Changes in rheology and mechanical properties of ultra-high performance concrete with silica fume content

Abstract

High silica fume content used in ultra-high performance concrete (UHPC) can increase viscosity and render agglomeration issue, leading to reduction in mechanical properties, including bond to fibers. This study investigates the effect of silica fume content, ranging from 0 to 25%, by mass of cementitious materials, on rheological, fiber-matrix bond, and mechanical properties of non-fibrous UHPC matrix and UHPC made with 2% micro-steel fibers. Binary mixtures consisting of cement and silica fume with targeted mini-slump flow were specifically prepared. The involved mechanical properties include compressive, flexural, and tensile behavior. Rheology, fiber-matrix bond, flexural, and tensile strengths of UHPC were linked to each other using fiber dispersion and orientation analyses or the Composite Theory. Test results showed that UHPC made with 10% to 15% silica fume obtained the highest fiber-matrix bond, flexural, and tensile properties. Such silica fume content was found to result in lower viscosity and more uniformly distributed fibers as determined by image analysis. The flexural and tensile strengths of UHPC made with 5%–20% silica fume can be effectively predicted using the Composite Theory considering obtained fundamental inputs, such as flexural or tensile strengths of matrix, fiber characteristics, and fiber-matrix bond strength. The experimental-to-predicted tensile and flexural strength ratios were in the range from 0.9 to 1.1. However, the predicted strengths of UHPC mixtures with 0 and 25% silica fume were greatly lower than the experimental values due to high viscosity and low packing density.