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The
Idiots' Guide to Highways Maintenance |
FOOTWAY
SLURRY SEAL
BUT, BE AWARE
You are responsible for making the decisions !
CONTENTS
SLURRY SEAL DESCRIPTION
COMPONENTS OF A SLURRY SEAL
CATIONIC EMULSIONS (The most
widely used emulsion type for footway slurry sealing)
CATIONIC SLURRY SEAL SYSTEMS
PRACTICAL CATIONIC FOOTPATH
SLURRY SEAL PRODUCTION
WORKMANSHIP AND EXPERIENCE
SPECIFICATIONS
ONE FINAL, MOST IMPORTANT,
POINT
SLURRY SEAL DESCRIPTION
This information sheet will supply you with some basic facts
about the slurry seal process, there seems to be a marked lack of
understanding/knowledge on this process and it is regarded as
something of a "black art'' instead of a normal highways
maintenance function.
Slurry seal is a mixture of a fine hard aggregate usually a 3mm.
"down'' well graded material, which is coated with bitumen
emulsion in a cold mix process, usually a wheeled cement mixer
for footpaths.
On mixing, the slurry will begin to "break" quite
quickly, i.e. the bitumen part of the emulsion will begin to coat
the aggregate and the slurry will solidify, the water fraction
becoming detached from the bitumen and eventually evaporating.
Application to the footpath surface and the brush texturing needs
to happen before the slurry has achieved its initial
"break", and the complete "break" or full
"walk on" stability of the material should take place
in about one hour.
Slurry seal will not add any strength to a footpath/carriageway,
but is an excellent preventative maintenance process that will
seal a surface against the ingress of water, prevent further
disintegration of the existing surface and add a texture to a
footway that has become
smooth.
The life of a sound existing footpath surface can be extended by
many years if slurry sealing takes place before serious surface
deterioration sets in, slurry seal will not repair footways that
have failed structurally, a "retread" process is more
suitable.
Although some proprietary slurry seal systems claim to be able to
be laid quite thickly, it is unlikely that a single application
of a conventional slurry seal will on average provide a laid
thickness greater than 3mm.
For pictures of Footway Slurry Sealing, Press -------------> HERE
COMPONENTS OF A SLURRY SEAL
1.
Fine Aggregate - to
specific grading requirements, in the U.K. the most usual grading
is that given in,
B.S.
434:Part 2:Table 8.
This is a 3mm. "down'' well graded aggregate.
But a 5mm. "down" aggregate is sometimes used to be
able to lay a greater thickness of slurry seal which will be
better able to fill depressions.
The aggregate is often referred to as "dust", but this
is an inaccurate description.
2.
Emulsion - normally
60% bitumen content emulsion supplied in bulk or drums; but for
some slurries where the emulsion has a 65% bitumen content the
emulsion is usually supplied in drums only.
Bitumen emulsion means a bitumen of a suitable base viscosity is
held in suspension as very fine bitumen droplets in a water /
emulsifier solution.
The very fine droplets are formed by subjecting the bitumen and
emulsifier solution to intense physical action in a mixer,
causing the bitumen to separate into droplets, i.e. emulsify.
In the process the bitumen droplets will receive a surface
electrical charge, either positive, CATIONIC, or negative,
ANIONIC according to what type of emulsifier has been used.
The electrical charge surrounding each droplet, be it positive or
negative, causes each droplet to remain separate, i.e. remain in
an emulsified state, this is because like charges repel each
other, similar to the way a "north pole'' of a magnet will
repel the "north pole'' of another magnet.
Be careful with the naming of the types of emulsion, although
having a positive charge, cationic emulsion is called cationic
because it means cathode seeking, i.e. it is looking for a
negative charge to satisfy its own positive charge.
The same principle applies to anionic emulsions, i.e anionic
emulsion droplets are anode seeking.
3.
Pre-wet Water - water
added to the aggregate used to make a slurry of the aggregate
before addition of the emulsion.
This must be clean water, impurities, e.g. clay, can accelerate
the "break", and perhaps discolour the slurry.
4.
Filler - usually
PORTLAND CEMENT - used to adjust aggregate grading and provide
fines. This will increase the final stiffness of the laid slurry
seal.
With cationic slurries it normally has the effect of reducing
slightly the amount of retard chemical needed, since the
positively charged calcium ions arising from the Portland Cement
also act as a retarding agent.
5. Stabiliser Chemical / Retard Agent / Dope - for cationic slurries.
This is usually a dilute solution of the emulsifier used in the
preparation of the emulsion. The action of the retard agent being
to supply extra positive ions to help satisfy the
electronegativity of the aggregate surface, and delay the
"break" of the slurry.
CATIONIC EMULSIONS (The most
widely used emulsion type for footway slurry sealing)
Based, usually, on a K-3 emulsion.
( See BS 434:Part 1 for further information on emulsion types
and properties. )
Since most aggregates have a surface negative charge, (i.e. they
are electronegative), the cationic emulsion "breaks" on
to the aggregate surface.
Differing aggregate sources will have differing degrees of
electronegativity, change the source and you change the natural
speed of the "break" of the slurry, which depends on
the degree of electronegativity of the aggregate and the ambient
temperature.
Some aggregate sources will not be suitable for cationic slurry
seal production.
The "break" of the emulsion can be retarded by the
addition of a cationic retarding agent, (sometimes called a
control additive, or more often "dope"), to the mix.
The amount of retard agent needed for a particular
emulsion/aggregate system can be determined in the laboratory by
means of simple mixing tests and the results can be translated
through to amounts for practical working.
The amount of retard agent may need to be altered slightly, all
other components remaining the same, with changes in weather and
ambient temperature, as slurry seal treatments can take place in
the range of temperature above 5°C.
CATIONIC SLURRY SEAL SYSTEMS
With the cationic slurry seal system a cationic bitumen emulsion
is mixed with a fine aggregate to produce a slurry.
The "break" of the emulsion is due to the electrical
neutralisation of the positively charged, cationic, emulsion
droplets by the negatively charged surface of the aggregate.
Because the slurry seal aggregate is finely divided it will have
a lot of surface area and so be highly electronegative.
The fineness of aggregate is sufficient to cause the emulsion to
"break" almost instantaneously on addition to the
aggregate.
The speed of the "break" can be retarded by the
addition of extra cationic chemical, (dope / retard agent /
emulsifier, whatever you wish to call it), either to the emulsion
or to the aggregate before the two are mixed.
In the design of a slurry seal an amount of retard agent will be
found that will give sufficient time after the addition of the
emulsion to allow complete mixing of the slurry and discharge /
laying on the footpath before "breaking" occurs.
However the mixing/laying process should be completed quickly and
initial emulsion "break" should occur immediately after
laying, complete "break" of the material, i.e. fit to
walk upon, may take up to an hour, or even longer depending upon
ambient temperature.
All other materials and proportions staying the same, increasing
the amount of retard agent will delay the "break".
All other materials and proportions staying the same, decreasing
the amount of retard agent will speed up the "break".
PRACTICAL CATIONIC FOOTPATH
SLURRY SEAL PRODUCTION
Assuming all components of the slurry are of suitable and
consistent quality it is first necessary to undertake trials to
produce the correct mix proportions for the slurry.
Once the mix has been correctly formulated it is most important
the materials and proportions remain constant.
The production of a consistent slurry is usually achieved by
having measured containers for each ingredient, aggregate,
cement, pre-wet water, emulsion and retard additive.
These containers should not become altered in any way either
through damage or build up of material.
If they are altered in any way they must be checked against
original measurements.
It is also important that each batch of slurry shall be mixed in
the same manner.
With cationic slurries this usually means aggregate first, cement
second, then the pre-wet water to produce a slurry paste then the
retard additive and lastly add the bitumen emulsion to produce
the final slurry seal.
Once the bitumen emulsion is added only sufficient mixing time is
needed to coat the aggregate before it is discharged on to the
footway and spread.
The initial "break" of the emulsion should be fairly
rapid, but allow enough time for the next batch of slurry to be
produced before the "wet edge'' is lost.
This is so that the work will look continuous and not a series of
individual mixes.
When a break in work occurs a straight joint is made using a thin
wooden strip or tape.
For your interest only, and to give you an idea of mix proportions
this is just one example of a typical design for a slurry seal
using a particular granite dust and a particular K3-60 cationic
emulsion, these proportions will not necessarily work for you :-
Water - 4 litres
Retard Agent, (Dope) - 75 grams
Aggregate, (Dust) - 30 kilograms
Cement - 300 grams
K3-60, (Cationic Emulsion) - 4.5 litres
WORKMANSHIP AND EXPERIENCE
Having spent a great deal of time giving technical details and
explanations on the materials and processes involved I cannot
stress too highly the benefit of having experienced operatives
doing this work.
Experienced gangs have developed over the years a knowledge of
how to control a slurry without understanding the chemistry.
The men involved have developed an "eye" for looking at
a slurry and wealth of practical knowledge in seeing something
that is wrong and knowing how to put it right.
And if you are setting up a new gang you must pick people who are
prepared to pay attention to detail, constantly.
SPECIFICATIONS
BS
434: Part 1: Specification for Bitumen Road Emulsions:-
This part specifies and describes the properties of the various
types of bitumen emulsions and the various tests to determine
these properties.
BS
434: Part 2: Code of practice for use of road bitumen emulsions:-
This part describes the uses of bitumen emulsions, and specifies
/ recommends the way in which they will be used.
This includes a section on slurry seals.
BS 3690: Part 1: Specification for bitumens for road purposes:-
In the process of producing a particular bitumen emulsion the
manufacturer will have emulsified a certain grade of bitumen as
specified in this document.
It is useful to know the properties of the bitumen used, as this
will ultimately have a great influence on the performance of the
laid slurry seal.
ONE FINAL, MOST IMPORTANT,
POINT
A 3mm./5mm. nominal size slurry seal treatment should be
3mm./5mm. thick, on average.
There are slurry seal gangs about who "pull" the slurry
as thinly as it is possible so to do, to obtain more square
metres to the individual batch of slurry and so increase their
productivity and area of work completed in a day.
This practice usually results in a poor quality finish, and of
course the customer does not receive the thickness of material
specified in the contract.
Be aware of this, and ensure you have adequate and knowledgeable
supervision on site to obtain the thickness you are paying for !
I
try to refer to as few commercial sites as possible in compiling my site, but
when a site offers particularly useful information about a subject I make an
exception.
For further information regarding bitumen emulsion and slurry seal production,
press ---> HERE
and HERE
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