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 show 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 a 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 of Crushing Test
- To determine the crushing value of aggregate for road aggregates;
- To determine the suitability of aggregates for use in different types of road pavements.
Apparatus for Crushing Test
The apparatus for the standard aggregate crushing test as per IS: 2386-1963 (Part IV) consist of the following:
- The test mold-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 up to 1 gm.
- IS sieve of sizes 12.5mm, 10mm, 2.36mm.
- A compression testing machine capable of applying load up to 40 tonnes at a uniform rate of 4 tonnes per minute.
- A cylinder measure having an internal diameter of 11.5 cm and height of 18 cm.
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 a surface dry condition before testing. The aggregates may be dried by heating at 100 oC – 110 o C for not more 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 a 12.5 mm sieve and retained on a 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 the tamping rod.
- Two more layers are added in a 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 the amount of aggregates that will be used to prepare the test specimen.
- Empty the cylinder and weight the aggregates, accurate up to 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 the compression testing machine.
- The load is applied at a uniform rate of 4 tonnes.
- Release the load
- Take the aggregate out of the cylinder and sieve them through a 2.36 mm IS sieve. Weight this fraction passing through it to an accuracy of 0.1 gm. This fraction is a quantity of the 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 the nearest whole number is reported as the “Aggregate crushing Value”.
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Test with Non-standard Sizes of Aggregates
If required, or if the standard size is not available, the test may be carried out with different grades. However, specifications will be different for such cases and should conform to those given in the following table:
- The plunger should be placed centrally and rest directly on the aggregates. Care should be taken that it does not touch the walls of the cylinder so as to ensure that the entire load is transferred onto the aggregates.
- In the operation of sieving the aggregates through 2.36 mm sieve and weighting care should be taken to avoid loss of fines. The sum of weights of fractions retained, and passing the sieve should not differ from the original weight of the specimen by more than 1 gm.
- The tamping should be done properly by gently dropping the tamping rod and not by hammering action. And the tamping should be uniform over the surface of the aggregates taking care that the tamping rod does not strike against the walls of the mould.
Record of Observation
( Note: The mean of the two results to the nearest whole number is reported as the ‘Aggregate crushing value’ of the material).
INTERPRETATION OF RESULTS
The suitability of aggregate is adjusted, dependent upon its proposed use in the pavement layers. The table given below lays down specified limits of percent aggregate crushing value, for different types of road construction.
Note:- For strong aggregate, crushing value should be low. For surface courses this value should not exceed more than 30% and for courses not more than 45%.
Questions for Discussion
- Which property of aggregates is measured by this test?
- How aggregate crushing value expressed?
- What variation in the result is expected if the smaller size aggregates are used in the test?
- Should the aggregates having high ACV be considered good for load construction?
- What are the uses of determining ACV?
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