Well. When Concrete cube test result fails at 28th day then 
go for concrete core cutting and check the compressive 
strength in it. Number of cores shall be as per the 
engineer incharge's decision but should not be less than 3 
nos. IS:456 says if the core test fails then go for load 
test. IS : 456 also says that NDT can be done . But with 
rebound hammer test only surface hardness will be known and 
hence this test result is not reliable and hardly anyone 
suggests for this test.