Reproduction on complex behavior of rocks in actual case studies is undoubtedly the main aim of numerical modelling applied to rock mechanics. However, to achieve a reliably realistic simulation ...process a good knowledge of rock behavior is needed together with calibration or matching processes of actual and controlled tests on rock. Within this framework, in a previous approach, the authors calibrated PFC (Particle Flow Code) micromechanical parameters to reproduce in a reasonably accurate way the complete stress-strain response of granite samples under triaxial conditions in the lab. In this previous study, it was found that the number of different micro-parameters needed to define a PFC model was large, and that the influence of some of these parameters on rock response was not clear. In this new study, a sensitivity program is carried out in order to assess the influence of micro-mechanic parameters on the macroscopic response of the numerical simulation of rock samples. The sensitivity approach carried out shows that there is a direct connection between some parameters, for example, the bond cohesion and the bond tensile strength influences the peak strength and tensile strength of the rock, respectively. However, it is not clear how these parameters may influence other geomechanical macro properties, neither the exact influence of other parameters as the friction coefficient or the stiffness ratio. The present study addresses some of these issues, and it sheds light on which are most relevant parameters and how they control the main macro-properties of the simulated rocks.
Because of the carbonation treatment of recycled coarse aggregates (RCAs), the properties of the weak multiple interface transition zones (ITZs) in recycled aggregate concrete (RAC) can be improved, ...contributing to the enhanced behaviour of the RAC. In this study, different carbonation conditions (carbonation pressure, initial moisture contents of the RCAs and carbonation duration) were considered. A model of the RACs (MIRAC) was prepared to accurately locate multiple ITZs. Nanoindentation tests, scanning electron microscopy (SEM) tests were conducted to study the effect of carbonation treatment on the microproperties of RAC. At the same time, the influence of the carbonation treatment on the compressive strength and chloride ion penetration resistance of the RAC was evaluated. Moreover, the relationship between the microproperties and macroproperties of the RAC was discussed. The results showed that the modulus of the ITZs, the old mortar matrix and the new mortar matrix increased while the thickness of the ITZs decreased when the carbonation pressure or carbonation duration were increased. However, there was no further significant change after the carbonation pressure exceeded 1 bar, and the carbonation rate began to decrease significantly after 3 h of carbonation. The carbonation efficiency was superior when the moisture content was 1.81%. The effects of different carbonation parameters on the microproperties and macroproperties of the RAC were consistent. The compressive strength and chloride ion penetration resistance of the RAC had a linear correlation with the modulus of the ITZ between the aggregate and old mortar.
Using waste powder (WP) derived from construction and demolition (C&D) waste as supplementary cementitious materials in the preparation of waste powder concrete (WPC) provides a new way of reclaiming ...C&D waste. This paper focuses on investigating the microproperties and the water permeability of the WPC. A series of micro experiments and water permeability related tests were conducted. The results show that the elements and compound compositions of WP are similar with those of cement and fly ash. The addition of high fineness WP promotes the hydration rate and improves the pore structure of WPC due to its microaggregate filling effect. The incorporation of WP decreased the workability and drying shrinkage behavior of WPC, and the WPC with up to 30%WP has a satisfied compressive strength. The water permeability of the WPC, under capillary absorption and pressure, is lower than that of the control group, and the WPC with approximately 30%WP has the lowest water permeability. Subjected to the harsh environment of freeze-thaw cycles and applied loading, the addition of WP increases the sensitivity of the WPC to the imposed damage and water permeability. Although the imposed damage increases the water permeability of the WPC, incorporating appropriate content of WP leads to a lower water permeability of WPC than that of the control group when the exposure condition is the same.
Joint properties play a controlling role in the strength of rock mass. In response to the situation that existing researches on bolting mechanism of bolted rock joints principally concentrate on the ...macroelements, such as rock properties, bolting angle and joint morphology, the direct shear tests on unbolted and single-bolted rock joints under the conditions of different normal stress and different joint microproperties by numerical calculation method of particle flow (PFC) were carried out in this paper to reveal the microbolting mechanism and study the influences of joint properties. Subsequently, a comprehensive comparison of microfailure characteristics between unbolted and single-bolted rock joint demonstrated that during the shearing process, a triangle extrusion reinforcement area emerges around the bolt, where microcracks are extremely developed and rock blocks are considerably fractured, but it also improves the anti-shearing efficacy of rock joint. Meanwhile, both the macroshear behavior and microfailure characteristics of single-bolted rock joints with different joint microproperties was analyzed by comparing the shear stress–shear displacement curves and crack development. Specifically, the shear stiffness of single-bolted rock joints enhances with the increase of joint tangential stiffness, and the augmentation of joint tangential stiffness or friction coefficient intensifies the shearing resistance of single-bolted rock joints, whereas the joint normal stiffness was proven to share a negative correlation with the shear strength of single-bolted rock joints. Besides, the shear stiffness of single-bolted rock joints decreases approximately linearly with the increasing joint thickness. In addition, the thicker the joint, the lower the peak shear stress of single-bolted rock joints. With the increase of joint thickness, the crack number of single-bolted rock joint failure decreases, and the particle confinement of bolt is enhanced.
A new methodology for the calibration of bond microparameters in rocks represented by a package of joined random spherical particles in the discrete element method (DEM) framework is presented. ...Typically, calibration is achieved through a trial-and-error procedure using several DEM simulations of uniaxial compressive tests (UCTs). The bond calibration model (BCM) does not need a time-dependent UCT-DEM simulation to establish the relation between the microproperties of the bond and the macroproperties of the rock specimen. The BCM uses matrices to describe the interaction forces exerted by bonds and, by means of an assembly process similar to the finite element method, it can describe the complex network of bonds, enabling the model to capture small variations in particle size and bond distribution as demonstrated in this work. In this work, the BCM is presented and compared with UCT simulations performed using Esys Particle software. Multiple simulations are done with constant bond properties and different particle size ratios (
D
M
A
X
/
D
M
I
N
)
that cause small variations in the specimen’s Young’s modulus; these variations are well captured by the BCM with an error of <10%.
Micromechanical modeling of a fragmented claystone-a difficult waste material produced by open-pit coal mines in Northwestern Bohemia-is presented in this article. The PFC2D code, which accounts for ...the discrete nature of geomaterials and represents them as an assembly of unbonded or bonded particles, has been used. First, synthetic claystone was generated and the deformability and strength parameters were calibrated via numerical testing and comparing the results with those of available laboratory and field tests. The pre-peak, peak, and post-peak behavior of synthetic claystone was studied, and microscopic indicators of macroscopic behavior were selected and visualized. In order to simulate the dump material, joints were introduced that divided the claystone specimen into fragments. The appropriate microproperties of synthetic dump material were selected by means of a similar numerical testing and calibration procedure. Distribution and redistribution of particle contact forces before, during, and after failure of the dump material specimen were visualized and velocities corresponding to strain localization plotted. According to the study and some previous references, the compressive contact force chain acting in the direction of major principal stress appears as a backbone of microstructure, and compression induced tension as its basic failure mode at particle level.
Water treatment plant residuals management Dharmappa, H.B.; Hasia, A.; Hagare, P.
Water Science and Technology,
1997, Volume:
35, Issue:
8
Journal Article, Conference Proceeding
Peer reviewed
Water treatment plants produce enormous amounts of sludge. Due to increased environmental concern, recently, there is considerable amount of pressure on the water authorities for the safe disposal of ...the sludge. Currently, the most acceptable form of disposal is sanitary landfilling, which requires sludge to be dewatered to 20.40% of solids. One of the most commonly used dewatering techniques is sand drying beds. This process is popular because of its reliability, ease of use and low cost. However, one of the basic concerns with this process is the requirement of a large area of land. This could be overcome by chemical conditioning of the sludge, which can accelerate the dewatering rate substantially. At this stage, however, there are no guidelines for the design of the sand drying beds for treating chemically conditioned sludges.
This study is conducted to investigate the design and performance of sand drying beds for treating chemically conditioned water treatment plant (WTP) sludge. Experiments are conducted using laboratory scale sand drainage columns. The results indicate that the drainage rates, after adding polymer, increased by 9 times for clarifier sludge and 20-25 times for filter backwash sludge. Thus, the land area requirement for sand drainage bed can be drastically reduced through polymer addition. Optimal dose of polymer is found to be between 3 and 6 kg/t of dry solids, with dilute sludges requiring high dosages. The cake solids concentration increased from 3 to 12% as the polymer dose increased from 0 to 16 kgA of dry solids. The variation of drainage rates and cake solids with solids/hydraulic loading rates are found to vary depending on the type of sludges and polymers. The filtrate quality is found to be independent of polymer dose and solids loading rates.
AbstractIn this paper, research was conducted to evaluate and analyze the macromechanical properties and micromechanism of stone mastic asphalt (SMA) containing an eco-friendly fibrous substance, ...calcium sulfate whisker (CSW). The investigation included the preparation of SMA specimens containing the lignin fiber (0.3% by wt. of mixture) and CSW (0%, 6%, 8%, and 10%, by wt. of asphalt binder). Then, wheel tracking, low-temperature flexural bending, Marshall immersion, freeze-thaw indirect tensile strength, and flexural bending fatigue tests were carried out to evaluate the pavement performance of these mixtures. Scanning electron microscopy (SEM) analysis were selected to observe the microstructure and micromorphology of the mixture. The results of laboratory tests revealed that the rutting resistance, moisture susceptibility, and fatigue performance of SMA were improved by the addition of CSW, while the crack resistance at low temperatures decreased slightly. The flexural bending tensile strain, tensile strength ratio, and fatigue life of the mixtures (under different stress levels) showed a trend of initial increase followed by a decrease with increasing CSW content, while optimal results were present at 8% CSW. SEM images revealed that CSW and lignin fiber were uniformly distributed and formed a three-dimensional network structure. Besides physical infiltration and adsorption effects, an anchoring force similar to the mechanical bonding force also existed between CSW and asphalt binder. The economic benefit analysis indicated that SMA with 8% CSW showed the most cost-effective performance. Consequently, about 8 percent should be the ideal CSW content. The outcomes provide critical insights into improving the performance of SMA pavement and serve as an important reference for the development of sustainable pavement.
The flat joint contact model (FJM) provides significant improvements over its predecessors, the parallel bond and contact bond models, for bonded particle modelling of rocks due to its unique ...microstructure that allows for the reproduction of the macroscopic compressive–tensile strength ratio,
σ
c
/|σ
t
|
; internal friction angle,
ϕ
; and the Hoek–Brown constant,
m
i
. However, the microproperty calibration process is tedious and time-consuming to perform manually due to the various microproperty interdependencies that exist in the FJM. Previous attempts at automating the bonded particle model microproperty calibration process have typically utilised advanced statistical methods, such as artificial neural networks, but they have not yet been widely applied to the FJM over a representative range of confining stresses for calculation of
ϕ
and
m
i
. In this study, a new method is proposed for automating the FJM microproperty calibration process based on a numerical root-finding algorithm and specific calibration sequencing. The new method is applied to a Rewan Sandstone case study with similar natural porosity to a 2D bonded particle model packed to a low initial mean stress. The resulting FJM microproperties are shown to reproduce both the target macroscopic laboratory properties and a realistic damage evolution, including a normalised crack initiation stress of 0.46 and a normalised crack damage stress of 0.83 coinciding with a reversal of the axial stress–volumetric strain curve in an unconfined compression test simulation. It is also demonstrated that the absolute change in the instantaneous lateral–axial strain ratio (Poisson’s ratio in the linear-elastic phase) provides a reasonable proxy to the acoustic emissions which may be measured in the laboratory.
Highlights
Successful convergence of a novel automated microproperty calibration method for a Rewan Sandstone flat-jointed bonded particle model case study.
Reproduction of realistic damage evolution, including crack initiation and crack damage stress thresholds.
Microstructural properties including number of flat joint radial elements and installation gap control the macroscopic compressive-tensile strength ratio and dilatancy of the bonded particle model.
Expanded polystyrene (EPS) concrete is a low-carbon green building material, which has the properties of thermal insulation, but its strength is not high at present. To improve the strength of EPS ...concrete, EPS particles were wrapped on the surface by magnesium phosphate cement, ultra-high performance concrete, and waterborne polyurethane, they were used to make concrete after aging, and then the strength and microproperties of concrete were tested. The test results show that the wrapping material form a shell structure on the surface of EPS particles; the hydrophilic property of the wrapped EPS particles and the uniformity of the concrete have been significantly improved; the contact between the wrapped EPS particles and the cement base has become more closer; the cube compressive strength of concrete is increased by 80–110%, the flexural strength of concrete is increased by 40–90%. The research results of this study are of great significance for EPS concrete used as structural material and fabricated lightweight wallboard.
PDF
