Ultra-high-performance concrete (UHPC) is a new and innovative concrete material associated with very high strength, low permeability to aggressive environment and self-compact ability. These characteristics enable innovative engineering solutions to be applied in practice which would have previously been thought to be impossible to manufacture. Many previous studies suggest relatively high dosage rates of micro silica (MS) (25% by weight of binder (wtob)). This paper focuses on optimizing and studying how the MS addition to UHPC affects the compressive strength and overall rheological properties of UHPC. The study was performed by doing compressive strength tests as well as concrete slump flow tests to evaluate the rheology and slump flow over a period of time to evaluate how well the mix retains its initial rheological properties. It was found that the compressive strength was enhanced the most by 3.75% by wtob of MS substitution. The best rheology for selfcompact ability was obtained by substituting MS between 5% and 7.5% by wtob. It was also found that increase an in MS dosage rate will reduce the open life of fresh concrete.
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 mixthe 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.
As concrete is one of the most widely used construction materials on the planet, it is essential to adopt its properties for a wide range of applications. Its high compressive strength makes it the ideal material for compressive load-bearing structures, such as walls and columns. The low tensile strength and brittle behaviour are considered the biggest disadvantages that considerably limit the application of the material. However, this can be overcome, when additional reinforcement, such as metallic rebars or fibres with high tensile strength, is introduced into the concrete matrix. This allows for the prevention of brittle fractures when concrete is subjected to tensile stress. There are certain differences in the application technology between rebars and fibres. Metallic rebars are designed and placed in the mould according to calculated tensile loads, thus requiring special preparation of formwork before pouring in fresh concrete. Whereas fibres are evenly distributed directly in fresh concrete during the mixing process. If the efficiency of rebars is mainly dependent on initial calculations and their spatial placement, then the efficiency of fibres is greatly affected by their distribution factor in concrete. As the density of metallic fibres is approximately 3 times higher than that of concrete, the prevention of sedimentation of the metallic fibres during curing is considered a major challenge for the application of such fibres. In our work, we are focusing on obtaining a uniform distribution of high tensile strength metallic microfibers. Self-compacting high-performance concrete with an aggregate diameter of less than 1.25 mm was used as a matrix and high tensile strength metallic fibres with an aspect ratio of 40 were used as reinforcement. Different superplasticizer concentrations were used to modify the rheology of the mix until uniform fibre distribution was obtained, and sedimentation of fibres was prevented.
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