Aggregate Crushing Value Test
The strength of aggregate parent rock is determined by preparing a cylindrical shape specimens of size 25 mm diameter and 25 mm height. This cylinder is subjected to compressive stress. Depending on the type of parent rock gives different compressive strength varying from a minimum of about 45 MPa to a maximum of 545 MPa.
It is a fact that parent rock compressive strength does not exactly indicate the strength of aggregate in concrete. For this reason assessment of strength of the aggregate is made by using a sample of bulk aggregate in a standardized manner. This testing method is known as an aggregate crushing value test.
Aggregate crushing value provides the resistance of an aggregate sample to crushing under gradually applied compressive load. Generally, the test is conducted on aggregate passing 12.5 mm and retained on 10 mm sieve. The aggregate sample is filled in a cylindrical mold and a load of 40 ton is applied through a plunger in a compression testing machine. The crushed aggregate sample which is finer than 2.36 mm is separated and expressed as a percentage of the original weight taken in the mould. The percentage of weight passed through 2.36m IS sieve is known as Aggregate crushing value.
“Aggregate Crushing values is a relative resistance of aggregates to crushing under gradually applied compressive load”
In situations, when aggregate value 30 or higher the result may be a mistake and in such cases, the “ten percent fines value” should be determined and used instead.
Aggregate Crushing Value Test:
The objective of this test is to:
- To find out Crushing Strength or Crushing value of Coarse Aggregate.
- A 15 cm dia. Steel cylinder with plunger and base plate.
- A straight metal tamping rod 16mm diameter and 45 to 60cm long rounded at one end.
- A Weigh balance of accuracy up to 1 gam.
- IS sieves of sizes 12.5mm, 10mm, and 2.36mm
- A compression testing machine.
- Cylindrical measure having a diameter of 11.5 cm and 18cm height.
- A compression testing machine having a loading capacity of 40 tones and which can be operated to give a uniform rate of loading so that the maximum load is reached in 10 minutes.
Sampling of Aggregates:
For the Aggregate crushing value test, an aggregate sample passing through 12.5 mm IS sieve and retained on 10mm IS sieve is selected and dried to a temperature of 105 ºC to 110 ºC then cooled to room temperature.
To fill the cylindrical measure mould about 6.5 kg of a sample of aggregate is sufficient.
The procedure of Aggregate Crushing Value Test:
- Take the Empty weight of cylindrical measure as W1.
- Fill aggregate sample passing through 12.5 mm and retained on 10 mm IS sieve in measuring cylinder in 3 equal layers such that each layer is subjected to 25 strokes using the tamping rod. Take the weight of aggregate with measuring cylinder as W2.
- Find out the weight of aggregate sample W = W2 – W1
- Now, fill the aggregate sample in 15 cm dia. and 13 cm height steel cylinder and level the surface of aggregate carefully and insert the plunger so that it rests horizontally on the surface.
- Place a steel cylinder with a plunger on the loading plate of the compression testing machine.
- Operate Compression machine such that 40 tonnes of the load is applied on aggregate in approximately 10 min.
- Release load and remove the steel cylinder from the machine.
- Take out the crushed aggregate sample and sieve on with 2.36mm IS sieve, care being taken to avoid loss of fines.
- Take off the weight of fraction passing through 2.36 mm IS sieve as (W3).
Calculation of Aggregate Crushing Value:
Aggregate crushing value can be found by the following formula,
“Aggregate crushing value is determined by taking a percentage of weight crushed aggregate sample passing through 2.36 mm IS Sieve divided by weight of aggregate sample taken for test”
Aggregate crushing value = (W3 / W) or W3 / (W2 – W1)
W1 = Empty weight of cylindrical Measure.
W2 = Weight of Aggregate with Cylindrical Measure
W = W2 –W1 = Weight of Aggregate Sample
W3 = Weight of crushed aggregate sample passed through 2.36 mm IS Sieve.
For accurate test result in conduct test at least 3 times and take an average of 3 values
Standard values for Pavement and Road Construction:
The below table shows the aggregate crushing value for different types of roads.
|Types of Roads / Pavements||Aggregate Crushing Value Limit|
|Water bound macadam||40|
|Bituminous surface dressing or thin premix carpet||30|
|Dense mix carpet||30|
|Other than wearing course||45|
|Surface or Wearing course||30|