Use of Construction and Demolition Waste as Aggregate: Properties of Concrete - Recycled Aggregate in Concrete - page 253

Fig. 5.17 The CS of RCAC
due to the replacement of
cement by other binder
materials (Ann et al. 2008 )
pulverized fuel ash (PFA) and gbfs with RCAC and conventional concrete mixes
(Fig. 5.17 ). They observed lower CS for RCA concrete containing PFA and gbfs
than for the control and RCAC mixes at the ages of 7 and 28 days, due to the lower
hydraulic reactivity of these binders. However, the CS of concrete containing gbfs
and PFA overtook the CS of the conventional concrete after 90 and 180 days of
curing, respectively.
Kou et al. ( 2011b ) observed a substantial reduction in CS due to the replace-
ment of cement by 55 % gbfs or 35 % FA. However, they observed an
improvement in the CS of RCAC with 50 and 100 % (by volume) replacement of
coarse NA by RCA due to the replacement of cement by 10 % SF or 15 % MK.
The CS of RCAC with SF and MK was similar to that of the conventional concrete
for a 50 % replacement ratio of NA by RCA (Fig. 5.18 ). However, for a 100 %
RCA replacement ratio, the 90-day CS of RCAC containing all type of mineral
additions was substantially lower than that of the conventional concrete. The gain
in CS of RCAC concrete with SF and MK was higher than that of the concrete
containing FA and gbfs in the early ages (up to 28 days); on the other hand, later
on (28-90 days), the gain in CS was higher for RCAC with FA and gbfs than for
RCAC with SF and MK. The improvement of ITZ due to the filling of pores as
well as cracks in RCA by the hydration products was the reason for the observed
improvement. The contribution of mineral additions to the improvement of CS of
RCAC was more than in conventional concrete. Ajdukiewicz and Kliszczzewicz
( 2002 ) observed a significant improvement in CS of RCAC due to the addition of
SF and superplasticizer. The improvement was more prominent for RCA produced
from high-strength concrete aggregates. Thangchirapat et al. ( 2008 ) reported that
the use of rice husk-bark ash (RHBA) to replace 20 and 35 % by weight of
Portland cement in RCAC yielded higher CS than in RCAC prepared without
RHBA.
Several other methods were adopted to overcome the negative effect of CDW
aggregate on the CS of concrete. Increasing cement content or lowering the water
to cement ratio in the mix and improving the concrete mixing process have already
been mentioned during the discussion of CS of concrete containing RA and RCA
aggregate. Since the major reason of the reduction in CS of RAC and RCAC is the