European Standards For Asphalt
An introduction to the new European Standards and PD document, and their application in the UK
By John B-Bullock, Aggregate Industries (technical Manager - Yeoman Asphalt)
In the UK, the new European Standards for asphalts came into force on 1 January 2008 and the previous standards, BS 4987 and BS 594, were withdrawn from use in December 2007. This paper gives an overview of the main requirements of the new standards and, in particular, the differences compared with the previous standards. While the asphalt mixtures themselves do not change, the descriptions and the methodology leading up to their manufacture, supply and conformity differ from those previously applicable.
The new European Standards for asphalt comprise two families of standards:
- the EN 13108 family of product and associated standards
- the EN 12697 family of testing standards.
These ENs (the UK versions of the ENs are designated ‘BS EN’ and are published by BSI) replace the current Part 1 of BS 49871, which addresses coated macadams, and Part 1 of BS 5942, which deals with hot-rolled asphalts. Additionally, the BS EN 12697 series of testing standards will, at the same time, replace the majority of those parts of BS 598 where an equivalent European test Standard exists. A list of the mixture standards is shown in table 1 with the testing standards listed in table 2.
European Standards (ENs) are drafted using a similar process to that used by the British Standards Institution. However, as a result of their wide applicability across the 27 member states of Europe that have agreed to adopt the European Standards for asphalt, their practical use without national guidance for individual countries would be almost impossible and would inevitably lead to the selection of inappropriate mixture compositions, resulting in performance levels that would be unacceptable to clients, producers and laying contractors. To address this, the UK has published a ‘national guidance’ document (designated ‘Published Document’ or PD) to address the applicability of the new EN 13108 series in the UK.
The guidance document – PD 6691, Asphalt - Guidance on the use of BS EN 13108 ‘Bituminous Mixtures - Material specifications’ – is particularly important as issues such as climate, traffic density and road safety demand specific properties and performance criteria to be selected and applied within the UK.
Development of the ENs started after the issue of the Construction Products Directive (CPD) in December 1988 with the work on harmonizing the standards commencing in 1989. The first EN construction standard published was that for cement in 2000. The bitumen standard EN 125913 was published later the same year. The European Standards for aggregates were published in 2002 and implemented in January 2004. It has taken until this year to reach a point where the asphalt standards can be implemented, some 18 years after work commenced.
The principle involves the harmonization of individual European countries’ specifications and test methods. While there are currently 27 countries represented and bound to implement the ENs, in 1989 there were only 12. Their implementation will remove the geographical barriers to trade within Europe and introduce a common approach to testing and performance assessment.
Key differences between the old British Standards and the new European Standards for asphalts
It is very important to note that the new standards have greater authority and legal standing than the previous British Standards, therefore requiring particular attention to the detail of product certification through CE marking.
The British Standards left compliance to be a requirement of the contract, whether directly with the customer/user of the road or pavement, or through other contractual supply arrangements including Sector Scheme 14, whereas the European Standards bring product compliance and conformity into the materials standards.
The ENs relate to the product ‘in the back of the lorry’, but in the UK some properties are installation trial dependent (the protocols for the trials are annexes within the new BS 594987, which addresses the issues covered in Part 2 of both BS 4987 and BS 594 and is discussed below). However, there will still be requirements written into contracts including, for example, proof of adequate compaction, and the same protocols applicable to what is called type testing, proof of conformity prior to supply, will be utilized.
The grading and binder content for the asphalt products are based on the principle of selecting specific target values and then applying the production control tolerances to these individual target values. This concept was first introduced in BS 4987 and BS 594 in 2005 as a lead in to the new standards but only against the 2mm sieve. The specification tolerances for grading and binder content are not included in the individual product standards; they sit within EN 13108-21 and, of course, PD 6691.
Target binder contents are adjusted according to the relative density of the aggregate combination used, thereby ensuring that the more critical aspect of the binder volume of the mixture is fully taken account of in the design of the mixture. Within PD 6691 the example specifications found in the annexes already take the nature of the aggregate into account and, therefore, state the actual binder contents that should be found on analysis.
In terms of product conformity, binder and filler contents are not adjusted to allow for any sampling bias. While this is particularly relevant to the larger 32mm lean binder content mixtures, in respect of all mixtures it increases the importance of ensuring that representative samples are taken from the material, whether it be in the lorry, a heap or even from the laid mat, for contractual audit testing.
There are provisions for the use of reclaimed asphalt in each of the asphalt product standards and reclaimed asphalt is formally recognized as an ingredient and covered under EN 13108-8, where it is identified and categorized according to any contaminants and the properties of the binder.
EME2 (Enrobé à Module Élevé), the very stiff and resilient base and binder course material that has now replaced HMB35 and which utilizes approximately 5.5% of a very hard 15/20 or 20/25 pen binder, is now officially covered by a national standard in addition to its inclusion in the Specification for Highway Works4,5. SMA (stone-mastic asphalt), which again was not covered under a British Standard specification, is now addressed by EN 13108-5 (SMAs also feature in a number of proprietary BBA/HAPAS-certified mixtures6 for use in a number of motorway and trunk road applications).
All mixtures supplied in compliance with one of the EN 13108 product standards are prefixed by an abbreviation which identifies the mixture type, the principal ones being: AC (asphalt concrete), HRA (hot-rolled asphalt) and SMA (stone-mastic asphalt). More details on mixture designations are given below.
The national guidance document for asphalt mixtures – PD 6691
This is the indispensable document for engineers, specifiers, contractors and suppliers, ie for all those involved in asphalt in the UK. It facilitates the selection of the appropriate asphalt mixture for the majority of paving applications and must be read and understood before attempting to grasp the contents of the individual standards in the BS EN 13108 series. Without the advice given in PD 6691 it would be almost impossible to specify asphalt mixtures capable of handling and performing in an equivalent manner to the mixtures used previously.
PD 6691 principally covers BS EN 13108-1 Asphalt Concrete, BS EN 13108-4 Hot Rolled Asphalt and BS EN 13108-5 Stone Mastic Asphalt, with limited reference to BS EN 13108-2 Asphalt Concrete for Thin Layers, which is covered primarily through BBA/HAPAS certification. PD 6691 does not cover BS EN 13108-3 Soft Asphalt, which is principally used in the Nordic countries, BS EN 13108-7 Porous Asphalt or BS EN 13108-6 Mastic Asphalt. While the use of porous asphalt on motorways and trunk roads is now limited in the UK, guidance can still be sought from the Specification for Highway Works. For mastic asphalt, contact should be made with the Mastic Asphalt Council7 and for asphalt specifically to be used on airfields, reference should be made to the series of Defence Estates Standards8.
For ease of use, the format of PD 6691 has been made very similar to that of BS 4987 and BS 594. While it provides guidance on the use and selection of asphalt mixtures, full details of the specification requirements for each of the principle mixture types used in the UK (AC, HRA and SMA) are covered and these take account of the requirements associated with UK climate, traffic and safety issues. Importantly, it also contains stand-alone example specifications for the most commonly used asphalts from BS 4987, BS 594 and from the Specification for Highway Works for SMA and EME2.
The UK will continue to follow the empirically-based recipe approach to mixtures with, initially, few ancillary property tests; the principal tests being voids content and, where necessary, wheel-tracking and stiffness. Some of the EN test methods will change, eg wheel-tracking, where the method used is dependant on the material being tested and a different format is used for reporting the results of AC and SMA. As a result, testing against the new BS EN test method will be carried out alongside testing undertaken in accordance with BS 598-1109. This will continue until sufficient data are available to correlate the results from the two methods, which, in time, will permit the specification of limits applicable to the new EN methods.
The more fundamental approach to mixture design will take a number of years to be introduced and can only be considered once the required experience with the new European Standard tests and how they relate to performance in the pavement has been gained.
As previously stated, target binder contents are adjusted according to the relative density of the aggregate combination used to ensure the binder volume relationship for the mixture is maintained. However, in the UK allowance has always been made for another key attribute of the aggregate – its microtexture. Accordingly, target binder contents in PD 6691 are recommended depending on the type and nature of aggregate used. For CE marking purposes, the binder content by mass used in the manufacture of the asphalt is back-calculated to ensure that it is declared in the correct format, in line with the requirements of the EN. This binder content is quoted as Bmin on the CE mark and is likely to be a different figure to the actual binder content used in the manufacture of the asphalt, which is known as Bact and is the binder content against which product conformity is adjudged.
With regard to the use of reclaimed asphalt, it is classified in terms of its content of foreign matter and the properties of its binder. Recommended maximum limits for its use in mixtures are 10% for surface course and 50% for bases and binder courses. Where more than 10% and 20% of reclaimed asphalt is used in surface courses or bases and binder courses, respectively, the recovered binder from the mixture is evaluated by penetration testing. Thus, there is little difference from the current requirements given in the Specification for Highway Works.
In the UK the normative product designations of the asphalt concretes have been further expanded to permit identification of the full grading range of the mixtures used. These descriptions will be familiar to the users of BS 4987 materials as:
- dense (dense)
- open (open graded)
- close (close graded)
- med (medium graded)
- fine (fine graded).
The terms ‘HDM’ (heavy-duty macadam) and ‘HMB’ (high-modulus base) will also continue to be used to differentiate between the mixtures providing improved performance characteristics in use. Additionally, the PSV of the aggregate can be added to the designation for surface course and binder courses exposed to traffic in their early life. This is not a mandatory requirement within the ENs but must be considered a safeguard from the customer’s perspective ensuring that appropriate skid-resistant mixtures are supplied. To identify design mixtures from recipe mixtures, as applicable to HRA and AC dense mixtures, the designation ‘des’ will be used.
Examples of these product designations are:
- AC 32 HDM base 40/60
- AC 10 close surf 100/150 PSV60
- HRA 35/14F surf 40/60 des PSV50
- SMA 10 surf 40/60 PSV65.
It should be noted that individual asphalt producers may add further information to the product designation/description. In most cases, the description of proprietary asphalts will be unchanged.
Product conformity (Attestation of Conformity) is evidenced by CE marking, which is similar to a product conformity certificate and works on similar principles to those applicable to the ENs for aggregates. However, it is anticipated that the CE marking for asphalt will be more rigidly applied and more widely used in the UK.
PD 6691 contains four very important annexes which provide detailed information and guidance on the mixture specifications. These annexes occupy some two-thirds of the document and are the key reference areas for the selection of appropriate target gradings and binder contents for the type of mixture that is required. The use of the tables in the annexes ensures that the mixture composition and performance do not change significantly when transferring the specification of the asphalts to the new EN Standards. The contents of the annexes are: Annex A: The importance of example mixture specifications; Annex B: Asphalt Concrete; Annex C: Hot Rolled Asphalt (including PCC); and Annex D: Stone Mastic Asphalt
The fundamental differences between specifying the grading using the European Standards and the UK method contained in BS 4987 and BS 594, and other documents, is easier to understand by way of an example. Table 3 identifies the process for selecting target gradings and binder content from the target grading ranges and applying the FPC tolerance using 32mm asphalt concrete dense binder course as an example. Readers should note the use of a comma, which replaces the decimal point traditionally used in the UK.
For the purposes of monitoring quality and checking on the deviation of the results from the declared target values used for type testing, the compliance specification is better reported as the declared target values with the tolerances alongside in a similar format to that shown in table 4.
To convert the Bact target values within PD 6691 back to the regulatory Bmin value required by the BS EN Standard, the following formula is used:
Bmin declared = b x Bact
here b is the mean particle density of the aggregate mixture in Mg/m3, determined in accordance with BS EN 1097-6, with the resultant binder content being declared to the nearest 0,2%.
Thus, if the mean particle density of the aggregate in a mixture is 2,730Mg/m3, the binder content for CE marking purposes (Bmin) would be calculated from the selected target binder content (Bact) used to manufacture the mixture using the above formula as follows:
Bmin declared = 2,730 4,0% Bact
(4,12 rounded to the nearest 0,2%)
The process for the approval of CE-marked asphalt mixtures on contracts is formalized through type testing disciplines covered by EN 13108-20 and the factory product control covered by EN 13108-21. The asphalt industry is identifying this process using the acronym ‘CE MAP’, which stands for the CE Marking and Approval Process.
The CE MAP - The methodology for type testing and factory production control
Section 13 of PD 6691 details the requirements for the Evaluation of Conformity and the obligations of the asphalt producer. Target grading values and the target binder content are selected from the tables in annexes B, C or D in PD 6691 and type testing is carried out in accordance with BS EN 13108-20 to prove mix conformity with the EN/PD requirements at the selected target composition. The protocols identified in this section and used as part of the type testing procedure are found in BS 594987.
The type test record is completed and includes:
- a declaration of the types and sources of constituent materials
- test data proving conformity of the constituents with the relevant requirements
- a declaration of the target composition of the mixture
- test data showing conformity of the mixture with any specified performance requirements.
It is important to note that type testing is mixture specific and does not have to be repeated where the same type, source and proportion of constituents are used on another plant. However, certain type tests will not have to be repeated where the change in a constituent leads to a result which will be ‘better’ than that obtained with the original constituent.
For example, a mixture containing 100/150 pen for wheel-tracking would be assumed to comply if the binder grade was changed to 40/60 pen, providing no other mixture constituents nor any mixture proportions were altered.
Under the requirements for factory production control covered in EN 13108-21, the type testing procedure for an asphalt mixture has to be repeated at intervals not exceeding five years.
The EN 13108-21 factory product control (FPC) tolerances are applied to the target composition to formulate the compliance specification for conformity assessment.
The documented FPC requirements to manufacture the product are followed. It is noteworthy that these are narrower in scope than those contained in BS EN 9001 as customer-related issues and continuous improvement are not covered in EN 13108-21. Industry Sector Scheme 1410 refers to EN 13108-21 and expands on the FPC requirements. The FPC system, including the additional requirements of Sector Scheme14, is currently ‘third party’ assessed by registered notified bodies (NoBo’s).
The Sector Scheme 14 requirements are considered in addition to those from the ENs and the FPC procedures are followed to check conformity of the product supplied. Conformity is checked against the single result criteria within PD 6691 (EN 13108-21). The mean deviation from target is a new concept in the UK and is determined as a check on product consistency to ensure production results do not wander too far from the target values proposed under type testing.
In place of the Q level previously used in the UK, a system to determine the OCL (operating compliance level) is used. This is determined from the last 32 results (Q level utilized the last 30 results). However, the OCL is also marked down by a failure in mean deviation. The OCL determines the sample rates for groups of mixtures and linked to this are recommended minimum test frequencies identified within Sector Scheme 14. CE marking is carried out by self-certification with the CE marks being subject to examination by the notified bodies.
To ensure that clients and customers are made fully aware of the supply specification, it is likely that the majority of asphalt suppliers will provide an additional product information sheet for each of their mixtures, which will show the types and sourcing of all the ingredients along with the compliance specification and any ancillary test data.
BS 594987 Asphalt for roads and other paved areas – specification for transport, laying and compaction and design protocols
This new standard replaces Parts 2 of BS 498711 and BS 59412 and covers all the EN 13108 asphalt mixtures found in PD 6691. It has been produced for UK use because there is no European Standard that addresses the issues of transport, laying and compaction. In a similar format to Parts 2 of BS 4987 and BS 594, it must be considered as the baseline document for all hot asphalt pavement applications. It details minimum and maximum compacted thicknesses along with the accuracy of finish required for each of the pavement layers, and it provides improved clarity on laying, the formation of joints and all the other key issues associated with site preparation.
Readers should note that, at the same time as the full implementation of the European Standards for Asphalt, the Highways Agency have had to amend their documents and have produced two Interim Advice Notes covering the European Standards for asphalt and the amended 900 series. These are covered in summary below.
The application of pre-coated chippings to hot-rolled asphalt surface course is detailed along with the rates of spread and the measurement of texture depth. As previously, the importance and the requirements for achieving compaction are detailed. An area not previously covered which is now included is the sampling and testing for type testing the asphalt through the use of trial strips. This is detailed within a series of very important informative annexes, a number of which cover protocols previously found within the Specification for Highway Works, as indicated by the SHW clause references in brackets.
The annexes are:
Annex A: Details delivery and rolling temperatures for a full range of mixtures including SMA
Annex B: Details the rates of spread including the range of Asphalt Concretes, Hot Rolled Asphalts, Stone Mastic Asphalt and EME2
Annex C: Details the procedure for the determination of DBM/HDM/HMB design voids (SHW 929)
Annex D: Details the procedure for the determination of DBM/HDM/HMB deformation resistance (SHW 952)
Annex E: Details of the EME2 design procedure and the properties testing (extracted from TRL 636)
Annex F: Details of the procedure for the determination of performance HRA properties (SHW 943)
Annex G: Details of the procedure for the determination of SMA binder course properties (SHW 937)
Annex H: Details of the procedure for the determination of the HRA surface course design (including the requirements of BS 598-107)
Annex I: Calibration and operation of indirect density gauges including both nuclear and other indirect gauges.
Changes to the Highways Agency Specification for Highway Works
During November 2007 two Interim Advice Notes (IANs) were published by the Highways Agency covering the introduction of the European Standards for Asphalt: IAN 102/0713, entitled ‘The introduction of BSEN standards for bituminous mixtures – site supervision aspects’, and IAN 101/0714, entitled ‘Revised MCHW specification 900 series’.
IAN 102/07 introduces the changes taking effect from 1 January 2008 focusing on product description changes, the introduction of reclaimed asphalt and, of course, the implications of initial type testing, factory production control and CE marking. The IAN also identifies and details the transitional arrangements on contracts and explains that CE mark certification from all suppliers will not necessarily be available from January due to the assessment work needing to be carried out by the notified bodies before certification can be issued by suppliers.
The significant changes to the 900 series are covered in IAN 101/07 with references to PD6691 in respect to all the key areas of the changes, including the important product designation/description changes.
However, further important changes have been brought in by the Highways Agency since 1 January concerning the durability of base and binder course layers. These have arisen from joint collaborative work and have been fully supported by the asphalt industry in full recognition of the early-life failures and the resultant significant costs arising from water ingress to the lower layers of flexible asphalt pavements. Within clause 903, in summary, the following are the most significant additional requirements to the base and binder course layers to enhance those of the new BS 594987.
- A detailed method statement is required for each contract covering laying and compaction of each of the layers along with details of longitudinal and transverse joint formation.
- Full interlayer bonding required between all flexible pavement layers.
- All longitudinal joints will be outside defined wheel-track zones and offset by 300m from any parallel joints in the underlying layer.
Additional, and specifically for the base and binder course layers:
- Compaction will be measured within 100m of the joints in each of the layers with the voids content being not more than 2% higher than that required for the layer in the contract.
- All joints will be overbanded with a bitumen sealant to a width of 75mm either side of the joint.
- The same sealant will be applied to the whole of any freestanding edge on the outside of the finished pavement on the high side of the camber and on the low side when specified in App 7/1.
These changes in clause 903 do involve significant additional costs and are therefore not mandatory for contracts that have already been priced, awarded or were under construction before 1 January 2008.
- BRITISH STANDARDS INSTITUTION: Coated macadam (asphalt concrete) for roads and other paved areas – Part 1: Specification for constituent materials and for mixtures, BS 4987-1:2005, BSI, London.
- BRITISH STANDARDS INSTITUTION: Hot-rolled asphalt for roads and other paved areas, Specification for constituent materials and asphalt mixtures, BS 594-1:2005, BSI, London.
- BRITISH STANDARDS INSTITUTION: Bitumen and bituminous binders, Specifications for paving grade bitumens, BS EN 12591:2000, BSI, London.
- HIGHWAYS AGENCY et al: Manual of Contract Documents for Highway Works, Vols 0-6, various dates, The Stationery Office, London.
- BRITISH STANDARDS INSTITUTION: Bituminous mixtures for roads and other paved areas, Methods of test for the determination of texture depth, BS 598-110:2004, BSI, London.
- UKAS: National Highways Sector Scheme for Quality Management in Highway Works, Sector Scheme 16, For the quality assurance of the production of asphalt mixes, March 2004, Sector Scheme Advisory Committee, Feltham, Middlesex.
- BRITISH STANDARDS INSTITUTION: Coated macadam for roads and other paved areas, Specification for transport, laying and compaction, BS 4987-2:2003, BSI, London.
- BRITISH STANDARDS INSTITUTION: Hot rolled asphalt for roads and other paved areas, Specification for transport, laying and compaction, BS 594-2:2003, BSI, London.
- HIGHWAYS AGENCY: Interim Advice Note 102/07 ‘The introduction of BSEN standards for bituminous mixtures – site supervision aspects’, Nov 2007. Available for download from: www.standardsforhighways.co.uk/ians/index.htm
- HIGHWAYS AGENCY: Interim Advice Note 101/07, ‘Revised MCHW specification 900 series’, Nov 2007. Available for download from: www.standardsforhighways.co.uk/ians/index.htm