Liverpool's Churchill Way flyovers are to be demolished at a cost of £6m after inspectors found significant defects and said they cannot be strengthened.
The two-lane concrete structures, which date back to 1970, were closed last autumn after design and construction flaws were discovered.
Engineers from Amey found the quality of construction using concrete and steel was poor, with tendons and ducts corroded and signs of structural distress including cracking over some supports. They said they should not carry vehicles or pedestrians.
They have calculated that strengthening the structures is not feasible and the cost of replacing them would be between £50m and £60m, compared to a demolition price of £5.7m.
Piece of loose concrete from the flyovers
James Noakes, Liverpool City Council’s cabinet member for Highways, said: ‘The Churchill Way flyovers are a relic of a plan from half a century ago that was never completed.
‘Public safety is absolutely paramount and despite the obvious inconvenience the demolition will cause, we can’t compromise on safety and it is simply not economically viable to make them safe.
‘Although there has been an increase in traffic at peak times in and around the Queensway Tunnel as a result of the closure, the city has been able to cope with it.’
Trevor Cherryholme of Amey Consulting said: ‘Our primary areas of concern are the poor quality of original construction, subsequent deterioration and the current signs of structural distress.
‘More specifically, poor steel placement and spalled concrete, collapsed or failed formwork, failed drainage and signs of overstress in the deck are among our most significant findings.
‘It is our view that there is no safe option other than demolition.’
Corroded reinforcement in the flyover
Following new legislation on major highways structures, a Post Tensioned Special Inspection (PTSI) began in 2016 to assess the northern and southern sections, both of which are more than 240 metres long.
This found problems with drainage, internal support, barriers and bearings, leading to the flyovers being shut last autumn for investigations into potential hidden defects and potential overstress.
Structural testing has now been carried out, involving removing the road surface, drilling into the decks and underground assessments of every supporting column.
Demolition work is expected to start the summer. In the meantime the council said it will be developing proposals to improve the Queensway Tunnel roundabout and the Hunter Street interchange to cope with increased traffic flow, at an estimated cost of £10m.