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 a weaker aggregate is used, it 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 Aggregate Crushing Value 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.
The aggregate sample 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 100C – 110C 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 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 then compute the aggregate crushing value. The mean of two observations, rounded to the nearest whole number is reported as the “Aggregate Crushing Value”.
Standard: comparable to BS 812:110
Comprising 150 mm nominal diameter steel cylinder, plunger, base plate, tamping rod and measure 115 mm diameter x 180 mm deep.
Used for aggregate passing 12.7 mm and retained by 9.52 mm sieve.
The complete assembly is cadmium plated for corrosion protection.
Weight: 20 kg approx.