Compaction Factor Test of Concrete is the workability test for concrete conducted in the laboratory. The compaction factor is the ratio of weights of partially compacted to fully compacted concrete. It was developed by Road Research Laboratory in the United Kingdom and is used to determine the workability of concrete. The compaction factor test is used for concrete which has low workability, for which the concrete slump test is not suitable.

Herein, we shall discuss the Compaction factor test, its apparatus, procedure and Advantages. This topic of Concrete Technology is important for upcoming exams like SSC JE CE and RRB JE Civil.

What is Compaction Factor Test?

The compacting factor test of cement concrete determines its workability. Workability refers to the ease with which concrete can be mixed, placed, and compacted without losing its desired properties. This test helps engineers and construction workers ensure that the concrete used in a construction project is of adequate consistency and will perform well when poured and compacted. 

The test measures the difference in the weight of the concrete before and after compaction. The compaction factor is calculated by dividing the weight of the compacted concrete by the weight of the partially compacted concrete. The compaction factor value indicates how easily the concrete can be compacted. A higher compaction factor suggests better workability, meaning the concrete is easier to handle and shape.

Apparatus for Compaction Factor Test

Fig: Compaction Factor Test Apparatus

The Compaction factor test apparatus consists of the following:

1. Trowels, 
2. Hand scoop (15.2 cm long),
3. A Tamping rod of steel or other suitable material (1.6 cm diameter, 61 cm long rounded at one end ) and,
4. A balance

The Compaction Factor Test uses gravity-based compaction to compare how much a concrete mix compacts under its own weight vs full manual compaction. Here’s how it’s done:

1. Prepare the Equipment: Ensure the upper and lower hoppers, and the cylinder are clean and dry.
2. Fill the Upper Hopper: Place fresh concrete into the upper hopper using a scoop, without applying any pressure or vibration.
3. First Drop: Open the trap door of the upper hopper so the concrete drops freely into the lower hopper. Let it settle by gravity — do not compact.
4. Second Drop: Now open the trap door of the lower hopper. The concrete will fall into the cylindrical mould placed just below.
5. Weigh the Concrete (W₁): Carefully remove excess concrete from the top of the cylinder, level it off, and weigh the mould filled with concrete. Subtract the empty mould weight to get W₁ = weight of partially compacted concrete.
6. Refill and Compact Fully: Empty the cylinder, clean it, and now refill it in layers. Each layer is compacted manually using a tamping rod.
7. Weigh Again (W₂): After full compaction, weigh the mould again. Subtract the mould weight to get W₂ = weight of fully compacted concrete.

Calculation of Compaction Factor Value

• The compaction factor compares the weight of partially compacted concrete to the weight of fully compacted concrete. 
• It is typically expressed to the nearest second decimal place.
• The formula to calculate the compaction factor value is:

Compaction Factor Value = (W1 - W) / (W2 - W)

Formula for Compaction Factor

 Compaction Factor (CF): W1/W2

Where:

• W₁ = Weight of partially compacted concrete (after free fall)
• W₂ = Weight of fully compacted concrete (manually compacted)

This ratio shows how easily the concrete can be compacted. A value closer to 1.0 means the concrete has high workability.

 Compaction Factor (CF): W1/W2

Apparatus Used in Compaction Factor Test

• Upper and Lower Hoppers: Funnel-shaped containers fitted with trap doors. Used to let concrete fall in two stages.
• Cylindrical Mould: Steel mould of 15 cm diameter and 30 cm height, where final compaction and measurement take place.
• Tamping Rod: Standard rod for manual compaction.
• Weighing Machine: For accurate weight measurements of concrete.
• Scoop and Trowel: To place and level the concrete properly.

Result of Compaction Factor

Compaction factor of the concrete = The Compaction factor values ranges from 0.7 to 0.95.

Why Compaction Factor Test is Preferred for Low Workability Concrete

Concrete workability means how easily concrete can be mixed, placed, and compacted. Some concrete mixes, especially those used in mass concreting, roads, and machine placement, are very dry or stiff. These mixes don’t flow easily and can’t be tested accurately using the Slump Test, because the concrete doesn’t slump properly — it either stays in place or breaks apart.

In such cases, the Compaction Factor Test is used. It helps measure how much effort the concrete needs to be fully compacted. The test gives a numerical value that tells how workable the mix is — higher the value, more workable the concrete. This makes it especially useful for low and medium workability mixes, where accurate measurement is important for safe design and construction.

Comparison of Slump Test and Compaction Factor Test

 Both the Slump Test and the Compaction Factor Test are used to measure the workability of fresh concrete. However, they are suited for different types of concrete mixes. While the Slump Test is ideal for concrete with high or medium workability, the Compaction Factor Test is more appropriate for drier, low workability mixes where the slump test fails to give reliable results. The table below highlights the key differences between the two:

IS Code Reference – IS 1199:1959

The Compaction Factor Test is officially described in the Indian Standard IS 1199:1959, titled "Methods of Sampling and Analysis of Concrete". This IS code is issued by the Bureau of Indian Standards (BIS) and serves as the official guideline for how concrete tests should be carried out across India.

Under this code, the Compaction Factor Test is included as one of the recognized methods to measure workability of concrete, especially low-workability mixes. The standard clearly defines:

• Apparatus Dimensions: Exact measurements of the hoppers and cylinder used in the test.
• Step-by-Step Procedure: Instructions for conducting the test correctly, from filling the hoppers to weighing the concrete.
• Method of Compaction: How to compact concrete in layers during the second stage.
• Calculation Formula: How to compute the Compaction Factor value accurately.
• Precautions: Guidelines to ensure results are consistent and errors are avoided during the test.

Using IS 1199:1959 ensures the test results are reliable, standardised, and acceptable for site use, lab reports, and construction quality control. Civil engineers and technicians must always refer to this code when conducting or validating Compaction Factor Tests

Advantages of Compaction Factor Test

The advantages of the Compaction Factor Test include the following:

• Simplicity: The test is relatively simple and easy to perform, requiring minimal equipment and expertise.
• Workability assessment: It provides a measure of the workability of concrete, allowing for adjustments in the mix design to achieve the desired consistency.
• Quality control: The test helps in assessing the uniformity and compatibility of concrete, ensuring that it meets the required standards and specifications for strength and durability.
• Accuracy: This test is more accurate as compared to the Slump test.

Disadvantages of Compaction Factor Test

The disadvantages of the Compaction Factor Test include the following:

• Limited applicability: The test is primarily suitable for assessing the workability of concrete with low to medium slump values and may not be as accurate for highly fluid or stiff mixes.
• Subjectivity: The test relies on the judgment and experience of the operator in determining the degree of compaction, which can introduce variability and subjective interpretation.
• Time-consuming: The test requires multiple steps and may take longer to perform compared to other workability tests, which can be inconvenient in time-sensitive construction projects.

This was all about the Compaction Factor Test, its procedure, apparatus and calculations. Enroll in the AE/JE Civil Coaching on the Testbook App in order to get confidence in the different topics of Civil Engineering.

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