Cold joint

A cold joint is a weakened interface formed when fresh material is placed onto concrete or mortar that has already stiffened too far for the two to merge into a continuous mass. In cast concrete cold joints are an occasional defect at pour boundaries. In layer-by-layer printing every layer interface is a potential cold joint, which is why interlayer behaviour is one of the defining structural questions of the process.

In practice

The controlling variable is the time between depositing one layer and covering it, often called the layer cycle time, together with the surface condition of the lower layer. Long cycle times, hot or windy conditions and a drying surface all reduce bond; research generally reports a loss of interlayer tensile and shear strength as the interval grows, though the numbers depend heavily on the mix and the test method. Practical countermeasures include keeping the layer time short and consistent, applying slight compression from the nozzle, controlling moisture loss, and in some cases spraying a bonding agent or fine water mist between layers. Restarts, pauses for maintenance and blocked hoses are the usual causes of an unplanned cold joint.

Related terms

• Interlayer bond — the strength actually achieved across a layer interface.
• Open time — the period during which fresh material can still accept the next layer.
• Layer cycle time — the interval between depositing successive layers at one point.