Drivers are being warned to expect diversions, speed restrictions and overnight closures at the Dunheved bridges on the A30 near Launceston.
The project to replace the structures over the River Tamar involves new steel and concrete composite decks.
One carriageway will have overnight closures, and there will also be a 50mph speed limit on both carriageways.
One of the new decks will be built alongside the present bridges, and over the next few days the bridge beams will be slid into place.
This will involve the closure of the A30 westbound carriageway overnight on Tuesday and Wednesday, as well as on 3 and 4 July. Westbound traffic will be diverted at Lifton Down.
The project is due to be completed in March 2007.
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