Avonmouth bridge on the M5 near Bristol is heavily trafficked and the deck of its steel central and anchor spans, to quote the Highways Agency, "displays significant movement". The crossing’s asphalt surfacing has a tendency to wear out fast as a consequence, and it requires maintenance with unseemly frequency.
The last time it was resurfaced was around the turn of the millennium. By 2006 it was apparent the bridge would soon need new asphalt and the Agency started a remarkable journey. It was determined to find a surfacing that would provide long life, require minimum intervention, resist water penetration and offer early life skid resistance.
The Agency and its agents scoured the world, ending up in Switzerland, talking to Aeschlimann International. This is the company that, in collaboration with others, surfaced Denmark’s 18km long Storebælt crossing.
Heinz Aeschlimann, the head of the asphalt specialist, took up the Avonmouth challenge. He claims his asphalt will last 20 to 30 years. And it has to be said that the quality of resurfacing underway over the Avon seems superb.
To be precise, Aeschlimann International’s material is Gussasphalt, or very dense mastic asphalt [as opposed to hot rolled asphalt or stone mastic asphalt – the other materials put under the microscope by the Agency]. The Gussasphalt being placed at Avonmouth looks intensely black, hard and – with its coated chips on top – "grippy". It is being laid by Aeschlimann International’s personnel using mastic asphalt plant, to high standards and tolerances.
Avonmouth bridge is the M5’s most important crossing and carries in the region of 120,000 vehicles a day high over the river Avon. In normal use, it provides four lanes in each direction plus a narrow central reserve, hard shoulders, a pedestrian walkway and a cycle way. It is 1.4km long, of which 400m constitutes the steel central section, the rest is made up of 17 approach spans (10 to the north, seven to the south) in reinforced concrete.
The list of players involved in the resurfacing project is complex. Ultimate client is the Highways Agency. Acting on its behalf is InterRoute, the joint venture of Mott MacDonald and Balfour Beatty Infrastructure Services which is service provider under the local (Area 2) enhanced managing agent contract (EMAC).
InterRoute placed a bridge deck waterproofing and resurfacing contract with Stirling Lloyd, which turned to Aeschlimann International for its Gussasphalt. The Agency specified that InterRoute keeps three lanes open in each direction during resurfacing, which is requiring some neat footwork in traffic management terms.
The timing of the activity – last autumn and early winter for the southbound carriageway, this autumn and early winter for the northbound – this takes into account the weather but more particularly peak traffic levels of summertime, when holiday makers head for the West Country.
Cost of resurfacing the Avonmouth bridge is put at £7M per carriageway, no small sum. "The Agency and its specialist technical arm Netserve have put a great deal of work into this, to ensure we get the best rate of return for the money we’re spending," says Highways Agency project sponsor Dave Stock. "The Gussasphalt ticks all the right boxes, including assured longevity, the likelihood of minimal intervention, resistance to water penetration, early life skidding resistance, good ride quality and relatively low noise."
Activity on the southbound deck began last September and asphalting has recently been completed. This means that motorists from the north now cross the Avon more smoothly comfortably than before. Vehicles continue to bump and clatter over the northbound deck.
Aeschlimann International opted for Hanson, operating out of its local Tytherington plant, to produce the Gussasphalt, designed by Aeschlimann himself, mixed to his exacting specification, and subject to his personal quality control. Helping Hanson get its asphalt supply contract from Aeschlimann International was the fact that it had a new Beninghoven mixing plant installed at Tytherington which could be turned over to producing the high-tech Swiss material. The Aeschlimann mixes for Avonmouth are very demanding ones, in terms of the temperatures required and complexity of ingredients.
Nominated as a supplier was the pan-European bitumen specialist Nynas, whose products and Swiss facilities are well known to Aeschlimann. Nypol 45MA is one of the new generation of polymer modified binders to come out of Nynas, purpose designed for specific applications. The company works closely with those it supplies with bitumen to help ensure the competence of asphalt mix design and production.
Nypol 45MA is a high performance binder particularly well suited to the kind of asphalt mixes designed by Aeschlimann. Details have not been formally released but it is known that a blend of sands, a very high proportion of limestone filler and a number of additives including Trinidad Lake Asphalt granules – along with the Nypol 45MA – go into making the Gussasphalt for the Avonmouth bridge’s binder and surface courses. Hanson reportedly has men dedicated solely to getting mixing and loading absolutely right.
The material arrives at site at over 200oC, delivered in mobile "Gussasphalt cookers" in which it can remain workable for up to 24 hours. It is viscous and spread rather than laid. Aeschlimann International’s paving machine – or "finisher" – runs on rails that are carefully positioned to achieve the correct thicknesses of asphalt, and ultimately levelled in by eye to ensure appropriate contours.
Such highly accurate pre-levelling, as opposed to levelling "on the run" which occurs with conventional paving machines, plus the human touch result in an exceptionally high ride quality for road users and relatively low levels of noise. This is in marked contrast to the surfacing provided before, as already mentioned. The steelwork had been surfaced in mastic asphalt, applied by hand, while the concrete decks had received hot rolled asphalt.
This time around, all the surfacing is with the Gussasphalt, from beginning to end. Surface and binder course add up to 50mm thick on top of the steel, 60mm on the concrete. Actual laying is simplicity itself. The material is merely dropped in front of the Gussasphalt finisher between the rails [which can be positioned up to 15m wide to suit circumstances]. The finisher spreads the Gussasphalt and, in the case of the surface course, immediately grits the new surface. A rolling system, patented by Aeschlimann, is then applied to bed in the grit. No other compaction is required. The material is quite impermeable enough, for instance, as placed, to keep out air and prevent ageing through oxidisation of bitumen.
The hot material adheres extremely well to the surface on which it is being laid. Given that preparation and levelling have already taken place, little can go wrong. InterRoute has been responsible for managing the Avonmouth project which involves operations other than just resurfacing and consequently a number of different interfaces.
Firstly the old surfacing and waterproofing has to be removed. This last is proving no easy task – Stirling Lloyd’s original Eliminator bridge deck waterproofing is proving to have worked well, not to have deteriorated since first applied, and difficult to take up. Not surprisingly it is being replaced by more of the same, another weather dependent task. Next comes the resurfacing itself. Lastly comes tricky reinstatement of the central reservation barrier. In between all these operations has been a fair amount of traffic management.
Time will tell if the Agency has got good value but the signs are that it will. The Agency cannot be seen to be endorsing a product but if Aeschlimann’s International’s Gussasphalt is still insitu in 30 years’ time, it will surely feel like doing so.
Heinz Aeschlimann– civil engineer, and artist
Google the name "Heinz Aeschlimann" and you get details of his works of public art. His beautiful shapes made from raw construction materials have been acclaimed in his home country of Switzerland and as far afield as North America and beyond.
He is particularly renowned for his Gussasphalt pyramids flecked with flakes of quartz. Less prominent is the acknowledgement that Aeschlimann, 61, has developed a small local building company (launched by his father in 1936) into a highly specialised global player, focusing among other things on bridge coatings. "Asphalt is in my blood," he says. "It is not just my profession but my passion as well, a hobby since I was a boy."
He graduated in civil engineering having studied in Lucerne and the United States. His materials have been used successfully on some of the world’s biggest crossings.