The floor slab is located in a laundry area of the unit and subject to periodic flooding. The floor slab was not protected by a waterproof membrane so any moisture or water on the floor was able to seep into the concrete initiating the corrosion of the steel reinforcement.
The photo shows that the drummy and spalled concrete has been removed by the builder and the steel exposed for inspection/approval prior to its treatment. Depending on the extent of the corrosion it is often necessary to add new steel reinforcement to provide adequate structural strength. After the steel is treated with an anti corrosion product the concrete will be reinstated with a special modified cement based repair mortar. Finally the concrete will be protected with a new waterproof membrane and as is required by current building codes.