Compacting factor test is performed using guidelines from past researches. However, BS 1881-103 has detailed the guidelines for performing the compacting factor test.
The objective of performing this test is to determine the degree of compaction of concrete. As an indirect course, the value of compacting factor dictates the concrete workability.
The concrete that free falls from one hopper to the other and finally into the cylinder gets partially compacted. This is because as the concrete falls from a height, it gets densified under its self-weight, thereby reducing the void initially present. The density of this concrete present in the cylinder, subject to two events of free-falling, is termed the density of partially-compacted concrete.
The concrete is placed in the cylinder in four layers whilst compacting each layer on a vibrating table. Consequently, the concrete gets even more densified and is termed fully-compacted concrete. The density of this concrete is greater than that of the partially-compacted concrete.
The compacting factor gives the degree of compaction of concrete and is defined as the ratio of the density of partially-compacted concrete to that of the fully-compacted concrete. Mathematically, it is given by,
The compacting factor is always less than 1 since the density of fully-compacted concrete is a step ahead in magnitude to that of partially-compacted concrete.
Relationship between concrete workability and compacting factor
The compacting factor indirectly informs about the workability of concrete. Concrete with high workability usually has a higher compacting factor than that of a mix with low workability. The table below gives the general values of compacting factors for concrete of different workability.
|Concrete Workability||Compacting factor|
- Two hoppers of known dimensions, having a tapered cross-section in the form of a cone frustum, and hinge doors at the bottom to allow the outflow of concrete from to underlying hopper or cylinder
- A cylinder of known dimension, present below the hoppers to collect the concrete when the hopper door discharges the concrete into it
- Weighing balance, to measure the cylinder weight with and without concrete
- Vibrating table, to fully compact the concrete
Fig.: Apparatus for compacting factor test
Fig.: Vibrating Table
This test is appropriate for concrete containing a maximum aggregate size of 40mm.
- Prepare a wet concrete sample using the guidelines provided by the code.
- Gently pour the wet concrete in the top-most hopper ensuring that the concrete is not dropped from a height, otherwise, the concrete will undergo some compaction.
- When the upper hopper gets filled with concrete, the door at the bottom of it is released open and the concrete falls freely into the hopper present underneath.
- Similarly, the door at the bottom of the lower hopper is opened and the concrete falls into the cylinder present below it.
- The excess concrete overflowing the cylinder is removed and the density of the partially-compacted concrete in the cylinder is determined using the weight-volume relationship.
- The concrete is then fully compacted by filling the same cylinder with concrete in 4 layers, each time placed on a vibrating table and subjected to vibrations for a certain duration of time. The density is again measured and this gives the density of fully-compacted concrete.
- The compacting factor of the concrete mix is determined by taking the ratio of densities of partially-compacted and fully-compacted concrete. Mathematically, it is given as,
Observations and Calculations
Diameter of cylinder (mm) = d =
Height of cylinder (mm) = h =
Volume of cylinder = V = (π*d2*h)/4
Weight of empty cylinder (kg) = W1
Weight of cylinder + concrete after partial compaction (kg) = W2
Weight of partially-compacted concrete in the cylinder (kg) = WPC = W2 – W1
Weight of cylinder + concrete after full compaction (kg) = W3 =
Weight of fully-compacted concrete (kg) = WFC =
Partially-Compacted Density (kg/m3) = ρPC = WPC /V
Fully-Compacted Density (kg/m3) = ρFC = WFC /V
Compacting factor = ρPC / ρFC
The compacting factor of the concrete mix is .
Workability of concrete (high, medium, or low) = .
- Care should be exercised to not compact the concrete while pouring it into the hopper.
- The internal surfaces of the hopper and cylinder must be greased to make them smooth.
- The door release swings must be frictionless so as to instantly discharge the concrete from one hopper to the other and then into the cylinder.
- The vibrations from the vibrator are to be halted when the top surface of the concrete in the cylinder attains a smooth or glazed finish, over-vibration of concrete must be avoided.
- Greater is the compacting factor of concrete, greater is its workability, and vice versa.
- The limit of 40mm on the coarse aggregate size is set because using particles larger than 40mm may lead to segregation of concrete as it free-falls from the hoppers under its self-weight.
- Greater is the density of a concrete sample as it gets compacted under its own weight, greater is its compacting factor.