Atkore Allied Tube & Conduit RCK300KON 3" Threadless Rigid Compression Coupling

That is what the Allied Tube & Conduit RCK300KON is for. It's a threadless compression coupling for 3 inch rigid conduit, so it clamps onto plain, unthreaded conduit ends. Handy for field cuts and repairs where you can't get a threader onto the pipe or can't rotate the conduit to spin a threaded coupling on.

Yes. A neoprene gasket in the coupling seals the joint concrete tight, so the 3 inch threadless compression coupling can be used on runs cast into concrete without wet mix pushing into the raceway.

Closely spaced runs are one of the stated uses for this coupling. Unlike a threaded union or split coupling that needs room to turn, the compression body tightens with clamping nuts, so you don't have to rotate the conduit or leave spinning clearance between adjacent pipes on the rack.

The coupling is zinc plated for corrosion resistance. That's the same protective approach used on galvanized rigid conduit itself, so the joint matches the run in damp indoor and general outdoor service. For continuously wet or chemically aggressive locations, spec a coupling rated for that environment instead.

One RCK300KON handles a single 3 inch trade size joint, coupling two conduit ends together. It's sold as a single model with no size or finish options to pick, so count your joints and order that quantity. It will not adapt between sizes, both ends are 3 inch.

Yes, and that's usually the reason to reach for one. A standard threaded coupling requires both conduit ends threaded and at least one length free to rotate. The RCK300KON compression coupling grips unthreaded ends, so it works on an existing run you've cut into mid-span where neither side can turn. If both ends are already threaded and one pipe is free to spin, a threaded coupling or union is the simpler choice.