Surface Hardness Test ( Rebound Hammer )

Principle : THe rebound of an elastic mass depends on the hardness of the surface on which the mass impinges .

THeory : Rebound number is higher , coimpressive strength is higher .

- THe appratus consists of a metal plunger , one ned of which is held against the concrete surface whle the free end is struck by a spring -loaded mass which rebounds to a point on a graduated scale .

- The amount of rebound increases in concrete strength for a particular concrete mix .

- The test is sensitive to the presence of aggregates & of voids immediately below the plunger .

- It is necessary to have at least 10-12 readings over the area to be tested in order for it to be representative of the member .

- Advantages :

QUcik to operate
Low cost
Non destructive
Hammer can be used horizontally , vertically or inclined .

Disadvantage
Results relate only to a surface zone of up to 30mm depth & may be affected by localised hardening due to carbonation .
Results may not accurate ( estimated error as high as 30% ) .
Finishes must be removed before testing since they tend to affect the results .
The test results are also influenced by the suface texture of the concrete , moisture , cement type , mix proportions & type of aggregates .