The effect of water-to-cement ratio on the hydration kinetics of tricalcium silicate cements: Testing the two-step hydration hypothesis

Recent efforts to model tricalcium silicate based cements assume a two-step hydration mechanism where, in the first step, the empty space between cement particles rapidly fills with a low density calcium silicate hydrate (C-S-H) and in the second step, the thus formed C-S-H densifies slowly. This gives rise to models that nicely mimic the shape of experimentally observed hydration calorimetry curves and explains the transition between Stage 3 (acceleration) and Stage 4 (decreasing rate) hydration as well as post Stage 4 continued slow reaction. If this mechanism is correct, however, one would expect that the amount of heat energy released should increase with available space between particles, i.e. with water-to-cement ratio. A series of experiments, wherein the water-to-cement ratio was systematically varied showed little or no increase in total heat released when the water-to-cement ratio was doubled or tripled suggesting that the two-step mechanism may need revision.     

Comb Polymer Architecture Effects on the Rheological Property Evolution of Concentrated Cement Suspensions

We have studied the rheological behavior of concentrated cement suspensions in the absence and presence of comb polymers comprised of a polyacrylic acid (PAA) backbone and charge-neutral, poly(ethylene oxide) (PEO) teeth. These species possessed a uniform backbone molecular weight and graft density, with varying teeth molecular weight. Both PAA, a linear polyelectrolyte, and PAA/PEO comb polymers imparted initial stability to concentrated cement suspensions above a critical weight fraction, w * of 4 mg/(g of cement). Cement–PAA suspensions, however, set prematurely. Their rapid, irreversible stiffening stemmed from deleterious interactions between PAA and multivalent counterions in solution. Interestingly, the presence of PEO teeth comprised of only a few monomer units in length mitigated such interactions. The rheological property evolution of concentrated cement–PAA/PEO suspensions exhibited complex behavior ranging from the reversible gel-like response observed at short teeth lengths to a remarkable gel-to-fluid transition observed during the deceleratory period for systems comprised of longer PEO teeth. At longer hydration times, all cement–PAA/PEO suspensions exhibited initial elastic modulus values, G_i '∼ exp( t /τ_c) before the onset of the acceleratory period, followed by initial set. Their characteristic hydration time, τ_c, and set time depended strongly on the concentration of "free" carboxylic acid groups [COO⁻] arising from non-adsorbed polyelectrolyte species in solution.     

Simplified Monthly Hydrology and Irrigation Water Use Model to Explore Sustainable Water Management Options in the Murray-Darling Basin

We aimed to provide whole-of-basin simulations of flows and diversions in the Murray-Darling Basin for economic and policy analysis. We describe a model based on a subdivision of the basin into 58 catchments. In each catchment, the monthly runoff, river flow and irrigation demand are modelled as lumped processes. This is the first single model of the whole of the Murray-Darling Basin. The model was calibrated using monthly flow and annual diversion records, both by trial and error and using an automated method. We use the model to examine the impact on the flow and diversions in the Murray-Darling Basin of proposed diversion reductions and climate change. The diversion reductions return water to the environment, increasing river flows below the main irrigation areas. However, a middle-of-the-range climate change projection may result in a 13 % decrease in total flows in 2030, which offsets the enhancement to flows gained by diversion reductions.     

One-Sided Smoothness-Increasing Accuracy-Conserving Filtering for Enhanced Streamline Integration through Discontinuous Fields

The discontinuous Galerkin (DG) method continues to maintain heightened levels of interest within the simulation community because of the discretization flexibility it provides. One of the fundamental properties of the DG methodology and arguably its most powerful property is the ability to combine high-order discretizations on an inter-element level while allowing discontinuities between elements. This flexibility, however, generates a plethora of difficulties when one attempts to use DG fields for feature extraction and visualization, as most post-processing schemes are not designed for handling explicitly discontinuous fields. This work introduces a new method of applying smoothness-increasing, accuracy-conserving filtering on discontinuous Galerkin vector fields for the purpose of enhancing streamline integration. The filtering discussed in this paper enhances the smoothness of the field and eliminates the discontinuity between elements, thus resulting in more accurate streamlines. Furthermore, as a means of minimizing the computational cost of the method, the filtering is done in a one-dimensional manner along the streamline.     

Discrete-event simulation based performance quantification of web-based and traditional bidder inquiry processes

This paper describes an innovative application of discrete-event simulation to model and evaluate differences in business processes following the introduction of new information technologies. Validated models can be used by offices considering the new technology to calibrate existing work loads and then to predict the expected impact of new information technology on measurable business metrics. Following the introduction of the technology, this model may assist in the real-time verification of user paths incorporated in software support tools. This paper will be of interest to those evaluating the impact of information technology and public construction contract administration.     

Two Systems for Direct Radiographic Miniaturization

The direct miniaturization of radiographic images is discussed, and two systems are analyzed in detail. The performance and relative advantages of a large, high-performance X-ray image intensifier (?18-in or 46-cm diameter) and an optically coupled radiographic intensifier are presented. These systems are estimated to have comparable performance and costs. The X-ray image intensifier system is superior in quantum efficiency and gain, while the radiographic intensifier is superior in contrast. While, as expected, the resolution, contrast, and sensitivity of these systems are not as good as a conventional screen-film cassette system under all circumstances, their performance may be adequate for all but the most demanding radiologic applications.     

Effect of various heat treatments on plasminogen activation in bovine milk during refrigerated storage

Plasmin and plasminogen‐derived plasmin activities were measured in heated milk with and without the addition of plasminogen activator, before and after storage at 4 °C for 96 h. The effect of a free sulfhydryl group donor, β‐lactoglobulin or cysteine, on plasminogen activation was investigated in a model system and milk. Heating milk to 75 °C enhanced plasminogen activation that was marked by a considerable increase in plasmin activity. Heating at 85 and 90 °C caused a significant decrease in plasmin and plasminogen‐derived plasmin activities. However, after storage, significant plasmin levels were restored because of the activation of remaining unfolded plasminogen. Both β‐lactoglobulin and cysteine significantly decreased plasmin and plasminogen‐derived plasmin activities in a model system. While endogenous β‐lactoglobulin was not sufficient to completely eliminate plasminogen activation in milk, cysteine addition prior to pasteurisation significantly decreased plasmin and plasminogen‐derived plasmin activities. Results highlighted the importance of the remaining plasminogen in heated milk systems.     

Phase relations in the Barium Titanate—Titanium Oxide System

Phase relations in the BaTiO₃—TiO₂ system were studied at temperatures above 1300°C in air. Quenching experiments were performed with high-purity reagents, and a new equilibrium phase diagram was constructed. Results include redetermination of the liquidus boundaries, the eutectic temperature, the melting or decomposition temperatures of the stable compounds in the system, the cubic—hexagonal transition in BaTiO₃, and the solid solubility of TiO₂ in BaTiO₃.     

Evaluating the solar resource: a review of problems resulting from temporal, spatial and angular variations

Accurate assessment of the availability of solar energy at the Earth's surface is hampered by difficulties associated with the variability of solar radiation in space and time and with the angular position in the sky hemisphere. This paper assesses each of these problem areas by highlighting the difficulties and the attempts to offset them. Major emphasis is placed on extrapolation and interpolation procedures, the sensitivity of inclined surfaces to variations in the solar input, numerical models for calculating the irradiance of inclined surfaces and the time scales for which these computations are valid.Finally, the reader is urged to consider the significance of these difficulties in light of both radiation measurement errors and the sensitivity of the application to imprecise determinations of the radiation environment.