Segregation in concrete is the term that mostly appears when concrete is placed while pouring structural elements or after the removal of formwork in the shape of a honeycomb. This happens because the separation of the concrete mix’s major components, such as aggregates and cement paste, produces a result of uneven distribution of hardened concrete.
The strength and durability of concrete consider the desired property of the concrete, which will compromise extensively if it does not provide proper attention to avoid segregation of concrete during construction. Further, the concrete will lose its potential to make itself impermeable.
Causes of Segregation in the Concrete
Concrete is a versatile construction material and can be cast in various shapes. But proper handling, transportation, and placement produce the result that is free from segregation. Mostly, segregation happens due to two major causes:
Separation of Coarser Particles
This case reflects the settlement of the coarser aggregate more quickly than the finer particles. The immediate settlement of coarse particles reflects the selection of size and shape of the aggregates, as the larger and heavier aggregates are more likely to settle and impact segregation. However, this happens when the concrete mix is too dry. Accordingly, the addition of water can make the mix more cohesive.
Separation of Cement Paste
This case exhibits the separation of cement paste, which occurs when the concrete mix is too wet.
Besides, another common cause of segregation is the application of mixing techniques. If improper mixing techniques are adopted, such as over-mixing or compaction with excessive use of a vibrator, which settled down the aggregates and brings up cement paste, or using inappropriate mixing equipment which is unsuitable for the mix. This can lead to a non-uniform distribution of the paste and aggregates, resulting in the uneven appearance of the concrete components somewhere with high concentration than other ones.
Effects of Segregation on Concrete
Segregation in concrete can have a range of negative effects on the material’s properties and performance. Here are some of the primary effects of segregation in concrete:
When the aggregates and cement paste are not uniformly distributed in the mixture, it can lead to areas of weak concrete. These weak areas are more prone to cracking and can reduce the overall strength of the structure.
The concrete in vertical elements such as columns is subjected to more segregation than in the case of horizontal elements such as beams. Columns are cast vertically in long, narrow forms, but the concrete in beams is cast in shallow, horizontal forms. Accordingly, the failure of a column in a structure is more critical than that of a floor beam.
Segregation can create voids and spaces within the concrete, which can increase its permeability. This can allow water and other substances to penetrate the concrete, which can cause corrosion of reinforcement steel and other structural damage.
Segregation can lead to uneven distribution of ingredients, which may result in some areas of the concrete being more exposed to the environment than others. This can cause some concrete areas to degrade more quickly than others, leading to reduced durability and a shorter lifespan of the structure.
Segregation can cause an uneven finish on the surface of the concrete, leading to poor aesthetics. This can be a concern for structures such as walls, floors, and countertops where appearance is essential.
Increased risk of cracking
When the paste and aggregates are not uniformly distributed, it can create areas of tension within the concrete. These areas are more likely to crack under stress, which can compromise the structural integrity of the concrete.
How to prevent segregation in concrete?
Preventing segregation in concrete is crucial to ensure its strength, durability, and overall performance. Here are some ways to prevent segregation in concrete:
Proper mix design
One of the most important factors in preventing segregation is proper mix design. The mix design should include the correct proportion of aggregates, cement, water, and other additives. The mix should be designed to be workable and to have good cohesion between the components.
Proper mixing techniques
Proper mixing techniques are essential to ensure that the components of the mix are uniformly distributed. The mixer should be of the correct size and type, and the mixing time should be sufficient to achieve a uniform mix.
Consistent water content
The water content of the mix is also critical in preventing segregation. The mix should be moist enough to ensure that the aggregates are properly coated with cement paste, but not so wet that the aggregates settle and separate.
Proper handling and transportation
Careful handling and transportation of the concrete can also prevent segregation. The concrete should be placed and compacted immediately after mixing, and it should be transported and placed using equipment that does not cause segregation.
Further, the height of the free fall of concrete should be limited while placing the concrete unless separation of coarse particles occurs, resulting in honeycombing. To avoid honeycomb, a limit of 3 to 4 feet may be adequate.
Use of appropriate aggregates
The size and shape of the aggregates used in the mix can also affect segregation. Smaller, more uniform aggregates are less prone to segregation than larger or irregularly shaped aggregates. The grading of the aggregates should also be appropriate for the intended use of the concrete.
Segregation in concrete is a significant concern for Engineers and those involved in construction and can have a significant impact on the performance and longevity of concrete structures. Proper understanding of the causes and taking actions to mitigate them, builders and engineers can ensure that their concrete is strong, durable, and able to withstand for a long time.