Concrete over broken concrete slab?

I have a very old 18' x 12' garage (really just a storage bldg now) with a badly cracked slab. A concrete contractor said that he could but a layer of sand over the old slab and put a new 4" reinforced concrete slab over that. I won't be using the bldg for anything heavier than a motorcycle and don't want to spend very much rehabbing this old thing (bid is $1400). He said this method is common and pretty reliable. Any comments? Thanks.
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Placing two inches of compacted sand over broken concrete is used for airfield reconstruction. The sand can be 100% compact by flooding with water .

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