FOOTWAY SLURRY SEAL

BUT, BE AWARE

This is only a GUIDE

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

[ Top of Page ]