as filler rocks blast furnace slag form the body reduce shrinkage 70-80% of the volume clean, hard, strong, durable graded in size

Transcription

1 UNIT 2 AGGREGATES

2 AGGREGATES AGGREGATES are the materials basically used as filler with binding material in the production of mortar and concrete They are derived from igneous, sedimentary and metamorphic rocks or manufactured from blast furnace slag, etc., Aggregate form the body of the concrete, reduce shrinkage and effect economy. They occupy 70-80% of the volume and have considerable influence on the properties of the concrete. They should be clean, hard, strong, durable and graded in size to achieve utmost economy from the paste.

3 Selection of aggregates Aggregates shall be hard, durable and clean and free from adherent coatings and organic matter and shall not contain appreciable amount of clay. Aggregates shall not contain harmful impurities such as iron pyrites, alkalis, salts, coal, mica, shale or other materials which will affect hardening and attack reinforcement.

4 Classification of aggregates On the basis of geological origin Natural aggregates Artificial aggregates On the basis of size Coarse aggregate All-in-aggregate Graded aggregate Fine aggregate On the basis of shape Rounded aggregates Irregular aggregates Angular aggregates Flaky aggregates Based on unit weight Normal-weight Heavy-weight Light-weight

5 On the basis of geological origin Natural aggregates They are obtained by crushing from quarries of igneous, sedimentary or metamorphic rocks. Gravels and sand reduced to their present size by the natural agencies also fall in this category. They require sieving and washing before they can be used in concrete.

6 Artificial aggregates Broken bricks, blast furnace slag and synthetic aggregates are artificial aggregates. Broken Bricks known as brick bats are suitable for mass concreting, for example, in foundation bases.they are not used for reinforced concrete works. Blast furnace slag aggregate is obtained from slow cooling of the slag followed by crushing. Synthetic aggregates are produced by thermally processed materials such a expanded clay and shale used for making light weight concrete.

7 Based on size: Fine aggregates - those aggregates which pass through 4.75 mm sieve or aggregates with size less than 5 mm. Coarse aggregates those aggregates passing through 75 mm sieve and entirely retained on 4.75 mm sieve or those aggregates with size greater than 5 mm. Graded aggregate aggregate most of which passes through a particular size of sieve are known as graded aggregate. All-in- aggregate naturally available aggregates of different fractions of fine and coarse sizes are known as all-in-aggregate.

8 Fine Aggregate Coarse Aggregate

9

10 Based on Density: Based on specific gravity or density measured in bulk, aggregate is divided into 3 types: Lightweight aggregate Normal-weight aggregate Heavyweight aggregate

11 Normal-weight aggregate Crushed stone, gravel and ordinary sand are examples of normal weight aggregate. They are commonly used in manufacture of normal weight concrete, asphalt concrete and roadway sub-base. The average values of specific gravity for sand and gravel are 2.6 and 2.65 respectively. Bulk density of normal weight aggregate is around 1520 to 1680kg/m 3.

12 Lightweight aggregate Lightweight fine aggregate is any aggregate with bulk density less than 1120kg/m 3 and lightweight coarse aggregate is any aggregate with bulk density less than 880kg/m 3. They are commonly used as ingredients in the manufacture of lightweight concrete, for making lightweight masonry blocks (to improved their thermal and insulating properties and nailing characteristics), and lightweight floor and roof slabs. 2 types of lightweight aggregate: Natural lightweight aggregates (eg: palm oil shell, rice husk, etc) Manufactured (also called synthetic) lightweight aggregates.

13

14 Heavyweight aggregate Those aggregate with high density and is used primarily in the manufacture of heavyweight concrete, employed for protection against nuclear radiation and as bomb shelter. The unit weight of heavyweight concrete varies from 2400 kg/m 3 with specific gravity range from 4.0 to 4.6. (e.g.: mineral ores and barite)

15 Based on shape

16

17 Fine aggregate Sand is used as a fine aggregate in mortar and concrete. It is a granular form of silica. Sand used for mix design is known as standard sand(is:650). The standard sand should be obtained from Ennore, TN. It should be quartz, light grey or whitish variety. It should be free from silt. It should (100%) pass through 2mm IS Sieve and should (100%) be retained on 90 micron sieve with the following distribution.

18 Particle size Per cent Smaller than 2mm and greater than 1 mm Smaller than 1mm and greater than 500micron Smaller than 500 micron but greater than 90 micron Sand used in mortars for construction purposes should possess at least 85% of the strength of standard sand mortars of like proportions and consistency.

19 Classification of Fine aggregate Depending upon the source, Natural sand resulting from natural disintegration of rocks or deposited by streams; Crushed stone sand produced by crushing hard stones. Crushed gravel stone produced by crushing natural gravel. Based on mineralogical composition, Quartz Feldspar and Carbonaceous varieties. Depending upon its size, Coarse sand Fineness Modulus Medium sand - Fineness Modulus Fine sand - Fineness Modulus

20 Based on particle size distribution

21 Coarse aggregate These may be uncrushed, crushed or partially crushed gravel or stone most of which is retained on 4.75mm IS sieve. They should be hard, strong, dense, durable, clear and free from veins and adherent coatings. They should be free from injurious amount of disintegrated pieces, alkali, organic matter and other deleterious substances. Flaky and elongated aggregate should be avoided.

22 Testing Of Aggregates Particle size distribution Test (IS 2386(part1)) Sieve Analysis Flakiness and elongation index test (IS 2386(part1)) Shape test Specific gravity and water absorption test (IS 2386 (Part 3)) Aggregate impact test (IS 2386(Part4)) Aggregate abrasion value test (IS 2386(Part4)) Crushing value test(is 2386(Part 4)) Bulking test for fine aggregate(is 2386(Part3))

23

24 Specific Gravity and Water Absorption Test (IS 2386(Part 3)) AGGREGATES LARGER THAN 10mm: 1. A sample of 2000g of aggregate is used for conducting the test. 2. The sample is thoroughly washed to remove finer particles and dust, drained and then placed in the wire basket and immersed in distilled water at a temperature between o C with a cover of atleast 50mm of water above the top of the basket. 3. Immediately after immersion the interrupted air is removed from the sample by lifting the basket containing it 25mm above the base of the tank and allowing it to drop 25 times at the rate of about one drop per second.

25

26 4. The basket and aggregate are kept completely immersed during the operation and for a period of 24±1/2 hours afterwards. 5. The basket and the sample are jolted and weighed in water (weight A 1 ). 6. These are removed from water and allowed to drain for a few minutes, after which the aggregate are gently emptied from basket to one of the dry clothes, and the empty basket is returned to the water, jolted 25 times and weighed in water (Weight A 2 ). 7. The aggregates placed on the dry cloth are gently surface dried with the cloth, and are completely surface dried. The aggregate are then weighed (weight B). 8. The aggregate are thereafter placed in an oven at a temperature of o C and maintained at this temperature for 24±1/2 hours. It is removed from the oven, cooled in air-tight container and weighed (Weight C).

27 Specific gravity = C B A Apparent Specific gravity = C C A B A water absorption % of dry weight = 100 C Where, A weight of the saturated aggregate in water (A 1 -A 2 ) (g) B - weight of the saturated surface dry aggregate in water (g) C- weight of the oven dried aggregate in air (g)

28 Sieve analysis Sieve analysis is the operation of dividing a sample of aggregate into various fractions each consisting of particles of the same size. The sieve analysis is conducted to determine the particle size determination in a sample of aggregate gradation. A convenient system of expressing the gradation of aggregate is one which the consecutive sieve openings are constantly doubled, such as 10mm, 20mm, 40mm, etc., Under such a system, employing a logarithmic scale, lines can be spaced at equal intervals to represent the successive sizes.

29

30

31 Grading pattern of a sample of C.A or F.A is assessed by sieved a sample successively through all the sieves mounted one over the other in order of size, with the larger sieve on the top. The material retained on each sieve after shaking, represents the fraction of aggregate coarser than the sieve in question and finer than the sieve above. Sieving can be done either manually or mechanically. In the manual operation, the sieve is shaken giving movements in all possible directions to give chances to all particles for passing through the sieve. Operation should be continued till such time that almost no particle is passing through. Mechanical devices are actually designed to give motion in all possible direction, and as such, it is more systematic and efficient than hand-sieving.

32 Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 to 150 micron and dividing this sum by an arbitrary number 100. Larger the fineness modulus, the coarser is the material.

33

34

35

36 Sieve Analysis of Fine aggregate

37 For crushed stone sands, the permissible limit on 150 micron IS sieve is increased to 20%. Where concrete of high strength and good durability is required, fine aggregate confirming to any one of the four grading zones may be used, but the concrete mix should be properly designed. As the fine aggregate grading becomes progressively finer, that is, from grading zones I to IV, the ratio of fine aggregate to coarse aggregate should be progressively reduced. The most suitable fine to coarse ratio to be used for any particular mix will, however depends upon the actual grading, particle shape and surface texture of both fine and aggregates. It is recommended the fine aggregates conforming to Grading Zone IV should not be used reinforced concrete unless tests have been made to ascertain the suitability if proposed mix proportions.

38 Test for determination of flakiness index Flakiness index of aggregate is the percentage by weight of particles in it whose least dimension(thickness) is less than three-fifths of their mean dimension. This test is not applicable to sizes smaller than 6.3mm This test is conducted by using a metal thickness gauge. A sufficient quantity of aggregate is taken such that a minimum number of 200 pieces of any fraction can be tested. Each fraction is gauged in turn for thickness on the metal gauge. The total amount passing in the gauge is weighed to an accuracy of 0.2 percent of the weight of the samples taken. The flakiness index is taken as the total weight of the material passing the various thickness gauges expressed as percentage of the total weight of the sample taken.

39 Thickness gauge

40

41 Test for determination of elongation index The elongation index on an aggregate is the percentage of weight of particle whose greater dimension (length ) is greater than 1.8 times their mean dimension. The elongation index is not applicable to sizes smaller than 6.3mm. This test is conducted by using metal length gauge. This test is conducted by using a metal thickness gauge. A sufficient quantity of aggregate is taken such that a minimum number of 200 pieces of any fraction can be tested.

42 Each fraction is gauged in turn for length on the metal gauge. The total amount retained in the gauge is weighed to an accuracy of 0.1 percent of the weight of the samples taken. The elongation index is taken as the total weight of the material retained on the various thickness gauges expressed as percentage of the total weight of the sample taken. The presence of elongated particles in excess of 10 to 15 per cent is generally considered undesirable, but no recognized limits are laid down.

43 Length gauge

44

45 Crushing Value Test (IS: 2386 (Part IV)) The material for the test should consist of aggregate passing 12.5 mm sieve and retained on 10 mm sieve.

46 About 6500 g of natural aggregate is required to provide samples for the 150 mm cylinder, or about 1000 g for the 75 mm cylinder. The aggregate is tested in a surface-dry condition. The weight of material comprising the test sample is determined (weight A). The cylinder of the test apparatus is positioned on the base-plate and the test sample is added in thirds, each being subjected to 25 strokes from the tamping rod. The surface of the aggregate is carefully levelled and the plunger is inserted so that it rests horizontally on this surface. The apparatus, with the test sample and plunger in position is then placed between the platens of the testing machine and loaded at an uniform rate as possible, so that the total load is reached in 10 minutes. The total load should be 400 kn. The load is released and the whole of the material is removed from the cylinder and sieved on a 2.36 mm sieve for the standard test, or on the appropriate sieve. The fraction passing the sieve is weighed.

47

48

49 The ratio of the weight of fines formed to the total sample weight in each test is expressed as a percentage, recorded to the first decimal place: Aggregate crushing value = B A 100 Where B = weight of fraction passing the appropriate sieve A = weight of surface-dry sample The aggregate crushing value should not be more than 45 % for aggregate used for concrete other than wearing surfaces, and 30% for concrete used for wearing surfaces such as runways,roads and air field pavements.

50 Aggregate Impact Value Test (IS:2386 (Part IV)) The aggregate impact value gives a relative measure of the resistance of an aggregate to sudden shock or impact, which in some aggregate differs from its resistance to a slow compressive load. The test sample consists of aggregate the whole of which passes a 12.5 mm sieve and is retained on a 10 mm sieve. The aggregate comprising the test sample is dried in an oven for a period of four hours at a temperature of C and cooled.

51 The measure is filled about one-third full with the aggregate and tamped with 25 strokes of the rounded end of the tamping rod. A further similar quantity of aggregate is added and a further tamping of 25 strokes is given. The measure is finally filled to overflowing, tamped 25 times and the surplus aggregate is struck off, using the tamping rod as a straight-edge. The net weight of aggregate in the measure is determined to the nearest gram (weight A).

52

53 A cup, 102 mm internal diameter and 50 mm deep, is fixed firmly in position on the base of the machine and the whole of the sample is placed in it and compacted by a single tamping of 25 strokes of the tamping rod. The hammer is raised until its lower face is 380 mm above the upper surface of the aggregate in the cup, and allowed to fall freely on to the aggregate. The test sample is subjected to a total of 15 such blows each being delivered at an interval of not less than one second. The crushed aggregate is then removed from the cup and the whole of it is sieved on 2.36 mm IS sieve until no further significant amount passes in one minute. The fraction passing the sieve is weighed to an accuracy of 0.1 g (weight B). The fraction retained on the sieve is also weighed (weight C) and, if the total weight (B + C) is less than the initial weight (A) by more than one gram, the result is discarded and a fresh test made.

54 Two tests are made. The ratio of the weight of fines formed to the total sample weight in each test are expressed as a percentage, recorded to the first decimal place: Aggregate impact value = B A 100 Where B = weight of fraction passing 2.36 mm sieve A = weight of oven-dried sample

55 Aggregate Abrasion Value Test (IS: 2386 (Part IV)) The abrasion value of coarse aggregate may be determined by either Deval Machine or by Los Angeles machine. Using Deval Machine Abrasive Charge: The abrasive charge consists of 6 cast iron or steel spheres approximately 48 mm in diameter, each weighing between 390 and 445 g. An abrasive charge of 6 spheres weighing 2500 ± 10 g is used with each test sample. The test sample consists of dry coarse aggregate made up of percentages of the various sizes conforming to one of the gradings. The grading used should be that most nearly representing the coarse aggregate furnished for the work.

56

57

58 Procedure The test sample and the abrasive charge are placed in the Deval abrasion testing machine and the machine is rotated for revolutions at a speed of 30 to 33 rev/min. At the completion of the test, the material is removed from machine and sieved on a 1.70 mm sieve. The material retained on the sieve is washed, dried and accurately weighed to the nearest gram. The loss by abrasion is considered as the difference between the original weight of the sample and the weight of the material retained on the 1.70 mm sieve, expressed as percentage of the original weight of the test sample. Loss byabrasion = Original weight weight of material retained on 1.70mm sieve original weight 100%

59 Using Los Angeles Machine Abrasive Charge: The abrasive charge consists of cast iron spheres or steel spheres approximately 48 mm in diameter and each weighing between 390 and 445 g. The abrasive charge, depending upon the grading of the test sample. The test sample consists of clean aggregate dried in an oven at C to substantially constant weight.

60

61

62 The test sample and the abrasive charge is placed in the Los Angeles abrasion testing machine and the machine is rotated at a speed of 20 to 33 rev/min. For gradings A, B, C and D, the machine is rotated for 500 revolutions; for gradings E, F and G, it is rotated for 1000 revolutions. The machine is so driven and so counterbalanced as to maintain a substantially uniform peripheral speed. If an angle is used as the shelf, the machine is rotated in such a direction that the charge is caught on the outside surface of the angle.

63 At the completion of the test, the material is discharged from the machine and a preliminary separation of the sample made on a sieve coarser than the 1.70 mm. The finer portion is then sieved on a 1.70 mm sieve. The material coarser than the 1.70 mm sieve is washed, dried in an oven at C to a substantially constant weight, and accurately weighed to the nearest gram. The difference between the original and the final weights of the test sample expressed as a percentage of the original weight of the test sample gives the percentage of wear

64 Bulking test for fine aggregate (IS 2386(Part 4)) Fine aggregate (sand) has a tendency to increase in volume (bulk) depending upon the moisture present in it. In making concrete mix, if the batching is done by volume, the actual quantity of sand in each batch will be less than the recommended volume and consequently the mix will be rich in cement, i.e. the proportions of cement and sand will be different to be provided. This will result in uneconomy and may affect adversely the concrete also. Therefore, the amount of sand will have to be increased by percentage bulking (B).

65 In the laboratory test sufficient quantity of oven dried sand is filled in graduated cylinder up to a certain fixed mark. The sand is emptied in a container and the weigh of dry sand is determined 1% water is added in the sand mixed thoroughly and filled in the graduated cylinder. The volume of sand will be found to increase. The process is repeated by increasing the percentage of water in steps of 1% each time till a decrease in the volume of sand is observed. Still this addition of water is continued till the volume of sand comes back to the original volume (fully saturated). A graph is plotted between moisture content and percentage increase in volume.

66