Autogenous deformations of cement pastes: Part I. Temperature effects at early age and micro-macro correlations

A micro-macro experimental study has been performed, from the end of mixing up to 2 years, on a set of plain cement pastes prepared with the same type I ordinary Portland cement (OPC) and various water-to-cement ratios (W/C), and cured at various constant temperatures. In this part I of the paper, volumetric autogenous shrinkage has been analysed in relation to various parameters characterizing the hydration process: chemical shrinkage, degree of hydration of the cement, Ca(OH)₂ content and Vicat setting times, within the early-age period (≤24 h). The effects of the curing temperature (ranging from 10 up to 50 °C) have in particular been investigated. Its effects recorded on both the rate and the magnitude of volumetric autogenous shrinkage vs. time have pointed out the irrelevance of the usual maturity concept to describe such phenomenon within the whole early-age period. An improved maturity concept has hence been proposed. It is based on separating the early-age period in different phases and on using chemical shrinkage data for the calculation of the apparent activation energy applied to the prediction of autogenous deformations occurring after the setting period. Furthermore, micro-macro relationships have been pointed out, illustrating in particular the determining role of Ca(OH)₂.     

Predicting Ca(OH)2 content and chemical shrinkage of hydrating cement pastes using analytical approach

A semiempirical model is proposed to predict the evolution of chemical shrinkage and Ca(OH)₂ content of cement paste at early age of hydration. The model is based on chemical equations and cement compound hydration rates. Chemical shrinkage and Ca(OH)₂ amount are computed using the stoichiometric results of the hydration reactions considered in the model and the density of hydration products and reactants. The model validation is conducted by comparison between computed and experimental results achieved on ordinary cement pastes with different water-to-cement (w/c) ratios (0.25, 0.30, 0.35 and 0.40) cured at 10, 20, 30, 40 and 50 °C, respectively. Hydration degree and Ca(OH)₂ content are determined using the thermogravimetric analysis (TGA) and chemical shrinkage evolution using a gravimetric method.The comparison reveals a good consistency between modelled and experimental data at early age of hydration.     

Study of cohabitation and interconnection effects on normal and leukaemic stem cells dynamics in acute myeloid leukaemia

On the basis of recent studies, understanding the intimate relationship between normal and leukaemic stem cells is very important in leukaemia treatment. The authors’ aim in this work is to clarify and assess the effect of coexistence and interconnection phenomenon on the healthy and cancerous stem cell dynamics. To this end, they perform the analysis of two time‐delayed stem cell models in acute myeloid leukaemia. The first model is based on decoupled healthy and cancerous stem cell populations (i.e. there is no interaction between cell dynamics) and the second model includes interconnection between both population''s dynamics. By using the positivity of both systems, they build new linear functions that permit to derive global stability conditions for each model. Moreover, knowing that most common types of haematological diseases are characterised by the existence of oscillations, they give conditions for the existence of a limit cycle (oscillations) in a particularly interesting healthy situation based on Poincare–Bendixson theorem. The obtained results are simulated and interpreted to be significant in understanding the effect of interconnection and would lead to an improvement in leukaemia treatment.Inspec keywords: cellular biophysics, cancer, blood, stability, patient treatmentOther keywords: healthy stem cell dynamics, cancerous stem cell dynamics, time‐delayed stem cell models, acute myeloid leukaemia, cancerous stem cell populations, leukaemia treatment, normal stem cells dynamics, leukaemic stem cells dynamics, intimate relationship, interconnection effects, cohabitation effects, decoupled stem cell populations, global stability conditions, haematological diseases, oscillations, limit cycle, Poincare–Bendixson theorem     

Etude expérimentale du couplage hydro-chimique dans les bétons en cours de maturation: incidence sur les retraits

The concrete cover has to protect the reinforcement of structures and therefore must withstand environmental damage. This resistance is clearly linked to the degree of hydration of the concrete, that is the proportion of hydrates produced from cement and mixing water. Natural drying of a structure, when forms are stripped, leads to an evaporation of free and adsorbed water and to the reduction of hydration in the external layers. The resulting hydration gradient has been determined by means of thermogravimetric analysis of samples taken from drying test bars in a controlled atmosphere (20°C, 50% relative humidity). At the same time the hydration and drying effects were measured:

- evolution of the air's relative humidity inside the cavities of the test bars at various distances from their dried surface. Hygrothermal probes were used for this purpose.

- evolution of autogenous and total shrinkage and consequently of drying shrinkage.

Drying and autogenous shrinkages have been linked to weight loss and internal relative humidity, respectively. These correlations can be justified theoretically.     

Numerical analysis of a finite volume scheme for two incompressible phase flow with dynamic capillary pressure

In this paper, we propose and analyze the convergence of a TPFA (Two Points Flux Approximation) finite volume scheme to approximate the two incompressible phase flow with dynamic capillary pressure in porous media. The fully implicit scheme is based on nonstandard approximation on mobilities and capillary pressure on the dual mesh. We derive a discrete variational formulation and we present a new result of convergence in a two or three dimensional porous medium. In comparison with static capillary pressure, the non-equilibrium capillary model requires more powerful techniques; especially the discrete energy estimates are not standard.     

Gas Turbine Modeling Based on Fuzzy Clustering Algorithm Using Experimental Data

The development of reliable mathematical models for nonlinear systems has been a primary topic in several industrial applications. This work proposes to examine the application of fuzzy logic to represent the control parameters of a gas turbine based on the fuzzy clustering method using Gustafson–Kessel algorithms. The results obtained from data classification of construction with associated models indicate applications in modeling the examined system.     

Dynamic and thermal study of air flow control by chicanes with inclined upper parts in solar air collectors

A dynamic and thermal simulation for two-dimensional model is developed on air flow and heat transfer control by chicanes in solar air collectors. New chicane form is adopted with two parts: the first is orthogonal to the air flow direction and the second is titled (α=60°). It is apparent that the turbulence created by introducing chicanes, resulting in greater increase in heat transfer inside the dynamic air vein with a rise of 23%. The effect of the variation of the Reynolds number in the range of 100<Re<4500, on the convective heat transfer coefficient, the pressure drop and Nusselt number are analysed and have shown good agreements with the literature results. Therefore, the mass flow rates effect on the velocity magnitude, temperature and the turbulent intensity is analysed. The Reynolds number variation showed a substantial effect on the mechanism of vortex development and separation phenomenon.     

First extraction of polyphenol oxidase from edible desert truffle (Terfezia leonis Tul.) and its thermal behavior

Polyphenol oxidase (PPO) is a common enzyme with large applications in food processing and analysis, especially based on their monophenolase activity. In this context, extraction and surfactant-mediated activation of PPO from desert truffle Terfezia leonis Tul. were successfully achieved. In the presence of l-tyrosine, the cresolase activity was optimal in the pH 5–6 domain and in the 35–45 °C temperature range. In the presence of pyrocatechol, the catecholase activity was optimal at neutral pH and 30 °C. Kinetics studies revealed higher affinity of PPO for l-tyrosine than for pyrocatechol. Both enzyme–substrate complexes were structurally robust, and their thermosensitivity was mainly related to entropy changes. These properties may reflect the adaptation to desert conditions where T. leonis grows and should be useful for the development of enzymatic catalysts and sensors.     

Early-age autogenous cracking of cementitious matrices: physico-chemical analysis and micro/macro investigations

High-performance cement-based materials, characterized by low water-to-cement (W/C) ratio and high cement content, are sensitive to early-age cracking because their autogenous shrinkage rate and magnitude are particularly high during this period. This article firstly presents experimental tools especially designed for the measurement of free and restrained autogenous shrinkage at early-age. Then, the results of a multi-parameter experimental study conducted on three different types of binder are analyzed. The physico-chemical deformations of cement pastes and mortars were measured from the very early-age up to several days in saturated and autogenous conditions to investigate the effects of binder, water-to-binder ratio, presence of aggregates and temperature on the driving-mechanisms leading to early-age autogenous cracking. Complementary tests such as hydration rate measurement and microscopic observations were also performed. Among the three binders used, the blast furnace slag cement shows higher chemical strain, for a given quantity of chemically-bound water, and higher early-age autogenous shrinkage. The presence of aggregates generates a local restraining effect of cement paste deformations, leading to the formation of microcracks in the surrounding cement paste. Ring test results reveal that the first through crack of cement pastes systematically appears for maximal internal stress values lower than the material tensile strength, estimated with three-point flexural tests. This phenomenon may be due to diffuse damage of the cementitious matrix, whose deformations are partially restrained.