DIGEST 2006 - Sabita

But the traditionalists doggedly
persist in specifying it for
low-traffic roads and parking areas
that:
• do not lend themselves to
production paving or rapid
compaction;
• have a fair amount of
handwork;
• have low quality base course
(G3 to G5, laterite, crushed
rubble, calcrete, all of
variable quality), that is not
finished to a high standard;
• have actual asphalt;
thicknesses that can easily
range from 15 to 35mm.
(Imagine the compaction
window time for a 15mm
thick area!).
The tradition joyfully continues by
making sure that most of these
jobs are paved in winter, when as
long as it is not raining, it's
regarded as a good paving day!
"The job HAS to be handed over
tomorrow y'know", followed by the
phrase that has supported the
movies for a generation “Don't
worry, everything will be OK".
Ja well no fine
And then the subtle assurance
“I'm sure the Engineer will
understand."
They don't say what it is the
Engineer will understand – it
usually turns out to be only what
the specification says! And when
the core results come out, they all
promptly forget about the
pressure-cooker situation at the
time.
The result? Typical problems
encountered are:
• erratic and variable density,
especially in winter;
• excessive permeability;
• erratic surface finish,
particularly with handwork;
• increased risk to the base
from the above.
The root cause of these problems?
The mix design is not appropriate
for the application and 25mm
thickness. (It is suitable for
production paving of a 40mm thick
layer.)
"Nonsense,” some of you will be
saying, “I've been using this mix
for 35 years, and can show you
many roads that are still perfect."
Ja well no fine!
Granted, this mix is not a
complete disaster, but:
• specs have tightened – 95%
Marshall has largely been
replaced with 92% Rice;
• 60/70 pen bitumen has
replaced the 80/100 used 35
years ago;
• in my experience testing has
become “more thorough” to
put it politely.
Certainly not every core will be a
failure, but the more thoroughly
you test this mix, the more
failures you generally find. I too
can name many roads and parking
areas that have failed to some
extent – failed in density and
failed to last a reasonable lifespan.
Am I a lone wolf howling in the
wilderness?
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