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similar to those observed in a conventional concrete. They observed CS about 2.5
and 0.4 % lower and 2 % higher for RAC than for conventional concrete after 7, 28
and 115 days of curing, respectively.
The detrimental factors that control the CS of RAC are the crushing strength
and mortar content in RA (Etxeberria et al.
2007a
). Gonzalez-Fonteboa and
Martinez-Abella (
2005
) specified three reasons, which controlled the CS perfor-
mance of RAC. The higher absorption of water by RA than NA reduced the
amount of free water in the mix. These decrease up to a certain level could increase
the CS; however, an excessive decrease in free water can deteriorate the CS of
RAC due to lesser hydration of cement particles and poor workability of the mix
than in conventional concrete. The weak bond of cement paste and RA also lowers
the CS of RAC. The addition of an extra amount of cement to the mix boosts the
CS by improving bond between the cement paste and the RA and improves
porosity and consistency.
The strength development of RAC was faster than for conventional concrete
after 28 days of curing (Gonzalez-Fonteboa and Martinez-Abella
2005
). Etxe-
berria et al. (
2007a
) observed gain of about 12-15 % in CS between 7 and 28 days
of curing of RAC prepared by replacing 25, 50 and 100 % (by volume) of NA by
RA in comparison to around 20 % in conventional concrete.
Compared to the studies on the use of RA in concrete, more information is
available on the use of RCA in preparation of concrete. Similarly to RA, the CS of
concrete containing RCA (RCAC) is normally lower than that of the corre-
sponding control concrete and increasing the addition of RCA to concrete further
lowers it (Table
5.8
). However, RCA addition to concrete does not have an
adverse effect on its strength development trend.
Frondistou-Yannas (
1977
) reported that the substitution of natural gravel by
RCA and recycled aggregate that contained mortar only led to a lower CS than that
of conventional concrete. The failure observed in both types of concrete was by
fracture in the aggregate. Eguchi et al. (
2007
) observed higher reduction in CS of
concrete due to the addition of RCA originated from a concrete which consisted of
low strength aggregate than that observed for RCA originated from a concrete
consisted of high-strength aggregate. Lopez-Gayarre et al. (
2009
) also observed a
strong dependence of CS of RCAC on the quality of RCA, mainly concerning the
adhered mortar content. However, they did not observe any effect of increased
addition of coarse RCA on the mean CS of RCAC if the water to cement ratio was
kept constant and the loss of workability due to addition of RCA was compensated
by using chemical admixtures.
Santos et al. (
2002
) observed a reduction of about 20 % in 7- and 28-day CS of
two types of RCAC from the corresponding strength of conventional concrete,
when both types of RCAC were prepared with coarse RCA originated from two
concrete mixes with different strengths. Gomez-Soberon (
2002
) observed that the
CS of concrete decreased with increasing content of coarse RCA. The 28-day CS
of concrete prepared by replacing coarse NA with 15, 30, 60 and 100 % coarse
RCA were respectively about 98, 95, 92 and 88 % that of conventional concrete.
After 90 days of curing, these values became 99, 94, 91, 89 %, indicating a similar
