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Effect of mixer type on fresh and hardened properties of Ultra-High-Performance Concrete (UHPC)

Ultra-high-performance concrete (UHPC) appears to be one of the key enabling construction materials of the 21st century. The term UHPC concrete refers to a new class of advanced cementitious composite materials with superior mechanical and durability properties when compared to conventional concrete. The production method of UHPC requires effective mixing of highly structured active components (reactive cement, micro silica, fly-ash and others). The development of the complex mixture of UHPC concrete occurs in lab conditions with small batches and lab-sized mixers. However, once a mix has been developed the large-scale production has to be transferred into a factory-based production. This means that the batch size is significantly increased, as well as the mixer is not the same as the one used in laboratory-based trials. This research paper focuses on the observable differences of the fresh and hardened UHPC properties that occur when different mixers and batch sizes are used. Tests, such as slump flow test, time for slump flow to reach a certain diameter and compressive strength tests were used to evaluate the differences between different mixers and batch sizes. The breaking time of the mix – the time it takes for the mix to become visually fluid and wet, was also noted. It was found that not only is the chosen mixer an important factor when scaling up the production from laboratory-based to larger-scale production, but also the chosen superplasticizer can greatly affect the fresh, as well as the hardened properties of a UHPC mix, when different mixers and batch sizes are used. It was also found that increased mixing speed reduced the breaking time of the mix for recipes with a high slump flow value. This behaviour, however, was not noticed for mixes with lower slump flow value.