Rebound Hammer Test of Concrete

1. Which of the following is not an indirect test?
2. The principle behind rebound hammer test is_____.
3. What is the minimum distance required between testing points and edge of sample as per IS:516?
4. What is the minimum number of readings should be taken on each face of sample in the experiment as per IS:516?
5. Rebound index doesn't depend on which of the following?