6.3 : Segregation in Fresh Concrete

Segregation in fresh concrete is a phenomenon where the components of the concrete mix separate, leading to uneven distribution and compromised structural integrity. This separation typically occurs when concrete is subjected to excessive horizontal movement within forms, or when it is dropped from considerable heights or forced through narrow, winding paths. As a result, heavier coarse aggregate particles settle at the bottom, while lighter, finer materials such as cement and water rise to the top.

Another form of segregation manifests as the grout (consisting of cement and water) detaches from the mix. This separation not only affects the concrete's appearance but also weakens its structural capabilities. To minimize the risk of segregation, concrete should ideally be prepared as close as possible to its final placement location, thereby reducing the need for extensive transportation that can disrupt the mix.

Additionally, overworking the concrete, such as prolonged use of vibrators to spread concrete over a large area, can lead to segregation. A practical method to test for potential segregation involves vibrating a concrete sample in a cube mold for about 10 minutes. Upon removing the mold, the distribution of coarse aggregate can be inspected to assess the degree of segregation. These preventive measures and tests are crucial for ensuring the uniformity and strength of the concrete structure.