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Brian,
 
Critical issues would be bond and shrinkage. I have seen problems with a 4" overlay debonding soon after construction, no experience with 1". specifying only a high strength will tend to get you a mix with lots of cement, and water to go with it, therefore shrinkage. A mix with low cement content that will not have as much dimensional changes to stress the bond, yet still having adequate strength to be durable is better.
 
Surface preparation is key, if there is soaked in oil or anything in the existing then the bond could be compromised. I have seen shot blasting for surface preparation, there are other methods of cleaning and roughening the existing surface. When you pour, the surface should be very clean and saturated surface dry. I have heard that bonding agents should be breathable so as not to restrict water vapor movement through the slab.
 
      A.E.