Dispersion of High Concentrations of Carbon Nanofibers in Portland Cement Mortars

This research focuses on creating and maintaining a stable dispersion of carbon nanofibers (CNFs) in portland cement based materials. A microfine cement is used in conjunction with an untraditional dispersion method to encourage and stabilize the dispersion of CNFs in concentrations up to 5% by mass of cement. A computational simulation was utilized to examine an effect called geometric clustering on the dispersion of CNFs among Type I/II and microfine cement grains. The geometric clustering simulation revealed a higher achievable dispersion for microfine cement than for Type I/II cement. Scanning electron microscopy (SEM) was used to quantify the dispersion of CNFs among Type I/II and microfine cement grains. SEM image analysis indicated excessive CNF clumping among Type I/II cement grains, while the dispersion of hybrid microfine cement mortar continued to improve as the concentration of CNFs increased up to 5% by mass of cement. Mortar cube elastic stiffness and mortar prism flexure tests revealed that high concentrations of CNFs had detrimental effects in hybrid Type I/II cement mortar, whereas similar concentrations of CNFs had negligible or beneficial effects in hybrid microfine cement mortar

Basalt Microfiber Reinforcement for Improving Desiccation Behavior of Clay Bentonite for Nuclear Waste Disposal: An Experimental Investigation

Each nuclear power reactor generates 20-30 tons of highly radioactive waste, annually. The preferred long-term solution for highly radioactive waste (HLW) disposal is containment underground in geological repositories. Canisters filed with radioactive waste are placed in tunnels, with one or more engineered barrier materials (EBM) encapsulating them and separating them from the natural rock (i.e., the natural barrier). Bentonite clay is commonly used as an EBM due to its many advantageous properties including low hydraulic permeability, which ensures that only slow diffusion-based transport of radionuclide contaminants is allowed. However, heat-induced desiccation cracking of bentonite clay may increase hydraulic permeability. An attractive solution may be to reinforce the bentonite clay to prevent desiccation cracking and resulting increase in hydraulic permeability. Fiber reinforcement has commonly been used as a method of improving the properties of various materials and has been effective in reinforcing desiccation cracking. Basalt fiber is a high-density material that has been used to provide physical reinforcement for composite materials (Bella, 2015; Dhand, 2015). There has also been some research on the use of basalt fibers for nuclear waste disposal application. However, there has been limited research on the drying shrinkage and desiccation behavior of basalt microfiber-reinforced bentonite clay. Basalt microfiber reinforcement was investigated as a method to reduce shrinkage cracking in bentonite clay EBM by generating frictional and tensile resistance within the composite inorganic microfiber reinforced engineered barrier material (IMEBM). Desiccation behavior was investigated using a free shrinkage and restrained ring test method with digital image correlation (DIC) for data collection and analysis. Free shrinkage was minimally affected by basalt microfiber reinforcement. However, basalt microfiber reinforcement was effective in “bridging” desiccation cracking, preventing complete separation of the material. Therefore, basalt microfiber-reinforced bentonite clay may be an effective solution to limit desiccation cracking in geological repositories and improve nuclear waste management and safety

Imperative Programming with Variational Effects

Variation is a commonly encountered element of modern software systems. Recent research into variation has led to increasing interest in the development of variational programming languages and corresponding variational models of execution. Variational imperative languages pose a particular challenge due to the complex interactions between side effects and variation. The development of interpreters for variational imperative languages has produced a number of techniques to address these interactions. This thesis builds upon and formalizes these techniques by defining a formal operational semantics for a simple imperative language that supports both variation and common side effects. We also provide an example of the successful implementation of these techniques in the form of the Resource DSL. One area in which variation is frequently encountered is in defining configurations and resource requirements for the deployment of modern software systems. To this end, we have developed the Resource DSL, a language that aids in the specification of resource requirements for highly configurable software systems

Evaluation of a permeability-porosity relationship in a low permeability creeping material using a single transient test

A method is presented for the evaluation of the permeability-porosity relationship in a low-permeability porous material using the results of a single transient test. This method accounts for both elastic and non-elastic deformations of the sample during the test and is applied to a hardened class G oil well cement paste. An initial hydrostatic undrained loading is applied to the sample. The generated excess pore pressure is then released at one end of the sample while monitoring the pore pressure at the other end and the radial strain in the middle of the sample during the dissipation of the pore pressure. These measurements are back analysed to evaluate the permeability and its evolution with porosity change. The effect of creep of the sample during the test on the measured pore pressure and volume change is taken into account in the analysis. This approach permits to calibrate a power law permeability-porosity relationship for the tested hardened cement paste. The porosity sensitivity exponent of the power-law is evaluated equal to 11 and is shown to be mostly independent of the stress level and of the creep strains

Insights from bond-slip investigations in different reinforced concrete mixtures for LNG containment

Understanding the concrete-steel interface's behavior at cryogenic temperatures is important for designing concrete for direct liquefied natural gas (LNG) containment; such behavior is currently unclear. Hence, the work conducted involved pull-out testing in providing insights on the bond-slip relationship and the development of internal strains, which have seldom been measured in cryogenic concrete. Deformed cryogenic steel rebars (16 and 19 mm diameter) were embedded in cylindrical concrete specimens made with either traprock or limestone aggregates, with and without air entrainment (AE). Embedded foil and vibrating wire gages monitored the concrete and rebar's internal strains during cooling and pull-out testing. Pull-out testing involving applied stresses ranging from 34.5 to 241.5 MPa was conducted at normal and cryogenic temperatures. Bond stress was similar in both aggregate types but increased with rebar diameter within the applied stress range. The gages indicated that the rebar and concrete showed similar strain patterns in AE and non-AE traprock, and the AE limestone concretes during cryogenic cooling. However, the rebar gages responded to expansive movements below −20 °C in the non-AE limestone concrete. Bond stiffness degradation occurred at higher applied tensile stresses with AE in limestone concrete but at similar tensile stresses in AE and non-AE traprock concrete

Modeling Sulfate Attack in Modern Concrete for Building Sustainable and Resilient Infrastructure

External sulfate attack is a complex phenomenon and is manifested in the form of large expansion, cracking, and spalling depending on the exposure solution and material constituent properties. Several models were developed in the past to demonstrate sulfate attack mechanisms that account for the diffusion of sulfate ions into the porous concrete and the successive deformation triggered by the chemical reaction and precipitation of expansive agents. However, none of these models accounts for the effect of the migration of solvent water from the low solute concentration solution to high solute concentration solution driven by the osmotic pressure. Osmotic pressure is believed to cause spalling and cracking of concrete substrates coated with semipermeable membrane that prohibits diffusion of ions from the surroundings into the porous body. In order to determine the effect of osmotic pressure on the deformation of concrete exposed to sulfate solution, a coupled poromechanical model has been developed. Sensitivity analysis has been performed to investigate the effect of material constituent properties and exposure solution on the osmotic pressure induced damage propensity of concrete. It has been found that concrete surface can exhibit high instantaneous tensile stress developed by the gradient in the salt concentration between the pore solution and external surroundings