A new approach for measurement of anisotropic tensile strength of concrete

Sarfarazi, Vahab,Faridi, Hamid R.,Haeri, Hadi,Schubert, Wulf Techno-Press 2015 Advances in concrete construction Vol.3 No.4

In this paper, a compression to tensile load converter device was developed to determine the anisotropic tensile strength of concrete. The samples were made from a mixture of water, fine sand and cement, respectively. Concrete samples with a hole at its center was prepared and subjected to tensile loading using the compression to tensile load converter device. A hydraulic load cell applied compressive loading to converter device with a constant pressure of 0.02 MPa per second. Compressive loading was converted to tensile stress on the sample because of the overall test design. The samples have three different configurations related to loading axis; 0, $45^{\circ}$, $-45^{\circ}$. A series of finite element analysis were done to analyze the effect of hole diameter on stress concentration of the hole side along its horizontal axis to provide a suitable criterion for determining the real tensile strength of concrete. Concurrent with indirect tensile test, Brazilian test and three point loading test were also performed to compare the results from the three methods. Results obtained by this device were quite encouraging and show that the tensile strengths of concrete were similar in different directions because of the homogeneity of bonding between the concrete materials. Also, the indirect tensile strength was clearly lower than the Brazilian test strength and three point loading test.

Numerical simulation of the effect of bedding layer on the tensile failure mechanism of rock using PFC2D

Vahab Sarfarazi,Hadi Haeri,Mohammad Fatehi Marji 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.69 No.1

In this research, the effect of bedding layer on the tensile failure mechanism of rocks has been investigated using PFC2D. For this purpose, firstly calibration of PFC2d was performed using Brazilian tensile strength. Secondly Brazilian test was performed on the bedding layer. Thickness of layers were 5 mm, 10 mm and 20 mm. in each thickness layer, layer angles changes from 0° to 90° with increment of 15°. Totally, 21 model were simulated and tested by loading rate of 0.016 mm/s. The results show that when layer angle is less than 15, tensile cracks initiates between the layers and propagate till coalesce with model boundary. Its trace is too high. With increasing the layer angle, less layer mobilizes in failure process. Also, the failure trace is very short. It’s to be noted that number of cracks decrease with increasing the layer thickness. Also, Brazilian tensile strength is minimum when bedding layer angle is between 45° and 75°. The maximum one is related to layer angle of 90°.

A fracture mechanics simulation of the pre-holed concrete Brazilian discs

Sarfarazi, Vahab,Haeri, Hadi,Shemirani, Alireza Bagher,Nezamabadi, Maryam Firoozi Techno-Press 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.66 No.3

Brazilian disc test is one of the most widely used experiments in the literature of geo-mechanics. In this work, the pre-holed concrete Brazilian disc specimens are numerically modelled by a two-dimensional discrete element approach. The cracks initiations, propagations and coalescences in the numerically simulated Brazilian discs (each containing a single cylindrical hole and or multiple holes) are studied. The pre-holed Brazilian discs are numerically tested under Brazilian test conditions. The single-holed Brazilian discs with different ratios of the diameter of the holes to that of the disc radius are modelled first. The breakage load in the ring type disc specimens containing an internal hole with varying diameters is measured and the crack propagation mechanism around the wall of the ring is investigated. The crack propagation and coalescence mechanisms are also studied for the case of multi-holes' concrete Brazilian discs. The numerical and experimental results show that the breaking mechanism of the pre-holed disc specimens is mainly due to the initiation of the radially induced tensile cracks which are growth from the surface of the central hole. Radially cracks propagated toward the direction of diametrical loading. It has been observed that for the case of disc specimens with multiple holes under diametrical compressive loading, the breaking process of the modelled specimens may occur due to the simultaneous cracks propagation and cracks coalescence phenomena. These results also show that as the hole diameter and the number of the holes increases both the failure stress and the crack initiation stress decreases. The experimental results already exist in the literature are quit agree with the proposed numerical simulation results which validates this simulation procedure.

Simulation of the tensile behaviour of layered anisotropy rocks consisting internal notch

Vahab Sarfarazi,Hadi Haeri,P. Ebneabbasi,Kourosh Bagheri 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.69 No.1

In this paper, the anisotropy of tensile behaviours of layered rocks consisting internal notch has been investigated using particle flow code. For this purpose, firstly calibration of PFC2D was performed using Brazilian tensile strength. Secondly Brazilian test models consisting bedding layer was simulated numerically. Thickness of layers was 10 mm and layered angularity was 90°, 75°, 60°, 45°, 30°, 15° and 0°. The strength of bedding interface was too high. Each model was consisted of one internal notch. Notch length is 1 cm, 2 cm and 4 cm and notch angularities are 60°, 45°, 30°, 15° and 0°. Totally, 90 model were tested. The results show that failure pattern was affected by notch orientation and notch length. It’s to be noted that layer angle has not any effect on the failure pattern. Also, Brazilian tensile strength is affected by notch orientation and notch length.

Investigation of ratio of TBM disc spacing to penetration depth in rocks with different tensile strengths using PFC2D

Vahab Sarfarazi,Hadi Haeri,Alireza Bagher Shemirani,Ahmadreza Hedayat,Seyed Shahin Hosseini 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.20 No.4

In this study, the effect of the tensile strength and ratio of disc spacing to penetration depth on the efficiency of tunnel boring machine (TBM) is investigated using Particle flow code (PFC) in two dimensions. Models with dimensions of 150×70 mm made of rocks with four different tensile strength values of 5 MPa, 10 MPa, 15 MPa and 20 MPa were separately analyzed and two “U” shape cutters with width of 10 mm were penetrated into the rock model by velocity rate of 0.1 mm/s. The spacing between cutters was also varied in this study. Failure patterns for 5 different penetration depths of 3 mm, 4 mm, 5 mm, 6 mm, and 7 mm were registered. Totally 100 indentation test were performed to study the optimal tool-rock interaction. An equation relating mechanical rock properties with geometric characteristics for the optimal TBM performance is proposed. The results of numerical simulations show that the effective rock-cutting condition corresponding to the minimum specific energy can be estimated by an optimized disc spacing to penetration depth, which, in fact, is found to be proportional to the rock’s tensile strength.

Effect of the circle tunnel on induced force distribution around underground rectangular gallery using theoretical approach, experimental test and particle flow code simulation

Vahab Sarfarazi,Reza Bahrami,Shadman Mohammadi Bolbanabad,Fariborz Matinpoor 국제구조공학회 2022 Structural Engineering and Mechanics, An Int'l Jou Vol.84 No.5

In this study, the effect of circle tunnel on the force distribution around underground rectangular gallery was investigated using theoretical approach, experimental test and Particle flow code simulation (PFC). Gypsum model with dimension of 1500×1500 mm was built. Tensile strength of material was 1 MPa. Dimension of central gallery was 100 mm×200 mm and diameter of adjacent tunnel in its right side was 20 mm, 40 mm and 60 mm. Horizontal distance between tunnel wall and gallery edge were 25, 50, 75, 100 and 125 mm. using beam theory, the effect of tunnel diameter and distance between tunnel and gallery on the induced force around gallery was analyzed. In the laboratory test, the rate of loading displacement was set to 0.05 millimeter per minute. Also sensitivity analysis has been done. Using PFC2D, interaction between tunnel and gallery was simulated and its results were compared with experimental and theoretical analysis. The results show that the tensile force concentration has maximum value in center of the rectangular space. The tensile force concentration at the right side of the axisymmetric line of gallery has more than its value in the left side of the galleries axisymmetric line. The tensile force concentration was decreased by increasing the distance between tunnel and rectangular space. In whole of the configurations, the angles of micro cracks fluctuated between 75 and 105 degrees, which mean that the variations of tunnel situation have not any influence on the fracture angle.

Direct and indirect methods for determination of mode I fracture toughness using PFC2D

Vahab Sarfarazi,Hadi Haeri,Alireza Bagher Shemirani 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.20 No.1

In this paper, mode I fracture toughness of rock was determined by direct and indirect methods using Particle Flow Code simulation. Direct methods are compaction tension (CT) test and hollow centre cracked quadratic sample (HCCQS). Indirect methods are notched Brazilian disk (NBD) specimen, the semi-circular bend (SCB) specimen, hollow centre cracked disc (HCCD), the single edge-notched round bar in bending (SENRBB) specimen and edge notched disk (END). It was determined that which one of indirect fracture toughness values is close to direct one. For this purpose, initially calibration of PFC was undertaken with respect to data obtained from Brazilian laboratory tests to ensure the conformity of the simulated numerical models response. Furthermore, the simulated models in five introduced indirect tests were cross checked with the results from direct tests. By using numerical testing, the failure process was visually observed. Discrete element simulations demonstrated that the macro fractures in models are caused by microscopic tensile breakages on large numbers of bonded discs. Mode I fracture toughness of rock in direct test was less than other tests results. Fracture toughness resulted from semi-circular bend specimen test was close to direct test results. Therefore semi-circular bend specimen can be a proper test for determination of Mode I fracture toughness of rock in absence of direct test.

Z shape joints under uniaxial compression

Sarfarazi, Vahab,Asgari, Kaveh,Mahshoori, Ali Reza Techno-Press 2021 Advances in concrete construction Vol.12 No.2

In this paper, the effects of angle of Z shape non-persistent joint on the compressive behaviour of joint's bridge area under uniaxial compression load have been investigated. Furtheremore experimental test and Particle flow code in two dimension (PFC2D) have been used. concrete samples with dimension of 50 mm×50 mm×100 mm were prepared. Tensile strength of model material was 1 MPa. Within the specimen, three Z shape non-persistent notches were provided. The notch length was 1.5 cm. when two upper and lower notch have horizontal direction, the middle joint angle were 45°, 90° and 135°. When dips of two upper and lower notch was 90°, the middle joint angle were 45°, 90° and 135° degree. When dips of two upper and lower notch was 135 degree, the middle joint angle were 45°, 90° and 135°. Totally, 9 different configuration systems were prepared for Z shape non-persistent joints. Also, 37 models with different Z shape non-persistent notch angle were prepared in numerical model. The axial load with rate of 0.005 mm/s was applied to the model. Results indicated that the failure process was mostly governed by the Z shape non-persistent joint angle. The compressive strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. In addition it was shown that the compresssive behaviour of discontinuities is depend on the number of the induced tensile cracks which are increased by increasing the middle joint angle. The middle joint has significant effect on the failure pattern. In fixed upper and lower joints angles, when the middle joint angle is 90° the failure stress has maximum value. On the other hand, it has minimum value when the middle joint angle was 135°. In fixed middle joint angle, the samples have minimum value when the upper and lower joints angles are 135°. At the end, the failure pattern and strength results of the experiential tests and the numerical simulations were similar.

Study of cracks in compressed concrete specimens with a notch and two neighboring holes

Vahab, Sarfarazi,Kaveh, Asgari,Shirin, Jahanmiri,Mohammad Fatehi, Marji,Alireza Mohammadi, Khachakini Techno-Press 2022 Advances in concrete construction Vol.14 No.5

This paper investigated computationally and experimentally the interaction here between a notch as well as a micropore under uniaxial compression. Brazilian tensile strength, uniaxial tensile strength, as well as biaxial tensile strength are used to calibrate PFC2d at first. Then, uniaxial compression test was conducted which they included internal notch and micro pore. Experimental and numerical building of 9 models including notch and micro pore were conducted. Model dimensions of models are 10 cm × 10 cm × 5 cm. Joint length was 2 cm. Joints angles were 30°, 45° and 60°. The position of micro pore for all joint angles was 2cm upper than top of the joint, 2 cm upper than middle of joint and 2 cm upper than the joint lower tip, discreetly. The numerical model's dimensions were 5.4 cm × 10.8 cm. The fractures were 2 cm in length and had angularities of 30, 45, and 60 degrees. The pore had a diameter of 1 cm and was located at the top of the notch, 2 cm above the top, 2 cm above the middle, and 2 cm above the bottom tip of the joint. The uniaxial compression strength of the model material was 10 MPa. The local damping ratio was 0.7. At 0.016 mm per second, it loaded. The results show that failure pattern affects uniaxial compressive strength whereas notch orientation and pore condition impact failure pattern. From the notch tips, a two-wing fracture spreads almost parallel to the usual load until it unites with the sample edge. Additionally, two wing fractures start at the hole. Both of these cracks join the sample edge and one of them joins the notch. The number of wing cracks increased as the joint angle rose. There aren't many AE effects in the early phases of loading, but they quickly build up until the applied stress reaches its maximum. Each stress decrease was also followed by several AE effects. By raising the joint angularities from 30° to 60°, uniaxial strength was reduced. The failure strengths in both the numerical simulation and the actual test are quite similar.

Study of compressive behavior of triple joints using experimental test and numerical simulation

Vahab Sarfarazi,Xiao Wang,Mojtaba Nesari,Erfan Zarrin Ghalam 국제구조공학회 2022 Smart Structures and Systems, An International Jou Vol.30 No.1

Experimental and discrete element methods were used to investigate the effects of triple joints lengths and triple joint angle on the failure behavior of rock mass under uniaxial compressive test. Concrete samples with dimension of 20 cm × 20 cm × 5 cm were prepared. Within the specimen, three imbedded joint were provided. The joint lengths were 2 cm, 4cm and 6 cm. In constant joint lengths, the angle between middle joint and other joints were 30°, 60°, 90°, 120° and 150°. Totally 15 different models were tested under compression test. The axial load rate on the model was 0.05 mm/min. Concurrent with experimental tests, the models containing triple joints, length and joint angle are similar to the experiments, were numerical by Particle flow code in two dimensions (PFC2D). Loading rate in numerical modelling was 0.05 mm/min. Tensile strength of material was 1 MPa. The results show that the failure behaviors of rock samples containing triple joints were governed by both of the angle and the length of the triple joints. The uniaxial compressive strengths (UCS) of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. Furthermore, it was shown that the compressive behavior of discontinuities is related to the number of the induced tensile cracks which are increased by decreasing the joint length. Along with the damage failure of the samples, the acoustic emission (AE) activities are excited. There were only a few AE hits in the initial stage of loading, then AE hits rapidly grow before the applied stress reached its peak. In addition, every stress drop was accompanied by a large number of AE hits. Finally, the failure pattern and failure strength are similar in both methods i.e., the experimental testing and the numerical simulation methods.