Introduction to Concrete Technology Part-III (Cement)

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