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The Idiots' Guide to Highways
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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
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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. |
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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. |
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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. |
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Compacting the laid 14mm. SMA with a
vibratory smooth wheeled roller. |
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LAYER THICKNESS OF STONE
MASTIC ASPHALT |
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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. |
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FINISHED
SURFACE OF A 14MM. SMA |
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Photographs of the same Particular Site showing the 14mm. Stone Mastic Asphalt nine years later - March 2010
CORES FROM A 14MM. STONE MASTIC ASPHALT SURFACE COURSE
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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. |
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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. |
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| 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 aggregate 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. |
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14MM. STONE MASTIC ASPHALT GRADING
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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. |
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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. |
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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 !
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