Segregation in concrete is one of the most common problems observed during mixing, transportation, placing, and compaction. Good concrete should have all ingredients uniformly distributed to form a homogeneous mix. However, due to improper handling or mix proportioning, separation of materials may occur.
Concrete consists of cement, coarse and fine aggregates, water, and admixtures. These materials have different specific gravities and particle sizes. If not properly controlled, they tend to separate from each other. This phenomenon is known as segregation of concrete.
What is Segregation in Concrete?
Segregation in concrete is the separation of constituent materials of concrete, resulting in non-uniform distribution of aggregates, cement paste, or water.
Generally, three types of segregation may occur:
- Separation of coarse aggregates from mortar.
- Separation of cement paste from aggregates.
- Separation of water from the mix (leading to bleeding).
Segregation reduces strength, durability, and surface quality of hardened concrete.
Effects of Segregation in Concrete
The following adverse effects occur due to segregation:
- Honeycombing in concrete members.
- Increased porosity and permeability.
- Reduction in compressive strength.
- Poor surface finish.
- Formation of voids and cavities.
- Lower durability and increased risk of reinforcement corrosion.
Therefore, segregation directly affects both structural performance and service life of concrete structures.
Causes of Segregation in Concrete
Segregation occurs due to improper mix design, excessive handling, or poor workmanship. The major causes are:
- Difference in specific gravity and particle size: Cement (~3.15), aggregates (2.6–2.7), and water (1.0) have different densities. High water-cement ratio reduces cohesion and allows heavier aggregates to settle.
- Excessive vibration: Over-vibration using mechanical vibrators causes heavier particles to settle at the bottom while cement paste rises to the top.
- Dropping concrete from excessive height: Placing concrete from heights greater than 1 meter causes coarse aggregates to separate.
- Poor mix proportioning: Lack of proper grading of aggregates leads to insufficient interlocking and increased segregation.
- High slump concrete: Very wet or pumpable concrete increases risk of segregation.
- Improper transportation: Long-distance hauling, rough handling, or improper chutes promote separation.
- Inadequate mixing: Insufficient mixing time results in non-uniform distribution of materials.
- Underwater concreting: Placement in water increases likelihood of material separation.
- Congested reinforcement: In heavily reinforced members, improper placing causes aggregates to block and separate.
How to Prevent Segregation of Concrete
Segregation can be prevented by proper mix design, controlled placement, and correct compaction techniques.
- Proper mix design: Use well-graded aggregates and maintain correct water-cement ratio.
- Controlled slump: Avoid excessively high slump unless using suitable admixtures.
- Limit free fall height: Concrete should not be dropped from more than 1 meter.
- Proper vibration: Use vibrators for appropriate duration only. Avoid over-vibration.
- Use air-entraining or pozzolanic admixtures: These improve cohesion and reduce segregation risk.
- Ensure adequate mixing time: Follow proper mixing duration in batching plants or mixers.
- Use proper transportation methods: Avoid excessive handling or long-distance free flow.
- Remix if necessary: If segregation is observed, remix the concrete before placing.
Conclusion
Segregation in concrete is a serious quality control issue that affects strength, durability, and surface finish. It mainly occurs due to improper mix proportion, excessive vibration, high water content, or poor handling. By maintaining proper mix design, controlled placing height, correct vibration practice, and well-graded aggregates, segregation can be effectively minimized. Preventing segregation ensures uniform concrete quality and long-term structural performance.