Just share some of few option for u,

normally if doing concrete top they will put a steel net first than come to a cement work,hardly people will put a rock inside the cement.
n for your case,just afraid the contractor won't do the whole new top for u,even if u cut it in the middle is not so strong too.

just give u the option if the contractor don't help u to do the new KC top.

before u doing the Kitchen cabinet. u may ask the carpenter to help u do the wood support below the area of the table too.that at least can support and help u to hold some weight.because the whole cabinet is able to support the top of the hob plus the concrete.( actually if good carpenter it won't affect the door)

plus,if u using granite,actually granite is the hardest tile among whole tiles,is more strong than concrete.

plus some tips for other about the KC top.

before you doing your kitchen cabinet top.u may ask your carpenter to cut u the size of the top plus the measurement of the sink and hob.so you won't get any trouble on that.