Explain the Limitations of Rebound Hammer?

The following characteristics of concrete can affect the correlation of the rebound number with the actual surface hardness and should be taken care of  for better results.

i. High rebound numbers are generally obtained for smoother surfaces and the scatter in the data tends to be less.  Minimising the data scatter increases the confidence in the test results.  Therefore, underwater concrete surfaces must be thoroughly cleaned and smoothened with a carborundum stone or similar abrasive before tests are conducted.

ii. Water saturated concrete tends to show rebound readings lower than for the same concrete tested dry.  This may affect the com- parison of data taken above and below waterline.  An assessment of reduction in rebound number in an underwater test should be evaluated, and necessary correction should be applied to rebound number.

iii. Type of aggregate and cement affects the correlation of the re- bound numbers with actual compressive strength of the concrete under test.  A calibration curve is required for each particular concrete mix to assure accuracy.

Because of these limitations, the estimation of concrete compressive strength obtained with a rebound hammer may vary up to ± 20% from the actual strength.  The rebound hammer is primarily useful for checking surface compressive strength or surface hardness and uniformity of concrete within a structure.  It can also be used to compare one concrete structure element against another if they are reasonably similar.