Civil Engineering Reference
In-Depth Information
Fig. 3.7 Relationship
between density and water
absorption capacity of CDW
aggregate (De Juan and
Gutiérrez
2009
)
due to the formation of a weak interfacial transition zone between the saturated
recycled coarse aggregates and the new cement paste (Etxeberria et al.
2007a
).
Etxeberria et al. (
2007a
) recommended a humidity level of 80 % of the total
absorption capacity of CDW aggregate for better performance of concrete con-
taining CDW aggregate. Oliveira and Vazquez (
1996
) reported that the mechanical
performance of concrete prepared by pre-saturated followed by 30 min air-dried
CDW aggregate is better than the performances of concrete containing oven dry as
well as saturated surface dry CDW aggregates. The humidity level of air-dried
CDW aggregate was about 90 % with respect to saturated surface dry aggregate.
In some investigations, an extra amount of water is added to the concrete mix
corresponding to the water absorbed by the CDW aggregate (Matias and de Brito
2004
; Oliveira and Vázquez
1996
; Santos et al.
2005
). Ferreira et al. (
2011
)
describe this method as mixing water compensation method. The amount of water
added depends on the initial water content and effective absorption of CDW
aggregate during the mixing period. Potential water absorption and absorption
evolution with time should also be known in order to predict the water to cement
ratio (W/C) after the mixing period. This ought to guarantee that the water added
does indeed correspond to the amount of water absorbed by the CDW aggregate.
The mixing water compensation method has the advantage of making it possible
to produce both concrete containing CDW aggregate and conventional concrete in a
similar way. However, Oliveira and Vázquez (
1996
) note that the water absorption
of CDW aggregate may not correspond to the free water absorption determined in
the laboratory, since the pores of CDW aggregate are filled with cement paste
during mixing, which may lead to an excess of water in the mix, and thus an
undesirable increase in the effective water to cement ratio (W/C).
Ferreira et al. (
2011
) used a different approach to keep the effective water to
cement ratio (W/C) constant in the different concrete compositions containing
CDW aggregate to ensure compensation during the mixing process. A schematic
diagram of the procedure is presented in Fig.
3.8
. To achieve this goal, these
authors added directly to the concrete mix an additional amount of water equivalent
to the absorption expected from CDW aggregate during the mixing process,
