Question

strength of aggregate types ,significance and tests
I need report with 5 page at less

Answer 1

(1) Concept and significance of the Aggregate crushing value test

The ‘aggregate crushing value test ` gives a relative measure of the resistance of an aggregate to crushing under a gradually applied compressive load. Aggregate crushing value is defined as the percentage by weight of the crushed (or finer) material obtained when the test aggregates are subjected to a specified load under standardized conditions, and the strength of the aggregate used in road construction is expressed by numerical index . Aggregates with lower crushing value shows a lower crushed fraction under load and would give a longer service life to the road and hence a more economical performance. If we used a weaker would get crushed under traffic loads, would produce smaller pieces not coated with binder and these would be easily displaced or loosened out resulting in loss of the surface / layer. Due to this reason the aggregates used in road construction must be strong enough to withstand crushing under roller and traffic.

• Objectives
• To determine the crushing value of aggregate for road aggregates;
• To determine suitability of aggregates for use in different types of road pavements.

• Apparatus

The appar atus for the standard aggregate crushing test as per IS: 2386-1963 (Part IV) consist of the following:

• The test mould-a 15.2 cm diameter open ended steel cylinder with square base plate; plunger having a piston of diameter 15 cm, a rod could be inserted for lifting or placing the plunger in the cylinder with a provided hole across the stem of the plunger.
• A straight metal tamping rod of circular cross-section 16mm in diameter and 45 to 60 cm long, rounded at one end.
• A balance of capacity 5 kg, readable and accurate upto 1 gm.
• IS sieve of sizes 12.5mm, 10mm, 2.36mm.
• A compression testing machine capable of applying load upto 40 tonnes at a uniform rate of 4 tonnes per minute.
• A cylinder measure having internal diameter of 11.5 cm and height 18 cm

• Procedure

  The aggregate sample: The material for the standard test consists of aggregates sized 10.0 mm to 12.5 mm . The aggregates must be in surface dry condition before testing. The aggregates may be dried by heating at 100 oC – 110 o C for not mor than 4 hours and cooled to room temperature before testing, if necessary.

• Sieve the material through 12.5 mm and 10 mm IS sieves. The aggregates passing through 12.5 mm sieve and retained on 10.0 mm sieve comprise the test material.
• Take about 3.25 kg of this material.
• Aggregates should be poured to fill about just more than 1/3 rd depth of the measuring cylinder.
• The material is compacted by giving 25 gentle blows with the rounded end of tamping rod.
• Two more layers is added in similar manner, such that the cylinder is full.
• Remove the excess material with a straight edge. Now this amount of quantity contained in the measuring cylinder is that amount of aggregates which will be used to prepare the test specimen.
• Empty the cylinder and weight the aggregates, accurate upto 1 gm.
• Transfer the whole of this weighted quantity to the test mould by filling it in three layers in the same manner as for cylindrical measure. The total depth of the sample is then about 10 cms and the surface a little below the top of mould.
• The surface is leveled and place the plunger over it so that it rests horizontally on the surface of the aggregates.
• Then place this assembly on the pedestal of compression testing machine.
• Load is applied at a uniform rate of 4 tonnes.
• Release the load
• Take the aggregate out of cylinder and sieve them through 2.36 mm IS sieve. Weight this fraction passing through it to an accuracy of 0.1 gm. This fraction is a quantity of loss of material due to crushing.
• Note down the observations in the proforma then compute the aggregate crushing value. The mean of two observations, rounded to nearest whole number is reported as the “Aggregate crushing Value”.
• (2)IMPACT VALUE OF AGGREGATE

• 1. Objective

  The aggregate impact value gives a relative measure of the resistance of an aggregate to sudden shock or impact, which in some aggregates differs from its resistance to a slow compressive load.

• Procedure

• The test sample shall consist of aggregate the whole of which passes a 12.5 mm IS Sieve and is retained on a 10 mm IS Sieve. The aggregate comprising the test sample shall be dried in an oven for a period of four hours at a temperature of 100 to 110°C and cooled.
• The measure shall be filled about one-third full with the aggregate and tamped with 25 strokes of the rounded end of the tamping rod. Further similar quantity of aggregate shall be added and a further tamping of 25 strokes given. The measure shall finally be filled to overflowing, tamped 25 times and the surplus aggregate struck off, using the tamping rod as a straight edge. The net weight of aggregate in the measure shall be determined to the nearest gram (Weight A).

• 5. Calculation

• The ratio of the weight of fines formed to the total sample weight in each test shall he expressed as a percentage, the result being recorded to the first decimal place:

  Aggregate Impact Value = (B/A) X 100

  where A = weight in g of saturated surface - dry sample,
  B = weight in g of fraction passing through 2.36 mm IS Sieve
• The impact machine shall rest without wedging or packing upon the level plate, block or floor, so that it is rigid and the hammer guide columns are vertical.
• The cup shall be fixed firmly in position on the base of the machine and the whole of the test sample placed in it and compacted by a single tamping of 25 strokes of the tamping rod.
• The hammer shall be 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 shall be subjected to a total of 15 such blows each being delivered at an interval of not less than one second.
• The crushed aggregate shall then be removed from the cup and the whole of it sieved on the 2.36 mm IS Sieve until no further significant amount passes in one minute. The fraction passing the sieve shall be weighed to an accuracy of 0.1 g (Weight. B).
• The fraction retained on the sieve shall also be weighed (Weight C) and, if the total weight (C+B) is less than the initial weight (Weight A) by more than one gram, the result shall be discarded and a fresh test made. Two tests shall be made.
• (3)

  The principle of Los Angeles abrasion test is to produce abrasive action by use of standard steel balls which when mixed with aggregates and rotated in a drum for specific number of revolutions also causes impact on aggregates.

  The percentage wear of the aggregates due to rubbing with steel balls is determined and is known as Los Angeles Abrasion Value.

• The Los Angeles abrasion test on aggregates are done for following purposes:

• To determine the Los Angeles abrasion value.
• To find the suitability of aggregates for use in road construction.

• Apparatus for Los Angeles Test

  The apparatus as per IS: 2386 (Part IV) – 1963 consists of:

• Los Angeles Machine
• Abrasive charge: Cast iron or steel balls, approximately 48mm in diameter and each weighing between 390 to 445 g; six to twelve balls are required.
• Sieve: 1.70, 2.36, 4.75, 6.3, 10, 12.5, 20, 25, 40, 50, 63, 80 mm IS Sieves.
• Balance of capacity 5 kg or 10 kg
• Drying oven
• Miscellaneous like tray
• PROCEDURE
• The test sample consists of clean aggregates dried in oven at 105° – 110°C. The sample should conform to any of the gradings shown in table 1.
• Select the grading to be used in the test such that it conforms to the grading to be used in construction, to the maximum extent possible.
• Take 5 kg of sample for gradings A, B, C & D and 10 kg for gradings E, F & G.
• Choose the abrasive charge as per Table 2 depending on grading of aggregates.
• Place the aggregates and abrasive charge on the cylinder and fix the cover.
• Rotate the machine at a speed of 30 to 33 revolutions per minute. The number of revolutions is 500 for gradings A, B, C & D and 1000 for gradings E, F & G. The machine should be balanced and driven such that there is uniform peripheral speed.
• The machine is stopped after the desired number of revolutions and material is discharged to a tray.
• The entire stone dust is sieved on 1.70 mm IS sieve.
• The material coarser than 1.7mm size is weighed correct to one gram.

• Observations of Los Angeles Test

  Original weight of aggregate sample = W1 g

  Weight of aggregate sample retained = W2 g

  Weight passing 1.7mm IS sieve = W1 – W2 g

  Abrasion Value = (W1 – W2 ) / W1 X 100