[ Top ] Hot Rolled Asphalt and Bituminous Macadam

The Idiots' Guide to Highways Maintenance
Copyright © 2000/08, C.J.Summers

A BROAD COMPARISON OF BITUMINOUS SURFACING MATERIALS
( with regard to the properties of durability, surface texture and noise generation )

 I am quite reluctant to create this table because selecting a bituminous surfacing material appropriate for a particular site is not an easy decision to make, and far more factors need to be taken in to consideration than are listed below, including cost.
Indeed, the first question I usually ask engineers who come to me for guidance on which materials to use or how to increase the strength of a road pavement is, "how much money have you got to spend".
But so many people are asking for information on the properties of the various materials on offer I thought it necessary to make an attempt at a table to at least provide a starting point for engineers and technicians to begin their quest. It will also make it easier for myself when receiving emails to just send a "one liner" pointing them in this direction. 

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 table. ( 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 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 "table" will remain as it is, with descriptions that actually meant particular/specified bituminous mixtures.
It is up to you the specify and purchase similar bituminous mixtures to those shown below if you believe they are suitable for a particular use, on a particular site.
It may not be possible to create such a table with new material descriptions, because each general material description could cover many variations of an actual bituminous mixture composition/recipe, and therefore have different engineering properties.

I really do believe that you need an experienced and competent Materials Engineer / Road Pavement Engineer close to hand that you can consult on these matters, because, "a little knowledge is a dangerous thing".




DISCLAIMER - ABSOLUTELY NO RESPONSIBILITY IS TAKEN FOR ANY DECISION TAKEN BY ANYBODY AS A RESULT OF READING INFORMATION IN THIS TABLE
THE INFORMATION IS OFFERED AS A GUIDE, THE DECISIONS ARE YOURS !

THIS TABLE IS ONLY TO BE REPRODUCED FOR PERSONAL EDUCATIONAL USE, 
PERMISSION FOR IT TO REPRODUCED BY ANY ORGANISATION, OR PUBLISHER, WILL NOT BE GIVEN !

I have referred to materials that can be laid 40mm. thick or less, in view of the recently introduced terminology that anything less than 40mm. is "thin".
However there are generic bituminous mixtures for surface courses that can be laid thicker than 40mm.that will provide appropriate ride qualities, texture depth, skid resistance and  most importantly increase the strength of the road pavement, this cost effective option should not be ignored on roads where it is permitted.
These comparisons are made on the basis that all the bituminous mixtures for surface course have been produced in compliance with the relevant specification or manufacturers instructions, and that they have been correctly applied.
This laying process may be on a fresh binder course (basecourse), or an existing road surface that has been suitably prepared to receive the new surface course, the road pavement being in a sound structural condition.

IMPORTANT ( just in case you missed it above ) 
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 table. ( 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 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 "table" will remain as it is, with descriptions that actually meant particular/specified bituminous mixtures.
It is up to you the specify and purchase similar bituminous mixtures to those shown below if you believe they are suitable for a particular use, on a particular site.
It may not be possible to create such a table with new material descriptions, because each general material description could cover many variations of an actual bituminous mixture composition/recipe, and therefore have different engineering properties.

Hot Rolled Asphalt Wearing Course to British Standard BS 594, and generic Stone Mastic Asphalt Surface Course
( for heavily trafficked road pavements, excluding motorways and trunk roads in England )
Material Description Matrix Properties Durability (4) Bit.
T.B.C.(d) %		 Texture (1)/ Skid Resistance (3) Noise (2)/ 
Ride Quality		 Considerations
"Recipe", 14mm. nominal size (30/14) Hot rolled asphalt wearing course, with 20mm. precoated chippings, BS 594 Predominantly a sand and stiff bitumen mixture stiffened with added limestone filler, the wheel loading is carried by the total stiffness of the whole matrix, not by mechanical interlock of the aggregate Impervious and extremely durable 7.8 +/- .6 POSITIVE TEXTURE 1.2mm. to 1.5mm. dependent upon rate of spread of precoats.
High PSV (a) precoat gives high SRV (b)  		 Regarded as the surface type that will generate the most noise. But this will be dependent upon texture depth and "Smoothness"  of ride. This is  much dependent upon the performance of the chipping spreader and quality of workmanship Still a popular material on county "A" & "B" roads due to its longevity, "whole life costing"
"Design",14mm. nominal size (30/14) Hot rolled asphalt wearing course, with 20mm. precoated chippings, BS 594 As above, but has undergone laboratory testing to insure it has specified "stability" and "flow" characteristics Impervious and extremely durable, but where lower bitumen % is present as a means of increasing stability surface fretting can occur prematurely Min. 6.5+/- 0.6 POSITIVE TEXTURE
A min. 1.5mm. was specified on motorways and trunk roads, this was often over chipped to ensure compliance and resulted in "noisy" surfaces  		 Not  permitted on motorways and trunk roads in England
Generic, Stone Mastic Asphalt  (SMA)
14mm. and 10mm. nominal size mixtures are available.		 A correctly designed SMA  has a predominantly single sized aggregate structure with the voids completely filled with bitumen, providing an impervious layer. As described it is a very durable surfacing material, it becomes less durable the thinner it is laid Min. 6.5+/-0.6 NEGATIVE TEXTURE, that will be dependent upon aggregate size and "openness" of grading, 1.2mm. is usually quite achievable with 14mm. aggregate Good ride quality, usually lower generation of tyre noise, but "bolder" 14mm. SMA's can generate significant noise Generic SMA is not  permitted on motorways and trunk roads in England -  when laid 40mm. it is possible for SMA's to be porous
 
Thin Surfacings  ( Proprietary materials laid less than 40mm. thick )
Material Description Matrix Properties Durability (4) Bit.
T.B.C.(d) %		 Texture (1)/ Skid Resistance (3) Noise (2)/ 
Ride Quality		 Considerations
Proprietary, Stone Mastic Asphalt (SMA), often sold as Thin Surfacing It is not possible to describe the matrix properties of these materials as these materials are many and varied and produced under "system" approvals. There will be many types of "branded" products with differing nominal sizes, aggregate gradings, and aggregate properties. So consult your supplier to determine which classification of the product is appropriate for your site.  A HAPAS certificate does not make a material suitable for all sites. This will be dependent upon the particular material selected and its engineering properties.
Thickness of layer is also an important factor, mixtures laid 40mm. thick are likely to be more durable, and will add some strength to the pavement. There are indications appearing that some Thin Surfacings have relatively short lives. 		 Target binder contents can range from 5.0% to 7.0% depending on the type of material and mix design. Bitumen content and type  has a large influence on the durability of a bituminous mixture. NEGATIVE TEXTURE
These mixtures are  required to achieve a minimum of 1,5mm. texture depth on motorways and trunk roads. Skid resistance will be highly dependent upon the polished stone value (PSV) of the aggregate. This means the complete layer has to consist of the high PSV aggregate, which is costly.		 All Thin Surfacings give excellent ride quality.
Noise generation of these surfaces will be dependent upon the particular mixture, but all are regarded as relatively quiet, but some will generate less noise than others.
Thin Surfacings are not all equal in this property.		 * It is currently Highways Agency policy that only proprietary Thin Surfacings having a HAPAS (c) certificate are permitted to be used as Surface Course on motorways and trunk roads in England.
Thin Surfacings
These by definition are proprietary bituminous mixtures sold under a brand name
NOTE  -  It is important to have a good tack coat on the receiving layer to all surface courses, but with thin surface course layers it is ABSOLUTELY ESSENTIAL, and this applies to proprietary and generic bituminous mixtures
GENERAL NOTES :- It is likely, but not necessary, that the load carrying ability and strength of these materials will derive from the mechanical interlock of the aggregate particles, this may be enhanced by using a modified binder and in some cases the addition of "fibres" to increase bitumen contents without binder drainage.
Texture is usually provided by using single size aggregate that creates a voided structure to give significant "negative" texture, but this in turn may create a porous matrix that will have the benefit of reducing spray but create the problem of allowing water through the surface course to the underlying road pavement unless an impervious binder course is employed or a bond coat of sufficient thickness to seal the receiving surface.
The passage of water and oxygen through the matrix of a bituminous material is known to hasten the ageing of the bitumen component of the mixture and the physical damage to the integrity of the layer structure, shortening the life of the surfacing. 
It is also widely recognised that porous surface courses need an increase in the spread rate of salt during the winter maintenance regime.
*  It is not possible to discuss proprietary Thin Surfacings fully without making reference to the,
Design Manual for Roads and Bridges,  Volume 7, Section 5, Part 1, HD 36/99 - Surfacing materials for new and maintenance construction,
and in particular,
Chapter 2 - Surfacing Options - Table 2.2E (England) - Permitted Pavement Surfacing Materials for New and Maintenance Construction,
issued in February 1999.
I hope you will take the time to seek out this document as it contains excellent information, you will also be made aware of the implications of Table 2.2E. 
 
 
Bituminous mixtures able to be laid thinly, i.e. 40mm. or less ( BS 594 and BS 4987 mixtures)
Material Description Matrix Properties Durability (4) Bit.
T.B.C.(d) %		 Texture (1)/ Skid Resistance Noise (2)/ 
Ride Quality		 Considerations
30%/10mm.  size, "Recipe", Hot rolled asphalt wearing course, with 14mm. precoated chippings,  Predominantly a sand and stiff bitumen mixture stiffened with added limestone filler, containing 30% aggregate of a predominantly single size grading  Laid 30mm. thick.
Impervious and extremely durable. Will add considerably less strength to a road pavement than a 40mm. layer		 7.8 +/- .6 POSITIVE TEXTURE1.2mm. to 1.5mm. dependent on rate of spread of precoats
High PSV (a) precoat gives high SRV (b)  		 Ride quality should be good, but is much dependent upon quality of chipping spreader and workmanship  Still a popular material on county "A" & "B" roads due to its longevity
55%/10mm. size High Stone Content Asphalt (HSCA)
 to BS 594  - Table 3/4		 An aggregate grading tending towards a single size with a sand, bitumen, filler matrix  Laid at 40mm. thick will be impervious, and with a 50pen bitumen it is known to be highly stable 6.5  +/- .6 NEGATIVE TEXTURE, initially texture can be lacking, i.e. 0.8mm. but this does improve with trafficking  Good ride quality, low noise generation characteristics Can be laid 25/40mm. thick
10mm. Close graded surface course, BS 4987 Cl.7.4 A well graded aggregate, i.e. a distribution of all aggregate sizes, with a high proportion of "fines" Load carrying ability by mechanical interlock, almost impervious, quite durable  5.3 +/- .5 NEGATIVE TEXTURE Not a highly textured surface, average texture depths around 0.8mm.  Good ride quality, low noise generation characteristics Can be laid 25/40mm. thick
10mm. Open graded surface course, BS 4987 Cl.7.2 A graded but "open" structure with few fines A voided matrix, as much as 20% voids, porous, a much shorter life compared to an impervious material  5.3 +/- .5 NEGATIVE TEXTURE Significant texture over 1.5mm., good SRV when a high PSV aggregate is used Good ride quality, low noise generation characteristics Can be laid 25/40mm. thick
10mm. Size porous asphalt surface course, 
BS 4987 Cl. 8.2		 A largely single sized grading with approx. 50% of 10mm. aggregate size, a requirement for 2% hydrated lime in filler to improve durability A voided matrix, approx. 20% voids, porous, a much shorter life compared to a well specified impervious material  5.2 +/- .5 NEGATIVE TEXTURE Significant texture over 1.5mm., good SRV when a high PSV aggregate is used Good ride quality, low noise generation characteristics, reduces spray in rain Can be laid 25/40mm. thick, a minimum porosity is often specified to guarantee spray reduction
6mm. Dense surface course, 
BS 4987 Cl. 7.5		 A well graded aggregate, i.e. a distribution of all aggregate sizes, with a high proportion of "fines" Load carrying ability by mechanical interlock, almost impervious, quite durable  6.3 +/- .5 NEGATIVE TEXTURE Not a highly textured surface, average texture depths around 0.8mm.  Good ride quality, low noise generation characteristics Can be laid 15/30mm. thick, more commonly used for footway surfacing
6mm. Medium graded surface course, 
BS 4987 Cl. 7.6		 A graded aggregate structure but with relatively few fines Load carrying ability by mechanical interlock, voided structure so porous, a modified bitumen is recommended  5.4 +/- .5 NEGATIVE TEXTURE  Reasonable texture approx. 1.0mm., good SRV when a high PSV aggregate is used Good ride quality, very low noise generation characteristics Can be laid 15/30mm. thick, used as a road surface in low speed situations
NOTE  -  It is important to have a good tack coat/bond coat on the receiving layer to all surface courses, but with thin surface course layers it is ABSOLUTELY ESSENTIAL, and this applies to proprietary and generic bituminous mixtures
 
"Surface Applied" road surfacing systems
(  Surface Dressing   -   Slurry Macadam / Micro-Asphalt  ) 
Material Description Matrix Properties Durability (4) Bit.
T.B.C.(d) %		 Texture (1)/ Skid Resistance (3) Noise (2)/ 
Ride Quality		 Considerations
Surface Dressing (SD) Not applicable to a strictly surface applied material A good dressing has a long life in the right situations, SD is not suitable for high stressed areas, e.g. tight bends, junctions and factory accesses carrying high numbers of HGV's Rates of spread of actual bitumen range between 1 & 2 litres per square metre POSITIVE TEXTURE Most often well in excess of 1.5mm. Skid resistance will also depend heavily on the PSV of the aggregate source but is usually high The ride quality largely depends upon the ride quality of the surface that is dressed, "bolder" dressings can be noisy These processes will extend the life of an existing strong road pavement by sealing the surface to the ingress of water and subsequent weakening of the road structure, they will arrest surface fretting, and they will restore surface texture and skid resistance, dependent upon aggregate size and its PSV
Slurry Macadam / Micro-Asphalt This may be applicable to this material where depressions in the road surface receive a greater than normal nominal  thickness layer, and it is likely the matrix will be similar to a bituminous macadam of the same nominal size These products establish a "settled" surface almost immediately, but their long term life is likely to be similar to SD, assuming a strong road pavement this can be up to 10/15 years depending upon site conditions  Bitumen contents, although initially in the form of an emulsion are quite similar to bitumen macadams POSITIVE TEXTURE Most likely in excess of 1.5mm., skid resistance will also depend heavily on the PSV of the aggregate source, where these materials are laid relatively thickly loss of texture can be hastened  The ride quality largely depends upon the ride quality of the surface that has received treatment but some improvement usually occurs, larger nominal sizes can be noisy

Notes

(1) Texture is stated as measured by Sand Patch
(2) Noise generation is more influenced by traffic speed and type of traffic, i.e. width of tyre and tyre pressure, than the surface characteristics of most commonly used surfacing materials.
It has been established that open textured surface courses, with a substantial void content, around 20%, can act as a sound absorbing layer while the voids remain open, and therefore tend to be a little "quieter" than dense materials.
(3) Skid Resistance of a surface results from a combination of texture depth (and perhaps type of texture), and the PSV of the aggregate in the bituminous mixture. 
(4) Durability of all bituminous mixtures can be improved by the the use of an APPROPRIATE bitumen modifier, or the use of an APPROPRIATE pre-modified bitumen.

Terminology

(a) PSV  -  Polished Stone Value
(b) SRV  -  Skid Resistance Value
(c) HAPAS  -  Highways Authority Product Approval Scheme
(d) TBC  -  Target Binder Content

Appropriate British Standards for Bituminous Mixtures, "Old and New"

BS 594:Hot Rolled Asphalt for Roads and other Paved Areas :-
Part 1:Specification for constituent materials and asphalt mixtures
Part 2:Specification for the transport, laying and compaction of rolled asphalt

BS 4987: Coated Macadam for Roads and other Paved Areas :-
Part 1:Specification for constituent materials and mixtures
Part 2:Specification for transport, laying and compaction

and the new specifications that were introduced on 1st. January 2008

BS EN 13108-1   : 2006:Bituminous mixtures. Material specifications. Asphalt Concrete
BS EN 13108-2   : 2006:Bituminous mixtures. Material specifications. Asphalt Concrete for very thin layers
BS EN 13108-3   : 2006:Bituminous mixtures. Material specifications. Soft asphalt
BS EN 13108-4   : 2006:Bituminous mixtures. Material specifications. Hot rolled asphalt
BS EN 13108-5   : 2006:Bituminous mixtures. Material specifications. Stone mastic asphalt
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,

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

Images of the typical surface appearance of some of the bituminous mixtures referred to above,
can be can be seen by viewing the web page,  BITUMINOUS MIXTURE SURFACES, A CLOSE STUDY

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