Civil Engineering Reference
In-Depth Information
Pereira et al. (
2012
) reported that the compressive strength of concrete con-
taining FRCAC could be improved by using superplasticizer and a lower w/c ratio.
The 28-day compressive strengths of conventional concrete and FRCACs con-
taining FRCA as the only fine aggregate with and without two types of superp-
lasticizers were respectively 39.5, 38.6, 45.1 and 63 MPa indicating a significant
increase in compressive strength due to the addition of superplasticizer in the mix
of RCAC. They also observed much a lower influence of FRCA on CS perfor-
mance in comparison to the change in w/c ratio. They proposed the following
relationship between CS
c
ef
(correlation coeffi-
f
ðÞ
and effective w/c ratio
cient of 0.96) from their experimental results:
230
:
3
25
:
9
f
c
¼
ðÞ
ef
1
0
:
077
ð
ð
Þ
W
24
r
Þ
ð
5
:
1
Þ
w
ð
Þ
where W
24
is 24 h water absorption capacity of concrete, r is replacement ratio and
numerical values are determined by regression analysis.
Kou and Poon (
2009a
) prepared two concrete series by replacing 25, 50, 75 and
100 % by weight of fine natural aggregate by fine recycled aggregate (FRA) with
particle size below 5 mm. The first and second series of concrete were prepared
using the same cement content at constant water to cement ratio (w/c) of 0.53 and
a close slump range of 60-80 mm. At same w/c, the CS of concrete decreased with
the content of FRA due to higher bleeding as well as poor aggregate-cement paste
bond owing to the higher initial free water content. At constant slump, the CS of
series II concrete also decreased with the FRA content; however, the deterioration
of CS was marginally higher than that observed in series I. According to the
authors, this was due to the weaker mechanical properties of FRA and FNA.
By replacing 100 % of fine natural aggregate by fine RCA, Kou and Poon
(
2009b
) prepared a self-compacting concrete, which can yield CS values as high as
64 MPa. The authors concluded that the inclusion of FRCA up to a ratio of 25-
50 % does not significantly change the CS of the resulting concrete. Dapena et al.
(
2011
) observed a drop of around 7.3-9.4 % in the CS of concrete due to the
replacement of 10 % coarse RCA by FRCA, where natural coarse aggregate was
replaced by 20, 50 and 100 % (by volume) of coarse RCA.
Kou and Poon (
2008
) reported up to 5 years experience of CS of concrete
prepared by replacing 0, 20, 50 and 100 % of coarse NA by an equal volume of
RA and RCA. They observed lower CS of RCA concrete than that of RA concrete
after 28-day but after 5 years the CS was highest for RCA for all substitution
ratios. The CS of concrete containing RCA and RA was always lower than that of
the conventional concrete; however, the reduction of CS decreases as curing time
increases. The 28-day to 5-year gain in the CS of concrete containing RA and RCA
was higher than that of the control concrete and it was highest for RCA concrete.
The strengthening of the paste-aggregate bond, the healing of cracks in interfacial
zone due to the deposition of new hydration products and the reduction in the
preferred orientation of Ca(OH)
2
crystals were the main causes for the observed
