DIGEST 2006 - Sabita

Out of the box thoughts on traditional mixes:
Old habits die hard –
especially in Cape Town!
Julian Wise
Director
Zebra Bituminous Surfacing cc
I've been a Capetonian for
more than 60 years and
yes, it is a great place to
live, work and play. I've been
here so long, I could probably
write a booklet entitled How to
enjoy Cape Town without
actually moving!
But what is it about the place that
makes us feel and act differently
from other areas – perhaps the
mountains and coast, the
(unpredictable) weather,
vineyards, sugarbirds, Tweede
Nuwe Jaar, the noon-day gun, the
laid-back attitude, the perception
that civilisation stops at Sir
Lowry's Pass and all those other
strange traditions that give this
city its great character.
One of these that I find as exciting
and enjoyable as visiting the
dentist is paving the traditional
SABS asphalt mix normally
specified for residential roads. This
is:
• 25mm thick;
• 13.2mm maximum size
stone;
• 5% bitumen
(60/70 pen);
• 4.5% voids in the mix.
When only 25mm thick, the
proportionally large size aggregate
and voids in the mix value cause
the mix to be:
• more difficult to compact
than need be;
• more permeable, thus putting
the asphalt and base at risk;
• more difficult to get good
surface finish with handwork.
Another fact not usually realised is
that the compaction window of a
25mm mat can range from as low
as:
• 6 or 7 minutes on a cold,
windy day;
• only 13 or 14 minutes on a
blinding hot, windless,
summer day.
Enough time for rolling in cold
weather? Enough time for placing
by hand, then rolling?
I don't think so mate!
119