CARRIAGEWAY SLURRY MACADAM / MICRO ASPHALT

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most commonly supplied as a proprietary product, although it can be produced in smaller quantities as described in the superseded,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 if you can still obtain a copy.

BS 434:Part 2:2006 has superseded the 1984 edition and it also is an excellent document and I would suggest that you obtain a copy as it contains a wealth of useful information on many highway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 a more "robust" surface on difficult sites and may be appropriate in "sensitive" urban areas.
However this process does produce 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 vehicle is purpose built to include a large amount of storage for all components needed to mix the slurry macadam so that significant amounts of work can be completed between re-stocking of the hoppers and holding tanks.
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. 
The final adjustments to ensure a "smooth" ride quality to the road surface will be done done by operators following the screed plate.

The pictures below should explain the process quite well.
 

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.

The slurry cannot be premixed as the "re-action" between the aggregate and the bitumen emulsion begins as soon as the two components are brought together, and there is then a limited amount of time before the initial "set" is complete and no further work can be performed on the laid slurry macadam / micro asphalt.

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 an 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.
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 it acts like a surface dressing and 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, however 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.

EARLY APPEARANCE
Immediately after laying the slurry macadam will be brown in colour, this indicates the emulsion component of the mixture has not "broken", i.e. the bitumen droplets are still in suspension in the water medium, when the the emulsion has "broken" the bitumen droplets will have bonded with the aggregate surface and created a bitumen coating around the aggregate.

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.

NEW SURFACE APPEARANCE
It is not easy to obtain good photographs of black road surfaces, but I hope this able to demonstrate the initial 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.

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 the nominal size of the aggregate can vary so the thickness of the 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.

SPECIFYING SLURRY SURFACING AND MICROSURFACING

With the exception of some slurry seal for footways, most slurry surfacing and micro-surfacing is procured/purchased as a branded product, and the document you will need to specify the performance characteristics of the finished product is,
BS EN 12273 : Slurry surfacing : Requirements

To provide guidance on the use of this specification, the Published Document, 
PD 6689 : 2009: Surface treatments - Guidance on the use of BS EN 12271 AND BS EN 12273
has been published to help clarify the implementation of BS EN 12273.

Other British Standards relating to the testing of Slurry Surfacing are listed below,

BS EN 12274-1 : 2002 : Slurry surfacing - Test methods - Part 1 : Sampling for binder extraction
BS EN 12274-2 : 2003 : Slurry surfacing - Test methods - Part 2 : Determination of residual binder content
BS EN 12274-3 : 2002 : Slurry surfacing - Test methods - Part 3 : Consistency
BS EN 12274-4 : 2003 : Slurry surfacing - Test methods - Part 4 : Determination of cohesion of the mix
BS EN 12274-5 : 2003 : Slurry surfacing - Test methods - Part 5 : Determination of wearing
BS EN 12274-6 : 2002 : Slurry surfacing - Test methods - Part 6 : Rate of Application
BS EN 12274-7 : 2005 : Slurry surfacing - Test methods - Part 7 : Shaking abrasion test
BS EN 12274-8 : 2005 : Slurry surfacing - Test methods : Part 8 : Visual assessment of defects

BS EN 13808 : 2005 : Bitumen and bituminous binders - Framework for specifying cationic bitumen emulsions (superseded BS 434:Part 1)
This document specifies the requirements for performance characteristics of cationic bitumen emulsion classes which are suitable for use in the construction and maintenance of roads, airfields and other paved areas.
This document applies to emulsions of pure bitumen, or of fluxed bitumen, or of cutback bitumen and to emulsions of polymer modified bitumen, or of polymer modified fluxed bitumen, or of polymer modified cut-back bitumen, which also includes latex modified bituminous emulsions.


NOTE : The introduction of PD 6689 is discussed in the Summer 2009 Newsletter.

A great deal of advice and guidance on the various uses of Bitumen Emulsion and the materials it can produce, can be obtained from :- 
The Road Emulsion Association Limited, including a large number of "DataSheets".

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