Cold joint
An unplanned seam formed when fresh concrete is placed against concrete that has already begun to set, preventing the two from bonding monolithically.
What does cold joint mean?
Cold joints create a plane of weakness and a path for water, and they are visible as a distinct line on the finished surface. They result from delivery delays, breakdowns, undersized crews, or hot weather accelerating set faster than placement proceeds. A planned construction joint with proper preparation is an engineered solution; a cold joint is the accidental version.
In practice
A truck that shows up ninety minutes late on a summer wall pour is how most cold joints get made.