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.
An existing five-year-old, four-inch slab has three inches of new cement added to it. The total thickness is seven 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.