Why cylindrical specimens are required
to be capped for compression testing whereas there is no such
requirement for
cubical specimens?
Article
contributed by: Ms Manisha,
M.Tech student , Kolkata
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 0.05 mm. 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.
