Rebound Hammer
This test measures the surface hardness of concrete. For new structures, the hardness reading is used to co-relate the strength of concrete. The hammer has to be used normal to the test surface, vertically (up or down) or horizontally depending on the structural element.
IS Codes applicable: IS 13311, (Part 2): 1992.
References: IS 516: 1959 Test for strength of concrete
IS 8900: 1978 Criteria for rejection of outlying observations.

Applications of the test:
Assessing the Compressive Strength of Concrete
Assessing the Uniformity of Concrete.
Applicable for new concrete. Applicable for old concrete only with special techniques.

Equipment Used:
Rebound Hammer

Test basis:
Harder the surface of the material tested, greater is the rebound. In new concrete, as it gains strength, hardness increases and as a result, the readings increase. The hammer can be used in 3 orientation, Vertical up or down (typically used for slab), horizontal (for columns). Depending on the orientation used and age of concrete, the results are interpreted for strength.

Identify test Locations.
Clean the concrete surface thoroughly (remove plaster, if present ).
Press hammer and release.
Note down the reading.
Take an average of multiple readings.
Repeat the test on multiple areas of the element if necessary.
Test at different members of the structure.

Influencing factors for readings:
Readings taken on directly concrete.
New/old construction in elements.
New + old material (e.g. jacketed columns where new and old concrete coexist).
Carbonation of surface.
Condition of the surface.
Moisture content.
Curing and age of concrete.
Cement and aggregates ( detailed mix design) used.

Understanding results:
The Rebound Hammer test measures surface hardness only (upto about 50 mm depth).
In order to assess continuity inside the element, UPV test is recommended.
The estimated strength may very from actual strength about +/- 10%.
In order to confirm the findings of tests, core tests may be conducted on a select sample of element.

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