Aggregate Crushing Value (ACV)
The aggregate crushing value test is conducted on coarse aggregates to determine their resistance to crushing under gradually applied compressive load. It is expressed as a percentage by weight of the crushed material obtained when test aggregates are subjected to a specified load under standardized conditions. The crushing value of aggregates is a numerical index of their strength, and it is used in the construction of roads and pavements. A lower crushing value is preferable for roads and pavements as it indicates a lower crushed fraction under load, resulting in a longer service life and greater economic efficiency. The aggregates used in road and pavement construction must be strong enough to withstand crushing under roller and traffic loads. In cases where the aggregate crushing value is 30 or higher, the result may be anomalous, and therefore, the ten percent fines value should be determined instead. The purpose of conducting this test is to determine the aggregate crushing value of coarse aggregates and evaluate their suitability for use in various types of roads.
Apparatus
Apparatus needed for the aggregate crushing value test includes a steel cylinder with a diameter of 15 cm, along with a plunger and base plate. A straight metal tamping rod with a 16 mm diameter and a rounded end, measuring between 45 and 60 cm in length is also required. A balance that can handle up to 3 kg and is readable and accurate to one gram is necessary for this test. Additionally, sieves in sizes of 14mm, 12.5 mm, 10 mm, and 2.36 mm are needed. A compression testing machine is also required. For measuring the aggregate, a cylindrical metal measure that can withstand rough usage is required. It should have a diameter of 11.5 cm and a height of 18 cm. Lastly, a dial gauge is needed for this test.
Sampling
Select coarse aggregate that passes a 14 or 12.5mm sieve and is retained on a 10mm sieve. Heat the aggregate at 100-110°C for four hours, or until constant mass is achieved, and let it cool to room temperature. The quantity of aggregate should be enough to fill the cylinder up to 10cm after tamping, as described below. To measure, fill the cylindrical measure in three layers of equal depth, tamping each layer 25 times with the tamping rod and leveling off the surface with the rod as a straight-edge. Take care not to break particles when working with weaker materials. Determine the weight of the material that makes up the test sample (Weight A) and use the same weight for the repeat test.
Procedure
To conduct the Aggregate Crushing Value (ACV) Test, follow these steps:
Place the cylinder onto the base plate and record its weight (W).
Take the sample and divide it into three equal layers. Use the tamping rod to subject each layer to 25 strokes, making sure to handle weak materials with care to avoid particle breakage. Weigh the sample after tamping (W1).
Carefully level the aggregate surface and place the plunger horizontally onto it. Ensure that the plunger does not get stuck in the cylinder.
Put the cylinder with the plunger onto the loading platform of the compression testing machine.
Apply a uniform load at a consistent rate to reach a total load of 40T over a 10-minute period.
Remove the load and extract the material from the cylinder.
Use a 2.36mm sieve to sieve the material, taking care not to lose any fines.
Weigh the fraction of material that passed through the sieve (W2).
Calculation
In each test, the percentage ratio of the weight of fines produced to the weight of the total sample must be calculated and recorded to one decimal place. This ratio is known as the Aggregate Crushing Value (ACV) and can be calculated using the following formula:
Aggregate Crushing Value (ACV) = (W2 x 100) / (W1 - W)
Example limits
Here is a list of the Aggregate Crushing Value limits for different types of road construction:
Types of Roads / Pavements:
Flexible Pavements
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
NOTE: there may be significant regional, product or project variation on the stated limits