The Scmidt hammer test is based on the elastic rebound of hammer which presses on concrete surface and it measures the surface hardness of concrete.
Since the test is very sensitive to the presence of aggregates and voids at the concrete surface, it is necessary to take more than 10 readings over the area of test.
However, it should be noted that Schmidt hammer test measures surface hardness only but not the strength of concrete.
Therefore, it may not be considered a good substitute for concrete compression test.
What is the indication of shear slump and collapse slump in slump tests? There are three types of slump that may occur in slumps test, namely, true slump, shear slump and collapse slump.
True slump refers to general drop of the concrete mass evenly all around without disintegration.
Shear slump implies that the concrete mix is deficient in cohesion.
Consequently, it may undergo segregation and bleeding and thus is undesirable for durability of concrete.
Collapse slump indicates that concrete mix is too wet ad the mix is deemed to be harsh and lean.