Bleeding, Segregation and Harshness Defects in Fresh Concrete

Fresh concrete should be uniform, cohesive, workable, and easy to place and compact. However, improper mix proportioning, excessive water, poor aggregate grading, inadequate mixing, and improper handling can produce several defects. Among the most common defects in fresh concrete are bleeding, segregation, and harshness. These defects can adversely affect the strength, durability, appearance, and performance of hardened concrete.

1. Bleeding

Bleeding is the phenomenon in which water rises to the surface of freshly placed concrete because the solid particles, particularly cement and aggregates, settle downward under gravity. As the concrete settles, excess water moves upward and forms a thin layer of water on the surface.

Bleeding is generally associated with excessive mixing water, low cement content, poor aggregate grading, and insufficient fine material. Excessive bleeding can create water channels within the concrete and may result in a weak and porous surface layer. It can also reduce the bond between concrete and reinforcement when water accumulates below reinforcing bars.

To reduce bleeding, the concrete should have proper water-cement ratio, adequate cementitious material, well-graded aggregates, and sufficient fines. Proper compaction and finishing practices are also important. Finishing should not be carried out while excess bleed water is present on the surface.

2. Segregation

Segregation refers to the separation of the constituents of fresh concrete so that the mixture loses its uniformity. Normally, cement paste, fine aggregate, and coarse aggregate should remain uniformly distributed. During segregation, heavier coarse aggregate may settle while cement paste and water move away from it.

Segregation can occur due to excessive water, poorly graded aggregates, improper mix proportions, excessive vibration, dropping concrete from excessive height, or improper transportation and handling.

Segregated concrete produces non-uniform sections and may contain zones with excessive coarse aggregate or cement paste. This can reduce strength and durability and may increase the risk of honeycombing and voids.

Segregation can be controlled by using properly proportioned and well-graded aggregates, maintaining an appropriate water-cement ratio, mixing the concrete thoroughly, and avoiding excessive vibration. Concrete should also be transported and placed carefully.

3. Harshness

Harshness describes a condition in which fresh concrete is rough, stiff, and difficult to mix, place, compact, and finish. Harsh concrete does not have adequate cohesion and often contains insufficient mortar to properly surround the coarse aggregate particles.

Harshness may result from poor aggregate grading, excessive coarse aggregate, insufficient cement paste, inadequate fine aggregate, or an unsuitable water-cement ratio. The mixture may appear dry and rough, and coarse aggregate particles may be difficult to move into position during finishing.

Harsh concrete can lead to poor compaction, honeycombing, surface defects, and reduced durability. Proper aggregate grading, adequate paste content, correct mix proportions, and suitable workability help prevent harshness.

Conclusion

Bleeding, segregation, and harshness are important defects in fresh concrete. Bleeding involves the upward movement of water, segregation involves separation of concrete ingredients, and harshness produces a rough and difficult-to-work mixture. Proper mix design, controlled water content, well-graded aggregates, adequate mixing, careful transportation, and appropriate compaction are essential for producing uniform and workable fresh concrete. Controlling these defects at the fresh-concrete stage helps ensure better strength, durability, and long-term performance of the finished structure.

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