[ Top ] Contents Page

  The Idiots' Guide to Highways Maintenance

Copyright © 2000/15, C.J.Summers


ROAD PAVEMENT DESIGN (TRRL Laboratory Report 1132)


BASECOURSE  (now referred to as BINDER COURSE)
ROADBASE  (now referred to as BASE)


This is a very, very, brief summary of the process of designing, and constructing a new road pavement and just hinting at the complexities involved, without even mentioning cost of construction.

The purpose of a good road pavement design is to effectively dissipate/spread the load from the tyre footprints of the vehicle over a wider area of the existing natural surface, so that the strength of the natural ground will be sufficient to carry the reduced loading without causing any type of failure, over the proposed route of the road.

This usually means a road pavement will be constructed of a number of layers, with each layer having a specific role, engineering purpose, in the road pavement construction.

A good road pavement design should cost effectively produce a durable road pavement that has a significant life before any attention is required to the matrix of the construction.

Below are examples of strong heavy duty road pavement designs, used in new constructions, that will provide a long life, possibly 40 years depending upon traffic, with the opportunity to overlay or replace the surface course if / when required.
TRL Report TRL639 - Guidance on the development, assessment and maintenance of long-life flexible pavements, is an interesting read.

In the image below and to the left the road pavement is regarded as a semi rigid construction, with the major stiffness component of the pavement provided by a cement bound material (CBM) lower base, often known as "lean concrete".

In the image below and to the right the design is of fully bituminous construction with the stiffness coming from the nature of the bituminous mixture and the use of 50pen. binder, in most, if not all material layers.

It is the case that the 50pen. bitumen will be significantly stiffer than 50pen. in the laid mixture due to the time it has been stored before mixing, elevated to high temperatures during mixing, stored for a considerable time in hot storage bins, and finally the time taken to transport it to site at a hot, able to be laid and fully compacted, temperature.
If bitumen is kept at elevated temperatures over time it will harden, if it is truly overheated i.e. exceeds the specified temperatures permitted for its storage and use, then the engineering properties of the bitumen will be damaged proportional to the amount of overheating.

It is permitted to use stiffer grade binders but I personally do not advise their use under UK procedures of laying and compaction, if you wish to know more you need to study the way that French highway engineers employ stiff, low penetration grade, bitumen in their road pavement construction.

The thickness of material layer will have been decided from reference to one of the various design documents available, some of which are listed below, relating to the amount, type and projected growth of traffic that will be using the road.
The design will also have considered the results obtained from preliminary site investigation of the route/site to determine the strength of the subgrade, and also the requirement and nature of the drainage to achieve and maintain a low water table below the construction.

The wearing/surface course can be overlaid or replaced as required, with the strength of the lower pavement remaining and perhaps even improving as materials compact under traffic and 50pen. bitumen binder hardens slightly, producing a stiffer road pavement.

Unfortunately the recent "modern" road pavement designs using initially much stiffer bitumen and thinner design thickness of the component layers, and subsequently the "bound" pavement thickness, have placed the length of life of an all bituminous road pavement in some doubt, and this topic is worthy of further study.
Although it appears the error of using this type of stiff bituminous road pavement construction has already been realised due to a number of major failures, and the use of the very stiff bitumen binder grades are no longer permitted options in bituminous base and binder course.

It is also extremely important to select the most appropriate surface course for your road pavement design, this is the layer which is the "front line" of your road pavement, if you choose the wrong bituminous mixture the whole road pavement could subsequently suffer.

However the basic parameters of any surface course are surface texture and skidding resistance, these topics are covered elsewhere on this website.

In my opinion durability is also of significant importance in a surface course, as the currently much discussed issue of pothole formation and cost of repair highlights.

I have chosen to use images of cores taken from road pavements that have hot rolled asphalt (HRA) surface course and precoated high polished stone vale (PSV) aggregate chippings because I am still of the opinion that a correctly specified, and laid, HRA and precoat surface course remains the premium surface course available for road surfacing.

These "bound" sections of the road pavement will have been constructed on top of the required thickness of sub-base, and if required, capping.


Semi rigid road design, click to enlarge.
A fully bituminous road pavement, click to enlarge.

ROAD PAVEMENT DESIGN (TRRL Laboratory Report 1132)

TRRL Laboratory Report 1132 : The Structural Design of Bituminous Roads

The current most appropriate document for general PAVEMENT DESIGN is
If you are aware of the current "normal" traffic flows, type of traffic and CBR VALUE of the SUBGRADE  "1132" will guide you through the pavement design for that road. 


TRL Report TRL615 - Development of a more versatile approach to flexible and flexible composite pavement design

This document deals with the idea of increasing the versatility of road pavement design to give the engineer a wider choice of materials and design configurations.
This increased versatility will lead to more economic designs by allowing new materials, recycled materials and a wider range of secondary aggregates and binders to be used.
It offers the potential to enable stronger foundations to be constructed, incorporating hydraulically bound materials, providing the option of reductions in the more expensive surfacing layers.
This document cross references to much of the work in, HD 26/06 and Draft HD 25/06.

I regard this as a need to read publication with the introduction of HD 26/06 and Draft HD 25/06, it contains much useful information, you may not agree with all of it, but you need to read it.
In my opinion much of the practical use of recycled and secondary material will depend upon the traceability and the uniformity of the product supplied, as well as meeting routine laboratory test requirements during placement and processing.
It is my concern that the good intentions of elements of this suite of publications may be open to abuse.
This caution does not mean I do not approve of recycling and reprocessing, I whole heartedly do, but it needs to be performed in a controlled manner in order that recycling/reprocessing builds a reliable reputation.
There have already been documented examples of the use of "new" reclaimed materials used in the construction of road pavements causing considerable problems after the passage of time, these problems being very expensive to rectify.


TRL Report TRL639 - Guidance on the development, assessment and maintenance of long-life flexible pavements

A very informative report that describes the concept of long life road pavements, the new concept of robust pavements, and how existing, less well constructed pavements, can be maintained so that they may become long life pavements.


Still using,
Road Note 29

Road Note 29 has been replaced by TRRL Report 1132.
But it is not a simple replacement as
TRRL Report 1132 was introduced to account for increased volume and weight of traffic, although even TRRL 1132 has been superseded for situations of extremely high traffic density and weight.
Road Note 29 designs may still be used in low traffic situations, if you choose to do so. 

There are also a number of excellent "DfT STANDARDS" and "DfT ADVICE NOTES" especially applicable to highly trafficked motorways and trunk roads, however they also tend to be used as a guide for more highly trafficked local road networks.
Most of these standards and advice notes are now included  in the,
Design Manual for Roads and Bridges, Volume 7, some of the more useful related to highway and footway design are listed below, and they can be downloaded in .pdf format from the Highways Agency website, accessed from the included link. 

This standard forms part of the Design Manual for Roads and Bridges - Volume 7 - Pavement Design and Maintenance, Section 2 - Pavement Design and Construction.
This informative part of Volume 7 covers a number of topics appropriate to assessing the strength (CBR) of the road pavement subgrade and designing the "foundation" thickness (capping and sub-base) relevant to the strength / weakness of the subgrade.
It also mentions the subject of drainage which is paramount in maintaining or even improving the strength of the subgrade.
(Able to be downloaded from the internet.)

A draft document
HD 25/06 - Pavement Foundations, is out for "consultation", copies are available if you go through the correct channels, it is well worth perusing if you can obtain a copy, but as yet I have not found copies on any website.

This standard forms part of the, Design Manual for Roads and Bridges - Volume 7 - Pavement Design and Maintenance, Section 2 - Pavement Design and Construction.
This is a "need to know" reference on road pavement designs, amongst many good background subjects it includes information on standard designs procedures, alternative design procedures and materials used in the road pavement. This is where you will find reference to designs for road pavements that carry high levels of "heavy" traffic.
(Able to be downloaded from the internet.)

This standard forms part of the, Design Manual for Roads and Bridges - Volume 7 - Pavement Design and Maintenance, Section 2 - Pavement Design and Construction.
This is a very comprehensive guide on the subject of footway design, and I regard it as necessary reading. 
It includes sections on design considerations, structural design and construction materials amongst a wealth of useful information.
(Able to be downloaded from the internet.)


This standard forms part of the, Design Manual for Roads and Bridges - Volume 7 - Pavement Design and Maintenance, Section 4 - Pavement Maintenance Methods.
This is an excellent reference source on the defects encountered on footways. It includes good information on the causes of defects, possible treatments, techniques and materials that can be used.
(Able to be downloaded from the internet.)


This is an exceptionally comprehensive guide on all aspects of the stated subject matter, and it includes many pictures of common maintenance problems, I believe it is a must for your technical library. 

This gives a total road pavement thickness of 450mm., the thickness usually regarded as the minimum needed to be resistant to frost damage.

* While I realise the current Highways Agency/DfT contract documents do not permit the use of  hot rolled asphalt and precoats as a surface course for the Strategic Road Network in England and Wales it remains my preferred option as the premium bituminous mixture for a surface course wherever traffic conditions or a road closure permits it to be laid, meaning the extra operating space required to supply the chipping machine hopper with precoated chippings.
It  is my opinion that it will provide the most durable surface course and the best use of scarce high PSV aggregate as the precoat layer, with the scarce high PSV layer not having to make up the entire layer thickness.

In view of the recent changes in terminology relating to the various road pavement layers , e.g. wearing course becoming surface course, to bring the UK in to line with Europe, be VERY, VERY CAREFUL that all parties know to what materials they refer to in the course of conversations, or you could easily find yourself in disputes.
I would suggest you quote clause and table numbers from British Standards for your bituminous materials, and preferably in writing, that way there should be no possibility of confusion.

The above statement may now no longer relevant since the introduction on January 1st. 2008 of
BS EN 13108 Bituminous Mixtures - Material Specifications "family" of standards.
In these new specifications there is no previous history of different descriptions for the same material, but I believe confusion is still possible so I still advise care in describing bituminous mixtures.

However please be aware that the description of a bituminous mixture/material, in any part of
BS EN 13108, does not refer to its actual specification, composition, recipe as it did with BS 4987 and BS 594 mixtures, you must specify the mixture that you actually require within the parameters laid down in the various new standards.

If you are still struggling with the new BS EN "European" standards introduced on the 1st. of January 2008 I suggest that you rely on,  
PD 6691:2010
(supersedes 2007 edition) :Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications
to keep you safe. 
In my opinion this document contains defined "sample" mixtures that will supply all you needs for a local highway network, but take care to select the most appropriate materia
The "sample" specifications are to be found in the appendices at the back of the document.

You must also be aware that you are only permitted to use proprietary "Thin Surface Course System" HAPAS approved bituminous mixtures on motorways and trunk roads in England and Wales.

If you need background information on this recent major change in bituminous mixture specification it can be found in the
Summer 2007 Newsletter.
I am leaving my original wording/descriptions in this page at this time and allowing the new terminology a little longer to become established before I refrain from including a reference to the old names/descriptions for the various pavement layers and bituminous mixtures, as in my opinion it will take a very long time before the old terminology is totally replaced/forgotten.

Even though I am not updating the content of this page with reference to the recent amendments in the,
Specification for Highway Works, Volume 1 :-
Series 800   - Road Pavements - Unbound, cement and other hydraulically bound mixtures
Series 900   - Road Pavements - Bituminous bound materials
Series 1000 - Road Pavements - Concrete materials 

but leaving the original fairly simple description of the following layers in a road pavement, it is most important that you make yourselves familiar with the significant changes that have occurred in these particular Series and the impact that it may have on the design of a road pavement and the nature of the materials that can now be utilised in road pavement construction.
The recent amended 800, 900and 1000 Series are able to be downloaded for study at the excellent Highways Agency website,

Manual of Contract Documents for Highways Works, Volume 1-Specification for Highway Works

Guidance for the use of Volume 1, can be found in,  
Volume 2-Notes for Guidance on the Specification for Highway Works
which is also able to be downloaded from the above website.

It goes with out saying that the support of a qualified and experienced Soils and Materials Engineer, or Road Pavement Engineer, with access to a well equipped laboratory would be of profound benefit in assessing the materials now available for road pavement construction. 
Personally, I do not see how you can assess new materials and procedures without such support at a local level.


The WEARING COURSE (surface course) is the top layer of the ROAD  PAVEMENT and as such is (usually, but not always) designed to be impervious to the ingress of water, have an even, (N.B. "even" NOT smooth) RUNNING SURFACE, be durable, and have a high resistance to skidding, and be chosen so as not to deform under the weight of traffic appropriate to the road.

Note, the designing of wearing courses to be impervious is becoming less of a feature with "modern" wearing courses, especially with "THIN SURFACINGS".
The very "thinness" of some proprietary materials and their open graded nature to provide texture of the "negative" kind and a claimed reduced noise running surface mean that you can no longer rely upon the wearing course to prevent water entering the road fabric. 
I have included HRA wearing course as the surfacing material in this example because I still believe this material to give the best "whole life costing" where noise is not a serious factor in the road design.
You always have the option of using a 14mm precoat to reduce noise generated by the action of the tyre on the road surface.

BASECOURSE  (now referred to as BINDER COURSE)

This is the layer of material below the WEARING COURSE and above the ROADBASE, except in some "new" high stability road designs that we won't cover here.

Be aware of the introduction/re-introduction of Enrobé à Module Élevé (EME)  from France, as a stiff bituminous load bearing material used as binder course and base.
It is being referred to in the new design guides listed above so you do require some knowledge on this subject, and as far as I am aware it is being promoted as only available as proprietary (branded) bituminous mixture, this is not true, although I am not aware of a BS EN specification for this mixture.

However, guidance on specifying this material is available as example specifications of defined EME2 mixtures in
Table B.8
PD 6691:2007
(supersedes 2007 edition) : Guidance on the use of BS EN 13108 Bituminous mixtures -  Materials specifications.
(Personally, I regard the binder contents rather low when compared with the binder contents of hot rolled asphalt (HRA) base and binder course mixtures of Table C1 of P.D. 6691.)

I, personally, do not like to hear engineers saying that they are bringing the roadbase (base) material up to the underside of the wearing course.
Why we cannot say we are increasing the stiffness of the basecourse (binder bourse) I do not know, because that is what is happening.

The BASECOURSE (binder course) is a load spreading layer, spreading the load imposed on the WEARING COURSE (surface course) over a wider area of the ROADBASE (base). 
It can also be a specialised layer, e.g. both STIFF and impervious under a POROUS ASPHALT wearing course to prevent the ingress of water into the road fabric.
Also the BASECOURSE (binder course) is laid to tight tolerances with a good surface so that the WEARING COURSE (surface course) can be laid accurately to "line and level" and so produce a better ride quality of the WEARING COURSE (surface course).
BASECOURSE (binder course) is most commonly a bituminous material, and can be either HOT ROLLED ASPHALT or DENSE BITUMEN MACADAM (asphalt concrete).

ROADBASE  (now referred to as BASE)

The ROADBASE (base) is the main load-bearing / load spreading layer in the road structure and is usually 100mm. or more thick depending on the loading of the traffic for which the road is designed, (we are not talking housing estate roads here).
The ROADBASE (base) is usually a bituminous material, dense bitumen macadam (asphalt concrete) or hot rolled asphalt.
It is quite usual to have an upper and lower base in a road pavement designed to carry very heavy traffic levels.

Details of bituminous mixtures are given in:-
The Specification for Highway Works, the 900  series,
BS EN 13108-4   : 2006:Bituminous mixtures. Material specifications -  Hot rolled asphalt (superseding BS 594)
BS EN 13108-1   : 2006:Bituminous mixtures. Material specifications - Asphalt Concrete  (superseding BS 4987)


However the ROADBASE (base) can also be, and quite frequently is, LEAN CONCRETE.
But I am afraid lean concrete is no longer called lean concrete, it is called CEMENT-BOUND MATERIAL although you will still come across the use of the term "lean concrete".

CEMENT-BOUND MATERIAL CATEGORY 4, CBM4, being the materials usually used as roadbase. CBM4 being stronger than CBM3.

The CBM's are specified in the,
Specification for Highway Works,  in the 1000 series, Road Pavements-Concrete and Cement-Bound Materials. 

The specifying of concrete for road pavements, as for use in structures, has gone through major changes recently, and many of them, so it is likely the terms I using here are out of date, so, be guided by the most recent relevant design guide listed above, and always remember to include your strength requirement, as insurance.


This particular layer in ROAD PAVEMENT construction, is quite often used to refer, some what misleadingly, to the material GRANULAR SUB-BASE.
GRANULAR SUB-BASE is usually the material used in the SUB-BASE LAYER, but materials other than GSB  can be used in the SUB-BASE layer.

GRANULAR SUB-BASE (GSB) is a graded granular material, usually a "hard rock".

There are now a number of types of GRANULAR SUB-BASE with GSB Type 1 and GSB Type 2 being the types most often quoted and used.
The various types of GSB are specified and described in the
Specification for Highway Works, the "800" Series, and you must refer to this document.

GSB Type 2 is a much finer material, has a much wider grading envelope and is of a lesser "engineering" quality than GSB Type 1 which has a "tighter" grading envelope allowing much less fines and encouraging a "well graded" material.

N.B. Be aware that these materials are now collectively know as "Unbound Mixtures, Type 1 or 2, or several other variations, again I leave the original description in this text but be aware of the changes, read the current "800 Series" for guidance, able to be download from the previously provided website link.

The SUB-BASE can also acts as a drainage layer, and some organisations have a "Type 3" GSB, designed to have less fines content to improve the drainage properties of the material.


And, if you do not realise how important drainage is in constructing and maintaining highway pavements you have missed out in a major part of your highways maintenance education, and you may be in the wrong job !

SUB-BASE is also a load spreading layer, especially during construction, carrying site traffic and protecting the SUBGRADE, to an extent, (as long as conditions are not too wet).
Since the introduction of a CAPPING LAYER material this role of GSB has become somewhat confused. 


When the California Bearing Ratio (CBR) of the subgrade is less than 5%, it is normal to require a suitable capping layer of low cost material.
This capping layer is usually a granular type material designed to provide a working platform on which sub-base construction can proceed with minimum interruption from wet weather, and capping is also used to minimise the effect of a weak subgrade on pavement strength. 

Be careful when specifying/buying CAPPING LAYER it can often be cheaper to increase the layer thickness of the TYPE 1  GSB. 
The specification requirements for capping are to be found in the,

Specification for Highway Works : Volume 1 : Series 600 : Tables 6/1 and 6/2. 
There are coarse and fine gradings, 6F1 and 6F2.

There is a good explanation of CAPPING LAYER in,
TRRL Laboratory Report 1132, The Structural Design of Bituminous Roads


The SUBGRADE is the layer of naturally occurring material the road is built upon, or it can refer to the imported fill material  that has been used to create an embankment upon which the road pavement is constructed.

The strength of the SUBGRADE or the material constituting the SUBGRADE is  commonly measured using the CBR test.

The strength of the SUBGRADE is an important factor influencing the thickness of the road pavement design. 

Where the SUBGRADE is weak, i.e. a low CBR, it will be necessary to have a CAPPING LAYER over the subgrade to increase the strength before the actual road pavement thickness is designed. 

For information on the California Bearing Ratio (CBR) test, press --------> HERE

For example of road design found on the web, press ---------------------------> HERE

[ Top of Page ]