Structure III

Structure IIILecture 2

Content Setting time of cementTests on cementSetting time of cementCement paste setting time is affected by a number of items including:cement fineness, water-cement ratio, chemical content(especially gypsum content) and admixtures.Setting tests are used to characterize how a particular cement paste sets.For construction purposes, the initial set must not be too soon and the final set must not be too late.Additionally, setting times can give some indication of whether or not a cement is undergoing normalhydration.Contd.Normally, two setting times are defined:-

Initial setting time Occurs when the paste begins to stiffen considerably.Final setting time Occurs when the cement has hardened to the point at which it can sustain some load.Note: These particular times are just arbitrary points used to characterize cement, they do not have any fundamental chemical significance. They describe the setting of the cement and are not tied to the setting time of concrete.

Review questionsWhat do you understand by setting time of cement ?What is the difference between initial setting time and final setting time of cement ?Tests on cementThe list of main physical tests conducted on cement are as-Fineness testSoundness testConsistency testSetting time testCompressive strength testFineness testTo determine the fineness of cement by dry sieving as per IS: 4031 (Part 1) 1996. The principle of this is that we determine the proportion of cement whose grain size is larger then specified mesh size.The apparatus used are 90m IS Sieve, Balance capable of weighing 10g to the nearest 10mg, A nylon or pure bristle brush, preferably with 25 to 40mm, bristle, for cleaning the sieve.IS Standard Sieves

ProcedureWeigh approximately 10g of cement to the nearest 0.01g and place it on the sieve.Shake the sieve by linear movements, until no more fine material passes through it.Weigh the residue and express its mass as a percentage R1,of the quantity first placed on the sieve to the nearest 0.1 percent. Gently brush all the fine material off the base of the sieve.Repeat the whole procedure using a fresh 10g sample to obtain R2.Then calculate R as the mean of R1 and R2 as a percentage, expressed to the nearest 0.1 percent. When the results differ by more than 1 percent absolute, carry out a third sieving and calculate the mean of the three values.Review questionsWhy fineness test is carried out ?Give the standard sieve size used in fineness test ?Consistency testThe basic aim is to find out the water content required to produce a cement paste of standard consistency as specified by the IS: 4031 (Part 4) 1988. The principle is that standard consistency of cement is that consistency at which the Vicat plunger penetrates to a point 5-7mm from the bottom of Vicat mould.Apparatus Vicat apparatus conforming to IS: 5513 1976, Balance, whose permissible variation at a load of 1000g should be +1.0g, Gauging trowel conforming to IS: 10086 1982.Procedure Weigh approximately 400g of cement and mix it with a weighed quantity of water. The time of gauging should be between 3 to 5 minutes.Fill the Vicat mould with paste and level it with a trowel.Lower the plunger gently till it touches the cement surface.Release the plunger allowing it to sink into the paste.Note the reading on the gauge.Repeat the above procedure taking fresh samples of cement and different quantities of water until the reading on the gauge is 5 to 7mm.

contd.Reporting of result-Express the amount of water as a percentage of the weight of dry cement to the first place of decimal.

Review questionsWhat do you understand by consistency of cement ?Why consistency test is carried out ?What is the name of apparatus used in this test ?Test for setting time of cementWe need to calculate the initial and final setting time as per IS: 4031 (Part 5) 1988. To do so we need Vicat apparatus conforming to IS: 5513 1976, Balance, whose permissible variation at a load of 1000g should be +1.0g, Gauging trowel conforming to IS: 10086 1982.Procedure Prepare a cement paste by gauging the cement with 0.85 times the water required to give a paste of standard consistency.Start a stop-watch, the moment water is added to the cement.Fill the Vicat mould completely with the cement paste gauged as above, the mould resting on a non-porous plate and smooth off the surface of the paste making it level with the top of the mould. The cement block thus prepared in the mould is the test block.(A) Initial setting timePlace the test block under the rod bearing the needle.Lower the needle gently in order to make contact with the surface of the cement paste and release quickly, allowing it to penetrate the test block.Repeat the procedure till the needle fails to pierce the test block to a point 5.0 0.5mm measured from the bottom of the mould.The time period elapsing between the time, water is added to the cement and the time, the needle fails to pierce the test block by 5.0 0.5mm measured from the bottom of the mould, is the initial setting time. (B) Final setting timeReplace the above needle by the one with an annular attachment.The cement should be considered as finally set when, upon applying the needle gently to the surface of the test block, the needle makes an impression therein, while the attachment fails to do so.The period elapsing between the time, water is added to the cement and the time, the needle makes an impression on the surface of the test block, while the attachment fails to do so, is the final setting time.Review questionsWhat is the main significance of this test ?How initial and final setting time is determined from test specimen ?How much quantity of water is used in this test ?Soundness testThis test is performed to determine the presence of uncombined lime in cement.If uncombined lime is present in cement, during the setting action it might slake causing increase in volume and will cause the development of cracks.In structure, cracks are objectionable as they deteriorate the structure.Contd.Soundness of cement is determined by Le-Chatelier method as per IS: 4031 (Part 3) 1988.Apparatus The apparatus for conducting the Le-Chatelier test should conform to IS: 5514 1969, Balance, whose permissible variation at a load of 1000g should be +1.0g and Water bath.Le-Chateliers Apparatus

Procedure Place the mould on a glass sheet and fill it with the cement paste formed by gauging cement with 0.78 times the water required to give a paste of standard consistency.Cover the mould with another piece of glass sheet, place a small weight on this covering glass sheet and immediately submerge the whole assembly in water at a temperature of 27 2oC and keep it there for 24hrs.Measure the distance separating the indicator points to the nearest 0.5mm (say d1).Submerge the mould again in water at the temperature prescribed above. Bring the water to boiling point in 25 to 30 minutes and keep it boiling for 3hrs.Remove the mould from the water, allow it to cool and measure the distance between the indicator points (say d2).(d2 d1) represents the expansion of cement.Review questionsWhy this test is carried out ?Give the practical utility of this test.Name the apparatus used in this test.Compressive strength test The compressive strength of hardened cement is the most important of all the properties. Therefore, it is not surprising that the cement is always tested for its compressive strength at the laboratory before the cement is used in important works.

Apparatus Required- 1. Non porous enamel tray2. Trowel3. Cube mould of size 7.06cm4. Platform vibrator (or) Equipment for hand compaction5. Compression testing machine6. Balance to measure weight

Contd.Strength test are not done on neat cement paste because of difficulties of excessive shrinkage and subsequent cracking of neat cement.Strength of cement is indirectly found on cement sand mortar in specific proportion.The cubes are prepared for this purpose.The cubes are then tested in compression testing machine at the end on three days and seven days.Testing of cubes is carried out on their three sides without packing.Thus the cubes are tested at each time. Procedure Preparation of cement mortar cubes:

Take 555 gms of standard sand, 185gms of cement (i.e. ratio of cement to sand is 1:3) in a non porous enamel tray and mix them with a trowel for one minute.Add water quantity (P/4 + 3.0) % of combined weight of cement and sand and mix the three ingredients thoroughly until the mixture is of uniform colour. The time of mixing should not be less than three minutes and not more than four minutes.Immediately after mixing fill the mortar into a cube mould of sizes 7.06cm.Compact the mortar either by hand compaction in a standard specified manner or on the vibrating table.Place the moulds in cabin at a temperature of 27 2 C for 24 hoursRemove the specimen from the moulds and submerge them in clean water for curing.

Contd.Testing of cement mortar cubes:

Take the cube out of water at the end of three days with dry cloth. Measure the dimensions of the surface in which the load is to be applied. Let be L and B respectively.Place the cube in compressive testing machine and apply the load uniformly at the rate of 35N/mm2.Note the load at which the cube fails. Let it be P.Calculate the compressive strength of the cube by using formula. The compressive strength at the end of three days should not be less than 16N/mm2Repeat the same procedure (steps 1 to 4) for other two cubes.Repeat the whole procedure (Step 1 to 5) to find the compressive strength of the cube at the end of 7 days and it should not be less than 22 N/mm2.

Review questionsGive the significance of this test.What is the size of cubes prepares for the test ?What is the minimum compressive strength recommended for the cubes after 3 days and 7 days ?