Crack Extension and Possibility of Debonding in Encapsulation-Based Self-Healing Materials

The breakage of capsules upon crack propagation is crucial for achieving crack healing in encapsulation-based self-healing materials. A mesomechanical model was developed in this study to simulate the process of crack propagation in a matrix and the potential of debonding. The model used the extended finite element method (XFEM) combined with a cohesive zone model (CZM) in a two-dimensional (2D) configuration. The configuration consisted of an infinite matrix with an embedded crack and a capsule nearby, all subjected to a uniaxial remote tensile load. A parametric study was performed to investigate the effect of geometry, elastic parameters and fracture properties on the fracture response of the system. The results indicated that the effect of the capsule wall on the fracture behavior of the matrix is insignificant for tc/Rc ≤ 0.05. The matrix strength influenced the ultimate crack length, while the Young’s modulus ratio Ec/Em only affected the rate of crack propagation. The potential for capsule breakage or debonding was dependent on the comparative strength between capsule and interface (Sc/Sint), provided the crack could reach the capsule. The critical value of Sc,cr/Sint,cr was obtained using this model for materials design

Geographical and Temporal Similarity Measurement on Location-based Social Networks

Using "check-in" data gathered from location-based social networks, this paper proposes to measure the similarity of users by considering the geographical and the temporal aspect of their geographical and temporal aspects of their "check-ins". Temporal neighborhood is added to support the time dimension on the basis of the traditional DBSCAN clustering algorithm, which determines the similarity among users at different scales using the classical Vector Space Model (VSM) with vectors composed of the amount of visits in different cluster area. The spatio-temporal similarity of the user behaviors are obtained through overlapping the different weighted user similarity values. The experimental results show that the proposed approach is effective in measuring user similarity in location-based social networks.QC 201505021</p

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ОБУЧЕНИЕПРЕПОДАВАТЕЛЬСКИЙ СОСТАВ МЕДИЦИНСКИХ УЧЕБНЫХ УЧРЕЖДЕНИ

Preparation and Self-Healing Properties of Clinker/PVP Microsphere in Cement Paste

This paper presents a new insight into the autolytic mineral self-healing method for cementitious materials. The clinker/PVP (polyvinyl pyrrolidone) autolytic microsphere was prepared via the film coating method with cement clinker as a healing agent and PVP as the autolytic coating film. The morphology and chemical structure of the microsphere were characterized by environmental scanning electron microscopy (FESEM) equipped with energy dispersive spectrometer (EDS) and Fourier transform infrared spectroscopy (FTIR), respectively. The clinker retaining original mineral healing composition was successfully coated with a PVP film confirmed by FTIR. The maximum film thickness was 7.54 &mu;m, which was determined by laser particle size measurement. The autolytic behavior was measured using isothermal calorimetry and successfully controlled by pretreatment degree (i.e., silane coupling agent amount). Experimental results showed that the compressive strength recovery of cement paste with a 30% microsphere was 54% higher than ordinary cement paste specimens. The damage degree of the specimen was also decreased by adding the autolytic microsphere

Study on Autolytic Mechanism and Self-Healing Properties of Autolytic Clinker Microsphere in Alkaline Environment

In this study, the autolytic clinker microsphere with clinker as core and polyvinyl pyrrolidone (PVP) as coating film was prepared. Pretreatment of clinker with silane coupling agent was firstly processed during the preparation. To investigate the autolytic mechanism, the microstructures of the autolytic clinker microsphere at different curing ages were observed using environmental scanning electron microscopy (ESEM), equipped with an energy dispersive spectrometer (EDS). The autolytic stages were also identified based on the microstructural evolution. The influence of pretreatment degree on autolytic behavior was also studied by measurements of micro-morphology and isothermal calorimetry. Experimental results indicated that the compressive strength recovery of specimens was increased by 15–19% due to the addition of autolytic clinker microspheres. The recovery of compressive strength was also improved with the increase of pH value. The improvements in compressive strength recovery of specimens with microspheres were in the range of 15–19%, 15–31%, 25–36%, and 29–50% with the pH value of 7, 8, 10, and 12, respectively. It was also found that inner damage of cement-based matrix had greater recovery when pre-cracked specimens were cured in alkaline environments

Self-Shrinkage Behaviors of Waste Paper Fiber Reinforced Cement Paste considering Its Self-Curing Effect at Early-Ages

The aim of this paper was to study how the early-age self-shrinkage behavior of cement paste is affected by the addition of the waste paper fibers under sealed conditions. Although the primary focus was to determine whether the waste paper fibers are suitable to mitigate self-shrinkage as an internal curing agent under different adding ways, evaluating their strength, pore structure, and hydration properties provided further insight into the self-cured behavior of cement paste. Under the wet mixing condition, the waste paper fibers could mitigate the self-shrinkage of cement paste and, at additions of 0.2% by mass of cement, the waste paper fibers were found to show significant self-shrinkage cracking control while providing some internal curing. In addition, the self-curing efficiency results were analyzed based on the strength and the self-shrinkage behaviors of cement paste. Results indicated that, under a low water cement ratio, an optimal dosage and adding ways of the waste paper fibers could enhance the self-curing efficiency of cement paste

Scaling of nanoscale elastic and tensile failure properties of cementitious calcium-silicate-hydrate materials at cryogenic temperatures: A molecular simulation study

International audienceThis paper investigates the scaling of the nanoscopic elastic and tensile failure properties of calcium-silicate-hydrate (C-S-H). We report a Zhurkov-like scaling behavior for disordered C-S-H of various compositions at cryogenic temperatures, using molecular dynamics simulations. To this end, we first propose a revised molecular construction route to generate C-S-H atomic configurations with varying compositions. Then, we investigate how the tensile behavior evolves with temperature, system size, and strain rate. Our simulation results show that tensile strength, Young’s modulus, fracture energy, and fracture-process zone (FPZ) length, all follow a Zhurkov-like scaling law providing a general temperature-size-time equivalence. Such scaling laws make it possible to extrapolate molecular simulation results to larger length and/or time scales. Detailed analysis shows that the typical FPZ length of C-S-H is about 150 Å, and the maximum reduction of activation energy barriers for tensile failure are 0.497 and 0.446 eV for Ca/Si of 1.7 and 1.1, respectively

Molecular simulation of the confined crystallization of ice in cement nanopore

Freezing of water under nanoconfinement exhibits physical peculiarities with respect to the bulk water. However, experimental observations are extremely challenging at this scale, which limits our understanding of the effect of confinement on water properties upon freezing. In this study, we use molecular dynamic simulations to investigate how confinement affects the kinetics of growth of ice and the thermodynamic equilibrium of ice-liquid coexistence. TIP4P/Ice water model and CSH-FF model were applied to simulate ice crystallization in a confined cement system at temperatures down to 220 K. We adapted an interface detection algorithm and reparameterized the CHILL/CHILL+ algorithm to capture ice growth. The confinement leads to a shift of the maximum growth rate of ice to a higher temperature than for bulk water. Both the confinement and surface impurities contribute to slowing down the ice growth. For the ice-liquid coexistence at equilibrium, we derive a formulation of Thomson’s equation adapted to statistical physics quantities accessible by molecular simulation, and we show that this adapted equation predicts accurately the melting line of bulk and confined ice Ih as a function of pressure. The confinement decreases systematically the melting temperature of ice of about 5 K compared with bulk ice Ih. A premelted water film about 1 nm thick is observed between the solid wall and ice, and its thickness is found to decrease continuously as temperature is lowered. We note that the surface impurities are key to the formation of the premelted water nanofilm when the temperature is lower than 250 K