Cementitious materials subjected to mechanical and environmental stressors: A computational framework

Despite the significant amount of concrete produced worldwide, there are long-standing issues with the long-term performance of concrete structures and facilities subjected to the mechanical and environmental stressors. To settle these issues, it is first imperative to understand the structural hierarchies and heterogeneous characteristics of concrete. While the structure of concrete at large length scales have been widely investigated in the literature, little knowledge is available about the structure, composition, and properties of the smallest building blocks of concrete, i.e., hydrated cement paste (HCP). This is mainly due to the complexities involved in the atomic structure of HCP phases that are often difficult to be characterized using conventional experimental methods. Atomistic simulations, however, can offer a promising solution, which not only plays a critical role to further interpret the experimental test results, but also advances the fundamental knowledge that is not accessible otherwise. In this dissertation, a robust bottom-up computational framework supported with experimental test data is established to address three categories of research needs. These research needs seamlessly connect the atomic structure of cement-based systems to the long-term performance of concrete structures at the macroscale. The first category of research needs attempts to understand the interplay between the structure and properties of the crystalline HCP phases, including portlandite, and the AFt and AFm phases. The second category of research needs deals with the characterization of the magnitude, sign, and directionality of the mechanical stresses produced as a result of the formation of the secondary sulfate-bearing minerals during the chemical sulfate attack reactions. Lastly, the third category of research needs is associated with the identification of the atomistic processes underlying the diffusion of water molecules and chloride ions at the interfaces of the main aluminum-rich phases in HCP. The outcome of this study (1) will extend the fundamental knowledge about the structure, dynamics, and properties of the HCP phases at the nanoscale, (2) will offer an invaluable addition to the existing experimental test data, and (3) can directly contribute to understanding and controlling the long-standing issues due to the deterioration of concrete structures subjected to the mechanical and environmental stressors

Nano-Scale Investigation of Mechanical Characteristics of Main Phases of Hydrated Cement Paste

Hydrated cement paste (HCP), which is present in various cement-based materials, includes a number of constituents with distinct nano-structures. The elastic properties of the HCP crystals are calculated using molecular dynamics (MD) methods. The accuracy of estimated values is verified by comparing them with the results from experimental tests and other atomistic simulation methods. The outcome of MD simulations is then extended to predict the elastic properties of the C-S-H gel by rescaling the values calculated for the individual crystals. To take into account the contribution of porosity, a detailed microporomechanics study is conducted on low- and high-density types of C-S-H. The obtained results are verified by comparing the rescaled values with the predictions from nanoindentation tests. Moreover, the mechanical behavior of the HCP crystals is examined under uniaxial tensile strains. From the stress-strain curves obtained in the three orthogonal directions, elastic and plastic responses of the HCP crystals are investigated. A comprehensive chemical bond and structural damage analysis is also performed to characterize the failure mechanisms of the HCP crystals under high tensile strains. The outcome of this study provides detailed information about the nonlinear behavior, plastic deformation, and structural failure of the HCP phases and similar atomic structures

Reactive Molecular Dynamics Simulations to Understand Mechanical Response of Thaumasite under Temperature and Strain Rate Effects

Understanding the structural, thermal, and mechanical properties of thaumasite is of great interest to the cement industry, mainly because it is the phase responsible for the aging and deterioration of civil infrastructures made of cementitious materials attacked by external sources of sulfate. Despite the importance, effects of temperature and strain rate on the mechanical response of thaumasite had remained unexplored prior to the current study, in which the mechanical properties of thaumasite are fully characterized using the reactive molecular dynamics (RMD) method. With employing a first-principles based reactive force field, the RMD simulations enable the description of bond dissociation and formation under realistic conditions. From the stress–strain curves of thaumasite generated in the x, y, and z directions, the tensile strength, Young’s modulus, and fracture strain are determined for the three orthogonal directions. During the course of each simulation, the chemical bonds undergoing tensile deformations are monitored to reveal the bonds responsible for the mechanical strength of thaumasite. The temperature increase is found to accelerate the bond breaking rate and consequently the degradation of mechanical properties of thaumasite, while the strain rate only leads to a slight enhancement of them for the ranges considered in this study

Cementitious materials subjected to mechanical and environmental stressors: A computational framework

Despite the significant amount of concrete produced worldwide, there are long-standing issues with the long-term performance of concrete structures and facilities subjected to the mechanical and environmental stressors. To settle these issues, it is first imperative to understand the structural hierarchies and heterogeneous characteristics of concrete. While the structure of concrete at large length scales have been widely investigated in the literature, little knowledge is available about the structure, composition, and properties of the smallest building blocks of concrete, i.e., hydrated cement paste (HCP). This is mainly due to the complexities involved in the atomic structure of HCP phases that are often difficult to be characterized using conventional experimental methods. Atomistic simulations, however, can offer a promising solution, which not only plays a critical role to further interpret the experimental test results, but also advances the fundamental knowledge that is not accessible otherwise. In this dissertation, a robust bottom-up computational framework supported with experimental test data is established to address three categories of research needs. These research needs seamlessly connect the atomic structure of cement-based systems to the long-term performance of concrete structures at the macroscale. The first category of research needs attempts to understand the interplay between the structure and properties of the crystalline HCP phases, including portlandite, and the AFt and AFm phases. The second category of research needs deals with the characterization of the magnitude, sign, and directionality of the mechanical stresses produced as a result of the formation of the secondary sulfate-bearing minerals during the chemical sulfate attack reactions. Lastly, the third category of research needs is associated with the identification of the atomistic processes underlying the diffusion of water molecules and chloride ions at the interfaces of the main aluminum-rich phases in HCP. The outcome of this study (1) will extend the fundamental knowledge about the structure, dynamics, and properties of the HCP phases at the nanoscale, (2) will offer an invaluable addition to the existing experimental test data, and (3) can directly contribute to understanding and controlling the long-standing issues due to the deterioration of concrete structures subjected to the mechanical and environmental stressors.</p

Free Vibration and Stability of Axially Functionally Graded Tapered Euler-Bernoulli Beams

Structural analysis of axially functionally graded tapered Euler-Bernoulli beams is studied using finite element method. A beam element is proposed which takes advantage of the shape functions of homogeneous uniform beam elements. The effects of varying cross-sectional dimensions and mechanical properties of the functionally graded material are included in the evaluation of structural matrices. This method could be used for beam elements with any distributions of mass density and modulus of elasticity with arbitrarily varying cross-sectional area. Assuming polynomial distributions of modulus of elasticity and mass density, the competency of the element is examined in stability analysis, free longitudinal vibration and free transverse vibration of double tapered beams with different boundary conditions and the convergence rate of the element is then investigated