Better Bridges

A surprise spin-off benefit from the hi-tech surfacing of Avonmouth bridge is helping the Swiss-designed asphalt surfacing find considerable favour with their UK bridge owners

In late 2008 the Highways Agency (HA) took a calculated risk that expensive asphalt technology would be worth importing to resurface Avonmouth bridge, for the advantages it would bring. Analysis indicated that the high initial cost of using Swiss Gussasphalt would be more than offset by longer life, less interventions, less disruption and a smoother ride.  

Avonmouth bridge on the M5 near Bristol is a lively structure, infamous for ‘shedding’ its surfacing. But, four years on from resurfacing with Gussasphalt, the signs are that the HA’s hopes for the material are being fulfilled in abundance. The bridge deck looks superb despite the pounding it gets from heavy traffic. In addition, a substantial benefit has become evident that was not factored into the original whole-life costing.

So smooth-running do the bridge’s twin carriageways remain that the ‘pounding’ they get is not translating into the same level of fatigue-inducing stress that existed before. It has become possible to cut back the amount of maintenance welding inside Avonmouth’s substantial box sections as a direct consequence. Fatigue cracks to be catered for in the course of routine inspection and maintenance are now less prevalent.

‘We have a very thorough inspection regime for Avonmouth bridge and – with a maintenance team permanently based nearby – the ability to carry out maintenance tasks on a day-to-day basis,’ said the HA’s Area 2 performance manager, Dave Sledge. ‘Some of these tasks involve welding to ensure the continuing integrity of the structure. Since the carriageways have been resurfaced we are carrying out less welding interventions than we did previously.’

This is good news for three reasons. First, less wear and tear on the Avonmouth structure is a good thing of itself. Secondly, less need for maintenance welding means less cost. Thirdly, there are health and safety benefits: welding in confined spaces is not necessarily a pleasant activity, so being able to reduce it means less exposure of operatives to discomfort.

‘We didn’t foresee this or take it into account when carrying out the initial cost-benefit calculations for using Gussasphalt,’ said Mr Sledge. ‘If we had done, the case for importing the technology would have been even stronger. We’re very happy with the performance of the material, which is withstanding the rigours of its location extremely well. At this time in its life cycle the previous mastic asphalt was already showing signs of distress and requiring interventions. There have been no interventions of any sort concerning the Gussasphalt to date.’

Gussasphalt is a highly durable, very dense mastic asphalt bound by a high-performance polymer-modified binder. Special recipes for both the asphalt and the binder came from Switzerland – from Aeschlimann International and European bitumen specialists Nynas respectively –  although both the materials used at Avonmouth were produced in England. Aeschlimann International claim their asphalt will last from 20 to 30 years – at least three times as long as Avonmouth bridge’s previous surfacing.

UK contractor Hanson produced the mix, which comprised a blend of sands, a high proportion of limestone filler and a number of additives, including Trinidad Lake Asphalt granules – plus the aggregate. The binder used was Nynas Endura N5, a high-performance material designed to ensure its suitability for the kind of mixtures designed by Aeschlimann International’s chief executive Heinz Aeschlimann. As Nynas UK southern region sales manager Miles Williamson said at the time: ‘Our Nynas colleagues in Switzerland made sure Mr Aeschlimann knew about, and had access to, our latest binders plus help in formulating the right Gussasphalt mixture. The material has gone down very well indeed.’ 

Speaking recently he said: ‘The surfacing seems to be standing the test of time. Gussasphalt is now being considered for other bridge projects around the UK where longevity and prevention of interventions are imperative. News about the structural maintenance benefits at Avonmouth is bound to advance the case for using the Swiss technology in Britain.’

Further to the west of Avonmouth, Gussasphalt is being trialled on the Taymar suspension bridge between Devon and Cornwall. Richard Cole of the Taymar Bridge & Torpoint Ferry Joint Committee said: ‘We laid the material last September on a 140m long side span that had been giving us a particular problem – the original 12-year-old mastic asphalt was cracking and crazing under low-speed traffic.’

Taymar bridge is jointly owned by Plymouth City Council and Cornwall Council. Mr Cole consulted with the HA about its Gussasphalt experiences at Avonmouth. ‘I was impressed by the way the Aeschlimann International crews laid the asphalt, on pavers mounted on carefully positioned rails – great for pavement formation,’ he said. ‘They did a very good job for us at Taymar, second to none I’d say. The material went down very well and seems to be performing excellently.’

As with the Avonmouth crossing, the same surfacing contractor and bitumen supplier – Hanson and Nynas – were involved with the Taymar bridge. Mr Cole and his colleagues were interested to learn of the seeming reduction in stress-inducing fatigue when using Gussasphalt and suggest that some of this may be down to the thickness that the material is laid to – 55mm at Taymar, some 15mm thicker than the previous surfacing. 

But they acknowledge that smoothness of the Gussasphalt surfacing and consequential reduction in wheel ‘thumping’ and fatigue is a likely beneficial bonus. Mr Cole said there was an intention to carry out measurements of Taymar’s orthotropic deck when surfaced with Gussasphalt and to compare the fatigue performance with that of more conventional surfacing. He said that there was a likelihood that the whole of Taymar bridge will be surfaced with Gussasphalt at some time in the future.

Kessock cable-stayed bridge across the Beauly Firth in Scotland is another structure for which the use of Gussasphalt is currently being considered. Again, this is an extremely lively bridge which has had issues with its surfacing. The deck is thin and – subjected as it is to high temperature differentials – prone to substantial thermal movement. The blackness of its asphalt surfacing is no help in this respect; as the deck expands in warm weather the asphalt tends to unstick.

Transport Scotland is looking into the use of Gussasphalt because of its stickiness, fine tolerances of level and the fact that it is so durable. The thoughts of England’s Highways Agency on fatigue reduction are also being taken into account. 

Meanwhile, back in England, the Severn river crossing is another structure for which use of Gussasphalt is in the frame. ‘We’re looking at it there too,’ said a spokeswoman for the HA.

Concerning Avonmouth, Dave Sledge had one further comment to make about yet another benefit of using the Swiss asphalt technology. ‘The perception of residents living below and alongside the bridge is that tyre noise has been reduced and life is a lot quieter. That really is a bonus,’ he said.