|[ Top ]||Hot Rolled Asphalt and Bituminous Macadam|
Idiots' Guide to Highways Maintenance
MIXTURES / PROCESS DIAGRAMS
(Important Background Knowledge before Considering Individual Types of Bituminous Mixture)
But to try and provide some clarity in what has become an extremely confusing situation with "specifying" / ordering bituminous mixtures.
All these bituminous mixtures
are the result of designing and blending aggregates, from sources of
aggregate (quarries) all over the UK, with differing amounts,
target binder contents, of bitumen.
The aggregates will have differing physical properties, and will be proportioned to different specifications/designs/"recipes" and mixed in different types of batching plants, with different bitumen viscosities supplied from several major suppliers.
All British Standard specifications will have working tolerances for each particular described mixture, with each particular described mixture filling a particular engineering role in the make up of a road pavement.
The range of British Standard bituminous mixtures can be found for bitumen macadam/asphaltic concrete on the page,
A GUIDE TO AVAILABLE (BITUMEN) COATED MACADAMS / ASPHALTIC CONCRETE
The range of British Standard bituminous mixtures for Hot Rolled Asphalt can be found on the page,
A GUIDE TO HOT ROLLED ASPHALT BITUMINOUS MIXTURES
The supplied bitumens, of various penetration grades and other engineering characteristics, are likely to be sourced from differing crude oil stocks and may be modified with various synthetic or natural modifiers.
The bitumen must be stored and used within certain time and temperature constraints to ensure the bitumens do not degrade before incorporation in any bituminous mixture, or indeed after mixing whilst in storage bins.
So, what am I saying is that there is the potential for hundreds, possibly thousands, of combinations to produce the sum of the various bituminous mixtures referred to on this page,
and that includes SMA, generic or proprietary.
The properties of the bituminous mixture will depend upon the quality, blending and mixing of its components, and length and temperature of storage, not on the name it may carry.
Just as a rose will smell as sweet if you call it a turnip, you will not get the same fragrance from a turnip if you call it a rose.
You probably know what I am going to say next, but I will say it anyway, you need a qualified and experienced Materials Engineer / Road Pavement Engineer to help you decide on the appropriate bituminous mixture for particular surfacing requirements, not an advertising campaign.
I hope that the brief bituminous mixture descriptions listed below will provide initial useful information relating to commonly used bituminous mixtures in road pavement construction and maintenance.
|CONTENTS (brief descriptions, specifications, for the main bituminous mixture "descriptions")|
|Close Graded Bituminous Macadam (CGM) - Asphalt Concrete||Stone Mastic Asphalt (SMA)|
|Hot Rolled Asphalt (HRA)||"Safepave" type thin surfacings|
|Porous Asphalt (Pervious Macadam)||Surface Dressing|
|IMPORTANT UPDATES / NEW INFORMATION THAT RELATES TO BITUMINOUS MIXTURES EMPLOYED IN HIGHWAYSMAINTENANCE|
|NEW - January 2008 Introduction of the new BS EN 13108 Family of Bituminous Mixtures, Replacing BS 594 and BS 4987|
|NEW - January 2011 A downloadable .pdf guidance document relating to Bituminous Mixtures for Surface Course laid 40mm. thick, or less|
|NEW - December 2011 Link to a brief summary of "Low Energy Asphalt" (LEA)|
|NEW - January 2013 Development of Standard Specification and Standard Details for Local Highway Maintenance - Appendix 1 - Specification Notes for Guidance - Version 1 - November 2012|
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 a dense/close graded 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.
Although these "larger"
British Standard BS 4987 bituminous macadams can no longer be
specified, when available and laid between 100mm. to 150mm. thick and
compacted with heavy rollers they produced an excellent load carrying and
durable base within the road pavement.
Unfortunately the promotion of thinner layers of stiffer, very low penetration grades of bitumen, to reduce cost, proved to be less durable and the poor performance was laid at the foot of all bituminous macadams.
Bitumen macadams (asphaltic concrete) are an excellent bituminous mixture, but personally I would not recommend a bitumen viscosity stiffer than a 50pen. for local authority highway networks.
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
A more comprehensive study of an example of a 14mm. Close Graded Macadam (CGM) Surface Course, now "renamed" asphaltic concrete (AC) can be found, HERE
Hot Rolled Asphalt & Precoats
(BS 594 : Parts
( 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.)
Diagram of the structure of a surface course
hot rolled asphalt layer with an applied application of high polished stone
value (PSV) 20mm. or 14mm. aggregate chippings precoated with bitumen.
The picture above (which can be enlarged) shows the various aggregates and their basic proportions after the bitumen has been removed and the total aggregate graded in the specified sieves.
Hot rolled asphalt with applied high
polished stone value precoated chippings
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" using
the existing HRA surface course as the binder course.
The "standard" bitumen binder for HRA is a straight run 50pen. grade bitumen, and there will be few sites where a different binder is required if the hot rolled asphalt mixture has been correctly designed.
The reporting of the steep "decline" in use of hot rolled asphalt surface course has only come about because, it is no longer a permitted bituminous mixture on motorways and trunk roads in England and Wales. HRA surface course, binder course and base bituminous mixtures as formerly specified in BS 594 and currently widely used on local highway networks.
Hot rolled asphalt and precoats as surface course can still be used on motorways and trunk roads in Scotland and Northern Ireland as a standard permitted option.
HRA as surface course, binder course and base bituminous mixtures as formerly specified in BS 594, and now PD 6691 are still widely used on local authority highway networks.
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.
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 the hot rolled asphalt 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 ( British Standard "recipe" binder content 7.8% +/- 0.6% ) 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.
The low void content and the high binder content of this material, in my opinion, make this the most suitable of all bituminous mixtures for recycling/remixing, and it does not matter if it has been surface dressed.
There is no aggregate interlock to distribute load in the 30% and 35% aggregate content wearing courses, the load bearing strength of HRA surface course comes from the stiffness of the matrix.
Note : The stiffness of the matrix will be compromised if lesser quality materials are used in the production of this bituminous mixture, e.g. limestone filler replaced with "quarry dust".
|The photograph to the right
shows a road surface that consists of a hot rolled asphalt
wearing course and 20mm. precoats, and the image can be magnified by "clicking"
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
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
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, and these proprietary bituminous mixtures had/have porous structures to ensure the when laid the road surface complies with the initial texture depth requirement on motorway and trunk roads.
( Although the 1.5mm. texture depth requirement, at the time of introduction of TSCS's, has recently been reduced for "Thin Surfacings", see your 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 requirement is not as comprehensive/stringent as it was when laying generic porous asphalt specified to a British Standard, indeed many TSCS's already have experienced premature failure.
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."
An example of a generic
14mm. stone mastic asphalt specification "discussion document", in .pdf format,
based on the above reports can be accessed,
Transport for Scotland TS 2010 - Surface Course Specification and Guidance
Transport for Scotland have recently introduced a surface course specification relating to the specifying of Stone Mastic Asphalt on trunk roads in Scotland, the link below will provide a few more details.
A small amount of further information can be found on the Winter 2010/11 Newsletter.
I am beginning to accept stone mastic asphalt, 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.
PERSONAL NOTE :
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.
It is usually suggested that for an SMA to be durable a voids content of below 5% is required.
Be aware that SMA designs can vary considerably, this is
just one example of an SMA mixture, and one which I believe is representative of a true SMA, but opinions
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.
There is a drawback with this specification in that it allows mixture "recipes" that have a high voids content which are specified to BS EN 13108-5 and therefore can still be described/called stone mastic asphalt.
I am not happy about this element of the new standard, as in my opinion it does not hold to the basic "German" principle that the voids in a SMA bituminous mixture, that has a predominantly single size aggregate, shall be filled with "mastic".
It follows that a correctly designed porous bituminous mixture will not be as durable as a correctly designed impervious bituminous mixture.
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 of exposed coarse 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.
There have been further more recent reports from TRL on this topic, and you will find reference to them on the TRL Reports Page.
I repeat, an example of a generic 14mm. stone mastic asphalt specification "discussion document", in .pdf format, based on TRL reports can be accessed, HERE.
"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
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 full explanation, and HERE for more pictures.
For a table comparing the various types of asphalt / bituminous surfacing materials, press -------> HERE
of the new BS EN 13108 Family of Bituminous Mixtures, Replacing BS 594 and BS
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 on this page will still include reference 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 in the new specifications 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,
PD 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.
NOTE, PD 6691 : 2010 : Guidance on the use of BS EN 13108 Bituminous mixtures - Materials specifications has superseded the 2007 edition.
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. 6691:2010:Guidance on the use of BS EN 13108 Bituminous mixtures - Materials specifications,
P.D. 6692:2010: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,
NOTE BS 594987:2010:Asphalts for roads and other paved areas -
Specifications for transport, laying and compaction and type testing protocols, has superseded the 2007 edition.
Interim Advice Note 101/07,
which is a complete, revised
900 Series of the Specification (MCHW1) for Road Pavements - Bituminous Bound Materials
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.
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 that you do, 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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