# Aggregate Crushing Value Test

Stone aggregates used in the construction of road pavements should be resistant to crushing under the roller during construction and to heavy wheel loads on the pavement during its service life.

The crushed (or finer) material obtained when the test aggregates are subjected to a specified load under standard conditions is described as aggregate crushing value, and the Aggregate crushing value is a numerical measure of aggregate strength that is used in the construction of roads and pavements.

The aggregate crushing test can be used to determine the strength of coarse aggregate. The aggregate crushing value is a relative measure of crushing resistance under a continuously applied compressive stress. Aggregates with strong crushing resistance or a low aggregate crushing value are preferred for use in high-quality pavements.

In other words, Lower crushing values are preferred for roads and pavements because they indicate a lower crushed fraction under load, resulting in a longer service life and more economical performance.

If we use weaker aggregates, they will be crushed under traffic loads, resulting in tiny particles that are not covered with a binder and can be easily displaced or loosened out, resulting in loss of the surface/layer. As a result, road construction aggregates must be strong enough to withstand crushing under rollers and heavy wheel loads.

##### Apparatus

- A 15.2 cm diameter steel cylinder with a plunger and a base plate.
- A straight metal tamping rod with a circular cross-section of 16 mm in diameter and a height of 45 to 60 cm, rounded at one end.
- IS sieves with diameters of 12.5, 10, and 2.36 mm.
- A 3 kilogram capacity balance that is readable and accurate to one gram.
- A compression testing machine capable of applying a load of 40 tonnes and operating at a consistent rate of loading to achieve the maximum load in 10 minutes.
- Cylindrical metal measure with enough strength to withstand hard use, 11.5 cm diameter, and 18 cm height.

##### Preparation of Sample

- The material for the standard test must be aggregate that passes a 12.5 mm IS Sieve and is retained on a 10 mm IS Sieve, and
- It must be properly separated on these sieves before testing.
- The aggregate shall be tested in a surface-dry condition. If necessary, the aggregates can be dried by heating at 100°C – 110°C for not more than 4 hours and cooled to room temperature before testing.
- Filling the cylindrical measure in three layers of approximately equal depth, each layer being tamped 25 times with the rounded end of the tamping rod and finally leveled off with the tamping rod as a straight-edge, gives the necessary quantity.
- The weight of the material that comprises up the test sample (Weight A) must be calculated, and
- The same weight of sample must be used for the repeat test.

##### Procedure

- Place the cylinder on the base plate and weigh it (Initial Weight W).
- Arrange the sample in three layers, with each layer undergoing 25 strokes with the tamping rod. In the case of weak materials, care must be taken not to shatter the particles and weigh them (Weight W1).
- Carefully level the aggregate surface and insert the plunger so that it lies horizontally on the surface. The plunger is being carefully monitored to ensure that it does not become stuck in the cylinder.
- Place the cylinder with plunger on the compression testing machine’s loading platform.
- Apply an uniform rate of load such that a total load of 40 Tonnes is applied in 10 minutes.
- Release the load
- Remove the material from the cylinder.
- Using a 2.36 mm IS sieve, sieve the material. Weigh this fraction going through it to 0.1 gram precision. This fraction represents the quantity of material lost due to crushing. Determine the proportion that passes through the IS sieve as (Weight W2).

##### Observations

In each test, the weight of fines generated to total sample weight must be represented as a percentage, The “Aggregate crushing Value” is determined with the mean of two observations, rounded to the closest whole value.

**Aggregate Crushing Value = 100(W2/W1) percent**

where,

W2 = Weight of fraction passing the appropriate 2.36 mm IS sieve, and

W1 = Weight of surface-dry sample.

##### Final Verdict

The aggregate crushing value for good quality aggregate used in base course shall not exceed 45 percent, and the value for surface course shall be less than 30 percent. The aggregate crushing value of coarse aggregates to be utilized for cement concrete pavement surface should not exceed 30%, according to the IRC and BIS.

However, neither the IRC nor the Ministry of Road Transport and Highways have prescribed aggregate crushing values for coarse aggregates to be utilized in flexible pavement/bituminous pavement construction methods.

In general, larger aggregate sizes provide a higher aggregate crushing value, and vice versa.

however, the relationship between the values obtained with different sizes differs from aggregate to aggregate. With larger aggregate sizes, extra care must be taken to ensure that the plunger does not jam in the cylinder.

##### Reference

- IS : 2386 (Part IV) – 1963 – Methods of Test for aggregates for Concrete – Part IV : Mechanical Properties

If you like this article, please do share this with your friends

And, tell us your queries in the below comment box.

**Also Read: **Aggregate Impact Value Test (AIV)