Influence of fiber orientation on the tensile behavior of UHPFRC and reinforced UHPFRC under static and fatigue loadings

Ultra-high performance fiber reinforced cementitious composite (UHPFRC) is a modern class of cementitious building materials. Because of its superior mechanical properties and durability, it is increasingly used globally to rehabilitate, strengthen and modify existing structures as well as to design and build new structures. In structural applications, the tensile behavior of UHPFRC is the critical property for structural design. The fiber volume and orientation influence the tensile properties of UHPFRC. Moreover, on structures like bridges, UHPFRC carries significant fatigue stress and is expected to have long lasting durability. Thus, fatigue damage, i.e., cracking due to fatigue loading, must be prevented by appropriate design rules. In view of the very long service duration of UHPFRC structures, the tensile fatigue behavior of UHPFRC and reinforced-UHPFRC (R-UHPFRC) elements must be well understood.This thesis aims to reveal the governing mechanisms underlying the static and fatigue tensile behavior of UHPFRC and R-UHPFRC elements utilizing five advanced measurement techniques. Based on the results, reliable models and guidelines for the rational design of UHPFRC structures are developed. In the experimental characterization, four measurement techniques are consistently employed, i.e., magnetoscopy, displacement transducers, digital image correlation and acoustic emission, while fiber-optic sensing is additionally used for R-UHPFRC. Firstly, two major intertwined questions concerning the elastic limit tensile stress of UHPFRC are conclusively addressed: i) based on refined characterization, the modulus-drop method is suggested as a representative and reliable means to determine the elastic limit; and ii) an original model to determine the elastic limit as a function of fiber orientation is proposed and validated based on own test data and literature.Secondly, the tensile fatigue behavior of UHPFRC preloaded within and beyond the tensile elastic domain is characterized. Based on the experimental results and literature, the governing mechanism underlying the tensile fatigue behavior of UHPFRC is identified. Furthermore, tensile fatigue resistance models and references are developed for the fatigue design of UHPFRC structural elements, describing the relation between number of cycles, maximum tensile fatigue stress and fiber orientation.Finally, the tensile fatigue behavior of R-UHPFRC elements, under a load ratio representative for structural applications, is investigated. The fiber orientation of UHPFRC is found to have minor influence on the tensile fatigue resistance of R-UHPFRC specimens, while it determines their loci of crack localization and thus of fatigue fracture. In addition, a S-N diagram of high correlation coefficient and a fatigue endurance limit are obtained, essential for the fatigue design in structural applications.