RAF MILDENHALL — An exit-only gate at this Suffolk air base is scheduled to close for 18 weeks starting Friday.

Gate 20, or the Shippea Hill gate, which allows motorists to leave the base near Beck Row will shut down as construction workers replace a guard shack there.

The current guard shack, a small hut usually manned by one security forces officer, will be demolished and swapped with a bigger, more solid structure as part of a $350,000 project.

The funds will cover ballistic hardening of the new structure’s shell and windows, said 1st Lt. Matthew Lapham, design and construction chief for the 100th Civil Engineer Squadron.

Outgoing traffic will be rerouted through Gate 19, near the 3rd Air Force building. Pedestrians will still be allowed in and out of Gate 20.