Investigating the Impacts of Ultra-Fine Calcium Carbonate in High-Volume Fly Ash Concrete for Structural Rehabilitation for Sustainable Development

An estimate shows that approximately 50% of global annual construction activities account for the repair and retrofitting of structures and constructed facilities. Therefore, structural rehabilitation and repair are becoming hot topics among researchers, in order to find innovative and comprehensive solutions. Ultra-high-performance concrete (UHPC) is designed to achieve high strength and long-term durability. Such types of concrete offer a solution for complicated repair and retrofitting jobs. Although many benefits of using UHPC have been derived, many concerns have also been identified with the use of UHPC—sustainability is considered a critical concern due to the requirement of large proportions of cement, which results in an increase in cost and environmental impacts. This paper presents a recipe for UHPC that contains a small ratio of cement and a large proportion of fly ash, i.e., up to 50%, as a cement replacement material (CRM). In order to achieve long-term durability and increase bonding with old concrete or brickwork for repair and retrofitting purposes, ultra-fine calcium carbonate (UFCC) is also added. In selecting an appropriate material for structural repair, it is essential to acquire an understanding of the material behavior. Therefore, this research was focused on providing a comprehensive guide to the behavior and strength performance of UHPC. The experimental results have shown that the highest strength of UHPC with low cement content can be achieved using a binary combination of high-volume fly ash (HVFA) and ultra-fine CaCO3 (UFCC) as a substitution for cement by up to a ratio of 50% in the recipe. The UHPC with low cement content displayed excellent repair and retrofitting potential for structural strengthening in regions of high stress by developing a strong bond with the existing concrete substrate.