Measurement of Bulk Mechanical Properties and Modeling the Load Response of Rootzone Sands. Part 3: Effect of Organics and Moisture Content on Continuous Sand Mixtures

The interactions between organics and sand particles at different moisture contents are important in understanding the general mechanical behavior of rootzone sand mixtures. Towards this end, eight rootzone sand mixtures (4 shapes ×2 moisture contents) used in golf green construction were tested using the cubical triaxial tester (CTT). These eight mixtures consist of sphagnum peat as the organic source and four sands of varying particle shape (round, subround, subangular, and angular). The sand-peat mixtures were tested at two moisture contents (air-dried and 30 cm tension). Of all the test samples, air-dried round sand with peat had the highest initial bulk density (IBD) value (1.49 g/cc), while moist angular sand with peat had the lowest IBD value (1.23 g/cc). These values influenced the compression behavior of samples, for example, the air-dried round sand with peat was least compressible while moist angular sand with peat was most compressible. Generally, moisture enhanced the compressibility of test specimens. At an isotropic pressure of 100 kPa, the volumetric strain value of moist round sand with peat was 47% higher than the volumetric strain value of the air-dried round sand with peat. Consequently, moisture and peat in bulk sand samples act as lubricants and assist in the compression process. In addition, bulk modulus values decreased with moisture. Due to the dominant effect of peat, there were no large differences between bulk modulus values of different particle shapes. The shear and failure responses of the above-mentioned eight compositions were also analyzed, compared, and modeled. Of all sand mixtures tested, air-dried angular sands with peat had the highest brittle-type failure stress value, 181 kPa at 34.5 kPa confining pressure, and moist subangular sand with peat had the lowest ductile-type failure stress value, 141 kPa at the same confining pressure. Shear modulus values increased with the increase of mean pressure, but in the case of sands containing both moisture and peat, shear modulus values increased gradually. Overall, peat and moisture content have a dominant effect on the compression and failure behavior of the rootzone sands.

rootzone sand mixtures moisture effect particle shape effect organics effect mechanical behavior compression response shear/failure response prediction models     

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 2: Effect of Moisture on Continuous Sand Mixtures

The compression and failure responses of four rootzone sand mixtures (with different types of particle shapes) were analyzed, compared, and modeled at two different moisture states (air dried and 30 cm tension). Differences in particle packing characteristics arising from particle shape and moisture were quantified. The air-dried and moist samples of the sand mixtures had initial bulk density (IBD) values ranging from 1.55 to 1.67g/cc and 1.23 to 1.48g/cc, respectively. The low IBD values observed for moist mixtures were attributed to the particle-particle agglomeration effects that take place in the presence of moisture. In addition, it was observed that the sand mixture's porosity increased with decreasing particle sphericity. During compression testing, moist samples underwent a greater volumetric deformation compared to the air-dried samples for the same pressure levels, e.g., at 69kPa, the volumetric strain of moist round sand mixtures was 8% higher than that of the air-dried round sand mixtures. Therefore, moisture acted as lubricant during volumetric compression of sand mixtures. Also, the bulk modulus values decreased with increasing moisture content and decreasing particle sphericity. During shear testing, the moist samples underwent a larger amount of strain deformation compared to the air-dried samples for the same stress difference values. This suggests that the presence of moisture makes the sand mixtures ductile during shear testing, unlike the usual brittle response in air-dried state. Shear modulus values linearly increased with the increase in mean pressure for the air-dried samples, whereas, for moist samples, the shear modulus values increased gradually or remained practically constant. The effect of pressure, moisture, and particle shape was also quantified for two elastoplastic parameters (consolidation and swelling indices). It was generally observed that the average consolidation index values decreased with pressure but increased with moisture and particle angularity. On the other hand, average swelling index values increased with pressure, moisture, and particle angularity. Overall, it was concluded that the moisture and particle shape had a decisive influence on the compression and shear profiles of continuous rootzone sand mixtures.     

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 1: Round and Angular Monosize and Binary Mixtures

The bulk mechanical properties of two different types of rootzone sands (round and angular) were measured using a cubical triaxial tester. Two monosize sands (d 50 = 0.375 mm and 0.675 mm) and their 50:50 binary mixtures (d 50 = 0.500 mm) were studied. The compression, shear, and failure responses of the above-mentioned six compositions were analyzed, compared, and modeled. Two elastic parameters (bulk and shear moduli) and two elastoplastic parameters (swelling and consolidation indices) of the six sand compositions were also calculated and compared. The angular sand was more compressible than round sand during isotropic compression. In addition, the angular sands tended to have lower initial bulk density and high porosity values. Among the three different size fractions, the 0.375 mm mixture was least compressible for both sand shapes. The failure strength and shear modulus of the angular sand were higher than the round sands. In addition, due to their simplicity, phenomenological models were developed to predict the compression and shear behavior of the sands. The prediction models were validated using subangular and subround sands. Average relative difference values were calculated to determine the effectiveness of the prediction models. The mean average relative difference values for compression profiles, i.e., volumetric stress vs. volumetric strain, were from 16 % to 39 %, except for the initial load-response portion (< 1 % volumetric strain). The predictive models were effective in reproducing the failure responses: at 17.2 kPa confining pressure, the mean of average relative difference was 23 %; at 34.5 kPa , the mean difference was 24 %.     

Investigation on the shear moduli and damping ratios of silica gel

Silica gel, a typical desiccant widely used in industry to absorb moisture, is a porous inert colloid with different sizes in beaded or angular shape. The mixture of silica gel and pore fluid of matched refractive index has been used as transparent media to mimic the behavior of sand. Previous studies have been focused on the static properties of transparent soil. In the current study, the dynamic properties of silica gel, including small-strain shear modulus and damping ratio, were examined through a series of resonant column tests. Four different gradations of silica gel were tested under confining pressures of 50, 100, 200, 300, and 400 kPa. The test results fully displayed the dynamic behavior of the silica gel. The test data also revealed that silica gel has a certain similar dynamic behavior as those of natural soils. With the test findings, silica gel could be used as a surrogate for natural soils in dynamic transparent soil model tests.     

Comparison of mechanical properties of ground corn stover,switchgrass, and willow and their pellet qualities

Mechanical and physical properties of ground corn stover, switchgrass, and willow were measured and compared in addition to the quality of pellets. Biomass was size-reduced with two different screen sizes (3.175 and 6.35?mm) and conditioned to obtain samples at two different moisture contents (17.5 and 20% on wet basis). Ground switchgrass had the smallest and willow had the highest D₅₀ when size-reduced with the same screen size. Hydrostatic triaxial compression tests were performed using the cubical triaxial tester to determine the bulk modulus, compression index, and spring-back index at specific unloading pressures (20, 45, 70, and 95?kPa). The trends of pressure vs. volumetric strain and void ratio vs. natural log of pressure were similar for all three materials; however, the magnitudes were different. Willow, size-reduced with 3.175?mm screen size at 17.5% wet basis, had the highest bulk modulus among different conditions of all the three biomass. Pellet durability values for all the three materials were higher than 80%. Corn stover pellets formed with 3.175?mm screen size at 20% wet basis had the highest diametral tensile and axial compressive strengths among different conditions for all the three biomass, however the values were not significantly different (p?>?0.05).     

Development of bio-cemented constructional materials through microbial induced calcite precipitation

Microbial induced calcite precipitation (MICP) is an environmentally friendly technology to bond sand particle together to form sandstone like materials. In this paper, MICP-treated bio-specimen was developed through MICP. The property of bio-specimen was compared with beams or bricks made through lime modification and cement modification. Ottawa sand was used in MICP-treated bio-specimen preparation. The proportion of lime or cement was in the range of 10–40% by weight of dry sand. The four-point bending tests, brick compression tests and unconfined compression tests were conducted. The test results indicated that flexure strength of MICP-treated bio-specimen was 950 kPa which was similar to flexure strength of 20–25% cement-treated sand beams, but was much higher than flexure strength of 30% lime-treated sand beams. The brick compression strength of MICP-treated bio-specimen achieved 500 kPa, which was similar to brick compression strength of 30% lime-treated sand bricks. The unconfined compression test results showed that the unconfined compression strength (UCS) of MICP-treated bio-specimen (1300 kPa) was higher than UCS of 10% cement-treated specimen (900 kPa), and much higher than UCS of lime-treated sample (around 140 kPa). The relative uniformity of precipitated CaCO₃ distribution was achieved through the sample immersing preparation method. SEM images showed that failure pattern of MICP-treated, cement-treated and lime-treated specimens were bond-particle failure.     

Static liquefaction of sandy soil: An experimental investigation into the effects of saturation and initial state

The effects of initial state of the samples and the saturation evaluated in terms of Skempton??s pore pressure B on the behavior of Chlef sand are studied in this article. For this purpose, the results of two series of drained and undrained monotonic triaxial compression tests on medium dense sand are presented. In the first test series, the influence of the specimen??s fabric and confining pressure has been studied. The tests were conducted at initial confining pressure of 50, 100, and 200?kPa. The specimens were prepared by two depositional methods that include dry funnel pluviation and wet deposition. All the samples were subjected to a monotonic loading after a consolidation phase. The results of the tests demonstrate that initial confining pressure and the specimen??s fabric have detectable effects on the behavior of the sand. In the second series of tests, the saturation influence on the resistance to the sand liquefaction has been realized on samples at an effective stress of 100?kPa for Skempton??s pore pressure coefficient varying between 13 and 90%. It was found that the increase in Skempton??s pore pressure coefficient B reduces the soil dilatancy and amplifies the phase of contractancy.     

Rate-Dependent Elasto-Viscoplastic Constitutive Model for Industrial Powders. Part 1: Parameter Quantification

ABSTRACT

The rate-dependent mechanical behavior of a dry industrial powder (MZF powder) was studied using a cubical triaxial tester (CTT) within the context of a new elasto-viscoplastic model (PSU-EVP model). The compression and shear properties of the powder were quantified at compression rates of 0.62, 6.21, and 20.7 MPa/minute with pressures up to 11 MPa. Test results demonstrated that the compression and shear responses of the powder were nonlinear, consistent, and reproducible (coefficient of variation or COV ≤ 15%). Also, MZF powder exhibited varying elastic and plastic deformation at different pressure levels that were quantified using statistical correlations (R² > 0.90). For example, the average bulk modulus and shear modulus values for MZF powder increased linearly with pressure (R² > 0.90) at all compression rates. The failure stress values also increased with the increase in mean pressure. For instance, at a compression rate of 0.62 MPa/minute, failure stress increased from 5.0 to 13.3 MPa as the confining pressure increased from 2.2 to 8.5 MPa. Similar effects were noted at compression rates of 6.21 and 20.7 MPa/minute. Overall, failure stress decreased with increasing compression rate. From the data collected, it was demonstrated that compression rate does have substantial effect on the compressibility and shear behavior of powders that can be quantified using the CTT and is suitable for use in the PSU-EVP model.     

DEM investigations of failure mode of sands under oedometric loading

It will be practically useful to explore the evolutions of the failure modes of sand grains within a sand specimen subject to compression for the particle breakage research. This paper attempts to deal with this challenge by conducting a discrete element method (DEM) simulation study on oedometric compression of two kinds of sands (spherical and non-spherical particles). In this study, particle morphologies reconstructed by the spherical harmonic (SH) analysis were created using the agglomerate method, and the micro-parameters used to define the contact model and the properties of walls and balls were adopted based on the single particle crushing tests. The effects of particle shape on the crushing behavior of granular materials and on the evolutions of failure modes of sand grains were captured, and the experimental data was used to evaluate the feasibility and reliability of the proposed DEM modelling strategy. The simulation results show that particle shape affects not only the number, type and orientation of cracks but also the evolution of the particle failure modes. The failure mode of chipping is the most common way to crush for both spherical and non-spherical particles. The particles that have less aspect ratio, sphericity and convexity are more likely to experience the failure mode of comminution. These findings shed light on the key role of particle shape in the investigation of the failure mode of sand grains and facilitate a better understanding of grain-scale behavior of granular materials.     

Flow Properties of Cohesive Powders Tested by a Press Shear Cell

The most important design parameters for roller presses can be referred to flow and compression characteristics of bulk materials. Usually the flow properties are measured in the low stress range 1–50 kPa at the shear rate of about 1 mm/min. But this does not fit the stress regimes in the roller press. Therefore, the compression and flow behavior of the powder have to be investigated at higher pressures, shear rates, and shear displacements. These properties of bulk materials in the so-called medium pressure range 50–1000 kPa can be analyzed using a press shear cell. Tests were implemented with limestone, bentonite, and microcrystalline cellulose at average 23°C powder bed temperature using shear rates from 0.00042 to 0.042 m/s and a more realistic preshear displacement from 0.1 to 2 m for practical applications in powder compaction. Physical observation based compression functions were developed for the low and medium pressure range, which include simple equations for the compression rate and specific compression work.     

Flow Properties of Cohesive Powders Tested by a Press Shear Cell

The most important design parameters for roller presses can be referred to flow and compression characteristics of bulk materials. Usually the flow properties are measured in the low stress range 1-50 kPa at the shear rate of about 1 mm/min. But this does not fit the stress regimes in the roller press. Therefore, the compression and flow behavior of the powder have to be investigated at higher pressures, shear rates, and shear displacements. These properties of bulk materials in the so-called medium pressure range 50-1000 kPa can be analyzed using a press shear cell. Tests were implemented with limestone, bentonite, and microcrystalline cellulose at average 23°C powder bed temperature using shear rates from 0.00042 to 0.042 m/s and a more realistic preshear displacement from 0.1 to 2 m for practical applications in powder compaction. Physical observation based compression functions were developed for the low and medium pressure range, which include simple equations for the compression rate and specific compression work.     

砂土强度和剪胀性的颗粒力学分析

砂土强度和剪胀性一直是土力学强度和变形研究的难点和重点,对其进一步认识的关键取决于对砂土颗粒状微观结构的洞察。砂土的颗粒性和散碎性使其适合采用颗粒力学来研究。该文从颗粒力学角度出发,利用平面离散元模拟砂土变形,建立并标定了砂土单元实验的一个颗粒力学模型。在此基础上,通过颗粒力学参数影响分析,研究了砂土无侧限双轴试验的三种表观强度指标(临界状态强度、峰值强度和特征应力强度)、剪胀性及剪切模量的颗粒力学影响因素。研究结果表明:砂土临界状态强度仅受砂土颗粒摩擦系数的影响,是材料属性,符合临界状态土力学理论;砂土峰值强度和特征应力强度不但与砂土颗粒摩擦系数相关,还与围压水平和相对密实度有关。峰值强度不受砂土颗粒自身刚度性质的影响,而特征应力强度受颗粒刚度性质的影响较大,但后者的影响规律不是简单的正比或反比的关系。砂土剪切模量主要受其颗粒自身刚度性质的影响,就目前研究来看,它与砂土相对密实度的关系并不显著。用颗粒力学方法对剪胀性的深入研究比较困难,主要是因为诸多颗粒力学参数(砂土颗粒摩擦系数和刚度、砂土样品的孔隙率及围压水平)均与之相关。该文尝试研究了砂土剪胀性与其颗粒转角的关系。最后,用该文标定的颗粒力学模型,研究了无重地基极限承载力普朗德尔-瑞斯纳问题,通过颗粒力学计算结果与普朗德尔-瑞斯纳解的对比,深化了对砂土地基极限承载力的理解,也为计算颗粒力学方法在岩土工程尺度上的应用提供了参考。     

Strength and deformation characteristics of a locked sand at low effective stresses

This paper describes the results of triaxial compression tests carried out at effective cell pressures ranging from 12.5 to 100 kPa to investigate the influence of fabric structure on the yield and failure of intact Reigate silver sand. In some of the tests, a digital image-based technique was used to determine the instant of onset of strain localisation, and the distribution of strain localisations within the specimen as overall deformation progressed. Comparative tests on intact and reconstituted specimens showed that fabric structure in the intact material allows the mobilisation of stress ratios close to peak before the onset of dilation, and increases the shear modulus at a given effective cell pressure and strain. Localisation was found to start at or after the onset of dilation, with a tendency to delay at increasing effective cell pressure. More localised deformation was observed at low effective cell pressures. Consistency between the critical state strengths of intact and reconstituted specimens is demonstrated, provided that the effect of shear band geometry is taken into account in stress analysis.     

内衬聚偏氟乙烯热塑层的纤维缠绕复合材料管径向平压性能

纤维缠绕角度、纤维缠绕层厚度及碳/玻纤维混杂比是影响内衬聚偏氟乙烯(PVDF)热塑层的纤维增强热固性复合材料缠绕管径向平压性能的重要因素,其性能直接决定复合材料缠绕管产品掩埋深度和抗碾压能力。将PVDF颗粒经挤出机制成PVDF管,然后以表面喷砂处理后的PVDF管为内衬芯管,采用湿法缠绕技术制备不同结构参数的复合材料缠绕管。利用管平行板外载平压性能测试方法,测试了3种结构参数对复合材料管径向平压性能的影响,并分析其破坏模式与失效机理。结果表明,随缠绕层厚度的增加,径向压缩强度和径向压缩模量逐渐增大;随着缠绕角度的增大,径向压缩强度和径向压缩模量先增大后减小;另外,随着碳/玻纤维混杂比的提高,复合材料缠绕管的压缩强度和压缩模量相应增加。     

Moisture distribution in drying ordinary and high performance concrete cured in a simulated hot dry climate

Adequate moisture is very important during early age of portland cement concrete. The Single Point Magnetic Resonance Imaging technique was used to study the effects of various lengths of moist curing, and the use of curing compound, on the amount and distribution of evaporable water during drying of ordinary and high performance concrete. The specimens subjected to six different curing regimes, were cast in triplicate for a total of 72. After moist curing at 38°C, the specimens were subjected to uniaxial drying in an environmental chamber at 38°C and 40% relative humidity that simulated hot dry climate conditions. As the specimens were drying, Magnetic Resonance Imaging was used to study the evaporable water distribution, non-destructively and with millimetric resolution. The Magnetic Resonance Imaging profiles indicated a reduced moisture loss with increasing length of moist curing. Extended moist curing was especially beneficial for the two self-compacting concrete mixtures, particularly for the cover concrete. In all mixtures the use of curing compound was only marginally better than one day moist curing, but was significantly better than air curing, particularly for the cover concrete. The moisture diffusivity was evaluated from the transient moisture distribution profiles using the Boltzmann transformation method. The results indicated a strong dependence of the moisture diffusivity on the moisture content when above 80% saturation, whereas below this value it remains almost constant. The moisture diffusivity is significantly reduced with increased moist curing period.     

Micromechanical finite element framework for predicting viscoelastic properties of asphalt mixtures

A micromechanical finite element (FE) framework was developed to predict the viscoelastic properties (complex modulus and creep stiffness) of the asphalt mixtures. The two-dimensional (2D) microstructure of an asphalt mixture was obtained from the scanned image. In the mixture microstructure, irregular aggregates and sand mastic were divided into different subdomains. The FE mesh was generated within each aggregate and mastic subdomain. The aggregate and mastic elements share nodes on the aggregate boundaries for deformation connectivity. Then the viscoelastic mastic with specified properties was incorporated with elastic aggregates to predict the viscoelastic properties of asphalt mixtures. The viscoelastic sand mastic and elastic aggregate properties were inputted into micromechanical FE models. The FE simulation was conducted on a computational sample to predict complex (dynamic) modulus and creep stiffness. The complex modulus predictions have good correlations with laboratory uniaxial compression test under a range of loading frequencies. The creep stiffness prediction over a period of reduced time yields favorable comparison with specimen test data. These comparison results indicate that this micromechanical model is capable of predicting the viscoelastic mixture behavior based on ingredient properties.     

Properties of self-compacting mortar made with various types of sand

Higher cement and fines content is needed in self-compacting mortars (SCMs) to increase their flowability and stability. Different inert fillers and supplementary cementitious materials are usually added. The use of sands rich in fines may be a cost effective alternative source of filler. This paper presents the results of an experimental study on the rheological and mechanical properties of self-compacting mortars (SCMs) made with various types of sands: crushed sand (CS), river sand (RS), dune sand (DS) and a mixture of different sands. The mini-slump flow, V-funnel flow time and viscosity measurement tests were used to study the rheological properties. The experimental results indicate that the rheological properties and strength improve with mixtures of crushed and river sands but decrease with mixtures of crushed and dune sands especially for higher dune sand content. Crushed sand with (10–15%) of limestone fines can be used successfully in production of SCM with good rheological and strength properties. However, a reduction in compressive strength with increasing dune sand content (up to 50%) in mortar with binary and ternary sands was observed.     

Properties of self-compacting mortar made with various types of sand

Higher cement and fines content is needed in self-compacting mortars (SCMs) to increase their flowability and stability. Different inert fillers and supplementary cementitious materials are usually added. The use of sands rich in fines may be a cost effective alternative source of filler. This paper presents the results of an experimental study on the rheological and mechanical properties of self-compacting mortars (SCMs) made with various types of sands: crushed sand (CS), river sand (RS), dune sand (DS) and a mixture of different sands. The mini-slump flow, V-funnel flow time and viscosity measurement tests were used to study the rheological properties. The experimental results indicate that the rheological properties and strength improve with mixtures of crushed and river sands but decrease with mixtures of crushed and dune sands especially for higher dune sand content. Crushed sand with (10–15%) of limestone fines can be used successfully in production of SCM with good rheological and strength properties. However, a reduction in compressive strength with increasing dune sand content (up to 50%) in mortar with binary and ternary sands was observed.     

Using the poker-chip test for determining the bulk modulus of asphalt binders

The properties of asphalt binders strongly influence the overall mechanical response of asphalt mixture composites. A thorough understanding of the mechanistic behavior of asphalt binders is important in order to fully and accurately characterize the behavior of the asphalt mixture. The mechanical properties of the asphalt binder, the matrix in the asphalt mixture composite, are time and temperature dependent and have a lower stiffness compared to the inclusions (aggregate particles). However, computational methods used to model the micromechanics of asphalt mixtures typically assume a constant bulk modulus or Poisson’s ratio for asphalt binders. This research investigates the time-dependence of the bulk modulus of asphalt binders. Several approaches for measuring the bulk modulus were explored and the poker-chip geometry was found to be the most suitable one. The boundary value problem for the poker-chip geometry was solved to determine the bulk modulus and Poisson’s ratio of asphalt binders as a function of time. The findings from this research improve our understanding of the viscoelastic behavior of asphaltic materials, and also guide important assumptions that are typically made during computational modeling of asphaltic materials.     

Behaviour of sand–clay mixtures for road pavement subgrade

Materials forming sand grains and colluvial soil deposits have a distinct structure, consisting of a composite matrix of coarse and fine soil grains. The influence of sand grains content on the behaviour of sand–clay mixtures was investigated by a series of intensive laboratory experiments. The California bearing ratio (CBR), unconfined compression strength (UCS) and compaction tests were carried out on various contents of sand and clay mixtures. The sand–clay mixtures were prepared with sand contents of 0, 10, 20, 30, 40, and 50% by weight. The laboratory tests on these mixtures have indicated that their behaviour will depend on the relative concentration of the sand and clay samples. The results of the tests showed a decrease in the UCS, and an increase the CBR values with an increase in the amount of sand. An increase in dry unit weight and a decrease in respective moisture content by an increase in the amount of sand were observed in the compaction tests.