Traffic restrictions will be put on a north-east bridge for 18 months.
Aberdeenshire Council has said a one-way system has been introduced at the B966 at the Abbeyton Bridge, near Fordoun.
The restriction is so assessments can be done on the structure after defects were found in an inspection.
Because the route is so close to the A90, a one-way system has been chosen because temporary traffic lights are likely to cause tailbacks on to the busy trunk road.
Structures manager Donald MacPherson said: “We will be undertaking some computer modelling of the structure to identify what remedial works might be required.
“The bridge is nearly 170 years old, so reducing traffic to one lane will help lighten the load on it while this work takes place and reduce the risk of further deterioration.”
Mr MacPherson added: “We will be continuing with structural assessment work, with the aim of clarifying the timescale for completion in due course.”