STONE MASTIC ASPHALT (SMA),  A STUDY IN PICTURES

The following are pictures of materials I have had practical experience with, they may or may not reflect the type or quality of products other engineers have worked with. 
So, the comments I make are in relation to these materials, and these photographs, and you may or may not find these comments useful in relation to products that you have purchased or propose to purchase.
I will continue to urge that you employ the services of an experienced Materials Engineer/Technician or Highways Pavement Engineer/Technician in assisting in the choice of bituminous mixtures you propose to use on the highways you are responsible for building or maintaining.


The information set out on the pages of this website is provided as a guide, and a guide only, you, who ever you are, where ever you are, are the ones responsible for the decisions ! ! !

IMPORTANT - From the 1st. of January 2008, the BS EN 13108:Bituminous mixtures -Material specifications, family of standards has been introduced for specifying bituminous mixtures in the UK.
Amongst these standards is,
BS EN 13108-5:Bituminous mixtures -Material specifications - Part 5:Stone Mastic Asphalt

It is now possible to precisely specify the nature of the SMA mixture that you require, with the parameters of nominal size, grading, binder content, void content, etc., being able to be stated.
Personally, I have to say that I am not sure that I would regard all the possible mixtures as "classic" stone mastic asphalt, but that is my opinion.
But you will be able to determine the nature/characteristics of the SMA that you wish to receive.
If the mixture that you receive does not present the characteristics that you required then the cause is with you, subject of course to the supplied material meeting the specification of your defined target mixture.
The stone mastic asphalt specifications defined using BS EN 13108:Part 5 are generic specifications and as such will not be permitted as surface course on motorways and trunk roads. They can, and I hope that they will be, used on local authority highway networks when you feel that you require the use of an SMA surface course on a particular site. 
If you wish an SMA type material for use on motorways and trunk roads these must still be supplied as a proprietary "Thin Surfacings" to comply with Highways Agency requirements.

If you are not yet aware of the introduction this new standard then you have some serious studying to commence, 
as you also have for BS EN 13108:Part 1 - Asphalt Concrete, and BS EN 13108:Part 4:Hot Rolled Asphalt.
Further information on the introduction of this range of standards for bituminous mixtures can be found in the
Summer Edition 2007 of my Newsletters.

CONTENTS

PICTURES OF A TYPICAL NEWLY LAID STONE MASTIC ASPHALT
CORE FROM A TYPICAL STONE MASTIC ASPHALT
STONE MASTIC ASPHALT SURFACE AFTER REMOVAL OF INITIAL SURFACE "MASTIC"
FRESHLY LAID STONE MASTIC ASPHALT
SAWN STONE MASTIC ASPHALT CUT-OUT, SHOWING SURFACE TEXTURE
INCREASING SURFACE TEXTURE BY LAYING THINNER SURFACE COURSE LAYERS

PICTURES OF A TYPICAL NEWLY LAID STONE MASTIC ASPHALT
	The low winter sunshine highlights the significant surface texture of this newly laid 10mm. SMA. This type of texture is often referred to as "negative" texture others do not like the use of the term.

A close-up of the surface showing that even though there is a "flat top" to the bituminous rich surface there are smaller particles of aggregate in the bituminous coating. These particles will be the passing 6.3mm. retained 2.36mm. fraction of the mixture, and also aggregate passing the 2.36mm. retained 75micron. It is the opinion of most materials engineers that all the aggregate in the mixture must be of  a high, polished stone value, (PSV), not just the coarser fraction of the mixture. It is my belief that the early life skid resistance of an SMA will be related to the quality of this finer aggregate contained in the binder coating of the larger aggregate.

It will be sometime before the larger aggregate particles will have their surface layer of binder removed by traffic, and it will not be until this time that the engineering characteristics of this aggregate will take effect. Having high PSV aggregate fines in the mixture will certainly be no detriment to the early life skid resistance of SMA, and is likely to be beneficial.

CORE FROM A TYPICAL STONE MASTIC ASPHALT

I hope this photograph is of sufficient quality so that you can actually see then size fraction of material I refer to in the last paragraph. Also in most 10mm. and 14mm. designs I am familiar with approx. 25% will pass a 2.36mm. sieve/screen, of which approximately 10% will be filler, i.e. passing a 75 micron sieve. The 15% which is aggregate will impart a very gritty nature to the aggregate binder covering. The filler should be specified as ground limestone, and therefore should be limestone and not predominantly quarry dust.

STONE MASTIC ASPHALT SURFACE AFTER REMOVAL OF INITIAL SURFACE "MASTIC"
It is interesting to note that with the wear from passing vehicles the rich binder coating, that gives the flat plane effect in the early life of an SMA, is worn away and the surface texture appears to achieve the characteristics of a "positive" textured surface.  How quickly this change in surface characteristics takes is more dependent on trafficking than time, so the two years mentioned in the photograph is a statement about that particular site, and not all sites. I believe you need to be aware of this change in nature of the surface qualities, because this is the nature of the material surface for the bulk of its life, and indeed is the type of surface that you require to produce good skid resistance.
You also need to be aware that it is the nature of the original surface that is claimed to achieve a reduction in tyre generated noise. But the significant change in the type of surface texture over the first few years is also likely to bring about a change in the volume of tyre generated noise from that surface. There are test vehicles, such as TRITON, at the TRL that can accurately record tyre generated noise from road surfaces, it may be wise to employ it before totally committing to a surface programme that may not deliver the results that you require.

FRESHLY LAID STONE MASTIC ASPHALT
	You will note that a correctly designed stone mastic asphalt will present an adequate initial surface texture, that will improve with trafficking, and have a binder content that will ensure long term durability of the surfacing.

	A correctly designed stone mastic asphalt can be laid at a "normal" thickness and still present adequate initial surface texture which will improve with trafficking. Laying at "normal" thickness will provide increased load spreading properties to your existing road pavement, which will not be provided by thin surface course layers.  Used in this manner you provide a new surface course and increased road pavement strength. However this may be an expensive, and wasteful use of high specification aggregate, if you are using a high polished stone value (PSV) aggregate to present to traffic tyres because of the nature of the site you are surfacing.
SAWN STONE MASTIC ASPHALT CUT-OUT, SHOWING SURFACE TEXTURE
	This section was recovered from a "stihl" saw cut joint of the laid mat. It shows the predominantly single sized nature of the mix design, with the voids completely filled with binder and the load spreading nature of the interlocking aggregate particles. But it also shows the smaller aggregate particles that are a part of the mixture and they will be an integral part of the binder coating on the larger aggregate at the road surface.

This is a photograph of the surface of the slab of material of which the photograph above is a cross section. And although the texture has become filled with the fine dust from the saw cutting process you can see there is considerable surface texture. It is possible to see the the finer aggregate in the binder coating preventing a flat, pure binder surface.

INCREASING SURFACE TEXTURE BY LAYING THINNER SURFACE COURSE LAYERS

The thinner the bituminous mixture is laid in relation to its nominal size, i.e. 14mm. in this instance, the greater the surface texture you will create. This is because the aggregate particles do not have the depth of material in which to move around, whilst the material is hot and plastic, during the compaction process. This is one method of achieving a porous matrix to a SMA surface course, a condition that was never intended during their original development on the continent.  This is a practice common in England to achieve surfaces that meet the requirement for a 1.5mm. (by sand patch) texture depth, but should these materials still be called SMA's.```

Useful information relating to stone mastic asphalt (SMA) can be accessed by pressing  -------------------------> HERE

Further useful information relating to SMA and other surfacing materials can be accessed by pressing --> HERE

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