Cementitious stabilization offers great advantages, such as beneficial utilization of in-situ materials or waste/byproducts. However, shrinkage cracking associated with stabilized materials limits the widespread use of this technology. The project is focused on the creation of an innovative sequential hydration procedure that could mitigate the development of shrinkage cracking of stabilized mixtures. The study will lead to a deeper understanding of the hydration mechanism of new stabilized mixtures and its effect on strength, shrinkage strain, relaxation, and shrinkage cracking potential. The behavior of stabilized mixtures will be characterized and modeled for optimal design and construction. In addition, the microstructure mechanism of hydration will be verified by the use of X-Ray computed tomography that allows non-destructive visualization of the 3-D microstructure of stabilized mixtures. The performance of the new mixture will be compared to that of traditionally stabilized mixtures.
The successful completion of the project will lead to the development of crack-free stabilized mixtures and thus significantly promote the technology for use in stabilizing in-situ materials and waste/byproducts. This in turn will contribute to the sustainable development by reducing the quarrying of virgin materials. Potential benefits include reduced construction costs, reduced energy consumption and reduced greenhouse gas emission. This project will provide cross-disciplinary educational opportunities for graduate and undergraduate students. The findings and results will be widely disseminated through publications and incorporated into a new graduate course.
Introduction: Problems associated with shrinkage cracking have been studied for decades, but with little success. It has been stated that shrinkage cracking is an intrinsic behavior of stabilized materials and cannot be avoided. It was reported that drying shrinkage is the major cause of shrinkage cracking associated with the use of cementitiously-stabilized materials (CSM) in pavement construction. Drying shrinkage was believed to be related directly to the moisture loss of cement-stabilized pavement layers. It is generally accepted that drying shrinkage is caused by capillary tension, solid surface tension, and the withdrawal of hindered adsorbed water and interlayer water movement from the cement gel. Research into concrete has shown that the correct prediction of the distribution of pore relative humidity (RH) in concrete specimens is critical to the determination of the shrinkage, creep, and thermal dilatation and their effects on the stress state. The moisture loss of CSM also is reported to be related directly to drying shrinkage, creep, and strength gain and, therefore, is the primary cause of shrinkage cracking. The modeling of moisture loss during drying is essential for assessing the shrinkage properties and related cracking. During drying, the drop in moisture content occurs first in the surface layers and much later in the core. The shrinkage gradient due to non-uniform moisture loss can result in non-uniform stress within the CSM. Therefore, stress gradient is present within the CSM and it keeps changing during the drying process. Although some studies on the drying shrinkage cracking of concrete exist, few of them focus on unsaturated soil and CSM. Even fewer studies consider the shrinkage stress distribution and the evolution of the stress profile during drying and the development of cement hydration in the modeling of shrinkage cracking. If the moisture gradient due to drying is high, the shrinkage stress could localize on the exposed surface and decrease sharply with depth. Therefore, evaluating shrinkage cracking without considering the stress gradient and its evolution due to drying could be misleading and result in significant errors. On the other hand, unbound geomaterials and CSM properties, such as strength, stiffness, creep, and shrinkage potential (the latter of which is defined in this study as the maximum possible drying shrinkage of a material at a given pore RH when there is no restraint), are affected significantly by the material’s moisture content and/or cement hydration. As a result of drying and hydration, the tensile strength, stiffness, and shrinkage potential within the specimen are neither uniform nor constant. The evolution of the gradients of these material properties needs to be characterized accurately in order to model and predict shrinkage cracking. Intellectual Merit Outcomes: This study examines the effects of pore RH and/or cement hydration on these material properties. Models are developed herein to predict the evolution of these material properties during drying and/or cement hydration. By measuring the humidity isotherm and diffusion coefficient and using the finite element (FE) method, the evolution of the pore RH gradient within the soil or cement soil specimens during drying can be fully captured. A coefficient of moisture shrinkage is proposed to bridge the knowledge gap between pore RH and shrinkage potential, which is defined in this study as the maximum possible drying shrinkage of a material at a given pore RH when there is no restraint. Then, the pore RH and/or cement hydration-dependent tensile strength, tensile stress-strain relationship, and shrinkage potential within the specimens during drying and/or cement hydration can be fully understood using the models developed in this study. Finally, FE modeling case studies were conducted to verify the models and procedure proposed in this study to predict shrinkage stress and shrinkage cracking. The results were compared with results from laboratory experiments and good agreement was found. Broader Impact Outcomes: Agriculture and pavement engineering are other disciplines that this project will have impacts. The soil shrinkage is also a critical behavior for agricultural soil. The findings of this study can be used to predict shrinkage cracking in a stabilized layer in a pavement. This project gave major funding support for graduate students, Xiaojun Li and Jingan Wang. As such the project has offered them the opportunity to investigate fundamental mechanism of shrinkage cracking and experiments. The training has now transformed into product and both PhD students have successfully finished his Ph.D. study. The project has also been pivotal in giving opportunities of hands on training for six undergraduate students. The students graduated from Washington State University with fundamental experimental skills on partially saturated granular soils. The skills they learnt under this project has helped six to secure professional positions in the job industry. In summary, it is evident that the intrinsic merits of the contributions of the research performed coupled with its applications to broader areas of science and technology makes the research a unique one.
