STRENGTH EFFECT OF F MINERAL ADMIXTURE ON CEMENT CONCRETE

F mineral admixture (FMA) is made of the fin- ely divided powder of natural zeolite with a bit of other agent. When FMA is used to displace about 10% (by weight) of the ordinary portland cement (OPC) (strength grade 575#) in concrete and mixed with a suitable amount of super plasticizer (w/c =0.31-0.35), then a high-strength concrete with compressive strength about 80 MPa and slump about 180 MM can be obtained. The strength of this concrete is about 10-15% higher than that of the corresponding concrete mixing with pure OPC, and its bleeding decreases greatly. It makes no segre- gation and separation, and thus it satisfies the requirement of pumping concrete in construction.

Strength weakening effect of high static pre-stressed granite subjected to low-frequency dynamic disturbance under uniaxial compression

This study aimed to elucidate the strength weakening effect of high static pre-stressed rocks subjected to low-frequency disturbances under uniaxial compression.Based on the uniaxial compressive strength(UCS)of granite under static loading,70%,80%,and 90%of UCS were selected as the initial high static pre-stress(σ_(p)),and then the pre-stressed rock specimens were disturbed by sinusoidal stress with amplitudes of 30%,20%,and 10%of UCS under low-frequency frequencies(f)of 1,2,5,and 10 Hz,respectively.The results show that the rockburst failure of pre-stressed granite is caused by low-frequency disturbance,and the failure strength is much lower than UCS.When theσp or f is constant,the specimen strength gradually decreases as the f or σ_(p) increases.The experimental study illustrates the influence mechanism of the strength weakening effect of high static pre-stress rocks under low-frequency dynamic disturbance,that is,high static pre-stress is the premise and leading factor of rock strength weakening,while low-frequency dynamic disturbance induces rock failure and affects the strength weakening degree.

Effect of Friedel's salt on strength enhancement of stabilized chloride saline soil

In the field of soil stabilization, only calcium silicate hydrate(CSH) and ettringite(AFt) as hydration products have been reported to directly contribute to the strength enhancement of the soil. A chloride dredger fill, an artificial chloride saline soil, and a non-saline soil were stabilized by Portland cement(PC) and PC with Ca(OH)_2(CH) with different contents. A series of unconfined compressive strength(UCS) tests of stabilized soil specimen after curing for 7 d and 28 d were carried out, and the hydration products and microstructure of the specimens were observed by X-ray diffractometry(XRD), scanning electronic microscopy(SEM), and energy-dispersive X-ray analysis(EDXA). The results showed that the strengths of PC+CH-stabilized chloride saline soils were much higher than those of PC-stabilized soils. A new hydration product of calcium aluminate chloride hydrate, also known as Friedel's salt, appeared in the PC+CH-stabilized chloride saline soils. The solid-phase volume of Friedel's salt expanded during the formation of the hydrate; this volume filled the pores in the stabilized soil. This pore-filling effect was the most important contribution to the significantly enhanced strength of the PC+CH-stabilized chloride saline soils. On the basis of this understanding, a new optimized stabilizer was designed according to the concept that the chloride in saline soil could be utilized as a component of the stabilizer. The strength of the chloride saline soils stabilized by the optimized stabilizer was even further increased compared with that of the PC+CH-stabilized soils.

DYNAMIC EFFECTIVE SHEAR STRENGTH OF SATURATED SAND

The dynamic effective shear strength of saturated sand under cyclic loading is discussed in this paper.The discussion includes the transient time depen- dency behaviors based on the analysis of the results obtained in conventional cyclic triaxial tests and cyclic torsional shear triaxial tests.It has been found that the dy- namic effective shear strength is composed of effective frictional resistance and viscous resistance,which are characterized by the strain rate dependent feature of strength magnitude,the coupling of consolidation stress with cyclic stress and the dependency of time needed to make the soil strength sufficiently mobilized,and can also be ex- pressed by the extended Mohr-Coulomb's law.The two strength parameters of the dynamic effective internal frictional angle φd and the dynamic viscosity coefficient η are determined.The former is unvaried for different number of cyclic loading,dy- namic stress form and consolidation stress ratio.And the later is unvaried for the different dynamic shear strain rate γt developed during the sand liquefaction,but increases with the increase of initial density of sand.The generalization of dynamic effective stress strength criterion in the 3-dimensional effective stress space is studied in detail for the purpose of its practical use.

In situ strength of coal bed based on the size effect study on the uniaxial compressive strength

In the early 1990 s, the Foundation for Science and Technology of Rio Grande do Sul State(CIENTEC)developed a pioneering study in Brazil, related to the simultaneous mining of multiple coal seams.One of the activities included detailed studies on the geomechanical characterization of materials present in the Irapua coal seam, under exploitation in the A-Sangao Mine, located near the city of Criciuma-SC,within the South-Catarinense coalfield. The goal of the laboratory tests was to define the behavior of the uniaxial compressive strength of the Irapua coal seam and establish a first approximation for the in situ strength value of this coal seam, since existing knowledge is solely based on practical mining experience over the years. Large samples of the coal seam were collected, using special techniques to maintain the integrity of the material, and a set of 56 uniaxial compression tests in cubic specimens, with side length ranging from 4.5 to 31 cm, were conducted in laboratory. This paper describes the experimental techniques used in the assays, and also presents the uniaxial compression strength results obtained.Moreover, important aspects of this type of study are considered, highlighting the size effect for the carbonaceous bed and the estimation of in situ strength values for the Irapua coal seam.

Strength criterion effect of the translator and destabilization model of gas-bearing coal seam

Coal seam destabilization inflicts damage to equipment, causes property loss and personnel casualties,and severely threatens mining safety and efficient production. To further understand this destabilization based on the basic theory of Lippmann seam destabilization, a mathematical model was introduced for gas pressure distribution by considering intermediate principal stress and support resistance.Subsequently, we established a translation model suitable for the entire roadway coal seam with rocky roof and floor by applying the unified form of yield criterion in the state of plane strain. We also obtained the analytic expressions of coal seam stress distribution on both sides of the roadway and the widths of plastic and disturbance zones. Afterward, we analyzed several typical cases with different material yield criteria, obtained the plastic zone widths of the coal seam under different gas pressures, and assessed the effects of support resistance, roadway size, and coal strength on coal seam destabilization. Results showed that: the results obtained on the basis of Wilson and Mohr–Coulomb criteria are considerably conservative, and the use of Druker–Prager criteria to evaluate the rockburst-induced coal seam destabilization is safer than the use of the two other criteria; coal seam stability is correlated with gas pressure;and high-pressure gas accelerates the coal seam destabilization.

Effect of intermediate principal stress on strength of soft rock under complex stress states

A series of numerical simulations of conventional and true triaxial tests for soft rock materials using the three-dimensional finite difference code FLAC3D were presented. A hexahedral element and a strain hardening/softening constitutive model based on the unified strength theory(UST) were used to simulate both the consolidated-undrained(CU) triaxial and the consolidated-drained(CD) true triaxial tests. Based on the results of the true triaxial tests simulation, the effect of the intermediate principal stress on the strength of soft rock was investigated. Finally, an example of an axial compression test for a hard rock pillar with a soft rock interlayer was analyzed using the two-dimensional finite difference code FLAC. The CD true triaxial test simulations for diatomaceous soft rock suggest the peak and residual strengths increase by 30% when the effect of the intermediate principal stress is taken into account. The axial compression for a rock pillar indicated the peak and residual strengths increase six-fold when the soft rock interlayer approached the vertical and the effect of the intermediate principal stress is taken into account.

Composite Model of Interracial Stepwise Ion/Coordination Particle Exchange-Electrostatic Exchange——Ionic Strength Effect on the Interaction Between Minor Metal Ion and Solid Particle in Seawater

It is the first time in experiment that we have found that the ionic strength effects on E(%)-pH curves of the systems with clay minerals as solid particles and those systems with hydrous oxides as solid particles, are 'right-left shift phenomenon' and 'left monodirectional shift phenomenon', respectively.The corresponding ionic strength effects on isotherms are 'downward-upward shift phenomenon' and 'upward monodirectional shift phenomenon', respectively.All the four phenomena are consistent with each other.In order to explain the above phenomena, the composite model of interracial ion/coordination particle exchange-electrostatic exchange is put forward to make a unified explanation and quantitative treatment.From the structures of the two kinds of materials——clay minerals and hydrous oxides, we know that the former has two kinds of exchange actions, i.e. when one increases,the other decreases, which makes the position of curves shift didirectionally,while the latter only has interfacial stepwise ion/coordin ation particle exchange,the curves of which shift therefore monodirectionally.The calculated results of the model accord with the experimental.

Influence of Soaking Effects to the Shear Strength of the Colluviums on Mt. Da-Lum

Landslide of colluviums often occurs during or after heavy rainfall because the huge quantity of precipitation induces the upsurge of groundwater table as well as the reduction of effective stress of soil stratum and the factor of safety from slope stability analysis. Moreover, the longer duration of heavy rainfall maintain the higher groundwater table, the shear strength parameters may be decline due to the soil being soaked for longer period of time. The campus of Huafan University is mostly covered by colluviums that located on Mt. Da-Lum, Shih-Tin Hsiang, Taipei County, Taiwan, China. Slope stability is one of the major concerned issues, for this reason, a slope monitoring test site has been established around the campus. According to the data collected, the magnitude of slope movement influenced by rainfall can be observed. In this study, the colluviums samples are collected from the campus of Huafan University and direct shear tests are performed to obtain the shear strength parameters of the remolded soil samples that soaked in the water for various periods of time, i.e. 0, 1, 2, 3, 5 and 7 days respectively. From these tests, the change of the shear strength and slope stability of colluviums with respect to the soaking time of soil has been further understood.

A Computational Investigation of the Group Effect of Pits on the Ultimate Strength of Steel Plates

In this work, we focus on assessing the group effect of localized corrosion on the ultimate strength of the marine structural plates and study the load-deformation behaviors of plates of various slenderness and uniaxial compression.Meanwhile, we investigate different corroded patterns from a single circular pit to 25 circular pits distributed over the plate and carry out hundreds of nonlinear finite element simulations by combining the number, depth, distribution of pits with imperfections and slenderness of plate. The distribution of multiple pits causes scattering of stress concentration on the plate, then the plastic section of plate changes with wider distribution of damage simultaneously. The ultimate strength arises when un-loading zone comprised of the yielding strips and holes extends across the plate. It can be concluded that the corroded condition defined as group effect of pits manipulates the deformation state and the loading capacity of plate at the ultimate strength mode that coincides with the proportion of effective loading area and section in the process of post-buckling. To validate the effect of pits group, we perform the numerical experiments of the post-buckling of steel plates containing pits in a row with different orientation.

The Study of the Grit-blasting Parameters and Their Effects on the Adhesive Strength of the Plasma Sprayed Coatings

Surface Preparation is very important in adhesive b on ding of spray coatings to the surface of a work piece. The common practice is gr it-blasting of the surface before subjecting it to the spray coating process. In this study, grit-blasting of an AISI 4130 steel (of different heat treatmen ts) with Al 2O 3 particles was studied. Various grit-blasting parameters such as blasting particle size, the distance between blasting nozzle and the work pi ece (25, 30 and 40 cm.), blasting pressure (3,4,5,6 and 7 bars), blasting time ( 3, 6 and 10 seconds), and the blasting angle (45° and 90°) were examined in or der to find the optimum roughness. The mean roughness (Ra) of the grit-blasted surfaces were measured and the vari ations of the roughness with respect to the above mentioned variables were studi ed. The results show that by increasing blasting time, surface roughness increas es up to a maximum and then slightly decreases it with further duration of t he process. On the other hand a lengthy blasting causes some undesirable results such as an increase in residual particles between surface irregularities. There fore an optimum blasting time is of great importance. Increasing the blasting pr essure also provides a rougher surface, but in grit blasting of harder specimens the surface roughness decreases when the pressure reaches a certain limit. About the blasting angle, it was noticed that an angle of 45° results in less r esidual particles between the surface irregularities, in comparison to the angle of 90°. After grit-blasting, the specimens were plasma spray coated with 80% ZrO 2-20 % Y 2O 3 powder. The adhesive strength of the coating to the substrate was the n measured according to the DIN 50160 standard. The results show that for a certain base metal, the adhesive strength is directl y related to the surface roughness of the base material. Residual particles afte r grit-blasting the surface of the specimens can also have a strong deteriorati ng effect on adhesive strength. Finally, it was shown that the hardness of the b ase material had a direct effect on the adhesive strength of the sprayed coating s.

Effective Yield Strength for Material Powder Consolidated at Stage II Compaction

This work is concerned with the estimation from the outside of effective yield strength for the stage II consolidated material package of axisymmetric solid particles. Once an appropriate simple representative axisymmetric unit cell is chosen, the kinematical approach of the yield design homogenization method is used in order to obtain external estimates which has been found depending on the loading history (isostatic and closed die compactions) as well as on the relative density of the material powder. For comparison purpose, finite element simulations that describe the behavior of spherical elastic plastic particles uniformly distributed inside the material powder are carried out.

Titrimetric Study of the Solubility and Dissociation of Benzoic Acid in Water: Effect of Ionic Strength and Temperature

The apparent acid dissociation constant (Kc) of benzoic acid in water has been determined ti-trimetrically under ionic strength values between 0.00 and 0.50 mol·L﹣1 at a range of temperatures between 16°C and 41°C. The thermodynamic dissociation constant (as pKa) of benzoic acid was determined as 4.176 at 25°C. No regular correlation between pKa of benzoic acid and the temperature in the range was used. The values of pKa are inversely proportional to temperatures between 16°C and 30°C. In this range of temperature, the values of thermodynamic quantities () for the dissociation process of benzoic acid in water were calculated by using Van’t Hoff plot. For this case the dissociation was not favoured through entropy and enthalpy changes. The values of pKa are directly proportional to temperatures between 30°C and 41°C.

Effect of Environmental Conditions on Flexural Strength and Fracture Toughness of Particulate Filled Glass-Epoxy Hybrid Composites

Multifunctional hybrid polymer composites were projected as novel solutions to meet the demands in various industrial applications ranging from automotive to aerospace. This investigation focuses on processing, flexural strength and fracture toughness characterization of the glass fabric reinforced epoxy (G-E) composites and graphite/fly ash cenosphere (FAC) modified interface between the epoxy matrix and glass fabric. Hand lay-up followed by compression moulding method was used to fabricate the laminates. Flexural and fracture toughness tests at room temperature, elevated temperature and cryogenic temperature were conducted to assess the flexural strength (FS) and mode-I plane-strain fracture toughness (KIC). The experimental and characterization efforts suggest that both graphite and FAC fillers improve bonding at the interface. The study showed that the graphite is more favorable for enhancing FS and KIC of G-E composites. Graphite filled G-E hybrid composites with significant FS and KIC to that of unfilled and FA filled G-E were successfully achieved by incorporating 10 wt% graphite. The incorporation of fillers resulted in improvement of FS, which increased by 43% and 37.7% for 10Gr+G-E and 10FAC+G-E hybrid composites respectively. All composites show a 26% improvement in KIC at cryogenic temperature and a decrease of 12.5% at elevated temperature. According to the SEM observations, fiber debonding from the matrix is suppressed due to the presence and uniform distribution of graphite. In addition, micro-pores, matrix shearing, active toughening mechanisms induced by graphite, such as crack deflection, layer breakage and delamination of graphite layers contributed to the enhanced KIC of hybrid G-E composites.

久坐与社区老年人下肢肌力:跌倒恐惧与年龄的中介和调节作用

背景:人体老龄化进程中,下肢肌肉力量会随着年龄的增长呈现显著的生理性下降,久坐、跌倒恐惧、年龄与下肢肌力之间可能存在一定关联,但它们之间的影响路径与效应关系尚不明确。目的:探究社区老年人久坐和下肢肌肉力量之间的关系,并探讨跌倒恐惧和年龄在上述两者关系中的作用。方法:共招募331名60岁及以上的上海市社区老年人。采用问卷调查法收集基本信息、人口学资料等;采用国际身体活动问卷-短卷评估久坐时间,采用30 s坐站测试测量下肢肌力,采用中文版国际跌倒效能量表测评跌倒恐惧。对数据进行描述统计、相关分析、基于回归的路径分析和中介效应检验。结果与结论:最终以318名老年人[78.9%为女性,平均年龄(67.8±5.5)岁]的有效数据纳入分析,其中久坐时间≥3 h的185名,<3 h的133名。①久坐和跌倒恐惧呈正相关关系(P<0.01),下肢肌力和久坐(P<0.01)、跌倒恐惧(P<0.001)均呈负相关关系。②久坐显著负向预测下肢肌力(β=-0.125,P<0.05),久坐显著正向预测跌倒恐惧(β=0.182,P<0.01);跌倒恐惧显著负向预测下肢肌力(β=-0.293,P<0.001)。③跌倒恐惧在久坐和下肢肌力之间起中介作用(β=-0.053,95%CI:-0.100至-0.018)。④久坐对跌倒恐惧的预测效应具有统计学意义(β=0.164,P<0.01),这说明年龄调节了久坐对跌倒恐惧的影响。

Shear strength criteria for rock,rock joints,rockfill and rock masses:Problems and some solutions

Although many intact rock types can be very strong,a critical confining pressure can eventually be reached in triaxial testing,such that the Mohr shear strength envelope becomes horizontal.This critical state has recently been better defined,and correct curvature or correct deviation from linear Mohr-Coulomb（MC） has finally been found.Standard shear testing procedures for rock joints,using multiple testing of the same sample,in case of insufficient samples,can be shown to exaggerate apparent cohesion.Even rough joints do not have any cohesion,but instead have very high friction angles at low stress,due to strong dilation.Rock masses,implying problems of large-scale interaction with engineering structures,may have both cohesive and frictional strength components.However,it is not correct to add these,following linear M-C or nonlinear Hoek-Brown（H-B） standard routines.Cohesion is broken at small strain,while friction is mobilized at larger strain and remains to the end of the shear deformation.The criterion ＇c then σn tan φ＇ should replace ＇c plus σn tan φ＇ for improved fit to reality.Transformation of principal stresses to a shear plane seems to ignore mobilized dilation,and caused great experimental difficulties until understood.There seems to be plenty of room for continued research,so that errors of judgement of the last 50 years can be corrected.

Effect of Cyclic Loading Frequency on Undrained Behaviors of Undisturbed Marine Clay

Based on a series of cyclic triaxial tests, the effect of cyclic frequency on the undrained behaviors of undisturbed marine clay is investigated. For a given dynamic stress ratio, the accumulated pore water pressure and dynamic strain increase with the number of cycles. There exists a threshold value for both the accumulated pore water pressure and dynamic strain, below which the effect of cyclic frequency is very small, but above which the accumulated pore water pressure and dynamic strain increase intensely with the decrease of cyclic frequency for a given number of cycles. The dynamic strength increases with the increase of cyclic frequency, whereas the effect of cyclic frequency on it gradually diminishes to zero when the number of cycles is large enough, and the dynamic strengths at different frequencies tend to the same limiting minimum dynamic strength. The test results demonstrate that the reasons for the frequency effect on the undrained soil behaviors are both the creep effect induced by the loading rate and the decrease of sample effective confining pressure caused by the accumulated pore water pressure.

Dynamic fracture toughness of high strength metals under impact loading:increase or decrease

An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.

Thickness-related synchronous increase in strength and ductility of ultrafine-grained pure aluminum sheets

To explore the specimen size effect of mechanical behavior of ultrafine-grained(UFG)materials with different structures,UFG Al sheets processed by equal channel angular pressing(ECAP)were selected as target materials and the dependency of tensile behavior on sheet thickness(t)was systematically investigated.The strength and ductility of ECAPed UFG Al sheets were improved synchronously as t increased from 0.2 to 0.7 mm,and then no apparent change occurred when t reached to 0.7 and 1.0 mm.The corresponding microstructure evolved from dislocation networks in equiaxed grains into the walls and subgrains and finally into the dominated cells in elongated grains or subgrains.Meanwhile,dense shear lines(SLs)and shear bands(SBs)were clearly observed and microvoids and cracks were initiated along SBs with the increase of t.These observations indicated that the plastic deformation of UFG Al sheets was jointly controlled by shear banding,dislocation sliding,and grain-boundary sliding.Furthermore,the propagation of SBs became difficult as t increased.Finally,the obtained results were discussed and compared with those of annealed UFG Al and UFG Cu.