The bleeding in concrete is referred as water gain. It is a particular form of segregation. This is a phenomenon in which some of the water from the concrete comes out to the surface of the concrete.The bleeding phenomenon is mainly found in the wet mix.
The rate of bleeding will increase with time. The bleeding can be considered as an inherent phenomenon in concrete.
Bleeding can be defined as the accumulation of water at the surface which will accompany the sedimentation of freshly mixed concrete. If the rate of evaporation from the concrete is equivalent to the rate of bleeding, the bleeding will not be considered harmful.
Bleeding can be called as a function of:
- Air Velocity
If the rate of bleeding is less than the rate of evaporation, the surface will become dry. This will result in cracks on the surface of the concrete.
The bottom restraints will encourage such types of cracks. The bleed water will collect under the longitudinal reinforcement. When this water evaporates, voids are formed that leads to a weak bond between the reinforcement and the surrounding and finally the permeability of the concrete is increased.
Types of Bleeding
There are mainly different types of bleeding that is observed in fresh concrete. They are:
- Normal Bleeding
- Channel Bleeding
The Normal Bleeding refers to the process of uniform seepage of water over the entire surface of the structure.
The channel bleeding is a process by which water rises through certain specific paths called as the channels.
Not all the bleed water will reach the concrete surface. Some of them may rise, while some will be trapped under the aggregates and the reinforcing. This can result in the weakening of the bond between the paste and those elements.
Causes of Bleeding
The primary cause of excessive water coming to the surface during the concrete placement is rooted in the concrete mix. Initially, the air content is checked. A lower of entrained air content will cause excessive bleeding.
If the air is within the design range, then we must look for a lack of fines as a result of a low cement content.
Read Article on: Effect of Concrete Ingredients on Bleeding
Test For Bleeding in Concrete
The procedure to determine the bleeding rate of concrete mix is as follows:
- A cylindrical container of capacity 0.01metre cube that has an inside diameter of 250mm and an inside height of 280mm is used.
- A sample of freshly mixed concrete is filled in 50mm layers for a depth of 250mm (5 layers)
- Each layer is tamped by giving strokes and smoothened the surface
- The specimen is weighed
- Knowing the water content in 1metre cube of concrete, the quantity of water in the cylinder container is also calculated.
- Cover the container with a lid. Water that is accumulated at the top is drawn by means of pipette at 10 minutes interval for the first 40 minutes, till the bleeding ceases.The bleeding water is then weighed.
% of bleeding water = Total quantity of bleeding water / total quantity of water in the sample