BITUMINOUS MIXTURES / PROCESS DIAGRAMS

CONTENTS

Close Graded Bituminous Macadam (CGM)  - Asphalt Concrete
Hot Rolled Asphalt (HRA)
Porous Asphalt (Pervious Macadam)
Stone Mastic Asphalt (SMA)
"Safepave" type thin surfacings
Surface Dressing

To understand the diversity of bituminous mixtures even when supplied under the same description read the April/March 2005 newsletter (all links worked at the time of writing the newsletter)

IMPORTANT - Introduction of the new BS EN 13108 Family of Bituminous Mixtures, Replacing BS 594 and BS 4987

Close Graded Macadam Wearing Courses & Dense Bitumen Macadam Basecourses / Roadbases, (BS 4987: Parts 1&2)

(The above mixtures are now known as Asphalt Concrete, from the 1st. of January 2008 they will be specified in accordance with, 
 BS EN 13108-1   : 2006:Bituminous mixtures. Material specifications - Asphalt Concrete, but they should be the same, or very similar to the  bituminous mixtures that you have previously been specifying and receiving, dependent upon your correct specifying of the actual material that you require. 
See below for further details of the introduction of the BS EN 13108 family of standards for Bituminous Mixtures.
Also note that, P.D. 6691:2007:Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications, provides a range of "standard" defined bituminous mixtures, very similar to those that you have become accustomed to, that will cover you most commonly required, and used, asphalt concrete (bitumen macadam) mixtures.
This document provides similar defined mixtures for hot rolled asphalt, stone mastic asphalt and porous asphalt.)

	The nature of an asphalt concrete (bitumen macadam) is that the aggregate is a through grading of a combination of all sizes of aggregate particles according to the largest size of the aggregate. The particles knit together to produce a tight aggregate mass held together by the bitumen binder. Wheel load is distributed through the layer by aggregate particle contact, and interlock.
Asphalt Concrete (Bitumen Macadam) provides a range of excellent cost effective materials for surface course, binder course and base. Below is a typical result from a sample of 14mm. size close graded macadam surface course, as specified in BS 4987:Part 1  For a look at this type of material, press -----------> HERE

Hot Rolled Asphalt & Precoats (BS 594 : Parts 1&2)

( From the 1st. of January 2008 the above mixtures will be specified in accordance with BS EN 13108-4 but they should be the same, or very similar to the  bituminous mixtures that you have previously been specifying and receiving, see below for further details of the introduction of the BS EN 13108 family of standards for Bituminous Mixtures.)

This is the premium highway surfacing material that we have used for many years and is responsible for a large percentage of our highway network being in such relatively good condition, even if it is covered by a surface dressing, or a proprietary bituminous mixture supplied as a "Thin Surface Course System". The reporting of its steep "decline" in use has only come about because, it is no longer a permitted bituminous mixture on motorways and trunk roads. The 35%/14mm HRA/SC is laid 50mm thick and not only provides a surface with good texture and a high PSV aggregate, it also imparts considerable strength and durability to the existing pavement when applied as an overlay. The more usual 30%/14mm. HRA/SC is laid 40mm. thick, but providing this is a minimum thickness this material will still add strength to the road pavement, if less than a 50mm. layer.

HRA surface course allows a lower PSV (45 min.) aggregate to be used in the matrix with the scarce and expensive high PSV aggregate only being used for the precoats, which is a very important factor when high quality, high PSV aggregate sources are becoming increasingly scarce. The strength of this material is from the stiffness produced as a result of the combination of a 50pen binder, limestone filler, and sand matrix. The higher binder content and low void content of this mixture results in a very durable material, with a "common" life of correctly designed materials in excess of 20 years. There is no aggregate interlock to distribute load in the 30% and 35% aggregate content wearing courses.

The photograph to the right shows a road surface that consists of a hot rolled asphalt wearing course and 20mm. precoats. The wearing course is at least 15 years old, possibly 18 years, but remains in good condition The texture is very good well in excess of the 1.5mm. specified by the Highways Agency The rate of spread of chippings is recommended to be 70% of total shoulder to shoulder cover, between 12kgs to 14kgs per square metre is usual, depending upon the "boldness" of the chipping For more information on hot rolled asphalt and precoat wearing courses, press -----------------> HERE For more information on bituminous mixtures, and associated topics, press ------------------> HERE

Porous Asphalt / Pervious Macadam (BS 4987:Parts 1&2)

(The above mixtures are now known specifically as Porous Asphalt, with a dedicated standard for these particular bituminous mixtures, and from the 1st. of January 2008 they will be specified in accordance with BS EN 13108-7 but they should be the same, or very similar to the  bituminous mixtures that you have previously been specifying and receiving in accordance with BS 4987, see below for further details of the introduction of the BS EN 13108 family of standards for Bituminous Mixtures.)

Porous asphalt is the same material that used to be called Pervious Macadam. If you want a material that is porous specify a material that has been designed to be porous, do not take another material i.e. stone mastic asphalt and make it porous by not filling the voids with mastic or by "dragging" it with the paver to a layer thickness less than it should be and hence creating voids in the matrix, that is not how a porous surface layer should be achieved.
Open graded macadams will also have a high degree of porosity. Any porous surface course material should be laid on an impervious binder course (basecourse) or thick tack-coat / bond coat to prevent water entering the road pavement. Unless that is, you are designing / constructing a road pavement or car park that is designed to be self draining, often referred to as Sustainable Urban Drainage (SUDS), then all layers will need to be porous, with a sub-base that is also porous and able to "hold" water until it soaks away naturally.

The photograph to the right is of a 10mm. Open Graded Macadam from, B.S. 4987 : Part 1, but in grading it is very similar to a 10mm. Porous Asphalt which is also specified in B.S. 4987 :Part 1. It does show that these materials are more or less single size in their grading, with few fines filling the voids. It is this open structure that creates the surface texture and allows water to pass thought connecting voids thus reducing spray, at least while the voids remain unclogged.

This material, with a stiffer and perhaps modified binder, may be used to provide a free draining and claimed "quiet" surface, "quiet" usually meaning just a little less noisy but whether the human ear can detect the difference is another issue. Porous surface course bituminous mixtures must must be laid on a strong and impervious base layer. Generic, i.e. specified to a British Standard, Porous asphalt is losing favour as a bituminous surfacing option. Although a material that may cause less surface noise to be generated and reduce spray, there have been problems in laying, and durability. Its life is much shorter than a hot rolled asphalt wearing course and there are problems with winter maintenance as porous asphalt needs salting at higher rates than impervious materials, as it is a "colder" surface. However, caution is needed because many of the proprietary products ( "Thin Surface Course Systems" ) are now seen as an alternative to porous asphalt, these proprietary bituminous mixtures have porous structures to ensure the 1.5mm. texture depth requirement on motorway and trunk roads. ( Although the 1.5mm. texture depth requirement has recently been reduced for "Thin Surfacings", see you Specification for Highway Works, 900 Series, as new texture depth requirements are variable according to site conditions. ) It is my belief therefore that many of these proprietary products will encounter similar problems to porous asphalt, especially as drainage detail is not as comprehensive as it was with generic porous asphalt, indeed many already have.

Stone Mastic Asphalt (SMA)

(From the 1st. of January 2008 stone mastic asphalt can be specified in accordance with BS EN 13108-5 this will allow you to specify the nature of the SMA that you wish to receive, it will be possible to specify the same, or very similar mixtures to the  bituminous mixtures that you have previously been specifying and receiving, see below for further details of the introduction of the BS EN 13108 family of standards for Bituminous Mixtures.)

"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".

Quotes from, TRL Report 314:Road trials of Stone Mastic Asphalt and other thin surfacings by J CNicholls, 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, is also excellent reading on the subject of SMA, especially if you are requiring information on its specification.

I am beginning to accept this material, as long as it is formulated and laid as it was originally intended to be, i.e. a binder rich, impervious material laid at a conventional thickness, but in this form it will not have the texture of a HRA and precoats, but a high texture depth is not necessary on all sites. However I personally still regard it as over priced compared to HRA and precoats, which is my favourite as you all know, probably because I have grown old using it and when laid correctly in county situations it has never really let me down.

Pictures of differing laid SMA mixtures can be accessed by pressing,  HERE A page showing the laying of SMA can be accessed by pressing,         HERE Further "web sourced" information on Stone Mastic Asphalt, in a .pdf download format, can be accessed by pressing,  HERE NOTE : I believe that a 55% High Stone Content Asphalt (BS 594) is an alternative to SMA in many situations, with eventual surface characteristics of these two materials being very similar after several years of trafficking.

Below is a result and grading "graph" of a generic  stone mastic asphalt that has been designed to have the "voids" filled with mastic and hence be impervious.
Note the steep fall of the plotted result indicating that it is a "gap graded" material, rather than a curve that would be indicative of a well graded mixture.
You will also note from the analysis figures that 96% passes the 14mm. sieve and 49% passes the 10mm. sieve, i.e. 47% of the aggregate is a single size passing 14mm. retained 10mm. This is a characteristic of SMA. and it is this predominant single size feature which creates the voids for the mastic to occupy.
If the design is such that there is not enough mastic of bitumen and fibres to fill these voids the matrix will be porous, and as such I do not believe should be called an SMA.

Be aware that SMA designs can vary considerably, this is one example, and one which I believe is representative of a true SMA, but opinions will differ.
Confusion over various "types" of SMA will be helped greatly now that,
BS EN 13108-5 : Bituminous mixtures - Material specification : Part 5 : Stone mastic asphalt,
has been introduced.
Engineers and engineering technicians should now be able to specify (produce a target mixture composition) of a Stone Mastic Asphalt mixture to meet their surfacing requirements and sample and test to see that it complies with the mixture that they have specified. 

Also note on the particular example above we have 14% "grit" passing 6.3mm. sieve retained on the 75micron. 
This "grit" will be completely incorporated in the bitumen/fibres mastic and as such will coat the surface aggregate. 
It is my opinion that it will only need a minimum of trafficking to expose this "grit" on the surface layer of aggregate and that providing this "grit" is of a similar PSV to the coarse aggregate employed in the SMA early life skid resistance should not be a problem. 
In the geographical area I work we are fortunate in the quality of aggregates commonly used, and I am not aware of any accident that has been attributable to poor early life skid resistance of SMA on the highway network that we have maintained.
I also prefer SMA's that have a 50pen. bitumen as the binder, especially now that the 100pen. option has been replaced by a 125pen., which with the tolerance on the target penetration can be considerably softer. 

I am aware research is being done on the early life skid resistance of SMA and other bituminous surface course mixtures, but I am not aware of any detailed published reports on the subject at this time.
I am aware that the report, TRL Published Project Report PPR060 - Early Life Skid Resistance of Asphalt Surfaces
was published by the Transport Research Laboratory in 2005, and although it discussed the issue at some length I felt few specific conclusions were achieved, so, in my opinion little advice or guidance could be obtained from it, but you should read it for yourselves and draw your own conclusions.

"Safepave" type materials

"The idea of an ultra-thin surfacing was originally proposed as a process which would not have the drawbacks associated with surface dressing and it can be considered as a paver-laid hot mix surface dressing." This extract from :- TRL Report 314: Road trials of Stone Mastic Asphalt and other thin surfacings, by J C Nicholls.

Surface Dressing (Road Note 39, 5th Edition, Revised)

Single size and racked in surface dressing are the most widely used surface dressing methods, and are extremely cost effective. They provide a highly textured surface and skid resistance of bends and approaches to, but not the actual, junctions can be improved by the minimal use of high PSV aggregate.  There are more complicated methods  of surface dressing (multi-dressings) which can approach the cost of basic paver laid generic  bituminous mixtures.

For more information on surface dressing, press  -----------------------------------------------------------------------> HERE

For a table comparing the various types of asphalt / bituminous surfacing materials, press -------> HERE

Introduction of the new BS EN 13108 Family of Bituminous Mixtures, Replacing BS 594 and BS 4987

IMPORTANT:- From the 1st. of January 2008 the materials descriptions used in this comparison will no longer apply to the bituminous mixtures referred to in the text above. ( For further information click ------> HERE )
This does not mean these bituminous mixtures will not be available, but it does mean that you must be aware of the new description, and more importantly that you have defined/specified the "target mixture composition" of the bituminous mixture description that you want to receive, when you purchase that particular mixture.
I fear there could well be a substantial amount of confusion in the whole (supply and use ) of the bituminous mixture industry, with the purchaser/user experiencing the greater difficulty.
At this time the text above will still refer to BS 594 and BS 4987 bituminous mixture descriptions, that actually meant particular/defined/specified bituminous mixtures, the use of the new mixture descriptions will not give you this clarity.
It is up to you to specify and purchase similar bituminous mixtures to those referred to above if you believe they are suitable for a particular use, on a particular site.
It may not be possible provide similar text as above in the future because each general material description could cover many variations of an actual bituminous mixture composition/recipe, providing significantly different engineering properties.

If you do not feel competent in converting BS 594 and BS 4987 defined generic bituminous mixtures to the equivalent mixtures using the new standards, be guided by,
P.D. 6691 : 2007 : Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications,
this may not necessarily provide an exact, optimum, equivalent to your "old" material but it should provide a "safe" alternative if you are not familiar with bituminous mixture composition and design.

The new specifications were introduced on 1st. January 2008

BS EN 13108-1   : 2006:Bituminous mixtures. Material specifications - Asphalt Concrete 
(This standard largely replaces BS 4987, for specifying Target Mixture Compositions for "macadam" bituminous mixtures.)

BS EN 13108-2   : 2006:Bituminous mixtures. Material specifications -  Asphalt Concrete for very thin layers 
(This standard can be used to provide Target Mixture Compositions for the finer "macadam" mixtures.)

BS EN 13108-3   : 2006:Bituminous mixtures. Material specifications -  Soft asphalt

BS EN 13108-4   : 2006:Bituminous mixtures. Material specifications -  Hot rolled asphalt 
(This standard replaces BS 594 for providing appropriate Target Mixture Compositions.)

BS EN 13108-5   : 2006:Bituminous mixtures. Material specifications -  Stone mastic asphalt 
(This standard can be used to provide particular Target Mixture Compositions for SMA.)

BS EN 13108-6   : 2006:Bituminous mixtures. Material specifications -  Mastic asphalt

BS EN 13108-7   : 2006:Bituminous mixtures. Material specifications -  Porous asphalt

BS EN 13108-8   : 2005:Bituminous mixtures. Material specifications -  Reclaimed asphalt

BS EN 13108-20 : 2006:Bituminous mixtures. Material specifications -  Type testing

BS EN 13108-21 : 2006:Bituminous mixtures. Material specifications -  Factory production control

P.D. 6691:2007:Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications,

P.D. 6692:2007:Guidance on the use of BS EN 12697 - Bituminous Mixtures - Test methods for hot mix asphalt,

BS 594987:2007:Asphalts for roads and other paved areas -
Specifications for transport, laying and compaction and type testing protocols,

Interim Advice Note 101/07,
which is a complete, revised
900 Series of the Specification (MCHW1) for Road Pavements - Bituminous Bound Materials   
and,
Interim Advice Note 102/7 - The introduction of BS EN Standards for bituminous mixtures - Site supervising aspects

Both the above IAN's introduced on the 1st. of January 2008 to provide the revision needed to support the changes produced by the introduction of, 
BS EN 13108 : 2006:Bituminous mixtures. Material specifications family of standards.
and,
BS 594987:2007:Asphalts for roads and other paved areas -
Specifications for transport, laying and compaction and type testing protocols,

FURTHER NEW CHANGE - It is now necessary to point out that both IAN 101/07 and IAN 102/07 have been superseded by the actual, 
Series 900 - Road Pavements - Bituminous Bound Materials of the Specification for Highway Works, Volume 1. 
This was released in August 2008, and can be download from the HA website.
It is necessary to draw to your attention to,
Clause 921-Surface Macrotexture of Bituminous Surface Courses, as in the revised Table 9/3, you will see that the initial texture depth on lower speed roads, including roundabouts on these roads has been decreased further to that stated in IAN 101/07.
The IAN's and the recent, August 2008 SHW  900 Series can be downloaded from the Highways Agency website, www.standardsforhighways.co.uk
To every Engineer and Engineering Technician who reads this page may I say, it really is necessary to download the recent SHW 900 Series and study it, for all the information, and changes to its predecessor, that it contains.
Especially, in my opinion, the major changes that it will allow in initial texture depth of a road surface and hence skid resistance, the PSV of the aggregate remaining the same.

Comments on my opinion of this reduction of initial texture on some parts of the Motorway and Trunk Road Network can be found in the Autumn 2008 Newsletter.

National Highways Sector Schemes for Quality Management in Highway Works,
No. 14 - For the quality management of the production of asphalt mixes

A revised edition of this Sector Scheme Document (SSD) No. 14 came into force on the 1st. of January 2008 to coincide with and support the introduction of the new British Standards mentioned above.
SSD No. 14 and BS EN 13108-21 : 2006:Bituminous mixtures. Material specifications -  Factory production control
are very similar in content but not exactly the same, with SSD No. 14 including elements of EN ISO 9001 as required by BS EN 13108:21.

Copies of SSD No. 14 are available from the United Kingdom Accreditation Service (UKAS), and can be downloaded from their website, www.ukas.com.

If you do visit this website, and I recommend it, I suggest that you also download,  
SSD No. 16 : For the laying of asphalt mixes
This document is published by the, Sector Scheme Advisory Committee for the Quality Management of the Laying of Asphalt Mixes.
I will remind you that these are "management" documents not actual specifications although elements of SSD No. 14 are contained in BS EN 13108:21, or vice versa, as previously mentioned, but in my opinion they are still well worth reading.
If you are associated in any way with purchasing large amounts of any generically specified bituminous mixtures (asphalt) I think you should take the time to read these documents.
In my opinion they have not been published by UKAS to be ignored, but to provide information and guidance on extremely important management practices in relation to the production of hot bituminous mixtures (asphalt).

TRL Report TRL656 : Implications of implementing the European asphalt test methods

With the implementation of the asphalt package of European standards and the withdrawal of the equivalent British Standards in January 2008, the basis of the tests called in the Specification for Highway Works will have to change. This harmonisation of test methods for bitumen and asphalt will mean that numerical values in requirements may need to be adjusted in order to maintain the same level of performance as prior to the change.
This report relates to work undertaken to determine the nature of any potential/necessary adjustments, in particular with the testing of the texture depth of pavement surfaces, wheel tracking and the storage stability of polymer modified binders, amongst others.

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