Shotcrete is routinely used in creating retaining walls or soil-nailed walls in this fashion. Designers and inspectors often confuse placement of multiple layers of shotcrete in building out a section with cold joints experienced in cast-in-place concrete construction. Unlike cast-in-place concrete, shotcrete provides thorough consolidation and densification by high-velocity impact of fresh concrete material on the receiving surface. The high-velocity impact of shotcrete on a hardened, previous shot layer (or existing concrete surface) provides a strong abrasive blast to open up the surface, and then provides an immediate exposure of that hardened surface to fresh cement paste. As a result, shotcrete exhibits excellent bond to concrete and previously shot surfaces. Thus, the structural action between the sections acts as a monolithic section without any weakened planes.

In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Section and that require: When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set. Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.

An experienced shotcrete contractor should routinely provide proper surface preparation between shotcreted sections, and use skilled crews with ACI certified nozzlemen to place and cure the shotcrete placements.