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Waste rubber aggregates 111

On the contrary, the addition of rubber sand reduced the micro-cracks caused by
elevated temperatures. The number of micro-cracks decreased as the rubber sand
content increases. Crumb rubber is able to reduce the initiation and development of
cracks in concrete when exposed to elevated temperatures. When the rubber melting

point (about 170 C) is reached, the space occupied by the rubber is replaced by air
pockets which allow the evaporated water to escape, without the formation of inter-
nal tensions.

4.6 Functional properties of concrete with rubber
particles

4.6.1 Thermal insulation
Sukontasukkul (2009) observed a reduction of 20% 50% in the thermal conductiv-
ity of concrete with partial replacement of sand with 10% 30% of crumb rubber,
by volume. Crumb rubber concrete exhibited lower heat transfer rate and higher
heat resistivity than the reference concrete. The heat transfer reduction was about
17%, 44% and 54% with the addition of 10%, 20% and 30% of rubber sand.
Paine and Dhir (2010) observed lower thermal conductivity of concrete contain-
ing rubber particles.
Hall et al. (2012) partially replaced natural sand with crumb rubber (2 6 mm) in
concrete, at up to 30%, by weight. The substitution of natural sand with rubber
caused a significant reduction in thermal conductivity.
Pelisser et al. (2012) found a reduction in the thermal conductivity of mortar
containing 40% recycled tyre rubber (0 2.4 mm) of about 13.8%.
Paine et al. (2012) observed that the addition of rubber in concrete as natural
sand replacement decreased its thermal conductivity.
Issa and Salem (2013) found that the addition of crumb rubber (0.075 2.54 mm)
as natural sand replacement in concrete improved its insulation properties. By
increasing the rubber sand content, the thermal conductivity decreased.
Fadiel et al. (2014) observed a reduction of the thermal conductivity of about
13%, 15%, 17% and 21% in mortar specimens containing 10%, 20%, 30% and
40% crumb rubber (0 0.6 mm), respectively, as natural sand replacement, by
weight.
In general, it can be concluded that the addition of rubber sand in concrete mixes
decreased their thermal conductivity, that is, an increase of their thermal insulation.
The thermal conductivity reduction in rubberised concrete is partly due to the
increasing air entrapment during mixing and to the lower thermal conductivity of
the crumb rubber particles. As the addition of crumb rubber increased, there was a
greater moisture-dependent effect on the saturated state thermal conductivity due to
the increase apparent porosity caused by air entrapment (Hall et al., 2012).
Concrete or mortars with rubber particles can be used where thermal insulation is
required.