Interlayer bond

Interlayer bond is the adhesion developed between two successive printed layers, and in most printed concrete it is the weakest plane in the element. Because the layers are placed without vibration and without formwork, the interface contains more voids and less continuous paste than the bulk of the filament. This is the main source of anisotropy in printed concrete: strength measured across the layers is generally lower than strength measured along them.

In practice

Interlayer bond is measured on sawn specimens by direct tension or pull-off, by splitting tension, or in shear, and reported values scatter widely between mixes and laboratories, so figures should be treated as mix-specific rather than general. The strongest influences reported in the literature are the time interval between layers, the moisture state of the receiving surface, the extrusion pressure and the standoff distance of the nozzle. Practical countermeasures on a print include keeping the layer cycle short and constant, protecting the work from wind and direct sun, avoiding unplanned stops, and, where design permits, placing reinforcement or connectors across the interface. A visible ridge or a dried, dusty layer surface is a reliable warning that the bond at that height will be poor.

Related terms

• Cold joint — the defect that results when the bond across an interface is essentially lost.
• Open time — the interval within which a layer still bonds properly to the next one.
• Layer height — the deposited thickness per pass, which sets how many interfaces an element has.