Cement Mortar – Preparation of Cement Mortar

Cement Mortar

The unit of measurement for cement is a bag of cement weighing 50 kg and this is taken as 0.035 cu. m. Sand is measured in boxes of size 25x35x40 cm on the basis of its dry volume.

Cement mortar shall be used as soon as possible after mixing and before it has begun to set, (and in any case within two hours, after water has been added to the dry mix), only that quantity of mortar which can be used within 30 minutes of its mixing shall be prepared at any time, care shall be taken not to add more water to the mixing than which shall bring the cement mortar to the consistency of a stiff paste,

In case cement mortar has stiffened because of evaporation of water, it may be retampered by adding water frequently as needed to restore the requirements of consistency, but this retampering shall be permitted only up to two hours.

Concrete Shrinkage – Shrinkage of Concrete

Concrete Shrinkage – Shrinkage of Concrete

Concrete shrinks during setting and drying due to hydration of cement and produces shrinkage cracks. The drying shrinkage increases with an increase in cement content or an increase in water content. Shrinkage is greater with richer mixes (more of cement) and also with aluminious cements. Other things being equal, shrinkage of concrete is almost directly proportional to the amount of-:water in the mix. The type of aggregate used does not generally affect the shrinkage seriously though it has an indirect effect due to the difference of water/cement ratio depending on the type of the aggregate ; with large size of aggregate shrinkage is low. Where shrinkage may give rise to high tensile stresses such as in road slabs, lean dry mixes are desirable. Rich mixtures are uneconomical and are used only for impermeable constructions to ensure water-tightness.



Slump Cone Test and its Advantages – Limitations

Slump Cone Test and its Advantages – Limitations

Although, the slump cone test is not entirely satisfactory since it gives widely varying results and also does not give a true measure of workability but it is of value in the field as a control test and is useful in comparing the consistence of successive batches of concrete made with the same ingredients and is one of the simplest tests to carry out. Provided no change is made in the aggregate or its grading, slump cone tests will indicate whether correct water and cement contents are being maintained. For a given slump and aggregate grading, the water required for unit volume of concrete is constant irrespective of the change of cement content. The amount of slump depends not only on the amount of water in the mix but also on the nature of the aggregate ; rounded stones give a greater slump than angular stones for the same mixture.

The slump cone test should not be used to compare mixes of different proportions or of different types of aggregates. This test is not applicable to lean dry mixes where the water/cement ratio is low as the slump recorded is very small. All aggregates of size 50 mm and above should be removed from the sample concrete before the test.

Slump Cone Test

The apparatus for determining the stump (slump cone) is a steel mould in the form of a truncated cone. Its top diameter is 10cm, the bottom diameter 20 cm, and the height 30 cm, open at both ends and fitted with handles and foot pieces on sides. The cone is placed on a smooth non absorbent surface and freshly mixed concrete is placed in the mould in four successive layers, each layer being rodded 25 times with a bullet-pointed rod 16 mm in diameter and 60 cm long. When tilled to top (after ramming) and top struck level, the mould is immediately with drawn and the slump or subsidence of the concrete measured from a straight edge held across the top of the mould.

“Slump” is the vertical settlement of the concrete after the mould has been withdrawn, i.e., the difference between the height of the mould and the highest point of the subsided concrete.

Slump Cone Test Table

A concrete with 0 to 25 mm slump has very low degree of workability, and with 100 to 175 mm slump a high degree of workability which is not normally suitable for vibrations.

Advantages of Slump Cone Test:

1. It is helpful in detecting the difference in water content of different batches of concrete of the same identical mix.

2. The slump cone apparatus is cheap, Convenient to handle, and moreover it is portable.

Limitations of Slump Cone Test:

1. It is not suitable for concrete in which maximum size of the aggregate exceeds 40 mm.

2. There is no direct relationship between the workability and the value of slump.

3. Different shapes of slump may occur and it is difficult to decide which is the correct value.

4. It is not suitable for dry mixes.

Storage of Cement

Storage of Cement

Cement can be safely stored in sacks for a few months if kept in dry and air-right room. If prolonged storage of cement is unvoidable, it is better to empty the bags and stock the cement in as deep a heap as possible in a damp-proof enclosed space. Paper sacks are better than jute sacks as regards deterioration by moisture. Cement stored for more than six months should be tested for soundness before use on all important works and which period may be three months when stored in jute bags. Concrete made with storage-deteriorated cement takes longer to harden.

Cement in bags should be stored in a dry room on a raised wooden platform 15 to 23 cm above the floor level and 30 cm away from walls. Bags to be stacked in not more than 10 layers high (max: 4.5 m) to prevent bursting of the bags in bottom layers. The bags should be placed close together to reduce circulation of air and all openings in the room should also be well closed. If the piles are to be more than seven or eight bags high, the bags should be placed in headers and stretchers, i.e., alternatively lengthwise and crosswise.

Test for Freshness of Cement

Test for Freshness of Cement

Indications of a damaged cement are given by the presence of large lumps of set cement, and when this happens the lumps should be screened out unless they are soft enough to be powdered when pressed in the fingers. It needs to be considered if this cement is to be rejected.

When cement is rubbed between fingers and thumb it should feel like a smooth powder such as flour. Grittiness may indicate deterioration. Such a cement should be tested for setting time. If caked material is screened out and the balance used with 10 per cent extra cement, the concrete will be good for common small works, though it will be slow setting.

Hydration of Cement & Evolution of Heat

Hydration of Cement & Evolution of Heat

When water is added to cement, the cement hydrates and during the chemical reactions which take place while the cement is setting an increase in temperature occurs and a considerable quantity of heat is generated. Hydration of cement is incomplete without an adequate quantity of water. Heat and humidity accelerate hydration of cement. The amount of heat and the rate at which it is generated depends mainly on the type (chemical composition) of the cement and affects the rate of hardening. The greater the heat generated the more rapid the rate of hardening. Shrinkage occurs on subsequent cooling of the cement mortar or concrete resulting in cracks. The more rapid the rate of hardening the more susceptible is a concrete to shrinkage cracks.

The amount of water required to hydrate cement is about 25 per cent of the weight of the cement. The amount of mixing water is rarely less than twice this quantity.

If water/cement ratio is less than 0.4 to 0.5 çomplete hydration of cement will not occur. Roughly water/cement ratio is 0.60 for a 1:2:4 mix, 0.5 for 1:1.5:3 mix, and 0.45 for 1:1:2 mix.

IRS-T 40 Special Grade Cement

IRS-T 40 Special Grade Cement

The IRS-T 40 Cement is a special type of cement which is mainly used in railway works only. This special cement is manufactured under the specifications that are approved by the ministry of Indian Railways. This cement contains high volume of C3S content which are finely grinded inorder to develop high early strength. The IRS-T 40 Cement is mainly used in the manufacture of railway sleepers and also can be used where high strength should be obtained. Beside that these cements are used in high rise buildings, prestressed concrete works.

Initial & Final Setting Time of Cement Test

Initial & Final Setting Time of Cement

Initial & Final Setting Time of Cement test is used to detect the deterioration of cement due to storage.This is a conventional type of cement test and it has no relation with that of hardening of concrete.

Initial Setting Time of Cement Test Procedure:

1. Take 300 gm of cement and mix it with water percentage as mentioned in consistency test of cement

2. Now the prepared cement paste is filled in the vicat mould.

3. The square needle of cross-section 1 mm x 1 mm is attached to the movable rod of the Vicat apparatus.

4. Then the needle is allowed to quickly release and allowed to penetrate in the cement paste. In initial stage, the needle penetrates completely. It is then taken out and dropped at a fresh place. The test procedure is repeated at regular intervals till the needle does not penetrate completely. The needle should penetrate upto about 5 mm measured from bottom.

5. The initial setting time is found out by taking  the interval between the addition of water to cement and the stage when needle stops to penetrate completely. The time should be about 30 minutes for ordinary cement.


Final Setting Time of Cement Test Procedure:

1. The initial test procedure is same that of initial setting time test.

2. Instead of square needle, annular collar is used. The annular collar is attached to the movable rod of vicat apparatus.

3. The annular rod is gently released. The time at which the annular rod makes an impression on test block and the collar fails to do so is noted.

4. The final setting time is found out by taking the difference between the time at which water is added to cement and time as recorded in.

5. The final setting time for ordinary cement should be 10 hours.


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Consistency Test of Cement

Consistency Test of Cement

Consistency Test of Cement is carried out inorder to find the determine the percentage of water required for preparing cement pastes for other tests.

Test Procedure to Find the Consistency of Cement:

1. Take an empty enameled pan free from dirt and other foreign particles.

2. Now take 300 gm of cement, and put that in the pan.

3. Add water, the quantity of water should be 30 per cent by weight of cement or 90 gm of water to it.

4. Mix the cement and water thoroughly.

5. Now, fill the cement paste in the mould of vicat apparatus, the interval between the addition of water to the commencement of filling the mould is known as the time of gauging and it should be between 3 to 4 minutes.

6. The Vicat apparatus is shown in the below figure. It consists of a frame to which is attached a movable rod weighing 300 gm and having diameter and length as 10 mm and 50 mm respectively. An indicator is attached to the movable rod. This indicator moves on a vertical scale and it gives the penetration. The Vicat mould is in the form of a cylinder and it can be split into two halves. The Vicat mould is placed on a non-porous plate. There are three attachments — square needle, plunger and needle with annular collar. The square needle is used for initial setting time test, the plunger is used for consistency test and the needle with annular collar is used for final setting time test.

Consistenct Test of Cement - Vicat Apparatus

7. The plunger is attached to the movable rod of Vicat apparatus. The plunger is gently lowered on the paste in the mould.

8. The settlement of plunger is noted. If the penetration is between 5 mm to 7 mm from the bottom of mould, the water added is correct. If penetration is not proper, the process is repeated with different percentages of water till the desired penetration is obtained. This is the test procedure for Consistency Test of Cement.

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Tensile Strength of Cement

Tensile Strength of Cement

Tensile Strength of Cement test was formerly used to have an indirect indication of compressive strength of cement. It is at present generally used for the rapid hardening cement.

Tensile Strength of Cement Test Procedure:

(i) The mortar of cement and sand is prepared. The proportion is 1:3 which means that (x) gm of cement is mixed with 3(x) gm of sand.

(ii) The water is added to the mortar. The quantity of water is 5 per cent by weight of cement and sand.

(iii) The mortar is placed in briquette moulds. A typical briquette is shown in below figure. The mould is filled with mortar and then a small heap of mortar is formed at its top. It is beaten down by a standard spatula till water appears on the surface. Same procedure is repeated for the other face of briquette. Such twelve standard briquettes are prepared. The quantity of cement may be 600 ml for 12 briquettes.

Tensile Strength of Cement

(iv) The briquettes are kept in a damp cabin for 24 hours.

(v) The briquettes are carefully removed from the moulds and they are submerged in clean water for curing.

(vi) The briquettes are tested in testing machine at the end of 3 days and 7 days. Six briquettes are tested in each test and average is found out. During the test, the load is to be applied uniformly at the rate of 35 kg/cm2 or 3.50 N/mm2.

(vii) It may be noted that cross-sectional area of briquette at its least section is 6.45 cm2. Hence the ultimate tensile stress of cement paste is obtained from the following relation:

Ultimate tensile stress = failing load / 6.45

viii) The tensile stress at the end of 3 days should not be less than 20 kg/cm2 or 2 N/mm2 and that at the end of 7 days should not be less than 25 kg/cm2 or 2.50 N/mm2.

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