Unformatted text preview: n 2.
9.4 Preparation of Test Specimens — The test specimens shall be
prepared in accordance with 2 and shall be stored in water at a
temperature of 24° to 30°C for at least 48 hours before testing. At least
three specimens shall be made and tested.
9.5 Age at Test — Normally test shall be made when the specimens
reach the age of 28 days.
9.6 In order to determine the compressive strength of the concrete, three
test specimens for compressive strength shall be made together with
each set of cylinders or prisms in accordance with 9.2. The test
specimens for compressive strength shall be made from the same sample
of concrete as the cylinders or prisms and shall be cured and stored
under identical conditions.
9.7.1 Testing Machine — The testing machine shall comply in all
respects with the requirements of 5.2.1 and in addition shall be capable
of maintaining the load at any desired value.
9.7.2 Extensometers — Two extensometers are required each having a
gauge length of not less than 10.2 cm and not more than half the length
of the specimen. They shall be capable of measuring strains to an
accuracy of 2 × 10–6. Mirror extensometers of the roller or rocker type
9.8 Procedure — The three test specimens for compressive strength
shall first be tested in accordance with 5 and the average compressive
strength shall be recorded. Immediately on removing the cylinder or
prism from the water and while it is still in a wet condition, the
extensometers shall be attached at the ends, or on opposite sides of the
specimen and parallel to its axis, in such a way that the gauge points are
symmetrical about the centre of the specimen and in no case are nearer
to either end of the specimen than a distance equal to half the diameter
or half the width of the specimen. The extensometers shall be fixed with
the recording points at the same end. The specimen shall be
immediately placed in the testing machine and accurately centred. The
load shall be applied continuously and without shock at a rate 18 IS : 516 - 1959
of 140 kg/sq cm/min until an average stress of ( C + 5 ) kg/sq cm is
reached, where C is one-third of the average compressive strength of the
cubes calculated to the nearest 5 kg/sq cm. The load shall be maintained
at this stress for at least one minute and shall then be reduced gradually
to an average stress of 1.5 kg/sq cm when extensometer readings shall
be taken. The load shall be applied a second time at the same rate until
an average stress of ( C + 1.5 ) kg/sq cm is reached. The load shall be
maintained at this figure while extensometer readings are taken. The
load shall again be reduced gradually and readings again taken at
1.5 kg/sq cm. The load shall then be applied a third time and
extensometer readings taken at ten approximately equal increments of
stress up to an average stress of ( C + 1.5 ) kg/sq cm. Readings shall be
taken at each stage of loading with as little delay as possible. If the
overall strains observ...
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- Fall '13
- Calcutta, specimen, indian standards, Bombay, Concrete Association of India