Concept of Bleeding of Concrete.
concrete mix design is a very precise science to achieve design concrete strength. If concrete ingredients are not mixed properly many concrete related problems can result and affect the strength and durability of concrete. Bleeding is one of the concrete related problems. It is mostly observed in a highly wet mix and badly proportioned concrete ingredients after placing of the fresh concrete. Free water in a mix rises upward to the concrete surface due to the settlement of solid particles by gravity action. This process is known as ‘Bleeding of Concrete’.
In certain situation though bleeding water does not come up to the surface but bleeding does take place. The bleeding water gets trapped on the underside of coarse aggregates or of reinforcements. This is known as internal bleeding.
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Effects of Bleeding of Concrete
- The main effect of is that the concrete mixture loses its homogeneity, Which results in weak and porous concrete.
- It affects the bond between hardened cement paste and aggregates for reinforcement on account of higher water cement ratio.
- Such concrete is easily prone to the micro cracking due to shrinkage stresses caused by dissipation of heat of hydration and drying shrinkage.
- If the bleeding water carries with it more amount of the cement particles, a layer of laitance will be formed.
- Due to bleeding the ability of pumping is very much reduced, which makes it difficult where concrete is to be pumped for higher elevations.
How to Prevent Bleeding of Concrete
- Bleeding of concrete depends on the properties of cement. Bleeding gets decreased by increasing the fineness of cement because finer particles hydrate earlier and also their rate of sedimentation is lower.
- The properties of cement are not an only soul factor influencing the bleeding of concrete.
- The presence of fine aggregates and higher water cement ratio also lead to bleeding. A higher rate of the water cement ratio can lead to excessive bleeding and if evaporation of water from the surface of the concrete is faster than the bleeding rate plastic shrinkage cracking may result.
- Use of air entraining admixtures can also reduce the bleeding in concrete.
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