The Influence of the Addition of Microsilica and Fly Ash on the Properties of Ultra-High-Performance Concretes

The paper presents experimental studies on the influence of a simultaneous, appropriately proportioned combination of microsilica and fly ash additives on the physical and mechanical properties of ultra-high-performance concretes (UHPCs). Concrete mixtures with the addition of microsilica in the amount of 6.7-14.7% and fly ash in the amount of 8.3-26.7% were analyzed, assuming a constant content of cement, water and superplasticizer. Experimental studies were carried out regarding the consistency of the fresh concrete mixtures and on the compressive strength, flexural strength, tensile splitting strength, secant modulus of elasticity, depth of penetration of water under pressure into hardened concrete and water absorption. The analysis of mechanical properties was carried out during a long maturation period from 2 to 90 days. Additionally, the influence of the cost of component materials on the final cost of concrete was taken into account. The test results indicate the effectiveness of the use of microsilica and fly ash additives in ultra-high-performance concretes and possible directions for optimizing their proportions in order to achieve the intended physical and mechanical properties. The best strength properties were obtained for concrete containing 16.7% fly ash and 13.3% microsilica. The highest resistance to water penetration and absorption under pressure was characterized by concretes containing an increased content of microsilica up to 14.7%.