How would you have repaired/cast the lower part of the beam?

[chali405]

I need to "fill up" with repair concrete about 2-3 cm under a beam. I have bought "weber rep 45" which should work to apply "overhead", i.e., underneath without dripping down (thixotropic). The blue is anti-corrosion coating that also works as adhesion.

The question is:
Is it easiest/best to place a board on each side of the beam, which is as low as I need to fill up with mortar? That way I can toss the mortar underneath, or are there any other good suggestions?

Thanks for all the tips

 

[chali405]

Adding this:
I think one of the following must be the best way?
i.e., build a form that holds up,

suggestions are welcome

 

[Pjosk75]

A form similar to the one used when casting bridges seems sensible. I assume everything needs to be cast simultaneously. So somewhere it needs to be possible to pour down the concrete.

 

[Matti_75]

Bygg en form som är lite bredare än balken och gjut med expanderande finbetong. Den flyter som ett flytspackel och har tillsatser som gör att den inte krymper som vanlig betong