In 1948, a Swiss engineer Ernst Schmidt developed rebound hammer for measuring the hardness of concrete based on the rebound principle.  Schmidt standardized the hammer blow by developing a spring-loaded hammer.  Schmidt devised a method to measure the rebound of the hammer and the rebound number could be correlated to the compressive strength of standard cubes.  Owing to its simplicity, the Schmidt rebound hammer is widely used for nondestructive concrete test.

Testing Equipment and Concrete Surface Preparation:
Rebound hammer	Equivalent cube strength chart for orientations and different testing angles	Testing surface on a wall.  Surface cement paste and laitance were removed	Testing surface on a column
Testing surface on a column in different concreted age	Demonstration of hammer test

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