How can i use the demolish tool in Revit to part of the slab.
Is it possible to create a boundary for demolition in the slab rather modelling separately.
You can't demolish "part" of something in Revit. You'll have to model it as two (or more) separate pieces, the piece(s) to remain and the piece(s) to be demolished. Then you demo the parts as needed.
rosskirby has it correct, although if you're wanting an "existing" plan that doesn't show the dividing lines, you can join the bit to remain with the bit to be demoed. They're still separate bits and will behave appropriately as you would expect with different view phase choices, but they will display as one (in those view phase choices wherein you would expect both to display).
xtn