Please Note: This is somewhat out of the norm and is not covered by the ASTM F2170 standard. The following answer is based on information we have received from experts.
A 5-year-old existing 4-inch slab has 3-inches of new cement. The total thickness is 7-inches.
Excessive moisture from any area in the combined slab can cause a failure. It is best to test the moisture in the center of the old slab as well as the center of the freshly poured slab.