Concrete Beam Repair

One wall in our kitchen is a concrete support wall so it was the only one we didn’t knock out all the plaster on. It was also the one most out of plumb; the room measures more than an inch wider at the ceiling than at the floor. There were some problems with the plaster surface over the concrete and we could feel some hollow gaps in between the 2 layers. That meant scraping the wall down, which lead to big chunks of the plaster falling off, all the way down to the rough concrete surface. Another repair not anticipated at the beginning of the renovation. 
Our contractor kept mentioning the steps we would be using to make the repair, but we were working on the other walls and when he left me on my own for a few days, I attempted the repair on my own. To fill those gaps back in bonding agent was needed to adhere the joint compound to the rough concrete. The concrete must first be clean & all loose particles removed. Then you coat the concrete with the bonding agent at full strength. Once it is tacky, but quickly before it dries, use an admix of bonding agent & joint compound and cover the raw concrete. You don’t try to fill the whole gap, but just cover the raw concrete. Once that dries then the next pass of joint compound will stick to the 1st coat.
Since the worst spots were at the corner joint, we needed to create a form on the underside of the beam to give a smooth edge to fill up to. My husband formed it up for me and I spread a very thick layer of compound across the wall and into those deep recesses. We had to let it dry for a day & a half before we removed the plywood form & even then, the underside of the mud was still damp and I even cut out a few big drips. Our contractor returned and decided to bring us a bag of durabond which is a dry mix of joint compound which is a bit more firm than the premixed stuff. Wish I had had it for the first layers; it was firmer & easier to work with. He had me add a few more layers of it to level out the corner as much as possible before adding the corner bead. Since the beam is solid, the plastic corner bead can’t be stapled into it, so I had to use the metal reinforced paper tape. Working again by myself, I smoothed on a layer of compound and climbed up ladders on each end of the beam to place the tape so that the bottom edge was level before I folded it under across the bottom side. There was of course a gap on the underside in the center that needed to be filled with joint compound too, but I’m pleased how well it came out. Now it just needs a few coats on each ace to smooth everything out nice & flat like the rest of the walls.