جستجوی مقالات مرتبط با کلیدواژه « Tensile strength » در نشریات گروه « زمین شناسی »

Today, shotcrete as a support system is widely applied in various surface and underground structures. Hence, in recent years, numerous studies have been conducted to improve the quality of shotcrete including the strength and ductility. Investigating the impact of adding different fibers to shotcrete is one of the most important research done in the field of shotcrete development. In the present research, the effect of using waste tire textile fibers on the tensile strength of shotcrete has been investigated by field implementation. For this purpose, the operation of shotcreting containing 0.5, 1, and 1.5% of waste tire textile fibers along with fiber-free panels was performed at the Baghan-Marivan tunnel project site. After preparing the required samples, the Brazilian tensile strength test was performed. The results demonstrated that waste tire textile fiber causes a significant increase in the energy absorption capacity of shotcrete and eliminate its brittle behavior. Other advantages of waste tire textile fibers include reduction of rebound, easy pumping, low weight, as well as high durability of fibers in acidic and wet environments. Forthermore, since this fiber is recycled, its utilizing in shotcrete industary, in addition to reducing the environmental problems, will lead to a significant reduction in the cost of the project.

Rock anisotropy plays an important role in engineering behavior of rocks. Slates are anisotropic rocks which have long been used for gable roof, floor tiles, borrow materials, and other purposes. The slates studied in this research are calcareous and have a porphyro-lepidoblastic texture. To determine the role of the anisotropy on the tensile strength and fracture pattern, two variables including ψ (the core axis angle to foliation) and β (the angle between the axis of loading and foliation) in the Brazilian tests were used. The angles were selected at 15° intervals. Thus, for both ψ and β, seven angles of 0˚, 15˚, 30˚, 45˚, 60˚, 75˚, and 90˚ were selected (i.e., there are 43 possible modes). In order to name and examine the failure pattern, 11 models were proposed. The average value of the failure strength for the three stations varies from 3.21 MPa to 20.94 MPa. Based on the obtained results, there is a direct relation between the average tensile strength and density. A comparison between Brazilian test data under dry and saturation conditions shows that the saturated Brazilian tensile strength is 30.8% less than the dry Brazilian tensile strength. Moreover, the changes in fracture length with the changes in ψ and β indicate an inverse relation. Eventually, the average of tensile strength (σt) and strength anisotropy index (Ia) demonstrates that the influence of orientation angle (ψ) is much larger than that of foliation-loading angle (β).

Extended Abstract (Paper pages157-174) Introduction Considering the fact that the estimation of mode fracture toughness by testing is time-consuming and expensive. It might be associated with certain practical difficulties. Therefore, many researchers have attempted to propose experimental relationships in order to capture these problems. Gunsallus et al. (1984) and Bhagat (1985) experimentally found that mode fracture toughness is related to tensile strength. Whittaker et al. (1992) have also proposed a number of relationships between mode I fracture toughness, tensile strength, point load index, uniaxial compressive strength and the velocity of sound waves. Bearman (1999) obtained an experimental relationship between mode I fracture toughness and point load index, while Brown et al. (1997) presented an experimental relationship between this parameter and density. Up to now no significant research effort has been made in this field in Iran, only Ayatollahi and Fatehi addressed rock fracture toughness. Although, Ayatollahi has not presented any experimental relationships. In the present research the three-point bending test was used on a cylindrical specimen containing a straight crack in order to determine the mode fracture toughness, and the Brazilian test was employed to determine tensile strength. Materials and Methods The tests were carried out on six types of rocks, namely gray sandstone, tuff, lithic tuff, travertine, andesite, and limestone. Sandstone, travertine, and limestone are sedimentary rocks, while andesite is an extrusive igneous rock, and tuff and lithic tuff are pyroclastic rocks (pyroclastic rocks resulting from volcanic eruptions that harden by sedimentation). Therefore, the studied rocks have different origins. In order to carry out the Brazilian and the three-point bending test, cores were prepared from these blocks. In order to perform the three-point bending test, specimens with diameter of 73 mm with a thickness of 30 mm were used. The samples were cut in two semicircular by a cutting machine, and a notch with length of 15 mm is created by a diamond saw. Notch is vertical in the center of the semicircular samples. The Brazilian test was performed on disc shaped specimens. In order to perform the Brazilian test, specimens with diameter of 51 mm and thick of 25 mm were used. The specimens are carefully placed under the curved jaws of the machine and then loaded until fracture. Results and Discussion A summary of the Brazilian and the three-point bending test results are presented in Table 1. The average value of test result pertaining to each rock is reported in Table 1. Table 1. Summary of the Brazilian and the three-point bending test results Specimen Tensile Strength (MPa) Fracture Toughness (MPa√m) Limestone 3.74 1.23 Sandstone 7.14 1.63 Tuff 16.36 2.17 Lithic Tuff 4.34 1.01 Andesite 13.25 1.86 Travertine 8.27 1.14 In this study, it was attempted to propose an experimental relationship between mode I fracture toughness and the tensile strength of the rock. In order to determine the relationship between the tensile strength and the fracture toughness, the tensile strength vs. fracture toughness diagram was plotted in Excel to obtain Eq. 1 and the coefficient of determination (R2) (Figure 1). K_IC=0.0812σ_t.7878 (1) R^2=0.7977 The coefficient of determination (R2) in Eq. 1 shows that almost 80 percent of the mode I fracture toughness variations can be estimated using the linear relationship (Eq. 1). The relationship is applicable for determining the mode I fracture toughness resulting from the three-point bending test on semicircular specimens containing a straight crack. In the following, the results of this study are compared to those reported by Whittacker (1992) and Zhang (2002). In order to examine the accuracy of the presented relationships, the Root Mean Square Error (RMSE) measure was used which is computed from Eq. 2. In the best case, RMSE is zero. RMSE=√(1/n ∑_(i=1)^n▒〖(y_i-〖y_i〗^')〗^2 ) (2) In the relationships, y_i represents the fracture toughness obtained from testing while 〖y_i〗^' is the fracture toughness estimated using the relationships. Comparison of the obtained results indicate that the proposed relationship has the capability of precise estimation of the mode I fracture toughness of rocks. Conclusion Given the many difficulties associated with the direct estimation of fracture toughness, indirect estimation methods have been proposed. One of such methods is the estimation of mode I fracture toughness using tensile strength. A linear relationship with a coefficient of determination of 0.7977 was proposed. The accuracy of this relationship has been verified by comparing its results to those from previous studies.

For estimating the rock strength and brittleness, Schmidt rebound hammer hardness value (SRH) is a useful and inexpensive technics. Brittleness depends on rock strength indicating the resistance of rock to elastic deformation. Since the direct methods for measuring of brittleness were not standardized, indirect methods such as Schmidt hammer were used in engineering geology. For this purpose, Schmidt rebound number, uniaxial compressive strength and Brezilian tensile test of seven granite rocks outcrops in Borujerd area were measured. Also, B1, B2, B3 and B4 concepts of brittleness and brittleness index (BI) as well as their relations with Schmidt rebound hammers hardness value (SRH) of the rocks were determined (in natural water content). The results show a good positive corrolation among Schmidt rebound hammers hardness value (SRH), brittleness index (BI) and brittleness concepts of these rocks(r2>0.7). Borujerd granites according to brittleness classificatoin system fall in to slighty brittle. The maximum and minimum brittleness index value in dry condition of rocks are 4.94 and 3.13, respectively. The study shows that the brittleness of the rocks decrease with an increase in water content. In saturation condition, the minimum brittleness concept(B4) is 5.27 Mpa. Monzogranites having B4
28 Mpa are very brittle and hard for drilling.