Concrete is a composite material that consists of cement, sand, coarse aggregates, and water. During handling and placement of concrete, segregation may occur, which is the separation of cement paste and aggregates from each other. Over-vibration or compaction of concrete may cause the cement paste to come to the top, while the aggregates settle at the bottom.
The segregation of concrete can have a negative impact on the strength and durability of structures. In a well-made concrete mix, all aggregates are evenly coated with sand and cement paste, resulting in a homogeneous mass. However, during the transportation and placement of concrete, the jerks and vibrations can cause the cement paste and sand to separate from the coarse aggregate, leading to segregation.
If segregation occurs during the transit of concrete, it should be remixed thoroughly before depositing. It is important to note that a concrete mix that has surpassed its initial setting time should not be used. Proper handling and placement techniques can help to prevent segregation and ensure the structural integrity and longevity of concrete structures.
Causes of Segregation of Concrete
The use of a high water-cement ratio in concrete is often observed in cases where unskilled workers are responsible for mixing the concrete on site. This can result in a weaker concrete mix that is more prone to cracking and deterioration over time.
Excessive vibration of the concrete with mechanical needle vibrators can also contribute to issues with the mix. This process can cause heavier particles to settle at the bottom, while lighter cement sand paste rises to the top. This can lead to a non-uniform mixture that is not as strong or durable as it should be.
Another situation that can lead to concrete segregation is when it is poured from a height, such as in the case of underground foundations or rafts. The force of the fall can cause the concrete to separate into its individual components, resulting in a mix that is uneven and weak in certain areas. This can compromise the stability and longevity of the structure, and it is important to take steps to prevent segregation from occurring during the concreting process.
How to Prevent Concrete Segregation?
When concreting is carried out at a depth greater than 1.5 meters, temporary inclined chutes should be used. The angle of inclination of the chutes can be kept between 1:3 and 1:2, which allows for smooth flow of concrete from the top to the bottom of the chute. To facilitate the smooth flow of concrete, a small amount of free water can be used at intervals to lubricate the path of the flow of concrete. The delivery end of the chute should be placed as close as possible to the point of deposit to avoid any segregation.
In the case of deep foundations and rafts with a thickness greater than 1 meter, segregation of concrete may occur near the bottom or in the center if proper supervision is not provided during the concreting process. Advanced methods of testing, such as ultrasonic testing, can be used to detect segregation. In case of doubt, random ultrasonic testing should be conducted, and if segregation is detected, the designer’s opinion should be sought. If segregation is found, it can be rectified by pressure grounding with special chemical compounds.
After any defect has been rectified by pressure grouting, a core test should be conducted to ensure that the strength of the concrete has reached the desired level.