Experimental and numerical investigation of the effect of bridge area and its angularities on the failure mechanism of non-persistent crack in concrete-like materials

Alireza Bagher Shemirani,M.S. Amini,V. Sarfarazi,K. Shahriar,P. Moarefvand,Hadi Haeri 국제구조공학회 2021 Smart Structures and Systems, An International Jou Vol.27 No.3

Experimental and discrete element methods were used to investigate the effects of distance between two preexisting cracks, bridge area (The length of the bridge area) and its angularities on the shear behaviour of bridge area. A punchthrough shear test was used to model the gypsum (concrete like) cracks under shear loading. Gypsum samples (concrete like) with dimension of 120 mm × 120 mm × 50 mm were prepared in the laboratory. Within the specimen model and near its four corners, four vertical notches were provided. Three different configuration systems were prepared for notches; i.e., paralell and in plane, inside echelon and outside echelon configuration systems, respectively. In these configurations, the length of cracks were taken as 2 cm, 4 cm and 6 cm based on the cracks configuration systems. Then, 9 specimens with different lengths of the bridge area and bridge area angles were prepared. Assuming a plane strain condition, special rectangular models were prepared with dimensions of 100 mm × 100 mm. similar to those for cracks configuration systems in the experimental tests i.e., 9 models with different lengths of the bridge area and bridge area angularities 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 lengths of the bridge area and bridge area angularities. The shear strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the lengths of the bridge area. The strength of samples decreases by increasing the crack length. Also, the outside echelon crack configuration system has the maximum value of strength while the inside echelon crack configuration system has the minimum value of specimen's tensile strength. The failure pattern and failure strength are similar in both methods i.e., the experimental testing and the numerical simulation methods.

The discrete element method simulation and experimental study of determining the mode I stress-intensity factor

Shemirani, Alireza Bagher,Haeri, Hadi,Sarfarazi, Vahab,Akbarpour, Abbas,Babanouri, Nima Techno-Press 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.66 No.3

The present study addresses the direct and indirect methods of determining the mode-I fracture toughness of concrete using experimental tests and particle flow code. The direct method used is compaction tensile test and the indirect methods are notched Brazilian disc test, semi-circular bend specimen test, and hollow center cracked disc. The experiments were carried out to determine which indirect method yields the fracture toughness closer to the one obtained by the direct method. In the numerical analysis, the PFC model was first calibrated with respect to the data obtained from the Brazilian laboratory test. The crack paths observed in the simulated tests were in reasonable accordance with experimental results. The discrete element simulations demonstrated that the macro fractures in the models are caused by microscopic tensile breakages on large numbers of bonded particles. The mode-I fracture toughness in the direct tensile test was smaller than the indirect testing results. The fracture toughness obtained from the SCB test was closer to the direct test results. Hence, the semi-circular bend test is recommended as a proper experiment for determination of mode-I fracture toughness of concrete in the absence of direct tests.

The discrete element method simulation and experimental study of determining the mode I stress-intensity factor

Alireza Bagher Shemirani,Hadi Haeri,Vahab Sarfarazi,Abbas Akbarpour,Nima Babanouri 국제구조공학회 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.66 No.3

The present study addresses the direct and indirect methods of determining the mode-I fracture toughness of concrete using experimental tests and particle flow code. The direct method used is compaction tensile test and the indirect methods are notched Brazilian disc test, semi-circular bend specimen test, and hollow center cracked disc. The experiments were carried out to determine which indirect method yields the fracture toughness closer to the one obtained by the direct method. In the numerical analysis, the PFC model was first calibrated with respect to the data obtained from the Brazilian laboratory test. The crack paths observed in the simulated tests were in reasonable accordance with experimental results. The discrete element simulations demonstrated that the macro fractures in the models are caused by microscopic tensile breakages on large numbers of bonded particles. The mode-I fracture toughness in the direct tensile test was smaller than the indirect testing results. The fracture toughness obtained from the SCB test was closer to the direct test results. Hence, the semi-circular bend test is recommended as a proper experiment for determination of mode-I fracture toughness of concrete in the absence of direct tests.

A review paper about experimental investigations on failure behaviour of non-persistent joint

Shemirani, Alireza Bagher,Haeri, Hadi,Sarfarazi, Vahab,Hedayat, Ahmadreza Techno-Press 2017 Geomechanics & engineering Vol.13 No.4

There are only few cases where cause and location of failure of a rock structure are limited to a single discontinuity. Usually several discontinuities of limited size interact and eventually form a combined shear plane where failure takes place. So, besides the discontinuities, the regions between adjacent discontinuities, which consist of strong rock and are called material or rock bridges, are of utmost importance for the shear strength of the compound failure plane. Shear behaviour of persistent and non-persistent joint are different from each other. Shear strength of rock mass containing non-persistent joints is highly affected by mechanical behavior and geometrical configuration of non-persistent joints located in a rock mass. Therefore investigation is essential to study the fundamental failures occurring in a rock bridge, for assessing anticipated and actual performances of the structures built on or in rock masses. The purpose of this review paper is to present techniques, progresses and the likely future development directions in experimental testing of non-persistent joint failure behaviour. Experimental results showed that the presence of rock bridges in not fully persistent natural discontinuity sets is a significant factor affecting the stability of rock structures. Compared with intact rocks, jointed rock masses are usually weaker, more deformable and highly anisotropic, depending upon the mechanical properties of each joint and the explicit joint positions. The joint spacing, joint persistency, number of rock joint, angle of rock joint, length of rock bridge, angle of rock bridge, normal load, scale effect and material mixture have important effect on the failure mechanism of a rock bridge.

A discrete element simulation of a punch-through shear test to investigate the confining pressure effects on the shear behaviour of concrete cracks

Alireza Bagher Shemirani,Vahab Sarfarazi,Hadi Haeri,Mohammad Fatehi Marji,Seyed shahin Hosseini 사단법인 한국계산역학회 2018 Computers and Concrete, An International Journal Vol.21 No.2

A discrete element approach is used to investigate the effects of confining stress on the shear behaviour of joint’s bridge area. A punch-through shear test is used to model the concrete cracks under different shear and confining stresses. Assuming a plane strain condition, special rectangular models are prepared with dimension of 75 mm×100 mm. Within the specimen model and near its four corners, four equally spaced vertical notches of the same depths are provided so that the central portion of the model remains intact. The lengths of notches are 35 mm. and these models are sequentially subjected to different confining pressures ranging from 2.5 to 15 MPa. The axial load is applied to the punch through the central portion of the model. This testing and models show that the failure process is mostly governed by the confining pressure. The shear strengths of the specimens are related to the fracture pattern and failure mechanism of the discontinuities. The shear behaviour of discontinuities is related to the number of induced shear bands which are increased by increasing the confining pressure while the cracks propagation lengths are decreased. The failure stress and the crack initiation stress both are increased due to confining pressure increase. As a whole, the mechanisms of brittle shear failure changes to that of the progressive failure by increasing the confining pressure.

Experimental and numerical simulating of the crack separation on the tensile strength of concrete

Sarfarazi, Vahab,Haeri, Hadi,Shemirani, Alireza Bagher,Zhu, Zheming,Marji, Mohammad Fatehi Techno-Press 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.66 No.5

Effects of crack separation, bridge area, on the tensile behaviour of concrete are studied experimentally and numerically through the Brazilian tensile test. The physical data obtained from the Brazilian tests are used to calibrate the two-dimensional particle flow code based on discrete element method (DEM). Then some specially designed Brazilian disc specimens containing two parallel cracks are used to perform the physical tests in the laboratory and numerically simulated to make the suitable numerical models to be tested. The experimental and numerical results of the Brazilian disc specimens are compared to conclude the validity and applicability of these models used in this research. Validation of the simulated models can be easily checked with the results of Brazilian tests performed on non-persistent cracked physical models. The Brazilian discs used in this work have a diameter of 54 mm and contain two parallel centred cracks ($90^{\circ}$ to the horizontal) loaded indirectly under the compressive line loading. The lengths of cracks are considered as; 10 mm, 20 mm, 30 mm and 40 mm, respectively. The visually observed failure process gained through numerical Brazilian tests are found to be very similar to those obtained through the experimental tests. The fracture patterns demonstrated by DEM simulations are mostly affected by the crack separation but the tensile strength of bridge area is related to the fracture pattern and failure mechanism of the testing samples. It has also been shown that when the crack lengths are less than 30 mm, the tensile cracks may initiate from the cracks tips and propagate parallel to loading direction till coalesce with the other cracks tips while when the cracks lengths are more than 30 mm, these tensile cracks may propagate through the intact concrete itself rather than that of the bridge area.

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.

Experimental and numerical simulating of the crack separation on the tensile strength of concrete

Vahab Sarfarazi,Hadi Haeri,Alireza Bagher Shemirani,Zheming Zhu,Mohammad Fatehi Marji 국제구조공학회 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.66 No.5

Effects of crack separation, bridge area, on the tensile behaviour of concrete are studied experimentally and numerically through the Brazilian tensile test. The physical data obtained from the Brazilian tests are used to calibrate the two-dimensional particle flow code based on discrete element method (DEM). Then some specially designed Brazilian disc specimens containing two parallel cracks are used to perform the physical tests in the laboratory and numerically simulated to make the suitable numerical models to be tested. The experimental and numerical results of the Brazilian disc specimens are compared to conclude the validity and applicability of these models used in this research. Validation of the simulated models can be easily checked with the results of Brazilian tests performed on non-persistent cracked physical models. The Brazilian discs used in this work have a diameter of 54 mm and contain two parallel centred cracks (90° to the horizontal) loaded indirectly under the compressive line loading. The lengths of cracks are considered as; 10 mm, 20 mm, 30 mm and 40 mm, respectively. The visually observed failure process gained through numerical Brazilian tests are found to be very similar to those obtained through the experimental tests. The fracture patterns demonstrated by DEM simulations are mostly affected by the crack separation but the tensile strength of bridge area is related to the fracture pattern and failure mechanism of the testing samples. It has also been shown that when the crack lengths are less than 30 mm, the tensile cracks may initiate from the cracks tips and propagate parallel to loading direction till coalesce with the other cracks tips while when the cracks lengths are more than 30 mm, these tensile cracks may propagate through the intact concrete itself rather than that of the bridge area.

Experimental investigating the properties of fiber reinforced concrete by combining different fibers

Ali Ghamari,Javad Kurdi,Alireza Bagher Shemirani,Hadi Haeri 사단법인 한국계산역학회 2020 Computers and Concrete, An International Journal Vol.25 No.6

Adding fibers improves concrete performance in respect of strength and plasticity. There are numerous fibers for use in concrete that have different mechanical properties, and their combination in concrete changes its behavior. So, to investigate the behavior of the fiber reinforced concrete, an in vitro study was conducted on concrete with different fiber compositions including different ratios of steel, polypropylene and glass fibers with the volume of 1%. Two forms of fibers including singlestranded and aggregated fibers have been used for testing, and the specimens were tested for compressive strength and dividable tensile strength (splitting tensile) to determine the optimal ratio of the composition of fibers in the concrete reinforced by hybrid fibers. The results show that the concrete with a composition of steel fibers has a better performance than other compounds. In addition, by adding glass and propylene fibers to the composition of steel fibers, the strength of the samples is reduced. Also, if using the combination of fibers is required, the use of a combination of glass fibers with steel fibers will provide a better compressive strength and tensile strength than the combination of steel fibers with propylene.

Simulation of fracture mechanism of pre-holed concrete model under Brazilian test using PFC3D

Vahab Sarfarazi,Hadi Haeri,Alireza Bagher Shemirani 국제구조공학회 2018 Smart Structures and Systems, An International Jou Vol.22 No.6

In the previous studies on the porous rock strength the effect of pore number and its diameter is not explicitly defined. In this paper crack initiation, propagation and coalescence in Brazilian model disc containing a single cylindrical hole and or multiple holes have been studied numerically using PFC3D. In model with internal hole, the ratio of hole diameter to model diameter was varied between 0.03, 0.17, 0.25, 0.33, and 0.42. In model with multiple hole number of holes was different in various model, i.e., one hole, two holes, three holes, four holes, five holes, six holes, seven holes, eight holes and nine holes. Diameter of these holes was 5 mm, 10 mm and 12 mm. The pre-holed Brazilian discs are numerically tested under Brazilian test. The breakage load in the ring type disc specimens containing an internal hole with varying diameters is measured. The mechanism of cracks propagation in the wall of the ring type specimens is also studied. In the case of multi-hole Brazilian disc, the cracks propagation and b cracks coalescence are also investigated. The results shows that breaking of the pre-holed disc specimens is due to the propagation of radially induced tensile cracks initiated from the surface of the central hole and propagating toward the direction of diametrical loading. In the case of disc specimens with multiple holes, the cracks propagation and cracks coalescence may occur simultaneously in the breaking process of model under diametrical compressive loading. Finally the results shows that the failure stress and crack initiation stress decreases by increasing the hole diameter. Also, the failure stress decreases by increasing the number of hole which mobilized in failure. The results of these simulations were comprised with other experimental and numerical test results. It has been shown that the numerical and experimental results are in good agreement with each other.