Numerical Simulation of Tensile Failure of Concrete in Direct, Flexural, Double Punch Tensile and Ring Tests

The present study considers the tensile strength of concrete samples in direct, flexural, double punch and ring tests using both of the experimental tests and numerical simulation (particle flow code 2D). It determined that which one of indirect tensile strength is close to direct tensile strength. Initially calibration of PFC was undertaken with respect to the data obtained from Brazilian laboratory tests to ensure the conformity of the simulated numerical models response. Furthermore, validation of the simulated models in four introduced tests was also cross checked with the results from experimental tests. By using numerical testing, the failure process was visually observed and failure patterns were watched to be reasonable in accordance with experimental results. Discrete element simulations demonstrated that the macro fractures in models are caused by microscopic tensile breakages on large numbers of bonded discs. Tensile strength of concrete in direct test was less than other tests results. Tensile strength resulted from punch test was close to direct test results. So punch test can be a proper test for determination of tensile strength of concrete in absence of direct test. Other advantages shown by punch tests are: (1) the punch test need less sample size compared with other tests, (2) less material is need for sample preparation, (3) sample preparation is easy and (4) the use of a simple conventional compression press controlled by displacement compared with complicate device in other tests

Numerical Simulation of Shear Behaviour of Non- Persistent Joints under Low and High Normal Loads

In this paper, the effect of rock bridge surface on the shear behavior of planar non-persistent joints under low and high normal loads has been investigated using particle flow code in 2 Dimensions. PFC2d was calibrated with respect to the data obtained from experimental laboratory tests to ensure the conformity of the simulated numerical models response. The models consisting non-persistent joint were simulated and tested by shear loading under low and high normal loads. The discrete element simulations demonstrated that the failure pattern was mostly influenced by normal load, while the shear strength was linked to the failure pattern and failure mechanism. The failure patterns were found reasonably similar to the experimentally observed trends

Investigation of the Effect of Bedding Layer Angle and Tunnel Number on the Stability of Tunnel under Uniaxial Compression Using PFC2D

In this paper the effect of bedding layer angle on the stability of tunnel under uniaxial compression have been investigated using particle flow code in two dimensions (PFC2D). For this purpose, numerical rectangle models with dimension of 100*100 mm have been prepared. These models consist of layers with different mechanical properties i.e., concrete layer and gypsum layer. The angle of these layers related to horizontal axis change from 0° to 90° with increment of 15°. These models are consisting of one, two and three tunnel. The diameter of tunnel change based on the tunnel number. The tunnel diameter was 6 m, when one tunnel exists in the model. The tunnel diameter was 3 m, when two tunnels exist in the model. The tunnel diameter was 2 m, when three tunnels exist in the model. These models were subjected to uniaxial compression. The results show that tensile cracks are dominant mode of fracture occurred in the models. The joint angle and tunnel number have important effect on the failure pattern and failure strength. Also, the mechanical properties of beddings control the crack growth path. The crack grows through the weak layers when bedding angle was equal to 45° and 60°, but it intersects the layer for any other bedding angels

Three-dimensional Discrete Element Simulation of Interaction between Aqueduct and Tunnel

In this investigation the effect of interaction between aqueduct and tunnel on the ground settlement has been examined using PFC3D. At first, the calibration of PFC3D was conducted based on UCS test results rendered from three different ground layer. Then intact model with dimension of 70 m × 20 m × 34.5 m (x × y × z) was built. These models are consisted of 8 layers with different mechanical and geometrical properties. Four different configurations for aqueduct were created in four models. Diameter of aqueduct was 2 m and its depth was different in four models. After aqueduct generation, tunnel with diameter of 9 m and length of 20 m was drilled in depth of 22 m. After tunnel drilling, the settlement data of ground surface were picked up. After tunnel simulation, the effect of support lining was investigated on the ground settlement. For this purpose, after each step of tunnel drilling, lining support with diameter of 35 cm was performed. The results show that the maximum value of settlement occurred when aqueduct reach to head of tunnel. Also the safety has maximum value when the distance between aqueduct and tunnel wall was 4.5 m. the ground settlement before and after support lining shows that ground settlement reach to zero by support application

Investigation of Separation Non-Persistent Faults in Fracture Mechanism of Rock Bridge

Rock mass is a heterogeneous material included joints, fractures and faults. The necessity of rock mechanics studies in conducting constructional issues has become important due to the increase in constructional works and the expansion of the structure’s dimension and especially creating underground spaces in rock masses. Faults are the most important discontinuous fractures in the earth's crust in which the two sides of the fracture have moved relative to each other. The purpose of this research is that if the non-persistent faults were situated adjacent to each other, how would be the shear failure mechanism of Rock Bridge surrounded between the faults. For this purpose, physical model consisting two horizontal edge faults and a surrounded angled fault was built; angularity of the central fault varies from 0° to 60° with increasing the 30°. The central fault places in 3 different positions. Along the lateral faults, 1.5 cm vertically far from the edge faults and 3 cm vertically far from the edge faults. All samples tested by uniaxial test machine so that shear load was distributed in the specimens due to special geometry of specimen. The results show that the failure pattern was mostly influenced by configuration of central joint, while the shear strength was linked to the failure pattern and failure mechanism

Fracture Mechanism of Brazilian Discs with Multiple Parallel Notches Using PFC2D

This study presents crack initiation, propagation and coalescenceat or near pre-existing open cracks in a numericalmodel under Brazilian test. Firstly, Particle Flow Code intwo dimensions (PFC2d) was calibrated with respect to thedata obtained from experimental laboratory tests to ensurethe conformity of the simulated numerical models response.Brazilian discs contain one, two, three, four, and five parallelcentred cracks (45° to the horizontal) under compressiveline loading. Models containing two and three cracks havedifferent joint spacing and joint configuration. In model consistingone flaws, tensile cracks initiated from notch tip andpropagates in direction of compressive loading till coalescewith model edge. By increasing the number of notch, first typeof tensile crack initiated at the tips of outer flaws and coalescedwith model edge. Also second type of tensile cracksinitiates from middle of inner flaws and coalesce with tip ofthe neighbouring flaws. The results show that joint spacingand joint configuration has important effect on the failure patternin model consisting two and three notch. Experimentaland numerical results rendered by other researchers showed agood agreement with the numerical results in the coalescencecharacteristics in cracked model. In addition, crack initiationand coalescence stresses in models were analyzed and comparedwith those in the single-flawed model

A New Approach for Measurement of Tensile Strength of Concrete

Tensile strength is one of the important mechanical properties of concrete. Indirect methods have been used up till now for its measurement. These methods though widely accepted, do not furnish the true tensile strength of concrete. In this paper, a compression to tensile load transformer device is developed to determine direct tensile strength of concrete. The cubic concrete samples with dimension of 15*19*6 cm containing internal hole with dimension of 75mm is prepared using the mixture of cement, fine sand and water. This model is subjected to direct tensile loading by special loading device. A series of finite element analysis were performed using FRANC2D to analyse the effect of hole diameter on the stress concentration around the hole and to render a suitable criteria for determination of real tensile strength of concrete. Concurrent with direct tensile test, Brazilian test is performed to compare the results of two methods. Results obtained by this test are quite encouraging and shows that the direct tensile strength is clearly lower than the Brazilian test. The dierence between the Brazilian and direct tensile strengths is about 33%

Crack growth mechanism in granite specimens with non-persistent joints under punch shear through test

Experimental and numerical methods (Particle Flow Code) were used to investigate the effect of echelon notches on the shear behavior of the joint’s bridge area in granite. A punch-through shear test was used to model the granite cracks under shear loading. Granite samples with dimension of 20 mm×150 mm×40 mm were prepared in the laboratory. Within the specimen model and near the edges, four edge notches were provided. Nine different configuration systems were prepared for notches. In these configurations, the length of each notch was taken as 3 cm, 4cm and 5 cm. Assuming a plane strain condition, special rectangular models were prepared with dimensions of 100 mm×100 mm using the particle flow code in two dimensions (PFC2D). Similar to those joints’ configuration systems in the experimental tests, i.e. 9 models with different rock bridge lengths and different rock bridge joint angles were prepared. The axial load was applied to the punch through the central portion of the model. This testing showed that the failure process was mostly governed by the rock bridge length and the rock bridge angle.  Shear strengths of the specimens were related to fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behavior of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the rock bridge angle.  The strength of samples decreases with increasing the joint length. The failure pattern and failure strength are similar in both methods, i.e. the experimental testing and the numerical simulation