The interface of a cold joint is called the "plane of weakness" or simply the "cold joint interface."

A cold joint occurs when fresh concrete is poured against concrete that has already begun to harden, preventing proper chemical bonding between the two layers . The interface is the surface where the older, partially set concrete meets the newly poured concrete. This boundary is often visible as a line or seam on the surface, sometimes with a slight difference in color or texture . Structurally, this interface represents a discontinuity in the concrete, which can act as a weak plane where stresses concentrate and water may infiltrate . Unlike a construction joint, which is intentionally planned and designed to maintain structural integrity, a cold joint interface is unplanned and can compromise the monolithic strength of the concrete element . Proper management of this interface involves techniques such as thorough vibration, use of bonding agents, or set-retarding admixtures to improve adhesion between the old and new concrete layers .