A quick google check to find the specific dates....
1948 - 1949 - The Berlin Blockade. (no barrier, just the roads and railways are blocked)
1949 - Massive population shift from East Germany to West, continues until 1961
1952 - East Germany closes its border with West Germany. Special passes are needed to travel. (no barrier)
The Berlin Wall went up on 16 August 1961.
The border is fully barricaded in the 1960's.
Oh, something I just found while checking the dates...
President Truman's Foreign Service Advisor, George Kennan, developed the concept of "containment" for the Truman Doctrin.