Entlüftungs- und Verdichtungsverhalten von Frischbeton

De-airing and compaction behaviour of fresh concrete – fundamental mechanisms and building component properties

The compaction behaviour of fresh concrete is a decisive characteristic in the production of high-performance and durable concrete components. It affects not only the mechanical properties and durability of the finished component, but also its aesthetics. Until now, however, the compaction behaviour can only be described indirectly by defining a consistency class of the fresh concrete, assuming that the compaction behaviour improves with increasing consistency. This approach, however, is impeded by the use of innovative concretes with reduced water content, as such concrete may have significantly altered rheological properties, which greatly affect the ability of air bubbles to rise in the fresh concrete. The objective of this paper was therefore to present new investigations on the fundamental mechanisms of air bubble rise in cement paste and mortar. This is followed by large-scale component tests in which the influence of compaction frequency and compaction time on surface porosity, compressive strength and microstructure porosity in hardened concrete were investigated. By linking the investigations of the basic mechanisms of air bubble rise with the transferability of the results to large-scale practical construction tests for concrete compaction, new insights into proper concrete de-airing can be demonstrated.