Bleeding in concrete, causes, remedies

Bleeding in concrete is a phenomenon that occurs in concrete when the mixture of cement, water, and aggregates is poured into a form or structure, and the water rises to the surface of the concrete, leaving behind a layer of cement paste. 

This layer of cement paste on the surface can cause a range of problems, including reduced durability, surface cracking, and a weakened bond between the surface of the concrete and any subsequent layers or coatings.

Causes of Bleeding in Concrete

1. Excess Water: Adding too much water to the concrete mix can increase the risk of bleeding. When the mix contains too much water, the excess water rises to the surface and separates from the rest of the mix.
2. Low Cement Content: If the cement content in the mix is too low, it can result in a mix that is more prone to bleeding. This is because the cement helps to bind the mix together and prevents the separation of water.
3. High Aggregate Content: If the mix contains a high proportion of coarse aggregates, it can make it more difficult for the mix to be compacted, resulting in a mix that is more prone to bleeding.
4. High Ambient Temperature: High temperatures can increase the rate of evaporation, causing the water in the mix to rise to the surface more quickly.
5. Prolonged Vibration: Over-vibrating the concrete during the placement process can cause the water to rise to the surface and bleed.
6. Inadequate Finishing: Improper finishing techniques can result in a weakened surface layer that is more prone to bleeding.

Effects of Bleeding on concrete

1. Reduced Durability: The layer of cement paste that forms on the surface of the concrete as a result of bleeding is weaker and less dense than the rest of the concrete. This can reduce the overall durability and strength of the concrete, making it more prone to cracking, spalling, and other forms of damage.
2. Surface Cracking: The layer of cement paste that forms on the surface of the concrete can crack as it dries, leading to surface cracking and other forms of damage.
3. Weakened Bond: The layer of cement paste that forms on the surface of the concrete can weaken the bond between the surface of the concrete and any subsequent layers or coatings that are applied.
4. Surface Discoloration: The layer of cement paste that forms on the surface of the concrete can also result in discoloration, making the surface of the concrete appear blotchy or uneven.
5. Increased Porosity: The layer of cement paste that forms on the surface of the concrete can be more porous than the rest of the concrete, increasing the risk of moisture penetration and other forms of damage.

How to reduce bleeding in concrete

1. Use a Suitable Mix Design: Choosing a mix that is properly proportioned with the right amount of cement, aggregates, and water is critical to reducing bleeding. A low water-to-cement ratio can help to minimize the amount of water in the mix and reduce the risk of bleeding.
2. Use Appropriate Aggregate Sizes: Using a mix of fine and coarse aggregates can help to reduce bleeding by improving the overall compaction of the mix. This can also help to prevent the formation of voids that can lead to bleeding.
3. Control the Ambient Temperature: Controlling the ambient temperature during the mixing, placing, and curing processes can help to minimize the risk of bleeding. Lower temperatures can slow down the rate of evaporation, reducing the amount of water that rises to the surface.
4. Avoid Over-Vibrating: Over-vibrating the concrete during the placement process can cause the water to rise to the surface and bleed. Using appropriate vibration techniques and equipment can help to minimize the risk of bleeding.
5. Proper Finishing Techniques: Proper finishing techniques can help to create a smooth, level surface that is less prone to bleeding. This can include using a bull float or trowel to level and smooth the surface of the concrete.
6. Use Concrete Admixtures: Certain concrete admixtures, such as air-entraining agents, can help to reduce the risk of bleeding by improving the overall workability and flow of the concrete.