Discrete element modelling of the fresh state behavior of pervious concrete

The objective of this paper is to propose a novel numerical approach to evaluate the fresh state behavior of pervious concrete (PC). A new constitutive law applied to Discrete Element Method was developed for the simulation of the interaction between aggregates connected by fresh cement paste. Several innovative aspects regarding the type of contact model, the consideration of the contact bridge and the inclusion of the rheology coefficients were proposed. A calibration was performed with data from the literature. Furthermore, an extensive experimental program considering different shapes of aggregates, grading curves and aggregate-to-cement paste ratios was conducted to evaluate the fresh state behavior and validate the numerical models. The good fit obtained between numerical and experimental results confirm the model and the constitutive law reproduce the under uniaxial compaction, thus representing a step forward in the design and application of pervious concrete mixes.     

Simulation of self-compacting concrete flow in the J-ring test using smoothed particle hydrodynamics (SPH)

A range of SCC mixes with 28-day cube compressive strength between 30 and 80 MPa has been prepared in the laboratory, and the time t_500J (the time when the mix spread reaches 500 mm) and diameter of the flow spread of each mix were recorded in the J-ring test. The entire test was then simulated from the moment the cone was lifted until the mix stopped flowing. An incompressible mesh-less smoothed particle hydrodynamics (SPH) methodology has been implemented in this simulation and a suitable Bingham-type constitutive model has been coupled with the Lagrangian momentum and continuity equations to simulate the flow. The aim of this numerical simulation was to investigate the capabilities of the SPH methodology to predict the flow of SCC mixes through gaps in reinforcing bars. To confirm that the mix flows homogeneously, the distribution of large coarse aggregates in the mixes has been simulated and examined along several cut sections of the flow pancake. It is revealed that all the simulated mixes meet the passing ability criterion with no blockage as in the laboratory J-ring test with respect to t_500J, the flow spread, and aggregate homogeneity.     

Mg2+-doped Na3V2(PO4)3/C decorated with graphene sheets: An ultrafast Na-storage cathode for advanced energy storage

NASICON-type Na₃V₂(PO₄)₃ is one of the most promising cathode materials for sodium-ion batteries, delivering about two Na⁺-ions extraction/insertion from/into the unit structure. However, the low electronic conductivity which leads to bad rate capability and poor cycle performance, limits its practical application for sodium-ion batteries. To overcome the kinetic problem, we attempt to prepare the carbon-coated Na₃V₂(PO₄)₃ nanocrystals further decorated by graphene sheets and doped with Mg²⁺ ion via the two steps of sol-gel process and solid-state treatment for the first time. Such architecture synergistically combines the advantages of two-dimensional graphene sheets and 0-dimensional Mg²⁺-doped Na₃V₂(PO₄)₃/C nanoparticles. It greatly increases the electron/Na⁺-ion transport kinetics and assures the electrode structure integrity, leading to attractive electrochemical performance. When used as sodium-ion batteries cathode, the hybrid composite delivers an initial discharge capacity of 115.2 mAh g⁻¹ at 0.2 C rate, and retains stable discharge capacities of 113.1, 109.0, 102.4, 94.0 and 85.2 mAh g⁻¹ at high current rates of 1, 2, 5, 10 and 20 C rate, respectively. Thus, this nanostructure design provides a promising pathway for developing high-performance Na₃V₂(PO₄)₃ material for sodium-ion batteries.     

Evaluation of China's local enforcement of energy efficiency standards and labeling programs for appliances and equipment

AimsThis paper aims to evaluate local enforcement of China's mandatory appliance and equipment energy efficiency standards and labeling programs, two increasingly important policies for meeting national energy and carbon reduction targets. The expected energy savings of efficiency standards and labels can be fully realized only with strong enforcement to ensure compliance for all products sold. This paper provides comprehensive retrospective evaluation of the methodologies, results, progress and remaining challenges in pilot enforcement projects initiated in the absence of consistent national check-testing focused on energy efficiency.ScopeThis paper's scope is focused on 2006–2009 pilot local check-tests conducted to verify appliance and equipment compliance with China's mandatory energy label and efficiency standards.ConclusionsThis paper finds both improvement and some backsliding in compliance rates over time. Compared to earlier efforts, 2009 check-tests covered a wider regional and product scope but demonstrated greater variation in compliance rates. Labeling display and energy efficiency compliance was generally high across regions and most products, but lower compliance rates were observed in less economically developed regions and for lighting and industrial products. Based on these findings, areas for improvement in local awareness, product sampling methodology, check-testing tools and procedures are identified.     

Correlation between the adsorption ability and reduction degree of graphene oxide and tuning of adsorption of phenolic compounds

Graphene oxide (GO) was prepared with a modified Hummers method and then reduced to different reduction degrees by using hydrazinehydrate. The obtained GO and reduced GO (RGO) were characterized. It was found that the reduction removed most of the oxygen-containing functional groups on the surface of GO. By using naphthalene as a probe, the interaction between RGO and organic molecules was evaluated with NMR. It was confirmed that the reduction of GO increased significantly the interaction between the π system of graphene and the π unit of organic molecules. The thermodynamic analysis indicated that the adsorption was a spontaneous, exothermic and entropy-decreasing process. It was observed that the adsorption capacities were generally increased with increasing the reduction degree of GO. The chemical structures of phenolics also affected their adsorption on RGO. The adsorption of the phenolics on RGO was enhanced by introducing electron-donating and withdrawing functional groups on the benzene ring. Depending on the chemical structures of phenolics, the surface reduction of GO to RGO-1 significantly increased the adsorption capacity for phenolics by a factor as large as 235%. A possible adsorption mechanism and correlation between the adsorption ability, reduction degree of GO and chemical structures of phenolics was discussed.     

Microstructural and bulk property changes in hardened cement paste during the first drying process

This paper reports the microstructural changes and resultant bulk physical property changes in hardened cement paste (hcp) during the first desorption process. The microstructural changes and solid-phase changes were evaluated by water vapor sorption, nitrogen sorption, ultrasonic velocity, and ²⁹Si and ²⁷Al nuclear magnetic resonance. Strength, Young's modulus, and drying shrinkage were also examined. The first drying process increased the volume of macropores and decreased the volume of mesopores and interlayer spaces. Furthermore, in the first drying process globule clusters were interconnected. During the first desorption, the strength increased for samples cured at 100% to 90% RH, decreased for 90% to 40% RH, and increased again for 40% to 11% RH. This behavior is explained by both microstructural changes in hcp and C–S–H globule densification. The drying shrinkage strains during rapid drying and slow drying were compared and the effects of the microstructural changes and evaporation were separated.     

Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics,microstructure changes and moisture properties

The purpose of this article is to investigate the carbonation mechanism of CH and C-S-H within type-I cement-based materials in terms of kinetics, microstructure changes and water released from hydrates during carbonation. Carbonation tests were performed under accelerated conditions (10% CO₂, 25 °C and 65 ± 5% RH). Carbonation profiles were assessed by destructive and non-destructive methods such as phenolphthalein spray test, thermogravimetric analysis, and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was carried out at different ages from 1 to 16 weeks of CO₂ exposure on cement pastes of 0.45 and 0.6 w/c, as well as on mortar specimens (w/c = 0.50 and s/c = 2). Combining experimental results allowed us to improve the understanding of C-S-H and CH carbonation mechanism. The variation of molar volume of C-S-H during carbonation was identified and a quantification of the amount of water released during CH and C-S-H carbonation was performed.     

Effects of sustained axial load and cooling phase on post-fire behaviour of reinforced concrete stub columns

In this paper is presented an experimental investigation of the effects of preload and cooling phase on the residual strength, stiffness and ductility of reinforced concrete stub columns which were heated and cooled down to room temperature under sustained axial load. Reinforced concrete stub columns were axially loaded and heated to designed temperatures in a specially built electrical furnace. After the specimens cooled down to ambient temperature with the axial loads kept constant, the stub columns were loaded to failure. The sustained preload led to significant residual deformations of reinforced concrete stub columns during the cooling phase. The test results showed that the mechanical behaviour of the fire-damaged reinforced concrete stub columns with preload was remarkably different from those without preload. The sustained axial loads resulted in obviously increased strength and stiffness during the loading phase, but reduced stiffness and deteriorated ductility in the unloading phase. Based on the test results, it is recommended that the effects of sustained axial loads during the fire and cooling phase should be taken into consideration in assessing the fire-damaged reinforced concrete columns.     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.     

Effect of sodium silicate- and ethyl silicate-based nano-silica on pore structure of cement composites

This study proposes using sodium silicate-based nano-silica (SS) in cement composites. The effect of the addition of the proposed nano-silica on cement composites was compared to that of conventional ethyl silicate-based nano-silica (ES) and silica fume (SF). This study found that the inclusion of SS in cement composites has mainly two effects on their properties: one is a fast pozzolanic reaction, and the other is a pore-filling effect in a cement matrix. As a result, SS dramatically improves the early-age strength of cement composites by up to 184% and 152%, compared to a control specimen and the specimen with ES inclusion, respectively. Calorimetry, X-ray diffraction (XRD), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP) tests were conducted to monitor the effects of these nano-silicas.