DIGEST 2006 - Sabita

seal, and everybody's happy! Or
were they?
Bob remembers that this mix was
made with 80/100-pen bitumen,
and was paved on primed
ferricrete.
"It was not an unqualified failure,”
he says ruefully, “but we had
endless problems with achieving
density."
What's changed?
OK, so now it's a nominal 25mm
thick, but is mixed with a stiffer
60/70-pen bitumen that needs
more compactive effort to get
density.
More rolling takes longer ... so the
mat must stay hotter for longer –
but it doesn't, does it!
Impermeability
Carl van der Merwe (UWP)
designed a mix for Khayelitsha
residential roads about 10 years
ago. His main priority was
impermeability to guard against:
• "grey” water damaging the
asphalt and base;
• flooding due to blocked storm
water catchpits.
The mix was basically a Colto Fine
paved 25mm thick using:
• 6.7mm maximum aggregate;
• 6% bitumen;
• 1.5 to 3% voids in the mix.
"Oh horrors!” I hear some of you
cry, “It will bleed, it will rut"...
No it won't – the light traffic does
not increase compaction over what
is achieved at construction. Good
compaction was achieved even
doing handwork, and ten years
later these roads are in really
excellent condition. It was a good
design and it worked.
Brian Roussouw (Kayad), that
singularly innovative engineer,
designed his township mix
because he was sick of the
problems using the traditional mix.
“I wanted a contractor-proof mix
that could be adequately
compacted and looks good even
with handwork,” he said.
Recommended mix
His “Kayad” mix is classed as a
9.5mm NMAS fine-graded mix.
Again this mix compacts more
easily and is less permeable than
the traditional.
If anyone is still reading and
doesn't consider this article the
demented ramblings of a biased,
senile contractor, my
recommended specification to
ensure compactibility and low
permeability for a 25mm thick
mat, using Cape Town aggregates,
is:
• a sound base with a high
quality, primed finish;
• a tack coat;
• a mix with 9.5mm max size
stone;
• 3 to 4% target voids in the
mix;
• bitumen content for this is
usually 5.5 to 5.6%;
• filler/binder ratio 1.1 to 1.2.
The advantages are:
123