Mô hình hóa tính chất hồ xi măng từ vi cấu trúc: Cơ học, co ngót, creep - Do Quang Huy, EPFL

Modelling Properties of Cement Paste from Microstructure: Porosity, Mechanical Properties, Creep and Shrinkage THÈSE NO 5881 (2013) PRÉSENTÉE le 9 AOUT 2013 À LA FACULTÉ DES SCIENCES ET TECHNIQUES DE L'INGÉNIEUR LABORATOIRE DES MATÉRIAUX DE CONSTRUCTION PROGRAMME DOCTORAL EN STRUCTURES ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Quang Huy Do acceptée sur proposition du jury: Prof. Geroliminis, président du jury Prof. Bishnoi, directeurs de thèse Dr G. Chanvillard, rapporteur Prof.

Lura, rapporteur Prof. Pichler, rapporteur Suisse 2013 Foreword The doctoral thesis of Dr. Do Quang Huy is an apt culmination of almost two decades of work in microstructural modelling of cements at EPFL. This work holistically tackles the phenomenon of autogeneous shrinkage through microstructural modelling.

In a first such attempt, the author has used the same microstructural model to simulate the microstructural development, elastic properties, creep and autogeneous shrinkage. The task of putting these models together was not simple. The author has successfully handled several problems at each step in an elegant manner. For example, although several earlier studies have pointed out that discrete models are unable to capture the late setting times of cements due to mesh effects, this study offers the most effective solution yet on the problem.

It is also the first time that creep has been modelled on a young evolving microstructure that is subjected to a time- variable load. Furthermore, each of these issues has been treated to a great depth and not just superficially. Despite the thoroughness of the models, the minimal variation of fit parameters required to reproduce experimental results demonstrates the tremendous development in our understanding of the hydration of cement. Throughout the work, it can be consistently seen that the introduction of microstructural effects such as flocculation and diffuse growth of C-S- H improves the quality of results.

It has also been seen that without introducing these effects, it is difficult to obtain the experimentally observed trends. At the same time, the results, especially on pore-structure, show that there are still large parts of hydration and microstructural development that we do not understand. As is often said, a good piece of research throws open many more questions than it answers. As models play an increasingly important role in the construction industry, continued efforts to understand these concepts will contribute much beyond mere satisfaction of academic curiosity.

At the end, I would like to congratulate Dr. Do Quang Huy for his hard work and his stubborn perseverance against the challenges he faced during this work. Delhi, August 2013 Shashank Bishnoi Acknowledgements I would like to thank all the people who helped me over the last four years in the work leading to this dissertation. I would like to acknowledge the Doctoral school at EPFL for accepting me as a PhD student and Swiss National Science Foundation for providing financial support for this research.

I wish firstly to address my great gratitude to Karen, my thesis director, for giving me the opportunity to work at LMC, for her precious advices, stimulating discussions, insightful comments and constructive criticisms, without which this work could not have been successfully carried out. Discussions with Karen not only brought new understandings but also opened new challenges that I needed to face with. She has given me the chance to learn from world leading scientists and approach advanced knowledge. The second person who has made impacts on my work is Shashank.

He was a first person to welcome me to LMC and my thesis co-director. I gratefully acknowledge him for his inspiring guidance, fruitful discussions, invaluable help and support and for his persistent encouragement and for also being my great friend. My research would not be running well without Shashank’s supervision. Looking back to the early stage of my doctoral study, I deeply appreciate how enormously patient Shashank was in teaching me.

I would not have such patience to teach a student with almost zero knowledge in cement science like me at that time. I have learnt a lot from Shashank when I was working with him despite our geographical distance. I would like to thank Amor, my thesis co-director, for his enthusiastic guidance and support and for sharing his expertise and knowledge from which my understandings of poro-mechanics have been enriched. The productive discussions with him equipped me to implement mechanical simulations and earned me wonderful results.

I am grateful to Cyrille for his enthusiastic and continuous support and for carefully reading through every line in my thesis and giving me extremely valuable feedback. I consider him as my thesis adviser and the encyclopaedia of all useful information. I offer my sincere thanks to Matthieu (at Navier, Université Paris Est) who lets me know the philosophy of his experimental data on the creep properties. I enjoyed our short-lasting but very interesting and informative meetings.

iii I would like to thank my thesis jury members, Gilles from Lafarge, Bernhard from TU Wien and Pietro from Empa, for their reviewing and correcting this research, which helped me to considerably improve this dissertation. I thank Sandra, Anna-Sandra, Maude, Marie-Alix, Christine for their administrative support, much more beyond their helping me with various logistic issues related to my conference travels and project meetings. I also thank Isabelle, Martina and Nikolas for helping me to submit this thesis in time. Thanks to Frédéric for his enthusiastic help related to human resource administration.

Thanks to all my LMC colleagues, my friends. Thanks to Ruzena for sharing her FE code and giving me a complete training. Thanks to Hui for letting me steal her hard-earned experimental results. Thanks to the geeks: Alain, Adytia, John for their computer tricks, and Olga, Arnaud, Théo, Trinh, Aude, Simone, Silke, Alexandra, 2 Philippe, Amélie, Cheng, Pawel, Berta, Aslam, Jaskanwal, Mohamad, Cedric, Christophe, Lionel, Elise, 2 Julien, Mathieu, Ruben, Mohammadhadi, Yaobo, Nicola, Mohsen, Patrick, Vanessa and Carolina for offering plenty of help, support and enthusiastic collaboration with cheerful attitude, and for all the good moments spent together outside the lab: Satellite, ski seminar, hiking, barbecues… I am thankful to my former classmates, special friends: Suresh, Raja, Dinesh and Deepak for chat, encouragement, both academic and more practical discussions, and for their kind help with this PhD application.

To my Vietnamese group: Xin cảm ơn anh chị em cộng đồng người Việt Nam, những người bạn tuyệt vời của tôi, vì những giúp đỡ trong cuộc sống, nguồn động viên chia sẻ và sự cảm thông sâu sắc của những người con Việt xa quê hương. To my family: Tình yêu và sự dạy dỗ của bố mẹ đã là cái nôi nuôi dưỡng cho con tri thức, niềm đam mê khoa học. Dù ở xa nhưng bố mẹ luôn là chỗ dựa tinh thần nguồn động lực vô cùng to lớn tiếp con sức mạnh vượt qua những khó khăn trong học tập nghiên cứu cũng như trong cuộc sống mà nhiều lúc tưởng chừng con không thể vượt qua. Em cảm ơn anh chị Quỳnh Thủy, cháu cảm ơn bà ngoại, các cô, bác, chú, thím, gì, cậu, mợ và các anh chị em từ hai bên nội ngoại đã luôn cổ vũ động viên trong suốt thời gian qua.

Und zum Schluss auch besonderen Dank an meine Freundin, Almut, für ihre Liebe, Fürsorge und Geduld in stressigen Zeiten, dass sie mit mir durch alle Hochs und Tiefs geht, immer zu mir steht, egal, was passiert und auch meine beste Freundin ist. iv Abstract Autogeneous shrinkage can be important in high-performance concrete characterized by low water to cement (w/c) ratios. The occurrence of this phenomenon during the first few days of hardening may result in early-age cracking in concrete structures. Although the scientific community has reached a fair level of agreement on the basic mechanisms and standard test methods, the prediction of autogeneous shrinkage is still a very challenging task.

Good prediction of autogeneous shrinkage is necessary to achieve better understanding of the mechanisms and the deployment of effective measures to prevent early-age cracking. The aim of this thesis was to develop a numerical, micromechanical model to predict the evolution of autogeneous shrinkage of hydrating cement paste at early age. The model was based on the three-dimensional hydration model µic of microstructure and the mechanism of capillary tension to simulate macroscopic autogeneous shrinkage. Pore-size distribution and Mercury Intrusion Porosimetry (MIP) were simulated.

Elastic and creep properties of the digital microstructure were calculated by means of numerical homogenization based on the Finite Element Method (FEM). Autogeneous shrinkage was computed by the average strain resulting from the capillary stress globally applied on the simulated microstructure. It was found that bulk density of C-S-H has to be assumed low at early age and gradually increased at later age to obtain an agreement between the experimentally measured and simulated total porosity. It was found that the experimentally observed break-through diameter from MIP is much lower than the values obtained by applying a numerical algorithm of MIP to the digital microstructure.

The effect of some of the most important input parameters on the pore-sizes in the simulated microstructure was explored. The reason which seems best able to explain this discrepancy is that C-S-H is not in fact a phase with a smooth surface as represented in microstructural models, but a phase which grows as needles into the pore space, leading to the formation of very small water filled capillary pores from early ages. This result indicates it will be extremely challenging to reproduce the pore structure of real microstructures in microstructural models on the scale of hundreds of microns necessary to study macroscopic transport. Consequently, it was necessary to use some experimental inputs in the later simulation of the autogeneous shrinkage.

v The first approach to determining elastic properties for the modelled microstructure gave values at early ages much higher than experimental ones, due to the connections formed in the microstructure as an artefact of the meshing procedure. Furthermore the percolation of the solids was found to occur even before hydration started. A procedure to remove these artefacts, on the basis of the information available in the vector microstructures was developed. Thanks to this improved procedure, a better agreement of the calculated and experimental results was obtained.

More realistic estimates of percolation threshold were obtained if either flocculation of initial placing of particles or a densification of C-S-H with hydration is assumed in the model. The basic creep of a simulated Portland cement microstructure is computed using Finite Elements. A generalized Maxwell model is used to describe the intrinsic C-S-H viscoelasticity as obtained by nano-indentation tests. It is found that if C-S-H is assumed to be homogenous with bulk density ρ = 2.

with a packing density η = 0.7), the numerical creep results of cement paste are in good agreements with experimental values for loading from 24 and 30 hours. However, the simulated creep for age of loading 18 hours appeared lower than the measured values: the input bulk density is much higher than its actual value at that time. In a refined model, C-S-H is assumed to have a creep response depending on η that varies with time. This latter model provides better predictions of early age cement paste ageing creep.

Autogeneous shrinkage was modelled using poro-elasticity and creep-superposition methods. It was found that the creep-superposition method provides a much better estimate of shrinkage than does the poro-elasticity method. The simulated results according to the creep- superposition method clearly show the effect of w/c ratio. This also suggested that the contribution of creep to shrinkage is considerable and should not be neglected.

Considering C-S-H densification in the simulations provides better predictions of autogeneous shrinkage in early age cement paste. Keywords: Autogeneous shrinkage, Modelling, Cement hydration, Cement microstructure, Mechanical properties, FEM, Homogenization methods, Ageing basic creep, Porosity analysis, C-S-H densification, Hydration model µic. vi Résumé Le retrait endogène est important dans les bétons à haute performance caractérisés par un faible rapport eau sur ciment (e/c).