CIB W116—Smart and Sustainable Built Environments - Test Input

the demand for natural aggregates too. Nowadays, these same buildings do not comply with the new
standards and many are demolished to take place to new ones. Therefore, large cities now are
converted into urban deposits of recycled aggregates. The production of recycled aggregates in The
United States is around 140 million metric tons per year, which correspond to 5% of total aggregate
market (Taylor, 2007). The machinery normally used for natural aggregates transformation can not be
used for recycled aggregates due to the metal content in the concrete; and metal must be sorted from
aggregates (Wilburn y Goonan, 1998). Concretes that will be recycled are previously sorted to remove
products which are not concrete or masonry at building site. From this point, there are basically two
methods to obtain recycled aggregates from concrete. One is crushed the concrete at a recycling plant
facility and the other one is with a mobile plant. Each method uses basically the same procedures.
The quality of materials which arrives at the recycling facilities is the part that recyclers have little or
no control, due to the wide range of variety of materials transported from different sources.
Construction and demolition wastes contain non metallic materials, such as wood and plastic that
must be collected. The energy types employed to processing and transporting the recycled aggregates
are electricity and diesel. Wilburn y Goonan (1998), estimated that the energy necessary to process
one metric ton of concrete into recycled aggregates is 34 MJ, for processing one metric ton of
recycled asphalt aggregate requires 16.5 MJ and to process one metric ton of natural aggregates is 5.8
MJ. The high difference in energy consumption between recycled and natural aggregates is due
mainly to the labour in identify and remove materials like wood, plastic and glass from construction
and demolition wastes for further processing. Energy values for transport are 2.7 MJ/metric tonkilometre
for fine aggregate and 3.8 MJ/metric ton-kilometre for coarse and recycled aggregates
(Wilburn y Goonan, 1998).
3. Carbonation of CO 2
Carbonation is a chemical reaction in which calcium hydroxide present in cement reacts with carbon
dioxide in the air and the result is calcium carbonate. Carbonation increases concrete resistance with
the past of the years. However, when steel is used as reinforced concrete carbonation can be a
problem, because carbonation decreases concrete pH. This can cause oxidation of steel and hence
structural problems. Dodoo et al (2009) investigated the carbon cycle of a building with reinforced
concrete and wood-frame structure. The work demonstrates that construction phase releases the most
carbon dioxide of all building life-cycle. These authors made calculations to determine the amount of
carbon dioxide absorbed by concrete through carbonation during the building life-cycle. Recycled
concrete has a high CO 2 absorption rate respect the same concrete used as structure (figure 1).
Carbonation in wood-frame structures showed in figure 1 is due to the concrete used as foundations.
The carbonation rate of concrete in wood-frame structure is low because concrete is underground land
and reacts slowly than concrete exposed to air.
183