X-ray microtomography (microCT) of the progression of sulfate attack of cement paste

High-resolution X-ray computed tomography (i.e., microCT or microtomography) was used to study the sulfate attack of cylinders of Type I cement paste cast with water-cement (w/c) ratios of 0.45, 0.50 and 0.60. Damage levels in samples exposed to a Na₂SO₄ solution with 10,000 ppm sulfate ion concentration were qualitatively rated from 0 (no damage) to 4 (extreme damage) based upon visual examination of the samples' exteriors and microtomography of the samples' interiors. The greater the w/c ratio, the more rapid the onset of sulfate damage. The corners of the cylinders appeared to be particularly susceptible to spalling, and damage may have continued into the cement paste by formation of subsurface cracks.     

Creep of UHPC in tension and compression: Effect of thermal treatment

Steel fiber-reinforced ultra-high performance concrete (UHPC) is of increasing interest for use in precast prestressed concrete highway bridge girders due to its superior durability and the potential for reducing or eliminating shear reinforcement, due to the presence of steel fibers. However, the contributions of creep, and especially tensile creep, to long-term performance must be better understood to develop appropriate design specifications. Due to practical considerations, it is also of interest to investigate the influence of varying thermal treatment, including temperatures lower than those recommended by the manufacturer (i.e. 90 °C), on the creep of UHPC. In this 1-year study, the effects of three different thermal treatment regimes on tensile and compressive creep performance of UHPC are examined, with complementary characterization by nanoindentation and scanning electron microscopy. Results show that UHPC creeps phenomenologically differently in tension and compression. Both thermal treatments examined resulted in similar tensile creep behavior, suggesting that a lower temperature applied over a longer period could effectively cure UHPC. For the non-thermally cured UHPC, a 10 μm wide region observed at the fiber/matrix interface was characterized by reductions in elastic modulus as well as greater porosity and microcracking than the bulk paste. It is suggested that the quality of the fiber/matrix interface is a major contributor to the measured increased creep of non-thermally treated UHPC as compared to UHPC treated at 60 °C or 90 °C.     

Water Efficiency and the Professional Plumbing Sector: How Capacity and Capability Influence Knowledge Acquisition and Innovation

Professional plumbers play an essential role in the implementation of water efficiency. If North America is to achieve high water efficiency standards and more ambitious plumbing codes, plumbers will need to be actively included in the water efficiency discussion. Repositioning the industry will required a cultural shift because the plumbing community has been mostly ignored in discussions of the larger environmental agenda and priorities. This repositioning will require substantial rethinking and retraining. New knowledge will need to be transmitted about emerging water efficient technologies, public policies and practices, as well as the rationale for use in residential and Industrial, Commercial, and Institutional (ICI) sectors. The GreenPlumbers Program (GPP) initiated this knowledge transfer process. Originating in Australia and expanded to the United States, the GPP is a national training and accreditation program for professional plumbers. Their focus has been on upgrading skills and awareness of water efficiency, conservation, and the professional plumbers’ roles in the contemporary environmental context. In this paper we report on the efficacy of the GPP’s curriculum and the process of transferring explicit water efficiency knowledge. Semi-structured interviews and a survey were used to gather the data. We considered how the program participants incorporated the GPP curriculum into their ‘day-to-day’ practices and operations post-certification. We also investigated participants’ motivating factors and cross-referenced these findings to their overall assessment of the program. Recommendations focus on how the GPP can best influence and contribute to a more comprehensive water efficiency agenda.     

A quasi-two-dimensional electrochemistry modeling tool for planar solid oxide fuel cell stacks

A quasi-two-dimensional numerical model is presented for the efficient computation of the steady-state current density, species concentration, and temperature distributions in planar solid oxide fuel cell stacks. The model reduction techniques, engineering approximations, and numerical procedures used to simulate the stack physics while maintaining adequate computational speed are discussed. The results of the model for benchmark cases with and without on-cell methane reformation are presented with comparisons to results from other research described in the literature. Simulations results for a multi-cell stack have also been demonstrated to show capability of the model on simulating cell to cell variation. The capabilities, performance, and scalability of the model for the study of large multi-cell stacks are then demonstrated.     

Laser scanning confocal microscopy for in situ monitoring of alkali–silica reaction

Alkali–silica reaction (ASR) occurs in concrete between reactive siliceous components in the aggregate and the strongly alkaline pore solution, resulting in the formation of a potentially expansive gel product. Lithium additives have been shown to reduce expansion associated with ASR, but the mechanism(s) by which lithium reduces expansion have not been understood. Therefore, development of an in situ method to observe reactions associated with ASR is highly desirable, as it will allow for non‐destructive observation of the reaction product formation and damage evolution over time, as the reaction progresses. A technique to image into mortar through glass aggregate by laser scanning confocal microscopy (LSCM), producing three‐dimensional representations of the sample was developed. This LSCM technique was utilized to monitor the progress of alkali–silica reaction in mortar samples prepared with alkali‐reactive glass aggregate both in the presence and in the absence of lithium additives: LiNO₃, LiCl or LiOH. The method proved to be effective in qualitatively monitoring crack formation and growth and product formation, within cracks and at the paste/aggregate interface. In particular, dendritic products were observed at the paste/aggregate interface only in those samples containing lithium, suggesting that these products may play a role in ASR mitigation.     

Characterization of elastic and time-dependent deformations in high performance lightweight concrete by image analysis

Image analysis and strain mapping were used to examine the nature of elastic, creep and shrinkage strains in high performance lightweight concrete (HPLC). The strain maps showed non-uniform deformations related to microstructural features. Both average strain and non-uniformity increased with time under testing. Paste-rich regions exhibited higher creep plus shrinkage than the lightweight aggregate (LWA) particles examined herein; it is suggested that LWA could have a role in reducing deformations of the paste. Compared to normal weight high performance concrete (HPC), the paste and LWA in the HPLC exhibited more gradual spatial differences in elastic deformations, creep and shrinkage. It is proposed that this difference results from the lower stiffness of the LWA compared to granite used in the HPC. The results indicate that improvement in elastic property matching between the lightweight aggregate and high performance paste reduces stress concentrations at the aggregate/paste interface and contributes to reductions in deformations of HPLC compared to HPC.     

Canadian remote community power generation: How reformer and fuel cell systems compare with diesel generators

More than three quarters of Canadian remote communities rely solely on diesel generators for electricity generation. The diesel dependency of remote communities has inflated local per capita greenhouse gas emissions and resulted in rising and inconsistent electricity prices that have made community viability reliant on government subsidies. As the diesel generators approach the end of their lifespan replacement, technologies must be considered that will help transition Canadian remote communities from diesel to renewables. Replacing diesel generators with steam reformer and solid oxide fuel cell systems would allow for more efficient diesel generation and would benefit the future implementation of renewable power. A model was generated in Honeywell's UniSim Design Suite to simulate the performance of a diesel fed steam reformer and solid oxide fuel cell system. System operating parameters in the model were optimized to minimize the expected payback period. The system model outputs were compared with expected diesel generator performance for a test case remote community. The test community demonstrated that replacing diesel generators with the proposed steam reformer and solid oxide fuel cell system would result in annual net efficiency improvements of 32%. The efficiency improvement could potentially translate to reductions in carbon dioxide equivalents of over 258 kt and 20‐year savings of over $450 million if all diesel‐reliant Canadian remote communities switched to steam reformer and solid oxide fuel cell systems. In addition to immediate environmental and economic savings, the improved low load performance of the reformer and fuel cell system would allow for the future integration of renewable energy to create highly efficient diesel‐renewable hybrid power plants.     

Application of Powers’ model to modern portland and portland limestone cement pastes

Powers’ model is a simple approach for estimating the relative volumes of hydration products, porosity, and chemical shrinkage present in portland cement paste as a function of its starting water‐to‐cement ratio (w/c) and current degree of hydration. It forms an important link between cement composition, microstructure, and performance, necessary for modeling cement‐based systems. Previous researchers have adapted Powers’ model for inert fillers to illustrate their effects on the hydration, porosity, and chemical shrinkage of blended cements; however, it is well‐documented that limestone is not, in fact, an inert filler, but rather participates in cement hydration through both chemical and physical processes. This research experimentally investigates the applicability of Powers’ model to modern portland cements containing up to 15% by mass finely divided limestone. The results demonstrate that the modified Powers’ model is insufficient for predicting the influence of finely divided limestone additions on the chemical shrinkage of both ordinary portland cement pastes and portland limestone cement pastes. Possible explanations for the discrepancy are discussed and a plausible source is proposed.     

Tree-ring elemental concentrations in oak do not necessarily passively record changes in bioavailability

Elemental concentrations in tree-rings from red and white oak trees at six sites across Southern Ontario, Canada, were assessed to determine whether they passively record changes in geochemical cycling in the presence of environmental stress. Periods of stress were defined as sustained periods with elevated delta(13)C values in tree-rings relative to atmospheric CO(2) during the same period. In some trees, nutrient concentrations (Ca, Mg, Mn) were erratic during historic periods of stress while chemically similar non-nutrients (Ba, Sr) and the anthropogenic pollutant Pb were not. Tree-ring concentrations of Ca and Sr were related to bedrock type and leachable concentrations in the soil. In contrast, tree-ring concentrations of Mg were not related to bedrock type, although Mg concentration in the soil leachate was. Tree-ring Mn, Ba and Pb concentrations were not related to bedrock type or soil concentrations, but were inversely related to soil pH. Erratic behavior of nutrient elements during historic periods of stress suggests that some nutrient concentrations in the environment were not always passively recorded by tree-rings.     

Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices

We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5-0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50-60% were also obtained.