Recycled Aggregates from Construction and Demolition Wastes

Classification

The three main types of material derived from most CDW are crushed concrete, crushed masonry, and mixed demolition debris. After crushing and undergoing beneficiation in certified recycling plants, the resulting aggregates may be assigned to one of the four following categories.

Recycled Concrete Aggregates (RCA)

Concrete is found in most RA because it is the most used construction material in structural applications. Organizations in various countries have developed specifications which include a definition for RCA. Many of them seem to agree that to be considered RCA they must comprise a minimum of 90%, by mass, of Portland cement-based fragments and NA. This category also includes mortar and concrete masonry units. It is intended that this will lead to grades of RA in which: (i) Rc P 90%, (ii) Rc P 70%, and (iii) Rc < 70%, where the quality of the RA is determined by the recycled brick (Rb) content.

Recycled Masonry Aggregate (RMA)

Masonry rubble is a collective designation for various mineral building materials resulting from the construction and demolition of buildings and civil engineering structures. This family of materials may include aerated and lightweight concrete blocks, ceramic bricks, blast-furnace slag bricks and blocks, and sand-lime bricks. Masonry rubble often contains mortar rendering and burnt clay materials such as roofing tiles and shingles. It is composed of a minimum of 90%, by mass, of all the materials mentioned above. RA with high recycled brick content are commonly produced by best-practice recycling centres in which a concerted effort has been made to separate concrete and asphalt to other stockpiles.

Mixed Recycled Aggregates (MRA)

This material is composed of crushed and graded concrete and masonry rubble. The resulting aggregate is a mixture of two main components obtained from the beneficiation of CDW. Some specifications establish its composition as less than 90%, by mass, of Portland cement-based fragments and NA. In other words, it may contain several other common CDW materials such as masonry-based materials (ceramic, light-weight concrete).

Construction and Demolition Recycled Aggregates (CDRA)

Throughout this investigation, it was found that, on the whole, the literature contains limited information on the origin and composition of aggregates and so, where it was not possible to fully categorize the RA they were deemed CDRA. In other cases, RA contained high levels of contamination (e.g. asphalt, glass, plastics, wood) and were also classified into this category since they did not belong to any of the others (RCA, RMA, MRA). These materials may be the result of waste coming from construction and demolition sites that have not been through any type of sorting and therefore may contain valuable materials as well as contaminants.

Contaminants

The variety of contaminants that can be found in RA from the demolition of existing structures can severely degrade the strength of concrete made with them. Such materials include asphalt, gypsum, metals, plastic, rubber, soil or wood.

Asphalt

Bituminous materials have a general effect of reducing strength, depending on their construction application. Hansen reported that the addition of 30% by volume of asphalt reduced the compressive strength by about 30%. Other authors noticed a 75% compressive strength loss with a replacement level of 64%, by weight of total aggregate content.

Glass

This material is usually removed from buildings prior to demolition and, given the recycling efforts in most of the UK and Europe, it tends not to be present in CDW and RA. This pre-sorting is vital because of its similar density to stone’s and brick’s, which makes it difficult to separate glass from the rest of the heavyweight materials through wet separation or air sifting procedures. Also, because glass is brittle it usually ends up in the fines content following the crushing procedures in recycling plants, which makes recycled sands more liable to having high percentages of this contaminant than the coarser fractions.

Other constituents

Organic materials, for example wood and plastic, are often difficult to separate from CDW prior to crushing. Good practice is to separate these materials using air blowers, water processing (they tend to float to the surface) or sometimes by hand from a conveyor belt moving between the primary and secondary crushing procedures. Any non-floating wood, paper and plastic remaining within RA are classified as ‘‘other constituents’’. Wood and plastic, which float in water, are classified separately as floating non-stone material and content may be limited to a maximum of 0.1% by mass.

Although smaller than 4 mm, soil and clay particles frequently stick to stone and brick. If not removed, their presence may adversely affect the properties of concrete. However, washing RA prior to use should remove most soil and clay. Like non-floating wood and plastic, clay and soil are classified as “other constituents”.

After jaw crushing, ferrous metals are usually removed from CDW by means of magnetic belts, whilst eddy currents may be used to remove non-ferrous metals. In addition, hand picking may be used at recycling plants to remove metals from a moving conveyor belt prior to the use of a secondary crusher. Both ferrous and non-ferrous metals are also classified as “other constituents”. It has been suggested that strict limits should be placed on the gypsum content to prevent sulphate expansion. In the Netherlands, CDW containing gypsum are regarded as contaminated. These contaminated CDW, along with sewer sand and contaminated soil, must be extensively washed before they can be used to produce RA.

Size and shape

The type of crushing devices used to break down larger pieces and the number of processing stages influence the size and shape of the resulting aggregates. The recycling process normally uses primary and secondary crushing stages. Jaw crushers, which are typically used in the primary crushing stage, provide the best grain-size distribution of RA for concrete production. A second crushing usually leads to rounder and less sharp particles. Therefore, if RA only undergo a primary crushing process they will tend to be somewhat flat and sharp, as observed by some researchers. Cone crushers are suitable as secondary crushers as they normally have a 200 mm maximum feed size and give a more spherical shape to RA. Impact crushers, also used for secondary crushing stages, produce aggregates with a good grain-size distribution and lower flakiness index.

It has been concluded that coarse aggregates meet the size specification range by simply adjusting the setting of the crusher aperture, and that it is reasonably easy to produce good quality coarse aggregates. However, it was found that during the production of fine RA, these tend to become coarser and more angular than any of the standard sands used in the production of concrete. Also, this coarseness and increased angularity are the reasons why the workability of concrete made with these materials may sometimes be a problem.

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