Fix holes in concrete slab (underside & outdoors)

[Ganlet]

Last summer, I poured a 13cm concrete slab (for a pizza oven) and made a small oversight... I didn't have enough spacers, so I made some improvised ones from some plastic pieces I found. But I didn't realize the concrete didn't have enough room to fill the cavities.

So now I have holes about 5 cm deep that more or less extend up to the reinforcement where I want to repair them appropriately, see images below. The focus isn't on cosmetics, but primarily to protect the reinforcement, I think.

Suggestions on the best way to address this to protect the reinforcement optimally?

The plan is to remove the plastic, of course, and then fill it in one way or another. I've seen various options with, for example, repair concrete and various other recommended additions, but it's hard to know what's the easiest and best. I have primer and Murburk B lying around, but I assume I'll have to buy something else to get it done properly.

 

[proffsrik]

I have had good experiences with combimix concrete filler for repairing chips in outdoor concrete stairs. It adheres well and is easy to work with. Sold at jem o fix. A bit expensive, but you don't need much at all.

 

[Ganlet]

I have good experiences with combimix concrete filler for repairing chips in outdoor concrete stairs. It adheres well and is easy to work with. Sold at jem o fix. A bit expensive, but you don't need very much at all.

Sounds convenient, but I get the feeling that "filler" might be a bit too weak (?). I want to protect the construction/reinforcement rather than something cosmetic.

From what I can read about larger/more important repairs, it seems to be some type of repair concrete. However, I don't know if it's as critical here or if it's possible to make it good since the repair is done from below.