BENEFICIATION
go to their consulting engineer, which
may in turn have the design verified or
recommend some tweaking of the mix.
Once the design is finalised, the plant
produces a physical sample of that design
to prove it can actually be manufactured
at the plant. That sample is then tested,
and if it again passes the prescribed tests, a
paving trial is conducted on the road. The
contractor will thereafter pave for two or
three days, performing trial tests during
that time which may require further
tweaking of the design based on actual
performance.
“We are currently moving over to an
entirely new bitumen specification and
asphalt mix design process, which is
performance based and is being driven
by the industry. The current bitumen
specification covers a number of empirical
tests, mainly for viscosity at different
temperatures. These tests are done before
and after short term ageing and although
they give an indication of ageing that
The importance of this lies in the fact that
bitumen is used in a wide range of climates
– even in South Africa from the Karoo
heat to the cold of the Eastern Free State
– requiring a different bitumen for each
climatic region, traffic type
and volume.
“The implementation of the new
PG bitumen specification is at quite
an advanced stage now – we’ve been
working at it for the past 10 years – and
the industry players have invested in the
advanced testing equipment required,”
says Marais.
“There are specs for the aggregate –
shape, strength and gradation, as well
as absorption level. It’s not just a matter
of how much material we can get from
the quarry. Often a quarry sits with huge
stockpiles of unsaleable materials until we
come and put up a plant, and when we
need 15 000t a month, it soon gets used
up. So the challenge from our perspective
is firstly the quality and
“South Africa is currently in the process of moving to a
performance-based system due to constantly increasing
road traffic growth.”
takes place during the asphalt mixing
and paving process, they do not cater
for prediction of long-term ageing that
happens on the road,” Marais explains.
“The new Performance Grade (PG)
specification classifies bitumen in
different temperature grades relating to
the maximum and minimum expected
pavement temperatures as well as
traffic class to which the asphalt will be
subjected. The new PG specification does
not only look at the viscosity of the binder
and the effect of short-term ageing, but
also incorporates a long-term ageing
process – using the Pressure Ageing Vessel
(PAV) – which simulates three to 10
years of ageing on the road. The testing
that is performed on the aged bitumen
also tests the response of the binder to
repeated stresses applied in the Dynamic
Shear Rheometer (DSR) as well as low
temperature performance in the Bending
Beam Rheometer (BBR).”
32_QUARRY SA| JANUARY/FEBRUARY 2019
second the quarry’s capacity to supply.
We often run into difficulty as soon as
the asphalt volumes pick up, the quarry
cannot keep up.”
Much Asphalt uses Afrisam’s Rooikraal
quarry, near Vosloorus, for about 95%
of the aggregate required at its Benoni
flagship plant. Marais points out that
Rooikraal supplies an aggregate that is
slightly rounded and not too flaky, which
Much Asphalt prefers.
“However, some contract specs require
a cubical aggregate for high volume
traffic and a maximum flakiness of 12
is specified. This is very tight and there
are not a lot of quarries that can crush
to that standard. The quarry sends us its
gradings and its test results, at which point
we do our own assessment. In the new
design process asphalt producers are being
guided to use what’s called ‘the Bailey
method’, which is similar to a concrete mix
design method and is a very useful tool to
optimise aggregate packing and to better
understand the volumetric properties of
your asphalt,” says Marais.
Bitumen and aggregate are combined
in a mixing facility where they are heated,
proportioned and mixed to produce the
desired paving mixture. The combination
of aggregates is weighed and dumped
into a mixing chamber called a pugmill
where it is thoroughly mixed. It is then
transferred into storage silos, trucks or
surge bins. The drum-mixing process heats
and blends the aggregate with bitumen in
a continuous process in the drum mixer.
When the mixing is complete, the hot-
mix is then transported to the paving site
and spread in a partially compacted layer
to a uniform, even surface with an asphalt
paver. While still hot, the paving mixture