- Before commencement of a test, the rebound hammer should be tested
against the test anvil, to get reliable results, for which the
manufacturer of the rebound hammer indicates the range of readings on
the anvil suitable for different types of rebound hammer.
- Apply light pressure on the plunger - it will release it from the
locked position and allow it to extend to the ready position for the
- Press the plunger against the surface of the concrete, keeping the
instrument perpendicular to the test surface. Apply a gradual increase
in pressure until the hammer impacts. (Do not touch the button while
depressing the plunger. Press the button after impact, in case it is not
convenient to note the rebound reading in that position.)iv) Take the
average of about 5 readings.
Interpretation of Results
The rebound reading on the indicator scale has been
calibrated by the manufacturer of the rebound hammer for horizontal impact,
that is, on a vertical surface, to indicate the compressive strength. When
used in any other position, appropriate correction as given by the
manufacturer is to be taken into account.
Hammer test can provide fairly accurate estimate of concrete compressive
strength, it performed with precautions and co- relations :
- Concrete Age : The concrete should be 14 to 56 days old.
- Surface Condition : The surface of the concrete at the point
tested must be smooth dry and free of honey combing. Otherwise, rebound
readings will be low indicating a weaker concrete than is actually the
- Location of Test Point : The concrete to be tested must be at
least 100 mm thick. Care must be taken that the test points are no
closer than 25 mm from the edge.
- No. of Readings : Calculate the average of 10 readings and
exclude extreme readings (if any) and recalculate average using the
A simple equipment known as Rebound Hammer or
Schmidt Hammer is used for this purpose. Surface hardness measured during
the test give an idea about the soundness and quality of cover concrete.
Locations having very low rebound numbers indicate weak surface concrete and
may be affected by corrosion. The quality of concrete may be interpreted as
shown in the above table.