Concrete bleeding is a problem that can affect any batch of concrete that has been improperly measured and mixed. It is the movement of water to the surface of the concrete after it has been poured. Water collects on the surface of a concrete slab or structure when the water-to-cement ratio of the mix is too high (when too much water has been added to the batch).
Bad proportioning of materials in a concrete batch is the main cause of bleeding. Too much water causes the concrete to segregate – a phenomenon where the heavy aggregates sink to the bottom of the mix, leaving the cement, sand and water at the surface of the structure. The water then seeps to the surface of the slab or structure, which creates big problems on the resultant integrity of the concrete.
When the water travels to the surface of the structure, it carries the fine particles of cement with it. This leaves a deficiency of cement within the rest of the structure. Since cement is the main binding material, the rest of the structure dries without much bonding between materials. Bleeding causes a weak, crumbly and dry concrete structure that cannot support any weight.
Bleeding affects the workability of concrete
There are two categories of concrete bleeding; normal and excessive bleeding. These are determined by the amount of water lost through the surface of the concrete structure. Both normal and excessive bleeding can affect the final quality of the concrete and the overall durability of the structure.
Normal bleeding is that which often occurs in all concrete projects. When you see freshly-poured concrete, it will often have a very thin layer of water on its surface. This is nothing to worry about and can actually enhance the workability of the mixture. The improvement in workability makes it easier to pour and work with before it sets. It also gives the concrete a smooth finish.
Excessive bleeding is an abnormal amount of water loss through the surface of the concrete. The mixture is usually too wet before it is poured. Contractors should keep an eye on their concrete batch and add more cement or dry aggregate if they think the mixture is too runny. Excessive bleeding leads to a high loss of water and a dry, brittle concrete structure.
Concrete that has suffered from excessive bleeding may also have large voids in the centre of the structure where heavy aggregates have moved the fine particles out of their way as they sink. This creates air bubbles and empty pockets that can severely affect the strength and durability of the concrete.
The best way to prevent concrete bleeding is to use the required amount of water, as printed on the bag of cement. Contractors can also carry out a slump test to check the mixture has the correct ratio of dry to wet ingredients. Bleeding is a normal occurrence with concrete, but excessive bleeding creates long-term problems that cannot be easily repaired.
LafargeHolcim is a leading building materials and solutions company that has been operating in international markets for decades. We produce cement and aggregates for construction projects, ranging from small affordable housing developments to large-scale infrastructure projects such as high-rise buildings, dams and bridges.
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