Introduction to Concrete Technology Part-III (Cement)
By Md. Yousuf Shaikh Ameer
Assistant Professor
Initial setting
Time: ( IS 4031, part 5 1988,
2000)
·
This test is performed on the Vicat‟s
apparatus, only this time the plunger is removed and
initial setting time needle is attached in its place.
·
It is very difficult to exactly know the moment when cement past starts losing plasticity, hence a continent but arbitrary limit is defined by the initial setting time.
The Procedure
for test is given below:
i. Take about 500 gram cement and mix it with 0.85 P of
water where P is the water percentage
required standard consistency to make smooth paste, Start a stop watch at the moment when water is added to cement.
ii. Fill and shake the vicat mould with the past with in 3 to 5 minutes, after adding water.
iii. Lower the initial setting
time needle till it touches
the surface of the cement paste
in mould.
iv. Release the weight so that needle penetrates the
paste. Initially, it will penetrate the complete depth,
i.e. 40 mm from top of mould.
v. Take readings after every 1 or 2 minutes and when the
penetration decreases, take reading
after every 20 seconds and then after 10 seconds, moving the mould different place every time.
vi. Record the time on the stop watch
when the penetration is 33 to 35 mm from the top surface.
vii. This time is known as the initial setting time.
Final setting
Time: (IS 4031, Part 5 1988, 2000)
Final setting time test is
conducted as a continuation of the initial setting and the same mould of cement
is used for the test.
The Procedure
for test is given below:
i) Remove the initial setting time needle and attach final setting
time needle, which is very similar to the initial
setting needle, only it has a collar with a rim attached to it. The center needle projects
0.5 mm more than the rim, the collar has hole known As air vent through which trapped air in the
rim can be escape so that it does not interfere with the reading.
ii) Lower the final setting time needle till it gently touches the paste and releases it.
iii) Observe the impression made by the attachment on the paste.
iv) Initially, both the rim and central needle will make
the impression. Turn the mould around and after every few minutes;
take the more number of readings on a different places on the surface of the paste.
v) When the impression of the rim stats becoming faint,
take readings at every short time intervals.
vi) Record the time when only central needle makes
impression but the surrounding rim does not make impression all.
vii) The time elapsed
between the moment when water is added
to the cement and moment
when only central needle makes an impression but the rim fails to make
impression is called the final setting
time.
Compressive Strength (IS 4031, Part 5, 1988, 2000)
i. Compressive strength of cement is the most important
parameter and hence this test is one of the most important tests.
ii. This test could not be performed on neat cement paste
due to the excessive shrinkage and subsequent cracking of neat cement paste insisted, a standard mortar
is prepared by mixing standard
sand conforming to IS 650-1960
The procedure
of the test is given below:
i.
Take 555 grams of standard sand and 185 grams of cement and mix it in dry condition for one minute.
ii.
Add water of [P/4 + 3.5] when ordinary sand is used and [P/4 +3] when standard sand is the standard consistency. Mix the all the ingredients thoroughly till mixture is
of
uniform color. The mixing time should be in
between 3 and 4 minutes. The mould is fitted on the table of the vibrating
machine immediately, after mixing and compacted at least for two minutes.
This process should be completed
within five minutes after mixing.
iii.
Immediately fill the mortar thus prepared into cube moulds
of size 7.06 cm is placed on non-porous
plate which is oiled from inside. Compact the mortar by standard means.
iv.
Keep the mould in 90% humidity and at 270C
+ or - 20 C f o r 24 hours. Where humidity room is not available, the mould can be kept under wet gunny bag for 24 hours.
v.
Remove the cubes from moulds after 24 hours and keep
immersed under clean water till the moment of testing.
vi.
Test the cubes under UTM for compressive strength.
vii.
The number of cubes to be tested
and the respective strength for some type of
cement.
Compressive strength of cement
|
Sr. no
|
Type of cement
|
Number of cubes
tested
|
1day strength N/mm2
|
3day strength N/mm2
|
7day strength N/mm2
|
28day strength N/mm2
|
|
1
|
Ordinary Portland
cement
|
2 cubes, 3
and 7 days
|
-
|
16
|
22
|
-
|
|
2
|
Rapid hardening cement
|
2cubes, 1day and 3
days
|
16
|
28
|
-
|
-
|
|
3
|
Low heat cement
|
3 cubes,
3,7 and 28 days
|
-
|
10
|
16
|
35
|
