This preview shows page 1. Sign up to view the full content.
Unformatted text preview: finished level with the top
of the mould, using a trowel, and covered with a glass or metal plate to
2.10.1 Compacting by Hand — When compacting by hand, the standard
tamping bar shall be used and the strokes of the bar shall be distributed
in a uniform manner over the cross-section of the mould. The number of
strokes per layer required to produce specified conditions will vary
according to the type of concrete. For cubical specimens, in no case shall
the concrete be subjected to less than 35 strokes per layer for 15 cm
cubes or 25 strokes per layer for 10 cm cubes. For cylindrical specimens,
the number of strokes shall not be less than thirty per layer. The strokes
shall penetrate into the underlying layer and the bottom layer shall be
rodded throughout its depth. Where voids are left by the tamping bar,
the sides of the mould shall be tapped to close the voids.
2.10.2 Compacting by Vibration — When compacting by vibration, each
layer shall be vibrated by means of an electric or pneumatic hammer or
vibrator or by means of a suitable vibrating table until the specified
condition is attained.
NOTE — The mode and quantum of vibration of the laboratory specimen shall be as
nearly the same as those adopted in actual concreting operations. 2.11 Capping Specimens — The ends of all cylindrical test specimens
that are not plane within 0.05 mm shall be capped. Capped surfaces
shall not depart from a plane by more than 0.05 mm and shall be
approximately at right angles to the axis of the specimens. The
planeness of the cap shall be checked by means of a straight edge and
feeler gauge, making a minimum of three measurements on different
diameters. Caps shall be made as thin as practicable and shall not flow
or fracture when the specimen is tested. Capping shall be carried out
according to one of the following methods.
*Specification for moulds for use in tests of cement and concrete. 7 IS : 516 - 1959
2.11.1 Neat Cement — Test cylinders may be capped with a thin layer of
stiff, neat Portland cement paste after the concrete has ceased settling
in the moulds, generally for two to four hours or more after moulding.
The cap shall be formed by means of glass plate not less than 6.5 mm in
thickness or a machined metal plate not less than 13 mm in thickness
and having a minimum surface dimension at least 25 mm larger than
the diameter of the mould. It shall be worked on the cement paste until
its lower surface rests on the top of the mould. The cement for capping
shall be mixed to a stiff paste for about two to four hours before it is to be
used in order to avoid the tendency of the cap to shrink. Adhesion of
paste to the capping plate may be avoided by coating the plate with a
thin coat of oil or grease.
2.11.2 Sulphur — Just prior to testing, the cylindrical specimens may be
capped with a sulphur mixture consisting of 2 or 3 parts sulphur to 1
part of inert filler, such as fire-clay. The specimens shall be securely held
in a special j...
View Full Document
This document was uploaded on 10/01/2013.
- Fall '13