•A 3D meso-scale model of concrete is constructed based on the grid mapping method.•Material parameters of HJC model for mortar, ITZ and aggregate are determined based on tests.•The meso-scale model ...is reliably validated both in the aspects of waveform data and specimen deformation process.•The SHPB tests of concrete specimen under various strain rates are successfully simulated.•The cracking mechanism of concrete in the SHPB test is revealed.•The formation mechanisms of different failure modes of the concrete specimen are revealed.
In the present study, a 3D meso-scale model of plain concrete has been developed with consideration of the aggregates, mortar and interface transition zone (ITZ) between them. Firstly, the generation methods of the random polyhedron aggregate and ITZ are introduced, as well as the process how to create the meso-scale finite element (FE) model of the concrete specimen through grid mapping method. Then, based on the experiment data and repeated numerical simulations, the appropriate material parameters of HJC model with respect to each ingredient in concrete are determined. Based on the waveform data and the specimen deformation and failure process in the split Hopkinson pressure bar (SHPB) test of the concrete specimen, the validation of the numerical model is well verified. Subsequently, the numerical SHPB tests of concrete specimen with the experiment size of Ф120 × 100 mm under various strain rates are performed. The formation processes of four typical failure patterns of the concrete specimen in the experiment are well reproduced, i.e. “slight-spalling”, “breaking”, “fragmentation” and “comminution”. What’s more, the cracking mechanism of concrete specimen in SHPB test is revealed. The formation mechanisms of various failure modes of the concrete specimen under different strain rates are further investigated. The research results show that the failure process of the concrete specimen in the SHPB test can be well simulated by the 3D meso-scale model developed in this paper. Furthermore, the accumulation of the tension strain damage in concrete is the main reason to initiate the cracks in the loading process. Under different loading conditions, due to the crush failure and the tension breaking failure derived from the compression expansion effect play different roles, the specimen deformation and failure processes of different modes will be exhibited.
Salt frost deterioration of concrete is an important durability problem to concern. Prior to clarifying the salt frost damage mechanisms, the salt effect on mechanical behavior of concrete needs to ...be clarified. To quantitatively understand the damage effects, this paper experimentally investigated meso-scale mortar samples suffered from sodium chloride (NaCl) attack, including mechanical degradation in meso scale and chemical analysis of calcium hydroxide and Friedel's salt in micro scale. The test results showed that calcium (Ca2+) leaching and salt crystallization occur along with the solution absorption process, and the kinetics is depending on water to cement ratio (W/C) and solution concentrations. Higher W/C has more severe Ca2+ leaching and strength loss, while the leaching amount and strength loss are changed in fluctuating manner with the solution concentrations. The amount of crystallized salt is increasing with the solution concentration. The mechanical degradation degree agrees with the Ca2+ leaching amount. The results indicated that the deterioration of mortar is mainly resulting from increasing porosity due to Ca2+ leaching.
The SAPO-34 zeotype with suitable physicochemical properties, was used to produce biodiesel as a clean fuel. The micropore nature of this catalyst limits the reaction because elainic acid molecules ...are too large to penetrate and access the internal pores of zeotype. The physicochemical properties of the synthesized catalysts were evaluated and compared using BET, XRD, TPD, EDX, FESEM, and HRTEM analyses. The simultaneous use of a carbon template and a heteroatom for doping in the synthesis of the catalyst (MCeAPSO-34) has a synergistic effect on increasing the pore diameter up to 4.1 nm. As expected, the MCeAPSO-34 zeotype gained 17, 12, and 27% more conversion rates than the CeAPSO-34 (doping effect), MSAPO-34 (carbon-template effect), and conventional SAPO-34 zeotypes in the production of biodiesel from elainic acid, respectively. Accommodation of CeO2 on the nanostructured MCeAPSO-34 zeotype by the solvothermal method increased the zeotype acidic sites from 73% to 89%. The use of ultrasound energy for distribution of the active phase increased the catalyst acidity and the conversion of 95% was obtained for the Ce/MCeAPSO-34 (U) catalyst. This catalyst has also converted 93% of the synthetic waste oil into biodiesel as a clean fuel.
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•The traditional smoothing kernel function in SPH method has been improved to realize the progressive failure processes of concrete.•The random aggregate generating method and the “region searching” ...method have been proposed to realize the generations of concrete meso-structures in SPH.•The SPH forms of humidity conduction equations have been derived, which can realize the simulations of humidity expansions in SPH.•The relationships between the humidity gradient and concrete shrinkage stress is defined, which is embedded into SPH constitutive equations to realize the simulations of concrete humidity – stress- damage coupling problems.
The dry shrinkage cracking of concrete structures has a great influence on their mechanical strengths. However, traditional numerical methods have some limitations in dealing with the shrinkage cracking processes of concrete. Based on this background, the traditional smoothing kernel function in Smoothed Particle Hydrodynamics (SPH) method has been improved, which can realize the simulations of progressive cracking processes of concrete; The SPH forms of humidity conduction equations have been derived, which can realize the simulations of humidity expansions in SPH; The relationships between the humidity gradient and concrete shrinkage stress is defined, which is embedded into SPH constitutive equations to realize the simulations of concrete humidity – stress- damage coupling problems; The random aggregate generating method and the “region searching” method have been proposed to realize the generations of concrete meso-structures in SPH. Firstly, the microscopic dry shrinkage cracking processes of a circular concrete model is simulated, and the rationality of the improved method is verified by the comparisons with previous experimental results. Then the influences of different factors (aggregate percentages, ambient humidity, shrinkage coefficient and moisture diffusivity) on the dry shrinkage cracking morphologies as well as the moisture transfer mechanisms of concrete are systematically discussed, results show that: The aggregate percentage affects the depths the “advantage cracks” penetrate into the concrete model; With the decrease of ambient humidity and moisture diffusivity, the density of shrinkage cracks increase sharply, and the crack length also increases; The penetration depth of the “advantage cracks” increases with the increase of the shrinkage coefficient. The research results can provide some references for the comprehensive understandings of the mechanisms of concrete shrinkage cracking and moisture migration. Meanwhile, it can also provide a certain understanding for the applications of SPH method into simulations of humidity – stress- damage coupling problems.
This paper proposes a meso-mechanical formulation aimed at explicitly simulating the contribution of both fibers and matrix in the cracking response of fiber-reinforced cementitious composite members ...subjected to bending actions. Specifically, based on an extension of a previous study inspired by the well-known non-linear cracked-hinge model, the present formulation is tailored for the case of High-Performance Concrete matrices whose specific features emerge from the observation of some experiments, presented in recent studies. The comparison between those experimental observations and the numerical results obtained in the present paper demonstrates the mechanical consistency of the aforementioned assumption. Moreover, they show the “saturation effect” of the fiber contribution, already observed experimentally in other studies available in the literature and points out the need to define an effective volume fraction for the specimens with higher quantities of fibers. A parametric analysis proposed in the final section of the paper shows the influence of the volume fraction on the main mechanical parameters characterising both the cracking phenomenon and the post-cracking response of High-Performance Fiber-Reinforced Concrete members in bending.
Expansive soils located in permafrost and seasonal frozen regions can easily suffer from the action of frost heaving and repeated freezing–thawing. When exposed to freeze–thaw (F–T) cycles, it may ...pose risk to civil engineering structures and thus causes heavy economic losses. In this study, a series of cylindrical expansive soil specimens were compacted at three different molding water contents (15%, 20% and 23%) and then subjected to a maximum of 12 closed–system F–T cycles. Besides, selected specimens compacted at the optimum water content were also tested under cyclic freezing–thawing with varying freezing temperatures (−5 °C, −10 °C and −20 °C). After each cycle of F–T, volume changes were measured and unconfined compression testing was also performed to estimate stress–strain behavior, resilient modulus and failure strength. Moreover, meso–structural analysis was conducted by using a simple optical test system to quantitatively extract the surface porosity and the pore orientation degree of expansive soil specimens after different F–T cycles. It is found that: 1) Volume changes for expansive soil specimens with higher and lower saturations present opposite directions and different magnitudes upon freezing, but show a similar trend of volume expansion after thawing. A moderate freezing temperature (i.e. –10 °C, in this study) has the greatest effect on volume changes. 2) Expansive soils tend to exhibit strain–softening behavior under unconfined compression conditions. The resilient modulus and failure strength decrease significantly at the first cycle of F–T and then reduce gradually to a stable value with increasing F–T cycles. The higher molding water content and a moderate freezing temperature will lead to a more pronounced degradation of mechanical behaviors with the F–T cycles. 3) It is suggested from the meso–structural analysis that the internal pores of expansive soils after a sequence of F–T cycles tend to become larger and more uniform, especially for the soil with higher water contents or experienced at a moderate freezing temperature.
•Volume changes and mechanical behavior at different water contents and freezing temperatures were investigated.•Freezing-induced volume changes is interpreted from the perspective of the inter- and intra-aggregate pores interaction.•Mechanical degradation upon F-T cycles are evident in higher water contents and a moderate freezing temperature.•Porosity and pore orientation of internal pores change upon F-T cycles.
There is urgent demand for the design of advanced coating materials for solid-phase microextraction (SPME) for water quality monitoring and assessment because of the global occurrence of ...chlorobenzenes (CBs). In this study, we proposed a dual-order activation method in which potassium hydroxide is used to modify pre-activated calcium citrate to synthesize a highly developed meso-/microporous carbon (MMC). The as-obtained MMC presented well-developed porosity with a super-high specific surface area (2638.09 m2 g−1), abundant meso-/micropores (0.5–10 nm), high hydrophobicity, excellent thermal stability (>720 °C), and a partly graphitized structure. As a coating material for headspace-SPME, the MMC-coated fiber exhibited outstanding extraction capability for CBs (up to 48.5 times higher than that of commercial fibers), which may be attributed to multiple interactions between the MMC and the pollutants, including size selectivity, micropore filling, π-π stacking and hydrophobicity. Finally, a satisfactory method using an MMC-coated fiber coupled with gas chromatography and electron capture detection was developed with good linearity (1–1000 ng L−1, R2 > 0.9982), high enrichment efficiencies (enrichment factors, 861–7819), low limits of detection (0.003–0.072 ng L−1), excellent repeatability (0.7–5.3%) and reproducibility (1.7–5.1%), and outstanding recoveries (90.18–103.02%) when applied to determine trace CBs in real water samples. These results suggest that MMC is a promising coating material for the SPME of CBs.
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•MMC was synthesized by a dual-order activation method.•MMC possess wide pore size distribution, graphene structure and hydrophobicity.•The MMC-coated fiber showed higher extraction performance than commercial fibers.•A satisfactory method was developed to determine trace CBs in water samples.
•The open-hole tensile properties of M3D woven composites are evaluated.•A macro-meso coupling and full-scale analysis model of OH-M3D specimen is proposed.•The failure mechanisms of OH-M3D specimen ...are analyzed using Micro-CT measurement.•Simulation of progressive damage evolution for OH-M3D woven composites is performed.
In the present work, the multiaxial 3D (M3D) woven composites and 3D layer-to-layer (3D LTL) interlock woven composites were designed and manufactured. Both unnotched and notched specimens of two composites were tested in quasi-static tension, and the macro mechanical behavior characteristics of all specimens were monitored using the digital image correlation (DIC) technique. Based on the meso-structure characteristics of the M3D woven composites, a macro-meso coupling and full-scale analysis model for the centrally notched specimens was proposed. Furthermore, the progressive damage analysis was performed to simulate the damage evolution processes of the open-hole (OH) specimens, and the failure mechanisms were revealed in cooperation with the CT measurement. The study concluded that the reduction in tensile strength for the OH-M3D composites was about 19.8% compared to the unnotched strength, which was significantly lower than the 3D LTL case. Besides, the strain field in the loading direction uniformly distributed on the surface of the OH-M3D specimen, and the notched plate failed along the + 45° and −45° directions, exhibiting a diamond like fracture behavior. The interwoven structure of M3D specimen with the + 45° and −45° yarns influenced the stress distributions in the vicinity of the hole and damage mode.
Porous media (PM) have the potential to enhance the flame stability and improve thermal performance in a micro/meso-scale burner. In this work, zirconia foam and high-porosity nickel foam are used in ...the meso-scale burners and partially premixed submerged combustion of liquid ethanol is achieved. An experimental investigation of the effects of foamed PM on flame structure, detailed temperature distributions, emissions, and thermal performance is undertaken. A blue flame can be observed in the nickel foam and the transient features of flame front is obtained. The flame front propagates intermittently when the combustion zone moves upstream. The maximum instantaneous flame speed in the PM is 6.519 mm/s and the average flame speed is 0.225 mm/s. The foamed PM in the meso-scale burners are shown to significantly improve the heat transfer efficiency. The burner in submerged combustion has a lower reaction zone temperature (806 °C vs. 1721 °C), lower exhaust gas temperature (419 °C vs. 1227 °C) and higher wall temperature than that with free flame. The heat transfer efficiency of the submerged flame is about twice that of the free flame.
•Submerged combustion characteristics of ethanol are investigated experimentally.•Zirconia foam and nickel foam increase the radiation output in a meso-scale burner.•A blue flame front is observed to move intermittently in the nickel foam.•Instantaneous flame speed in the foamed porous media is obtained experimentally.
•This is 3D discrete meso-scale approach and cracks are simulated directly.•3D shape of the reinforcement including its ribs is modeled.•Failure of RC beam column joint is successfully simulated by ...3D RBSM.•The failure process is revealed through the study of simulation results.•The effects of stirrups and an additional concrete block are discussed.
There is as yet no clear design method for a rational reinforcement arrangement in a beam-column knee joint with mechanical anchorages because internal stresses, internal cracks, and the failure process have not been well understood. Previous experiments have shown that local reinforcement along the anchorages or the placing of an additional concrete block on the top surface of the joint can prevent anchorage failure due to local stresses from the anchorage plates. In this study, a meso-scale discrete analysis using 3D rigid body spring model (RBSM) is conducted to investigate the failure process and to investigate the effect of these measures. By studying the internal stresses and cracks in a beam-column joint with mechanical anchorages, the failure process is revealed: with bonding along the development length of the anchorages, diagonal cracks begin to occur and these cracks propagate to the top surface of the beam-column joint where they open at the surface, and finally, the opening of diagonal cracks takes place. Local reinforcement placed along the anchorages is shown to have two effects: to increase bond performance along the development length of the anchorages and to restrict the opening of the diagonal cracks. An additional concrete block placed on the top surface of the beam-column joint also has two effects: to increase bond performance along the development length of the anchorages and to restrict crack penetration to the top surface of the joint.
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