Epoxy Adhesive for garage door threshold?

While many epoxies will claim to bond many things I would start by testing a small piece in a inconspicuous area. Clean concrete is a must.

Don't be lured by the quick setting versions of epoxy in the hardware store. In my experience the 90 min-4 hr types will bond best.

If your test fails and the epoxy stays on the concrete, try passing a flame over the surface of the rubber (be careful not to melt or burn it). Use proper safety instructions and do it outside. In many cases this will open the surface to the adhesive.

Special Polyurethanes (caulking or liquid) may also be useful for this application. Urethane is more flexible and may survive the traffic effect better. An added plus is they like to bond to concrete