Concrete floor made with a weak mix, solutions?

Either smash it out and have another go or, if height allows, lay 4" of good reinforced concrete over the top. My standard mix, as taught to me in childhood by engineer grandfather, is 3 parts clean crushed gravel (not round river pebbles cos that's just rubbish), 1 part clean sand, and 1 part cement all mixed with the bare minimum of water consistent with workability. Should give you around 45MPa, at least that's what my classmates and I got when we tested a sample made to that mix at Polytech late last century.

Its an open garage port, looks like a dig out & replace......

Would you care to expand on that interesting theory?

No cracks etc, Ill check that out thanks

Professional general contractor here. If (big IF) the slab itself isn’t cracked or heaved (sections popping up) and you aren’t putting a tremendous amount of load on it you could overlay it with 2” of a fiber mesh reinforced concrete slab. This isn’t the best fix, but it’s one we’ve used on small office and retail space slabs. And weight wise I’m talking about a medium size car, anything like tractors and big trucks will break up the topping slab.
Best way is to break it out and re pour it.