Civil Engineering Reference
In-Depth Information
Fig. 5.18 CS of concrete due
to the replacement of cement
by mineral addition (Kou
et al.
2011b
)
weaker bond between the recycled aggregate and the cement paste, some studies
are also directed at improving the bond strength by using some other materials.
Out of these, coating the aggregate's surface with silica fume or nano-silica is most
promising one.
Chen et al. (
2003
) observed a 10-15 % higher CS of concrete containing
washed RCA than that of concrete containing unwashed RA at various w/c ratios.
The RA was washed to remove the sand content and other impurities like bricks
and tiles. Shayan and Xu (
2003
) observed that the surface treatment of coarse RCA
by silica fume slurry can substantially improve the CS of the resulting concrete;
however, sodium silicate treatment did not have any beneficial effect. They
reported that structural concrete with 50 MPa strength grade could be produced by
replacing all the coarse NA with silica fume treated coarse RCA and by replacing a
maximum 50 % by weight of fine NA with untreated fine RCA. Katz (
2004
) also
observed an increase of about 23-33 and 15 % in 7- and 28-day CS due to the
impregnation of SF on the surface of coarse RCA. The improvement of the
microstructure of the ITZ between the cement paste and the RCA aggregate sur-
face and the mechanical performance of RCA can be the major cause for the
observed increase in CS. Although it was not as prominent as SF impregnation,
ultrasonic treatment to remove unbound particles of RCA also increased by about
7 % the 7- and 28-day CS of RCAC.
Akbarnezhad et al. (
2011
) observed significantly smaller reduction in the CS of
concrete due to the incorporation of RCA obtained after microwave heating
(MRCA) than that observed for normal RCA incorporation due to the removal of a
part of the adhered mortar content as well as of weak RCA particles (Fig.
5.19
).
The RCA was heated using microwaves to remove adhered mortar. The differences
in 28-day CS between conventional concrete and mixes containing MRCA were
negligible up to 40 % replacement of coarse NA. These differences were
respectively 10 % and around 30 % for the mixes containing RCA and MRCA as
sole coarse aggregate.
Tam et al. (
2007
) observed up to 21 % improvement in 28-day CS of concrete
with 20 % by volume replacement of coarse natural aggregate by RCA using a
