Compaction Factor Test
The compaction factor test helps to define the concrete workability performed in the laboratory.
The weight ratio of partially compacted concrete to fully compacted concrete is termed the compaction factor.
It was invented by Road Research Laboratory in the United Kingdom and is practised to define concrete workability.
This test is performed for concrete with low workability(slump less than 50 mm), for which the slump cone test doesn’t provide accurate results.
The compaction factor test is much accurate than the slump cone test, but it is less prevalent. The apparatus size makes it challenging to perform the test in the field.
It measures the degree of workability of fresh concrete concerning the internal energy demanded by concrete for compacting it thoroughly.
Apart from the compaction factor test and slump cone test, various other tests are practised to determine the workability of concrete, such as the Kelly ball test, Vee-Bee consistometer test, K Slump test and Flow table test.
Where do we use low workability Concrete?
Concrete with low workability is suitable for mass concreting work such as foundations, sections with light reinforcement.
Low workability concrete helps to minimize the thermal stresses developed due to the massive amount of heat of hydration caused by mass concreting.
Low water-cement ratio removes weak zone and bleeding. Though, the desired workability is achieved by adding admixtures.
According to the IS1199:1999 Compaction factor test apparatus comprises two conical hoppers, a bottom cylinder ordered, as shown in the image above.
A tamping rod of steel diameter 1.6 mm and length 61 cm, and a weight balance to weigh the concrete.
- Prepare the concrete according to the design mix in the laboratory.
- Clean the conical hopper’s inner surface to ensure it does not have any moisture, and apply oil or grease to it then close the trap door of the upper conical hopper.
- Now weigh the empty bottom cylinder and note it down as W1.
- Then fill the upper conical hopper with freshly prepared concrete with the help of a trowel without compaction.
- Now open the trap door of the upper conical hopper and allow the concrete free fall to the lower hopper. Gently push the concrete sticking on the hopper’s inner side with the help of a rod.
- Now open the lower conical hopper’s trap door and let the concrete free fall on the cylinder.
- Level the concrete with the top level of the cylinder by cutting off excess concrete with the help of a trowel and clean the outer surface of the cylinder.
- Now weigh the cylinder filled with concrete and note it down as W2. this weight is referred to the weight of partially filled concrete.
- Now empty the cylinder and filled it again with the same sample concrete mix in layers (depth approximately 50 mm) and compact each layer by 25 blow with the help of a tamping rod to achieve fully compacted concrete.
- Now level the top surface of the cylinder, measure the cylinder’s weight with fully compacted concrete, and note it down as W3. The weight W3 is termed as fully compacted concrete weight.
For measurement, the test is sensitive enough to enable variation in workability starting from the initial cement hydration process.
If strictly comparable results are obtained, each test should be performed at a constant interval after mixing concrete.
The suitable time for delivering the concrete from the upper hopper is two minutes after mixing.
Calculation and Observation Table
The compaction factor is determined as the ratio of partially compacted concrete weight to fully compacted concrete weight. Usually, its value is stated to be near about the second decimal place.
|S.No.||Description||Sample 1||Sample 2||Sample 3|
|1||Empty Cylinder Weight W1|
|2||Cylinder + Free Fall Concrete Weight W2|
|3||Cylinder + Compacted Concrete Weight W3|
|4||Partially Compacted Concrete Weight |
Wp = (W2- W1)
|5||Fully Compacted Concrete Weight |
Wf = (W3 – W1)
|6||Compaction Factor = Wp/Wf|
The Compaction factor value for concrete varies from 0.7 to 0.95.
Compaction Factor For The Concrete =
Conclusion Based on Value of Compaction Factor
|Standard of Workability||Value of Compaction Factor||Slump Value (mm)|
|Good/High||0.95||100 – 175|
|Low||0.85||25 – 50|
|Very Low||0.78||0 – 25|
We can also determine the weight of fully compacted concrete by identifying materials proportions, specific gravities, and cylinder volume.
As per my knowledge and research, it gives a minimal variation in compacting factor value compared to the weight of fully compacted concrete calculated theoretically or found out practically after 100% compaction.
The compaction factor test evaluates the internal concrete properties closely connected to the workability need of concrete. It is the most helpful test to describe concrete with low workability.