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Civil Engineering - Interview Questions
What do you understand by bleeding 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.
 
Effects of Bleeding of Concrete : 

* The main effect of is that the concrete mixture loses its homogeneity, Which results in weak and porous concrete.
* If the bleeding water carries with it more amount of the cement particles, a layer of laitance will be formed.
* It affects the bond between hardened cement paste and aggregates for reinforcement on account of higher water cement ratio.
* Due to bleeding the ability of pumping is very much reduced, which makes it difficult where concrete is to be pumped for higher elevations.
* Such concrete is easily prone to the micro cracking due to shrinkage stresses caused by dissipation of heat of hydration and drying shrinkage.
 
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.
* Use of air entraining admixtures can also reduce the bleeding in 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.
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