Civil Engineering Reference
In-Depth Information
cement paste/aggregate binding due to the penetration of cement paste into the
porous tile and brick aggregate, filling of pores by fine tiles and brick aggregate as
well as the presence of more tiles and bricks aggregate in concrete due to their
lower density were the major causes for this improvement.
Nagataki et al. (
2004
) observed comparable STS for RCAC incorporating RCA
with minimum adhered mortar content to that of conventional concrete made with
the original aggregates. The RCA with minimum adhered mortar content even
exhibited higher STS than conventional concrete. The smaller size, lower sand
content as well as the elastic compatibility between RCA and cement paste were
the causes of the good performance of RCAC. Padmini et al. (
2009
) observed
lower STS in RCAC than in conventional concrete and the difference narrowed
down as the CS decreased. In contrast to interfacial bond failure between cement
mortar and aggregate observed in conventional concrete, RCAC exhibited both
interfacial bond failure and aggregate failure in the STS tests (Padmini et al.
2009
;
Rao et al.
2011
).
Tabsh and Abdelfatah (
2009
) reported that the STS for 50 and 30 MPa classes
of conventional concrete as well as that of RCAC were similar when the RCA was
generated from 50 MPa concrete. On the other hand, a drop by 25-30 % and 10-
15 % in STS was observed for both concrete classes when RCA was generated
from 30 MPa concrete. Tavakoli and Soroushian (
1996
) observed a negligible
effect of aggregate size or dry mixing time on the STS of RCA concrete. The 28-
day STS of RCAC with two types of RCA is either higher or statistically com-
parable to that of the control concrete for limited ranges of various experimental
parameters such as size of coarse RCA, mixing time and w/c ratio.
The addition of several mineral admixtures such as silica fume, fly ash, rice
husk ash does not have prominent beneficial effect on STS improvement as
observed in CS (Gonzalez-Fonteboa and Martinez-Abella
2008
; Thangchirapat
et al.
2008
). Gonzalez-Fonteboa and Martinez-Abella (
2008
) observed around
6.8 % higher STS in RCAC prepared at w/c of 0.55 than in conventional concrete
due to the incorporation of silica fume as mineral admixture into cement but the
improvement was not as significant as for CS (around 11.6 %). Ajdukiewicz and
Kliszczzewicz (
2002
) observed improvement in STS of RCAC due to the addition
of SF and superplasticizer, but the improvement was not as significant as in CS.
Kou et al.
2007
) also observed lower STS for RCAC using a blended cement
prepared by replacing 25 % (by weight) OPC by FA than for RCAC using OPC.
The increasing addition of FA into 35 % further lowered the strength. On the other
hand, the same authors in another publication (2008) reported that the addition of
fly ash as a replacement of 25 % of cement by weight can increase the STS. The
major difference between these two studies was the larger amount of binder
content in the mix containing FA in the later study than in the former one. The
improvement was due to the pozzolanic activity of FA which densified the con-
crete matrix by improving porosity.
The replacements of 10 % OPC by SF or 15 % OPC by metakaolin (MK) gave
higher STS to the resulting mixes prepared by replacing 50 and 100 % (by vol-
ume) of coarse natural aggregate by RCA than that of the control and of the RCAC
