SALT CORROSION of steel reinforcement in the deck of a road bridge in Montreal is the likely cause of a collapse which killed five people on Saturday, engineers said this week Heavy road salting, required during Canadian winters, was thought to have created a corrosive cocktail that easily penetrated the movement joint where the road deck slab met the cantilever supports - an acknowledged weak spot (see diagram).
Motorists contacted emergency services at around 11am last Saturday, as chunks of concrete fell onto the highway from the overpass.
A spokesperson for highway authority Transport Quebec, confi med that a highway patroller arrived at the scene before midday, made a visual inspection, but did not conduct a full survey of the structure.
The patroller assumed that the falling concrete was not a structural threat and declared that the bridge did not need to close.
But debris continued to fall and at around 12.40pm the structure collapsed. Witnesses said that light traffic prevented further loss of life.
'The inspection was not adequate. We need to know what sort of inspection was carried out, what sort of inspection would have been adequate, and how prevalent is this fault, ' said Dr Ghani Razaqpur, chair of civil engineering at Canada's McMaster University.
The bridge was a half joint deck structure comprising two short cantilever spans supporting the bridge's central span on either side. The central span is simply supported between bearings on the side cantilevers.
Engineers explained that the most likely cause of failure was salt corrosion over time.
This would have led to reinforcement deterioration which would then have caused a shear failure of the concrete from the joint to the point of contact with the support.
Chloride from salt is causing havoc on both concrete and steel. On this bridge there would have been the release of oxygen from the steel. In addition there would have been accidental chemical spills, seeping into the concrete and reaching the steel, ' said University of Toronto civil engineering Professor Shamim Sheikh.
'This is consistent with other [salt] damage I have seen.' Roger Dorton, former chief bridge engineer from neighbouring Ontario, defended the inspection programme in Quebec.
But he said detecting salt corrosion was time consuming.
'For deterioration due to salt corrosion, you can check cores and the structure for salinity and signs of deterioration, but this cannot be done all the time on 15,000 bridges in your area, ' he said.
Engineers also said that sulphate impurities could have weakened the concrete.
Concrete expert Donald Pearson-Kirk from highway service firm AccordMP said the crumbling concrete falling onto the road was evidence of, compression failure or sulphate attack.
'We have evidence of this from bridges in the UK, ' he said.
An inquiry has been established by the Quebec government led by a former premier Pierre-Marc Johnson.
Razaqpur said he hoped the inquiry would yield lessons for the future.
'We must assure the public - we need more money.
Public money will help fix the problem but we have an additional problem of expertise.
We have to learn more about how things deteriorate, ' he said.
A full inspection was last carried out in May 2005.
Ed Owen Flawed designs no longer used Half-joint deck structures are no longer used in the UK or Canada.
'There will be a few about, and you can bet these bridges will be getting a very close examination, ' said former Ontario chief bridge engineer Roger Dorton.
Such designs were popular in the 1970s, but have since been phased out because, 'they are generally very highly stressed areas, which need great care in their design and detailing to be successful, ' said Benaim director Simon Bourne.
'They are often badly affected by water penetration from the deck, with all the problems also caused by road salts. The half-joints sit, by definition, right beneath a roadway expansion joint.
They, therefore, always need to be particularly well protected and maintained, ' he said.