Defects in concrete is caused by varies reason like separation of aggregate, poor handling, improper mixing, improper selection of material etc. Defects in Concrete and how to avoid them is explained below,
Segregation in concrete
Bleeding in concrete
Honeycombing in Concrete
1. Segregation in concrete:
Segregation in concrete is a case of particle segregation in concrete applications, in which particulate solids tend to segregate by virtue of differences in the size, density, shape and other properties Segregation usually implies separation of:
- coarse aggregate from fine aggregate,
- paste from coarse aggregate, or water from the mix and the ingredients of the fresh concrete no longer remain uniformly distributed.
Causes of Segregation in concrete
- Poorly graded aggregate & excessive water content is the major cause of segregation.
- A badly proportioned mix, where sufficient matrix is not there to bond and contain the aggregate cause aggregates to settle down.
- Insufficiently mixed concrete with excess water content shows a higher tendency for segregation.
- When height of dropping of concrete is more (ex. In case of concreting long column) it will result in segregation.
- If a mixer used for mixing concrete is badly designed or a mixer with worn out blades, then the concrete shows a tendency for segregation.
- If a high slump concrete or pump able concrete are not compacted with sufficient care then it is likely to result in segregation of concrete.
- Immediate working on the concrete on placing, without any time interval is likely to press the coarse aggregate down, which results in movement of excess matrix or paste towards the surface.
- Chances of segregation are more when concrete is to be placed under water.
How to Avoid Segregation in concrete:
- The concrete mix should be properly designed with optimum quantity of water i.e. not too wet nor too dry.
- Make sure the concrete is properly mixed at the correct speed in a transit mixture for at least two minutes.
- Regularly check the performance of mixer with respect to adequate uniformity of distribution of constituents in each batch.
- Transport the concrete mix correctly. Choose the shortest route for transportation of concrete mix.
- Place the concrete in its final position as soon as possible. Never place a concrete from large heights.
- Form-work should be water tight so that no paste should leak from the forms. Do not vibrate form work
- Do not allow concrete to flow Use the vibrator correctly and never use the vibrator to spread a heap of concrete over a large area. Also Vibrate for just the right time-not too long, not too less
- Use chemical admixtures such as air en-training agent in the mix. Entrained air reduces the danger of segregation
- If any segregation is observed in concrete, remixing should be done so to make it homogeneous again
2. Bleeding in concrete:
Bleeding in concrete refers to the process wherein free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water. Some bleeding is normal but excessive bleeding can be problematic.
Effect of Bleeding on Concrete:
As mentioned above, water move upwards in bleeding, sometimes with this water certain amount of cement moves along with water to the concrete surface. When the top surface is worked up with the trowel, the aggregate goes down and cement paste forms at the top surface this is called ‘Laitance in concrete. Due to the formation of Laitance, structures may lose its wearing capacity and decreases its life.
- Water while moving from bottom to the top, forms continuous channels. Due to these channels, concrete becomes permeable and allow water to move, which forms water voids in the matrix and reduces the bond between aggregate and the cement paste.
- Forming of water at the top surface of concrete results in delaying the surface finishing.
- Concrete becomes permeable and loses its homogeneity.
- Excessive bleeding breaks the bond between the reinforcement and concrete.
Measures to Control Bleeding:
- Bleeding in concrete is controlled by Adding minimum water content in the concrete mix.
- Encouraging the use of air en-training admixtures in the mix.
- By adding more cement in the mix
3. Honeycombing in Concrete
When concrete is poured into the structural element forms it does not just flow in like water and fill up the forms to the top. If it is not vibrated properly it may leave voids called “honeycombing.” The exposed aggregate leaves a honeycomb look and hence the named ‘Honeycombing of concrete’.
- A lack of integrity at the perimeter form boards at the time the concrete is being poured.
- Improper cement to water ratio that causes poor workability
- Poor consolidation practices or inefficient means of vibration
Remedies of Honeycombing in concrete:
- Concrete fall should be kept to the minimum.
- Use a mix with appropriate workability for the situation in which it is to be placed.
- Ensure the mix has sufficient fines to fill the voids between the coarse aggregate
How to Fix Honey Combing:
- The common approach includes:·
- Isolate the affected area by removing layers of honeycombing until suitable concrete is exposed.
- Thoroughly clean the area to be repaired and remove all dirt and loose aggregate. Wet the cleaned area prior to applying non-shrink grout