High-cycle fatigue of ultra-high performance fibre reinforced concrete in tension

Ultra-high performance concrete fibre reinforced concrete (UHPFRC) is known by its high-strength capacity and the design of slender and lightweight concrete structures. This is due to the material's low porosity, which contributes to the increase in strength, and the use of high-strength fibres that bridge the cracks and allow for the stress to be transferred in a cracked region. These characteristics have facilitated the attendance of most recent design procedures that require an increase in structural service-life and reduction in maintenance, leading to an improvement in the sustainability of reinforced concrete structures. However, such effectiveness of UHPFRC structures in the long-term is only possible if considerations of service effects known to cause crack widening and increases in deflection over time are also included in the structural design of UHPFRC. Approaches for the inclusion of time-dependent effects such as creep and shrinkage in the serviceability design of UHPFRC flexural members are relatively well researched. However, little research has been devoted to the impact of highcycle fatigue at either the material, mechanism (tension stiffening) or member level, and this is probably due to the complex and time-consuming nature of the laboratory campaigns required to generate test data. The tension stiffening mechanism impacts the serviceability behaviour of reinforced concrete members because it directly influences member deflection and crack width, and this mechanism is highly sensitive to the presence of fibre reinforcement. This thesis, therefore, aims to understand the tensile behaviour of UHPFRC with straight type ultra-high strength fibres under high-cycle fatigue and to develop models at material and mechanism (tension stiffening) levels that will contribute to serviceability design. To this end, an extensive experimental campaign was performed to create material models to identify the impact of highcycle fatigue on the tensile stress-crack width and bond stress-slip behaviour of UHPFRC at the material level. This work was then extended to the mechanism level, where tension stiffening is investigated using UHPFRC tension elements with continuous steel reinforcing. Finally, the material models are used as the basis for the development of a partial-interaction (PI) model to simulate tension stiffening under high cycle fatigue. The contribution of this thesis is presented through three journal publications that describe the behaviour of UHPFRC under high-cycle fatigue for: i) concrete under direct tension; ii) the bond between reinforcement and concrete; and iii) the tension stiffening mechanism.Thesis (Ph.D.) -- University of Adelaide, Civil, Environmental and Mining Engineering, 202