Is it normal for the first shotcrete layer on a monolithic dome to look this patchy?

The nozzleman is actualy right about the **flash coat** being thin, but seeing the mesh completley bare in large patches isn't ideal. It might look like a moisture farmer's hut in the making, but that first layer is critical for the structural integrity of the whole dome. A proper first pass should be about 1/4 to 1/2 inch thick—just enough to stiffen the wire or mesh so it doesn't vibrate like a tuning fork when the heavier structural layers go on. If the mesh is bouncing or 'ghosting' during the main spray, you’re going to get voids behind the steel, wich is where the real structural problems start.

Regarding the rebound, seeing a pile at the base is normal, but the volume tells the story. For wet-mix, you should generaly see about 5% to 15% waste. If it looks like half the material is on the floor, the mix might be too dry or the nozzleman is standing too far back. That waste isn't just lost money; it’s the large aggregate bouncing off becuase it didn’t have enough paste to stick. You want to make sure they aren't trying to shovel that rebound back into the hopper, because it's basically just rocks now and will ruin the strength of the wall. The uneven thickness is actually a benefit for the mechanical bond between layers. You want a rough, craggy surface. If they smoothed it out like a finished floor, the next layer whould have a harder time sticking. But 'dusting' isn't a coat. If it’s just a light spray of gray water, it’s not doing anything. You need enough material to encapsulate at least one side of the wire so the whole cage becomes rigid.

Keep an eye on the **slump** of the mix. If it’s too wet, it’ll sag and pull away from the mesh; too dry and the rebound goes thru the roof. Don't be afraid to ask them to beef up those thin spots before they move to the next section. It's way easier to fix a thin area now than to try and patch a hollow pocket later once the dome is closed in. Grab a glass of blue milk and watch the nozzle distance closely, and make sure they blow off any loose rebound or dust before the next pass so you don't get sand pockets.

Spot on about the vibration. One thing I'd add is to watch the nozzle angle. If the guy isn't spraying perpendicular to the mesh, you'll get 'shadowing'—basicly empty pockets on the backside of the wire. Even if it looks covered from where you're standing, those voids are structural killers. Make sure he's moving the nozzle to hit the mesh from slightly diferent angles to wrap that material all the way around the steel.

Have you ruled out the ambient temprature or the humidity during the spray? I’m always a bit skeptical when 'flash coat' is the go-to answer for patchy coverage. If the mix is losing moisture too fast before it even makes contact, that coud be why you’re seeing so much dusting and rebound. Also, have you checked the tension on the mesh itself? If the cage isn't tight, you're going to get sagging regardless of the coat thickness. Might want to give it a poke and see if there's any play before they start the heavy lifting.

While everyone is focusing on the visual thickness, you realy need to verify that the compressor is providing sufficient CFM to maintain the required nozzle velocity, as inadequate air pressure often results in a lack of proper compaction where the cementitious paste fails to fully densify around the reinforcement. If the nozzleman is throttling down to reduce rebound, he might inadvertently be creating a friable, porous interface that lacks the necessery compressive strength to bond effectively with the subsequent structural passes, wich essentially renders the entire 'flash coat' concept moot if the primary interface is structurally compromised from the outset.

Quick update—I talked to the crew about the nozzle angle to avoid shadowing, and that’s helped the coverage look much more consistent. The mesh is finally getting buried. However, the amount of rebound at the base is still huge. Is there a standard percentage of waste for wet-mix, or is the air pressure maybe too high?