The pouring mark was reached last week in one of the monoliths that is on the west wall of the middle lock chamber.
It follows last month's arrival from Italy of the second shipment of four rolling gates for the third set of locks.
Construction of the two lock complexes requires a total of 4.3 million cubic metres of concrete. The volume placed at the Atlantic side currently stands at 1.8 million cubic metres.
The new lock complexes in the Pacific and Atlantic sides will feature three chambers, three water-saving basins per chamber, a lateral filling and emptying system, and rolling gates.
Overall progress on the project was 76.3% as of 31 May.