Cemented Granular Materials Research Articles

Characterizing interfacial behavior of structurally cemented granular material using a large three-dimensional contact surface testing machine and a mesoscale bond model in DEM

Structuralized cementing technology takes full advantage of non-pressure controlled grouting technology to fill pores in coarse aggregates using self-compacting material with no need for vibration or mixing. By cementing the aggregates in a structured manner, structurally cemented material has properties between those of a discrete granular material and a continuous medium. The interface between different component structures exerts an important effect on the physical properties of the whole material. In this paper, the monotonic and cyclic mechanical properties of the contact interface properties of a structurally cemented material under different normal stresses are studied by performing experiments and numerical simulations. Through the study, the deformation displacement time history and volume change with the shear process are provided, the deformation process of the sample near the contact interface during the shear process is discussed, and the shear mechanics law with the strength mechanism under the simple shear condition is summarized.

Geometry smoothing and local enrichment of the finite cell method with application to cemented granular materials

AbstractIn recent times, immersed methods such as the finite cell method have been increasingly employed in structural mechanics to address complex-shaped problems. However, when dealing with heterogeneous microstructures, the FCM faces several challenges. Weak discontinuities occur at the interfaces between the different materials, resulting in kinks in the displacements and jumps in the strain and stress fields. Furthermore, the morphology of such composites is often described by 3D images, such as ones derived from X-ray computed tomography. These images lead to a non-smooth geometry description and thus, singularities in the stresses arise. In order to overcome these problems, several strategies are presented in this work. To capture the weak discontinuities at the material interfaces, the FCM is combined with local enrichment. Moreover, the L$$^2$$ 2 -projection is extended and applied to heterogeneous microstructures, transforming the 3D images into smooth level-set functions. All of the proposed approaches are applied to numerical examples. Finally, an application of cemented granular material is investigated using three versions of the FCM and is verified against the finite element method. The results show that the proposed methods are suitable for simulating heterogeneous materials starting from CT scans.

Micro and macro mechanical characterization of artificial cemented granular materials

Micro and macro mechanical characterization of artificial cemented granular materials

Anisotropic Breakage Mechanics for cemented granular materials

The anisotropy of granular geomaterials is sensitive to their fabric, which exhibits anisotropic mechanical properties as a function of deposition history, microscopic fabric, and loading paths. Here, a new fabric-enriched continuum breakage model is proposed to examine the relation between elastic and inelastic anisotropy in granular materials and cemented granular materials. A microstructure model is first implemented in the framework of fabric-enriched continuum breakage mechanics (F-CBM), where the anisotropic behaviour prior to yielding is introduced through a symmetric second-order fabric tensor embedded in the expression of the elastic energy potential. The anisotropic strain energy storage prior to grain crushing leads to the rotation and distortion of the yield surface of cemented granular materials. Parametric analyses are performed to assess the overall capability of the model to characterize the anisotropic inelastic processes in cemented granular. It is shown that the proposed model can accurately predict the strong correlation between anisotropic elasticity and breakage-damage processes in cemented granular materials. When damage involving the skeleton is the dominant inelastic process, the size of the elastic domain contracts and the material exhibits augmented brittleness with the disintegration of cement. While breakage processes are predicted to dominate the response of lightly cemented granular materials resulting in hardening behaviour. This work can be further extended to dynamically capture the anisotropic response of cemented granular materials with water-sensitive mineral constituents by accounting for the evolution of microstructural anisotropy.

Exploration on electrical resistance tomography in characterizing the slurry spatial distribution in cemented granular materials

This study investigated the application of electrical resistance tomography (ERT) in characterizing the slurry spatial distribution in cemented granular materials (CGMs). For CGM formed by self-flow grouting, the voids in the accumulation are only partially filled and the bond strength is often limited, which results in difficulty in obtaining in situ samples for quality evaluation. Therefore, it is usually infeasible to evaluate the grouting effect or monitor the slurry spatial distribution by a mechanical method. In this research, the process of grouting cement paste into high alumina ceramic beads (HACB) accumulation is reliably monitored with ERT. It shows that ERT results can be used to calculate the cement paste volume in the HACB accumulation, based on calibrating the saturation exponent n in Archie’s law. The results support the feasibility of ERT as an imaging tool in CGM characterization and may provide guidance for engineering applications in the future.

Acoustic monitoring of compaction in cohesive granular materials.

We study the transition from cohesive to noncohesive states of cemented granular materials (synthetic rocks) under oedometric loading, combining simultaneous measurements of ultrasound velocity and acoustic emission (AE: microseosmicity). Our samples are agglomerates made of glass beads bonded with a few percent of cement, either ductile or brittle. These cemented granular samples exhibit an inelastic compaction beyond certain axial stresses likely due to the formation of compaction bands, which is accompanied by a significant decrease of compressional wave velocity. Upon subsequent cyclic unloading-reloading with constant consolidation stress, we found the mechanical and acoustic responses like those in noncohesive granular materials, which can be interpreted within the effective medium theory based on the Digby's bonding model. Moreover, this model allows P-wave velocity measured at vanishing pressure to be interpreted as an indicator of the debonding on the scale of grain contact. During the inelastic compaction, stick-slip-like stress drops were observed in brittle cement-bonded granular samples accompanied by the instantaneous decrease of the P-wave velocity and AEs which display an Omori-like law for foreshocks, i.e., precursors. By contrast, mechanical responses of ductile cement-bonded granular samples are smooth (without visible stick-slip-like stress drops) and mostly aseismic. By applying a cyclic loading-unloading with increasing consolidation stress, we observed a Kaiser-like memory effect in the brittle cement-bonded sample in the weakly damaged state which tends to disappear when the bonds are mostly broken in the noncohesive granular state after large-amplitude loading. In this paper, we show that the macroscopic ductile and brittle behavior of cemented granular media is controlled by the local processes on the scale of the bonds between grains.

Mesoscale FEM approach on cemented sands: Generating and testing the digital twin

The mechanics of Cemented Granular Material (CGM) have been studied by means of geotechnical experimental testing, whose output consists the basis of mathematical models which approach the material response in various loading states. The information derived from standard experimental response curves is the basis of understanding and handling the material. Still, it is intuitive to analyse the CGM down to the mesoscale and dray conclusions over the interaction of the constituent material phases. Diverging from the practice of equivalent continuum, the alternative description of a three phase composite of sand particles, cement binder and void pores has been realised in this study. In order to implement the specific morphology of this multiphase, geomaterial, X-ray Computed Tomography is used to capture the internal structure and quantify it into a three dimensional greyvalue map (or a three dimensional image). The distinction of the material phases is made possible by the application of a developed filter, which corrects the artefacts caused by beam hardening phenomena and allows for the generation of a phase segmented equivalent image. An image adapted meshing algorithm has been utilized to transform the labelled image into a tetrahedral mesh, grouped into sets that correspond to the different materials. The tetrahedral domain was assigned boundary conditions and was numerically tested under uniaxial compression using the finite element method. The kinematics of the simulation proved that the mesoscale approach, which carries internal structure information of the granular fabric and the cement paste distribution, provides a output which captures the kinematics of the granular skeleton.

Effects of cementing matrix characteristics and particle size on the adhesion rule and mechanical properties of cemented granular materials

The process of filling self-compacting cement grout (SCCG) into granular packing without disturbing the granular skeleton culminates in the formation of a well-defined cemented granular materials (CGMs). The inherent attributes of both the cementing matrix and granular packing critically dictate the adhesion behavior and mechanical performance of CGMs. A succession of cement grout flow tests was conducted to elucidate the influence of particle size and the SCCG flowability on SCCG adhesion mechanism within granular packing. Furthermore, the influence of particle size and the cementing matrix volume on the mechanical characteristics of CGMs was scrutinized via uniaxial compressive testing. The results illuminate that the adhesion behavior of SCCG within granular packing is predominantly dependent on the yield stress of SCCG and the average particle size. Additionally, it is established that the peak strength of CGMs is intimately intertwined with the cementing matrix volume and the average particle size, as corroborated by a substantial aggregate of 135 uniaxial compression tests.

On the load bearing mechanisms of cemented granular material: A mesoscale FE approach

AbstractThe numerical investigation of cemented granular material (CGM) is a demanding task that requires the combination of various scientific disciplines. At the mesoscale, CGM is decomposed into the constituents of particles, cement matrix and void pores. The combination of these constituents, as found in nature or in civil infrastructure projects, creates a highly heterogeneous structure, whose morphology defines the response of the CGM under mechanical loading. The composite internal structure is quantified by means of x‐ray Computed Tomography, which provides 16‐bit grayscale 3D images of the scanned microsamples. These images are decomposed into the three constituent materials to be investigated and modeled. Still, the ability of simple Cartesian grids to adequately carry information about curved geometries and provide accurate representations is questioned. In order to overcome staircase effect errors, the interfaces of the material inclusions are smoothed and re‐defined, making use of Gaussian blurred signed distance fields (SDF). These level‐set derived interfaces serve as a reference for the volume reconstruction of the image via tetrahedra. The comparison of the raw and the smoothened images to analytical geometry Standards suggests that the above reconstruction alleviates the grid's inherited uncertainities. In addition, a comparison between a non smoothed and a smoothed meshed image is performed in terms of mechanical response, utilizing the Finite Element (FE) Method. The results suggest that even a slight change in the granular contact fabric can alter the unfolding stress transmission mechanisms.

Investigation of the load sustaining micro mechanisms of cemented sand using the mesoscale FEM approach

While the mechanics of cemented granular material have already been approached by means of constitutive modeling, the capacity of introducing the precise internal structure of multiphase media in numerical simulations offers an alternative perspective of investigation. In specific, means of X-ray computed tomography are able to quantify the internal irregular geometry of the composite material in the form of three-dimensional images. Image analysis can be applied in order to distinguish the topology of each material phase and advanced meshing techniques are able to form a twin meshed domain. The current paper deals with the quantitative description of the morphology of small cemented sand samples, the application of developed image analysis algorithms to treat imaging artefacts (mainly due to beam hardening), the equivalent mesh generation via the image adapted meshing technique and the numerical testing using the Finite Element Method. Four samples are imaged, meshed and numerically tested in uniaxial compression; the mesoscale finite element simulations are able to display the intergranular stress transmission chains. A statistical analysis of the load carried by the granular skeleton provides evidence of the influence of the cementation degree and the compaction of the aggregates on the uniformity of the stress distribution.

A DEM-based approach for modeling the thermal-mechanical behavior of frozen soil

The mechanical characteristics of frozen soil under various temperatures have theoretical and engineering significance for improving the efficiency of permafrost excavation and ensuring the stability of permafrost area engineering. A novel approach based on Discrete Element Method (DEM) is proposed to simulate the temperature effect of frozen soil. An existing 3D contact model for cemented granular material is extended to include the effects of temperature on bond strength and modulus of ice cementation, which are verified by existing analytical solutions and experimental data, and a simplified method is employed to simulate the influence of ice crystal expansion on the void ratio of frozen soil, whereafter, the modified contact model is calibrated by the results of the temperature-controlled triaxial test. This approach was proven to be effective by comparing the simulation results with the experimental results. The micro mechanical behavior of frozen soil samples was studied. The results showed that different types of bond contact had different effects in the shearing stage. The structural ‘weakening’ and ‘strengthening’ of the frozen soil occur simultaneously during the loading process, and the structural damage is mainly caused by the plastic damage induced by the relative slip between soil and ice particles.

DEM analysis of crushing evolution in cemented granular materials during pile penetration

The present study demonstrates that the 3D discrete-element method provides a practical model approach to visualize the cemented grain crushing evolution under pile penetration. A combined method using a rigorous breakage criterion based on octahedral shear stress (OSS) was implemented in the particle flow code PFC3D. First, the pile penetration is simulated by considering the grains as uncrushable with a screening of highly stressed grains exceeding the threshold defined by the OSS failure condition. Then, the simulation is repeated where crushable agglomerates replace the highly stressed grains. This method is more accurate than the replacement method and more efficient than the agglomerate method. A quarter of the numerical model was considered after validation to achieve an acceptable computational time for parametric studies. Parametric investigations were performed on the effects of particle crushing, boundary conditions, pile tip shape, and pile penetration velocity on the penetration resistance behavior. In agreement with the observations of the physical calibration chamber, the present results indicate that the proposed modeling approach is reliable in reproducing the concentration of crushed granular material particles in the vicinity of the pile tip and shaft. In addition, grain crushing has been found to reduce both penetration and shaft resistance.

X-ray Microtomography Study on the Cemented Structure of Cemented Granular Materials

X-ray Microtomography Study on the Cemented Structure of Cemented Granular Materials

A novel peridynamics modelling of cemented granular materials

A novel peridynamics modelling of cemented granular materials

A mesoscale bond model for discrete element modeling of irregular cemented granular materials

Cemented granular materials, formed by pouring self-compacting cement into coarse aggregate, have been widely used in geotechnical engineering. In this study, we developed a mesoscale bond model to study the mechanical behavior of cemented granular particles, considering the amount and distribution of cement. A discrete element method with clumped particles is used to simulate irregularly shaped particles. Voronoi tessellation is used to analyze the void structure, which forms the basis for determining the cement distribution and geometric properties of the cement bond between the clumped particles. Numerical simulations are compared with experimental uniaxial compression tests, with the cement ratio ranging from 5 to 50%. The numerical model is also used to study the fracture development and breakage patterns of cemented granular samples. The numerical results are consistent with the experimental results, indicating that this numerical model can simulate the behaviors of cemented granular materials with a wide range of cement ratios.

Realistic modeling of cemented granular materials with a lattice spring model by developing a digital 3D reconstruction approach

AbstractThree‐dimensional (3D) geometric reconstruction is a fundamental requirement to realistically predict the mechanical behavior of cemented granular materials (CGMs). In this work, a four‐phase geometric reconstruction method for CGMs, involving aggregate, cement, in‐between interfacial transition zone (ITZ), and void phases, is developed by using digital image processing techniques and an optimization algorithm. The reconstruction method includes four steps. (1) The planar outlines of aggregates are extracted from a two‐dimensional (2D) aggregate image (e.g., gravels, pebbles, and crushed stones) based on the image segmentation algorithm. (2) Through spatial geometric transformations on the acquired planar outlines, the 3D aggregates are generated, whose reliability is verified by comparison with those obtained by a 3D scanner. (3) According to the volume ratio of each phase, the initial four‐phase model is reconstructed by randomly placing aggregates into the user‐defined domain with considering the size distribution of aggregates, followed by the generation of the ITZ represented by their surrounding minimum envelope surface, and the insertion of voids in the rest cement phase. (4) The final realistic CGM is reconstructed by optimizing the spatial distribution of the void and cement phases based on a simulated annealing algorithm. Afterwards, the rationality of the reconstructed CGM is verified by comparing two‐point probability functions between the physical model and the reconstructed model. With the aid of a four‐dimensional lattice spring model (4D‐LSM), the feasibility of the reconstructed CGM to reproduce its mechanical behavior is demonstrated by numerical examples and a comparison with existing experimental or other numerical solutions.

Experimental study on the macroscopic and microscopic mechanical properties of simulated cemented granular materials

As solid matrices can bond with other materials and enhance the mechanical properties of engineering structures, investigating the mechanical properties and cementation mechanism of cemented granular materials has become increasingly significant. However, most experiments were conducted only at macroscopic or microscopic levels. Further, it is difficult to compare and analyze the results at macroscopic and microscopic levels because of the complex spatial and mechanical effects between cemented particles at the mesoscale. In this study, we created a well-defined cohesive granular material with controllable cementation held together by mixing glass beads and a solid matrix. First, we explored the influence of the confining pressure and solid matrix content on the mechanical strength of the cemented granular materials through a consolidated undrained triaxial compression test to address the cementation effect quantitatively. Second, through tensile and shear tests on cemented units with different matrix volumes, the cementation mechanism of the cemented units was explored for the first time from the perspective of the stress–strain relationship. Finally, we qualitatively explained the mechanical phenomena of macroscopic cemented granular materials through the microscopic interaction between particles and realized a multi-scale analysis of the mechanical response of cemented granular materials, which may provide new ideas for the study of cemented granular materials.

A breakage–damage framework for porous granular rocks in surface-reactive environments

High-porosity granular rocks are moisture-sensitive solids, in that their strength and deformability are modulated by the relative humidity of their environment. Here, a novel continuum breakage–damage framework is developed to characterize and simulate the inelasticity of cemented granular materials subjected to changes of relative humidity. A microstructural model is proposed to describe the evolution of the solid–fluid interfaces emerging from grain breakage and cement disintegration. The proposed thermodynamic framework links the microstructural model to the energy dissipation and macroscopic rate-dependence of sandstones . The performance of the model is assessed against experimental data for sandstones subjected to loading under both dry and wet conditions. It is shown that the proposed model can accurately predict the yielding and stress–strain response of sandstones by capturing the moisture-weakening effects. The simulations of the model indicate that the increase of moisture lowers the yield stress and reduces the brittleness of the post-yielding response of variably saturated cemented granular materials. It is shown that the rate of damage and breakage growth control the distortion of the yield surface. When inelasticity is dominated by damage, cement bonds are disintegrated and the yield surface shrinks, thus resulting into augmented brittleness. By contrast, and in agreement with experimental evidence, the response of lightly cemented granular solids is found to be dominated by the breakage of the skeletal grains. As a result, changes in relative humidity are predicted to be accompanied by hardening behavior.

Influence of cementation on the yield surface of rocks numerically determined from digital microstructures

Digital Rock Physics has reached a level of maturity on the characterisation of primary properties that depend on the microstructure – such as porosity, permeability or elastic moduli – by numerically solving field equations on μCT scan images of rock. After small deformations or at depth though, most rocks eventually reach their limit of elasticity and the complementary plastic properties are needed to describe the full mechanical behaviour. Currently, determination of a rock’s yield surface from its microstructure is often restricted to semi-analytical criteria derived by limit analysis or numerical simulations performed on idealised geometries. Such simplification lacks representativeness, particularly for processes that affect directly the pore-grain interface such as the cementation phenomenon, happening during diagenesis. Eventually, only direct numerical simulation of elasto-plasticity performed on digitalised microstructures can be used to assess the strength of different cemented materials and its evolution with the alteration of the microstructure. In this study, we provide a comprehensive parametric study on the impact of cementation on rock strength for real microstructures of cemented granular materials. Compared to most previous studies, the whole yield surface is determined numerically (using Finite Element Method) in order to assess the influence of cementation for different stress-paths. The previously known tendency of rock to strengthen with increasing cementation volume is verified. New results on the influence of cement property namely Young’s modulus, friction and cohesion on the rock’s yield surface are explored. The envelopes obtained are compared to the ones obtained by experimental data and existing models. The framework presented in this study showcases the wider possibility of determining any rock’s or porous material’s yield surface from its microstructure.

Adhesion behavior and deposition morphology of cement grout flowing through granular materials

The liquid distribution of unsaturated wet granular media composed of granular materials and Newtonian fluid has been in detail studied. However, when a granular material is mixed with a certain amount of Bingham fluid, the effect of rheological properties of Bingham fluid on the deposition morphologies is not clearly explored. In this study, the adhesion rules of fresh cement grout flowing through granular materials without considering the filtration effect is investigated, and the deposition morphologies of cement grout in random packing of spheres is observed with the help of X-ray microtomography device. The results show that the deposition mechanism of cement grout is significantly dependent on its yield stress. The cement grout spontaneously forms open structures with large liquid–air interface. The morphologies of cement grout within cemented granular materials vary with grout content. With the increase of the yield stress of cement grout, neighbouring smaller clusters gradually touch each other and coalesce into more complex structure, and the largest cluster can contain almost all the cement grout.