DIGEST 2006 - Sabita

The publication, Interim Guidelines
for the Design of Hot Mix Asphalt
in South Africa (prepared in 2001
as part of the Hot Mix Asphalt
Design Project launched in 2001
by SANRAL, the CSIR and Sabita),
supports this. A few extracts are:
• Clause 2-16: Current
specifications limit the
maximum aggregate size to
not more than half the
compacted mat thickness.
Designers should consider
decreasing this limit;
whenever conditions are
anticipated in which
compactibility or segregation
might pose problems during
construction;
• Clause 2-18: Selection of
maximum size aggregate i.e.
9.5mm for 25mm thick mats;
• Clause 4-8: Minimum voids in
the mix for light traffic is
3.5%.
Impermeability
What should be the main purpose
of such a thin layer on low quality
base in a heavy winter rainfall
area?
Impermeability for a start!
Impermeability and good
compaction go hand in hand. We
don't want to pave thicker do we,
that would be too much of a break
in tradition. But we can easily
design for higher density and low
permeability.
Design the mix for quick, easy
compaction – remember the very
short compaction window time.
• Use a small maximum size
aggregate;
• Use a high bitumen content
to provide good lubrication
for compaction;
• Target a low voids in the mix
– light traffic will only
marginally increase the
construction density, so there
is no risk of rutting or
bleeding.
What were the origins of this
stubborn, ingrained tradition? Bob
Kingdon tells a fascinating story.
The construction of the freeway
system in Cape Town in the 60's
prompted the establishment of
three production HMA plants. As
freeway projects tailed off, the
producers looked for alternative
markets to keep their plants busy.
Cracked windscreens
In addition, the standard surfacing
for residential roads then was a
single seal. To allow immediate
opening to traffic, chips were
spread at a rate 1.2 times higher
than was called for by the design.
Result was a period of flying chips
and cracked windscreens until the
team came back to sweep up the
excess.
So the authorities were looking for
an alternative, especially for the
then emerging Michells Plain.
The wearing course for the
freeways had been approved and
established – 40mm thick,
13.2mm maximum size aggregate
CCC mix.
So just reduce the thickness to
20mm to make it economical
enough to compete with the single
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