Mechanics-based approaches for the flexural and shear behaviour of ultra-high performance fibre reinforced concrete beams

Abstract

This thesis presents a series of journal articles outlining a mechanics-based analysis approach for the flexural and shear behaviour of ultra-high performance fibre reinforced concrete beams. These solutions apply the mechanical principles of partial interaction, shear friction and segmental analysis to the design of fibre reinforced concrete (FRC) and ultra-high performance fibre reinforced concrete (UHPFRC) beams. The analysis techniques are developed for both normal strength FRC and UHPFRC, which is important as these materials have in the past often treated separately, but should be treated together the mechanical principles do not change, rather only the material properties. Further, because of their mechanics foundation, these approaches can also be applied to conventional reinforced concrete without modification by simply ignoring all terms relating to fibre properties. In the first part of this thesis the bond, tension and shear friction properties of UHPFRC are obtained through material testing. A significant part of this research is the development of a new apparatus for determining the shear friction properties. The development of this apparatus is important as it allows for the precise control of the confining force applied to the shear plane and because tests can be conducted using standard cylinders, it allows for rapid, low-cost testing of the large number of samples required to understand the impact of different fibre types and volumes. In the second part of the thesis closed form mechanics solutions are developed for the tension stiffening properties including crack spacing and the crack opening stiffness. These are then used to develop closed form solutions for the deflections and crack widths at the serviceability limit state. Next, experimental work is conducted to investigate the impact of hybridising fibres by testing UHPFRC beams with varying cross sections and fibre types. This is followed by tests to investigate the impact of prestressing with either steel or fibre reinforced polymer (FRP) tendons. Having experimentally investigated this behaviour, a segmental analysis technique, is developed to predict deflections at all load levels and crack widths. Finally, closed-form solutions are developed for predicting moment redistribution behaviour of continuous reinforced concrete beams including those constructed of UHPFRC at all limit states. Having investigated flexural performance at both the serviceability and ultimate limit state, a numerical and analytical approach which is mechanically consistent with the proposed flexural analysis is developed to predict member shear capacity. The solutions are validated against 31 tests, including two conducted by the author on UHPFRC I-sections in order to demonstrate accuracy compared to codified solutions and those available from the literature. Simplified solutions are then developed in a form which can be implemented in routine design. In the final section of this thesis further applications of partial interaction theory are developed. In this section closed-form solutions are derived for FRP to substrate joints with and without anchors. In this section it is shown that the same theory used to analyse conventional, FRC and UHPFRC reinforcement can also be applied, without fundamental modification to predict the behaviour of FRP retrofitted sections.