Learn : Testing portland cement,Consistency of standard cement paste with the help of Vicat’s apparatus ,Soundness.with the help of “Le Chatelier’s apparatus”
TESTING PORTLAND CEMENT
In this article learn 9 tests 1.samples for testing 2.Chemical composition 3.Fineness 4.Temperature for testing 5.Consistency of standard cement paste with the help of Vicat’s apparatus 6.Soundness.with the help of “Le Chatelier’s apparatus”7.Setting time.with the help of Vicat’s apparatus 8.compressive strength 9.Tensile strength with the help of briquette testing machine.
The Indian Standards Institution, New Delhi, has prescribed the following tests so as to control the quality of Portland cement.
Samples for testing.
Each sample for testing shall consist of an intimate mixture or approximately equal portions selected from at least 12 different bags or packages when the cement is not loose or 12 different positions in the heap or heaps when the cement is loose. Selection of samples shall be done in such a manner so as to obtain a fair average sample. The weight of final sample shall be at least 5 kg.
The sample so taken shall be stored in air tight container till the time of test.
Loss on ignition.
Heat 1.0 gm of the sample for 15 minutes in a platinum crucible (or for one hour in porcelain crucible) at a temperature of 900ºC to 1000ºC. Cool and weigh. Loss on ignition should not be more than 4%.
Boil for ten minutes a well stirred mixture of one gram of cement, 40 cc of water and 10 cc of concentrated hydrochloric acid (sp. gravity 1.18). Filter the solution. Rinse the container five times and wash the filter ten times with hot water. Wash the residue on filter with hot water and boil for ten minutes with Na2Co3 solution (2N). Filter the solution again through the same filter paper and wash live times with water. It is now washed with Hydrochloric acid (2N) and finally with water till it is free from chlorides. The filter paper should be dried, ignited and weighted to give the insoluble residue.
The insoluble residue should not be more than 1.5 per cent.
Lime and alumina.
he percentage of lime to silica, alumina and iron oxide when calculated by the formula :
shall not be greater than 1.02 nor less than 0.66. The ratio of the percentage of alumina to that of iron oxide shall not be less than 0.66. An excess of free lime shall cause unsoundness of cement.
(iv) Magnesia. If free magnesia exceeds five per cent then it makes the cement unsound.
Finer cements react quicker with water and develop early strength, though the ultimate strength is not affected. How ever, finer cements increase the shrinkage and cracking of concrete. The fineness is tested by any one of the following two methods.
(i) By sieve analysis, and (ii) By specific surface.
By sieve analysis.
Break with hands any lumps present in 100 gm of cement placed in IS sieve No. 9 and sieve it by gentle motion of the wrist for 15 minutes continuously. The residue when weighed should not exceed 10 per cent by weight of the cement sample.
By specific surface.
Specific surface shall not be less than 2250 sq cm/gm as determined by Air Permeability method and 1600 sq cm/ gm as found by Wagners’ Turbidimeter method.
Temperature for testing.
The following physical tests should be carried out, as far as possible, between the temperature rage of 25ºC to 29ºC.
Consistency of standard cement paste.
This test is performed to find out the correct amount of water to be added to a given quantity of cement so as to get a paste of “normal consistency”. This test precedes the test of cement for soundness, setting time, tensile strength or for compressive strength. It is done with the help of Vicat’s apparatus shown in Fig. 5.3. The frame A of Vicat Apparatus has a moveable rod B weighing 300 gm. Diameter of the rod is 1 cm and is 5 cm long. At its lower end is attached a detachable needle 1 mm suare or 1.13 mm in diameter and 5 cm long. There is a verticle scale C graduated from 0 to 40 mm in either direction to measure the vertical movement of the rod.
To start with 25 per cent of clean water is mixed with about 300 gm of neat cement in a crucible. The mixing is done with a standard spatula shown in Fig. 5.4. After about 30 seconds it is thoroughly mixed with hands for at least one minute. The kneaded paste is tossed about six times from one hand to the other and pressed into the hard rubber mould D, E through its bigger end. Fill the mould completely with paste and remove the extra paste by a single movement of the palm. Place the inverted mould (with larger end on glass plate F) and slice off extra paste from top by a single movement of trowel. Place the mould resting on glass plate under the needle. Bring 1 cm diameter end of needle in touch with the paste and release it without any jerk or force and note the penetration. Time taken from adding of water in cement to filling of mould should be between three to five minutes. Repeat experiment with trial pastes made with varying percentages of water. The paste giving a penetration of 33 to 35 mm is said to be of normal consistency. The amount of water mixed is expressed as a percentage by weight of dry cement. This is usually in the neighbourhood of 30 per cent for a paste of normal consistency.
It is essential that cement concrete does not undergo large changes in volume after setting. This change in volume is known as Unsoundness and may cause cracks, distortion and disintegration of concrete.
The test is carried our with the help of “Le Chatelier’s apparatus” shown in Fig. 5.5. It consists of a split brass cylinder 30 mm high, 30 mm internal diameter and 0.5 mm thick. Two pointers AA, 165 cm in length up to the axis of cylinder are attached to the cylinder, one on each side of the split. Cement paste prepared with 0.78 times the water required to prepare a paste of normal consistency and 100 gm of cement is filled in the mould resting on a glass plate. Another glass plate is placed on the mould and weighed down. The whole is immediately placed in a water bath maintained at a temperature of 27ºC to 32ºC. After 24 hours the distance between the pointers is measured and the mould is transferred to a beaker of water heated to boiling point in 25 to 30 minutes, and kept at this temperature for one hour. After cooling the increase in the distance between the pointers is noted.
The increase in this distance should not be more than 5 mm for cement that had been aerated for seven days in a humidity of 50 to 80 per cent before test or 10 mm if the cement had been kept in airtight containers.
To enable the concrete to be laid in position properly the initial setting of cement should not start too quickly. Once the concrete has been laid it should harden rapidly so that the stage in the process of hardening after which any cracks that may appear do not reunite. Final setting is that when it has attained sufficient strength and hardness.
Vicat appartus shown in Fig. 5.3 is used to find the setting time for cement. The paste of 300 gm cement made with 0.85 times the amount of water required for a paste of normal consistency is filled in the mould. At the lowr end of the rod is fitted 1 mm square needle. This needle is brought in contact with the surface of paste and released. The initial set is said to have taken place when the needle falls to penetrate beyond a point 5 mm above the glass plate. The time taken from the instant water was added to cement to the moment when the needle fails to penetrate beyond 5 mm above the glass plate is known as initial setting time. It should not be less than 30 minutes for Ordinary Portland Cement.
For finding out the final setting time the 1 mm square needle is replaced by the needle F (Fig. 5.3.). This needle has an annular attachment around 1 mm square needle and projecting by 0.5 mm below it. To find final setting time the needle shall be brought in touch with the paste in the mould and released instantly. Finbal set shall be considered as having taken place when the attachment fails to make any impression on the surface of paste whereas the needle makes one. The time from the moment water was added to cement to the moment the circular attachment fails to make impression on the surface of cement paste is known as final setting time.
Fir ordinary Portland cement the final setting time should not be more than ten hours. The test should be performed in an air-conditioned room with 90 per cent humidity and at temperature between 25ºC to 29ºC.
The compressive strength of cement is judged by finding the compressive strength of cement and sand mortar. For this purpose one part by weight of cement is mixed dry with three parts by weight of IS sand. To this dry mixture of cement and sand is added water given by the following formula :
where P is the percentage of water by weight of dry materials and P is the percentage of water required for making a cement paste of normal consistency. Cement, sand and water shall be intimately mixed to give paste of uniform colour but the mixing should not be for less than three minutes or for more than four minutes. Cubes of 7.06 cm sides are then moulded out of this pate and are kept in an atmosphere of 90 per cent humidity at 25ºC to 29ºC temperature for 24 hours. These are then removed from the moulds and kept submerged in clean water till the time of test and should not be allowed to dry.
Three cubes each are tested in a compression testing machine after three days and seven days. Compressive strength of ordinary Portland cement should not be less than the following values :
After 72+1 Hour 160 kg/sq cm
After 168+2 Hours 220 kg/sq cm
Tensile strength of cement sand mortar is tested to judge the tensile strength of cement. To do so briquettes of standard dimensions as shown in Fig. 5.6. are prepared. Briquettes have a uniform thickness of 25.1 mm and a minimum sectional area 645 sq mm at the central section. For preparing
one part by weight of cement and three parts by weight of water are mixed with the quantity of water found from the following formula :
Cement, sand and water are mixed intimately so as to get a uniform colour of the mortar. A small heap of mortar is placed on a briquentte mould and filled as usual. It is then beaten down with the standard spatula (Fig. 5.4.) till water appears on the surface. The mould is now turned upside down and as before again a small heap of mortar is placed and beaten down. The surfaces are smoothed with the blade of a trowel. The briquettes are taken out of moulds after keeping them in an atomsphere of 90 per cent humidity and a temperature of 25ºC to 29ºC for 24 hours. Six such specimen each are tested in a briquette testing machine (Fig. 5.7.) after three days and seven days.
Tensile strength for good Portland cement should be as follows:
After 3 days not less than 20 kg/sq cm
After 7 days not less than 25 kg/sq cm