Sunday, February 6, 2011

DETERMINATION OF COMPRESSIVE STRENGTH OF CEMENT.( IS : 4031 – Part – 6 )

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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