I have a 24 in. (610 mm) thick concrete dome that serves as an enclosure to protect extremely sensitive and important equipment that needs to withstand high impact demands such as tornados or missiles. The contractor is proposing to use the shotcrete method with the following sequence: shoot approximately 1 in. (25 mm) (to achieve reinforcing bar cover); let stand for 8 hours; then place a reinforcing mat; then shoot the majority of the dome thickness; let stand for 8 hours; then place the other mat of reinforcing; then shoot the remaining concrete cover. I am concerned that, with an 8-hour duration between concrete placements, the three layers of concrete will not be adequately bonded such that they behave monolithically. In particular, I would be concerned that the aggregate of the concrete that is shot onto a mat of reinforcement will not be able to make its way “behind” the bar’s deformation, thus causing voids. Please let me know your thoughts on the aforementioned concerns, whether it would be reasonable to shoot a 24 in. (610 mm) dome with a minimum of two layers of reinforcement all at once, and whether any of the ACI codes or standards speak to shotcrete joints parallel to reinforcement.

Multi-layer buildout of shotcrete sections is very common and has decades of successful performance in existing structures. Shotcrete applied to a properly prepared, existing hardened concrete substrate (such as a previously shot shotcrete layer) develops an excellent bond. The high-velocity impact of shotcrete on the surface is in effect like sandblasting, and opens up the receiving surface immediately before exposing it to the fresh cementitious paste. Cores taken through multiple layered shotcrete sections exhibit no signs of reduced bond. Often it is nearly impossible to identify where one layer stops and the next starts.
Incremental placement of reinforcing bars in layered application is also common. Proper shotcrete consistency, nozzleman technique, and air velocity will force fresh cement paste around the back of the bar and fully encase the reinforcing bar, even when in contact with the previous hardened concrete surface.
Shooting a 24 in. (610 mm) thickness at one time with two layers of reinforcement in the mostly overhead orientation of a dome would require use of special concrete mixture designs with chemical accelerators, and would be very difficult to execute with consistent quality. Also, depending on the formwork design, unbalanced loading on the dome by shooting very thick sections adjacent to sections not yet shot would be a potential concern.
For more information on the performance of shotcrete in layers, you can review this article from Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.”