Aggregates impact value test
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 100-110°C and cooled. 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). 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. 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
Aggregates abrasion value test (IS 2486 Part 4).
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.
Procedure. The test sample and the abrasive charge are placed in the Deval abrasion testing machine and the machine is rotated for 10000 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. In case of crushed gravel, the percentage by weight of crushed fragments is determined, and the permissible percentage or wear is calculated as:
W = AL+(100-A)L'/100
Where W = permissible percentage of wear
A = percentage of uncrushed fragments
L = maximum percentage of wear permitted by the specifications for gravel consisting entirely of uncrushed fragments
100 – A = percentage of crushed fragments
L'= maximum percentage of wear permitted by the specifications for gravel consisting entirely of crushed fragments
Using los angles machines.
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 test sample consists of clean aggregate dried in an oven at 105 -110°C to substantially constant weight.
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 counter-balanced 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.
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 105 -110°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.
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