Initial set
The point at which concrete stiffens enough that it can no longer be placed or vibrated, though it is not yet hard and carries no meaningful load.
What does initial set mean?
Initial set marks the practical end of the placement window and the beginning of the finishing sequence. Its timing swings widely with temperature, cement type, and admixtures, which is why the same mix behaves completely differently in July and November. Placing fresh concrete against concrete that has reached initial set produces a cold joint.
In practice
Crews read initial set by footprint depth: when a boot leaves only a shallow impression, it is time to work.