2. Segregation and Bleeding

The separation of coarse aggregates from the cement paste or mortar during handling. It leads to non-uniform concrete and reduced strength.

Types of Segregation: • Coarse particles settling down. • Paste separating from aggregates. • Water rising to the top (linked to bleeding).

Causes: • Poorly graded aggregates. • Excessive vibration. • Dropping concrete from height. • High water-cement ratio.

Segregation refers to the separation of different components of concrete, particularly the coarse aggregates from the cement paste or mortar. This issue can lead to a non-uniform mixture, which results in varying strength across the concrete structure.

There are several types of segregation:

1. Coarse particles settling down: When the heavier aggregate materials drop to the bottom, they leave behind a layer of weaker cement paste at the top.
2. Paste separating from aggregates: This occurs when the cement paste and aggregates do not bond properly, causing patches of different strengths within the concrete.
3. Water rising to the top (linked to bleeding): This is often caused by the excess water in the mix that moves upward, forming puddles on the surface, which can reduce the overall quality of the concrete.

Several factors can cause segregation, including poorly graded aggregates (which do not mix well), excessive vibration during handling that can displace materials, dropping concrete from significant heights, and having a high water-cement ratio in the mix that disrupts the cohesion of the materials.

Imagine making a salad with various ingredients like lettuce, tomatoes, and cucumbers. If you mix them gently, everything blends nicely. But if someone shakes the bowl too hard, the heavier cucumbers might settle at the bottom while the lighter lettuce floats on top. Just like in this salad, when concrete components separate, it can lead to a 'salad' that's uneven and not very appealing, resulting in poor-quality concrete.

Bleeding is the movement of water to the surface of freshly placed concrete.

Effects: • Formation of laitance. • Weak surface layer. • Increases permeability. • Causes water pockets below aggregates and reinforcement.

Control Measures: • Use of air-entraining agents. • Proper mix design. • Reduced water content. • Use of finer cement or mineral admixtures.

Bleeding occurs when excess water in freshly placed concrete rises to the surface. This phenomenon can have several negative effects on the concrete's properties. For instance:

1. Formation of laitance: This is a weak surface layer that can weaken the concrete's structural integrity and bonding with additional layers.
2. Weak surface layer: The layer created by bleeding may not hold up well under stress, making it vulnerable to damage.
3. Increases permeability: Water can create pathways in the hardened concrete, inviting more water or chemicals that can lead to deterioration over time.
4. Causes water pockets below aggregates and reinforcement: These pockets can lead to weak spots where the concrete does not bond well, making the overall structure less durable.

To control bleeding, certain measures can be taken, such as using air-entraining agents to improve the mix’s consistency, ensuring a proper design of the concrete mix, reducing water content, or including finer cement and mineral admixtures to enhance cohesiveness.

Think of bleeding as the way water sometimes pools on the surface of wet soil after heavy rain. Just as that excess water can cause issues for plants growing in the soil, the water rising to the surface of fresh concrete can create problems for the overall strength and durability of a building. By adjusting how we mix concrete, similar to ensuring soil has good drainage, we can prevent this 'water pooling' and make sure everything stays strong.

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