If Core test failed for slab which happens very rare, consult the designer for his formulated advise whether the strength is UN-acceptable as per the Design codes.
Calculate the total and various type of load to be taken by Slab and arrange of Loading Test.
If the test failed area is of mass concrete, retain the concrete with rectification like Grouting method.
Grouting can be considered to improve concrete quality followed by Hammer test before deciding for dismantling the slab as Final decision.
