A concrete cold joint forms when a pour delay lets one lift set before the next is placed, so the two never bond. It is a placement and timing issue, not a plywood defect. Here are the real causes, the time limit, how it differs from a planned construction joint, and how to. A cold joint in concrete is an area or surface with a structural discontinuity caused by the delayed concrete pouring between two layers of concrete. The visible change between the two concrete surfaces could be a slight difference in color or texture, or as bad as unsightly exposed aggregate. Repairing cold joints is vital for maintaining structural integrity. If you plan your pour for a single day but run out of daylight, you'll likely end up with a cold joint.