THE LAYING AND APPEARANCE OF 14MM. STONE MASTIC ASPHALT (SMA)

CONTENTS

Introduction
Mixing and transporting stone mastic asphalt
Laying stone mastic asphalt
Stone mastic asphalt design
Layer thickness of stone mastic asphalt
Finished surface of a 14mm. SMA
Saw-cut through a laid 14mm. SMA

14mm. Nominal Size 55% High Stone Content Asphalt

Personal Note

INTRODUCTION
It is important to note that there is now, and in fact has been for several years, a specification for generic stone mastic asphalt (SMA), this being,
BS EN 13108-5 : 2006:Bituminous mixtures. Material specifications -  Stone mastic asphalt 
This standard is one of the family of standards for bituminous mixtures introduced on the 1st. of January 2008.
This standard can be used to provide the particular Target Mixture Composition for the bituminous mixture that you require to provide the particular engineering characteristics that you need for the site in question.
You do need to make yourself familiar with this specification because bituminous mixtures of vastly different engineering properties can be designed/specified using BS EN 13108-5 and yet be supplied under the general description of stone mastic asphalt, this can cause confusion if you are not aware of it.
Personally I prefer to specify/use an SMA that has the engineering characteristics of the "original" low void German designed material.
If I wish to specify an "open"  bituminous mixture I will use a different standard to specify the material, such as,
BS EN 13108-7 : 2006:Bituminous mixtures. Material specifications -  Porous asphalt
 

	MIXING AND TRANSPORTING STONE MASTIC ASPHALT The mixing of SMA is performed in conventional mixing plants, with a preference to batch mixing plants as opposed to drum mixing plants. However because of the many variations in basic plant design some drum mixers are more able to mix bitumen rich mixtures than others. It is reasonable to note that in Germany, where SMA was developed, the material is primarily produced in batch mixers. Inadequate mixing effort may produce material of variable consistency which may not comply with the specification although feed to the plant is accurate.
	LAYING STONE MASTIC ASPHALT SMA is laid with a conventional paver without any problems. The ride quality of SMA is similar to a bituminous macadam (BS 4987), or a high stone content asphalt (BS 594) depending on the nominal aggregate size of any of these materials. Hand laying of SMA can leave an "untidy" surface appearance from raking what is a predominantly single sized material and is not to be recommended. This hand laying problem will be influenced by the grade (viscosity) of bitumen used in the SMA design, the stiffer the bitumen the greater the difficulty of hand laying.
	STONE MASTIC ASPHALT DESIGN The design of an SMA is critical in providing an aggregate grading that will accept the high bitumen content that provides durability without binder drainage. If this design is not correct it will result in binder drainage in the lorry bodies and fatted areas of the surfacing. Conversely an aggregate design that requires a lower binder content to prevent binder drainage will result in a bituminous mixture that will be less durable and have a reduced life.
	LAYER THICKNESS OF STONE MASTIC ASPHALT SMA was developed to be laid at a conventional thickness, here it is being laid as a nominal 45mm. layer. SMA laid at this thickness will add strength to the road pavement. A conventional SMA surfacing course, with its high binder content, will also be impervious to the ingress of water into the road pavement. It is necessary to be cautious when Thin Surfacing materials are offered to you as an SMA as these materials may not possess the basic design concept of an SMA, which is that it shall be a binder rich mixture with a load carrying aggregate matrix and bitumen filled voids. Note the good coating of tack /bond coat on the existing surface.
	FINISHED SURFACE OF A 14MM. SMA This photograph shows the surface characteristics of this material as it emerges from behind the paver and before rolling, so the texture shown here will reduce an amount after rolling. This material however will have significant "negative" texture, probably above the 1.5mm. (by sand patch) required by the Highways Agency on motorway and trunk roads. IMPORTANT It is necessary to read,  Clause 921-Surface Macrotexture of Bituminous Surface Courses, of Interim Advice Note 101/07,  as there have been changes to initial texture depth requirements for Thin Surfacing Systems. This may, or may not refer, to generic SMA. Read the Summer 2008 Newsletter for further information.
SAW-CUT THROUGH A LAID 14MM. SMA Below is what I believe a "genuine" stone mastic asphalt should look like, a load bearing aggregate matrix, with not a high amount of fine aggregate filling the voids, but the voids filled with binder to create a binder rich, impervious, durable bituminous mixture. But there are some aggregates fines (grit) in the "mastic" which provides initial micro texture to the road surface, providing that you use fines from an aggregate source that has good engineering properties with regard to the polished stone value (PSV) and aggregate abrasion value (AAV) The scale is 1cm. squares, so you will see this was laid almost 50mm. thick, ( although the nominal thickness was specified at 45mm ), however with a 50pen. binder there have been no problems with deformation after three years. With a correctly designed SMA, sufficient surface texture is achieved without the need for a porous matrix.

Further information and pictures relating to stone mastic asphalt can be accessed by pressing ------------------------> HERE

14mm. NOMINAL SIZE 55% HIGH STONE CONTENT ASPHALT Note - I thought out of interest and to increase your knowledge on bituminous mixtures this would be a good opportunity to include a photograph of a core from a, 14mm. / 55% High Stone Content Asphalt (HSCA), as specified in the superseded BS 594, and can still be specified by referring to,  BS EN 13108-4   : 2006:Bituminous mixtures. Material specifications -  Hot rolled asphalt You may find the similarity of the profile between a 14mm. SMA  and a 14mm. HSCA interesting. In a HSCA the voids between the larger aggregate is a fines (usually natural sand), ground limestone, and bitumen mixture. A correctly designed material has a relatively high binder content, is impervious to water, durable, and will have a high stability. However surface texture with this material does not usually achieve the 1.5mm. (sand patch) required in many locations, but it usually achieves around a 0.8mm. which with an aggregate of suitable polished stone value (PSV) will be adequate for low speed areas.  Surface texture does improve with trafficking as fines surrounding the larger aggregate is worn away by vehicle tyres.

Consult superseded BS 594 : Part 1, for further details, and then BS EN 13108-4 to produce a similar Target Micture Composition, if you wish to specify this bituminous mixture. Or, select a "sample" bituminous mixture from, P.D. 6691:2007:Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications, I must explain that the thickness of layer in this case was due to also using the surface (wearing) course material as a regulating course. This practice is not unusual when using HSCA, as the material does have a high stability especially when using a 50pen. bitumen as the binder.  So, in fact there is a thicker basecourse (binder course) below a conventional (50mm. for this material) wearing course (surface course) layer. TIP - Using one good "universal" bituminous mixture is often a practical solution to difficult on site decision making when working in sensitive areas with road closures.  This is usually in busy city/town areas where you are not always sure what you are going to find when you start planing out, and you only have a limited time to complete the work.

Further information on the appearance of road surfaces from differing bituminous mixtures can be seen, -----> HERE

Personal Note

I feel it is necessary for me to suggest to you that when the words "SMA" are used to describe a material, especially if "SMA" and "Clause 942 - Thin Surface Course" are mentioned in the same breath, that you need to be cautious.
And some of you will not even be aware that there are now SMA bituminous mixtures for use as binder course and Regulating course.
You may, or may not, (perhaps not) seek some insight into the confusion over what is, or what is not, SMA by studying, 
"Clause 937 - Stone Mastic Asphalt (SMA) Binder Course and Regulating Course", 
which is to found in the recently revised MCHW Specification 900 Series, August 2008 amendment.

You have to be aware that any one of these 16 "described" generic bituminous mixtures will not be the same composition from all suppliers unless you specify the Target Mixture Composition that you require for the bituminous mixture supplied to you, although you can defer to BSI PD 6691 : Annex D, which should give some amount of conformity.
You also need to specify the engineering requirements of the aggregate in the mixture.
The lack of knowledge as to what actual attributes a bituminous mixture needs to possess to be correctly described as a stone mastic asphalt is now not only causing confusion in marketing some proprietary bituminous mixtures, but also generic mixtures specified using BS EN 13108-5 : 2006:Bituminous mixtures. Material specifications -  Stone mastic asphalt .
The incorrect use of terminology may be due to a lack of knowledge, or sad to say it could be being used by the "marketing boys" to gain an advantage over competitors when advertising the proprietary products for which they have been asked to create marketing material.
I know I am not alone amongst engineers and engineering technicians who know a little bit about bituminous materials, that the content and information provided in some modern bituminous material advertising leaves us less than impressed.

I sincerely believe that some of you reading this page really do need guidance from an experienced and up to date Materials Engineer/Road Pavement Engineer if you are to be prevented from making inappropriate engineering decisions that will prove costly in one form or another.
The advice I will suggest is that if you have been using bituminous mixtures that have proven themselves on the highway network that you maintain, remain with them by specifying the necessary Target Mixture Compositions, using the appropriate BS EN 13108 standard.
Failing this course of action you may choose to specify the "sample" bituminous mixtures quoted in, 
P.D. 6691:2007:Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications.

However I would prefer that you choose my first suggestion, if you regard yourselves as practical highway maintenance Engineers or Engineering Technicians.
If the "suits" are making the decisions then you should not to be blamed for failures, but you will be !

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