What is the main risk associated with over-vibration of concrete during placement?

The main risk associated with over-vibration of concrete during placement is segregation. Vibration is used to consolidate concrete, removing entrapped air and allowing the concrete to flow and fill the formwork completely. However, excessive vibration can cause the heavier aggregate particles to settle to the bottom of the form, while the lighter cement paste and water rise to the surface. This separation of the concrete components is known as segregation. Over-vibration disrupts the carefully designed mix proportions, leading to non-uniform concrete with variations in strength, durability, and surface finish. The bottom portion of the concrete element may become aggregate-rich and cement-paste deficient, while the top portion may become weak and prone to scaling due to an excess of water and cement paste. In severe cases, over-vibration can also cause excessive bleeding, where water migrates to the surface, carrying fine cement particles with it and creating a weak, dusty layer called laitance. Proper vibration techniques involve inserting the vibrator vertically, allowing it to penetrate deeply into the concrete, and withdrawing it slowly to avoid creating voids. The duration of vibration should be just long enough to consolidate the concrete, typically until the surface becomes glossy and air bubbles cease to appear. Over-vibrating beyond this point can lead to segregation. For instance, in a wall pour, over-vibration can cause the coarse aggregate to settle at the bottom, making the upper part of the wall weak and susceptible to cracking.