Humboldt H-2987 Concrete Rebound Hammer - Manual (Page 5)

Rebound Number
Correlation curves. This diagram shows the relationship between the
compressive strength for concrete, measured directly in a laboratory using
concrete cylinders or concrete cubes, and the rebound numbers you receive
using your Rebound Hammer.
A refers to using the Rebound Hammer against a concrete floor (downward).
B refers to using it against a concrete wall (forward).
C refers to using it against a concrete ceiling (upward).
The strength values shown in this diagram are directly based on compressive
strength testing in a laboratory, using concrete 14 to 56 days old. The tests use
concrete cylinders, 6 inches wide and 12 inches high (15cm x 20cm), or concrete
cubes six inches high.
According to ASTM C-805-08, you should discard any readings that differ
from the average value rebound number by more than six units. Then, take an
average for the remaining readings. If more than two readings differ by more
than six units from the average value, throw out all of your readings and perform
the test over again. Take ten new readings with your Rebound Hammer in a
different section of the concrete but in the same test area.
Figure 4: Concrete Rebound Hammer correlation curves