The Interface Transition Region (ITR) in 3D Printed Concrete: A Mechanistic Taxonomy and Constitutive Requirements for Interlayer Stress Transfer

3D concrete printing (3DCP) creates interlayer interfaces that often govern structural performance under tensile and shear loading. Despite extensive research on manufacturing parameters, the field lacks a unified framework for interfacial stress transfer, and terms such as bond, adhesion, cohesion, and interlocking are used inconsistently. This paper addresses that gap in three steps. First, it distinguishes the interface, an idealised zero-thickness boundary, from the finite-thickness Interface Transition Region (ITR) formed where two cementitious surfaces of the same material meet with different physicochemical histories at deposition. Second, it proposes a taxonomy of four interacting stress-transfer mechanisms (adhesion, cohesion, friction, and interlocking). Third, the taxonomy is confronted with an orthotropic constitutive model for 3DCP concrete, interpreted as an adhesion-dominated limit case under tensile stress perpendicular to the printing layers; coupled normal–tangential transfer reported for an analogous discrete interface; and six constitutive formulations, evaluated for the mechanisms each represents or neglects. The synthesis yields five physically grounded requirements for future constitutive treatments of these interfaces.