Civil Engineering Reference
In-Depth Information
Fig. 5.15 Effect of curing
conditions on the CS of
RCAC (Rao et al.
2011
)
coarse RCA was 10-14 % higher than that of the control concrete when RCA was
used to replace partially or fully coarse NA (limestone and gravel). This is due to
the addition of a water reducing admixture to the RCA mix, which improved the
quality of the mortar more than observed in conventional concrete. One advantage
of this method was that the amount of cement necessary to make concrete was
about 10 % less than for conventional concrete.
Ferreira et al. (
2011
) compared the CS of mixes containing coarse RCA using
two mixing methods. In the pre-saturation method, the RCA was initially pre-
soaked before mixing. In the water compensation method, extra water absorbed by
RCA was added during mixing (Fig.
5.16
).
The 7- and 28-day CS of RCAC using the pre-saturation method was lower than
that using the other method, possibly due to a lower ''nailing effect'', which results
from the penetration of cement paste into the superficial pores of aggregate par-
ticles. Before mixing, pre-saturated RCCA exhibited not only a high level of
humidity but also water on the surface and within surface pores. This might have
impaired the penetration of the cement paste into the pores, leading to a decrease
of the ''nailing effect'' and, consequently, to a weaker ITZ between cement paste
and RCCA. However, the differences of CS between the mixes prepared by two
methods became insignificant as the RCCA's incorporation ratio increased, which
might be a consequence of a higher number of weak zones in concrete.
Using a certain amount of mineral additions such as fly ash (FA), metakaolin
(MK), silica fume (SF), rice husk ash or latent hydraulic materials such as ground
blast furnace slag (gbfs) into cement have some beneficial effect on the CS per-
formance of RCAC.
Kou and Poon (
2006
) observed an improvement in 90-day CS of RCAC due to
25 % replacement of OPC by FA; however, the addition of 35 % FA had a
negative effect on the CS of RCAC. The gain in CS between 28 and 90 days of
RCAC containing 25 and 35 % FA is higher than that observed in conventional
concrete. The strength gain of concrete containing RCA between 28 days and
