LABORATORY TESTING OF CEMENT AT SITE

1.Specific Gravity of Cement

Object

To determine the specific gravity of cement.

Apparatus

Weighing balance, specific gravity bottle (or) Le Chatelier Flask, kerosene free from water, etc.

Theory

Specific gravity is normally defined as the ratio between the mass of a given volume of material and mass of an equal volume of water. One of the methods of determining the specific gravity of cement is by the use of liquid such as water free kerosene which does not react with cement.

Procedure

1.            Weigh the specific gravity bottle dry. Let the mass of empty bottle be W1.

2.            Fill the bottle with distilled water and weigh the bottle filled with water. Let the mass be W2.

3.            Wipe dry the specific gravity bottle and fill it with kerosene and weigh. Let this mass be W3.

4.            Pour some of the kerosene out and introduce a weighed quantity of cement (about 50 grams) into the bottle. Roll the bottle gently in inclined position until no further air bubbles rise to surface. Fill the bottle to the top with kerosene and weigh it. Let this mass be W4.

5.            Let the Mass of cement be W5.

6.            From these data calculate the specific gravity of the cement.

Observations and calculations

(Specific Gravity of Cement is ranging between 3.1 - 3.16 g/cc.)

2.            Fineness of cement

Object

To determine the fineness of a cement sample by sieving through a 90 micron IS Sieve.

Theory

The degree of fineness of cement is a measure of the mean size of the grains in cement. The rate of hydration and hydrolysis and consequent development of strength in cement mortar depends upon the fineness of cement. To have same rate of hardening in different brands of cement, the fineness has been standardized. The finer cement has quicker action with water and gains early strength though its ultimate strength remains unaffected. However, the shrinkage and cracking of cement will increase with the fineness of cement.

Apparatus

90 micron IS sieve, rice plate, weighing balance, bristle brush (25 or 40 mm brush with 250mm handle). The sieve has mesh openings of 0.087 mm.

Procedure

1.            Weigh accurately 100 gm of cement in a plate and transfer it to a clean dry IS test sieve and break down any air set lumps.

2.            While holding the sieve and pan in both hands, sieve with gentle wrist motion until most of the fine material has passed through and the residue looks fairly clean. This usually requires three to four minutes.

3.            Place the cover on the sieve and remove the pan. With sieve and cover held firmly in one hand, the other side of the sieve is tapped with the handle of the brush which is used for cleaning the sieve. Sweep clean the underside 0f the sieve.

4.            Empty the pan and wipe it clean with a cloth. Replace the sieve in the pan and remove the cover carefully. Return any coarse material that had been caught in the cover during tapping the sieve.

5.            The sieving is continued as described above for 15 minutes, rotating the sieve continuously throughout the sieving operation, involving no danger of spilling the cement.

6.            Weigh the residue.

Observations and calculations

Result

Residue of cement is _____ percent.

Precautions

a.            Any air set lump in the sample should be broken down with fingers, but do not rub on the sieve.

b.            The sieve must be cleaned thoroughly before starting the experiment.

c.             The care should be taken to ensure that no cement is spilled. After sieving all residues must be taken out carefully and weighed.

References

1. IS 4031 (part 1):1996               ---           Procedure for conducting the test

3.            Standard Consistency and Setting Time

Object

To determine

1. Standard consistency and

2. Initial and Final setting times of a given cement sample by vicat apparatus.

Theory and scope

1. Standard Consistency

The object of conducting this test is to find out the amount of water to be added to the cement to get a paste of normal consistency, i.e., the paste of a certain standard solidity, which is used to fix the quantity of water to be mixed in cement and before performing tests for setting time, soundness and compressive strength.

2. Setting Time

In order that the concrete may be placed in position conveniently, it is necessary that the initial setting time of cement is not too quick and after it has been laid, hardening should be rapid so that the structure can be made use of as early as possible. The initial set is a stage in the process of hardening after which any crack that may appear will not re-unite. The concrete is said to be finally set when it has obtained sufficient strength and hardness.

Apparatus

Vicat apparatus with vicat plunger, vicat needles and vicat mould, gauging trowel, measuring jar (100

to 200ml capacity), weighing balance, stop watch, rice plates, rubber gloves and glass plates.

The Vicat apparatus consists of a frame bearing a movable rod with a cap at one end and detachable needle or plunger at the other end. The movable rod carries an indicator which moves over a graduated scale having graduations in mm from zero to 40 on either direction to measure the vertical movement of the plunger. The scale is attached to the frame. The movable part with all attachments, i.e., the cap and rod with needle or plunger, weighs 300gm.

The Plunger required for determining the consistency, is of polished brass 10mm in diameter and 50 mm long with the lower end flat and small projection at upper end for insertion into movable rod.

The Needle A, required for determining the initial setting time, is 1mm square or 1.13 mm in diameter with the lower end being flat.

The Needle B, required for determining the final setting time, is the same as needle A but with a metal attachment hollowed out so as to leave a circular cutting edge 5mm in diameter, the end of the needle projects by 0.50 mm.

The vicat mould for cement paste consists of a split ring 80mm in diameter and 40mm in height and rests on a non-porous plate.

Procedure

Standard Consistency

The standard consistency of a cement paste ( the amount of water expressed as percentage by mass of the dry cement) which permits the Vicat plunger to penetrate to a height 5 to 7mm from the bottom of the vicat mould when the cement paste is tested as described below.

1.            For preparing one mould take 400gm of cement passing 850-micron IS sieve and prepare a paste of cement with a weighed quantity of water (100ml) taking care that the time of gauging is between 3 to 5 minutes. The gauging time is counted from the time of adding water to the dry cement until commencing to fill the mould.

2.            Fill the vicat mould resting upon non-porous plate with this paste. After completely filling the mould, smooth off the surface of the paste by single movement of palm making it level with the top of the mould. The mould may be slightly shaken to expel air.

3.            Place the test block in mould with the non porous resting plate under the rod attached with the plunger. Lower the plungers gently to touch the surface of the test block and release it quickly, allowing it to sink into the paste.

4.            Prepare the trial paste with varying percentage of water(firstly at an interval of 4%, that is of 24%,28%, and 32% and then at an interval of 1% and 0.25% between the percentage range determined by the previous test) and test as described above until the amount of water necessary for the standard consistency as defined is obtained.

Setting Time of Cement

1.            Prepare a neat cement paste by gauging the cement with 0.85 P water, where P = standard consistency as found before. The gauging time is again kept between 3 to 5 minutes. Start the stop watch at the instant when the water is added to the cement.

2.            Fill the Vicat mould and smooth off the surface of the paste making it level with the top of the mould. The cement block thus prepared is known as test block.

3.            For the determination of initial setting time, place the test block confined in the mould and resting on non-porous plates under the rod attached with the needle A, lower the needle gently in contact with the surface of the test block and release quickly, allowing it to penetrate into the test block.

4.            Repeat this procedure until the needle fails to pierce the block for about 5mm measured from the bottom of the mould. The period elapsed between the time when water is added to the cement and the time at which the needle fails to pierce the test block by about 5 mm is the initial setting time.

5.            For the determination of Final setting time replace the needle A of the vicat apparatus by the needle with an annular attachment B. The cement is considered finally set when, upon applying the needle B gently to the surface of the test block, the needle makes an impression thereon, while the attachment fails to do so. In the event of scum forming on the surface of the test block, use the underside of the block for the determination of final setting time.

 Observations and Calculations

 For Standard Consistency

Mass of cement taken for one mould = 400 gm.

Note: Initial reading is the indicator reading when the lower end of plunger touches the bottom of non porous surface of the mould.

For Setting Time

Mass of cement taken = 400 gm

Mass of water taken = 0.85 * P * 400 gm.

 Results

1.            Standard Consistency of cement =            percent   (3.1 - 3.16 g/cc is good)

2.            Initial setting time of cement =    Mts.        ( Min. 30 minutes is good)

3.            Final setting time of cement =     Mts.         (Max. 60 minutes is good)

Precautions

1. The experiment should be conducted at a room temperature of 27 +- 20C and at a relative humidity of 90 percent.

2. After a half minute from the instant of adding water, it should be thoroughly mixed with fingers for at least one minute. A ball of this paste is prepared and then it is pressed into the test mould, mounted on the non- porous plate.

3. The plunger should be released quickly without pressure or jerk, after the rod is brought down to touch the surface of the test block.

4. For each repetition of the experiment fresh cement is to be taken.

5. Plunger should be cleaned during every repetition and make sure that it moves freely and that there are no vibrations.

References

1. IS 4031 (part 4): 1988              ---           Procedure for conducting Standard Consistency

2. IS4031 (part 4): 1988               ---           Procedure for conducting Initial and Final setting Time

3. IS 5513:1996                ---           Specification for Vicat Apparatus

4. IS 10086:1982             ---           Specification for Gauging Trowel

4. Compressive Strength of Cement

Object

To determine the compressive strength of 1: 3 cement-sand mortar cubes after 3 days, 7 days and 28 days curing.

Scope

The compressive strength of cement mortar is determined in order to verify whether the cement conforms to IS specification (IS 12269 & IS 8112) for 53 & 43 grades respectively) and whether it will be able to develop the required compressive strength of concrete. According to IS, the ultimate compressive strength of cubes of cement sand mortar of the ratio 1 : 3, containing ((P/4)+3) percent of water should be as follows:

Apparatus

Compressive testing machine, cube moulds, vibrating machine, crucible for mixing cement and sand, measuring cylinder, trowels, non-porous plate and weigh balance.

Description of the Apparatus

Vibrating Machine

Vibrating machine (12000+/- 400 rpm, amplitude of vibration 0.055 mm, and 3phase motor with automatic cut-off).    

It consists of a heavy frame, on one side of which is fixed an electric motor and on the other side there is a set of four springs. Above these springs is fixed a mould on another frame and this mould is removable. With the frame carrying mould, a pulley is attached and the belt runs on the pulley and the motor. The mould is fitted with a detachable hopper at the top. Through the hopper mortar or concrete can be put into the mould without any loss of sample. A weight is attached to the frame to keep the mould in balance. When motor is started, the belt moves the pulley and gives vibrations to the mould at the rate of 12000 +/- 400 cycles per minute. These vibrations are simple harmonic at 90o out of phase. The normal running speed of electric motor is 12000 +/- 400 rpm. Due to the load attached to the frame, the C.G. of machine falls near the weight.

Cube Moulds

The moulds are of special shape and dimensions. The cube mould for compression test has 70.5 mm side (5000 mm2 face). It is constructed in such a way that it can be split up in parts in order to take out the cube without any damage. The base plate is non-porous and of such a size that there should be no leakage of water from the bottom.

The side of the cube mould is 70.5 +/- 1.27mm and angle between adjacent interfaces should be 90 +/- 0.5 degrees.

Procedure

1.            Calculate the material required. The material for each cube shall be mixed separately and the quantities of cement and standard sand shall be as follows:

Cement = 200 gm

Standard sand   = 600 gm

Water    = ((P/4) + 3) percent of total mass Where P is percentage of water for standard consistency.

2.            Place the mixture of cement and standard sand in the proportions of 1:3 by mass on a non-porous plate or china dish and mix it dry with a trowel for one minute and then with water until the mixture is of uniform colour. The percentage of water to be used shall be ((p/4) +3). The time of mixing (gauging) in any event shall not be less than 3 minutes and if the time taken to obtain a uniform colour exceeds 4 minutes, the mixture shall be rejected and the operation is repeated with a fresh quantity of cement, sand and water.

3.            Place the assembled mould on the table of the vibrating machine and firmly hold it in position by means of suitable clamps. Securely attach the hopper at the top of the mould to facilitate filling and this hopper shall not be removed until completion of the vibration period.

4.            Immediately after mixing the mortar as explained above, fill the entire quantity of mortar in the hopper of the cube mould and compact by vibration. The period of vibration shall be 2 minutes at the specified speed of 12000 +/- 400 cycles per minute.

5.            Remove the mould from the machine and keep it at a temperature of 27 +/- 2 0C in an atmosphere of at least 90 percent relative humidity for 24 hours after completion of vibrations.

6.            At the end of this period, remove the cube from the mould and immediately sub merge it in clean and fresh water and keep there until taken out just prior to breaking. The water in which the cubes are submerged shall be renewed after every 7 days and be maintained at a temperature of 27 +/- 2 0C. Keep the cubes wet till they are placed in machine for testing.

7.            Test the specimens at the required periods.

Observations and calculations

 Precautions

1. The mortar shall not be compressed into the moulds with hand.

2. Neglect the results which fall outside by 15 percent of the average results on either side.

3. The cubes should be tested on their sides and not on their faces.

4. The inside of the cube mould should be oiled to prevent the mortar from adhering to the sides of the mould.

5. The size of sand particles should be such that not more than 10 percent by mass shall pass a 60 micron IS sieve and shall completely pass through an 85 micron IS sieve.

6. The time of wet mixing shall not be less than 3 minutes. If the time of mixing exceeds 4 minutes to bring a uniform colour, the mixture shall be rejected and fresh mortar should be prepared.

References

1.            IS 4031 (part 6)-1988    ----         Procedure for conducting the test

2.            IS 10080 :1982 ----         Specification for vibration Machine

3.            IS 10086 :1982 ----         Specification for moulds & gauging Trowel

4.            IS 650:1991       ----         Specification for Standard Sand