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Binding agreement

Highways Materials

Greater harmony ushered in by a new European bitumen standard could be just the tonic for future UK bitumen and asphalt developments.

Bitumen and asphalt design has become more scientific during recent years, but remains partially empirical, open to an element of interpretation and dependent on many variable factors - including national ones. A new European bitumen code which came into force at the beginning of this month is intended to harmonise specification of bitumen grades across Europe.

The next step involves developing scientific prediction of bitumen and asphalt behaviour.

This will allow a further significant swing towards the ultimate goal of performance-based specification. However, regional differences in bitumen and asphalt design must first be overcome.

In the UK, technological developments in bitumen and asphalt have been imported from the continent and adapted to British design methods and materials, many with great success. But, occasionally, application of European technology to UK roads would appear to be less successful.

Recent problems with high modulus base (HMB) asphalts provide a case in point. In spring 2000 the Highways Agency (HA) suspended use of two grades of HMBs after tests on core samples showed lower than expected performance. The HA has since been investigating the problem.

HMBs incorporate stiff binders to produce a material that provides either equal design life from a thinner construction or extended durability from the same layer thickness.

Exxon Mobil has more than a passing interest in HA endorsement of HMBs. According to the company's technical manager Chris Rayner, Exxon Mobil is Europe's largest supplier of hard grade bitumen. Successful use of these materials on the continent points to how they may be applied to UK roads, he says.

'HMBs used in the UK have a larger nominal stone size and lower binder content, which generally produce a material more sensitive to durability.' he says.

HMBs are claimed to work well in France, in conjunction with a smaller stone size and a more bitumen rich mix that is easier to lay. 'French specifications demand a more thorough approach to compaction of the layer, ' says Rayner. He thinks more of the French methodology may have to be imported to ensure satisfactory performance in the UK.

Exxon Mobil is one of five main UK bitumen suppliers, with Shell, Nynas Bitumen, Total Fina Elf and BP, all of which are members of the Refined Bitumen Association (RBA). According to RBA technical director Tony Harrison, adoption of the new bitumen standard is expected to increase trade and exchange of best practice between European countries. Work to advance bitumen technology and specifications will continue, he says.

'Virtually all major players in bitumen and asphalt supply are multinational companies with the ability and incentive to apply their knowledge to all countries they operate in.

'These efforts have been helped by the new specifcation developed by CEN (Committee for European Normalisation) and the consequent removal of trade barriers and increased European harmonisation. But we need more of a handle on performance related specifications to speed up exchange of technical knowledge.'

The next generation of bitumen specs will rely on greater understanding of relationships between scientific properties and real performance behaviour.

This is a major target occupying CEN and the main bitumen suppliers, Harrison says.

Rayner says the UK bitumen and asphalt industry has effectively taken two steps forward and one step back. The old British Standard 50 pen bitumen has traditionally been the material of choice for road bases, and although the much stiffer 15 pen grade is temporarily out of HA commission, a 35 pen mix is now being used.

RBA members are looking forward to getting HMB 15 back in use as they represent a good approach to long life roads, adds Harrison. 'We should be using 15 pen HMBs again this year.' The bitumen and asphalt industries are currently working together to that end.

Making the grade UK aggregate and coated stone products supplier RMC Aggregates has produced a handy calculator for converting superseded bitumen grades into their nearest European equivalents.

The European bitumen specification BS EN 12591 replaced the British penetration grade spec BS 3690 Part 1 on January 1 2002. There are no direct equivalents to many of the outdated penetration grades, although some, including 200, 50 and 35 pen grades, have very similar European alternatives

The guide gives approximate rates of asphalt spread appropriate to the new European bitumen grades, and indication of how minimum and nominal asphalt concrete layer thicknesses now apply to aggregate size is also provided.

RMC has added a note to point out that the calculator can only be used as a guide, such is the variety of influential factors.

For a copy of RMC's calculator, contact: ian. southcott @rmc.co.uk or tel 01527 57 57 77.

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