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- All Subjects: Civil Engineering
- Genre: Doctoral Dissertation
- Creators: Mobasher, Barzin
- Resource Type: Text
- Status: Published
The current study focuses on the fundamental understanding of such functional composites, from their microstructural design to macro-scale application. More specifically, this study investigates three different categories of functional cementitious composites. First, it discusses the differences between cementitious systems containing interground and blended limestone with and without alumina. The interground systems are found to outperform the blended systems due to differential grinding of limestone. A novel approach to deduce the particle size distribution of limestone and cement in the interground systems is proposed. Secondly, the study delves into the realm of ultra-high performance concrete, a novel material which possesses extremely high compressive-, tensile- and flexural-strength and service life as compared to regular concrete. The study presents a novel first principles-based paradigm to design economical ultra-high performance concretes using locally available materials. In the final part, the study addresses the thermal benefits of a novel type of concrete containing phase change materials. A software package was designed to perform numerical simulations to analyze temperature profiles and thermal stresses in concrete structures containing PCMs.
The design of these materials is accompanied by material characterization of cementitious binders. This has been accomplished using techniques that involve measurement of heat evolution (isothermal calorimetry), determination and quantification of reaction products (thermo-gravimetric analysis, x-ray diffraction, micro-indentation, scanning electron microscopy, energy-dispersive x-ray spectroscopy) and evaluation of pore-size distribution (mercury intrusion porosimetry). In addition, macro-scale testing has been carried out to determine compression, flexure and durability response. Numerical simulations have been carried out to understand hydration of cementitious composites, determine optimum particle packing and determine the thermal performance of these composites.
The results of this work prove the feasibility of PCMs as a temperature-regulating technology. Not only do PCMs reduce and control the temperature within cementitious systems without affecting the rate of early property development but they can also be used as an auto-adaptive technology capable of improving the thermal performance of building envelopes.
The utilization of these materials has relied primarily on the dual benefits of reduced presence in landfills and cost. These have also proven to yield a higher service life as opposed to conventional ordinary portland cement (OPC) concrete due to an enhanced microstructure. The use of such materials however has not been readily acceptable due to detrimental early age behavior. The influence of design factors is studied to understand the reaction mechanism. Silicon polymerization at the molecular level is studied to understand the aluminosilicate interactions which are responsible for prevention of any leaching of ions. A comparative study between fly ash and slag binders is carried out to evaluate the stable states of sodium, aluminum and silicon in both these binders, since the likelihood of the sodium ions leaching out is high.
Compressive and flexural strength have been reported in previous literature, but the impact of crack resistance was unevaluated from an approach of characterizing the fracture process zone. Alternative routes of activation are explored with an intent to reduce the high alkalinity by use of neutral salts such as sodium sulfate. High volume OPC replacement by both class C and F fly ash is performed to evaluate the differences in hydration phase formation responsible for its pore refinement and strength. Spectroscopic studies have also allowed to study the fundamental material structure. Durability studies are also performed on these samples to understand the probability external sulfate attacks as opposed to OPC mixes.
The use of reinforcing fibers in asphalt concrete (AC) has been documented in many studies. Published studies generally demonstrate positive benefits from using mechanically fiber reinforced asphalt concrete (M-FRAC); however, improvements generally vary with respect to the particular study. The widespread acceptance of fibers use in the asphalt industry is hindered by these inconsistencies. This study seeks to fulfill a critical knowledge gap by advancing knowledge of M-FRAC in order to better understand, interpret, and predict the behavior of these materials. The specific objectives of this dissertation are to; (a) evaluate the state of aramid fiber in AC and examine their impacts on the mechanical performance of asphalt mixtures; (b) evaluate the interaction of the reinforcement efficiency of fibers with compositions of asphalt mixtures; (c) evaluate tensile and fracture properties of M-FRAC; (d) evaluate the interfacial shear bond strength and critical fiber length in M-FRAC; and (e) propose micromechanical models for prediction of the tensile strength of M-FRAC. The research approach to achieve these objectives included experimental measurements and theoretical considerations. Throughout the study, the mechanical response of specimens with and without fibers are scrutinized using standard test methods including flow number (AASHTO T 378) and uniaxial fatigue (AASHTO TP 107), and non-standard test methods for fiber extraction, direct tension, semi-circular bending, and single fiber pull-out tests. Then, the fiber reinforcement mechanism is further examined by using the basic theories of viscoelasticity as well as micromechanical models.
The findings of this study suggest that fibers do serve as a reinforcement element in AC; however, their reinforcing effectiveness depends on the state of fibers in the mix, temperature/ loading rate, properties of fiber (i.e. dosage, length), properties of mix type (gradation and binder content), and mechanical test type to characterize M-FRAC. The outcome of every single aforementioned elements identifies key reasons attributed to the fiber reinforcement efficiency in AC, which provides insights to justify the discrepancies in the literature and further recommends solutions to overcome the knowledge gaps. This improved insight will translate into the better deployment of existing fiber-based technologies; the development of new, and more effective fiber-based technologies in asphalt mixtures.
A mesoscale micro-structural framework is proposed in Multiphysics Object-Oriented Simulation Environment (MOOSE) finite element framework which represents the first step in this direction. As part of the framework, a coupled creep damage algorithm was developed and implemented in MOOSE. The algorithm considers creep through rheological models, while damage evolves exponentially as a function of elastic strain and creep strain. A characteristic length is introduced in the formulation such that the energy release rate associated with each element remains the same to avoid vanishing energy dissipation with mesh refinement. A creep damage parameter quantifies the effect of creep strain on the damage that was calibrated using three-point bending experiments with varying rates of loading.
The creep damage model was also validated with restrained ring shrinkage tests on cementitious materials containing compliant/stiff inclusions subjected to variable drying conditions. The simulation approach explicitly considers: (i) moisture diffusion driven differential shrinkage along the depth of the specimen (ii) viscoelastic response of aging cementitious materials (iii) isotropic damage model with Rankine′s failure initiation criterion, and (iv) random distribution of tensile strengths of individual finite elements.
The model was finally validated with experimental results on neutron-irradiated concrete. The simulation approach considers: (i) coupled hygro-thermal model to predict the temperature and humidity profile inside the specimen (ii) radiation-induced volumetric expansion of aggregates (RIVE) (iii) thermal, shrinkage and creep effects based on the temperature and humidity profile and (iv) isotropic damage model with Rankine’s criterion to determine failure initiation.