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Waste rubber aggregates 89

4.2.2 Physical properties

Typical values of waste rubber particles are: specific gravity between 0.8 and
3
1.20 g/cm and water absorption of less than 3% (Kumar et al., 2017).

4.2.3 Geometrical properties
Shredding rubber waste reduces the volume of used tires. Generally, the cost of
shredding increases with the need to obtain pieces as small as possible. For grinding,
rubber wastes are initially processed through mechanical cutters, roll crushers and
screw shredders. To obtain finer particles, shear crushers and granulators are used.
The final processing of rubber wastes is with high-temperature shredding equipment,
such as rotary shredders, where degradation occurs during compression simulta-
neously with shear and wear (Mikulionok, 2015). In the initial phase, shredding rub-
ber wastes results in dimensions of approximately 7.623 10.16 cm. These pieces are
then placed in cutters that reduce the size to 0.633 0.63 cm (Rafique, 2012).
Granulators are used in the second step of the recycling process, where pieces of
waste tyres are grinded in the large-sized granulators to produce a large quantity
of granules. The use of pulverises can reduce the rubber granulated material into
fine powder. The rubber particles size can range from a few micrometres up to
centimetres.

4.3 Properties of rubberised mixes in the fresh state

4.3.1 Workability, setting time, segregation and bleeding
4.3.1.1 Mortars with waste rubber aggregates
The effect of tyre rubber ash (size up to 0.15 mm) on different properties of cement
mortar was explored by Al-Akhras and Smadi (2004) by replacing virgin sand with
tyre rubber ash at 0%, 2.5%, 5%, 7.5% and 10%, by weight. The results showed a
reduction in the workability by increasing rubber ash sand content. The reduction in
the flow was up to 25% with the addition at maximum rate of 10% of rubber ash
sand. The setting time increased by increasing the rubber content.
Pierce and Blackwell (2003) partially replaced the natural fine aggregate in
mortar mixes with rubber particles (up to a size of 0.6 mm) at 32% 57%, by
volume. They reported that waste rubber contents as high as 57% can be incorpo-
rated without noticeable rubber segregation, but there was measurable bleeding.
Topcu and Demir (2007) studied the flowability of mortar containing rubber
¸
with particles sizes distribution of either 0 1or1 4 mm as virgin sand replace-
ment at 0%, 10%, 20%, 30% and 40%, by volume. The results showed a work-
ability reduction at increasing rubber content up to 24%.
Marques et al. (2008) partially replaced natural sand in mortar mixes with rubber
(particle size under 0.8 mm) at 0% and 12%, by volume. They found that the
addition of rubber particles reduced the workability.