3D Characterisation of Microcracks in Concrete
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3D Characterisation of microcracks in concrete A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy and Diploma of Imperial College London by Monika Jolanta Mac Concrete Durability Group Department of Civil and Environmental Engineering Imperial College London December 2018 Declaration I hereby declare that this thesis, submitted for the degree of Doctor of Philosophy and Diploma of Imperial College London is the result of my own work, and that all else is appropriately referenced. The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. Signature: Date: December, 2018 2 Abstract The nature of microcracks that developed in concrete is not well understood. One reason for this is the lack of suitable techniques to detect and characterise the microcracks. Conventional methods include imaging polished cross sections with scanning electron microscopy and optical microscopy. However, these techniques only provide a two-dimensional representation of a three-dimensional structure, which significantly reduces the insights from such analysis. Another reason is that the development of microcracks may be associated with various complex forms of concrete deterioration during service life, e.g. due to mechanical loading, drying, thermal effects and chemical reactions. This complicates laboratory scale experiments and inducing “realistic” microcracks in concrete samples becomes very difficult. The aim of this study is to develop new techniques for three-dimensional quantitative characterisation of microcracks and to apply these to understand the properties of microcracks in concrete. A thorough literature review was conducted to identify the causes of microcracking in concrete, mechanisms of microcrack initiation and propagation, transport properties of micro-cracked concrete and methods to characterise microcracks in two dimensions (2D) and three dimensions (3D). Materials and experimental procedures for inducing different types of microcracks, sample preparation for imaging and image analysis of microcracks are discussed. The feasibility of three-dimensional techniques such as focused ion beam nanotomography (FIB-nt), broad ion beam combines with serial sectioning (BIB), X-ray microtomography (µ-CT) and laser scanning confocal microscopy (LSCM) for imaging microcracks were investigated. A new approach that combines LSCM with serial sectioning was proposed to enhance the capability of LSCM for imaging microcracks in 3D. A major focus of this thesis was dedicated to microcracks induced by autogenous shrinkage because this has been previously neglected due to the dominant role of drying shrinkage. Nonetheless, the increasing use of high strength concretes containing low water/binder ratio, complex binder systems and multiple chemical admixtures in recent years has highlighted the problem of autogenous shrinkage in these concretes. This study presents a first attempt on direct characterisation and understanding of the microcracks caused by autogenous shrinkage in 3D. Various concrete samples were produced and sealed cured to induce autogenous shrinkage. The water/binder ratio, cement type and content, and aggregate particle size distribution were varied to vary the magnitude of autogenous shrinkage and degree of microcracking. Linear deformation measurement was performed to correlate autogenous shrinkage with degree of microcracking. Samples were imaged in 2D using laser scanning confocal microscope (LSCM) and in 3D with X-ray microtomography (µ-CT). Subsequently, 2D and 3D image analysis was employed to quantify microcracks > 1 µm in width. A major challenge was to isolate the microcracks that are inherently connected to pores and air voids. Therefore, an algorithm was developed to separate microcracks from pores, and to extract quantitative data such as crack density, orientation degree, distribution of width and length, as well as connectivity and tortuosity. The results show that use of supplementary cementitious materials and low water/binder ratio can increase linear deformation and the amount of the microcracks. The thesis discusses the effect of autogenous shrinkage on the characteristics of the induced microcracking, which is critical to understanding the transport properties and long- term durability of concretes containing supplementary cementitious materials. Keywords: microcracking, 3D imaging techniques, image analysis, autogenous shrinkage 3 Acknowledgements First of all, I would like to thank the TRANSCEND recruiting committee led by Professor Karen Scrivener, for having offered me the opportunity to pursue my PhD in such a unique collaborative environment within the TRANSCEND Marie Curie Initial Training Network programme. I would like to acknowledge the European Commission since the research leading to these results has received funding from the European Union Seventh Framework Programme (FP7 / 2007-2013) under grant agreement 264448. I would also like to thank my supervisors, Dr Hong Wong and Professor Nick Buenfeld for their guidance and support throughout the study and at the same time for allowing me to do my research with freedom. Professor Karen Scrivener and Dr Hong Wong deserve a special mention for believing in me. This work would not have been completed without their encouragement and help. Throughout the programme, I had invaluable opportunities to undertake secondments at several leading research institutions. I was initially hosted by the Centre for Research in Nano-Engineering at Polytechnic University of Catalonia – Barcelona TECH. I undertook secondments at Sika Schweiz AG in Zurich and ZAG, the Slovenian National Building and Civil Engineering Institute in Ljubljana and visited the Institute of Structural Geology, Tectonics and Geomechanics at RWTH Aachen University. At all of these places, I have been surrounded by many great people, to whom I would like to express my greatest gratitude. This applies especially to Professor Ignasi Cassanova, Professor Patricia Pardo and Dr Trifon Triffonov from UPC, Dr Emmanuel Gallucci and Dr Arndt Eberhard from Sika, Professor Andraž Legat and Dr Lidija Korat from ZAG, and Dr Guillaume Desbois and his students from RWTH. I thank all of them for their co-supervision, collaboration and all the ideas and suggestions to improve my experiments during the first years of my PhD. My thanks are also extended to the National History Museum in London for providing me access to their 3D CT- scanner, and to the Facility for Imaging by Light Microscopy at Imperial College London for the access to their two-photon microscope. I appreciate all the assistance provided by the staff in the Department of Civil and Environmental Engineering at Imperial College: International Office, Central Library and Student Hub, but mostly, the help and engagement with our lab technician, Mr Andrew Morris. I am deeply grateful to my TRANSCEND colleagues: Agata, Arnaud, Elena, Luis, Merlin, Noemi, Vadim, Serge-Henri, Mohamed, Wu-Min, Saeed, Mahsa, Zhidong, Xiaomeng and Pierre-Vincent and all Nanocem members. I thank you all for the shared moments during our training courses, meetings and discussions. Above all, thank you for the time we spent out of working hours, chatting, dinners, road trips, sightseeing and travelling. I also thank my office mates, colleagues and friends: Vanessa, Aneta, Agustin, Francesca, Elena, Nuria, Raul and Christian (Barcelona), Camille (Zurich), Lidija and Alenka (Ljubljana), Marcus, Ima, Fadhilah, Sukina, Zhigen, Saeed, Han, Libin and Alfredo (London), for their great company and invaluable support! Thanks for the lab help and technical discussions, for all the lunches, coffee-/ tea-breaks and dinners, and especially for the motivation to keep going. Thanks are also due to my friends that I left behind in Poland for their support and friendship. And finally, I would like to thank my son Wojtek, my parents and sister and everyone in my family for being so supportive, understanding and loving during this long journey. This work is dedicated to you. 4 Notation Abbreviations: 2D two dimensions 3D three dimensions ASR alkali – silica reaction AU Airy Unit av air voids BIB broad ion beam DSLR digital single-lens reflex FIB-nt focused ion beam nanotomography FOV field of view FT Freeze -thaw GGBS ground granulated blast furnace slag HPC high-performance concrete LICM line intercept count method LSCM laser scanning confocal microscope MG morphological gradient MSA maximum size of aggregates OM optical microscope OPC ordinary Portland cement PSF point-spread function PMT photomultiplier tube REV representative elementary volume RH relative humidity SC sealed curing SCMs supplementary cementitious materials SEM scanning electron microscope SF silica fume SLR single-lens reflex SSD surface dry condition UHPC ultra-high-performance concrete UV ultraviolet light VFA voxel face area XRF X-ray fluorescence µ-CT X-ray micro-computed tomography 5 Roman letters: AA area fraction Acrack total area of the microcracks Aimage area of the image Aind. crack area of an individual microcrack AR aspect ratio C1 connectivity of the largest microcrack C10 connectivity