A surprise spinoff benefit from the surfacing of Avonmouth bridge is less wear and tear on the massive structure below. Ty Byrd reports.
In late 2008 the Highways Agency took a calculated risk that it would be worth importing expensive asphalt technology 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 (NCE 15 January 2009). The 1.4km long viaduct with its 164m main span on the M5 near Bristol is a lively structure, infamous for “shedding” its surfacing. Three years on from resurfacing with Gussasphalt, the signs are that the Agency’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.
“Since the carriageway has been resurfaced, we have been carrying out less welding interventions”
Dave Sledge, Highways Agency
It seems the ride quality of the bridge’s twin carriageways is so good 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 steel 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,” says Highways Agency Area 2 performance manager Dave Sledge (see box).
“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. Less wear and tear on the Avonmouth structure is a good thing in itself.
Secondly, less need for maintenance welding means less cost. Lastly 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 the fatigue reduction or take this into account when carrying out cost benefit calculations”
Dave Sledge, Highways Agency
“We didn’t foresee the fatigue reduction or take this into account when carrying out the initial cost benefit calculations for using Gussasphalt,” says 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,” he adds.
At this stage in its lifecycle, 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,” he says.
Gussasphalt is highly durable, a very dense mastic asphalt bound by a high performance polymer modified binder.
Special recipes for the asphalt and the binder came from Switzerland - from Aeschlimann International and European bitumen specialist Nynas respectively - although both the materials used at Avonmouth were produced in England.
Aeschlimann claims its asphalt will last for between 20 and 30 years, or in other words at least three times as long as Avonmouth bridge’s previous surfacing material.
UK asphalt supplier Hanson produced the mix which was 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 is Nynas Endura N5, a high performance material designed to ensure its suitability for the kind of mixtures designed by Aeschlimann.
Other clients have already spotted the benefits. Further to the west of Avonmouth, Gussasphalt is being trialled on the Tamar suspension bridge between Devon and Cornwall.
Tamar Bridge & Torpoint Ferry Joint Committee engineering manager Richard Cole says: “We laid the material last September on a 140m long side span that has been giving us a particular problem - the original 12 year old mastic asphalt cracking and crazing under low speed traffic.”
Tamar bridge is jointly owned by Plymouth City Council and Cornwall Council. Cole consulted with the Highways Agency about its Gussasphalt experiences at Avonmouth.
Carefully positioned rails
“I was impressed by the way the Aeschlimann crews laid the asphalt, on pavers mounted on carefully positioned rails - great for pavement formation. They did a very good job for us at Tamar, second to none I’d say. The material went down very well and seems to be performing excellently,” says Cole.
The same asphalt producer and bitumen supplier - Hansson and Nynas - were involved with the Tamar bridge as the Avonmouth crossing. Cole and his colleagues were interested to learn of the seeming reduction in stress inducing fatigue in using Gussasphalt and suggest that some of this may be down to the thickness that the material is laid to - 55mm at Tamar, 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. Cole says there is an intention to carry out measurements of Tamar’s orthotropic deck when surfaced with Gussasphalt and compare the fatigue performance with that of more conventional surfacing.
He remarks that there is a likelihood that the whole of Tamar 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 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; and as the deck expands in warm weather the asphalt tends to debond.
Avonmouth bridge is of high importance as a major link between the south west of England and the rest of the UK.
Because of this, a maintenance team is permanently based close to the structure, allowing maintenance tasks to be carried out on a daily basis and greatly simplifying the logistics for works and inspections.
The principal inspection regime is risk based depending on the criticality of the particular element of the structure.
High risk elements are formally inspected every 12 months, medium risk elements every 24 months and low risk elements every 36 months.
All components of the orthotropic deck are deemed to be high risk. This includes welded connections between trough stiffeners and deck plate and cross girder; the trough stiffener splices; the deck plate splices; and the deck plate to web connections.
All inspections of the orthotropic deck are “Close visual inspections” in accordance with BS EN 970.