Following tests are carried out in laboratory on the samples of aggregates to ascertain their some of the properties:
The quality of aggregates plays a great role in the performance and long-term economy of the structure. The aggregates also bear the main stresses occurring in the R.C.C items like slab, beam, columns, etc. and resists wear from surface abrasion. The maximum use of locally available aggregates also helps in the economic utilization of the natural resources.
The main aim of carrying out suitable tests on the aggregates is to know their physical characteristics and to classify them into different groups accordingly.
The aggregates are held in position by binding materials like bitumen and cement. Thus a strong structure is formed by the combination of aggregates and bituminous materials or cement.
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In most of the techniques of construction, the stone aggregates are commonly used. The serviceability of aggregates is generally assumed to be related in some degree with certain inherent qualities such as hardness, resistance to abrasion, impact, toughness, soundness, specific gravity, water absorption, compressive strength, etc.
Following tests are carried out in laboratory on the samples of aggregates to ascertain their some of the properties:
(1) Abrasion test
(2) Crushing test
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(3) Impact test
(4) Shape test
(5) Soundness test
(6) Specific gravity and water absorption test
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(7) Stripping value test.
Each of these tests for the aggregates will now be briefly described.
(1) Abrasion Test:
In order to determine the hardness or resistance to wear, the abrasion test is carried out by anyone of the following three methods:
(i) Deval abrasion test
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(ii) Dorry abrasion test
(iii) Los Angeles abrasion test.
It may be mentioned that the Los Angeles test is the most commonly used method for abrasion test as its results have been correlated with the performance studies.
(i) Deval Abrasion Test:
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Fig. 8-2 shows the Deval abrasion testing machine. It consists of generally two cast-iron cylinders which are closed at one end and provided with an iron cover at the other end. The cover is being capable of fitting tightly. The inside diameter and length of the cylinder are respectively 200 mm and 340 mm. The cylinders are mounted on a shaft which makes an angle of 30 degrees with the axis of rotation.
The test is carried out as follows:
(a) The specified quantity of dry aggregate specimen (usually 5 kg) of any one of the specified gradings is placed in each cylinder. It may be noted that duplicate test will be carried out simultaneously as there are two cylinders and the final result is obtained as an average of two values.
(b) The abrasive charge is then placed. It consists of six cast-iron or steel spheres of about 48 mm diameter with weight of each being 390 gm to 445 gm. Thus the total weight of the abrasive charge is adjusted very near to 2500 gm.
(c) The covers are tightly fitted and the cylinders are rotated for 10000 revolutions at an average speed of 30 to 33 r.p.m.
(d) The material is then removed from the cylinders and it is sieved on 1.7 mm IS sieve. The material which is retained on the sieve is washed, dried and weighed to the nearest gram.
(e) The percentage wear is worked out by the following expression –
Where, W1 = Original weight of sample
W2 = Weight of material retained on IS sieve.
As the desirable limits of percentage wear by Deval test are not specified, this test has limited application in practice. However, the I.S.I has standardized this test for abrasion of the coarse aggregates.
The Deval test is of the rattler type of test and it is the modified form of the British attrition test in which no abrasive charge is used.
(ii) Dorry Abrasion Test:
This test is carried out commonly in U.K. and a few other countries. It decides the suitability of rock as road material.
The Dorry abrasion machine consists of a flat circular cast-iron or steel disc of not less than 600 mm diameter which revolves in a horizontal plane about a vertical shaft. The arrangement for holding two test specimens in diametrically opposite direction is made and the specimens are so placed in vertical direction that their lower ends are pressed with a prescribed pressure against the surface of the disc.
A convenient funnel is also attached for continuously feeding a standard sand upon the disc. The distance from the centre of the disc to the centre of the specimen is 260 mm.
The test procedure is as follows:
(a) The test specimens in the form of cylinders with diameter as 25 mm and height as 25 mm are prepared from the rock to be tested. Two samples are used for each test.
(b) The specimen is made to press on the top of disc with a pressure of 1250 gm.
(c) The disc is then revolved through 1000 revolutions at a speed of about 28 r.p.m., i.e., nearly for about 35 minutes.
(d) The dry standard sand is fed continuously on the disc through the funnel immediately in front of the specimen and it is not to be re-used.
(e) The specimens are taken out and weighed again. The average coefficient of hardness is calculated from the following expression –
If the specimen is gravel, the pressure to be applied is of 2000 gm and it is to be rotated for 500 r.p.m.
For a good stone to be used for road construction, the coefficient of hardness should not be less than 17. If it is between 14 and 17, it indicates a stone of medium quality. The stone having coefficient of hardness less than 14 is considered too soft for road work,
(iii) Los Angeles Abrasion Test:
The Los Angeles abrasion test was developed in mid 1920’s in the municipal testing laboratory of the city of Los Angeles, U.S.A. It rapidly became accepted worldwide as a better measure of both abrasion resistance and toughness.
The machine consists of a steel hollow drum with 12 mm thick side walls, an interior length of 500 mm and an internal diameter of 700 mm. A radial steel shelf of 89 mm width extends for the full drum length. An opening is provided in the cylinder for putting test sample inside the cylinder and it is so arranged that when the opening is closed with a cover, the entire assembly becomes dust-tight and the interior surface is perfectly cylindrical.
The distance of the opening measured along the circumference of cylinder is 1250 mm from the steel shelf. Fig. 8-3 shows the cross-section of Los Angeles testing machine.
The test has been standardized by the I.S.I, and it is carried out as follows:
(a) The specified weight of aggregate is placed in the machine. It is 5 kg for grading A, B, C or D and 10 kg for grading E, F or G.
(b) The abrasive charge consists of cast-iron cylinders about 48 mm in diameter with weight varying from 390 gm to 445 gm. The number of spheres will also depend on the grading and the specified spheres are also placed in the cylinder.
(c) The cover of the machine is fixed dust-tight and it is then rotated at a speed of 30 to 33 r.p.m. The revolutions for the grading A, B, C or D are 500 and those for E, F and G are 1000.
(d) The machine is then stopped and the material is taken out. It IS sieved on 1.7 mm I.S. sieve and the weight of material retained on the sieve is found out. The percentage wear is worked out as follows –
Where W1 = Original weight of aggregate
W2 = Weight of material retained on sieve.
(e) Table 8-4 shows the specifications for the Los Angeles test. The Los Angeles test is more dependable than other abrasion tests. It measures the resistance of aggregate to rubbing as well as pounding actions. The rubbing occurs due to the rotation of the tightly closed drum and the pounding occurs due to the impact of the abrasive charge being picked up and dropped from the shelf.
Table 8-5 shows the maximum allowable abrasion values of aggregates as specified by the I.R.C. for different methods of construction.
(2) Crushing Test:
This test is carried out to assess the strength of coarse aggregate when compressive load is gradually applied.
Fig. 8-4 shows the apparatus used in this test. It consists of a hollow steel cylinder of internal diameter 152 mm and height of 130 mm to 140 mm. One end of the cylinder is placed on the base plate having diameter of 200 mm to 230 mm. At the other end, the arrangement of piston and plunger is provided to compress the material.
The test is carried out as follows:
(i) The sample is broken into chips passing 12.5 mm I.S. sieve and retained on 10 mm I.S. sieve. The material is heated to a temperature of 100°C to 110°C and it is then cooled to make it of dry surface before testing.
(ii) The material is placed in steel cylinder upto one-third height and it is tamped 25 times by the plunger. The second and third layers are also subsequently laid in a similar way and after the tamping of third layer, the surplus aggregate is struck off by a straight edge.
(iii) The plunger is placed on the upper surface and a load of 40 tonnes is applied at the uniform rate of 4 tonnes per minute by the compression machine.
(iv) The crushed aggregate is removed and sieved on 2.36 mm I.S. sieve. The material passing through this sieve is collected and weighed. The aggregate crushing value is obtained by the following expression –
Where W1 = Total weight of dry sample
W2 = Weight of material passing 2.36 mm I.S. sieve.
The aggregate crushing value provides an indirect measure of the crushing strength of the road aggregate. The strong aggregates possess low aggregate crushing value. The aggregate crushing value for surface course should be within 30 per cent and for base course, it should not exceed 45 per cent.
(3) Impact Test:
The toughness of rock means resistance to fracture from impact or absence of brittleness. The impact test is designed to evaluate the toughness of stones.
This test can be carried out in the following two ways:
(i) The page impact test
(ii) The aggregate impact test.
(i) The Page Impact Test:
This test is carried out on cylindrical stone specimens. For this purpose, a cylindrical specimen of 25 mm diameter and 25 mm height is prepared from the stone to be tested. It is put up on the anvil of the machine. A spherically headed plunger weighing 1 kg rests on the specimen. Then a hammer weighing 2 kg is allowed to fall on the plunger.
The first blow is from a height of 10 mm, the next from 20 mm, the next from 30 mm, etc. The specimen will break after some nth blow of hammer. The value of n represents the toughness index of the stone.
If n < 13, the stone is not tough enough;
when n = 13 to 19, it is called moderately tough; and
when n > 19, the toughness is said to be high.
This test is now seldom specified. But it may be used occasionally to indicate the relative difficulty of various rocks to be crushed.
(ii) The Aggregate Impact Test:
This test has now replaced the Page impact test and it has been standardized by the I.S.I. The test is carried out on stone aggregates in a machine known as impact testing machine. Fig. 8-5 shows the impact testing machine. It consists of a metal circular base firmly fixed on the floor.
A cylindrical steel cup which is detachable is rigidly fastened to the base plate. The internal diameter and depth of cup are 102 mm and 50 mm respectively. A metal hammer of weight about 13.5 kg to 14 kg is free to move between the vertical guides.
The lower end of hammer is cylindrical in shape with diameter as 100 mm and height as 50 mm and its lower edge is provided with 2 mm chamfer. The arrangement is so made that the hammer can fall freely from a height of 380 mm on test specimen, the height of fall being adjustable to 5 mm. A key is also provided to support the hammer while fastening or removing the cup.
The test is carried out as follows:
(a) The sample is broken into chips passing 12.5 mm I.S. sieve and retained on 10 mm I.S. sieve. The material is heated in an oven at a temperature of 100°C to 110°C for 4 hours and then cooled.
(b) The sample is placed in cup and the hammer is allowed to fall from a height of 380 mm for a total of 15 blows. It should be seen that each blow is delivered at an interval of not less than one second.
(c) The crushed aggregate is then removed from the cup and it is sieved on 2.36 mm I.S. sieve. The material passing through the sieve is collected and weighed. The aggregate impact value is obtained by the following expression –
Where W1 = Original weight of oven dry sample
W2 = Weight of material passing 2.36 mm I.S. sieve.
With respect to the aggregate impact value, the stones can be classified in table 8-6:
It has been found that for the majority of aggregates, the aggregate impact values are more or less numerically the same as the aggregate crushing values. But in the case of fine grained highly siliceous aggregates which are less resistant to impact than crushing; the aggregate impact values are somewhat higher to the extent of about 5 than the aggregate crushing values.
Table 8-7 shows the recommended aggregate impact values by the I.R.C. for different types of pavement.
(4) Shape Test:
The shape test is carried out to get a rough idea of the relative shapes of the aggregates.
The evaluation of the shape of the particles is made in the form of the following three terms:
(i) Angularity number
(ii) Elongation index
(iii) Flakiness index.
(i) Angularity Number:
The angularity number of an aggregate indicates the amount by which the percentage voids exceeds 33 per cent after being compacted in a prescribed manner. It can be worked out by the following simple equation –
Angularity number = 67 – per cent solid volume.
The angularity number is expressed to the nearest whole number and its range for aggregates used in construction is from 0 to 11. The higher the number, the more angular is the aggregate and less workable is the aggregate mix. The aggregates with high angularity number are preferred for bituminous and W.B.M. roads because they grant higher stability due to better interlocking and friction.
(ii) Elongation Index:
The term elongation index is used to indicate the percentage by weight of particles whose greatest dimension or length is greater than one and four-fifth, i.e., 1.8 times their mean dimension. This test is not applicable to aggregate sizes smaller than 6.3 mm. The elongated particles are likely to break under smaller loads and the aggregates having elongation index value greater than 10 to 15 are generally considered undesirable for pavement construction.
(iii) Flakiness Index:
The percentage by weight of aggregate particles whose least dimension or thickness is less than three-fifth, i.e., 0.6 times their mean dimension is known as the flakiness index of aggregate. The test is not applicable to aggregate sizes smaller than 6.3 mm.
The presence of flaky materials in pavement construction, especially the surface course, is undesirable because such particles would easily break down under the loads. The flakiness index for the aggregates to be used in road construction should be less than 15 and in no case, it should exceed 25.
(5) Soundness Test:
This test originated in Europe in the early nineteenth century and it is intended to assess the resistance of the aggregate to weathering. The test is carried out by immersing dry and clean specimen of specified size range in saturated solution of either sodium sulphate or magnesium sulphate. It is then taken out and oven dried at 105°C to 110°C to a constant weight.
Thus one cycle is completed and after the specified number of cycles (usually 10) are over, the aggregates are sieved in a standard fashion to determine to what extent the sample has disintegrated. As a rule, the average loss in weight after 10 cycles should not exceed 12 per cent when tested with sodium sulphate and 18 per cent when tested with magnesium sulphate.
The soundness test is not a precise test and many aggregates which fail to meet common specification requirements have been found to perform quite adequately. In such cases, the service record should take precedence over this test and the aggregates or aggregate sources should not be rejected solely on the basis of the behaviour of samples in the sulphate soundness test.
(6) Specific Gravity and Water Absorption Test:
The inherent properties of specific gravity and water absorption are often important to get an idea of strength or quality of the material. The test methods for determining these properties are based on what is traditionally known as Archimedes’ Principle, namely, that a body of any kind, when immersed in water, loses weight by an amount exactly equal to the weight of water it displaces.
The term specific gravity is used in indicate the ratio of dry weight of aggregate to the weight of equal volume of water. Its value for aggregates commonly used in construction varies from 2.5 to 3, the average being 2.68.
The higher the specific gravity, the stronger is the aggregate. However, the aggregate should not be recommended only on its value of specific gravity. But other mechanical properties such as aggregate crushing, impact and abrasion values should also be taken into consideration.
The water absorption of an aggregate is the percentage by weight of water absorbed in terms of oven dry weight of the specimen and it is an accepted measure of porosity. For aggregates normally used in construction, the water absorption value varies from 0.1 to 2 per cent. In some cases, the aggregates having 4 per cent water absorption have also been used in base courses.
In general, a value of less than 0.6 per cent is considered desirable for surface course. The I.R.C. has specified the maximum water absorption of value of 1 per cent for aggregates used in bituminous surface dressing.
(7) Stripping Value Test:
This test is also known as bitumen affinity test and it is carried out to know the behaviour of aggregates towards bitumen. The aggregates can be classified in two categories — hydrophilic and hydrophobic.
The hydrophilic aggregates arc water liking and they lose their bituminous coating in the presence of water. On the other hand, the hydrophobic aggregates have a dislike for water and they retain their bituminous coating even in presence of water.
In order to determine the affinity of aggregate towards bitumen, several laboratory tests have been developed. The static immersion test is very commonly used as it is easy and simple. The aggregates coated with bitumen are immersed in water bath at 40°C for 24 hours.
The percentage of stone surface that is stripped off after the specified period is estimated and as per the I.R.C., it should not exceed 25 per cent for material to be used as aggregate in bituminous surface dressing, penetration macadam and bituminous macadam construction.