The bridge is one of those being renovated on the international highway.
Road officials said that vibrations of traffic nearby would be enough to compromise the concrete. Even now heavy vehicles are being routed through Cañas, Tilarán and La Fortuna.
The road has been reduced to one lane. Officials said they expected the bridge work to be finished by the end of December. Part of the job is to install walkways for pedestrians.
The closing will be from 8 p.m. Monday until 8 p.m. Tuesday, said officials. Scheduled bus passengers will be met with a second vehicle on the opposite side of the 101.5-meter (333-foot) bridge, the agency said.
Specifically the job is to pour concrete in the center of the bridge to join two lanes of concrete that already are there.