Bleeding of Concrete – Types | Effects | Causes | Measurement and Prevention

Bleeding of Concrete

Bleeding of Concrete: concrete has consisted of mainly four elements are Cement, Aggregate (Coarse and fine), Water, and Admixtures. The proportion of these elements is called concrete mix design. The mixing of their ingredients is very important. If these are not mixed properly, flaws in structure may occur which will cause a reduction in the strength and durability of concrete.

Bleeding is one of the concrete problems which occurs after laying and compaction fresh concrete if the concrete is not designed according to design. It is predominant in very high water-cement ratio concrete. Excess water in the concrete mix will rise upward to the concrete surface due to the settlement and solidification of cement. This process is called bleeding.

The water that came up to the surface makes a paste called “laitance” which on future drying makes a powder-like material called dusting of concrete.

Types of Bleeding

There are two types of bleeding

1. Normal or Uniform bleeding: In this water comes up uniformly through the surface.
2. Non-Uniform bleeding: In this water comes out in patches. It is more harmful than uniform bleeding.

Bleeding is not harmful if the rate of evaporation is equal to rate of bleeding. Normal bleeding helps in improving workability of concrete. Bleeding also replaces evaporation loss. Nominal bleeding prevents plastic shrinkage also. Use of highly wet aggregate increases bleeding in concrete.

Segregation of concrete

The process of separation of solid particles especially coarse aggregate from fresh concrete resulting in uneven casting of structure and reduction in strength and durability is called Segregation of Concrete. It is caused due to a very high slump concrete.
Transportation, handling, and placing of concrete cause a jerking effect which causes segregation. If concrete is segregated during transportation, it should be remixed before placement.

If the height of pouring of concrete is more than 1.5-meter, segregation will occur. Use of a low water-cement ratio and the use of finer cement reduce this. The casting of concrete should be done in batches to prevent segregation. Direct pouring of concrete also causes segregation. That’s why concrete should be poured in a 1:3 or 1:2 slope.

Also Read: 11 steps in Construction of Multi Storey Buildings

Honeycombing of concrete

After the removal of shutting, there are hollow spaces left in the concrete surface which looks like honeybees’ nests. This is known as the honeycombing of concrete. Improper vibration and a bad slump of concrete cause this problem.
Junction points like beam to beam or beam to the column are the spots where this is predominant where there is jumbling of reinforcement.

The presence of more amount of larger aggregate from average size also hinders the reachability of concrete mix hence entrapping more air hence causing the honeycomb effect.
The application of proper vibration and use of a thin needle of 25mm or less improves the reachability of concrete hence fewer air voids. Use of aggregate less than 20mm helps is the workability of concrete hence fewer air voids.

Effects of bleeding in concrete

1. Bleeding reduces cement particles which get deposited on top of the surface as the water evaporates hence causing the problem of dusting and significantly reducing the strength of concrete.
2. Top surface takes longer for the setting than the bottom.
3. Concrete losses its homogeneity.
4. The structure is more prone to cracking.
5. Particle distribution of concrete gets disturbed.
6. Internal bleeding may also occur which will cause a reduction in bond strength of reinforcement with concrete.
7. Delay in finishing work.
8. Increases permeability of top surface.

Difference between Bleeding and Segregation

Causes of Bleeding of Concrete

1. Segregation is the main reason for the bleeding. Due to settlement of heavy aggregate in concrete causes water to rise to the surface.
2. Bleeding also depends on the quality of cement and aggregate. Use of rounded aggregate will also cause segregation ultimately leading to bleeding.
3. Use of excess admixture.
4. Improper aggregate proportion also causes bleeding.

Measurement of bleeding of concrete

1. Take fresh concrete of known water-cement ratio and pour it in a cylinder as 5 layers of 50mm. Each layer should be compacted by 60 blows.
2. After compacting the final layer, level the top surface.
3. Note the time at which levelling is completed.
4. Measure and collect the amount of water accumulated after 10 minutes.
5. After 40 minutes of the test collect the water after every 30 minutes.
6. Perform the experiment for 3 hours.

\[\text{Bleeding=}\frac{\text{B}}{\text{W}}\text{ }\!\!\times\!\!\text{ 100 }\!\!%\!\!\text{ }\]

Where,

B=  Ammount of water bleeded

W= Ammount of water used for making of concrete.

Prevention of bleeding of Concrete

1. By reducing the water-cement ratio.
2. Use of water-reducing admixtures reduces water content but didn’t affect the workability of concrete.
3. Use of finer cement helps in reducing porosity hence less capillary action.
4. Finer cement also helps in reducing permeability.
5. Proper mixing of concrete improves the binding of materials hence reducing air voids.
6. Increase amount of fine sand reduces air voids hence reducing capillary action.
7. Use of air-entraining admixtures can also reduce the bleeding in concrete.
8. Pozzolana elements like Fly Ash, Slag, and Silica Fumes reduce bleeding by improving bond strength.
9. Proper compaction helps in reducing bleeding.

Also Read: How to calculate cement sand and aggregate quantity in concrete?

FAQ’s on Bleeding of Concrete