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The Idiots' Guide to Highways
Maintenance |
FOOTWAY
SLURRY SEALING BY PICTURES
INTRODUCTION
Please do not be mislead by what may appear to be a "rough and ready
process", IT IS NOT !
Considerable attention to detail is needed to produce a good slurry seal
surface, and a great deal of knowledge and experience is needed to make those
slight adjustments for things such as weather conditions, ambient temperature
and slight differences in the moisture content of the dust.
In my experience good footway slurry seal surfacing is more "gang"
dependent than company dependent, by which I mean just because you have good
work from one company one year you will not necessarily receive the same
standard of work the following year..
But I have found that if you have the same gang you had the year before it is
likely you will obtain the same standard of work, and the good gangs are usually
"known" and in demand.
This process, performed correctly, is an excellent cost effective surfacing. It
is regarded as a preventative maintenance process which will prolong the life of
a footway that is in sound structural condition, it is not able to strengthen a
footway that has failed structurally.
As always I am going to recommend strong site supervision to ensure good
quality work.
Slurry seal and its production is/was described in
BS 434-1 : 1984 : Bitumen road
emulsions (anionic and cationic)
Part 1 : Specification for
bitumen road emulsions
This part specifies the properties of various types of emulsion and the
various types of test to determine these properties.
and
BS 434-2 :
1984 : Bitumen road emulsions (anionic and cationic)
Part 2 : Code of practice for
use of bitumen road emulsions
This former edition described the uses of bitumen emulsions, and
specified/recommended the way in which they where to be used. e.g. surface
dressing, emulsion macadams, slurry sealing, etc..
There has recently been issued a new edition of BS 434:Part 2,
this new edition,
BS 434-2 : 2006 : Code of Practice
for the use of cationic bitumen road emulsions on roads and other paved areas
may lack some of the precise detail that the superseded copy contained, but it is an excellent document and I would suggest you obtain a
copy as it contains a
wealth of useful information relevant to the successful use of bitumen emulsion
in many highways maintenance applications.
I am not aware if BS
434-1:1984 has yet been
superseded.
I would also suggest you retain your "old" standards, as a
reference documents, particularly as a guide to the production of slurry seal
mixtures, should you wish to produce small amounts yourself, although suppliers
of the bitumen emulsions are usually prepared to help in the design of appropriate
slurry seals and slurry macadams if you will be buying reasonable amounts of
their emulsion.
CONTENTS
Mixing
and Applying the Slurry Seal
Materials
Required for Slurry Sealing
Bitumen
Emulsion and Retard Agent ("Dope")
Applying
the Slurry
Spreading
the Slurry Evenly
Finished
Surface of a Typical Footway Slurry Seal Surface
Surface
Finishing and Setting Time
Poor
Quality Slurry Sealing
THE PRINCIPLES OF SLURRY SEALING
Mixing and Applying
the Slurry Seal
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The aggregate, pre-wet water, cement, dope and lastly the bitumen
emulsion have all been added to the mixer and mixed until there
is a thorough mixing of components before discharging onto the
footway for screeding.
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Materials required
for Slurry Sealing Lorry showing the materials needed to carry out footway slurry sealing. This will include the coarse aggregate ("dust"), the bitumen emulsion, the retard agent ("dope"), and the cement. Please note all the various size containers for the accurate proportioning of the slurry. |
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| The electro-negativity of the aggregate source is very important in the process of slurry sealing using cationic bitumen emulsion, you must be aware of this characteristic and understand that it is not just a question of buying "dust" that has the correct grading, the surface of the aggregate particles must also exhibit a significant negative charge. |
| Bitumen
Emulsion and Retard Agent ("Dope") Many companies produce excellent bitumen emulsions and slurry seal additives, these photographs show the particular products used on this site and are not meant to be an endorsement. However some contractors believe that some products work better with particular aggregate sources than others, I am not aware of any evidence to support this, but I am prepared to listen to their counsel if the quality of work supports their view. |
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Finished Surface of a Typical Footway Slurry Seal
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Surface Finishing
and Setting Time
After initial screeding to distribute the slurry evenly over the
footway it is "brushed" to establish a uniform
thickness and appearance to the final surface.
At a reasonable ambient temperature (ten degrees centigrade) it
will be approximately one hour before it is safe to walk on, with
care.
It will be several hours before it is fully "set".
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For
an explanation of the Footway Slurry Sealing process, Press
-------> HERE
POOR
QUALITY SURRY SEALING
I
include this section as much to support those contractors who carry out good
work as to encourage others to improve, and to give guidance to those who are responsible
for overseeing this type of footway refurbishment.
Slurry sealing is an excellent, cost effective, preventative maintenance process
for the highways maintenance engineer to employ where footways are structurally
sound but the surface is beginning to fret through bitumen oxidation and general
wear and tear.
I would not want to see the process lost because of occasional poor quality.
Below are pictures of how slurry
sealing should NOT be carried out.
You will note that, for whatever reason, the coarser component of the slurry
seal aggregate has been "pulled out" / segregated from the matrix of
the slurry seal layer, leaving a rough none uniform surface.
This segregated material is likely detach from the surface leaving loose
aggregate on the footway, and you will note some areas that have little if any
coarser aggregate cover.
Possible causes of low quality surfacing such as this are :-
1) The aggregate did not meet the specification, in that it had a proportion of
large, above the specification limit, aggregate
2) The aggregate was poorly graded, in that it had a larger proportion of the
permissible larger aggregate sizes than the specification allows.
3) Operatives trying to achieve a larger coverage from a given amount of slurry
than a particular formulation will allow without segregation. The slurry should
be laid at a thickness that allows the coarser aggregate component to be accommodated
in the thickness of the layer to produce a uniform appearance to the surface.
If you require a satisfactory thinner layer it is necessary to use a different
aggregate grading.
4) Inexperienced operatives who have not received correct training.
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A great deal of advice and guidance
on the various uses of Bitumen Emulsion, can be obtained from :-
The
Road Emulsion Association Limited,
including a large number of
"DataSheets".
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