 INTRODUCTION
Slurry
macadam or micro asphalt as it is quite often called is most commonly
supplied as a proprietary product, although it can be produced in smaller
quantities as described in the superseded,
BS 434 : Part 2:1984 - Bitumen road emulsions (anionic and cationic)
Part 2 : Code of practice for use of bitumen road emulsions.
This standard described the uses of bitumen emulsions, and
specified/recommended the way in which they will be
used, and it included information on bitumen emulsion macadams, it is
therefore a useful source of information on this type of product.
 BS 434:Part
2:2006 has superseded
the 1984 edition and it also is an excellent document and I would suggest that you obtain a
copy as it
contains a wealth of useful information on many highway maintenance
materials and procedures that use bitumen emulsion in the process.
I would suggest that you retain your "old" standard (BS 434:Part
2:1984) as a
reference document, particularly as a guide to slurry seal and slurry
macadam mixtures, should you wish to produce small amounts yourself,
although suppliers of the emulsions are usually prepared to help in the
design of appropriate slurry seals and slurry macadams.
Samples of "established" slurry macadam that I have sampled and analysed have shown
aggregate gradings and bitumen contents very similar to conventional hot
mix 6mm. and 10mm. "dense" macadams, but of course slurry
macadams are produced cold, and not all aggregate is suitable for this
process.
In being able to produce "cold" macadams the base binder of the
emulsion tends to be of a lower viscosity, and hence the laid mixtures are
unlikely to have the stiffness of conventional hot mix bituminous macadams.
Although an excellent process, it is more expensive than surface dressing, it
does tend to be more "robust" surface on difficult sites and may be
appropriate in "sensitive" urban areas.
However there is often some surface "shedding" of larger
aggregate very soon after laying, this is not failure, but it may be
commented on
The most common mode of failure, if it happens, is that the surface will
lose its texture due to the lower viscosity of the base binder compared to
that used in a hot mix bituminous mixture. Therefore it is wise to be
cautious on high speed sites that take carry a large amount of heavy
vehicles.
As with all highways maintenance processes it is up to the Engineer /
Engineering Technician to make a judgment as to the suitability of the
product for the site in question, and that includes budget considerations.
CONTENTS
THE VEHICLE
THE "MIXING BOX"
LAYING THE "MACADAM"
EARLY APPEARANCE
NEW
SURFACE APPEARANCE
THICKNESS
THE
PRINCIPLES OF SLURRY MACADAM ARE THE SAME AS SLURRY SEALING
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THE VEHICLE
The vehicle is purpose built to include a large amount of
storage for all components needed to mix the slurry
macadam.
It has the ability to mix the materials, to produce the
"macadam", immediately before applying, and the means of
spreading it evenly across the road surface.
With final adjustments to ensure a "smooth" ride quality to
the road surface being done by operators.
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THE "MIXING BOX"
All materials are kept separate until immediately prior
to laying when they are brought together in a controlled fashion and mixed
vigorously in the "mixing
box" before being discharged on to the road.
The materials being a possible combination of the
following :-
Coarse aggregate, fine aggregate, ordinary portland
cement, bitumen emulsion (possibly with a polymer
modifier), an amount of retard agent according to ambient temperatures.
The bitumen emulsion used in this process is almost always cationic,
some anionic emulsions may be used in hotter drier climates.
The slurry macadam may even contain a small percentage of
"chopped fibres" to improve the binding strength of the
slurry matrix over weaker surfaces.
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LAYING
THE "MACADAM"
When slurry macadam is first laid it behaves as a liquid,
therefore the actual thickness deposited is more or less
controlled by the nominal size of the coarse aggregate.
The screed plate at the back of the machine is mainly to
ensure even distribution of the material across the road
surface, by lifting the plate significantly you will
NOT be able to lay thick layers of this material as you
can with a hot mix bituminous material and a "Blaw
Knox" paver.
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However, if the road
surface has depressions or the transverse profile of the
road falls away from the crown thicker layers of the
material will be laid down and the road shape improved.
This process is not
thought of as adding any significant strength to a road
pavement, but like a surface dressing by sealing an
oxidised and fretting surface it will extend the life of
the road pavement.
The applied layer is capable of sealing minor, non structural cracking,
but in my opinion isolated serious cracking associated with road
pavement weakness should be rectified prior to applying the treatment.
The process should improve the skid resistance, ride quality and appearance of the
road.
The skid resistance of the finished surface will depend upon the
properties of the coarse aggregate used, and the ability of the mixture
to retain its surface texture. |
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EARLY APPEARANCE
Immediately after laying the slurry macadam will be brown
in colour, this indicates the emulsion component of the
mixture has not "broken".
After about 20 minutes in reasonable ambient temperatures
the material will have established an initial set and
will have turned black indicating the bitumen has come
out of the emulsion phase and deposited itself on the
aggregate components of the mixture.
The material should be safe to traffic after about one
hour, it could be sooner in good summer weather.
The larger nominal size materials can shed some loose
aggregate in the initial stages and look a little
"untidy" before they settle down.
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NEW
SURFACE APPEARANCE
It is not easy to obtain good
photographs of black road surfaces, but I hope this able to
demonstrate the intial appearance of a typical 6mm. slurry macadam
surface.
I hope you are able to
observe the significant texture, and also some "shedding" of
coarse aggregate not held in the matrix of the macadam.
In my experience the retention of the initial surface texture largely
depends on the depth of the applied layer.
It is not unknown for these types of material to lose texture where
the applied layer has been too thick in relation to the nominal
aggregate size. |
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THICKNESS
The photograph on the left shows the achieved thickness
of a 6mm. slurry macadam laid on a flat road surface with
a one pound coin as comparison.
The thickness achieved is generally regarded to be approximately one
and a half times the nominal aggregate size, and as nominal size of
the aggregate can vary so thickness of layer can vary.
As stated previously material will be laid thicker where there are
depressions. This is not always a good thing as deeper areas can lose
texture over time depending upon the nature of the trafficking. |