Biaxial Compressive Strength Research Articles

Dynamic strength and failure modes of sandstone under biaxial compression

Geomaterials are increasingly being subjected to the quasi-static confinements and dynamic loadings, and thus it is critical to characterise dynamic behaviour under the coupled static-dynamic loading conditions. In this study, a series of dynamic biaxial compression tests was performed on cubic specimens of sandstone by using a triaxial Hopkinson bar system, high-speed three-dimensional digital image correlation (3D-DIC) and synchrotron-based micro-computed-tomography (μCT). This testing apparatus allows for determining dynamic stress-strain behaviour of the specimen under biaxial static pre-stress (σ1, σ2) conditions and different impact velocities (i.e. 15, 20 and 26 m/s), corresponding to average axial strain rates of 80, 200 and 250 s−1. Both σ1 and σ2 applied in the tests varied from 0 to 40 MPa with an interval of 10 MPa. Dynamic strength and fracture behaviours of biaxially confined rocks were systematically characterised from macroscopic to microscopic, surface to interior and 2D to 3D by using the 3D-DIC and synchrotron-based μCT techniques. Experimental results show that, under the same biaxial pre-stresses, dynamic biaxial compressive strength of sandstone increases with strain rate, i.e. rate dependence. At a given impact velocity, dynamic biaxial compressive strength decreases with increasing pre-stress values (σ1) along the impact direction, while it enhances with the increase of intermediate principal stress (σ2) in the lateral direction, which is so-called confinement dependence. Besides, two types of dynamic stress-strain curves of rocks are observed under biaxial compression, which is associated with post-peak fracturing characteristics. High-speed 3D-DIC shows that rock fragments are ejected out of the free surface (σ3 = 0) with certain velocities including the translation and rotation, which resembles the rock ejection observed during drilling and blasting and even strong rockbursts. μCT reveals that the double “V” shapes of shear fractured zones are symmetrically generated under biaxial compression. The crack surface area, volume and dynamic fracture energy are also estimated under different confinements to characterise the damage of impacted rock.

Investigation on recycled aggregate concretes exposed to high temperature by biaxial compressive tests

Recycled aggregate concretes (RACs) made of building wastes can achieve good environmental and economic benefits. Since fire is one of the most common disaster in modern high-rise buildings, the strength, deformation and bearing capacity of recycled concrete structures under high temperatures should be carefully studied in design. The mechanical performances of RACs exposed to high temperatures under biaxial compressive actions have been studied in this research. The RAC specimens with three replacement rates (0%, 50%, 100%) exposed to high temperatures (200 °C, 400 °C) have been tested. The influences of stress ratios, replacement rates and high temperatures have been investigated intensively. Failure criterions with improved accuracy have been proposed to describe the strength failure characteristics of the RACs. The results indicate that biaxial compressive strength and peak strain after high temperature show an increase compared with that under normal temperature, and the biaxial compressive strength and peak strain have opposite trend with the variation of replacement rate and temperature. The study can provide a scientific instruction for the design of RACs exposed to high temperature.

Numerical study of the biaxial compressive strength of high strength concrete based on a yield design micromechanical approach

SummaryThis paper presents a numerical study of high strength concrete microstructure effects on its uniaxial and biaxial compressive strengths. Concrete is first represented as a set of angular aggregates interacting within a cement paste matrix. Then, a yield design kinematic approach is conducted at the mesoscopic scale in order to determine the concrete compressive strength for a given loading path. The proposed model, having a low computational cost, is able to capture the main microstructure effects already observed in literature on concrete uniaxial compressive strength, in particular, the aggregates volume fraction and maximal size effects. Finally, the proposed model also predicts the biaxial failure envelope of high strength concrete and confirms some experimental trends observed in literature.

Theoretical and Numerical Study of Shear Strength of Concrete Material

The shear strength of some concrete materials should be analyzed based on elastic-plastic theory in petroleum, water conservancy, tunnel engineering, and so on. The majority of researches concentrate mainly on the tensile and compressive strength of concretes, but few have studied the shear strength. Concrete materials have been increasingly applied broadly to geotechnical engineering. Thus, investigating the shear strength characteristics of concretes is of great importance. To study the characteristics of shear strength of concrete materials, the theoretical relationship between concrete’s compressive and shear strengths was discussed in the uniaxial, biaxial, and triaxial stress states. The concrete strength envelopes under the biaxial and uniaxial compressive strength were studied. Given the concrete shear strength, the overload method and the finite difference software FLAC3D were used to investigate the concrete failure modes and ultimate bearing capacities. Results show that the theoretical formula under the 3D stress-bearing condition is only applicable to the circumstance with equal compressive strengths under the biaxial and uniaxial conditions, which conforms to 3D Mohr’s circle theory. 3D Mohr’s circle theory is not totally applicable to concrete materials where the concrete compressive strength under the biaxial condition is larger than that under the uniaxial condition. Concrete material gains its shear strength 47 percent from its frictional force while the rest form cohesive force. The study results can provide a certain basis and reference for analyzing the shear strength characteristics of concrete materials.

The Effect of Inner Friction on Concrete Fracture Behavior under Biaxial Compression: A 3D Mesostructure Study.

The mechanical behavior of concrete under biaxial loading condition (especially biaxial compression) is one of the most important indexes to evaluate the quality of concrete. To study the mechanical behavior of concrete under biaxial compression at mesoscale, we adopted our recently developed 3D numerical model based on Voronoi tessellation and cohesive elements. A constitutive model considering the friction effect is used in the model to characterize the fracture behavior of all potential fracture surfaces inside the concrete. A series of numerical experiments with different biaxial compression stress ratios were carried out. It was found that with the increase of the biaxial compression ratio, the proportion of energy increment caused by friction stress increases. The effect of inner friction coefficient on the biaxial relative strength was also investigated, and this kind of study is hard to be carried out through laboratory experiments. The results show that the inner friction coefficient has a great influence on the biaxial relative strength of concrete, and there is a positive correlation between these two parameters. Based on the above rules, a conservative biaxial relative compression strength envelope is obtained by setting the inner friction coefficient as zero.

Combined effects of steel fiber and strain rate on the biaxial compressive behavior of concrete

Understanding the biaxial mechanical behavior of steel fiber reinforced concrete (SFRC) is of critical significance when the concrete structures are subjected to dynamic loading. This paper presents the results of an experimental investigation on the dynamic mechanical behavior of SFRC subjected to biaxial compressive loadings. Aiming at the combined effects of steel fiber and strain rate on strength criterion, a series of dynamic biaxial compressive experiments on 100 mm-length SFRC cubic specimens with various steel fiber contents (0%, 1%, 2%, and 3% by volume) were conducted by using a large dynamic triaxial servo-hydraulic testing apparatus. The specimens were loaded by biaxial compression with five stress ratios (i.e. 0:1, 0.25:1, 0.5:1, 0.75:1 and 1:1, respectively) at various strain rates ranging from 10−5/s to 10−2/s. A comprehensive dynamic biaxial strength criterion taking into account the combined effects of uniaxial strength, strain rate and stress ratio was proposed. The effect of steel fiber content and strain rate on the biaxial mechanical behavior of SFRC specimens was discussed, including the failure mode, stress-strain curve, biaxial compressive strength, dynamic strength criterion. It suggested that at given strain rate, the biaxial stress ratio is the major factor dominating the strength increment, and the incorporation of steel fiber has a remarkable enhancement on the strength for all stress ratios. Additionally, the dynamic compressive strengths of SFRC were found to increase with the increase of strain rate, while the failure pattern and dynamic ultimate strength were closely dependent on the magnitude of lateral stress exerted on the specimens. The comparison indicates that the proposed model results of dynamic strength criterion were in good agreement with the experimental data for SFRC specimens.

The Effect of Joint Dip Angle on the Mechanical Behavior of Infilled Jointed Rock Masses under Uniaxial and Biaxial Compressions

Due to the complex formation process of a rock mass, a large number of fissures, joints, faults, other defects exist and the defects commonly contain infilled materials. The jointed rock masses are in a complex geological environment, in which the geometric distribution and the boundary condition can greatly affect the mechanical behavior of the infilled jointed rock mass. In this study, the infilled jointed rock mass specimens with different dip angles are prepared using similar materials, and the uniaxial and biaxial compression tests on the specimens are conducted. The effect of the joint dip angle on the mechanical behavior of the infilled jointed rock mass under uniaxial and biaxial compressions is investigated. The results show that the uniaxial compressive strength shows a W-shaped variation, and the biaxial compressive strength shows a V-shaped variation with an increase in the dip angle. Most of the cracks appear in pairs around the joint and occur symmetrically in a bilateral distribution, and the existence of the infilled joints induces a nonlinear mechanical behavior in the specimen. In addition, the specimens exhibit three failure modes under uniaxial compression: splitting failure, step-path failure and planar failure. The specimens present two failure modes under biaxial compression: splitting failure and planar failure.

Mechanism-based strength criterion for 3D needled C/C–SiC composites under in-plane biaxial compression

ABSTRACTBiaxial compressive experiments for 3D needled C/C–SiC composites parallel to the nonwoven cloth were conducted. The failure mechanisms and mechanical properties of the composites were greatly related to the biaxial compressive stress confinement ratio, R. It was found that out-of-plane shear failure controlled the failure of the composites. The failure shear plane was aligned with one of the major loading axes for R (0:1 or 1:3), while the failure shear plane occurred along both loading axes for R (1:2 or 1:1). Compared with uniaxial strength, the biaxial compressive strength increased obviously, which also significantly depended on the value of R. Based on the failure modes, a modified twin-shear strength criterion was established to predict the failure surface of 3D needled C/C–SiC composites under biaxial compressive loading.

An experimental investigation on mechanical property and anchorage effect of bolted jointed rock mass

Apparent discontinuities can be easily found in natural rock medium owing to the constant motion and change of the Earth’s crust, which contains large amount of discontinuous surfaces such as faults, joints, cracks and so forth. Rock bolt is one of the most effective and economical reinforcing tools in practical geotechnical engineering for a long time. This paper investigates the mechanical properties, anchorage effect, cracking, and coalescence process of intact rock-like specimens, rock-like specimens containing flaws, and bolted rock-like specimens containing flaws. A series of uniaxial compression tests, splitting tests, and biaxial compression tests are performed on these specimens. Some findings can be observed from this study. (1) The number of rock bolt(s) and the anchoring angle have a great influence on the anchorage effect of rock bolt(s). With the increasing number of rock bolt(s), the uniaxial compressive strength (UCS), the splitting peak strength (which can be converted to the tensile strength), and biaxial compressive strength (BCS) can all be improved, whose variation tendencies do not follow a linear relationship. (2) The contributions to the tensile strength for rock bolt is greater than that to the UCS for the same-type specimen. (3) In the biaxial compression tests, with the increasing of the lateral pressures, the anchorage effect of rock bolt gradually declines and the lateral pressure plays a dominant role in improving the strength of the specimen. The failure characteristics of three types of laboratory tests have also been systematically analyzed in this paper.

Effects of frost-damage on mechanical performance of concrete

The aim of this study is to provide the quantificational change laws of strength, stiffness, and deformation capacity of frost-damaged concrete relating to a united index, the data were obtained by different researchers. Then the index of relative compressive strength (RCS) was introduced as the indicator of frost damage and a large number of mechanical performance testing data of frost-damaged concrete were collected and analyzed. By curve fitting, the correlations between RCS and the initial elastic modulus, the strain at peak compressive stress, and biaxial compressive strength, and tensile strength, and the strain at peak tensile stress were established. Thereafter, the analytical stress-strain response of frost-damaged concrete under monotonic loading was presented using RCS and compared with that of the experimental data. Moreover, an isotropic elastoplastic damage model of frost-damaged concrete subjected to repeated loading was established. Finally, we can systematically estimate the effects of frost-damage on the mechanical performance of concrete, which can be provided for the numerical simulation of frost-damaged concrete structures.

Mechanical and Failure Criteria of Air-Entrained Concrete under Triaxial Compression Load after Rapid Freeze-Thaw Cycles

The experiment study on the air-entrained concrete of 100 mm cubes under triaxial compression with different intermediate stress ratioα2=σ2D : σ3Dwas carried out using a hydraulic-servo testing system. The influence of rapid freeze-thaw cycles and intermediate stress ratio on the triaxial compressive strengthσ3Dwas analyzed according to the experimental results, respectively. The experimental results of air-entrained concrete obtained from the study in this paper and the triaxial compression experimental results of plain concrete got through the same triaxial-testing-system were compared and analyzed. The conclusion was that the triaxial compressive strength is greater than the biaxial and uniaxial compressive strength after the same rapid freeze-thaw cycles, and the increased percentage of triaxial compressive strength over biaxial compressive strength or uniaxial compressive strength is dependent on the middle stress. The experimental data is useful for precise analysis of concrete member or concrete structure under the action complex stress state.

Strength criterion of plain recycled aggregate concrete under biaxial compression

This paper presents results of biaxial compressive tests and strength criterion on two replacement percentages of recycled coarse aggregate (RPRCA) by mass for plain structural recycled aggregate concrete (RAC) at all kinds of stress ratios. The failure mode characteristic of specimens and the direction of the cracks were observed and described. The two principally static strengths in the corresponding stress state were measured. The influence of the stress ratios on the biaxial strengths of RAC was also analyzed. The experimental results showed that the ratios of the biaxial compressive strength 3f to the corresponding uniaxial compressive strength for the two RAC are higher than that of the conventional concrete (CC), and dependent on the replacement percentages of recycled coarse aggregate, stress states and stress ratios; however, the differences of tensile-compressive ratios for the two RAC and CC are smaller. On this basis, a new failure criterion with the stress ratios is proposed for plain RAC under biaxial compressive stress states. It provides the experimental and theoretical foundations for strength analysis of RAC structures subject to complex loads.

Discussion of “Effect of Uniaxial Strength and Fracture Parameters of Concrete on Its Biaxial Compressive Strength” by E. Chen and Christopher K.Y. Leung

Discussion of “Effect of Uniaxial Strength and Fracture Parameters of Concrete on Its Biaxial Compressive Strength” by E. Chen and Christopher K.Y. Leung

Closure to “Effect of Uniaxial Strength and Fracture Parameters of Concrete on Its Biaxial Compressive Strength” by E. Chen and Christopher K. Y. Leung

Closure to “Effect of Uniaxial Strength and Fracture Parameters of Concrete on Its Biaxial Compressive Strength” by E. Chen and Christopher K. Y. Leung

Numerical calibration of a yield limit function for rock materials by means of the Brazilian test and the uniaxial compression test

Numerical investigations of rocks require a realistic material description, preferably one with a constitutive law that includes damage-plasticity. The constitutive law for this work can only be set up correctly with the correct yield function. It needs a biaxial compressive strength ratio rσ and, more important, the uniaxial tensile strength σ¯t0. Difficulties in determining these parameters on rock specimens experimentally lead to indirect methods or simply estimation of the parameters. The direct uniaxial tensile test is replaced by the indirect determination of the tensile strength by means of the Brazilian test. However, the stress states at failure in the direct uniaxial tensile test and the Brazilian test are not comparable. The biaxial compressive strength ratio is generally estimated. Numerical analyses of the Brazilian test show significantly different results for 2D and 3D calculations. A distinct stress peak at the surface is observable in 3D calculations only. High-speed camera images are taken from the fracture process. They support the theory that cracks emerge from the point exhibiting the distinct stress peak, instead from the center of the specimen. The complete stress state at failure is evaluated at this critical point with numerical linear elastic calculations. The yield function is then fitted to this critical stress state for all tests and evaluated for the σ¯t0 and rσ needed as input parameters. Numerical calculations of the Brazilian test are then performed with the calibrated yield function in the damage-plasticity constitutive law. They match the experimental results.

Nonlinear multiaxial dynamic strength criterion for concrete material

Basic strength parameters of concrete material which are uniaxial compressive strength, uniaxial tensile strength, biaxial compressive strength and biaxial tensile strength are used to replace the four material parameters of generalized nonlinear strength theory in this study. The change rules and value ranges of four material parameters are acquired by studying the basic strength characteristics of concrete materials. Nonlinear multiaxial dynamic strength criterion for concrete material is proposed by four rate effect expressions of basic strength parameters for concrete material, and the four rate effect expressions are established on the basis of S criterion. The rate effect rules of dynamic strength parameters express that with the increase of strain rate, strength properties of concrete gradually transform to that of metal material. Dynamic strength of concrete is significantly influenced by the strain rate in the range of -3 to 3, and dynamic strength does not increase indefinitely, instead, there is a peak dynamic strength. Compared with dynamic loading test of biaxial compressive-compressive and biaxial tensile-compressive of concrete, nonlinear multiaxial dynamic strength criterion can describe the biaxial dynamic strength rule of concrete reasonably and the nonlinear strength characteristics at a certain strain rate. Strength rules are not influenced by coupling between the strain rate effect and multiaxial stress state, in other words, the dynamic strength surfaces dose not intersect at different strain rates.

Mechanical behaviour of different types of concrete under multiaxial compression

The strength of concrete under biaxial compression and triaxial compression is higher than the strength under uniaxial compression, but the level of increase is affected by factors such as the type of concrete, differences in testing machines, loading rate, etc. Based on this, an experimental investigation into the strength of normal air-entrained concrete (NAEC) under biaxial and triaxial compression loading conditions was carried out. The experimental results on NAEC were compared with the multiaxial compression strengths of other types of concrete obtained using the same testing machine. According to the test data, failure criteria and multiaxial ultimate strength envelopes in principal stress space were proposed. The test results indicated that biaxial compressive strength is higher than uniaxial compressive strength and the maximum increase of ultimate strength occurred at a biaxial compression stress ratio of 0·5 for all types of concrete. A relationship between biaxial compressive strength, triaxial compressive strength and stress ratio was developed. This work provides theoretical and experimental foundations for strength analysis of concrete structures.

Strengthening a dental gypsum model by infiltration of cyanoacrylate

To explore a simple but novel method of strengthening gypsum material by cyanoacrylate infiltration. To evaluate the influence of cyanoacrylate on the mechanical properties of dental gypsum models. Gypsum specimens were polished to the dimension of 35 mmx4 mmx4 mm. Butyl-cyanoacrylate was diluted with chloroform at different concentrations, namely 20% and 30% cyanoacrylate. Gypsum specimens were infiltrated by diluting one component of cyanoacrylate at different concentrations for 8 h and then dried for analysis. The changes in elastic modulus, fracture toughness, compressive strength, biaxial strength, brinell hardness were measured. The data were analyzed using software OriginPro 8. The viscosity measurements indicated that diluted cyanoacrylate were Newtonian fluids and the viscosity increased slightly within the 48 hours of preparation but still similar as water at room temperature, which could be used to infiltrating gypsum. The gypsum infiltrated with cyanoacrylate exhibited good physicochemical properties. The biaxial strength, fracture toughness, compressive strength and brinell hardness of the gypsum were improved by 39%, 30%, 63% and 18%, respectively. Cyanoacrylate can significantly improve the strength of gypsum model which indicates the potential clinical application.

Effect of Uniaxial Strength and Fracture Parameters of Concrete on Its Biaxial Compressive Strength

AbstractThe biaxial compressive behavior of concrete has been widely studied in the literature, and the biaxial envelope has always been reported for a particular uniaxial compressive strength. However, there is no theoretical or experimental evidence that concrete with similar strength exhibits similar biaxial behavior. In this study, several kinds of concrete with different maximum aggregate sizes were tested for their biaxial compressive strength under several load ratios. To ensure a uniform stress state in the cubic specimens, four layers of plastic membrane separated by three layers of butter were inserted between the steel loading platens and concrete surfaces to reduce the end friction during the test. In addition to biaxial strength, the uniaxial compressive strength, fracture energy, and characteristic length of each concrete mix were also determined. It was found that the biaxial compressive strength of concrete was dependent on both the uniaxial strength and the fracture energy (or characteris...

Experiment and Analysis on the Strength of Large Aggregate Concrete under Biaxial Compression after Freezing-Thawing Cycles

Biaxial compression tests are performed on 450mm*450mm*450mm cubic specimens of big aggregate concrete at five kinds of stress ratios,0:-1, -0.25:-1, -0.5:-1, -0.75:-1 and-1:-1 after exposure to freeze-thaw cycles of 50, 100, 150, 200, 250 and 300 times by employing a large static-dynamic true triaxial machine. Failure modes of the specimens are observed and described. The two principally static compressive strengths are measured. Based on the test data, the influences of the freeze-thaw cycles and stress ratios on the biaxial strengths of big aggregate concrete after exposure to freeze-thaw are analyzed respectively. The relationships between the ultimate compressive strength and freeze-thaw cycles, stress ratios are given respectively. The unified failure criterion with consideration of the influence of freeze-thaw cycles and stress ratios is proposed, which provides the experimental and theoretical foundations for strength analysis of big aggregate concrete structures subject to complex loads in cold environment. Key words: big aggregate concrete; freeze-thaw cycle; stress ratio; biaxial compressive strength; failure criterion