What Is A Concrete Segregation? Causes Of Segregation In Concrete
What Is A Concrete Segregation?
Segregation in concrete is the separation of the cement paste and aggregates during handling and placement. This can occur due to over-vibration or compaction, resulting in the cement paste rising to the top and the aggregates settling at the bottom.
This can negatively impact the strength and durability of structures. Proper handling, transportation, and placement, as well as proper mixing, can help prevent segregation. However, if segregation occurs in transit, the concrete should be remixed before being placed.
If the initial setting time has passed, the concrete should not be used. Segregation can also lead to the formation of honeycomb in the concrete, which can affect its strength and porosity.
Understanding and preventing segregation is important for those constructing buildings or working on construction sites. A concrete sample that shows signs of separation of coarse aggregates from the rest of the ingredients may indicate segregation, depending on the wetness or dryness of the mix.
Types Of Segregation Of Concrete
Concrete segregation refers to the separation of different components within the mix. There are two main types of segregation:
1) Coarser and heavier particles separating from the rest of the mix due to faster settling or travel along a slope, which can occur when the mix is too dry.
2) Separation of grout (water and cement) from the other materials due to its lower specific gravity, which can occur when the mix is too wet. Proper mixing and batching can prevent segregation from occurring in well-designed concrete.
Causes Of Segregation In Concrete
Proper care during the pouring of concrete can prevent segregation. Causes of segregation include uneven proportion of concrete ingredients, insufficient mixing time, poor handling of the concrete mix, and bad construction practices during concrete placement.
To avoid segregation, it is important to follow the concrete mix design, use a batching machine for accurate proportioning, mix the ingredients properly for the appropriate duration, handle the concrete carefully during pouring, and avoid pouring from a height or through a chute.
Additionally, testing of aggregates can ensure that the proper proportion of ingredients is used.
How To Prevent Concrete Segregation
When placing concrete at a depth greater than 1.5 meters, temporary inclined chutes should be used. The angle of the chutes should be between 1:3 and 1:2 to allow the concrete to flow smoothly to the bottom.
Adding a small amount of water at the top of the chutes can also help with this. The delivery end of the chute should be as close as possible to the point of deposit.
In deep foundations and rafts that are more than 1 meter thick, there is a risk of segregation of concrete near the bottom or in the center if proper supervision is not in place.
This can be detected through advanced testing methods such as ultrasonic testing. In case of doubt, random ultrasonic testing should be conducted, and if segregation is present, the designer’s opinion should be sought.
This type of segregation can be corrected by pressure grouting with special chemical compounds. After any defects have been corrected by pressure grouting, a core test should be performed to ensure that the strength of the concrete has reached the desired level.
Effects Due To Segregation Of Concrete
The negative consequences of segregation in concrete include the formation of air pockets (honeycombing), increased porosity, reduced resistance to compression, and rough surface finishes. Honeycombing occurs when large aggregates become segregated and cause voids in the set concrete. This creates a weak point that can easily crumble or be damaged under pressure or severe weather conditions.
The increased porosity resulting from segregation leads to a decrease in concrete strength, meaning it is much less able to resist compression forces. This means it can be more easily cracked or crumbled due to external pressures.
Finally, segregation causes rough surface finishes which lead to an unsightly appearance and make it difficult for other materials such as paint or stucco to properly adhere.