Increase in Concrete Air Content due to Excessive Internal Vibration

Internal vibration is a commonly used method for consolidating fresh concrete. Proper vibration causes entrapped air voids to float to the surface of the concrete or collapse at the water-air interface. However, if vibration is excessive, it may introduce air into the concrete under certain circumstances. The consequence of this is an uneven distribution of air voids, resulting in heterogeneous concrete and subsequently undesirable strength and freeze-thaw durability. This study investigated the mechanism by which air voids are induced in concrete due to vibration. Concrete mixtures were prepared and vibrated at several frequencies for a range of durations. Air-void distribution within different zones from each sample was obtained using the linear traverse method. It was found that rotary movement of a high-frequency, vertical vibrator created a vortex inside the liquefied fresh concrete that sucked air into the concrete from the surface. In general, increasing the frequency of vibration will cause more air addition, and the amount of air added with increasing duration of vibration is dependent on the amount of unreacted air-entraining agent (AEA) remaining in the system.