THE LAYING AND APPEARANCE OF 14MM. STONE MASTIC ASPHALT (SMA) - A CASE STUDY

AN INTRODUCTION TO STONE MASTIC ASPHALT
"The aggregate grading is similar to that of Porous Asphalt, but with the voids filled with mortar."

"The process of designing a SMA mixture involves adjusting the grading to accommodate the required binder and void content rather than the more familiar process of adjusting the binder content to suit the aggregate grading."

"A very high binder content is essential to ensure durability and laying characteristics".

The above quotes are from,
TRL Report 314:Road trials of Stone Mastic Asphalt and other thin surfacings by J CNicholls,
this is a very informative report and well worth reading.
TRL Project Report 65:Evaluation of stone mastic asphalt (SMA):A high stability wearing course material, by M E Nunn,
this report is also excellent reading on the subject of SMA, especially if you are requiring information on its specification.

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 generic 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, (download the .pdf file above for further guidance).
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

The case study I describe below is of one particular generic 14mm. nominal size stone mastic bituminous mixture, that was supplied to a specification included in the contract document. The specification was very similar to that contained in this .pdf file, which you can download and read. It was not complicated and stated quite simply what is required of the supplied bituminous mixture, how it shall be laid, and the characteristics of the laid material. The laying at this site was carried out on a "brisk" bright morning early in February 2001, hence the amounts of steam you can see coming of the tipped material, which was supplied by a quarry just a few miles from the site. The later photographs were taken in March of this year, 2010, so the stone mastic asphalt surface course is now nine years old. The island is a busy island, but does not take a large proportion of heavy commercial traffic. It is the distribution route to and from two large local villages, with associated large housing estates, and this island provides access to/from the city of Leicester, the A50, and the A46 trunk road. You will note that the general surfacing is in good condition with a now "positive texture" established to the surface where the trafficking of vehicle tyres has removed most of the surface mastic in the most trafficked areas. I have tried to tell readers, many times, that there is no such thing as one stone mastic asphalt, one hot rolled asphalt and precoats, one porous asphalt, one close grade macadam, etc. etc., there are just two many variables involved in the process. Such as :- choice of bitumen, choice of aggregate, choice of sand, choice of filler, storage time of the bitumen, type of mixing plant, mixing temperatures, storage time of the mixed material, thickness of laying, workmanship of laying, etc. etc., and of course the competency of the people that run the bituminous mixture production plants. And this can all apply to the same particular specification, if you change some aspect of the specification, i.e. the binder content and/ or aggregate grading and you change everything again. Perhaps you will begin to understand why I keep saying that any reasonably large organisation involved in purchasing and laying bituminous material must have access to a knowledgeable and experienced Materials Engineer. I can understand why "managers" who are not knowledgeable and experienced engineers or technicians choose "supercallafragalisticmastic" with a "guarantee". They somehow think that they are no longer responsible for the quality of the material. This is not my view, if they are being paid good money to make decisions on what is used on "their" site then the correct choice of the bituminous mixture for the surface course is their responsibility. If somebody "higher up" makes a "management decision" to purchase such products, taking the Engineer's choice away from him, then in my opinion it is the "management's" responsibility. It may possibly even be "Governments" responsibility if the Design Manual for Roads and Bridges states what you can and cannot use as your surface course bituminous mixture, and that is with a capital "G". I say again, this is one individual case study that is proving successful except for some lengths of joint in some areas. The success of the actual bituminous mixture, I am pleased to say is indicative of the majority of the 14mm. stone mastic asphalt laid in the authority where I worked. But I will say that the generic 10mm. stone mastic asphalts have proved far more durable, but did provide less surface texture. I cannot show you pictures of the failed 14mm. stone mastic asphalt sites that had low binder contents, I am afraid they failed early in their life and had to be replaced, although some were "rescued" by early application of 6mm. surface dressings that proved remarkably effective. (Now I could show you photographs of that, before and after, perhaps another time.) I leave you with my usual proviso, although I am trying to provide genuine information on bituminous mixtures, in this case stone mastic asphalt, so that you can make up your own mind on which bituminous mixture you wish to purchase. My preferred bituminous mixture for sites such as this is hot rolled asphalt surface course and the appropriate PSV precoated chippings. In my opinion HRA and precoats is a more durable bituminous mixture and makes far more effective use of scarce, and expensive, high PSV aggregate Do not forget that with generic SMA, and/or any proprietary Thin Surface Course System (TSCS) material the whole of the aggregate in the mixture will need to be of a high PSV aggregate to ensure that you obtain the required surface friction / skid resistance on highly stressed sites. Also, a point often forgotten but becoming far more important is that hot rolled asphalt is by far the most "recyclable friendly" bituminous mixture because of its higher binder content and impervious nature i.e. little oxidation of the bitumen in the matrix of the surface course. I did not choose the bituminous mixture for this site. However I may have been influential in selecting a 14mm. SMA rather than the 10mm. SAM option to achieve higher surface texture on, and the approaches to,  the island. You will note that no high friction surfacing has been applied to this site, so I am assuming that the surface friction for vehicle braking, has been, and is adequate.

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, and some drum mixers are really not up to the job of producing binder rich stone mastic asphalt.
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, and not thoroughly distributing the cellulose fibres uniformly throughout the mixture, which may result in samples from the produced material failing to comply with the specification although the various feeds to the plant are accurate.

Photographs of a Particular Site where 14mm. Stone Mastic Asphalt is being laid - February 2001

LAYING STONE MASTIC ASPHALT SMA is laid with a conventional paver without any problems, but any hand laying required is difficult. 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 a "rich" 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.

The images above show the laying of 14mm. stone mastic asphalt on a "fresh" morning in February 2001, hence the clouds of steam that you can see coming off the material. Most of it, but not all, coming from the water used in rolling.

The image above shows the edge of the newly laid mat before rolling, the nominal thickness of mat for this particular material was 45mm., with a minimum thickness anywhere of 40mm.  The average binder content of the sample taken on this site was within the specified requirement of 6.5% +/- 0.6%, but it was under the 6.5% mid-point, which is usually the case, as modern bituminous mixture production plants are usually very efficient at weighing in components.	A view of the surface of the laid mat before compaction. Note the "irregular" nature of the "mastic" coating indicating the presence of "grit" in the "mastic." My reference to "mastic" meaning the combination of bitumen, limestone filler, cellulose fibres and the "grit" (passing the 2.36mm. sieve). It is this "mastic" that fills, or should fill, the large voids between the predominantly single sized aggregate structure. It will also coat the surface aggregate until trafficking form vehicle tyres has removed it.
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 of the bituminous mixture without causing binder drainage problems in storage and transporting. If this design is not correct it will result in binder drainage in the lorry bodies and fatted areas of the surfacing, usually at the ends of loads. Conversely a mixture design that deliberately has a lower binder content to prevent binder drainage, rather than a correctly designed mixture able to take the increased binder, will result in a bituminous mixture that will be porous, be less durable and have a reduced life.

Compacting the laid 14mm. SMA with a vibratory smooth wheeled roller.
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 (German) 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 Course System (TSCS) 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. But other "variations" are available, see my comments on BS EN 13108-5 : 2006:Bituminous mixtures. Material specifications -  Stone mastic asphalt, at the top of this page. Note the good coating of K1-40 tack coat on the existing surface.

The image above shows the finished, compacted 14mm. SMA surface, you will see that it has significant "negative" texture. In my judgement above 1.5mm., by sand patch.	It will be seen better after enlargement that there has been a "token" painting of the joint. You have to realise that was still in the relatively early days of laying stone mastic asphalt, and no one was too sure whether to cut the joint or not, and even whether to paint it (with hot 50pen. bitumen) or not. In hindsight I would not cut a 14mm. SMA but would specify a complete coating of the joint of a low air void bituminous mixture. However I would cut and paint a 10mm., low air void, SMA. However I am sure that other engineers and technicians may hold differing views, and they may be correct for the bituminous mixtures "local" to their highway network.
FINISHED SURFACE OF A 14MM. SMA The photographs above show the surface characteristics of this material as it emerges from behind the paver and after rolling, indicating that this particular bituminous mixture has significant surface texture, of a "negative" characteristic as it is now widely described. This material has significant "negative" texture, probably above the 1.5mm. (by sand patch - see following IMPORTANT note) required by the Highways Agency on motorway and trunk roads, although on this site and to this specification, the amount required was a minimum of 1.2mm., by sand patch. IMPORTANT It is necessary to read,  Clause 921-Surface Macrotexture of Bituminous Surface Courses, of the 900 Series, of Volume 1, of the Specification for Highway Works - Amendment August 2008, as there have been changes to initial texture depth requirements for Thin Surfacing Systems. This may, or may not refer, to generic SMA on local highway networks. Read the Summer 2008 Newsletter for further information.

Photographs of the same Particular Site showing the 14mm. Stone Mastic Asphalt nine years later - March 2010

Above is an overall view of the site, showing that the surface is generally in good condition.	Above is an overall view of the site, showing that the surface is generally in good condition. However if you look closely you will see lengths of joint overbanding.

The area marked, is an area that has had very little traffic over the nine years.	This image shows a more detailed photograph of the untrafficked area showing the surface retention of the original "mastic". The "mastic" that has been removed is likely to have occurred by oxidation of the bitumen. Please note the exposed "grit" in the retained surface "mastic", you will see it better in the enlarged image.

The image above showing failure of the joint, while the actual surface is in good condition.	A length of joint that has already received some overbanding treatmnet, but has continued to fail under the "screwing" action of vehicle tyres.

An area of typical trafficked surface showing the now almost complete removal of surface mastic leaving a "positive" textured surface, as opposed to the original "negative" textured surface.	This is a higher magnification of the image to the left which better shows the established nature of the surface and its current surface texture. With the large aggregate fully exposed the polished stone value of this aggregate can contribute its full potential.
It is a fact that all bituminous mixtures will have the newly laid surface coated with binder, to a greater or lesser degree. This is rarely a problem with vehicles complying to road speed limits. However there have been recorded instances where the "mastic" coating of some SMA surface courses, whether generic or proprietary have taken an unusual length of time to be worn away by traffic. There are documented cases of this to be found on the "web", or there were, if you search diligently. Having never experienced this problem on work that I was involved with I put this down to the binder that was used in the supplied mixtures, and where I actually worked and where my supplies came from. By this I mean that only straight run 50pen. or 100pen binder was used in the supplied mixtures, and all "local" quarry aggregates had relatively high PSV's, some being in the higher PSV range. You will find that in reading other of my web pages relating to SMA surface course that I place great store by the quality of the fines/grit (passing the 2.36mm. sieve) which will be present in the surface mastic and should be very quickly exposed to vehicle tyres with a minimum of trafficking. In fairness, this is my opinion, and not held by all engineers. I just take the evidence that before my retirement I was not made aware of any increased problems with the initial friction properties of SMA surface course laid on the local authority highway network, under the control of the local authority Highways Department. It is now becoming a more widely practised procedure to carefully apply high PSV natural aggregate "grit" to the newly laid, still hot, SMA surface before rolling to overcome any possible early life skid resistance problems. Information on the "gritting" of newly laid SMA surface course can be found in the German SMA information download. But do consider the effect "gritting" of SMA may have on initial surface texture of the surface course. I keep saying that Highways Maintenance is not easy, it is down right difficult at times, but the "people in power" still think that any politically correct "suit" or "PR" person can do it, they cannot. Let me just remind you of the Winter Maintenance/Service debacle over the last two years, but I digress as usual, just trying to support an opinion. The exception to an initial bitumen coated surface layer, of course, is surface dressing, when uncoated aggregate chippings are applied to the sprayed bitumen layer, so an exposed aggregate surface from the time that it is laid, I thought that I would just mention that.

CORES FROM A 14MM. STONE MASTIC ASPHALT SURFACE COURSE

Above is a core taken from a laid 14mm. SMA surface course, the specification in the contract document called for a nominal 45mm. thickness with a minimum at any point of 40mm.. You will see that in the area that this core was taken the laid thicknes was only 35mm. resulting in the surface course being heavily voided. This is not what was wanted from this surface course, but shows what can result when the specification is not complied with, the specification is there for a reason.	The above core from the same site has a laid thickness of 41mm. and the SMA has been able to be fully compacted. The thickness is above the minimum 40mm. required in the specification but is still not the nominal (average) 45mm. that is required for the site.
This has occurred even though the stone mastic asphalt surface course material was purchased on a laid tonnage basis. If the client had been paying on a "square metre" basis it is much more likely that there would have been more thickness "issues", because the thinner the contractor lays the surface course (but within the permitted tolerances) the more money he will be making, but the more likely he will be putting himself in a situation where some areas will be thinner than the required. The sign of a "skilled" laying contractor is one who can consistently lay the bituminous mixture, what ever it is, when paid on a "square metre" basis within the laid surface tolerance but always below the tolerance. This ability is often factored into the bid price for the work, to secure the work. If the client pays on a tonnage basis plus the cost of laying, this should exclude the deliberate intention of laying the surface course thinner than the nominal thickness stated in the specification / works drawings. It may cost the client more, but you will be getting what you are paying for, and I would suggest that you have on site supervision taking delivery tickets, but you should have on site supervision any way for a number of good reasons. Please note that in the case study above there was no problem with the thickness of the laid 14mm. SMA surface course, and there has been no problems with the durability of the actual laid "mats" of material.

14MM. STONE MASTIC ASPHALT GRADING

The image above shows the results of a sample taken form a load of stone mastic asphalt supplied to a laying site. I have include this to show you that a 14mm. stone mastic asphalt bituminous mixture is a material where the aggregate will be predominantly passing a 14mm. test sieve (quarry grading screen) and retained on a 20mm. test sieve (quarry grading screen). However in this "real" world of highways maintenance that I keep trying to explain to you, most of this "14mm. aggregate" could be only just passing the 14mm. sieve, i.e. giving a "bold" material, or the majority of the aggregate could only just be retained on the 10mm. sieve, i.e. giving a "fine" material. Although I am currently writing about stone mastic asphalt this same principle applies to all quarry supplied materials, it can be a particular problem with surface dressing chippings. If you have experienced and knowledge engineers and engineering technicians, especially a Materials Engineer, in your team, they will often factor in this knowledge when they are purchasing from local suppliers, that is if they are still able to purchase from local suppliers of their choice, of course bearing cost in mind. I will also take this opportunity to point out the significant "grit" fraction, 12%, in the aggregate grading i.e. passing the 2.36mm. sieve retained on the 75 micron sieve (yes I know the sieve sizes have changed a little but that is not important for this explanation). This is a significant amount of "grit" and it will become an even larger proportion of the "mastic" that will be surrounding the surface exposed larger aggregate. It is my opinion/theory that the quality, i.e. polished stone value (PSV) and the aggregate crushing value (ACV), of this "grit" is just as important as the quality of the nominal size aggregate, perhaps more so, and reference to the specifying of these properties in relation to the "grit" fraction was included in the generic specification. This is because it is the "grit" in the mastic that will be the first aggregate exposed to vehicle tyres, after minimal trafficking. My opinion/theory seems sensible in its logic, but also because I was fortunate enough to work in an area where the aggregate from all suppliers was good, that includes the "grit", and some suppliers very good, and until my retirement I was not aware of any early life skid resistance issues with generic stone mastic asphalt on sites under the control of the local authority.

Further information and pictures relating to stone mastic asphalt can be found on the page, Bituminous Mixtures / Specifications / Processes / Diagrams

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 or PD 6691:2007:Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications, 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 stiff 50pen. or 100pen. 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 initially 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.  However the 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 Mixture Composition, if you wish to specify this bituminous mixture. Or, select a "sample" bituminous mixture from, PD 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.

See the page Bituminous Mixture Surfaces, a Close Study in Pictures, for views of typical surfaces.

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, or indeed the same supplier but from a different production plant, unless you specify the Target Mixture Composition that you require for the bituminous mixture supplied to you, although you can defer to BS PD 6691 : Annex D, which should give some amount of conformity of bituminous mixture.
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 that 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, 
PD 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 !

[ Top of Page ]