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Article based on contribution by: Ms Manisha, M.Tech student , Kolkata
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Cubical
specimens are required to be placed in compression testing machine in such a
manner that the load is applied to opposite sides of the cubes-as cast, that
is, not to the top & bottom. This means that the top surface of the test
cube (which comes into contact with the platen of the testing machine) is obtained by casting against a machined
plane.
In
case of cylindrical specimens, the top surface of cylinder (which comes into
contact with the platen of the testing machine) is finished by a float i.e this
surface is not obtained by casting against a machined plane.
Thus
the top surface of cylindrical specimens is somewhat rough & not truly
plane. We know that such imperfections in surface would result in introduction
of stress concentrations. This reduces apparent strength of concrete. Lack of
planeness of 0.25 mm may lower strength by as much as one third. The loss of
strength is particularly high in high strength concrete. Convex end surfaces
generally lead to higher concentrations. As such, convex end surfaces cause
greater reduction in strength compared to concave surfaces.
Plane end surfaces are essential to avoid this loss of strength. Many
standards including ASTM C 617 require end surfaces of a cylinder to be plane
within 0.05mm. Capping is required to be done to obtain plane end surfaces. IS
516 stipulates that capped surfaces shall not depart from a plane by more than
0.05 mm. As already discussed, the end surfaces of cubical specimens are plane
as they are cast against a machined plane whereas the end surfaces of
cylinderical specimens are not truly plane since they are finished by float. As
such, capping is prescribed only for cylindrical specimens.
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