal natural aggregates. In order to improve workability of recycled concrete aggregates, the water absorption must be reduced. To do so, the RCA should be cleaned prior to making of concrete. The cleaning of these aggregates helps to reduce the amount of mortar dust that is on their surface. The reduced absorption does not only improve workability, but also reduce the mass of concrete, reduce the air content and improve the compressive strength [32]. CSIRO provides an important guide for engineers when using RCA in their concrete mixes [38]. The guide provides the acceptable level of contaminants in each class of RCA. According to the guide, a 30% replacement of natural aggregates with RCA of Class 1A should have similar strength properties as 100% Page 20 of 70 Recycled Aggregate natural aggregates [37]. For Grade 2 reinforced concrete, the binder content should be added in order to achieve similar compressive strength as mixes with 100% Class 1A RCA. The Dutch Standard VBT 1995 specifies that 20% recycled concrete can be added to natural aggregates without any significant change in the strength properties of concrete [38]. However, the RCA should have a minimum density of 2000 kg/m3 to qualify for replacement of natural aggregates. 2.5.1. Production The coarse recycled concrete aggregates are normally produced through the crushing of demolition waste. Ideally, the aggregate crusher and the concrete plant should be located on the same site in order to minimise transportation costs [38]. Mobile crushing plants can be used in larger projects. The demolition waste must be sound, and should be composed of at least 95% of concrete by weight [32]. The contaminant level in such waste should be lower than 1% when measured by mass. Recycled concrete aggregates may be composed of other materials such as crushed stones, gravels, and cement [32]. This concrete waste can be obtained from aged concrete building from elements such as foundations, pavements and bridges[37]. The concrete is hence crushed to specified sizes. Materials such as reinforcing steel are carefully removed and care taken to prevent the contamination of the recycled waste by materials such as gypsum and plaster. The recycle concrete should hence be stored separately from other construction materials to avoid incidences of contaminations[32]. The majority of the available recyclers use jaw crushers to crush large sizes of concrete into smaller pieces and to sort out reinforcement steel. Secondary crushing is conducted using impact crushers in order to reduce the size of the aggregates and to separate mortar from the aggregates [38]. The primary crusher is normally used to crush the waste concrete to particles ranging from 60mm to 80mm in diameter [37]. The resulting aggregates are then fed into a secondary crusher. This crusher is able to reduce the size of the aggregates further and then empties it content to a series of screens[21, 22]. The screens are used to separate different sizes of aggregates. The screens use Page 21 of 70 Recycled Aggregate sieves of different sizes ranging from 19mm to 7mm. Particles of sizes lesser than 7mm are removed and can be used as road backfill materials [21, 22]. The figure 8 and 9 below illustrate an aggregate crushing plant and a mobile impact crusher respectively. Figure 8: Static aggregate crushing plant Source:[39] Page 22 of 70 Recycled Aggregate Figure 9: Mobile impact crusher Source: [9] Aggregates can also be reclaimed from trucks returning from construction sites through washing out the discharge. The coarse aggregates and sand can be reclaimed from wash out and reused [21, 22]. The coarse aggregate recovered through this method are normally adequately clean and can be considered to be equivalent to virgin aggregates. Many concrete mixing plants have recyclers that can be used to wash out aggregates from trucks [21] [22]. The recovered aggregates are dried and reused while smaller particles are dried and disposed in landfills. The wash water can also be recycled and reused [37]. In Australia, the recycled concrete aggregates (RCA) are the most recycled demolition waste. The aggregates are obtained as coarse or fine and reused in concrete production. It is approximated that about 5 million tonnes of RCA is available in Australia annually especially in Sydney and Melbourne. Page 23 of 70 Recycled Aggregate 2.5.2. Current challenges in the used of RCA The use of recycled concrete aggregates is relatively new in many countries. It is not generally accepted in many structural uses due to its associated problems that results from weak bonds between the aggregates and cement paste. The presence of dust around the recycled aggregates makes them to form weak interfacial transition zones with the cementing paste [40]. The presence of transverse cracks and large number of pores in demolished concrete represents a significant challenge to engineers using RCA. The RCA also have high levels of chlorides and sulphates that can corrode the concrete and reinforcement steel [21] [22]. Presence of impurities such as rust and plaster remains causes a lot of variation in the quality of concrete [40]. The use of RCA also presents challenges in grading. Despite these challenges, the use of recycled concrete aggregates has been found to be more economical and environmentally friendly [37]. The lack of specifications and guidelines for the use of recycled concrete also poses significant challenges. In order for the use of RCA to be accepted, standards should be developed to guide engineers when using these aggregates [40]. The lack of awareness, databases, specifications and standards has also derailed the dispersion of this concept. This has led to the lack of confidence among researchers and engineers on the ability of recycled concrete to replace the use of natural aggregates. The involvement of the Australian government can help to eliminate the obstacles hindering the large-scale utilisation of recycled concrete. The government can help by organising seminars to educate and create awareness in the use of recycled aggregates. Providing incentives to the organisations that are recycling their wastes can also help to improve the practice. 2.6. Environmental impacts of concrete Concrete is the most widely used construction material in the world. It is used in construction of major infrastructure for commerce, habitation, transportation and industrial use. The benefits Page 24 of 70 Recycled Aggregate of concrete to the modern society are many [41] (Babor, Plian and Judele 2009). Concrete is manufactured with expenditure of several raw materials[16]. Additionally, it requires a processing plant, a source of power and transportation of raw materials and finished concrete [42]. The production and the use of concrete are meant to improve the lives of people [21] [22]. It is however unavoidable that there are environmental impacts from production and use of concrete [16]. It is the responsibility of the industry players to ensure the benefits outweigh the negative impacts. Although many people perceive environmentally friendly methods to be more costly, this is not always the case[41] (Babor, Plian and Judele 2009). Some organisations that prepare ready mixed concrete have found ways in which to reuse water instead of allowing it to pollute the watercourses [41] (Babor, Plian and Judele 2009). Reuse of water has lowered water consumption and reduced operation costs of these organisations. Proper planning and development of environmental protection laws can be helpful in reduce the impacts of construction on environment [42]. Several countries have passed regulations requiring an environmental impact assessment to be done on any proposed development [42]. Quarries and mining sites are required to backfill the sites after extracting minerals and aggregates[41] (Babor, Plian and Judele 2009). Environmental laws can help prevent impacts of concrete production activities on local level [16, 42]. All aspect of construction should be evaluated in order to understand their associated environmental impacts[42]. For instance, development of infrastructure leads to loss of agricultural land and destruction of land due to extraction of aggregates [42]. Transportation of aggregates and other materials to site lead to pollution through emission[41] (Babor, Plian and Judele 2009). Energy is consumed in running and maintaining buildings. Energy is also used in demolition and disposal of demolition waste [16]. In order to control pollution resulting from the production and use of concrete, it is important to take into consideration the full life cycle of concrete from conception to grave Page 25 of 70 Recycled Aggregate [42]. It is important to consider each constituent materials of concrete. The Figure 10 below shows the excavations and emissions that result from production and use of concrete. Figure 10: Excavations and emissions caused by production of concrete Source:[42] 2.6.1. Portland cement Portland cement is produced by burning clay and limestone in kiln. The resulting clinker is then grounded with gypsum to produce cement [42]. The process consumes a lot of energy [16]. The Table 3 below summarises the major environmental concerns in the production and the use of cement. Table 3: Environmental concerns associated with manufacture of cement Source: [41] Stages in the cement-making process Major environmental concerns Quarrying and processing raw material • Scarring of landscape • Emission of dust • Noise pollution during transportation Production of clinker through burning of raw materials • Emission of carbon dioxide from burning of fuel and limestone. Page 26 of 70 Recycled Aggregate • Emission of greenhouse gases such as sulphur dioxide and nitrogen dioxide. • Use of energy • Emission of dust Grinding of clinker • Use of electrical energy Transportation of cement to concrete mixing plants, to builders, and to precast works • Use of fuel • Traffic congestion • Noise pollution 2.6.2.Aggregates Aggregates are the major constituents of concrete. They are derived from rocks such as limestone and granites. The rocks are quarried and then crushed into the desired sizes [42]. In Australia, marine sand and land-based gravels are readily available [41] . However, the aggregates must be processed through washing and grading before they can be used in production of concrete [16]. The table 4 below summarises the environmental concerns associated with production of aggregates. Table 4: Environmental issues associated with production and processing of aggregates Source: [41] Stage in aggregate processing Major environmental concerns Quarrying and processing of raw materials • Scarring of landscape • Noise and dust pollution • Loss of agricultural land • Energy consumption Delivery of aggregates to concrete production plants • Fuel consumption • Traffic congestion • Noise pollution Coarse aggregates have particles with sizes greater than 5mm. The size of coarse aggregates can be up to maximum of 300mm depending on use such as dams. However, the normal maximum size is 20mm [16]. Fine aggregates refer to sand where particles are less than 5mm [41]. Old concrete can be recycled to produce both fine and coarse aggregates. However, the recycling processes require the removal of deleterious substances[41]. Legislation has been put Page 27 of 70 Recycled Aggregate in place in Australia to encourage recycling of construction and demolition waste. The rise in the cost of landfills has made contractors to turn to recycling. 2.6.3.Water The amount of water in concrete affects the compatibility and fluidity of fresh concrete [42]. Water reacts with cement to harden and become strong [41]. Some of the water fills the capillary pores within concrete [16]. Water to cement ratio affects the strength and durability of concrete. The higher the water content the weaker and less durable is the concrete [41]. Admixtures are used to reduce the amount of water required in concrete. Recycling of water can help to reduce the demand for water in construction activities. 2.6.4.Admixtures Admixtures are added to improve the properties of fresh and hard concrete. For instance, plasticisers are used to improve the fluidity of concrete without reducing its physical properties [16]. The use of such admixtures helps to reduce direct contact between the workers and concrete[41]. As a result, the working environment is improved by improving the health and safety of workers. The impact on environment can thus be lessened. 2.7. Properties of Recycled Concrete Coarse Aggregates The use of recycled concrete aggregates must ensure that they can sufficiently replace natural aggregates without compromising on the strength of the concrete. If these aggregates can sufficiently replace the need to extract virgin resources, then they can help address the issue of environmental degradation and sustainability [16]. In order to ascertain this, there is a need to ensure the recycled waste have the desirable mechanical properties [42]. Research has shown that there is a lot of variation in mechanical properties of recycled aggregates depending on the source [37]. Since aggregates make up about 70% to 80% of the concrete, they have significant contribution to the overall strength of concrete [41]. Therefore, it is important to check the Page 28 of 70 Recycled Aggregate quality of recycled aggregates before they can be used in structural concrete. The basic mechanical properties such as shape, size, absorption, texture, permeability, specific gravity, deleterious substances, strength characteristics and resistance to free should be investigated [41]. It is through this process that one can ascertain the desirable level of substitution of natural aggregates that specific recycled aggregates can achieve for a certain desired concrete strength. The basic properties of recycled aggregates are outlined below: 2.7.1.Adhered mortar Research shows that most of the recycled concrete aggregates has mortar adhered to their surfaces [43]. This mortar is undesirable because it usually weakens the bond between the aggregates and the cement paste by creating an interfacial transition zones. The coat of mortar on recycled aggregates has higher porosity than the aggregates. This high porosity has the effect of reducing the strength and the quality of the resulting concrete. According to Yadav and Pathak (2009) the amount of adhered mortar is highly dependent on the texture, shape and the size of the aggregates. The amount of adhered mortar is higher for smaller aggregates than for larger aggregates. In a research conducted by Yadav and Pathak (2009) using thermal method, it was found that for aggregates ranging from 4mm to 8mm they have 33-55% mortar composition. On the other hand, aggregates with diameters ranging from 8mm to 16mm had 23 to 44% mortar composition [37]. This adhered mortar normally ranges from 25 to 65% depending on the size of the aggregates and the method used to measure the composition[41] (Babor, Plian and Judele 2009). The presence of adhered mortar around the recycled concrete aggregates is the major reason for the high water absorption in these concretes. The absorption rate of these aggregates has been observed to range from 4% to 12% depending on the size of aggregates and the percentage composition of mortar. The adhered mortar thus consequently has a significant impact on the compressive strength of concrete. 2.7.2.Particle size distribution Page 29 of 70 Recycled Aggregate Several studies have carried sieve analysis to investigate the particle size distribution in recycled concrete aggregates[43, 44]. In one of these studies conducted by Wagih, El-Karmoty, Ebid and Okba (2013), fifteen samples were taken from different RCA crushing plants. The RCA was separated into different categories depending on the sizes and other properties during crushing and sieving. A sieve analysis of the fifteen samples indicated that the majority of the finer aggregates fell under the diameter of 4.75mm [37]. The particles falling under this region were between 20% and 44% for different samples of RCA[41]. This particle size distribution was found to be influenced by the crushing method and the original quality of demolished concrete. 2.7.3. Specific Gravity and Water Absorption The specific gravity is an important aspect of an aggregate since it has a lot of influence on the strength of concrete. Similarly, the water absorption of aggregates influences the water cement ratio and hence the final compressive strength of the resulting concrete. Several researchers [43-45] have carried out experiments to investigate how the specific gravity and water absorption of RCA compares with the same parameters in natural aggregates. All these studies [43-45] the specific gravity of recycled concrete aggregates is significantly lower than that of natural aggregates. This phenomenon has been explained by the presence of mortar coating adhering on the surface of recycled concrete aggregates. According to Yadav and Pathak (2009), the specific gravity of RCA ranges from 2.35 to 2.58 depending on the source concrete. This specific gravity is lower than that of natural aggregates. The water absorption in RCA has also been found to be higher than that of natural aggregates [43, 45]. Wagih, El-Karmoty, Ebid and Okba (2013) explain this outcome by highlighting the presence of mortar coating as the cause of higher water absorption ratio. In their experiment, Wagih, El-Karmoty, Ebid and Okba (2013) find that the water absorption in RCA ranges from 2.15% to 7.15%. This high water absorption is undesirable in aggregates since it leads to increase in water-cement ratio and Page 30 of 70 Recycled Aggregate hence decrease in compressive strength in the resulting concrete. Ideally, the water absorption should not be more than 2.5% [44]. In order to attain desirable concrete strength when using RCA, it advisable to ensure the surfaces of the aggregates is sufficiently dry before doing the mixing. 2.7.4.Bulk Density The bulk density of RCA has been found to be lower than that of natural aggregates by different researchers [43-45]. In a study by Tiwari (2015) the average bulk density of the RCA was found to be 1324.98 and that of natural aggregates to be 1470.8. The researcher attributed the lower bulk density in RCA to the higher porosity in the RCA than in the natural aggregates. This is an explanation that is also backed by several other researchers such as Yadav and Pathak (2009); and Wagih, El-Karmoty, Ebid and Okba (2013). The Table 1 below provides a summary of the findings on the specific gravity, water absorption and bulk density that were obtained by Tiwari (2015). Table 5: A comparison of physical properties of RCA and natural aggregates Source: [45] Physical properties Water absorption Bulk density Specific gravity Natural aggregates 1.58 1470.8 2.65 RCA 6.6 1324.98 2.2 2.7.5.Abrasion Index and Impact Values In various abrasion and impact tests carried by different researchers [21, 32, 46], it has been found that RCA are relatively weaker than the natural aggregates. According to Tabsh and Abdelfatah (2009) and Yadav and Pathak (2009) the abrasion of aggregates is an important indicator of its strength. Aggregates with a high abrasion resistance have higher compressive strengths. A high percentage of abrasion in aggregates indicates low compressive strength [42]. The abrasion resistance of an aggregate is influenced by its size and the strength of the source concrete. The source concrete with low abrasion also produced aggregates with low abrasion Page 31 of 70 Recycled Aggregate [42]. The presence of mortar adhering on the surface of recycled aggregates increased their susceptibility to abrasion. Thus, there was higher abrasion recorded in RCA than the natural aggregates. According to Yadav and Pathak (2009) recycled concrete aggregates can have abrasion of up to 48% leading to a loss in strength of between 20% and 35%. The Figure 8 below was plotted by Yadav and Pathak (2009) to illustrate how increase in abrasion leads to decrease in compressive strength of recycled concrete aggregates. According to Wagih, El-Karmoty, Ebid and Okba (2013) the abrasion of aggregates for structural concrete should not exceed 30%. On the other hand, the researchers[44] note that the impact values should be lower than 45% in aggregates for structural concrete. In a study conducted by Wagih, El-Karmoty, Ebid and Okba (2013), it was found that the impact values of RCA ranged from 13% to 38%. These values were lower than the recommended maximum impact value of 45%. This observation indicates that RCA are good in withstanding impacts. The presence of mortar adhering on the surface does not influence the impact strength of RCA. 2.7.6.Chemical properties Chemical properties of concrete have significant influence on the overall durability of a structure. Presence of chemical can corrode the concrete and the steel present in concrete. It is important investigate the presence of chemicals such as chlorides and sulphates in concrete. In study conducted by Wagih, El-Karmoty, Ebid and Okba (2013), an experiment was carried out to determine the sulphate and the chloride content of RCA. The researchers found that the amount of chlorides in the RCA samples ranged from 0.004% to 0.027% of the water soluble portion of the aggregates. However, a higher level of chloride content was found in RCA obtained from marine structures. The chloride content in the aggregates obtained from marine structures was found to range between 0.07% and 0.244%. The difference in the chemical composition in the tested aggregates indicates that the chemical properties of RCA are influenced by the source concrete. RCA that have been sourced from marine structures should Page 32 of 70 Recycled Aggregate be checked for chloride content. On the other hand, Wagih, El-Karmoty, Ebid and Okba (2013) found the sulphate contents in all the samples to range from 0.01% to 0.05%. This range is lower than the limit value for 0.4%. It is however advisable to mix recycled aggregates with natural aggregates in order to achieve the desirable chemical characteristics of the final aggregates. Page 33 of 70 Recycled Aggregate 3: RESEARCH 3.1. Introduction This research methodology outlines the procedures that were followed in this research. It explores how the case studies were carried out and how experimental work was conducted by different researchers. The section gives details of the tests that were reviewed on the recycled concrete fine aggregates and the resulting concrete. It was important to verify if the recycled concrete fine aggregates had the desired properties to substitute the natural aggregates. 3.2. Design mix Numerous trial mixes are conducted in order to establish the optimum mix. The determined optimum mix is then used to produce concrete with 0%, 25%, 50%, 75% and 100% recycled concrete content [47]. In a study conducted by Topcu and Şengel (2004), the optimum mix design was found to be in the ratio of 1:1.5:3 (C: S: G). Thus, the materials that were required for this experiment include 125kg of natural coarse aggregates, 125kg of coarse aggregates, 150kg of sand,150 kg of recycled fine aggregates and 100 kg of cement. 3.3. Sourcing of cement, fine and coarse aggregates The cement used in reviewed studies [47, 48] is Ordinary Portland Cement (OPC) 43 grade. In a study by Topcu and Şengel (2004) natural fine aggregates were obtained from river sand of zone II type. Natural coarse aggregates were obtained from microtonalite rock. Recycled concrete fine and coarse aggregates were obtained from crushed concrete. 3.4. Sourcing recycled fine aggregates Recycled fine aggregates are produced form processors such as polishing and crushing processors. Crushing processors break demolished concrete waste through the application of external force [47]. There two types of crushers: one that applies compressive force and one Page 34 of 70 Recycled Aggregate that applies impact through a revolving metal object. The polishing process involves breaking the crushed aggregates further through applying frictional force. This force makes the grains to grind each other into finer particles. The recycled fine aggregates were obtained by using three types of processors. The three processors were a jaw crusher, a ball miller and a granulator. The jaw crusher was used to break down large concrete waste blocks, while the ball miller and the granulator [49]. The jaw crusher helps to reduce the size of particle into smaller ones. It reduced the recycled concrete into particles of 40mm or less. The granulator helps in polishing fine aggregates by allowing them to rub against each other. It has a roller and a drum that helps to generate the friction needed to polish the aggregates. In a study by Levy and Helene (2004) the crushed concrete was two years old at the time of crushing. The resulting material was then passed through a 5mm sieve, and very fine particles of dust with sizes less than 0.15mm were blown off using an air separator. This recycled fine aggregate was labelled appropriately and used in the experiments. 3.5. Sourcing recycled coarse aggregates The physical properties of natural aggregates and recycled concrete aggregates were compared. The specific tests that were reviewed include the water absorption, specific gravity and bulk density. In a study it was found that, the quality of recycled coarse aggregates was improved by firstly washing, heating and drying before they were used in concrete [50]. These processes were conducted on the crushed concrete as outlined below 3.5.1. Washing In a study by Padmini, Ramamurthy and Mathews (2009) water under pressure was directed on the aggregates in order to wash them. This process was considered important in removing adhered mortar and dust from the surfaces of the aggregates. Water was applied at a pressure of about 500psi for duration of about 10 to 15 minutes. The cleaning helped to clean the Page 35 of 70 Recycled Aggregate aggregates to acceptable standards. The cleaned aggregates were then placed under the sun for about 30 minute to dry the excess water. 3.5.2. Heating The recycled concrete aggregates were dried further by placing them in an oven at a temperature of around 150oC for one hour [51]. In order to so, the aggregates were firstly packed in small flat aluminium pans. The heating process helps to dry and clean the aggregates even further. The resulting recycled concrete aggregates were ready to be used in making concrete. 3.5.3. Casting of cubes and cylinders The cleaned aggregates were batched for 7 and 28 days for preparing cubes and 28 days cylinders [51]. The natural aggregates were replaced in different batches using 0%, 25%, 50%, 75%, and 100% recycled concrete aggregates. From each of the five batches made six cubes and three cylinders were made. Three cubes from each batch were crushed after 7 days and the other three after 28 days to test for compressive strength. The three cylinders from each batch were tested for tensile splitting strength. 3.5.4. Test for concrete In studies conducted by Padmini, Ramamurthy and Mathews (2009), and Ajdukiewicz and Kliszczewicz (2002) tests were conducted for both fresh and hardened concrete. A fresh concrete is one that has been mixed and has not set. During it fresh state, concrete can be handled, transported, place and compacted. On the other hand, hardened concrete must have the strength to carry service and structural loads [42]. It should also be durable to resist deterioration due to the exposure to environmental factors. Page 36 of 70 Recycled Aggregate 3.6. Test for fresh concrete It is important to investigate the properties of fresh concrete because they influence the final properties of hardened concrete [52](Ajdukiewicz and Kliszczewicz 2002). The following are some of the tests that should be conducted on fresh concrete. a) Consistency: this is a measure of how sloppy or how stiff or fluid is a mixture of concrete. The ideal consistency should ensure it is easy to handle, place, and compact the fresh concrete [42]. The consistency should be the same in all the batches of concrete. Consistency should thus be measured at regular intervals to ensure it is not affected by errors. Consistency can be measure using slump test. b) Workability: this property describe the relatively ease with which concrete can be mixed, handled, placed, compacted and finished without the segregation or separation of specific components from the mix. Consistency and workability are different properties of concrete [42]. A mix can have same workability but different consistency. Workability is affected by elements such as stone sizes, water content, stone content, plasticity, and cohesiveness of concrete. A concrete with smaller stone are considered to be more workable. It is not possible to directly measure workability, but the slump test can give an important clue. c) Settlement and bleeding: the high density of aggregates when compared to water makes them to settle and consequently make water to move upwards. This water collects on the top surface of the fresh concrete [34]. This movement of water to the surface of concrete is what is referred as bleeding and the water is referred as bleed water. d) Plastic shrinkage: if fresh concrete losses water as it sets, it volumes decreases with the amount of water lost. The volume reduction caused by lost water is referred as plastic shrinkage of concrete[34]. The water in concrete mostly lost through evaporation or through absorption by dry surfaces such as formworks, old concrete or soil. Page 37 of 70 Recycled Aggregate e) Slump loss: after the concrete is mixed, it slowly loses its consistency. The loss of consistency can become a problem if the concrete cannot be properly handled, placed and compacted [34]. The loss in slump is caused by evaporation of water, absorption of water by aggregates, absorption of water by surfaces in contact with concrete, and cement hydration. 3.7. Tests for hardened concrete The hardened concrete should be tested to ensure it has attained the desired strength [52] . In experiments conducted by Ajdukiewicz and Kliszczewicz (2002), concrete cubes and cylinders were tested at 7 and 28 days for compressive and tensile splitting strength. 3.7.1. Compressive strength test This test is important in evaluating the characteristic strength of a concrete sample. In order to perform this test, a cube measuring 150 mm is used to cast a concrete mould. The test is carried out by loading on one surface of a cube until it crashes [34]. The compressive strength of the concrete is taken as the average of the three outputs of concrete tested at 7 and 28 days. Compressive strength is calculated by dividing the average maximum strength required to crash the cubes by the area of the cube. Thus, compressive strength (C) can be calculated using the formula below; C=P/A [38] Where P = maximum average load applied on the cube before crashing, and A = the cross sectional area of the cube under test. For instance, if the average load applied on the cube of 150mm before it crashed is 50,000 Kg, and then compressive strength can be obtained as illustrated below; C = (50,000X 9.81)/ (150X150) Thus, C= 22 MPa Page 38 of 70 Recycled Aggregate 3.7.2. Tensile splitting test This test is conducted to investigate the ability of a concrete sample to withstand tensile stresses. It is conducted on cylindrical concrete specimens. These specimens are normally placed on the flat surface on their horizontal axis and a force applied along the length of the cylinder. The standard length of the cylinder is 300 mm and a diameter of 150mm [38]. Wooden strips that are slightly longer than the cylinder with a thickness of 3.2mm and a width of 25 mm are used to secure the cylinder in place. The same machine used in the compressive test is used to applied load on the cylinders. Figure 11 below shows the machine that was used to test for compressive and tensile splitting strength of the concrete samples. Figure 11: Concrete compressive crushing strength machine at Charles Darwin University Source:[13] Page 39 of 70 Recycled Aggregate 4: A CASE STUDY ON THE USE OF RECYCLED AGGREGATES CONCRETE IN DARWIN CITY 4.1. Introduction This chapter examines the existing solid waste management and recycling in Darwin city in Australia. In order to do so, the chapter examines the following issues a) The current waste management in the construction industry in Darwin and its challenges b) Evaluate the importance of recycling solid waste in Darwin c) Examine the operations of construction demolitions sites, recycling plants and landfills in order to understand the existing recycling practices d) Outlines the challenges that face the current waste management and recycling in the Darwin city e) Provides a proposal for recycling the current waste produced by the construction industry in the city 4.2. Waste management and recycling in Darwin, NT, Australia There are a lot demands by the residents of the city for organisations to adopt environmentally friendly practices. Darwin city is well known for its waterfronts, beaches and green areas such as the Bicentennial Park. The city is also the gateway to the famous Kakadu National Park. Due to the ecological importance of this city, it is important for organisations to adopt more environmentally friendly practices. One of the major industries that have experienced pressure to enhance sustainable practices is the building and construction. Many organisations are adopting the hierarchy of waste management that entails six levels: 1) reduce, 2) reuse, 3) recycle, 4) compost, 5) incinerate, and 6) landfill. The Figure 12 is a schematic showing the waste management hierarchy from the most desirable to least desirable strategy. Reduce Reuse Recycle Low Page 40 of 70 Recycled Aggregate Source: [30] Reduce is the most preferred strategy for waste reduction while landfill is the most undesirable [30]. In order to reduce waste generation in construction and demolition sites, organisations are encouraged to coordinate with professionals in order to develop designs and construction processes that are energy efficient. The practice of recycling is strategy that is being used by organisations in Darwin to minimise the amount of solid waste that goes to landfills (Poon and Chan 2006). The major benefits of recycling include reducing the demand for quarrying new construction materials from virgin resources, reduce the energy that is consumed when transporting waste and virgin materials, and reducing the environmental pollution by reducing the amount of materials in landfills [30]. Traditionally, the council in major cities of Australia do not evaluate the specific composition of builders or rubble waste that is disposed into landfills. The waste that is generated from construction and demolition of facilities such as roads and houses, and other activities such as excavation is generally considered as rubbles. However, this is not the case because demolition waste generally depends on site specific issues such as the style of architecture, design, construction method and materials used in the construction [30]. The construction and demolition waste include components such as masonry, wood, bricks, concrete, glass, pipes, Page 41 of 70 Recycled Aggregate soils, rocks, insulation, wires, and roofing tiles. All these materials have different chemical and mechanical properties that can affect the environment in different ways. The Table 4 below shows the average percentage composition of construction and demolition waste. Table 6: The average percentage composition of construction and demolition waste Source:[30] Type of construction material Construction waste percentage composition by weight (%) Demolition waste percentage composition by weight (%) Concrete 11 21 Timber 15 19 Pavers and bricks 10 16 Paper/cardboards 9 10 metals 8 9 plasterboards 12 8 Earthen materials 23 6 Green waste materials 7 5 Plastics wastes 3 4 Glass materials 2 2 Total 100 100 4.3. Current waste management strategies in Darwin, Northern Territory, Australia There are very few companies in Darwin that recycle aggregates [53]. Darwin, as the capital of the Australia’s Northern Territory, has a lot of construction projects taking place annually. As a home to some of the most iconic beaches and green parks, such as the Bicentennial Park, it is essential to ensure the construction activities are sustainable. Hyder Consulting (2011) indicates that there very little data relating to the recycling and reuse of construction and demolition materials in the Northern Territory. This lack of data can only be attributed to the absence of recycling activities in the territory. The Northern Territory has very low population when compared to other regions in Australia. In 2014, the population in the territory was estimated at 243,000 people. 60% of this population is found in Darwin. The generation of waste per capita in the territory is low at 1.32 tonnes per head per annum. Thus the total waste produced in the region is around 320,000 tonnes annually. Thus NT is responsible for about Page 42 of 70 Recycled Aggregate 0.6% of the total pollution produced in Australia [20]. Despite the low waste production, NT has the highest number of landfills per capita of the entire jurisdiction in the country. This implies that Darwin has the highest number of landfills of the settlements in NT. The high number of landfills is attributed to the pattern of population. The jurisdiction has scattered but sparsely populated settlements that have their own landfills [20]. The major form of waste management in NT is thus landfills. The jurisdiction has low waste recovery rate of approximately 9% [20]. Therefore, it is ranked among the worst performing jurisdiction in terms of recycling. The low recycling rates in NT is attributed to the high cost due to distances in the region. The major settlement centres in NT include Darwin, Litchfield, Palmerston, Alice Springs, Katherine region, Barkly region, and East Arnhem. The largest numbers of Territorians live in regions such as Darwin, Litchfield, and Palmerston. The second most populate areas are the central regions such as the Alice Springs. Residents in major cities such as Darwin normally have the highest number of waste facilities [20]. There are specialised organisations that collect, transport and dispose waste. Organisations are also appointed to treat and recycle waste. Remote areas have lesser access to waste recycling facilities. The presence of accessible waste collection facilities reduces the chance of waste going into landfills [20]. The presence of kerbside collection services is one of the ways used in Darwin to segregate waste depending on it type. This system helps to remove recyclable waste before going into landfills. These kerbside rubbish collection and segregation units are available in the major cities such as Palmerston, Darwin, Tennant Creek, Nhulunbuy and Katherine [20]. The high relative population of Darwin city makes it the highest contributor of waste in the territory. The Council of the City of Darwin disposed approximately 170,000 tonnes of waste materials in the landfill at the Shoal Bay Waste Management Facility. Although the Australian average resource recovery is 60%, the city has as little as 5% rate of resource recovery [20]. The major deterrent Page 43 of 70 Recycled Aggregate towards recycling in the jurisdiction is the lack of recycling plants. Recyclables must be transported across states for processing. Waste from the Darwin is normally disposed by the MACHAHON Holdings in the Shoal Bay Landfills that are owned and operated by the City of Darwin [12]. The waste disposal company is normally paid by the volume of waste it disposes to the landfills. This arrangement does not offer any incentive to the company to divert any waste from the landfills. There are no concrete blocks or any other inert materials removed before the waste is disposed to landfills [20]. The rate paid for disposal of waste in the fills ranges from $32 to $47 per tonne depending on the type of waste [12]. In an effort to promote recycling, the City of Darwin awarded the NT Recycling Services a contract to extract recyclable materials from the waste[12]. However, the efforts by the company are hampered by lack of incentives for landfill operators to reduce the volume of waste disposed in the fills. Noguchi and Tamura (2001) note that RUB Group is one of the few companies that are involved in the recycling of aggregates in the City of Darwin. The Figure 13 below shows some of the crushed concrete stockpile at R.U.B. Group. Figure 13: Concrete Block ready for crushing at RUB site Source: [13] The group is involves in the crushing and recycling of concrete aggregates. It operates a crusher that can produce approximately 200 tonnes of aggregates in an hour. However, when compared Page 44 of 70 Recycled Aggregate to the HB Group, a company that processes natural aggregates, the output by R.U.B. Group is insignificant [14]. The HB Group processes an estimated 500 tonnes of natural aggregates in an hour. In order to compete favourably, the R.U.B Group must increase its output in order to match that produced from natural aggregates [14]. That lack of proper technology and investment in recycling of aggregates in Darwin has made many construction projects to use natural aggregates The remote regions with low populations face significant challenge of recycling waste. These regions are not served with kerbside rubbish collection and segregation facilities. These facilities are essential in separation of hazardous waste from the recyclable one. Due to lack of such facilities, the residents normally dispose of waste through burning or burying it landfills [20]. Investments in recycling plants where there are small communities are not a viable solution. This is because there is very little of recyclable material generated from such communities. The limitations such as high transport costs and long distances have prevented businesses from investing in waste management practices. The lack of economic incentives for the community has dissuaded from engaging in environmentally friendly practices. In order to overcome such challenges, the small communities should use integrated strategies such as avoidance, reuse, recycling and treatment of waste before disposal. The use of such integrated strategies can help to bring together the communities and the council to discuss the must feasible alternative for each specific type of waste [20]. The adoption of such approach may help reduce the instances of landfills and hence a more sustainable approach in waste management. The avoidance of landfills has economic benefits such as reducing the land occupied by landfills. This system allows waste to be disposed at a transfer station of a common landfill for the community. The organisation of waste management system is an important aspect of managing waste in small communities. Page 45 of 70 Recycled Aggregate 4.4. Site visits Site visits were carried out in several construction, demolition, landfills and recycling organisations and facilities in order to examine the current practices. The focus of the site visits was within the city of Darwin. In order to current recycling methods for waste generated from construction and demolition activities. These sites were one construction sites, one demolition sites, one recycling facilities, and one landfill. The purpose of the site visits was to identify the composition of materials supplied, material received, and the rate of production of aggregates, applications of recycled aggregates, the equipment used, testing methods for recycled aggregates, and costing methods. The site visits indicated that the construction and demolition sites produce mixed waste. Contractors do not make effort to separate different types of waste before disposal. 4.5. Barriers to recycling construction and demolition waste The recycling of construction and demolition waste has not been prevalent in Darwin due to several barriers. Unless these barriers are addressed, recycling will fail to achieve its goals. Some of the barriers are discussed here. a) Loss of jobs: there is a perception that recycling of concrete waste will lead to loss of jobs. Many people will lose jobs at landfills and quarry when recycled aggregates are used. However, the recycling of aggregates will create new jobs in recycling. b) Characteristics of recycled concrete: the recycled concrete aggregates have high content of cement and therefore tend to absorb more water than normal aggregates. The high water absorption creates problems such as prolonging the curing time and altering other properties of fresh concrete. Page 46 of 70 Recycled Aggregate c) Contaminants in aggregates: the recycling industry is often faced with the challenge of contaminants in waste concrete. Contaminants such as asbestos found in waste concrete are a health and safety hazard. Several inspection methods should be developed to help remove contaminants before the aggregates are used in concrete production. d) Perceptions: there are negative perceptions about the use of recycled aggregates in the city. Many constructor think that the recycled materials are difficult to use and inferior to virgin ones. However, these perceptions have been changing in recent times as more field trials demonstrate that recycled aggregates can perform as well as virgin materials. The change in perception has also been driven by the increasing scarcity and cost of virgin materials in the Northern Territory. Page 47 of 70 Recycled Aggregate 5: ANALYSIS OF PROPERTIES OF COARSE RECYCLED AGGREGATES CONCRETE 5.1. Introduction The properties of recycled coarse aggregates concrete were investigated from previous studies and are provided in this chapter. In these studies[23, 24], experimental results were obtained from different mix proportions from the replacement ratio of recycled aggregates of 0% to 100%. The properties of concrete were also investigated at different water to cement ratios and aggregate to cement ratios. The water to cement ratios used was 0.45, 0.5 and 0.60 [23, 24]. The aggregate to cement ratio used was 4.5. The four major properties of concrete that were investigated were workability, strength, density, and deformation. 5.2. Mixing proportions In a research conducted by Sagoe-Crentsil, Brown and Taylor (2001), prior to the use of the aggregates, they were washed, dried and heated to ensure they did not have adhered mortar. Ordinary Portland cement was used in all the experiments [23, 24]. The mixing ratios: aggregates to cement ratios, and water to cement ratios issummarised in the table shown below: Table 7: Mixing ratios Source: [23] Water to cement ratio Aggregate to cement ratio RA replacement ratio Cement (Kg) RA 20mm (Kg) NA 20mm (kg) Sand (kg) Water (kg) 0.45 4.5 0 40 0 108 72 18 20 40 21.6 86.4 72 18 40 40 43.2 64.8 72 18 60 40 64.8 43.2 72 18 80 40 86.4 21.6 72 18 100 40 108 0 72 18 0.5 4.5 0 40 0 108 72 20 50 40 54 54 72 20 100 40 108 0 72 20 0.6 4.5 0 40 0 108 72 24 50 40 54 54 72 24 100 40 108 0 72 24 5.3. Mixing procedure Page 48 of 70 Recycled Aggregate The mixing of concrete in studies by Sagoe-Crentsil, Brown and Taylor (2001) and Kou and Poon (2009) was conducted as per AS 1012. The first material to be batched was half the coarse aggregates, then fine aggregates were added followed by cement and finally the other portion of coarse aggregates. The aggregates were mixed for two minutes before water was added. 5.4. Properties of recycled aggregate concrete The following major properties of concrete were investigated by Sagoe-Crentsil, Brown and Taylor (2001), and Kou and Poon (2009) as outlined below: a) Workability was investigate using slump test as stipulated in AS 1012.3.1- 1998 b) Density was measured as per AS 1012.12.1- 1998 c) Compressive strength was investigated as per AS 1012.9 -1999 d) Tensile splitting strength as per AS 1012.10- 2000 e) Flexural strength as per AS 1012.11- 2000 f) Dry shrinkage as per AS 1012.13 – 1992 5.4.1. Workability The state of fresh concrete is often describes by its consistency or workability. Although workability cannot be measured directly, the slump test provides an important indicator of workability [24]. The slump test is a simple and convenient test for measuring workability. When concrete has big slump, it indicates it is less cohesive and hence more prone to separation or segregation [23]. Segregation of concrete can eventually affect the strength of hardened concrete. However, when the slump of concrete is very small, it indicates the mix is dry and the concrete can therefore be very difficult to place, compact and finish [21] [22]. Poor workability increases the risks of having a large number of voids and hence low compressive strength. Page 49 of 70 Recycled Aggregate The workability of concrete is affected by the amount of water used, the volume of aggregates, and the nature of aggregates[21] [22]. When the amount of water and cement are assumed to be constant, then the factors that affect workability are the size shape, texture, grading and size of aggregates. When design concrete mixes, it is important to pay attention to the water to cement ratio, coarse to fine aggregate ratio, and aggregate to cement ratio. Due to the influence of the nature aggregates on workability, then concrete made from recycled aggregates has complex workability. Previous researchers show that concrete made from coarse recycled aggregates needs 5% more water than concrete made from natural aggregates in order to achieve similar workability [21, 22, 24, 32]. In this research, different recycled aggregates replacement ratio, aggregate to cement ratio, and water to cement ratio are used. According to a study by Etxeberria, Vázquez, Marí and Barra (2007) recycled concrete aggregates can absorb up to 90% more water than normal aggregates. The review of aggregates used in the above researches indicates that they are washed, dried and heated to ensure they do not have adhered mortar and they are sufficiently dry. The workability of the recycled aggregates concrete was found to be similar when replacement ratios increased from 0% to 100% when the water content and aggregates to cement ratios were maintained constant at 0.45 and 4.5 respectively [21] [22]. It is expected that the workability of concrete to increase with increase in aggregates to cement ratio. Higher aggregates to cement ratio implies that there lower total surface area from aggregates and cement. When aggregates to cement ratio is fixed and water content is increase, the workability increases. It is important to consider the amount of water that is absorbed by fine and coarse aggregates in concrete. The calculation of free water allows the designer to allow for enough water for mixing. According to Etxeberria, Vázquez, Marí and Barra (2007) all slumps of fresh recycled aggregate concrete should lie between 35mm and 125mm for effective workability and sufficient concrete strength. Page 50 of 70 Recycled Aggregate 5.4.2. Density In a study by Etxeberria, Vázquez, Marí and Barra (2007) the density of concrete with different RA replacement ratio was measured from three samples. The density of the concrete obtained in the study is summarised in the table below. It was noted that recycled aggregates concrete with high recycled aggregates replacement ratios had lower density than those with lower replacement ratios [27]. This low density is caused by the low density of mortar adhered around the recycled aggregates. The increase in water content and aggregates to cement ratio also led to decrease in density due to decrease in aggregates and increase in volume of water [27]. The lowest density obtained by Etxeberria, Vázquez, Marí and Barra (2007) was 2,226 kg/m3 , which was obtained for concrete with 100% recycled aggregates replacement ratio, 4.5 aggregate to cement ratio, and 0.6 water to cement ratio. On the other hand, concrete with 0% recycled aggregates replacement ratio was found to have the highest density at 2419 kg/m3 Etxeberria, Vázquez, Marí and Barra (2007). The density of all concrete was found to lie within the Australian Standards recommended range of 2100kg/m3 to 2800kg/m3 . Table 8: Concrete density at different water cement ratio and RA replacement ratios Source:[21] [22] Water to cement ratio Aggregate to cement ratio RA replacement ratio (%) Concrete density (kg/m3 ) 7 days 14 days 28 days 0.45 4.5 0 2411 2416 2419 4.5 20 2382 2396 2392 4.5 40 2372 2376 2378 4.5 60 2342 2349 2358 4.5 80 2329 2335 2339 4.5 100 2306 2317 2325 0.5 4.5 0 2370 2385 2395 4.5 50 2345 2358 2365 4.5 100 2265 2270 2292 0.6 4.5 0 2335 2357 2368 4.5 50 2301 2304 2306 Page 51 of 70 Recycled Aggregate 4.5 100 2226 2236 2249 5.4.3. Concrete Strength The majority of literature reviewed [22, 25, 26] conducted three tests were conducted to investigate the compressive, flexural and tensile strength of concrete at 7, 14 and 28 days. The results are summarised in Table 6, Table 7 and Table 8 below. According to some researchers [21, 32], high substitution of recycled aggregates leads to reduction in strength of concrete. The more recycled aggregate is added in concrete, the more it loses it strength. For instance, at 100%recycled aggregates replacement ratio, water to cement ratio of 0.45 and aggregate ratio of 4.5, concrete found to have compressive strength, tensile, and flexural strength of 41.86MPa, 3.95MPa, and 3.39Mpa respectively. When 0% recycled aggregates were used for similar water and aggregate to cement ratios, the compressive, tensile and flexural strength were found to be 58.75 Mpa, 5.25 Mpa, and 5.60 Mpa respectively at 28 days. The strength of concrete is also affected by the change in water to cement ratios[25]. For instance, at 20% replacement of recycled aggregates and aggregates to cement ratio of 4.5, the compressive, tensile and flexural strengths were found by Debieb and Kenai (2008) to be 56.25Mpa, 4.85Mpa and 5.13Mpa respectively with water to cement ratio 0.45. When the water content was increased to 0.60, the compressive, tensile and flexural strengths were found to decrease to 29.27 Mpa, 3. 55 Mpa, and 3.55 Mpa respectively at 28 days. Table 9: Compressive strength of RAC with water to cement ratio between 0.45 and 0.60 Source: [25, 26] Water to cement ratio Aggregates to cement ratio RA replacement ratio Compressive strength (MPa) 7 days 14 days 28 days 0.45 4.5 0 50.85 52.70 58.75 4.5 20 45.00 47.68 56.25 4.5 40 41.78 45.75 52.08 4.5 60 41.23 44.21 48.00 Page 52 of 70 Recycled Aggregate 4.5 80 37.95 41.20 47.50 4.5 100 33.50 38.85 41.86 0.5 4.5 0 33.22 40.63 46.05 4.5 50 25.45 31.27 35.12 4.5 100 23.52 29.00 32.59 0.6 4.5 0 24.46 31.21 32.98 4.5 50 22.59 25.26 29.27 4.5 100 18.23 21.70 25.92 Table 10: Tensile split strength of RAC with water to cement ratio between 0.45 and 0.60 Source:[25] Water to cement ratio Aggregates to cement ratio RA replacement ratio Tensile strength (MPa) 7 days 14 days 28 days 0.45 4.5 0 4.50 5.02 5.25 4.5 20 4.00 4.30 4.85 4.5 40 4.25 4.49 4.58 4.5 60 4.20 4.35 4.60 4.5 80 3.59 3.91 4.23 4.5 100 3.36 3.58 3.95 0.5 4.5 0 3.81 3.96 4.27 4.5 50 3.51 3.73 4.02 4.5 100 3.01 3.18 3.37 0.6 4.5 0 3.35 3.65 3.87 4.5 50 3.05 3.52 3.55 4.5 100 2.42 2.69 2.92 Table 11: Flexural strength of RAC with water to cement ratio between 0.45 and 0.60 Source:[25] Water to cement ratio Aggregates to cement ratio RA replacement ratio Flexural strength (MPa) 7 days 14 days 28 days 0.45 4.5 0 4.83 5.27 5.60 4.5 20 4.50 5.05 5.13 4.5 40 3.90 4.28 4.40 4.5 60 3.87 4.02 4.39 4.5 80 3.48 3.71 4.12 4.5 100 2.87 3.25 3.39 0.5 4.5 0 4.11 4.51 4.65 4.5 50 3.45 3.60 4.26 4.5 100 2.76 2.90 3.17 0.6 4.5 0 3.34 3.59 4.28 4.5 50 3.15 3.25 3.55 4.5 100 2.59 2.63 2.81 5.4.4. Deformation There are two tests that can be used to measure deformation of concrete: creep and dry shrinkage [26]. When concrete is placed and allowed to cure, it is exposed to environment. When the environment has humidity that is lower than the saturation level, moisture is lost from the concrete. The loss of the absorbed water from the calcium silicate in cement leads to Page 53 of 70 Recycled Aggregate shrinkage strain in concrete [26]. When the shrinkage strain is sustained for a long period, creep strain develops. The factors that influence shrinkage and creep of concrete include the relative humidity, time of drying, applied stress, rate of drying, duration offload, stiffness of aggregates, amount of aggregates, thickness of concrete, volume to surface ratio, admixtures, moisture content, age of paste, curing temperature, porosity, and cement composition [26]. This research investigated how dry shrinkage occurs when taking concrete with replacement ratio of 0%, 40% and 100% and taking concrete with lowest and highest water to cement ration (0.45 and 0.6). The Table 9 below provides a summary of the dry shrinkage deformation in concrete. Table 12: Dry shrinkage deformation in concrete Source: [47] Age of concrete (day) Water cement ratio =0.45 Water cement ratio = 0.5 Water cement ratio = 0.6 0% RA 40% RA 100% RA 0% RA 50% RA 100% RA 0% RA 50% RA 100% RA 0 0.00000 % 0.00000 % 0.00000 % 0.00000 % 0.00000 % 0.00000 % 0.00000 % 0.00000 % 0.00000 % 7 0.00295 0.00356 0.03124 0.00172 0.00521 0.00827 0.01171 0.00756 0.00725 14 0.02373 0.02576 0.01679 0.02754 0.02847 0.02895 0.00897 0.01465 0.00962 28 0.03754 0.04215 0.00364 0.05213 0.05160 0.4157 0.03164 0.02608 0.02765 It has been shown that the deformation of aggregates has an influence on deformation of concrete [47]. Recycled concrete aggregates deform more than virgin aggregates and hence do not provide sufficient restraint to cement paste against shrinkage. This study shows that the shrinkage deformation of concrete increases with increase in the recycled concrete aggregates in the matrix. Increasing the water to cement ratio was also found to increase the magnitude of deformation. Increasing the replacement ratio of recycled aggregates at the same water to cement ratio was found to increase the shrinkage deformation. There was no significant difference that was noted for shrinkage deformation for 0% and 40% RA replacement ratios. Page 54 of 70 Recycled Aggregate In a study by Topcu and Şengel (2004) the highest shrinkage was noted at day 28. The dry shrinkage at 100% RA replacement ratios at day 28 for water to cement ratio 0.45, 0.5, and 0.6 was found to be 0.00364%, 0.4157% and 0.02765 % respectively. 5.5. Summary In the studies investigated in this research, the recycled aggregates replacement ratios varied from 0% to 100%, while the water to cement ratio varied from 0.45 to 0.6. The six major properties of recycled coarse aggregates concrete that were reviewed were workability, compressive strength, tensile splitting strength, flexural strength, density, and deformation. It was noted that the workability of recycled aggregates concrete increases with increase in water to cement ratio. The slump of the fresh recycled aggregates concrete was found to fall between 35mm and 125mm. The density of concrete was found to fall with increase in recycled aggregates and water to cement ratio. The fall in density with increase in recycled aggregates replacement ratio indicates that recycled aggregate concrete has lower density than normal aggregate concrete. It has been shown that an increase in the amount of recycled aggregates in concrete leads to decline in the concrete strength. Recycled aggregate concrete with high recycled aggregates replacement ratios indicated lower compressive, tensile and flexural strength. Recycled aggregate concrete with 100% replacement of aggregates indicate the weakest compressive strength at 28 days. However, the compressive strength of this concrete was found to be still more than 25MPa at 28 days. The strength of concrete also declined with the increase in water to cement ratio. The studies reviewed did not show any significant deformation behaviour of recycled aggregate concrete[25, 47]. However, it was noted that the increase in recycled aggregates replacement ratio led to an increase in shrinkage deformation of the concrete. Page 55 of 70 Recycled Aggregate 6: ANALYSIS OF PROPERTIES OF FINE RECYCLED AGGREGATES MORTAR 6.1. Introduction This chapter outlines a review of tests that have been conducted by several researchers on fine recycled aggregates mortar in order to investigate their properties when used to replace natural sand. The chapter explains how recycled fine aggregates are collected, tested and evaluated. 6.2. Collection of recycled fine aggregates In studies conducted by Chen,Yen and Chen (2003) and Lin, Tyan, Chang and Chang 2004), recycled fine aggregates were collected through crushing of old concrete using a crusher. Multiple processes were used in crushing the fine aggregates. Concrete demolition waste was first crushed using a large jaw crusher. The resulting mixture of aggregates was then separated using a vibrating sieve. The sieve was meant to allow through particles with diameter lesser than 50mm [54]. Particles with diameter greater than 50mm were returned to the jaw crusher for further crushing. The process was repeated until all the aggregates were less than 50 mm in diameter. A smaller jaw crusher was then used to crush the resulting aggregates into smaller particles. The small particles were passed through a roller crusher before being taken to a vibrating screen. The aggregates with sizes exceeding 4.75 mm were sieved and taken back to the small jaw crusher and the roller crusher until finer particles were produced [54]. A wheeled san washer was then used to remove sludge with sizes less than 600μm [54]. The resulting fine aggregates were sized between 600μm and 4.75mm. These aggregates were then preserved in air tight containers. The fine aggregates were stored in air-tight container to prevent the hydration of un-hydrated part of the cement by air moisture [54]. The recycled fine aggregates were then sieved through #200 sieves in order to obtain very fine aggregates. The resulting fine aggregates were used in preparing concrete. The same procedure was followed for Page 56 of 70 Recycled Aggregate preparing natural fine aggregates. The Figures 14 and 15 below show the recycled and natural fine aggregates respectively. Figure 14: Recycled fine aggregates Source:[54] Figure 15: Natural fine aggregates Source: [54] The collected and sieved fine aggregates were used for making mortar cubes that were tested to evaluate the cementing properties of recycled fine aggregates when used to substitute for natural fine aggregates in concrete. Page 57 of 70 Recycled Aggregate 6.3. Mix proportions The mix proportions used in making mortar by Debieb and Kenai (2008) are summarised in the table below. The water to cement ratios used was 0.35 and 0.55. To improve flowability of mortar with 0.35 water to cement ratios, super plasticizer was added at 0.5% of the cement weight. The replacement ratio of natural fine aggregates with recycled concrete fine aggregates was set at 0%, 25%, 50% and 100%. The mixing ratio of cement to fine aggregates in the control batch is 1:2. Table 13: Mix proportions Source:[25] Mix number Water to cement ratio Fine RCA ratio Water (kg) Cement (kg) Fine Natural aggregates Fine recycled aggregates Super plasticizer A0 0.35 0 245 700 1400 0 4 A25 0.35 25 245 700 1050 350 4 A50 0.35 50 245 700 700 700 4 A100 0.35 100 245 700 0 1400 4 B0 0.55 0 340 620 1240 0 0 B25 0.55 25 340 620 930 310 0 B50 0.55 50 340 620 620 620 0 B100 0.55 100 340 620 0 1240 0 6.4. Fabrication of specimens The fine recycled concrete aggregates have high water absorption than the fine natural aggregates due to presence of cement [27]. It was therefore necessary to pre wet the recycled aggregates 24 hours before they were used in the mix. This was necessary to achieve saturated surface dry conditions before using the aggregates. Mixing was done as per the determined proportions. The resulting mortar was used to make 6 cylindrical specimens with a height of 50mm and a diameter of 100mm. Three cylinders with a height of 200mm and 100mm were also produced. Nine cubes of 50mm and four prismatic specimens measuring 25x25x285 mm were done. [27] repeated this process for each mix number. The cast specimens were covered using plastic materials to prevent loss of moisture. They were allowed to stay in their moulds Page 58 of 70 Recycled Aggregate for 24 hours before they were removed and allowed to cure under lime water at a temperature of 25oC until they were tested. 6.5. Properties of fine recycled aggregates mortar The characteristics of the mortar were investigated using different studies. The tests that were reviewed were absorption test, flow test, dry shrinkage, density, and compressive strength tests. 6.5.1. Flowability In a test conducted by Fan, Huang, Hwang and Chao (2015), the flow test was done as per the guidelines in ASTM C1437-13. In order to conduct this test, a mould of a bottom diameter of 100mm was placed on a flow table and fresh mortar poured in. The flow table was vibrated as the mould was lifted. The test was repeated four times and the mean diameter of the resulting flow was calculated [55]. The flow was calculated using the following formula; Flow (%) = (Da – Do)/Do x 100 (%) The maximum diameter of the flow table was 254mm. Thus the flow value of mortar was computed as follows; (254-100)/100x 1000 = 154 The flow rate values of all the mortar produced represented a good flow rate irrespective of water content. The flow rate values exceeded 154%. The figure below illustrates the flowability of mortar with water to cement ratio of 0.35. Page 59 of 70 Recycled Aggregate Figure 16: Flow rate values at water to cement ratio of 0.35 Source: [55] The flow values of mortar containing fine recycled aggregates had lower flow values than those of natural fine aggregates. The flow values also declined with increase in the fine recycled aggregates replacement ratios. The mortar that was made using water to cement ratio of 0.55 has a flow value exceeding 154%. 6.5.2. Density In a study by Fan, Huang, Hwang and Chao (2015), the density of mortar was done as per the guidelines in ASTM C642-13. A cylindrical sample with a height of 50mm and a diameter of 100mm was used. The sample were taken at 28 days and allowed to dry before density was measured. The samples were suspended in boiling water (Wa) and the weight retaken after being removed from water at 25oC (Wb). The following formula was then used to calculate the density of mortar; Density (kg/m3 ) = [Ws/(Wa-Wa)] x 1000 (kg/m3 ) The mortar samples with recycled fine aggregates had lower density than that with natural fine aggregates. The increase in the recycled fine aggregates replacement ratios led to a decrease in density of mortar[29, 56]. Braga, De Brito and Veiga (2012), Pereira, Evangelista and De Brito 0 20 40 60 80 100 120 140 0.00% 20.00% 40.00% 60.00% 80.00% 100.00% 120.00% Flow values (%) Recycled aggregates replacement (%) Page 60 of 70 Recycled Aggregate (2012), and Poon and Chan (2006) also observed similar trends with addition of fine recycled aggregates in mortar. The Figure 17 illustrates the average density of mortars at 28 days with different replacement ratios. Figure 17: density of mortars at 28 days with different replacement ratios Source: [55] 6.5.3. Absorption An experiment to measure the absorption of mortar was conducted by Fan, Huang, Hwang and Chao (2015) as per ASTM C642-13. The cylinder used measured 50mm in height and100mm in diameter. The absorption rate was measured at 28 days curing. The sample was removed from water a dried using an oven. This allowed the sample to achieve a constant weight (Wd). The dried sample was then soaked in water to attain saturated surface dry condition and weight taken (Ws). The following formula was then used to calculate water absorption. Absorption (%) = (Ws – Wd)/ Wd x 100 (%) The water absorption of mortars with fine recycled concrete aggregates exceeded that of natural fine aggregates at 28 days. The absorption rate of mortar increased with increase fine recycled concrete aggregates. The increase in absorption rate can be attributed to high absorption 2060 2080 2100 2120 2140 2160 2180 2200 2220 2240 2260 0% 20% 40% 60% 80% 100% 120% Density at 28 days (Kg/m3) Recycled aggregates replacement ratio (%) Page 61 of 70 Recycled Aggregate capacity of fine recycled concrete aggregates[57] (Amjadi, Monazami, Mohseni, Azar Balgouri and Ranjbar 2016). These findings agree with previous studies done by Poon and Chan (2006) and Pereira, Evangelista and De Brito (2012). The Figure 18 below illustrates the absorption of mortars at different replacement ratios at 28 days. Figure 18: Absorption of mortars at different replacement ratios at 28 days Source: [55] 6.5.4. Dry shrinkage The dry shrinkage of mortar was tested by Fan, Huang, Hwang and Chao (2015) as specified in ASTM C596-09. A sample measuring 25x25x285mm was used for this test. The initial length of the sample (Li) was firstly measured. This is the length at 3 days of curing. The sample was then cured under 25oC and at a relative humidity of 50%. The specimen was measured it length at 7 days, 14days and 28 days (Lx). The dry shrinkage of mortar was computed using the formula below; Dry shrinkage = (Li – Lx)/G; Where G is the effective length of the mould (250mm). All the samples with high fine recycled concrete aggregates had higher drying shrinkage than those with natural fine aggregates. The recycled fine aggregates have high porosity that allows 0 2 4 6 8 10 12 14 16 18 20 0% 20% 40% 60% 80% 100% 120% Absorption at 28 days (%) Recycled aggregates replacement (%) Page 62 of 70 Recycled Aggregate water to evaporate easily [57]. Increase in the recycled fine aggregates in mortar led to an increase in the drying shrinkage. 6.5.5. Compressive strength Fan, Huang, Hwang and Chao (2015) investigated the compressive strength of mortar in accordance to ASTM C109-13. The test was conducted using 50mm cube samples. The specimens were removed from water, allowed to dry and tested at 7, 14 and 28 days. The compressive strength of samples with high content of recycled fine aggregates was lower than that of samples containing natural fine aggregates[55]. Increase in the replacement ratio led to decrease in compressive strength. This phenomenon can be attributed to presence of cement paste in recycled aggregates that led to high porosity. High porosity in mortar leads to reduction in compressive strength. Page 63 of 70 Recycled Aggregate 7: CONCLUSION AND FUTURE WORKS 7.1. Introduction This conclusion and future works chapter provides a summary of all the findings that were made in the study. The chapter summarises the major conclusions made from the sites visits, the desk studies, and the review of experimental works. The chapter also provides a recommendation on the way forward. 7.2. Conclusion The conclusion is made on the use of recycled concrete aggregates in Darwin, the use of recycled coarse aggregates, and the use of recycled fine aggregates. 7.2.1. Use of recycled concrete aggregates in Darwin The awareness of the impact of construction and demolition waste on the environment has risen across the world. Many countries are promoting recycling in order to prevent this waste from going into landfills. Despite these efforts, landfills are still available for construction and demolition waste in Darwin. Such landfills include the Shoal Bay Waste Management Facility in the city. The lack of recycling activities in the city has been driven by availability of adequate supply of virgin raw construction materials. However, there is a strong argument in favour of reduction, reuse and recycling of construction and demolition waste in order to preserve natural resources and prevent adverse environmental destruction. Site visits were conducted in order to examine the current methods used in management of construction and demolition waste in Darwin. The investigations were also done to examine how concrete waste is recycled and reused. The site visits were done to two demolition sites, two construction sites, two recycling plants, and one landfill in Darwin. It was found that contractors do not make any effort to separate waste into dangerous one and recyclable one. Instead, waste is comingled and disposed in landfills. The demolition sites recycled aggregates Page 64 of 70 Recycled Aggregate and used them for backfilling and construction of pavements. There was little reuse of recycled aggregates for structural applications. They also separated steel from concrete and sold it as scrap metal. Interviews conducted on sites indicated some major challenges in the use of recycled aggregates in structural applications. Some of the barriers highlighted are listed below: a) Landfill disposal fees are very low in Darwin. The fees range from AU$18 to AU$20 per tonne. There were little incentive given to contractors who recycled construction and demolition wastes b) The cost of transporting virgin materials from quarries is lower than that of transporting recycled aggregates from centralised recycling plants. c) The local council does not have guidelines for use of recycled materials in construction of structural elements. d) Many people think that recycled materials are inferior and cannot match the strength of provided by virgin materials. e) The contractors lack experience in recycling aggregates and reusing them. 7.2.2. Use of coarse recycled concrete aggregates The properties of recycled concrete aggregates were investigated from earlier experimental works in order to understand their performance when used as replacement for natural coarse aggregates. Properties such as workability of fresh concrete, density, strength and deformation were reviewed. The experimental studies indicated three major issues: a) The strength of concrete decreases with increase in the recycled coarse aggregates replacement ratio from 0% to 100%. b) The increase in water to cement ratio from 0.45 to 0.6 cause a decrease in the strength of recycled aggregates concrete Page 65 of 70 Recycled Aggregate c) Recycled concrete aggregates can be used to replace virgin coarse aggregates. The replacement ratio should be determined by the strength requirements of the structural applications and the need to protect the environment. 7.2.3. Use of fine recycled concrete aggregates The study investigated how fine recycled aggregates (600μm to 4.75mm) can be used to replace natural fine aggregates in production of mortar and concrete. I was noted that fine recycled aggregates are obtained through a process of crushing waste concrete using large jaw crusher, small jaw crusher and a roller. The large particles are sieved out using vibrating screen. Various moulds of mortar are prepared using different fine recycled concrete aggregates replacement ratios. The samples are tested for flowability, density, and strength and the following conclusions can be made: a) The increase in the fine recycled aggregates in mortar lead to a decrease in properties of mortar such as flow values, density and strength. This observation indicates that the replacement ratio used in making mortar or concrete is an important consideration. b) The observations made in this study indicate that fine aggregates can be used to replace the use of natural fine aggregates. The replacement ratio used should provide a balance between strength requirements and the subsequent impact on the environment. 7.3. Future work This paper focused on field studies and desk studies. The field studies were conducted to examine the current uses of recycled fine and coarse recycled concrete aggregates in Darwin, Northern Territory, Australia. 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The post According to the Australian government report on sustainable aggregates, approximately 9 million tonnes of aggregates were recycled from demolition in Australia between 2008 and 2009 [1]. The recycled materials were used to replace the use of virgin crushed rocks in construction appeared first on Term Paper Tutors.

The post According to the Australian government report on sustainable aggregates, approximately 9 million tonnes of aggregates were recycled from demolition in Australia between 2008 and 2009 [1]. The recycled materials were used to replace the use of virgin crushed rocks in construction first appeared on Term Paper Tutors.

 

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