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The Idiots' Guide to Highways
SLURRY SEALING BY PICTURES
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.
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may lack some of the precise detail that the superseded copy contained, but it is an excellent document and I would suggest you obtain a copy as it contains a wealth of useful information relevant to the successful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I am not aware if BS 434-1:1984 has yet been superseded, even if it has, older documents such as this contain very useful information, even if you can no longer use them as a means of specifying.
So, 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.
|Mixing and Applying the Slurry Seal||Bitumen Emulsion and Retard Agent ("Dope")|
|Applying the Slurry||Finished Surface of a Typical Footway Slurry Seal Surface|
|Spreading the Slurry Evenly||Surface Finishing and Setting Time|
|Materials Required for Slurry Sealing||Poor Quality Slurry Sealing|
|THE PRINCIPLES OF SLURRY SEALING|
Mixing and Applying
the Slurry Seal
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.
|Applying the Slurry|
|Spreading the Slurry Evenly|
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.
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
The fact that this particular aggregate has a pink/red colour is purely coincidental and has no significance with regard to the characteristics of the aggregate.
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.
Finished Surface of a Typical Footway Slurry Seal
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".
For an explanation of the Footway Slurry Sealing process, Press -------> HERE
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.
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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