The purpose of this method is to determine the crushing value of the given sample of aggregate for the road construction works.
The Principal mechanical properties required in road stones are:
- Satisfactory resistance to crushing under the roller during construction and
- Adequate resistance to surface abrasion under traffic.
Also stresses under rigid tyre rims of heavily loaded animal drawn vehicles are high enough to consider the crushing strength of road aggregate as an essential requirement.
Crushing strength of road aggregate may be determined either on aggregate or on cylindrical specimens cut out of rocks.
These two tests are quite different in not only the approach but also is the expression of the results.
Aggregate used in road construction, should be strong enough to resist crushing under traffic wheel loads.
If the aggregate are weak, the stability of the pavement stretches is likely to be adversely affected, the strength of coarse aggregate is assessed by aggregate crushing test.
The aggregate crushing value provides a relative measure of resistance to crushing under gradually applied compressive load.
To achieve a high quality of pavement, aggregate possessing low aggregate value should be preferred.
Equipment for Crushing Value Test
Below apparatus is necessary for conducting the crushing value test.
Steel cylinder with open ends and a square plate
Plunger with piston
Sieves (12.5 mm, 10.0mm & 2.36 mm)
Compression testing machine
Steel Tamping Rod
Crushing Value Test Procedure
Select clean and dry aggregate passing through 12.5 mm and retained on 10.0 mm sieve.
Weight the empty cylindrical measure. Let the weight be ‘a’ grams.
Fill the aggregate in the cylindrical measure in three layers, tamping each layer 25 times with the rounded end of the tamping rod.
Weigh the cylindrical measure with aggregate. Let the weight be ‘b’ grams.
Thus the weight of aggregate = W1 gram
Transfer the aggregate into the steel cylinder again in three layers tamping each layer 25 times
Place the plunger in the steel cylinder such that the piston rests horizontally over the aggregate surface.
Keep the assembly of steel cylinder with plunger in the compression testing machine.
Set the pointer to read zero and apply the compressive load of 40 tons.
Stop the machine. Take out the assembly.
Sieve the crushed material on 2.36 mm sieve and find he weight of material passing this sieve. Let the weight be W2 gram.
Then Aggregate crushing value=W2 / W1 * 100 %
Recording in the Tabular Column
Weight of empty cylindrical measure = ‘a’ g
Weight of cylindrical measure with aggregate = ‘b’ g
Weight of aggregate taken in the cylinder before test =(b-a) = W1 g
Weight of crushed material Passing IS 2.36 mm sieve = W2 g
Aggregate crushing value = W2 / W1* 100 %
Desirable Aggregate Crushing value
Strong aggregates give low aggregate crushing value. Preferably the aggregate crushing value of the coarse aggregate used for cement concrete pavement at surface should not exceed 30 percent.
For aggregates used for concrete other than for wearing surfaces, the aggregates crushing value shall not exceed 45 percent.
However aggregate crushing values have not been specified in some countries for coarse aggregates to be used in bituminous pavement construction methods.