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

 

Introduction to Concrete Technology Part-II (Cement)

 Introduction to Concrete Technology Part-II (Cement)

By Md. Yousuf Shaikh Ameer

Assistant Professor

Hydration of cement

·   When Portland cement is mixed with water its chemical compounds undergoes a series of different chemical reactions that cause it to harden. This chemical reaction with water is called "hydration". Every reaction occurs at a different time and rate. Together, the results of these reactions show how Portland cement hardens and gains strength.

 

OPC hydration

Hydration starts as soon as the cement and water are added and mixed.

·         The rate of  heat of hydration and the heat emmitted by the reaction of each compound is different

           ·         Each compound goes under different reactions and produces different products when it hydrates.

·         Tri-calcium silicate (C₃S). Hydrates and hardens rapidly and is wholly responsible for initial set and early strength of cement. Portland cements with higher percentages of C₃S will gain high early strength.

·         Tri-calcium-aluminate ( C₃A). Hydrates and hardens the quickest. Liberates a large amount of heat almost immediately after mixing and contributes somewhat to early strength. Gypsum is added to Portland cement to retard the amount of C₃A hydration. Without gypsum, due to C₃A Hydration it causes Portland cement to set as quickly after adding water.

·         Di-calcium silicate (C₂S). Hydrates and hardens slowly and is mainly responsible for strength increases beyond one week.

·         Tetra-calcium-alumino-ferrite (C₄AF). Hydrates rapidly but contributes very little to strength. By using it allows lower kiln temperatures in Portland cement manufacturing. Most of the Portland cement color effects are due to C₄AF.

Heat of Hydration

·         The heat of hydration is the heat evolved when water and Portland cement react. Heat of hydration is mostly influenced by the proportion of C₃S and C₃A in the cement but is also influenced by water-cement ratio, fineness and curing temperature. As any one of these factors is increased, heat of hydration increases.

·         For usual range of Portland cements, about one-half of the total heat is emmited between 1 and 3 days, about three-quarters in 7 days, and nearly 90 percent in 6 months.

           ·         The heat of hydration of cement depends on the chemical composition of cement.

Physical Properties of OPC and their tests

·         Portland cements are commonly checked by their physical properties for quality control purposes. Its physical properties can be used to classify and compare Portland cements. Now the challenge in physical property characterization is to develop physical tests that can satisfactorily characterize key parameters.

The physical properties of cement

 

·           Setting Time

·           Soundness

·           Fineness

·           Strength

 

Fineness:

 

·      Fineness of cement mostly refers to the size of grains or particles of cement. Lesser the grain size, finer the cement. Due to small grain size, more surface area is available for contact with water and hence the reaction of hydration becomes very faster, gaining of strength is more rapid and rate of evolution of heat increases.

Test for Fineness                                                                                                            

There are two methods prescribed in IS code for testing of fineness.

I)     Sieve Test (IS 4031, part 1 1996):

II)   Air permeability Method (IS 4031, part 2 1999)

 

Sieve Test (IS 4031, part 1 1996):

This is very simple test. The procedure is given below

 

·         Weight 100 gram of cements correctly and takes it on a standard IS sieving number 9, i.e. a sieve size 90 microns.

·         Break down if any air set lumps found in the sample with fingers.

·         Continuously sieve the sample giving circular and vertical motion for a period of period at least 5 minutes. Mechanical sieving device may also be used.

·         Weigh the residue left on the sieve.

·         This residue shall not exceeds by the limits given as per IS CODE below

·         OPC                                        10 gm 

       Rapid Hardening Cement         5gm

Air permeability Method (IS 4031, part 2 1999)

·        In this method, the permeability of a cement  bed for passage of air current is released with the specific surface of the grains of cement.

·         The finer the grains, the more the specific surface.

·       Lea and nurse permeability apparatus is used for the test, which is shown in below fig. the diameter of permeability cell is 2.5 cm.

·         knowing the density of cement the weight of cement required to make a cement bed of 0.47 porosity can be calculated. The procedure of the test is given below.

·        Fill the weight of cement as calculated above for making a bed of 0.475 porosity into the permeability cell.

·         Slowly slowly pass dry air through the cement bed at a constant velocity

·         Read the difference in manometer limbs and note it h₁

·         Read the difference in flow meter limbs and not it has h₂. It should be between 30 to 50 cm.

·         Repeat the readings untill steady state is achieved.

·         Specific surface is calculated by following formula

 

K = √Ɛ³A/CL

Where     Ɛ     = Porosity =0.475

L    = length of cement bed (cm)

D    = density of cement in gm/cm³ C             = flow meter constant.

A     = Area of c/s of cement bed (cm²)

Generally value of C and K are provided be the manufacturer for different value of porosity.

·         This specific surface should not be less than the limits given below. OPC: Not less than 2250 cm²/gram

Rapid hardening cement: Not less than 3250 cm²/gram Low heat Cement: Not less than 3200 cm²/gram.

Setting Time:

Cement paste setting time is influenced by a number of factors including:

·         Cement fineness, water-cement ratio, chemical content (specially gypsum content) , type of cement and admixtures. Setting tests are used to determine how a particular cement paste sets.

·         For construction purposes, the initial setting time must not be too soon and the final settting time must not be too late.

Normally, two setting times are define:

1)      Initial setting: Occurs when the paste begins to stiffen considerably and when cement will lose its plasticity.

2)      Final stetting: Occurs when the cement has hardened to the point at which it can sustain some load.