Placing old slabs under new concrete slab

[Jim54]

Hi,
In my garden, I've some old slabs and I'm planning on pouring a concrete slab to make a new patio. I was thinking of putting the old slabs on top of some dpm (after going over with a whacker), pouring about 50-75mm of concrete, dropping in some rebar and then putting same amount of concrete again on top? Does this sound reasonable?
Thanks,

 

[Crystalclear]

Just a thought...

Will the poured concrete dry in different times on the DPM compared to the broken slabs?
If so, will this effect the new concrete structure?; possibly cause hairline cracks where the two different foundations are absorbing the water differently from the new concrete?