BENEFICIATION
used in pothole repair products.
The primary customers of asphalt are
contractors acting on behalf of national,
provincial and municipal road authorities
as well as private developers.
The dawn of asphalt
Asphalt is one of the world's oldest
engineering materials, having been used
since around 6 000BCE by the Sumerians
who had a thriving shipbuilding industry
that produced and used asphalt for
caulking and waterproofing. As early as
2600BCE the Egyptians were using asphalt
as a waterproofing material and also to
impregnate the wrappings of mummies
as preservative.
Asphalt used by early civilisations
occurred naturally and was found in
geologic strata as either soft, workable
mortars or as hard, brittle black veins of
rock formations (also known as asphaltic
coal). Natural asphalts formed when
crude petroleum oils worked their way up
through cracks and fissures to the earth's
surface. The action of the wind and sun
drove off the lighter oils and gases, leaving
a black residue. Natural asphalts were
extensively used until the early 1900s. The
discovery of refining asphalt from crude
petroleum and the exponential increase
in automobile ownership served to vastly
expand the asphalt industry. Modern
asphalt has the same qualities of durability
as natural asphalt, with the added
advantage of being refined to a uniform
condition free from organic and mineral
impurities.
An exacting process
million tonnes a year. We’re currently in
a weak market where demand is no more
than about 3.5 million tonnes a year.
The aggregate has to be of an exact and
consistent quality. Asphalt manufacturers
therefore emphasise that when they find a
quarry capable of consistently producing
the required standard of aggregate – they
become joined at the hip, at least as far as
the asphalt manufacturer is concerned,
which remains relatively minor customers
of quarries compared to concrete
manufacturers. Herman Marais, technical
director at Much Asphalt, points out that
www.quarryonline.co.za
many of its plant are in fact located at its
partner quarry, or at least nearby.
Most of the petroleum-based bitumen
produced today is used for highway
surfacing (tar comes from coal). Marais
says that apart from South Africa’s main
road network (national, provincial and
municipal), asphalt is also used for
parking lots, airport runways, large storage
tank bases, domestic driveways, as well as
levelling layers to CRCP pavements and
Astro turf surfaces are also sometimes laid
on a porous asphalt layer such as a hockey
field. And of course, asphalt is extensively
There are various asphalt mixes: hot-mix,
cold-mix and warm-mix. Hot-mix asphalt
is commonly used for heavier traffic areas
while cold-mix asphalt (generally mixes
made with emulsified or cut-back binders)
is usually used for light to medium traffic
secondary roads, or for remote locations
or maintenance.
Hot-mix asphalts are a mixture of
suitable aggregate coated with bitumen.
The term comes from the process
of drying and heating the aggregate
and bitumen before mixing to ensure
moisture is removed from the aggregate
and to obtain low enough viscosity of
the bitumen for proper coating of the
aggregate and workability.
The first step in the process, says Marais,
is the mix design which is typically done at
the Much Asphalt main laboratory.
“If it is for a major contract like
SANRAL or a city council then it will
QUARRY SA | JANUARY/FEBRUARY 2019_31