Civil Engineering Reference
In-Depth Information
terisation. In fact, aggregate occupies more than 75 % volume of concrete mix and
therefore the characterisation of properties of a new material to be used as aggregate
should be evaluated properly, namely because it is produced from different types of
materials. To classify the different types of CDW aggregates, BS 8500 (2002)
defines two types of CDW aggregates. The concrete aggregate containing a mini-
mum of 95 % crushed concrete is defined as recycled concrete aggregate (RCA) and
100 % crushed masonry-based aggregate is defined as recycled aggregate (RC).
However, here, all types of aggregates will be considered as CDW aggregates.
The grain size, specific gravity/density, water absorption, Los Angeles abrasion
and crushing value of CDW aggregates are studied in detail. As CDW contains
several contaminants, such as wood, plastics and gypsum, these contaminating
materials should be removed before the application of CDW as an aggregate in
concrete. In this section, the properties of CDW as aggregate will be discussed in
detail from the existing literature data.
3.2 Preparation of CDW Aggregate
In several countries recycling plants have been established to produce CDW
aggregate. For the preparation of aggregates, in general, waste concrete elements
are mechanically broken into small-sized pieces. The small pieces are further
crushed into small-sized pieces using crushers. After crushing, different sized
fractions are screened using a sieving device and used as aggregates. Rubble from
demolished concrete buildings is generally contaminated with mortar paste, gyp-
sum and minor quantities of other substances such as wood, plastics, metals and
glass. These impurities have several deleterious effects and therefore are unsuit-
able for concrete production. Consequently, in most of the cases, the impurities
present in CDW must be separated during the process. The concrete produced in
the laboratory is also used to prepare CDW aggregate. The use of laboratory
produced concrete allows control of its production which can then be characterised
thoroughly (Fonseca et al.
2011
). A schematic diagram of two typical plants that
recycle CDW as aggregate is presented in Fig.
3.1
.
The production process of CDW aggregate affects its quality and composition.
The original concrete used to make CDW aggregate plays a very important role in
the aggregate properties. Further processing and higher quality source concrete
result in better quality aggregates (Nagataki et al.
2004
).
3.3 Composition of CDW Aggregates
In the literature, two types of CDW aggregates are reported. Some CDW aggregates
contain natural aggregates with adhered mortar. These are produced from recycled
precast concrete and test specimens. On the other hand, in some CDW aggregates,
