A nonlinear meso–macro approach to modelling delayed ettringite formation and concrete degradation

We show how a two-scales FE model—within a sequential approach—leads to consistently simulate the DEF macroscopic consequences: cracks pattern and opening, residual modulus decrease. The mesoscopic part of this model is adapted to heterogeneous materials and is based on two kinematics enrichments. The first one leads to a very fast meshing process along the randomly chosen set of aggregates and, the second leads to directly compute the cracks openings. Assuming DEF is driving to a progressive and homogeneous mortar expansion, we show how the macroscopic—concrete—expansion as well the cracks pattern are consistent with the experiments. For the latter we are interested in the characterization of the aggregates effect on the amplitude of DEF swelling, so that we focus on parameters such as the size and the volume fraction of granular inclusions.     

Review of effect of P and Sr on modification and porosity development in Al–Si alloys

Abstract

The benefits of Sr additions to Al–Si alloys to modify the eutectic are often impaired by the development of porosity, sometimes to the degree that benefits are negated. Experimental reports are reviewed in this paper, suggesting an explanation in terms of the oxide population in the melt. The unmodified silicon particles are nucleated by AlP, which has in turn nucleated on oxide bifilms. The oxide bifilms, which are essentially cracks, are straightened by the crystalline growth of Si particles, leading to increased crack size and consequently reduced mechanical properties. The addition of Sr improves properties by suppressing the formation of Si on bifilms and thereby preventing the straightening of the pre-existing cracks. Si is now forced to precipitate at a lower temperature as a coral-like eutectic. Unfortunately, the bifilms are now freed (the primary Si particles no longer exist to grow around and sequester the bifilms), remaining in suspension in the liquid metal, allowing them to act to block interdendritic flow and aid the initiation of the formation of pores, countering the benefits of the improved structure.     

The hydration and microstructure of ultra high-strength concrete with cement–silica fume–slag binder

This study investigated the flowability, compressive strength, heat of hydration, porosity and calcium hydroxide content of ultra-high-strength concrete (UHSC) with cement–silica fume–slag binder at 20 °C. The composition of the binder was designed using seven-batch factorial design method. The relationships between the binder composition and the properties were expressed in contours. Results showed that proper silica fume content could improve the flowability and compressive strength of UHSC, reduce the porosity and calcium hydroxide content of UHSC. Slag reduced the flowability, compressive strength, porosity, and calcium hydroxide content of UHSC to certain extent. The silica fume and slag demonstrated positive synergistic effects on the flowability and 3 d compressive strength, but have negative synergistic effects on the total heat of hydration, hydration heat when the time is infinitely long(P₀), 56 d compressive strength, porosity and calcium hydroxide content of UHSC.     

A combined experimental–numerical approach for generating statistically equivalent fibre distributions for high strength laminated composite materials

A technique is presented where actual experimental distributions, measured from a high strength carbon fibre composite, are considered in the development of a novel method to generate statistically equivalent fibre distributions for high volume fraction composites. The approach uses an adjusted measure of nearest neighbour distribution functions to define inter-fibre distances. The statistical distributions, characterising the resulting fibre arrangements, were found to be equivalent to those in the actual microstructure. Finite element models were generated and used to determine the effective elastic properties of the composite and excellent agreement was obtained. The algorithm developed is simple, robust, highly efficient and capable of reproducing actual fibre distributions for high strength laminated composite materials. It does not require further heuristic steps, such as those seen in fibre stirring/shaking algorithms, in order to achieve high volume fraction microstructures and provides a useful alternative to both microstructure reproduction and random numerical models.     

A fuzzy multi-criteria decision-making approach for managing performance and risk in integrated procurement–production planning

Nowadays in Supply Chain (SC) networks, a high level of risk comes from SC partners. An effective risk management process becomes as a consequence mandatory, especially at the tactical planning level. The aim of this article is to present a risk-oriented integrated procurement–production approach for tactical planning in a multi-echelon SC network involving multiple suppliers, multiple parallel manufacturing plants, multiple subcontractors and several customers. An originality of the work is to combine an analytical model allowing to build feasible scenarios and a multi-criteria approach for assessing these scenarios. The literature has mainly addressed the problem through cost or profit-based optimisation and seldom considers more qualitative yet important criteria linked to risk, like trust in the supplier, flexibility or resilience. Unlike the traditional approaches, we present a method evaluating each possible supply scenario through performance-based and risk-based decision criteria, involving both qualitative and quantitative factors, in order to clearly separate the performance of a scenario and the risk taken if it is adopted. Since the decision-maker often cannot provide crisp values for some critical data, fuzzy sets theory is suggested in order to model vague information based on subjective expertise. Fuzzy Technique for Order of Preference by Similarity to Ideal Solution is used to determine both the performance and risk measures correlated to each possible tactical plan. The applicability and tractability of the proposed approach is shown on an illustrative example and a sensitivity analysis is performed to investigate the influence of criteria weights on the selection of the procurement–production plan.     

Prediction of porosity contents and examination of porosity formation in Al–4.4%Mg DC slab

A new method for predicting porosity contents in Al–4.4% Mg DC sheet ingot (slab) is proposed. Hydrogen supersaturated mass in the inter-dendritic liquid and the pressure drop were taken into account for the porosity prediction. Using the method, the porosity contents were calculated and they well agreed with the experimental results. Thus it becomes possible to predict the porosity contents in the slab quantitatively. Furthermore, the porosity generation mechanism was observed from the surface to the center in the slab. The quantities and area fractions of the porosity were measured quantitatively by using an image analysis apparatus (Luzex). An explanation of this kind of porosity generation mechanism and the distribution of the porosity was attempted by using a local equivalent pressure and hydrogen supersaturation in dendritic solidification. The effects of parameters such as thermal gradient and cooling rate on the porosity contents were investigated analytically and the distribution of porosity and porosity contents in the slab of Al–4.4% Mg alloys were determined.     

Prediction of porosity contents and examination of porosity formation in Al–4.4%Mg DC slab

A new method for predicting porosity contents in Al–4.4% Mg DC sheet ingot (slab) isproposed. Hydrogen supersaturated mass in the inter–dendritic liquid and the pressure drop were taken into account for the porosity prediction. Using the method, the porosity contents were calculated and theywell agreed with the experimental results. Thus it becomes possible to predict the porosity contents in the slab quantitatively. Furthermore, the porosity generation mechanism was observed from the surface to the center in the slab. The quantities and area fractions of the porosity were measured quantitatively by using an image analysis apparatus (Luzex). An explanation of this kind of porosity generation mechanism and the distribution of the porosity was attempted by using a local equivalent pressure and hydrogen supersaturation in dendritic solidification. The effects of parameters such as therma gradient and cooling rate on the porosity contents were investigated analytically and the distribution of porosity and porosity contents in the slab of Al–4.4% Mg alloys were determined.     

Non-saturated ion diffusion in concrete – A new approach to evaluate conductivity measurements

Non-saturated ion diffusion properties of cementitious materials were evaluated in an experimental study. To assess these properties, resistivity measurements have been performed on mortars with different binders (ordinary Portland cement – OPC, OPC with 5% silica fume, 40% slag and 70% slag, respectively) and different water-to-binder ratios (w/b, 0.38 and 0.53). Specimens have been conditioned to eight different climates with relative humidity (RH) from 100% to 33% RH in order to assess an effective diffusion coefficient. The results from the resistivity measurements have been corrected for changes of the conductivity of the pore solution when drying to different degrees of saturation.The diffusion coefficients for Portland cement binders within the range 100–59% RH are presented. They showed that the diffusion coefficient of the mortar with high w/b ratio was higher at high RH, but at low RH the opposite trend was found. By comparing these results with the corresponding desorption isotherms, it is shown that the diffusion coefficient for the two w/b ratios have the same dependency on the degree of saturation.     

Capillary absorption in concrete and the Lucas–Washburn equation

Capillary absorption kinetics of concrete–ethylene glycol system was studied with respect to concrete matrix porosity and liquid viscosity. Porosity of specimens was altered by air-entraining agents and superplasticizers. Liquid which doesn’t react with cement gel was chosen for the experiment in order to study the reasons for deviation from Lucas–Washburn equation observed in concrete–water system. Viscosity of ethylene glycol changes from ～23 to 2 mPa s in the temperature range from 20 to 100 °C. The values of the capillary coefficient were determined at 20, 60 and 100 °C using Neutron Radiography and were found to be in the range from ～1.5 to 4.9 mm h^?1/2. The results show that the Lucas–Washburn equation in concrete–ethylene glycol system is valid only for ～25 h, which indicates that swelling and rehydration of cement gel are not the main reasons for deviation observed in concrete–water system.     

A model for a long-term diffusion of multispecies in concrete based on ion–cement-hydrate interaction

A model for simulating a long-term diffusion in concrete under submerged conditions has been developed. The model focuses on the interaction between cement hydrates and electrolyte solution adopting the physical properties of concrete as alternative parameters for estimating long-term diffusion. The model was verified by the large variety of long-term experimental data involving verification of cement hydrates, porosity properties, pH value, element distribution, and chloride penetration for sulphate-resistant Portland cement (SRPC) concretes. Evaluating impacts of hundreds of years’ exposure on concrete durability, a simulation was also performed for an exposure time of 500 years. The results confirmed the importance of ion–cement-hydrate interaction in the evaluation of a long-term diffusion of harmful substances such as chlorides into concrete. The simulation results show also that the solid phases of the SRPC concrete mix that was used are stable in the long-term. The threshold concentration of chlorides which may initiate reinforcement corrosion defined as Cl^?/OH^? ratio could be exceeded in concrete after moderate exposure period under the conditions investigated with typical protective concrete layers. Using of case-specific values for a threshold chloride content and evaluating the method used for estimating the initiation time of reinforcement corrosion are recommended in the structures with a long-design service life.     

The ITZ in concrete – a different view based on image analysis and SEM observations

The traditional picture of the ITZ in concrete involves an approximately 30 μm zone around each aggregate, within which the porosity increases as the aggregate interface is approached. The results of the writers' extensive image analysis investigations, and examinations of SEM specimens from various concretes provide a very different picture. While the “wall effect” excluded much of the ground cement from the vicinity of the aggregates, the great increases in pore content within a few μm of the aggregate, up to approximately 30% porosity, that have been reported by others, are not found. On average, only modestly higher porosities are observed within the ITZ than in the bulk paste. This is true even in the innermost areas immediately adjacent to the aggregates. In part, the extra space produced by the wall effect is filled in by CH deposits, many of which are anchored directly on the surfaces of aggregates and are essentially non-porous. Strong indications exist that the ITZ contains as high a proportion of C–S–H per unit volume as the bulk paste; thus some of the extra space that was created by the wall effect is filled in by through-solution deposits of C–S–H derived from elsewhere in the system. It is considered that the structure of the ITZ in ordinary concretes is not different enough from that of the bulk cement paste to provide any basis for significant effects on permeance or mechanical properties.     

A dimension reduction approach for conditional Kaplan–Meier estimators

Many quantities of interest in survival analysis are smooth, closed-form functionals of the law of the observations. For instance, the conditional law of a lifetime of interest under random right censoring, and the conditional probability of being cured. In such cases, one can easily derive nonparametric estimators for the quantities of interest by plugging-into the functional the nonparametric estimators of the law of the observations. However, with multivariate covariates, the nonparametric estimation suffers from the curse of dimensionality. Here, a new dimension reduction approach for survival analysis is proposed and investigated in the right-censored lifetime case. First, we consider a single-index hypothesis on the conditional law of the observations and propose a \(\sqrt{n}-\)asymptotically normal semiparametric estimator. Next, we apply the smooth functionals to this estimator. This results in semiparametric estimators of the quantities of interest that avoid the curse of dimensionality. Confidence regions for the index and the functional of interest are built by bootstrap. The new methodology allows to test the dimension reduction assumption, can be extended to other dimension reduction methods and can be applied to closed-form functionals of more general censoring mechanisms.     

Modeling mass loading effects in industrial cyclones by a combined Eulerian–Lagrangian approach

Cyclone separation is studied by means of numerical simulations. While the gas flow is modeled by a modified Reynolds stress (RS) model, the behavior of the particles is pictured by a combined Eulerian–Lagrangian approach. A mono-disperse Eulerian particle phase is utilized to account for inter-particle collisions, while the effects of fractional separation and particle-wall collisions are considered by poly-disperse Lagrangian particles. The above particle models interact in two ways. On the one hand, the Lagrangian particles determine the local mean diameter of the substitute Eulerian particle class. On the other hand, especially in regions of high particle concentration, the Eulerian particle phase exerts an additional collisional force onto the Lagrangian particle trajectories. An industrial cyclone is chosen as a test case and the numerical results are evaluated with respect to pressure drop as well as to global and fractional separation efficiency. In this context the influence of the cyclone’s mass loading and wall roughness is highlighted. Simulations indicate that the separation efficiency improves with increasing mass loading until an excess loading is reached while at the same time the pressure drop is reduced. Furthermore, it can be shown that rough walls lead to a reduction of separation efficiency while simultaneously the pressure drop decreases. The simulations results are compared with both an analytic theory of Muschelknautz [Die Berechnung von Zykonabscheidern für Gase. Chem Ing Techn 44, (1+2):63–71, 1972] as well as with real plant measurements.     

Serviceability Limit State criteria based on steel–concrete bond loss for corroded reinforced concrete in chloride environment

This paper will focus on the study of reinforced concrete beams stored in a chloride environment for a period of 14–23 years under service loading. According to the experimental results, a Serviceability Limit State (SLS) criteria is proposed based on an excessive steel–concrete bond reduction. Corrosion of reinforcement in chloride environment leads to a specific local steel cross-section loss as well as a steel–concrete bond loss. Experimental results have shown that, in the first stage of corrosion propagation period, the deflection is more sensitive to chloride-induced corrosion than the ultimate capacity due to the effect of the tension steel–concrete bond loss even if both are correlated. Given this high sensibility of the bending stiffness to corrosion pitting attacks, it appears that a Serviceability Limit State (SLS) criteria based on excessive deflection of structural members is an adequate factor for SLS assessment. Later in corrosion propagation period, when the bond is already significantly reduced, only the ultimate capacity is affected by the steel cross-section loss. This does not affect the serviceability, because pitting attacks are very localised with an insignificant influence on the global deflection. Then, once the steel–concrete bond is lost in critical parts of the beams (high bending moment areas), pitting corrosion propagation does not affect anymore serviceability (stiffness reduction, bending or corrosion cracks patterns) but still leads to an ultimate capacity reduction, which is not acceptable. As a result, excessive steel–concrete debonding can be considered as the SLS criteria.     

A combined BEM–FEM model for the velocity–vorticity formulation of the Navier–Stokes equations in three dimensions

This paper describes a combined boundary element and finite element model for the solution of velocity–vorticity formulation of the Navier–Stokes equations in three dimensions. In the velocity–vorticity formulation of the Navier–Stokes equations, the Poisson type velocity equations are solved using the boundary element method (BEM) and the vorticity transport equations are solved using the finite element method (FEM) and both are combined to form an iterative scheme. The vorticity boundary conditions for the solution of vorticity transport equations are exactly obtained directly from the BEM solution of the velocity Poisson equations. Here the results of medium Reynolds number of up to 1000, in a typical cubic cavity flow are presented and compared with other numerical models. The combined BEM–FEM model are generally in fairly close agreement with the results of other numerical models, even for a coarse mesh.     

A robust nano-mechanics approach for tensile and modal analysis using atomistic–continuum mechanics method

Nanoscale engineering has been developing rapidly. However, experimental investigations at the nanoscale level are very difficult to conduct. This research seeks to employ the same model to investigate an atomic-scale structure for tensile and modal analyses, based on atomistic–continuum mechanics (ACM) and a finite element method (FEM). The ACM transfers an originally discrete atomic structure into an equilibrium continuum model using atomistic–continuum transfer elements. All interatomic forces, described by the empirical potential functions, can be transferred into springs to form the atomic structure. The spring network models were also widely utilized in FEM based nano-structure studies. Thus, this paper attempts to explore ACM using three examples including silicon, carbon nanotube, and copper. All of the results are validated by bulk properties or literature.