Analyzing the impact of concrete composition on air void distribution in fresh concrete using ultrafast X-ray computed tomography

Understanding the compaction behaviour of fresh concrete is essential, as improper de-airing can lead to unwanted air voids, diminishing mechanical strength and posing durability issues. Despite conventional approaches focusing primarily on concrete workability, this paper investigates the interaction of concrete composition - including cement (paste) content, superplasticizer dosage, aggregate size distribution and compaction duration. As concrete is a highly opaque system with time dependent properties due to both thixotropy as well as chemical cement hydration, an ultrafast X-ray computed tomography (UFXCT) system was used to study air void characteristics in fresh concrete before and after compaction. The study reveals that while the initial concrete workability affects formwork filling, factors such as paste content and compaction duration significantly influence the air void distribution and reduction, particularly for voids trapped in the concrete bulk, thus affecting the strength and durability of the casted concrete element. Incorporating these insights into practical concrete processing procedures could lead to more targeted compaction strategies, ultimately enhancing concrete durability and sustainability.