BENEFICIATION
South Africa has to make do with a -5mm to -7mm dust, whereas in Europe quarries produce zero
to -2mm and zero to -4mm.
lasting asphalt mix. Depending on the
specific area of use, a set of performance
criteria on the final asphalt has to be met.
“Of the total aggregate component, the
dust comprises the larger portion between
40% and 60% and the behaviour of the
asphalt mixture is largely dependent on
the consistency; that is its quality, shape
and gradation.”
Marais explains that South Africa has
to make do with a -5mm to -7mm dust,
whereas in Europe quarries produce
zero to -2mm and zero to -4mm. “We
would prefer to have similar fractions as
in Europe, but we have to take what is
available as we are not the main clients
of quarries.” One of the products used
regularly by Much Asphalt is
‘continuously graded medium’, a 0 -10mm
34_QUARRY SA| JANUARY/FEBRUARY 2019
mix used by all city councils.
Quality is so important that testing
is a huge component of the asphalt
manufacturing process, with tests done
every 100t (at Much Asphalt), says
Marais. This is one of the highest testing
frequencies in the world, including the US,
Europe and Australia.
“Due to the large volumes manufactured
in an asphalt plant, the smallest change
can have a major impact on the asphalt
mix behaviour and performance. It is
therefore essential that gradations, shape
and texture remain consistent with the
gradations, shape and texture used during
the asphalt design stage. Allowable
variation limits should be agreed to and
adhered to.”
The viscosity of bitumen and asphalt
changes with temperature – less viscous
at higher temperature and more viscous
at lower temperature – and it is therefore
important that the asphalt arrives on site at
the correct temperature to ensure proper
workability and compaction. Careless
temperature and mixing control can cause
asphalt to harden at elevated temperatures,
losing many years of service on a roadway.
Barriers to entry
Marais explains that the capital barriers to
entry ‘are not that high’ but the primary
barrier to entry is skill, particularly good
quality control. It is not that difficult to
produce a batch of asphalt, but if not
done exactly to specs the asphalt can
be rejected by the client and will be