DETERMINATION OF COMPRESSIVE STRENGTH OF
DETERMINATION OF COMPRESSIVE STRENGTH OF CEMENT.
( IS : 4031 – Part – 6 )
Object:
Determination
of the compressive strength of standard cement mortar cubes compacted by means
of a standard vibration machine.
Apparatus:
Vibration
machine, cube moulds of size 7.06 cms(confirming to IS : 4031 – 1968), and
Standard Sand to be used in the test shall be confirm to IS : 650 – 1966.
Procedure:
Mix
Proportions and Mixing: Clean
appliances shall be used for mixing and the temperature of the water and that
of the test room at the time when the above operations are being performed
shall be 270 +/- 20C. Place in a container a mixture of
cement and standard sand in the proportion of 1 : 3 by weight; mix it dry, with
a trowel for one minute and then with water until the mixture is of uniform
colour. The quantity of water to be used shall be as specified below. In any
event, it should not take more than 4 minutes to obtain uniform coloured mix.
If it exceeds 4 minutes, the mixture shall be rejected and the operation repeated
with a fresh quantity of cement, sand and water.
The
material for each cube shall be mixed separately and the quantity of cement,
standard sand and water shall be as follows: Percentage of water to be added to
the cement and sand in ( 1:3 ) cm (P/4 + 3) X % combined weight of cement and
sand = (P/4 + 3) X 800/100.
Cement
200 gms, standard sand 600 gms, water (P/4 + 3) per cent of combined weight of
cement and sand, where P is the percentage of water required to produce a paste
of standard consistency.
Moulding
Specimens:
In
assembling the moulds ready for use, cover the joints between the halves of the
mould with a thin film of petroleum jelly and apply a similar coating of
petroleum jelly between the contact surfaces of the bottom of the mould and its
base plate in order to ensure that no water escapes during vibration. Treat the
interior faces of the mould with a thin coating of mould oil. Place the
assembled mould on the table of the vibration machine and firmly hold it in
position by means of suitable clamps. Securely attach a hooper of suitable size
and shape at the top of the mould to facilitate filling and this hooper shall
not be removed until completion of the vibration. Immediately after mixing the
mortar, place the mortar in the cube mould and rod with a rod. The mortar shall
be rodded 20 times in about 8 seconds to ensure elimination of entrained air
and honey combing. Place the remaining quantity of mortar in the hooper of the
cube mould and rod again as specified for the first layer and then compact the
mortar by vibration. The period of vibration shall be two minutes at the
specified speed of 12000 +/- 400 vibrations per minutes. At the end of
vibration remove the mould together with the base plate from the machine and
finish the top surface of the cube in the mould by smoothing surface with the
blade of a trowel.
Curing
Specimens:
Keep
the filled moulds at a temperature of 270C +/- 20C in an atmosphere of atleast 90% relative
humidity for about 24 hours after completion of vibration. At the end of that
period remove them from the moulds immediately submerge in clean fresh water
and keep them under water until testing. The water in which the cubes are
submerged shall be renewed every 7 days and shall be maintained at a
temperature of 270C +/- 20C. After they have been taken
out and until they are tested, the cubes shall not be allowed to become dry.
Testing:
Test
three cubes for compressive strength at the periods mentioned under the
relevant specifications for different hydraulic cements, the periods being
reckoned from the completion of vibration. The compressive strength shall be
the average of the strengths of the three cubes for each period of curing. The
cubes shall be tested on their sides without any packing between the cube and
the steel platens of the testing machine. One of the platens shall be carried
base and shall be self-adjusting and the load shall be steadily and uniformly
applied, starting from zero at a rate of 350 kgs/cm2/min.
Calculation:
Calculate
the compressive strength from the crushing load and the average area over which
the load is applied. Express the results in kgs/cm2 to the
nearest 0.5 kg/cm2.
Compressive
strength, kg/cm2 = P/A, where
‘P’is the crushing load in kg, and ‘A’ is the area in cm2.
DETERMINATION OF COMPRESSIVE STRENGTH OF CONCRETE.
(IS: 516 – 1959)
Object:
Determination
of compressive strength of concrete.
Apparatus:
Testing
Machine: The testing machine may be
of any reliable type of sufficient capacity for the tests and capable of
applying the load at the specified rate. The permissible error shall not be
greater than 2 percent of the maximum load. The testing machine shall be
equipped with two steel bearing platens with hardened faces. One of the platens
shall be fitted with a ball seating in the form the portion of a sphere, the
center of which coincides with the central point of the face of the platen. The
other compression platen shall be plain rigid bearing block. The bearing faces
of both platens shall be at least as larger as, and preferably larger than the
nominal size of the specimen to which the load is applied. The bearing surface
of the platens, when new, shall not depart from a plane by more than 0.01mm at
any point, and they shall be maintained with a permissible variation limit of 0.02mm.
the movable portion of the spherical seated compression platen shall be held on
the spherical seat, but the design shall be such that the bearing face can be
rotated freely and tilted through small angles in any direction.
Age
at test: Tests shall be made at
recognized ages of the test specimens, the most usual being 7 and 28 days. The
ages shall be calculated from the time of the addition of water of the dry
ingredients.
Number
of Specimens: At least three
specimens, preferably from different batches, shall be made for testing at each
selected age.
Procedure:
Specimens
stored in water shall be tested immediately on removal from the water and while
they are still in the wet condition. Surface water and grit shall be wiped off
the specimens and any projecting find removed specimens when received dry shall
be kept in water for 24 hours before they are taken for testing. The dimensions
of the specimens to the nearest 0.2mm and their weight shall be noted before
testing.
Placing
the specimen in the testing machine the bearing surface of the testing machine
shall be wiped clean and any loose sand or other material removed from the
surface of the specimen, which are to be in contact with the compression
platens. In the case of cubes, the specimen shall be placed in the machine in
such a manner that the load shall be applied to opposite sides of the cubes as
cast, that is, not to the top and bottom. The axise of the specimen shall be
carefully aligned with the center of thrust of the spherically seated platen.
No packing shall be used between the faces of the test specimen and the steel
platen of the testing machine. As the spherically seated block is brought to
bear on
the
specimen the movable portion shall be rotated gently by hand so that uniform
seating may be obtained. The load shall be applied without shock and increased
continuously at a rate of approximately 140 kg/cm2/min.until the
resistance of the specimen to the increasing load breaks down and no grater
load can be sustained. The maximum load applied to the specimen shall then be
recorded and the appearance of the concrete and any unusual features in the
type of failure shall be noted.
Calculation: The measured compressive strength of the specimen
shall be calculated by dividing the maximum load applied to the specimen during
the test by the cross sectional area, calculated from the mean dimensions of
the section and shall be expressed to the nearest kg per cm2.
Average of three values shall be taken as the representative of the batch
provided the individual variation is not more than +/-15 percent of the
average. Otherwise repeat tests shall be made.
A
correction factor according to the height / diameter ratio of specimen after
capping shall be obtained from the curve shown in Fig.1 of IS:516-1959. The
product of this correction factor and the measured compressive strength shall
be known as the corrected compressive strength this being the equivalent
strength of a cylinder having a height/diameter ratio of two. The equivalent
cube strength of the concrete shall be determined by multiplying the corrected
cylinder strength by 5/4.
DETERMINATION OF FLEXURAL STRENGTH OF CONCRETE.
(IS: 516 – 1959)
Object: Determination of
the flexural strength of concrete specimen.
Apparatus:
a)
Standard moulds of size 15 X 15 X 70 cms for preparing the specimen.
b)
Tamping bar.
c)
Testing Machine.
Procedure:
Test specimens stored in water at a temperature of 250C to
300C for 48 hours before testing shall be tested immediately on
removal from the water, whilst they are still in a wet condition. The
dimensions of each specimen shall be noted before testing. No preparation of
the surface is required.
Placing the specimen in the testing machine: The bearing surfaces of
the supporting and loading rollers shall be wiped clean, and any loose sand or
other material removed from the surfaces of the specimen where they are to make
contact with the rollers. The specimen shall then be placed in the machine in
such a manner that the load shall be applied to the upper most surface as cast
in the mould, along two lines spaced 20 or 13.30 cms apart. The axis of the
specimen shall be carefully aligned with the axis of the loading device. No
packing shall be used between the bearing surface of the specimen and the rollers.
The load shall be applied with shock and increasing continuously at a rate such
that the extreme fiber stress increases at approximately 7 kgs/cm2/mm
for the 10 cm specimens, the load shall be increased until the specimen falls,
and the maximum load applied to the specimen during the test shall be recorded.
The appearance of the fractured faces of the concrete and any unusual features
in the type of failure shall be noted.
Calculation:
The flexural strength of the specimen shall be expressed as the modules
of rapture ‘ fb’ which if ‘a’ equals the distance between the line of fracture
and the nearer support measured on the centerline of the tensile side of the
specimen, in cm, shall be calculated to the nearest 0.5 kg/cm2 as
follows.
fb = ( p X l ) / ( b X d2 )
When ‘a’ is greater than 20.0 cm . for 15.0 cm specimen or greater than
13.30 cm for a 10.0 cm specimen, or
fb
= ( 3p X a ) / ( b X d2 )
When ‘a’ is less than 20.0 cms. but greater than 17.0 cms for 15.00 cms
specimen, or less than 13.30 cms but greater than 11.0 cms for a 10.0 cms
specimen, where b = measured width in
cms of the specimen, d = measured depth
in cms of the specimen at the point of failure,
l = length in cm. of the span on
which the specimen was supported, and p
= maximum load in kg. applied to the specimen.
If ‘a’ is less than 17.0 cm. for a 15 cm specimen or less than 11.0 cm
for a 10.0 cm specimen, the result of the test shall be discarded.
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