Bleeding in concrete detail explanatation

bleeding in concrete

1. Bleeding in concrete Bleeding in concrete is the result of excess of water being rise to the surface of concrete due to settlement of aggregate. Bleeding in concrete is referred as water gain. It is a particular form of segregation. Bleeding is not harmful if the rate of evaporation is equal to rate of bleeding. Fig: Bleeding in concrete

2. Types of bleeding Normal or Uniform bleeding:  In this water comes up uniformly through the surface.  Normal bleeding helps in improving workability of concrete. Rate of  evapouration of water on the surface of concrete is equal to rate of bleeding. Channel bleeding : The channel bleeding is a process by which water rises through certain specific paths called as the channels.  It is more harmful than uniform bleeding. Rate of  evapouration is less than rate of bleeding.

3. Causes of Bleeding in Concrete High value of w/c ratio. Generally, 0.6 or more. Too much finishing. Segregation. Segregation is the phenomena in which heavy aggregate particles settles down, due to settling of heavy particles, water rises up to the surface and forms a layer. Depth of placed concrete. Insufficiently mixed concrete. low  cement composition. Higher value of fineness modulus( coarser the aggregate).

4. Effects of Bleeding in concrete Bleeding is responsible for causing permeability in concrete. Reduces the bond between the reinforcement and concrete. The accumulation of water at the top, results in delayed surface finishing. Reduces bond between the aggregate and cement paste and hence reduces the strength of concrete. If the rate of evapouration from the surface of bleeding water is faster than bleeding rate, plastic shrinkage would develop. Formed the laitance at the top surface. Laitance is the weak, powdery layer of cement dust, and sand fines. fig(a): bond between R/F & concrete fig(b): shrinkage in concrete fig(c): Laitance fig(a)

5. Methods of Reducing Bleeding Add minimum water content in the concrete mix. Increase the cement content. Mix the concrete properly. Use fly ash or other supplementary cementitious materials. Proper proportion of concrete. Increase the amount of fine aggregate. ASTM (American Society for Testing and Materials) rate of bleeding is measured by following procedure: Firstly, take sufficient quantity of freshly mixed concrete of known water content A sample of concrete is placed in a container of 250mm diameter and 280mm height. Fill the Cylindrical container with concrete mix in 3 layers. Each layer should be tamped or compacted by giving minimum 25 strokes using tamping bar after filling.

6. Hence, Bleeding = total sum of bleed water/net mixing water Draw off the water accumulated at the top surface of concrete using a pipette for every 10 minutes & collect the water. This should be done for 1 st 40 minutes and after this continuously for every 30 minutes until bleeding stop.