ABSTRACT
Abstract Application of vibration, of appropriate amplitude and frequency, to fresh concrete reduces its yield value to zero and permits it to flow under its own weight to pass between reinforcement, fill formwork and release air bubbles. Previous work suggested that the peak velocity is the most important characteristic of the vibration and a re examination of results obtained in a vertical pipe apparatus confirms that there is a linear relationship between fluidity of the vibrated concrete and the peak velocity (given by amplitude x frequency) of the vibration. The proportionality constant is termed the vibrational susceptibility and is characteristic of the material, being influenced by the workability of the unvibrated concrete, but decreases as frequency increases. The greatest fluidity and hence the most rapid placement of concrete is therefore achieved at low frequencies (16-30 Hz) rather than at the frequencies commonly employed in industrial vibrators for concrete (50-200 Hz). Keywords: Fresh concrete, rheology, vibration
1 Introduction
Vibration has long been recognised as necessary for effective compaction of concrete and arising from much early work done on the effect of vibration on the properties of the hardened concrete recommendations for current practice exist in many countries. The general consensus seems to be that the higher the acceleration at moderate frequencies the more effective the vibration. However, it is much more fundamentally sound to base recommendations on the performance of fresh concrete and advances in the understanding of the rheology of fresh concrete have permitted this. This paper discusses the relationship between rheology and the behaviour under vibration together with some important implications.
