Multi-scale investigation of microstructure, fiber-matrix bond, and mechanical properties of ultra-high performance concrete

The main objective of this study is to provide new insights into enhancing fiber-matrix bond and mechanical properties of ultra-high performance concrete (UHPC). Three main strategies were investigated: 1) use of supplementary cementitious materials; 2) use of nano-particles; and 3) use of deformed fibers. A multi-scale investigation involving the evaluation of non-fibrous UHPC mortar phase (matrix), fiber-matrix interface phase, and then UHPC composite material was undertaken to determine microstructural characteristics, fiber bond to matrix, and key mechanical properties of the UHPC matrix and UHPC. Test results indicate that the incorporation of 10%-20% silica fume effectively improved the fiber-matrix bond and mechanical properties due to increased C-S-H content and decreased porosity. The flexural and tensile strengths of UHPC were found to increase by approximately 15%-30% and 35%-70%, respectively. Optimal nano-CaCO₃ and nano-SiO₂ contents were found to be 3.2% and 1.0%, respectively, by mass of binder. High nano-particle content decreased the fiber-matrix bond and mechanical properties of matrix and UHPC due to the increased porosity and introduction of weak zone at fiber-matrix interface. The use of deformed fibers enhanced bond given additional mechanical interlocking and enhanced friction. Compared to straight fibers, the corrugated and hooked fibers improved the bond strengths by approximately three and seven times, respectively. Such values were three and four times greater for pullout energy. The flexural strengths of UHPC made with 2% corrugated and hooked fibers were enhanced by approximately 10%-30% and 15%-50%, respectively. The tensile/flexural strengths of UHPC can be predicted based on the composite theory given fiber-matrix bond strength, strength of matrix, and fiber characteristics --Abstract, page iii