Use of Construction and Demolition Waste as Aggregate: Properties of Concrete - Recycled Aggregate in Concrete - page 244

water absorption capacity of RCA. Evangelista and de Brito ( 2007 ) also observed
higher CS of concrete incorporating fine RCA, which will be discussed later.
Some studies indicated a relationship between aggregate to cement ratio and the
CS of RCAC (Fig. 5.5 ): decreasing the aggregate to cement ratio is beneficial for
the CS of RCAC (Poon and Chan 2007 ; Poon and Lam 2008 ). The authors pointed
out that the low crushing strength of the aggregate as well as the weak cement
paste-aggregate bond are the causes of these results. In fact, the cement content in
concrete plays a vital role in the CS (and other mechanical) behaviour of RCAC. A
typical example is presented in Fig. 5.6 (Courad et al. 2010 ).
Similarly to conventional concrete, a linear relationship between water to
binder ratio and CS also exists in RCAC (Corinaldesi 2010 ; Nagataki and Iida
2001 ). Nagataki and Iida ( 2001 ) observed this relationship for RCAC up to a
binder to water ratio of 2.5-3.3 depending on the strength of the source concrete
from which RCA originates (Fig. 5.7 ). Kou and Poon ( 2006 ) observed a decrease
in CS as the content of coarse RCA in concrete increases at water to cement ratios
of 0.45 and 0.55. However, the CS of the control concrete and RCAC increased
with curing time and decreased with water to cement ratio. However, in a few
studies, it was pointed out that the strength characteristics of RCAC were not
significantly affected by the quality of RAC at high water to cement ratio, since it
was affected only when the water to cement ratio was low (Ryu 2002 ; Padmini
et al. 2002 ).
Chen et al. ( 2003 ) observed much lower differences between the CS of con-
ventional concrete and that of RAC, when the w/c ratio increased to a certain value
and they became lowest for w/c values between 0.58 and 0.80 (Fig. 5.8 ). Katz
( 2003 ) observed that the CS of RCAC was comparable to that of reference con-
crete up to the replacement level of 75 % at a w/c ratio between 0.6 and 0.75.
Padmini et al. ( 2009 ) specified several factors that can affect the CS perfor-
mance of RCAC: to achieve the design CS RCAC required lower w/c ratio and
higher cement content than concrete containing granite aggregate; for a targeted
mean CS, the actual strength of RCAC increased with the maximum size of RCA
used as the maximum size of RCA in concrete decreased and the strength of the
source concrete increased.
Fig. 5.5 Relationship
between aggregate to cement
ratio and CS of RCAC
(Poon and Lam 2008 )