Compacting factor test is performed using guidelines from past researches. However, **BS 1881-103** has detailed the guidelines for performing the compacting factor test.

Contents

## Objective

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.

## Related Theory

### Partially-Compacted Concrete:

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.

### Fully-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.

### Compacting factor:

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 |

Very Low | 0.78 |

Low | 0.85 |

Medium | 0.92 |

High | 0.95 |

## Apparatus

- 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

## Test Procedure

#### Test Limitation:

This test is appropriate for concrete containing a maximum aggregate size of 40mm.

## Procedure

- 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*d^{2}*h)/4

Weight of empty cylinder (kg) = W_{1}

Weight of cylinder + concrete after partial compaction (kg) = W_{2}

Weight of partially-compacted concrete in the cylinder (kg) = W_{PC} = W_{2} – W_{1}

Weight of cylinder + concrete after full compaction (kg) = W_{3} =

Weight of fully-compacted concrete (kg) = W_{FC} =

**Partially-Compacted Density (kg/m ^{3}) = ρ_{PC} = W_{PC} /V**

**Fully-Compacted Density (kg/m ^{3}) = ρ_{FC} = W_{FC} /V**

**Compacting factor = ρ _{PC} / ρ_{FC}**

## Results

The compacting factor of the concrete mix is .

Workability of concrete (high, medium, or low) = .

## Precautions

- 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.

## Discussion

- 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.