Concrete segregation is the undesirable separation of cement paste and aggregates within the concrete mix during handling, transportation, and placement. It often occurs when the concrete is subjected to jerks, vibrations, or excessive compaction. Over-vibration, for example, causes the heavier aggregates to settle at the bottom while the lighter cement paste rises to the top. This separation compromises the concrete’s strength and durability, as a well-mixed concrete should consist of aggregates uniformly coated with cement paste, forming a homogeneous mass.
Causes of Segregation of Concrete
Several factors contribute to segregation in concrete:
- High Water-Cement Ratio: This is common in site-mixed concrete by unskilled workers. Excess water reduces the cohesion between the cement paste and aggregates, causing the materials to separate easily.
- Excessive Vibration: Using mechanical needle vibrators excessively can cause heavier particles to settle while lighter paste moves upward.
- Placement from Height: Pouring concrete from significant heights, such as in underground foundations or raft foundations, often leads to segregation due to the impact forces separating the components.
Effects During Handling and Transport
During transit, the jolts and vibrations can cause the cement paste and sand to separate from coarse aggregates. If segregation occurs, it’s crucial to remix the concrete properly before placement to restore uniformity. However, if the concrete has already begun setting, it should not be used as remixing will no longer be effective.
Preventing Concrete Segregation
To minimize segregation, especially when concreting depths exceed 1.5 meters, the use of temporary inclined chutes is recommended. The chute should have an angle of inclination between 1:3 and 1:2 to allow smooth flow of concrete from top to bottom. Additionally, lubricating the chute path intermittently with small amounts of free water can ease the concrete flow.
It’s important that the delivery end of the chute is positioned as close as possible to the point of deposit to reduce free fall and prevent segregation.
Detecting and Rectifying Segregation in Deep Foundations and Rafts
In thick foundations or rafts exceeding 1 meter, segregation is likely near the bottom or center if proper supervision is absent. Advanced non-destructive testing methods such as ultrasonic testing can detect such segregation early.
If segregation is detected, the project designer should be consulted for guidance. One effective remedy is pressure grouting using special chemical compounds to restore the concrete’s integrity.
After remediation, core testing must be performed to ensure the concrete strength meets the desired specifications and that the defect has been successfully rectified.
Conclusion
Segregation of concrete is a critical issue that can weaken structural elements if not controlled properly. Understanding the causes, taking preventive measures during placement, and employing advanced detection and repair techniques ensure concrete quality and durability. Supervision and proper handling during mixing, transporting, and placing concrete remain the most effective ways to avoid segregation and maintain the strength and longevity of concrete structures.