Pull-out of hooked end steel fibres : experimental and numerical study

The reinforcement of concrete with steel fibres changes the failure of the composite material from catastrophic brittle failure to pseudo-ductile behaviour as a result of crackbridging by the fibres, and the additional work which is absorbed by fibre pull-out. A good understanding of the properties of the fibre-reinforced concrete depends on an understanding of the fibre pull-out process. The main aim of the current study is to investigate, both experimentally and numerically, the pull-out behaviour of a single hooked end steel fibre from epoxy matrix, where epoxy was chosen to replace concrete in order to enable visualisation of the pull-out process. The experimental and numerical results both contribute to the development of a physical understanding of the mechanism of pull-out. Experimental studies included the evaluation of the mechanical properties of hooked end steel fibre and epoxy matrix by means of tensile tests, the manufacturing of pull-out specimens consisting of a single hooked end steel fibre embedded in epoxy matrix, and the experimental characterisation of the fibre pull-out. The significant features (peaks and minima) of the load vs. displacement graph were correlated to stills taken from a video of the pull-out process, in which the plastic deformation of the fibre is evident. Small deformations (spalling) were also observed in the matrix. A model is proposed for the mechanisms which interact during the pull-out process. © University of Pretoria ii Numerical analysis has been performed using a 3D finite element analysis implemented in ANSYS, incorporating nonlinear contact between the fibre and the matrix and plastic deformation of the fibre. Sensitivity studies investigated the effects of material and interface properties on the resultant load vs. displacement curve of the pull-out. Results of these studies demonstrated the importance of accurately modelling the plastic behaviour of the steel fibre in predicting the pull-out response of the system. The numerical and experimental pull-out behaviours capture the same fundamental behaviour. The fibre displacement at critical intervals is well-correlated, and the numerically-predicted load is within the range of the experimental results at these intervals. The finite element analysis allows investigation of the stress and strain distributions. The plastic strain accumulated in different regions of the fibre, and the maximum stress experienced in the matrix are studied, and correlated with the global response, and further understanding is gained into the mechanisms of fibre pull-out. © University of Pretoria iii Acknowledgements A successful life does not depend on self-effort, but it is a result of different contributions coming from good intentioned people that surround us. I thank God that I had wonderful people, around me, and it would not have been possible to pursue the goal of this work without their help. I would like to express my deepest gratitude to my study leaders, Ms Inglis, Prof Kok, and Mr Pietra, for your unyielding support and enthusiastic encouragement. Thank you for your valuable guidance and constructive suggestions. Your willingness to provide generously the best of your knowledge and your quality time have been much appreciated. I would also like to express my great appreciation to Prof Kearsley, for the suggestion that she has made regarding the research topic and more importantly for the necessary financial support provided for this research. It would be ungrateful on my side if I do not mention the technical help that I have received in the Civil laboratory at the University of Pretoria, I thank all the technicians in general and particularly to Mr Mostert for his guidance. My special thanks are also extended to the committee of the University of Pretoria, in general, and to the Department of Mechanical and Aeronautical Engineering, in particular. The laboratory, library and computer facilities have been the indispensable resources for the completion of my research work. I would not be the man I am today without the support and encouragement of my father Kangaj Floribert-Richard, my mother Mpanga Cécile, and with the love of my entire family, brothers and sisters, nephews and nieces. I am heartily grateful for the assistance given by my brother Tshal Flavien, his advice and consideration are much appreciated. To my fellow friends, you have helped me tremendously. I am so thankful to God for having you as companions, you are wonderful people. Thank you for all the support you have bestowed. © University of Pretoria iv Table of contents Abstract . i Table of contents . iv List of Figures . vii List of Tables . xi List of Symbols . xii Abbreviations . xiii