Aggregate Crushing Value Test | Transportation Engineering Laboratory Experiment
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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

1. To determine the crushing value of aggregate for road aggregates;
2. 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: 23861963 (Part IV) consist of the following:

1. The test molda 15.2 cm diameter openended 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.
2. A straight metal tamping rod of circular crosssection 16mm in diameter and 45 to 60 cm long, rounded at one end.
3. A balance of capacity 5 kg, readable and accurate up to 1 gm.
4. IS sieve of sizes 12.5mm, 10mm, 2.36mm.
5. A compression testing machine capable of applying load up to 40 tonnes at a uniform rate of 4 tonnes per minute.
6. A cylinder measure having an internal diameter of 11.5 cm and height of 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 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”.

The table below shows limits of aggregate crushing value for different types of road construction:

Types of Roads / Pavements Aggregate Crushing Value Limit
Flexible Pavements
Soling 50
Water bound macadam 40
Bituminous macadam 40
Bituminous surface dressing or thin premix carpet 30
Dense mix carpet 30
Rigid Pavements
Other than wearing course 45
Surface or Wearing course 30

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