Page 25 - New Trends in Eco efficient and Recycled Concrete

2 New Trends in Eco-efficient and Recycled Concrete

1.2 Processing of construction and demolition waste

In general, CDW includes a wide variety of materials that are generated from dif-
ferent sources, such as concrete, bricks, bituminous materials, soil, steel, wood and
gypsum that may come from the construction of new buildings, roads and bridges
and other structures, renovation-related activities, demolition of end-of-life struc-
tures and also from natural or man-caused disasters (USEPA, 2016; Arisoy and
Sgem, 2016; Woolley, 1994). CDW that is normally taken to treatment plants can
be classified into three types depending on their nature and composition (Fig. 1.1):
Concrete blocks: this type of CDW is the most studied and applied to obtain high-quality
recycled aggregates (RAs). In general, this type of CDW is extracted during the demoli-
tion process of concrete structures as bridges, concrete buildings, breakwaters, walls,
among others.
Mixed and clean CDW: this waste contains mainly concrete, masonry or bituminous mate-
rials. This type of CDW is usually obtained with source identification, separating different
waste in the demolition process.
Mixed demolition debris: this type is usually associated with lack of selection at the
source and needs to receive specific pre-treatment for the elimination of some
components.
Ideally, the treatment processes applied to CDW should depend on their initial
characteristics as well as on the intended quality of RA. However, experience has
shown that the processes implemented in some CDW recycling plants are inade-
quate for the production of RA, wherein some do not apply effective segregation of
different constituents or even crushing processes (Rodrigues et al., 2013). Typical
processing steps required for CDW in treatment plants in order to achieve appropri-
ate RA include the following (Hiete, 2013):
1. Separation of the different fractions of waste (e.g., removing wood, plastics, metals from
aggregates, garbage remains, paints);
2. Removal of contaminated waste (e.g., asbestos, gypsum, heavy metals);
3. Pre-screening process (segregation of fine fractions containing soil and lightweight
particles);
4. Screening to achieve appropriate particle sizes;
5. Sorting devices, such as air separators, to remove light particles;
6. Electromagnets for metal separation; and
7. Hand-picking lines to eliminate materials that are difficult to be removed with other
techniques.

Figure 1.1 Main types of CDW: concrete blocks (left); mixed and clean (middle); mixed
demolition debris (right). CDW, Construction and demolition waste.