Page 120 - New Trends in Eco efficient and Recycled Concrete

94 New Trends in Eco-efficient and Recycled Concrete

Fadiel et al. (2014) replaced natural sand in mortars with crumb rubber (size
0 0.6 or 0.84 2) at 10%, 20%, 30% and 40%, by weight. A fixed w/c ratio was
used. Results showed a reduction in the dry density of mortar specimens with the
addition of rubber sand. The reduction in the density reached 8%, 20%, 28% and
39% with the addition of 10%, 20%, 30% and 40% of rubber sand (size
0 0.6 mm), respectively, while the addition of rubber sand with sizes of
0.84 2 mm led to a reduction of 3.25%, 13.94%, 21.21% and 30.1%, respectively.
Ling (2011) investigated the hardened density of concrete blocks by varying the
w/c ratios (from 0.45 to 0.55) and the replacement of natural sand with rubber (size
1 5 mm) at 5% 50%, by volume. Results showed a reduction in the hardened den-
sity by increasing rubber sand content at all w/c ratios.
Eiras et al. (2014) found a reduction in the dry bulk density of mortars by partially
replacing natural sand with crumb rubber (size 0.08 1.3 mm) at 40%, 50% and 60%,
by volume. The reduction in the dry bulk density was approximately 18%, 23% and
27% with the addition of 40%, 50% and 60% rubber sand, respectively.
It can be noted that the addition of rubber sand in concrete mixes in substitution
of natural aggregates decreased the fresh and hardened density. The reduction is
directly related to the physical properties of rubber, which has lower density than
natural sand (Albano et al., 2005). The ability of rubber particles to entrap air in its
jagged surface texture could be another explanation for the reduction of concrete
unit weight (Taha et al., 2008). The possibility to reduce the concrete density by
partially replacing natural aggregates with rubber could be an important factor in
order to produce lightweight concrete.

4.4.2 Mechanical strength
4.4.2.1 Replacement of natural aggregates with crumb rubber in
mortars
The effect of the substitution of natural aggregates with waste rubber aggregates on
the mechanical properties of cement-based mortars has been investigated by several
researchers.
Al-Akhras and Smadi (2004) reported an increase in the compressive and flex-
ural strength by replacing natural sand in mortars with rubber ash (size 0.15 mm) at
2.5%, 5%, 7.5% and 10%, by weight. At 28 days of curing the compressive strength
increase was 12%, 14%, 23% and 40% with the addition of 2.5%, 5%, 7.5% and
10% of rubber ash sand, respectively, while the improvement of flexural strength
was 12%, 27%, 32% and 43%, respectively.
On the contrary, Segre et al. (2004) carried out experimental investigations on
mortars containing 10% rubber (size 0.2 mm) as a natural sand replacement, by
weight. Flexural strength reduction with the addition of rubber sand was about 25%.
Turatsinze et al. (2006) partially replaced natural sand (0 4 mm) in mortars
with shredded non-reusable tyres (0 4 mm) at 0%, 20% and 30%, by volume. The
reduction of the compressive strength after 28 days of curing was 57% and 79%
with the addition of 20% and 30% of rubber sand, respectively, while the tensile