The pair of steps up from the loading bay into the main workshop have
long been broken. They're concrete and it seems that the top inch was
poured separately, and is now breaking away. Like most of the ad hoc
concrete work in this place, it's not well done. Clearly I need to re-
pour a new top.

Any advice? I know nothing of concrete or wet trades stuff.

What sort of mix? There's a gallon of SBR additive here that I've
been using to weatherproof where I've been grouting my outdoor crack
repairs. Any use?

Any bonding additive? I'd expect to use PVA, but given that it's a
thin layer on a step, do I need to do anything else? Drill and stud
the underlying substrate?

Any advice on technique for step building?

Thanks