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Civil Engineering Tests

VARIOUS TESTING OF CEMENT

Cement we use may go bad during transportation and storage prior to its use in work.We need to test the cement for a good quality and better performance in concrete mixtures. Broadly there are two tests for cement-

  • Field Testing
  • Laboratory Testing

Field Testing
It is generally done when cement is used for small or minor works. Following are the field tests-

  • After opening the bag of cement, we should give a good look, There should not be any visible lumps present in it. Colour of cement should be greenish grey.
  • It should give a smooth and gritty feeling when pinch of cement is feel between the fingers.
  • If we thrust our hand into the cement bag, it must give a cool feeling.There should not be any lumps inside.
  • If we throw some amount of cement into a bucket of water, it should float sometime before sinking.
  • A cube made from a stiff paste of 100gm cement and little water is to be put in a glass plate. The glass plate is taken slowly under water in a bucket. The shape of the cube should not be deformed while taking into water. After 24 hours cube should retain its original shape.

If a sample of cement truly satisfies the above tests we may say that the cement is not bad. But the above tests do not really indicate that the cement is really good for important work or big structure. For using cement in important and major structure, it should satisfy the Indian Standard specifications. It is possible only in laboratory testing to confirm the requirements of specified physical and chemical properties.

Laboratory Tests

Following are the laboratory tests for cement:

(i) Fineness Test
(ii) Setting time Test
(iii) Strength Test
(iv) Soundness Test
(v) Heat of Hydration Test
(vi) Chemical Composition Test

Fineness Test
The fineness of cement has great importance on the rate of hydration and hence rate of gain of strength. Finer cement has greater surface area for hydration and hence strength develops at a faster rate. Fineness of cement is tested in two ways-

  • By sieving
  • By determination of specific surface area.

Sieve Test:
In this test, about 100 gm of cement is taken on 90 micron sieve. Air-lumps are broken down by fingers. The sample is sieved continuously with circular and vertical motion for 15 minutes. The residue left on the sieve is weighed. The value should not exceed 10% for ordinary cement. This test is rarely used now-a-days.

Apparatus for fineness of cement
Fig 1: Apparatus for fineness of cement.
Courtesy: civilalliedgyan.com

Air Permeability Test:
Here fineness of cement is represented by specific surface area(sq cm/gm or ).Fineness of cement is represented by specific surface expressed as total surface area in sq cm/gm of cement. It is also expressed as /kg. Lea and Nurse Air Permeability Apparatus is used in measuring specific surface area of cement. The principle is based on the relation between the flow of air through the cement bed and the surface area of the particles comprising the cement bed.

Fineness can also be measured by Blains Air Permeability Apparatus. This is most commonly used in India. In this method a cement sample of 2cm height is placed on a perforated plate and pressure is applied. The size of perforation is 40.The manometer is connected to the top of the permeability cell and air is turned on. The lower end of the permeability cell is then slowly connected to the other end of the manometer. The rate of flow is so adjusted that the flowmeter shows a pressure difference of 30-50 cm. The reading of manometer is recorded. The process is repeated till the ratio between reading of manometer to pressure difference attains a constant value.

Air Permeability Apparatus
Fig 2: Air Permeability Apparatus

Setting time Test:
During the time of actual construction dealing with cement paste, mortar and concrete should remain in plastic state. Certain time is required for mixing, transporting, placing, compacting and up to finishing. Time interval between the moment that water is added to the cement to the time that the paste starts loosing its plasticity is called initial setting time of cement. Normally a minimum of 30 minutes is given foe mixing and handling operation. Vicat’s Apparatus is used for setting time test. Here 500 gm of cement sample is taken having consistency 0.85P and made a cement paste. The paste shall be gauged and filled into the Vicat’s mould in specified manner within 3-5 min. The square needle to the Vicat’s plunger is attached and lowered it gently to make contact with the surface of the mould and it is quickly released. As the needle penetrates only to a depth of 33-35 mm from the top, the test is completed. The time should not be less than 30 minutes for OPC and 60 minute for low heat cement.

The final setting time is the time elapsed between the moment the water is added to the cement and the time when the paste has completely lost its plasticity and has attained sufficient firmness to resist certain definite pressure. In this test, square needle is replaced by a needle with annular collar. Here the paste should attain such hardness that the needle doesn’t pierce through the paste more than 0.5 mm. The final setting time should not be more than 10 hours.

Vicat’s Apparatus for setting time test
Fig 3: Vicat’s Apparatus for setting time test

Strength Test
The compressive strength of hardened cement is most important of all the properties. In this test 555 gm of standard sand, 185 gms of cement (1:3;ratio of cement to sand) is taken in a tray and mix them with a trowel for 1 minute and water is added of quantity ()% of combined weight of cement and sand. The three ingredients should me mixed thoroughly until the mixture shows uniform colour. The time of mixing should not be less than 3 minutes. Immediately the mortar is filled into a cube mould of size 7.06 cm. The area of the face of the cube will be equal to 50 sq cm. The mortar should be compacted for 2 minutes. The compacted cube should be kept at a temperature of and at least 90% relative humidity for 24 hrs. Three cubes are tested for compressive strength at 1 day, 3 days, 7 days and 28 days where the period of testing being calculated from the completion of vibration. The compressive strength shall be average of strengths of three cubes for each period respectively. The compressive strength of 33 grade OPC at 3 days, 7 days and 28 days is 16 MPa, 22 MPa and 33 MPa respectively. Load applied gradually as 0 to 35 N//min on cubes by using UTM machine.

Soundness Test
Cement after setting should not undergo any appreciable volume change. It causes serious difficulties for the durability of structures. The cement having some quantity of free lime, magnesia and excess sulphates undergoes large change in volume as the time elapses. This test is performed to ensure that the cement does not show any appreciable expansion. The soundness of cement is determined by “Le Chatelier’s method” or by mean of “Autoclave test”.

Le Chatelier’s Apparatus
Fig 4: Le Chatelier’s Apparatus

Heat of Hydration Test
The reaction of cement with water is exothermic. The reaction liberates a considerable quantity of heat. It is estimated that about 120 calories of heat is generated in the hydration of 1 gm of cement. The apparatus used to determine the heat of hydration of cement is known as calorimeter.60 gm of cement and 24 ml of distilled water are mixed for 4 minutes at a temperature between 15C. Three specimen glass vials 100mm are filled with this mixture, corked and sealed with wax. The vials are then stored with mixture in a vertical position at . The heat of hydration is obtained by subtracting the respective heat of solution of hydrated cement from the heat of solution of unhydrated cement calculated nearest 0.1 calorie.

Heat of Hydration Apparatus
Fig 5: Heat of Hydration Apparatus

Chemical Composition Test
Here, ratio of different composition of cement is calculated. It should be present within the limit. Ratio of percentage of lime to percentage of silica, alumina and iron oxide is known as Lima Saturation Factor (LSF) shall not be greater than 1.02 and not less than 0.66.

The weight of insoluble residue residue shall not be more than 4%.

Weight of magnesia shall not be more than 6%.

Total loss on ignition shall not be more than 5%.

Total sulphur content calculated as sulphuric anhydride shall not be more than 2.5%

Plate Load Test

This is an in situ test for settlement computation in granular soils. It is difficult to procure the undisturbed samples of granular soils which are non-cohesive. Hence, the laboratory tests cannot be used to obtain the compressibility characteristics of granular soils. Plate Load Test is one of the most frequently used field tests where the test plate simulates a foundation. The method may also be used to evaluate the ultimate bearing capacity, the shear strength and deformation parameters of the soil at the ground surface, in trenches, or in pits.

The plate load test may be carried out in all soils, fills and rocks. The suitable soil for the test is coarse grained and composite soils as well as stiff to firm fine-grained soil. The test should normally not be used for very fine-grained soil. In the progress of the test, care should be taken to ensure that the loading plate is not placed directly on the particles than one-quarter of its diameter.

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Unconfined Compression Test

The unconfined compression test is a type of triaxial test in which the confining pressure is taken as zero. The test is onlybe conducted in clayey soil specimens which can stand without confinement because of this case. The test is generally performed on intact (non-fissured), saturated clay specimens. Although the test can be conducted in a triaxial test apparatus it is more convenient to perform it in an unconfined compression testing machine. There are two types of machines as described below:

1. Machine with a spring
In this type of testing machine a loaded spring is used. It consists of two metal cones which are fixed on horizontal loading plates supported on the vertical posts. The upper loading plate is fixed in position, whereas the lower plate can slide on the vertical posts. The soil specimen is placed between the two metal cones

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Consolidation Test of Soil

The consolidation test for soil samples is conducted in laboratory scale to determine the compressibility behavior of the soil specimen. The test is performed in the consolidation test apparatus, known as the consolidometer or an oedometer. Consolidometer consists of a loading device and a cylindrical container which is called as a consolidation cell. The soil specimen is placed in the consolidation cell between the top and bottom porous stones. The consolidation cells are of two types as follows:

1. Floating or free ring cell in which both the top and bottom porous stones are free to move. The top porous stone can move downward and the bottom stone can move upward as the sample consolidates.

Consolidation Test of Soil
Source: Soil Management India

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Elongation Index Test For Aggregates

AIM OF THE EXPERIMENT:
To determine the Flakiness Index of the given sample of course aggregate.

CODE OF REFERENCE:
• IS 2386(Part 1):1963 Methods of Test for Aggregates of Concrete- Particle Size and Shape. Reaffirmed Dec 2016
• ASTM 4791-10: Standard Test Method for Flat Particles, Elongated Particles or Flat Elongated Particles in Coarse Aggregates, American Society for Testing and Materials.
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