The Government-owned company has stated it is working "flat out" to finish the repairs between the M6 to M5 in time for the holiday period.
The waterproofing work is being carried out between the M6 junction 8 at Walsall to the south and the M5 at West Bromwich, but the £6.4m viaduct project has already been delayed for five months due to unforeseen circumstances.
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Highways England Project Manager Jess Kenny, said: "We understand this has been frustrating for drivers, but unfortunately there was no way for us to tell the full extent of the damage until we'd already started work.
"We were only able to assess the true extent of the concrete damage and the steel corrosion inside the structure once we had removed the surface.
"While the amount of damage is roughly as much as we predicted, the most damaged areas are over the parts which carry the weight of the viaduct, which are crucial to the integrity of the bridge. So repairs have to be done with painstaking precision and care to prevent the bridge becoming weak.
"We have had to completely change our work plan, and only treat small sections of the road at a time which has lengthened the timescale of the work. But we are working hard to complete this as soon as possible."
Diversions will remain in place throughout the works.
(LM/JP)
