Concrete is one of the most widely used materials in modern construction, valued for its strength, versatility, and durability. To achieve the desired quality, it’s essential to ensure the right level of workability, which refers to how easily the concrete can be mixed, placed, and compacted without segregation. The Compaction Factor Test is one of the most reliable laboratory methods used to evaluate this property, especially for mixes with low workability.
In this blog, Brick & Bolt explains what the Compaction Factor Test is, the step-by-step procedure for performing it, and the relevant IS codes that govern the testing process to ensure accurate and consistent concrete quality.
What is the Compaction Factor Test?
The compacting factor test for concrete is used to evaluate its workability and consistency. Workability is understood as the ease with which concrete can be placed and compacted, keeping its desired properties. Based on this workability test value, engineers and construction workers can maintain the concrete’s adequate consistency for a construction project.
The compaction factor test will be used for the concrete mixes that are low to medium in workability, where the slump test might not give accurate results. It uses gravity-based compaction to compare the weight difference of the concrete before and after compaction. By doing so, it helps determine how easily the concrete can compact under its own weight. The compaction factor of concrete is then determined by dividing the weight of partially compacted concrete by that of fully compacted concrete. If the compaction factor is higher, the workability will be better.
What are the Compaction Factor Test Apparatus?
The compacting factor test apparatus commonly involves the following:
- The Compaction Factor Apparatus consists of two conical hoppers with hinged trap doors that allow controlled flow of concrete, along with a cylindrical receiver of 0.005 cu. m capacity positioned below the hoppers.
- Trowels – 2
- Hand scoop (15.2 cm long) – 1
- Tamping steel rod (16 mm in diameter and 600 mm in length)
- Ruler
- A balance to weigh
Stepwise Compaction Factor Test Procedure
The compaction test on concrete will be conducted following this procedure:
- Prepare the Apparatus setup: Clean and dry the entire apparatus setup, including the upper, lower hopper, and bottom cylinder, for the ease of conducting the test.
- Prepare and Mix Concrete: Prepare the concrete as per the suitable concrete mix ratio for the particular project requirements, and use a hand scoop to place it into the upper hopper.
- Fill the Upper Hopper: The Upper Hopper will be gently filled with fresh concrete, and the surface will be levelled without compacting it.
- Release the Concrete to Lower Hopper: Open the trap door of the upper hopper so that the concrete drops freely into the lower hopper. If the concrete mix sticks in the hoppers, use a tamping rod to gently push it. Let it settle by gravity without compacting.
- Release Concrete into Cylindrical Receiver: Open the trap door of the lower hopper to allow the concrete to fall into the cylindrical receiver below. Use a trowel to remove the excess concrete and level the surface evenly with the top of the cylinder.
- Weigh the Concrete (W₁): Carefully clean the outer surface, level the top, and weigh the mold filled with concrete to record the weight of the partially compacted concrete. Subtract the weight of the empty mold to get accurate W₁.
- Refill and Compact Fully: Empty and clean the cylinder, then refill it in layers. Compact each layer properly using a tamping rod.
- Weigh Again (W₂): After complete compaction, weigh the mold again. Subtract the weight of the empty mold to get W₂, the weight of fully compacted concrete.
Calculation of Compaction Factor Value
To calculate the compaction factor of concrete, use this formula:
Compaction Factor (CF): W1÷W2
Where:
W₁ = Weight of partially compacted concrete (after free fall)
W₂ = Weight of fully compacted concrete (after manual compaction)
Result of Compaction Factor
The following table indicates the compaction factor values along with the respective workability features:
| Compaction Factor Test Values | Workability Features | Applications |
| 0.75-0.80 | Very low | Vibrated concrete for roads and pavements. |
| 0.80-0.85 | Low | Mass concreting. |
| 0.85-0.92 | Medium | General RCC works like beams, slabs, and walls. |
| Above 0.95 | High | Highly reinforced concrete with congested reinforcement. |
Factors Affecting Compaction Factor
The following are different factors impacting the compaction factor of concrete:
- Aggregate Cement Ratio: If the cement content is higher in the concrete, the friction between aggregates will be minimal, allowing for smoother movement of aggregates during mixing, transportation, placement, and compaction, thereby increasing overall workability.
- Water/cement Ratio: A higher water-cement ratio increases the workability of concrete, making it easier to mix and place. However, using a very small quantity of water can reduce strength development, while excessive water weakens the concrete. The ideal water-cement ratio for good workability typically ranges between 0.45 and 0.55.
- Aggregate Shape and Size: Aggregates with a rounded shape improve workability. Rounded and slightly irregular particles are preferred for easy mixing and placement. In contrast, elongated, angular, and flaky aggregates are harder to mix, have a larger surface area to cover, and thus result in lower workability of concrete.
- Temperature: An increase in temperature accelerates the setting, hardening, and heat evolution of concrete, which raises the fluid’s viscosity and reduces its workability. However, above 30°C, this trend may reverse. Conversely, low temperatures slow down the setting process, and when the temperature falls below 0°C, both setting and hardening stop completely. The ideal temperature range for proper setting and hardening is 14°C to 18°C.
- Use of Admixtures in Concrete: Admixtures such as plasticizers and superplasticizers significantly enhance concrete workability without affecting strength. Additionally, mineral admixtures like pozzolanic materials also help improve the flow and performance of concrete mixes.
IS Code for the Compaction Factor Test (IS 1199 (Part 2): 2018 )
IS 1199 (Part 2): 2018 provides updated guidelines for conducting the Compaction Factor Test, which is used to measure the workability of concrete with low to medium consistency. The standard outlines the apparatus dimensions, testing procedure, sample handling, and calculation methods to ensure accuracy and consistency. It is mainly applicable for concrete with a maximum aggregate size of 40 mm or less, particularly when vibration compaction is used on-site. These guidelines promote uniform testing and reliable results in concrete quality assessment across India.
Advantages of the Compacting Factor Test
The following are the benefits of conducting the Compaction Factor Test:
- Easy to Perform: Compared to other tests for the workability of concrete, this test is simple and easy to perform, requiring fewer pieces of equipment and less expertise.
- Adjusting the Mix Ratio of Concrete: Based on the test results, an engineer or construction worker can directly adjust the concrete mix to the required consistency.
- Quality control: Performing this test helps to evaluate the uniformity and compatibility of concrete, ensuring that it meets the required standards and specifications for maintaining strength and durability.
- Accuracy: Compared to the slump test, this test gives more precise results.
Disadvantages of the Compacting Factor Test
The following are the disadvantages of the compaction factor test:
- Time-taking test: This test involves multiple steps and may require more time to complete compared to other workability tests, which are not as practical or suitable for time-sensitive construction projects.
- Minimal Applicability: This test is only suitable for assessing the workability of concrete with low to medium slump values. It may not provide accurate results for highly fluid or very stiff concrete mixes.
In conclusion, the compaction factor test is a simple but effective technique for checking the workability of concrete. Checking the compaction factor will help ensure that concrete mixes meet the required specifications for workability and consistency, which in turn enhances the overall quality and durability of the building. Although this test has its own limitations, it is often used and considered essential in current concrete quality control. For professional construction services that prioritise thorough concrete testing and quality construction work, consider working with Brick & Bolt, where expert engineering meets reliable construction solutions for your dream home.
Frequently Asked Questions
1. Why is the Compaction Factor Test Preferred for Low Workability Concrete?
Some concrete mixes, especially those used in mass concreting, road construction, and machine placement, tend to be dry and stiff. Since these mixes do not flow easily, the Slump Test cannot measure their workability accurately. In such cases, the Compaction Factor Test is preferred, as it provides a more reliable measure of workability for low to medium-consistency concrete.
2. What apparatus is used for the Compaction Factor Test?
The compaction factor test apparatus is used in the testing. This includes two conical hoppers with trapdoors, a cylindrical receiver, a steel stand, trowels, a hand scoop, and a tamping rod, all in accordance with IS 1199 standards.
3. How is the Compaction Factor calculated?
Compaction Factor = (Weight of partially compacted concrete) / (Weight of fully compacted concrete).
It typically ranges from 0.75 to above 0.95, indicating workability.