The Interaction between Shrinkage, Creep and Cracking of Cement Material

Cement is the most important binder material in concrete structures, which can also be used alone for certain types of structures such as pavements, domes, shells, borehole wells etc. Nowadays, people are increasingly recognizing the importance of durability problems of cement and concrete structures. The shrinkage, creep and cracking of the cementitious material can impair the service life of structures by lowering the strength of materials, reducing the capacity of structural members, producing large deformations in structures, accelerating deterioration of structures, etc. Thus, it is critical to better understand the phenomena of shrinkage, creep and cracking of cement material and reduce the harm of these phenomena. Shrinkage is the time-dependent deformation of cement material without loading. It includes autogenous shrinkage, drying shrinkage, thermal shrinkage, plastic shrinkage, carbonation shrinkage and so on. Shrinkage in cement will not only cause unintended deformation of structures, but will also cause cracking when the shrinkage is restrained by itself or boundary conditions. Creep is the time-dependent deformation of cement material under continuous loading. It is more of a chemical phenomenon rather than the instantaneous mechanical deformation caused by loading. Creep is usually harmful for structures since it will increase the deformation of the members under loading, which can cause cracking or buckling of structures, lower the prestress in the prestressed structures or even lead to the failure of structures. Cracking is a common phenomenon that will directly harm the durability of cement and concrete structures. It opens new surface areas of the materials to the environment, which will reduce the bearing capacity of designed load, accelerate the corrosion of steels, and offer access to chemical attacks etc. Cracking can be caused by various reasons such as nonuniform shrinkage, creep, excessive loading and so on. The shrinkage, creep and cracking are usually not independent phenomena, but can interact with each other over time. Thus it is meaningful to study these phenomena and better understand the interactive mechanism between them. This can help us to reduce the disadvantageous aspects of these phenomena and improve the design and maintenance strategies. Four projects are covered in this thesis. The first project is to study the interaction between early age cracks and the size effect of drying shrinkage for cement paste. This study explicitly takes the cracking network into consideration of nonuniform drying shrinkage, which has not been taken into account in existing design codes. The second project is to study the viscoelastic behavior of early age hydrating cement paste by explicitly considering the multiple phases composition of cement paste, the distribution of C-S-H properties based on nanoindentation test results, and different patterns of microcracks in the microstructures. The third project is to study the restrained shrinkage cracking of early age hydrating well cement. The shrinkage cracking development process and the cracking network in the oil & gas well cement annulus are captured and analyzed by the restrained ring test, the image analysis method and numerical simulation. The last project describes the design and application of a full sized well cement testing system developed in a borehole remediation project. The newly developed well cement testing system can test well cementing structures under realistic geometrical and environmental conditions. The effectiveness of the nanoparticle injection technique for remediation of well cement cracking have been studied on specimens cured under realistic working conditions. The findings from these studies can help to better understand the complex behavior of early age cement materials and can be considered in future industrial design of related structures