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Rebound Hammer Test - Common Method Adopted for Checking Strength of Concrete Surface
Rebound  Hammer Test :-  ( IS – 13311 ( Part 2 ) : 1992 )
 
Principle :
This is the most common method adopted for checking strength of concrete since 1940. The test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. The plunger of hammer is pressed strongly and steadily against the concrete surface at right angles to its surface, until the spring loaded mass is triggered from the locked position. The spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The distance traveled by the mass as a percentage is defined as rebound number.
 
Rebound  Hammer  Cross Section
 
Different  Rebound Hammers along with Calibration Anvil
 
Procedure :
The test is sensitive to local variations in the concrete, for e.g. If plunger strikes the aggregate underneath, then it would indicate high rebound number, visa versa if it strikes a void / cement paste , then it will indicate low rebound number. For this reason it is desirable to take at least 12 readings per 300 Sqcm. ( as per BS 1881 : Part 202 (21)) and discard abnormally high / low readings from the average one. The impact / test points shall not be closer than 20 mm to each other.
 
Influence of Test Conditions :
 
The test is significantly influenced by following factors :
 
Type of Cement :-
Concrete made with high alumina cement can give strength 100 % higher or that with supersulphated cement can give 50 % lower strength than that with ordinary Portland cement.
 
Type of Aggregate :-
The influence of aggregate type and proportion can be considerable.
 
Surface condition & Moisture content :-
Trowelled & floated surfaces are harder than moulded surfaces. Exposed aggregate surfaces are unsuitable for this methods.
A wet surface will give rise to underestimation of strength of concrete by about 20 % lower than calibrated under dry condition.
 
Curing & Age of concrete :-
The relation between hardness and strength varies as a function of time, subsequent curing and exposure condition will further influence this relationship. But this effect can be ignored for concrete up to 3 months old.
 
Carbonation of Concrete surface :-
Concrete exposed to atmosphere will normally form a hard carbonated skin. The strength predicted for carbonated concrete may overestimate up to 60 %.
 
Application :-
The rebound hammer method may be used for
  • Assessing the likely compressive strength of concrete with the help of suitable calibration charts.
  • Assessing uniformity of concrete
  • Assessing the quality of concrete in relation to specified standard requirements.
  • Assessing the quality of one element of concrete to another.
 
 
Rebound Hammer Test on Column
 
Reliability & Limitations :-
The test determines the hardness of the surface only. The impact of hammer is sensitive to the surface layer of about 25 to 50 mm only. The reliability of this test as alone, for compression strength is less, but it can generally indicate uniformity of concrete.

The rebound hammer results are very sensitive to the spring constant, thus after a use of hammer for about 15000 impacts, it is recommended to calibrate the hammer on Anvil.

The influence of aggregate type and proportion can be considerable. Thus it is strongly recommended that every lab, NDT consultant using rebound hammer shall develop their own graphs for compressive strength verses Rebound number, and shall not use blindly, the graphs given by manufacturers.

The probable accuracy of prediction of concrete strength in laboratory level is ± 15 % and that of structure is ± 25 %. It is recommended to use of Rebound hammer along with ultrasonic concrete testing or core test.