Processing of Recycled Aggregates

Processing of Recycled Aggregates

4

Main Headings

• Benefits of selective demolition • Environmental impacts of construction and demolition waste processing • Production and collection of construction and demolition waste • Construction and demolition waste recycling plants

Synopsis This chapter presents an overview of the main types of methods and procedures that have been developed within the recycling industry to produce good-quality recycled aggregates in a cost-effective manner. It states the main benefits of the selective demolition approach and its environmental impact in comparison with the conventional demolition approach, and how it affects the quality of recycled aggregates coming from construction and demolition waste recycling plants. It also describes many of the procedures normally used to obtain debris from construction and demolition sites. The chapter finishes with a comprehensive explanation of the typically employed recycling procedures as well as the different types of crushers and the processes for sorting and contamination removal. Keywords: Construction and demolition waste, Selective demolition, Environmental impact, Recycling process, Recycled aggregates.

Sustainable Construction Materials: Recycled Aggregates. https://doi.org/10.1016/B978-0-08-100985-7.00004-2 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Sustainable Construction Materials: Recycled Aggregates

4.1  Introduction In the early years of the demolition industry, the process of demolishing a structure was a low-technology, low-skill, labour-intensive, and poorly regulated activity. However, like all other major industries, that of the demolition of structures has developed automated procedures by replacing manual labour with complex machines. This evolution is mainly due to the increasing complexity in building design, financial pressures from clients, health and safety and other regulatory and legal requirements and advances in plant design. Most of the work involved in construction and demolition tends to involve small and medium-sized enterprises, wanting to carry out their operations as promptly as possible and in a seemingly cost-effective manner. Naturally, this type of management will result in unsorted wastes with lower potential for recycling and can be more easily disposed of in landfills, rather than being subjected to beneficiation processes. Nevertheless, reclamation of reusable materials and redirecting recyclable wastes to certified beneficiation plants instead of landfills may result in higher revenues compared with the conventional demolition approach (Coelho and de Brito, 2011; Hurley et al., 2001). The construction and demolition industries are at a turning point in several countries concerning the management of their waste. In some countries, the waste management sector has been challenged to reduce its landfill dependency and instead offer reclamation and recycling services for handling construction and demolition waste (CDW). In addition, pressure stemming from rising disposal costs and the heightened awareness of clients and the public with respect to the environment, as well as tighter legislation, have put the waste management sector in a key position regarding the long-term sustainability of the construction, demolition and manufacturing industries. In an ideal situation, this would mean that all stages of a project’s life cycle (design, planning, manufacture, construction, deconstruction and reconstruction) will become an integrated operation (Hurley et al., 2001). This chapter provides an overview of the main types of methods and procedures developed to produce a high volume of good-quality recycled aggregate (RA) in a costeffective manner. It gives the main benefits of the selective demolition approach and its environmental impact in comparison with the conventional demolition approach, and how it affects the quality of RA produced. It also describes the procedures normally used to obtain CDW from demolition sites. The chapter ends with a comprehensive assessment of the typically used recycling procedures, as well as the different types of crushers and the processes for sorting and contamination removal.

4.2  Benefits of Selective Demolition There are two distinct approaches for the demolition of structures: conventional undifferentiated demolition and selective demolition (or deconstruction). The latter is normally based on four strategies that promote recycling of materials, reprocessing of components, reuse of elements and adaptability or relocation of buildings (Figure 4.1). A wide range of studies have been carried out internationally on selective demolition

Processing of Recycled Aggregates

59

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$VVHPEO\LQWR EXLOGLQJV %XLOGLQJ XVH

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Figure 4.1  Four strategies for material reuse and recycling in deconstruction. Adapted from Crowther (2000).

and its technical, economic and environmental implications (ACWMA, 2013; Roussat et al., 2009; Dantata et al., 2005; Guy and Gibeau, 2003; Guy, 2006; Coelho and de Brito, 2011). Even though selective demolition is already a standard practice in some countries, it is still poorly consolidated in the industry, of debatable economic appeal, with little practical impact (Duan et al., 2015). The economic feasibility of the selective demolition approach, compared with a conventional demolition system, largely depends on a number of factors that are site specific, including labour costs, market prices and tipping fees for recovered products (Coelho and de Brito, 2011; Dantata et al., 2005; Guy, 2006; ACWMA, 2013). Nevertheless, adoption of the selective demolition approach is likely to be more cost effective than a system in which all materials are undifferentiated and disposed of as a single product. Furthermore, recent trends have shown that the use of such approach may result in a decline in waste generation in most countries and that efforts to encourage a ‘greener’ construction industry would materialise (Tam and Lu, 2016). From an environmental point of view, there are clear benefits from applying selective demolition compared with conventional demolition, mainly due to the direct reduction of waste sent to landfill (ACWMA, 2013; Roussat et al., 2009). This results in the reduction of a number of different impacts, specifically those arising from climate change, acidification, summer smog, nitrification and release of heavy metals (Coelho and de Brito, 2012). Still, meaningful environmental impacts may be realised only when the structure is subjected to an almost complete selective demolition. When there is only partial selective demolition, the environmental impact may intensify, in comparison with conventional demolition, because of increased transportation

60

Sustainable Construction Materials: Recycled Aggregates

distances (Coelho and de Brito, 2012), which represent a considerable portion of the overall ecological footprint of construction and demolition activities (Marrero et al., 2017). Therefore, to mitigate the environmental impact, it is estimated that the recycling rate has to increase to over 90% and the resulting materials must be incorporated in the new construction. Furthermore, from a technical feasibility viewpoint, in light of the different natures of the elements normally encountered in CDW (de Brito and Silva, 2014; Silva et al., 2014, 2017), the high quality of control associated with the selective demolition approach is extremely effective at minimising contamination of the resulting materials (Noguchi et al., 2015). Since this has fundamental importance to the improved quality of output from CDW recycling plants, selective demolition should be encouraged, including the introduction of strict control procedures to ensure proper storage of CDW and employment of variable gate fees, according to the amount of contaminants, the composition and the origin of the CDW (Vyncke and Rousseau, 1993). This would ensure that the CDW would largely consist of separated inert materials, specifically concrete, mortar, bricks and other ceramic materials.

4.3  Environmental Impact of CDW Processing The operations leading to the recovery and recycling of CDW have four main benefits: reduced use of natural resources, reduced transportation to and from mining and quarrying sites, reduced energy consumption and reduced volume of CDW sent to landfill (Table 4.1). A substantial amount of work has been carried out on the environmental and amenity impacts of construction, demolition, quarrying and mining activities, as well as on the benefits of recycling or reusing CDW materials (Bond, 2005; Dhir et al., 2006; Table 4.1  Benefits of using construction and demolition waste (Bond, 2005) Positive Environmental Impacts Reduced use of natural resources

• Reduced • Less

damage to habitat visual damage

Reduced transportation of natural resources

• Reduced

greenhouse gas emissions pollution emissions • Less use of fossil fuel resources

Reduced energy consumption

• Reduced

• Reduced

greenhouse gas emissions pollution emissions • Less use of fossil fuel resources • Reduced

Reduced amount of construction and demolition waste sent to landfill

• Less • Less

damage to existing habitat visual amenity damage

Processing of Recycled Aggregates

61

Coelho and de Brito, 2013c,d; DETR, 2000; SymondsGroup, 1999; Guthrie, 1997; Faleschini et al., 2016). Apart from the obvious rapid depletion of natural resources, quarrying and mining operations also involve the generation of other environmental and amenity impacts, which include:

• Noise. • Dust. • Atmospheric emissions from combustion engines. • Surface and groundwater pollution caused by, e.g. fuel and chemical spillages. • Vibration from, e.g. blasting. • Visual and aesthetic impacts. • Landform changes. • Destruction of natural habitats and historical artefacts.

Naturally, the scale and detail of the impacts would depend on the product quarried. Sand-digging operations tend to be much quieter and less dusty than blasting and crushing hard rock. Quarries in remote areas lead to lower amenity impacts compared with sites near urban or suburban areas. However, remote quarries must rely on longer haulage operations to deliver aggregates to their final locations, which generates other impacts in the form of noise, vibration, dust and air pollution. A CDW recycling plant is an industrial facility that transforms mixed and uncontaminated CDW into RA suitable for use in the production of new construction materials, the output of which is expected to offset an equivalent amount of natural resources (Coelho and de Brito, 2013c). The installation of such a facility may bring in substantial environmental benefits (Faleschini et al., 2016). It was observed that the processes involved may use up to 85% less energy than the conventional approach, and the use of RA from these plants in new construction applications may result in almost 90% lower CO2 equivalent emissions, compared with the use of natural resources (Coelho and de Brito, 2013c,d). Nevertheless, since the availability of CDW and possible locations for its use after having been beneficiated are more likely to occur in urban settings, CDW recycling facilities have their own carbon footprint and may affect the environment in a manner that is similar to that of natural aggregate (NA) producers (Gayarre et al., 2016). The impacts related to transportation and delivery of CDW are basically the same as those associated with NA delivered by road, unless it can be treated and used on-site. The manufacturing process of RA may also present considerable energy-related impacts (i.e., high-temperature treatment). In addition, the process of washing aggregates consists of removing contaminants (e.g., wood, plastic and gypsum), by using large quantities of potable water that is likely to

62

Sustainable Construction Materials: Recycled Aggregates

pollute groundwater. Therefore, breaking, crushing, sorting and stockpiling of CDW recycling operations are likely to generate the following main environmental impacts (DETR, 2000):

• Land take and ancillary development (visual impacts related to the plant’s presence, material stockpiles taking up land space, loss of habitats).



• Dust (processing, storage and transportation of materials). • Noise, vibrations, gas and odour emissions. • Land contamination and water pollution (internal combustion engines and lubricants used by the equipment).



• Other transportation-related impacts (e.g. congestion, poorer safety).

4.4  Production and Collection of CDW There have been significant changes within the demolition industry. Initially, the demolition of a building structure involved the use of intensive low-skill manual labour, using very simple technologies, and it was poorly regulated. Today, the industry has increased automation with the use of multifaceted equipment. This evolution is mainly due to the increasing complexity in building design, advances in plant design, financial pressures from clients and health, safety and other legal requirements. The ACI Committee 555 (Lamond et al., 2002) developed a report on the removal of concrete elements using techniques that go very well with the idea of selective demolition. This report describes the stages of the deconstruction of a building structure as well as necessary equipment. One of the first steps required for a successful selective demolition consists of the evaluation of existing materials, which may be carried out using petrographic studies or non-destructive and semi-destructive testing (Bonifazi et al., 2017), to assess the quality and strength of the concrete. These include:

• Surface hardness (to estimate the compressive strength of the concrete). • Pull-off tests (to determine the strength of adhesion to the support or

direct tensile

strength of the material).



• Magnetic methods (to determine the cover and location of reinforcements). • Electrical methods (to assess reinforcement corrosion, thickness of concrete pavements, moisture content and moisture penetration).



• Radioactive methods (to evaluate density, voids, composition and segregation). • Ultrasonic pulse velocity and pulse echo techniques (to find cracks and voids in mass concrete).



• Micro X-ray fluorescence on drilled core samples (to determine the chemical composition of concrete).

Processing of Recycled Aggregates

63

Table 4.2  Factors affecting the choice of demolition equipment (Kasai, 1998) Factor

Description

Structural form of the building

Shape of structure as well as technology and materials used

Scale of construction

Larger structures may make a complex method more economic and faster, while small buildings can be demolished using simple techniques

Location of the building

Access to the building and its location in urban and nonurban settings

Acceptable levels of nuisance

Noise, vibration and dust tolerance levels

Use of the building

Contaminated structures are treated different to ordinary structures

Safety

Safety of workers, the public and the environment must be ensured with the choice of proper equipment

Time period

Longer periods allow more material separation and reuse, yet short periods may mean a faster, but not necessarily greater, return on investment

A number of factors influence the choice of demolition method (Table 4.2). Depending on the type of concrete structure (general, mass concrete structures, underground structures, reinforced concrete structures, prestressed/post-tensioned structures, pre-tensioned members, monolithic structures, separately stressed precast units, progressively prestressed structures), different demolition methods and support structures are required. Demolition methods and concrete removal techniques may be divided into manual labour, mechanical methods, thermal cutting methods, mechanical cutting and grinding methods and expansion-based methods. The demolition equipment used in each of those techniques is presented in Table 4.3. Manual labour-based demolition, using simple hand tools, was often applied after the First and Second World Wars in heavily bombarded areas. It is still used in countries with cheaper manual labour when faced with the cost of renting or buying significantly more expensive demolition equipment. Mechanical demolition methods are normally associated with heavy demolition plants. These machines may use impact, crushing or shear-based methods to demolish a structure. Thermal cutting equipment is used in demolition operations that normally involve the division of structural elements into smaller parts by creating narrow slots. Iron and steel are cut by heating them to high temperatures to initiate combustion and then maintaining the combustion. Another common method is to heat the material to melt it.

64

Table 4.3  Demolition tools (Lamond et al., 2002; Silva et al., 2017; Hendriks and Pietersen, 2000) Heavy Demolition Equipment

Thermal Cutting Equipment

Mechanical Cutting and Grinding Equipment

Expansion-Based Methods

• Manual

• Impact

• Cutting

• Core

• Explosive

• Powder

• Diamond

• Gas

electrical tools hydraulic tools • Manual pneumatic tools • Drop hammers/blades • Petrol-driven tools • Manual

breakers and hammers • Spring-action hammers • Wrecking ball • Mechanical splitters • Ripper • Concrete crushers

torch cutting torch • Powder cutting lance • Plasma cutting torch • Electrical heating

drills saws • Hand-held diamond saws • Walk-behind diamond saws • Rideable pavement saws • Wall saws • Diamond wire saws • Stitch drilling

blasting expansion • Solid non-explosive demolition agents

Sustainable Construction Materials: Recycled Aggregates

Hand-Operated Demolition Tool

Processing of Recycled Aggregates

65

For mechanical cutting and grinding, a structure is divided into smaller elements using drills and saws. Some of these apparatuses use hard-cutting diamond tools, which can create smooth holes or surfaces. These tools have minimal vibration and, when water-cooled, minimise dust. However, hard aggregates or high concentrations of steel reinforcements can greatly reduce the cutting speed and life of a drill bit or saw. Expansion methods are based on the disruption of elements by considerable volume increase, which may occur at varying speeds. Explosives, gases and solid non-explosive agents may be used for expansion-based demolition. Generally, after expansion breakage, the product needs to be subjected to further size-reducing processes using other equipment. Water-jet blasting (hydro-demolition) is normally used for the partial removal of concrete whenever the steel reinforcements need be reused (e.g., rehabilitation). This method is especially advantageous in an urban context as it causes little vibrationrelated damage and it avoids any risk of fire, unlike the thermal demolition methods.

4.5  CDW Recycling Plants CDW recycling plants essentially present the same layout as plants that manufacture crushed NA from quarries. They make use of various crushers, screens, transfer equipment and devices for removing foreign matter, to produce a relatively clean granular product with a particle size distribution within given limits. The degree of processing is determined by the level of contamination of the initial CDW and its intended future application, such as general bulk-fill sub-base, base or surface material in road construction; hydraulically bound materials or mortar/concrete manufacture (Hansen, 1992). Recycling plants can be mobile or stationary. Normally, mobile plants (Figure 4.2) consist of one crusher and some sorting equipment, with lower effectiveness at removing contaminants compared with stationary plants. The process for removing contaminants and steel is somewhat precarious because it is mainly conducted by hand and with self-cleaning electromagnets. On the other hand, a stationary recycling facility normally comprises a large primary crusher working in parallel with secondary and/or tertiary crushers and includes various cleaning and sieving devices to produce high-quality RA. The choice as to whether CDW processing should be done in stationary or mobile recycling plants is complex and needs to be evaluated on a case-by-case basis, taking into account several technical, financial and environmental aspects (i.e., plant capacity, transportation cost, haulage distances, CDW amount, economy of scale, NA price and tipping fees) (Zhao et al., 2010; Faleschini et al., 2016). Recycling CDW in situ using a mobile crusher is considerably more advantageous than using stationary facilities; apart from the benefits of not sending the debris to a landfill, short haulage distances between the demolition site and the processing equipment also reduce costs and carbon emissions,

66

Feed hopper

Grizzly bars

Feed materials

Jaw crusher

Vibrator Chute

Selector door Fixed teeth

Figure 4.2  Example of a mobile crusher. Adapted from KHM (2017).

Movable Swing jaw teeth

Conveyor belt

Sustainable Construction Materials: Recycled Aggregates

Grizzly feeder

Magnetic separator

Processing of Recycled Aggregates

67

whereas off-site recycling in a certified facility is highly dependent on the haulage distance (Bovea and Powell, 2016; Faleschini et al., 2016). Table 4.4 presents the main advantages and disadvantages of using either of these recycling plants (O’Mahony, 1990). Owing to its various components, CDW is difficult to treat, and high contamination negatively influences the properties of the final product, the low quality of which is one of the main barriers to its wider use in construction. As shown in the literature (Teranishi et al., 1998; Zhao et al., 2010; Yanagi et al., 1998; Müller, 2004; Nagataki et al., 2004; Dhir et al., 1999; Dosho et al., 1998; Eguchi et al., 2007; Mas et al., 2012; Gokce et al., 2011; Muscalu and Andrei, 2011; Vyncke and Rousseau, 1993; Hosokawa, 1999; Li et al., 2009; Nagataki and Iida, 2001; Müller and Winkler, 1998; Kohler and Kurkowski, 1998; Jungmann and Quindt, 1998; Chen et al., 2003; Cho and Yeo, 2003; BCA, 2008; Sim and Park, 2011; Kasai, 1998), the treatment procedure affects the quality of RA considerably. Furthermore, as the type and techniques applied at different plants can vary greatly, the characteristics of RA in the market can differ Table 4.4  Advantages and disadvantages of using mobile or stationary recycling plants Recycling Plant Type Advantages

Mobile Recycling Plant

Stationary Recycling Plant

• Transport

• This

within the vicinity of the construction or demolition site is reduced, particularly if the CDW is produced, recycled and reused at the same site. disposal and landfill fees due to less dumping.

• The

efficiency of the plant is better because it can produce RA of various sizes.

• Lower

• Lower

• Local

• Local

supply of aggregates improves, and thus smaller amounts need to be transported to the area.

• This

recycling plant is relatively easily moved to another site.

Disadvantages

recycling plant is capable of producing high-quality RA.

• There

are few contamination removal processes in this type of installation and therefore the final product is of lower quality.

• The

recycling plant causes high levels of noise and dust, which may be unacceptable in residential areas.

• This type of plant is viable for use

only if there are sufficient quantities of CDW on-site to justify the expense of setting up the recycling plant. CDW, construction and demolition waste; RA, recycled aggregate.

disposal and landfill fees due to less dumping. supply of aggregates improves, and thus smaller amounts need to be transported to the area.

• High

initial investment.

• Longer

transport distances in the vicinity of the recycling plant.

• This

recycling plant may cause high noise levels.

• The

efficiency of production depends on the local constant supply of CDW, which demolition contractors cannot guarantee.

68

Sustainable Construction Materials: Recycled Aggregates

significantly (Rodrigues et al., 2013; Bravo et al., 2015, 2016). It is also important to note that a given recycling facility can manufacture products with significantly different characteristics, due to varying composition of demolished source structure. In a case study in Chongqing, China, the economic viability of a CDW recycling plant was evaluated in light of a nearby ongoing construction work and the demand for recycled materials, which created a market opportunity for significant growth in the recycling of the material (Zhao et al., 2010). It was also observed that recycling plants with new equipment may have an uncertain viability because the profit margins become limited by high fixed costs. However, the economic feasibility can be improved with lower production costs, benefitting from using larger stationary plants; as the size of the plant increases, the unit production costs decrease, as the higher production rate allows gradually writing off the initial equipment cost over a period. Operational efficiency also improves with increasing capacity of the equipment, leading to lower variable costs. Similar findings were observed in a project dealing with a CDW recycling plant in the Lisbon region, Portugal (Coelho and de Brito, 2013a,b), where running a plant at higher capacity, whilst risking overburdening, would be preferable to having several smaller recycling plants. Furthermore, it was found that the profitability was highly dependent on the gate fees and the condition of the delivered CDW (i.e., the most favourable conditions were when the CDW was completely mixed, which translated into higher gate fees).

4.5.1  Recycling Procedure The quality control of CDW recycling facilities may minimise the existence of contamination to a point that allows the use of the product in higher grade applications. This stricter quality control system normally follows a standard procedure for acceptance and processing, from the source to the buyer’s ownership (Table 4.5). Owing to the large size of stationary plants, and with an operation similar to that of conventional NA producers, these CDW facilities can build up stocks of differentquality materials for immediate supply to larger contractors. Figure 3.10, in Chapter 3, presents the concept of a performance-based approach to the use of RA in construction applications with varying requirements, in which, by categorising RA based on its intrinsic properties rather than on composition alone, it is possible to maximise the incorporation of RA into the most suitable application without significant loss in performance. Furthermore, classifying RA in easily understandable categories (Silva et al., 2014), followed by certification, also drives future purchases, since clients will feel that they will be buying a fit-for-purpose material (e.g., structural concrete would need RA of class A, whereas RA of class D would be for subgrade in road construction). As mentioned, CDW delivered to recycling facilities may consist of a single material (e.g., concrete), but it usually contains several other components because of an undifferentiated demolition approach. After an initial appraisal of the CDW composition, it can be subjected to a wide range of possible recycling procedures with the aim of maximising the quality of output and minimising costs. The flow

Processing of Recycled Aggregates

69

Table 4.5  Standard procedure for acceptance and processing of construction and demolition waste at recycling plants Inspection at location of source of supply

If possible, the CDW should be inspected at the construction/ demolition site to register and assess suitable materials for reprocessing.

Transport and arrival of CDW

The CDW arrives at the recycling plant and is ready to be assessed.

Evaluation of materials at arrival

The cargo is then visually assessed for quality and type; the incoming material must meet predetermined requirements before it is accepted.

First acceptance

If the cargo meets the requirements of the preceding phase, further handling may follow.

Weighing

The cargo is weighed and administratively incorporated.

Dumping of CDW

After the cargo is dumped, the CDW is further scrutinised.

Second acceptance

After the secondary and more intense inspection, the cargo can be approved or refused.

Approved CDW

The CDW materials are then stored separately by type.

Recycling process

The next stage is the recycling process of the CDW materials, in which they undergo crushing, sieving and contamination removal.

Final product – RA

The generated RAs are then tested according to predetermined requirements; this determines their final quality and type.

Storage

Depending on the results of the preceding stage, the RAs are stored separately by type or class (this depends on the classification process); the RAs must be stored in such a way that contamination is impossible.

Purchase

The RAs are then sold in accordance with the requirements or specifications of the buyer.

CDW, construction and demolition waste; RA, recycled aggregate. Adapted from Hendriks (1998).

diagram presented in Figure 4.3 consists of a generic recycling process that is capable of producing good-quality RA with a minimum amount of contaminants and adhered mortar, without spending too much energy. As pointed out, the type of recycling procedure mainly depends on the types of materials and contaminants involved. For example, if plain concrete elements (without steel reinforcements) are introduced, processes that involve the use of manual or mechanical removal of contaminants may be bypassed, thus saving energy. Also, since the CDW may exhibit a very low degree of contamination, it may be possible to make good use of material finer than 10 mm in the primary screening stage, instead of disposing of it. Provided there is a strict quality control during the mixing procedure, the use of fine RA has been found to be feasible in mortar production (Ledesma et al., 2015) and also structural concrete (Evangelista and De Brito, 2004; Evangelista and de Brito, 2007, 2014; Evangelista et al., 2015).

70

Sustainable Construction Materials: Recycled Aggregates

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0DQXDORUPHFKDQLFDOVHSDUDWLRQ UHPRYDORIODUJHSLHFHVRIZRRG LURQSDSHUSODVWLFVHWF  3ULPDU\VFUHHQLQJUHPRYDORIDOO ILQHPDWHULDOOHVVWKDQPPVXFK DVVRLOJ\SVXPHWF  3ULPDU\FUXVKLQJ XVXDOO\DMDZFUXVKHU  (OHFWURPDJQHWLFUHPRYDORIIHUURXV PDWHULDOV 6HFRQGDU\VFUHHQLQJ 0DQXDORUPHFKDQLFDOUHPRYDORI FRQWDPLQDQWV 6HFRQGDU\FUXVKLQJ LPSDFWRUFRQHFUXVKHU :DVKLQJRUDLUVLIWLQJUHPRYDORI OLJKWZHLJKWFRQWDPLQDQWV  )LQDOVFUHHQLQJDQGVWRUDJHRI YDULRXVVL]HIUDFWLRQVFRQIRUPLQJ ZLWKWKHFRVWXPHU VUHTXLUHPHQWV Figure 4.3  Example of a recycling procedure for construction and demolition waste.

A tertiary crushing stage may also be applied in addition to the rest of the process presented in Figure 4.3, which would produce better quality RA as a result of the rounder particles and lower adhered mortar content. Nevertheless, this improved quality may not be sufficient to justify subjecting RA to a tertiary crushing stage, as the resulting concrete produced with it may present similar or only slightly improved performance compared with that of products made with RA subjected only to a secondary crushing stage (Nagataki et al., 2004; Gokce et al., 2004). This analysis needs to be carried out on a case-by-case basis taking into account the end-use application of the RA, since it is possible to manufacture a good-quality material with lower energy expenditure and with a higher ratio of coarse to fine aggregates if a tertiary crushing stage is not used.

Processing of Recycled Aggregates

71

Regarding the final contamination removal stages, either air sifting or wet separation can be used. Air sifting can be as effective as wet separation and, at the same time, eliminates the use of large quantities of water. However, wet separation has the advantage of removing water-soluble chlorides and sulphates (Van Der Wegen and Haverkort, 1998; Weimann and Müller, 2004). This means that aggregate washing is a better contaminant-removal method for the production of RA for rendering or masonry mortars, or structural concrete, than air sifting. However, since the performance of some applications is blind to the existence of sulphate or chloride contamination, the air-sifting method can be used instead of wet separation.

4.5.2   Crushers Depending on the size of the debris, it may either be ready to enter the recycling process or need to be broken down to obtain a product with workable particle sizes, in which case hydraulic breakers mounted on tracked or wheeled excavators are used. In either case, manual sorting of large pieces of steel, wood, plastics and paper may be required, to minimise the degree of contamination of the final product. The three types of crushers most commonly used for crushing CDW materials are the jaw crusher, the impact crusher and the gyratory crusher (Figure 4.4). A jaw crusher consists of two plates, with one oscillating back and forth against the other at a fixed angle (Figure 4.4(a)) and it is the most widely used in primary crushing stages (Behera et al., 2014). The jaw crusher can withstand large and hard-to-break pieces of reinforced concrete, which would probably cause the other crushing machines to break down. Therefore, the material is initially reduced in jaw crushers before going through any other crushing operation. The particle size reduction depends on the maximum and minimum size of the gap at the plates (Hansen, 2004). An impact crusher breaks the CDW materials by striking them with a high-speed rotating impact, which imparts a shearing force on the debris (Figure 4.4(b)). Upon reaching the rotor, the debris is caught by steel teeth or hard blades attached to the rotor. These hurl the materials against the breaker plate, smashing them into smaller particle sizes. Impact crushers provide better grain-size distribution of RA for road construction purposes, and they are less sensitive to material that cannot be crushed, such as steel reinforcement. Generally, jaw and impact crushers exhibit a large reduction factor, defined as the ratio of the particle size of the input to that of the output material. A jaw crusher crushes only a small proportion of the original aggregate particles but an impact crusher crushes mortar and aggregate particles alike and thus generates a higher amount of fine material (O’Mahony, 1990). Gyratory crushers work on the same principle as cone crushers (Figure 4.4(c)). These have a gyratory motion driven by an eccentric wheel. These machines will not accept materials with a large particle size and therefore only jaw or impact crushers should be considered as primary crushers. Gyratory and cone crushers are likely to become jammed by fragments that are too large or too heavy. It is recommended that

72

Sustainable Construction Materials: Recycled Aggregates

D

E

F

Figure 4.4  Examples of: (a) a jaw crusher, (b) an impact crusher and (c) a cone crusher (Nordberg, 1994).

wood and steel be removed as much as possible before dumping CDW into these crushers. Gyratory and cone crushers have advantages such as relatively low energy consumption, a reasonable amount of control over the particle size of the material and production of low amounts of fine particles (Hansen, 2004). For better control of the aggregate particle size distribution, it is recommended that the CDW should be processed in at least two crushing stages. First, the demolition methodologies used on-site should be able to reduce individual pieces of debris to a size that the primary crusher in the recycling plant can take. This size depends on the opening feed of the primary crusher, which is normally bigger for large stationary plants than for mobile plants. Therefore, the recycling of CDW materials requires careful planning and communication between all parties involved. A large proportion of the product from the primary crusher can result in small granules with a particle size distribution that may not satisfy the requirements laid down by the customer after having gone through the other crushing stages. Therefore, it should be possible to adjust the opening feed size of the primary crusher, implying that the secondary crusher should have a relatively large capacity. This will allow maximisation of coarse RA production (e.g., the feed size of the primary crusher should be set to reduce material to the largest size that will fit the secondary crusher). The choice of using multiple crushing stages mainly depends on the desired quality of the final product and the ratio of the amounts of coarse and fine fractions (Yanagi et al., 1998; Nagataki and Iida, 2001; Nagataki et al., 2004; Dosho et al., 1998; Gokce et al.,

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2011). When recycling concrete, a greater number of crushing processes produces a more spherical material with lower adhered mortar content (Pedro et al., 2015), thus providing a superior quality of material to work with (Lotfi et al., 2017). However, the use of several crushing stages has some negative consequences as well; in addition to costing more, the final product may contain a greater proportion of finer fractions, which may not always be a suitable material.

4.5.3  Sorting and Contamination Removal The threat of contamination in RA is one of the most significant barriers to its use in construction products. It is widely recognised that CDW is difficult to process, as it can contain various contaminants (i.e., wood, soil, asphalt, glass, metal and plastic) that can affect the handling of the final product and its properties to the extent that it is considered of inferior quality compared with NA (Noguchi et al., 2015). The process of removing contaminants from CDW can be done prior to crushing (pre-crushing separation) and/or after crushing (post-crushing separation). For the first, CDW debris can be initially sorted throughout the process of demolishing a structure (i.e., selective demolition) or at the recycling plant before the crushing stages (O’Mahony, 1990). Once the CDW reaches the recycling plant, the debris is stockpiled according to its major constituents and/or the amount of contamination present, which allows the plant operator to take the necessary measures. This initial sorting can help to optimise the crushing time and energy use. For example, if large quantities of clean debris have accumulated in a stockpile, they can be crushed in a single, continuous run. It is also possible that some stockpiles will be made of materials with small enough size that they require no further crushing and thus can bypass the primary crusher. Post-crushing separation, on the other hand, is performed after the first crushing stage using several contamination removal techniques. The most direct approach is hand sorting, which entails the removal of contaminants by hand from the conveyor belts. The concentration of the operator and speed of the conveyor belt are vital factors for the efficiency of the hand sorting system. The main advantage is that the human eye can recognise contaminants that would be difficult to remove by mechanical means, for example, glass. Recent advances have allowed a more enhanced sorting of CDW by using computerassisted identification of constituents (Mulder et al., 2007), as well as estimation of the final grain-size distribution instead of traditional sieving (Di Maria et al., 2016). One of such technologies is the near-infrared sorting method, which identifies specific constituents, such as organic material and gypsum, which are known to affect adversely the quality of RA (Vegas et al., 2015). This method can be used, in conjunction with strategically placed powerful air jets, to remove such materials from the CDW stream and divert them to different containers. After the primary crushing stage, the use of self-cleaning magnets, positioned in various strategic locations on the conveyor belts, separates small pieces of steel reinforcements and other ferrous metals from the stream of crushed material. The

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efficiency of the magnets is greatly dependent on their distance from and positioning angle with respect to the CDW on the conveyor belt, the speed of the conveyor belt and the volume of the debris. For example, a magnet is more efficient when it is positioned directly above and parallel to a slow-moving conveyor belt with a low concentration of material. The electromagnets placed above the conveyor belt may accumulate the ferrous metals (Figure 4.5) or take the form of a rotating magnetic belt (Figure 4.6). The latter has the advantage of carrying the metals to the side, removing them from the CDW stream, instead of accumulating them within the magnet. In addition to ferrous metals, CDW may also contain various amounts of non-ferrous metals, such as aluminium, brass, copper, lead and zinc, which should also be removed. This work is usually carried out using eddy current separators. By placing these devices at the end of the conveyor belt, with thin layers of mixed waste passing, metallic pieces are thrown off the belt, whilst other materials simply fall off the belt, due to gravity.

Figure 4.5  Fixed electromagnets (Nordberg, 1994).

Figure 4.6  Rotating magnetic belts (Nordberg, 1994).

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It is possible to eliminate gypsum, dirt and other fine impurities at a later stage by laying the crushed material over scalping screens. Based on the amount of contamination, this finely crushed material can be considered either waste or fine RA. The dry screening process is an effective method to separate the product by size, which can be recombined at a later stage to produce well-graded RA. Most lightweight materials, as well as finer fractions, can be removed from CDW debris by wet separation or air sifting. Wet separation can remove lightweight contaminants such as clay, wood, cardboard, plastics, straw, roofing felt, water-soluble chlorides and sulphates and asbestos fibres (Kohler and Kurkowski, 1998; Xiao et al., 2016) from heavier bulk material by directly applying water jets in combination with a float–sink technique and vibration (Jungmann and Quindt, 1998). The floating lightweight contaminants are then removed by combs, which move from one end of the tank to the other. This technique effectively removes contaminants from materials with particle sizes between 10 and 40 mm. The use of a 10-mm screen prior to washing is recommended, because the 0- to 10-mm fraction produces large quantities of undesirable sludge in the washing water (O’Mahony, 1990). Wet separation techniques also have the advantage of leaching water-soluble chlorides and sulphates (Van Der Wegen and Haverkort, 1998; Weimann and Müller, 2004; Galvin et al., 2014; Rodrigues et al., 2013). The air separation technique, which can be carried out both vertically and horizontally using powerful air currents (Mulder et al., 2007; Kohler and Kurkowski, 1998; Lotfi et al., 2014), separates fine powder from the heavy bulk materials, as well as other lightweight materials (i.e., paper, cardboard, plastics, insulating materials and wood) (Hansen, 1992). This can be as effective as some wet techniques in terms of the removal of lightweight contaminants and would also avoid the use of large quantities of water. To obtain an adequate level of separation, the crushed material must be divided into different fractions that are then sifted separately. Air jets flowing upwards may also be used to segregate the CDW constituents by means of their different particle densities using constant mass airflow, which fluidises the material (Sampaio et al., 2016). This process results in a stratified product, wherein the denser particles (usually recycled concrete aggregates) accumulate at the bottom. The main disadvantage of air separation techniques is the excess dust, which must be controlled by means of closed systems and air filters.

4.5.4  Additional Recycled Aggregate Quality Enhancement Techniques Apart from the aforementioned and widely practiced separation methods, there are other less used techniques that can further enhance the performance of RA, which include:

• Autogenous cleaning methods. • Underwater high-performance sonic impulses. • Wet grinding.

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• Microwave heating. • Heating and grinding. • Screw grinding. • Shear–compression. • Mechanical grinding. • Wet heavy media separation. • Acidic presoaking treatment. • Carbon curing.

The autogenous cleaning method consists of placing the RA in a rotating mill drum, wherein the aggregate particles collide with one another (Pepe et al., 2014). This process allows the removal of weakened adhered mortar from the RA’s surface. Longer periods of rotation lead to cleaner aggregates exhibiting a rounder shape, higher density and lower water absorption. This process can be especially effective at removing adhered mortar from RA of small particle size (Pepe et al., 2014). The high-performance sonic impulse treatment (Linß and Mueller, 2004) starts by placing pre-crushed materials into a water-filled container, and sonic impulses are generated underwater. These sonic waves generate stresses between the adhered mortar and the aggregate, destroying the interface bond. Using a similar technique, other researchers (Narahara et al., 2007; Maeda et al., 2008) reported that, after a number of pulse discharges, the resulting aggregates exhibited properties that were almost constant and similar (density, water absorption, resistance to fragmentation) to those of the control NA. It is claimed that the treatment, apart from reducing the particle size and the adhered mortar content, which can be controlled by varying the voltage and number of impulses, also results in the leaching of water-soluble chlorides and sulphates, as the material is treated in a water medium. It is also possible to improve the main properties of RA by applying the wet grinding method, which reduces the adhered mortar content. In this method, concrete is crushed by a rotor inside a cylindrical shell. Fine RA, of 5 mm or less, is manufactured by passing through a screen. Wood chips and powder are then removed using a wet highspeed centrifuge (Dosho, 2007). Another method developed for the removal of adhered cement paste uses microwave heating (Akbarnezhad and Ong, 2010; Akbarnezhad et al., 2011; Ong et al., 2009, 2010). It is possible to control the extent and pattern of the microwave heating by varying the microwave power and frequency. This method works based on the differential temperatures developed within recycled concrete aggregate (RCA). The material is exposed to concentrated microwave heating at relatively high frequencies, and high temperatures develop in the surface layer whilst the interior temperature remains more or less unaffected. This thermal differential leads to high stresses and fast water evaporation, causing the delamination and spalling of adhered cement paste.

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The heating and grinding method consists of ‘softening’ the cement mortar adhered to RCA by heating the material to high temperature (Shima et al., 2005; Voinitchi et al., 2014). This thermal treatment produces fine cracks in the interfacial transition zone between the cement paste and the original NA. The material is then ground to separate the old mortar from the NA, resulting in a relatively clean aggregate. Voinitchi et al. (2014) showed that as the temperature of the process increases, the amount of mortar detached from the RA decreases. Some also suggested quenching the material before the grinding process (Pandurangan et al., 2016). In the quenching process, the sudden thermal variation can be effective at separating further the old adhered mortar from the RA. The screw grinding method uses a shaft screw with an intermediate part, followed by an exhaust part with a warping cone, which removes the adhered cement paste from the RA (Matumura, 2005). The shear–compression method is based on the application of a combination of shearing and compression forces by a vertical cylinder into a ring-shaped container filled with CDW exhibiting particle size below 20 mm. This method allows the removal of a higher amount of adhered mortar content and results in RA with improved quality, capable of producing concrete with performance comparable to that of conventional concrete (Lotfi et al., 2015, 2017). The mechanical grinding method uses a drum body which separates the aggregate using partition boards with holes (Noguchi et al., 2015). Steel balls move horizontally and vertically when the drum rolls and, upon impact, will separate the adhered mortar from RA. This improves the quality of coarse RA, which exhibits increased particle density and reduced water absorption (Letelier et al., 2016; Pandurangan et al., 2016). The aggregate quality can be adjusted by narrowing the space partition (Kajima, 2006). This method is preceded by a jaw crushing stage and is capable of producing high-quality fine RA, but at a higher energy cost compared with other procedures (Gomes et al., 2015) The use of magnetite (Fe3O4) had been suggested as a cost-effective pseudo-heavy material for the wet heavy media separation of CDW to upgrade the quality of coarse mixed RA for use in the manufacture of structural concrete (Kang and Kee, 2017). A Fe3O4-based suspension is used in the coal and iron industries because of its high density, cost effectiveness, resistance to corrosion and abrasion and recoverability for reuse. By controlling the amount of Fe3O4 it is possible to obtain a wet separation medium with a given density and thus separate RA based on its density. It has been observed that the use of a medium with a density of 2.4 g/cm3 allowed separating RA with oven-dried density of over 2500 kg/m3 and with water absorption below 3% as per JIS-5021 (2011). The use of acid has been suggested as a possible means of separating old adhered mortar from the original NA (Tam and Tam, 2007; Yagishita et al., 1993; Juan and Gutiérrez, 2004; Pandurangan et al., 2016; Kim et al., 2016) and thus improving the quality of RA (Katkhuda and Shatarat, 2017; Saravanakumar et al., 2016). However, this method

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should be avoided in limestone-containing RA, because of its susceptibility to acid. In this process, the material is immersed in an acidic solution (nitric, hydrochloric, sulphuric or phosphoric acid) for some time (normally 24 h) and then immersed in water to remove the acidic solvents (Saravanakumar et al., 2016; Wang, 2016; Ismail and Ramli, 2014). This treatment can significantly decrease the adhered cement paste content, yielding RA concrete with improved mechanical performance (Tam and Tam, 2007; Katkhuda and Shatarat, 2017). The use of a mechanical rubbing method after having placed the RA in an acidic solution can be even more effective at further removing the adhered mortar (Wang et al., 2017). There have been some developments in the quality enhancement of recycled concrete aggregate by means of accelerated carbonation (Zhang et al., 2015a,b). The concept behind this process is based on placing RA in a CO2-enriched environment, which induces calcite (CaCO3) precipitation of calcium-bearing phases existing in the old adhered mortar. The precipitation of CaCO3 occurs in the RA’s porous network, which results in a material with lower water absorption, higher density and higher resistance to fragmentation (Tam et al., 2016; Zhang et al., 2015a,b). From a practical point of view, the method results in the production of RA concrete products with improved performance. Other less used methods to further improve the quality of RA include soaking it in a sodium silicate (Na2SiO3) solution (Yang et al., 2016; Ondova and Sicakova, 2016), soaking it in a calcium metasilicate solution followed by oven drying (Mohammad et al., 2016; Ismail and Ramli, 2014), coating the surface with sodium metasilicate pentahydrate (Na2SiO3·5H2O) (Katkhuda and Shatarat, 2016), coating it with silica fume (Saravanakumar et al., 2016; Liang et al., 2015; Tam and Tam, 2008), impregnating the RA with silanes and siloxanes (Ondova and Sicakova, 2016; Zhu et al., 2013; Tsujino et al., 2006) and CaCO3 precipitation by microbial activity on the surface of the RA (Sahoo et al., 2016; Qiu et al., 2014).

4.5.5  Storage of CDW Before and After Processing Many entities within the construction and demolition industry and operators in recycling plants do not adequately categorise or store CDW. Naturally, this results in a highly contaminated material, which will increase gate fees. When handling CDW, the following aspects should be taken into consideration (Shukla et al., 2000):

• CDW must be properly separated and deposited in suitable containers in situ, to prevent scattering and to preserve its characteristics, enabling its future reuse or recycling.



• CDW that can be reused on-site should be placed in separate containers from materials that will be sent to a landfill or sold.



• In large projects (e.g., bridges, dams), a considerable amount of planning is needed for

waste management. Depending on the site storage capacity, the transportation of CDW needs to be thoroughly calculated, since it can be a constraint to ongoing works or could interfere with road traffic.

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After the beneficiation process of CDW, recycling facility operators should take care when storing the RA, to prevent contamination or mixing of products belonging to different categories. Therefore, the following recommendations should be followed (Kasai, 1998):

• RA from materials exhibiting significantly different quality should be stored separately. • RA produced by different recycling procedures should be stored separately. • RA of different types should be stored separately. • RA of different size fractions should be stored separately. • Owing to the self-cementing properties of unhydrated cement particles within RA, it is recommended that materials are kept dry, as long as possible, until their use.



• RA should be transported in a way that prevents breakage and segregation and that takes into consideration the previous recommendations.

4.6  Conclusions Most operations in the construction and demolition industry are performed by small and medium-sized enterprises, which are more interested in cheap and quick processes, and thus it is vital that they are controlled by regulating authorities when engaged in these types of work. Apart from enforcing the use of a selective demolition approach, which minimises contamination of the debris, that external entity could also evaluate the CDW produced and ascertain its best possible destination. The selective demolition approach, though slow, with seemingly non-existent economic advantages and practicality, is by far the most cost-effective approach to achieve sustainability in the construction industry, as demonstrated by several experiences worldwide. There is a wide range of demolition techniques and concrete removal methods, the selection of which should be made according to the type of structure, location, surrounding limitations, size of construction, time and safety. The existing technology for separating materials and demolishing elements in the decommissioning of buildings or structures has developed to a point at which selective demolition becomes a wise approach, from economic and environmental viewpoints, in comparison with conventional undifferentiated demolition. When the demolition stage and storage of debris are performed correctly, CDW may consist mostly of one type of material only. However, it often is a complex mixture of various components and, for this reason, CDW must be evaluated before being submitted to a recycling process to ascertain the most suitable procedure. This will prevent excessive costs incurred by unnecessary steps in the recycling procedure, reduce processing time, produce higher quality RAs and increase work rate and revenue.

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There are different types of crushers that may be used in different circumstances and in a varying number of crushing stages. The use of three or more crushing stages may indeed improve the quality of RA, but only slightly, in contrast to the obvious improvement of adding a second stage to an initial primary one. In light of this, the use of a tertiary crushing stage must be seriously considered, as it will have minimal effect on the quality of RA, decrease the coarse-to-fine aggregate ratio, and increase costs and energy spent. There are a number of effective techniques for separating and removing contaminants from the main CDW stream. Since some contaminants cannot be removed with mechanical methods alone, a manual separation stage in the recycling procedure should be considered to minimise contamination in the output. However, the application of manual labour would significantly increase the cost of the final material and thus there have been several developments in the increasing automation of the recycling process. Although several approaches have been developed, in addition to those normally used, to further enhance the quality of RAs, some have been tested only under laboratory conditions and may not be environmentally friendly and/or economically viable in practice. These aspects should always be taken into consideration in the production of fit-for-purpose RAs following the requirements of their desired application.

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